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  • 1.
    Agrawal, Vishal
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Solid Mechanics.
    Scapin, Nicolo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Department of Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway.
    An efficient isogeometric/finite-difference immersed boundary method for the fluid–structure interactions of slender flexible structures2024In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 418, article id 116495Article in journal (Refereed)
    Abstract [en]

    In this contribution, we present a robust and efficient computational framework capable of accurately capturing the dynamic motion and large deformation/deflection responses of highly-flexible rods interacting with an incompressible viscous flow. Within the partitioned approach, we adopt separate field solvers to compute the dynamics of the immersed structures and the evolution of the flow field over time, considering finite Reynolds numbers. We employ a geometrically exact, nonlinear Cosserat rod formulation in the context of the isogeometric analysis (IGA) technique to model the elastic responses of each rod in three dimensions (3D). The Navier–Stokes equations are resolved using a pressure projection method on a standard staggered Cartesian grid. The direct-forcing immersed boundary method is utilized for coupling the IGA-based structural solver with the finite-difference fluid solver. In order to fully exploit the accuracy of the IGA technique for FSI simulations, the proposed framework introduces a new procedure that decouples the resolution of the structural domain from the fluid grid. Uniformly distributed Lagrangian markers with density relative to the Eulerian grid are generated to communicate between Lagrangian and Eulerian grids consistently with IGA. We successfully validate the proposed computational framework against two- and three-dimensional FSI benchmarks involving flexible filaments undergoing large deflections/motions in an incompressible flow. We show that six times coarser structural mesh than the flow Eulerian grid delivers accurate results for classic benchmarks, leading to a major gain in computational efficiency. The simultaneous spatial and temporal convergence studies demonstrate the consistent performance of the proposed framework, showing that it conserves the order of the convergence, which is the same as that of the fluid solver.

  • 2. Ahmed, Z.
    et al.
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Costa, Pedro
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Muradoglu, M.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Turbulent bubbly channel flows: Effects of soluble surfactant and viscoelasticity2020In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 212, article id 104717Article in journal (Refereed)
    Abstract [en]

    Interface-resolved direct numerical simulations are performed to examine the combined effects of soluble surfactant and viscoelasticity on the structure of a bubbly turbulent channel flow. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is achieved through a non-linear equation of state which relates the surface tension to the surfactant concentration at the interface. The two-fluid Navier-Stokes equations are solved using a front-tracking method, augmented with a very efficient FFT-based pressure projection method that allows for massively parallel simulations of turbulent flows. It is found that, for the surfactant-free case, bubbles move toward the wall due to inertial lift force, resulting in formation of wall layers and a significant decrease in the flow rate. Conversely, a high-enough concentration of surfactant changes the direction of lateral migration of bubbles, i.e., the contaminated bubbles move toward the core region and spread out across the channel. When viscoelasticity is considered, viscoelastic stresses counteract the Marangoni stresses, promoting formation of bubbly wall-layers and consequently strong decrease in the flow rate. The formation of bubble wall-layers for combined case depends on the interplay of the inertial and elastic, and Marangoni forces. 

  • 3.
    Ahmed, Zaheer
    et al.
    Koc Univ, Dept Mech Engn, Istanbul, Turkey..
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lu, Jiacai
    Johns Hopkins Univ, Dept Mech Engn, Baltimore, MD 21218 USA..
    Tryggvason, Gretar
    Johns Hopkins Univ, Dept Mech Engn, Baltimore, MD 21218 USA..
    Muradoglu, Metin
    Koc Univ, Dept Mech Engn, Istanbul, Turkey..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Effects of soluble surfactant on lateral migration of a bubble in a pressure driven channel flow2020In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 126, article id 103251Article in journal (Refereed)
    Abstract [en]

    The effects of soluble surfactant on the lateral migration of a bubble in a pressure-driven channel flow are examined by interface-resolved numerical simulations. The interfacial and bulk surfactant concentration evolution equations are solved fully coupled with the incompressible Navier-Stokes equations. A non-linear equation of state is used to relate interfacial surface tension to surfactant concentration at the interface. Extensive computations are performed to investigate the bubble dynamics for a wide range of parameters. It is found that surfactant dramatically changes the bubble dynamics. In the clean case, the bubble position depends on its deformability, characterized by the Eotvos (Eo) and the capillary (Ca) numbers. The spherical bubble moves towards the wall, while the deformable one migrates away from it. On the other hand, in the presence of the surfactant, even the spherical bubble moves away from the wall. It is also found that the contaminated bubble stays away from the wall for Eo = 0.1 and Eo = 1.5 while it migrates towards the wall for 0.1 < Eo < 1.5. Also, at high Eo, the onset of path instability is observed for both the clean and the contaminated cases. However, adding surfactant to the system triggers the path instability earlier and amplifies the oscillations afterwards.

  • 4. Alegre, Cesar
    et al.
    McNally, Donal
    Choi, Kwing-So
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The effect or arterial flow elasticity on the flow through a stenosis2016In: Proceedings of the International Conference on Bionic Engineering 2016, 2016Conference paper (Refereed)
  • 5. Alegre-Martínez, C.
    et al.
    Choi, K. -S
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    McNally, D.
    On the axial distribution of plaque stress: Influence of stenosis severity, lipid core stiffness, lipid core length and fibrous cap stiffness2019In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 68, p. 76-84Article in journal (Refereed)
    Abstract [en]

    Numerical simulations of blood flow through a partially-blocked axisymmetric artery are performed to investigate the stress distributions in the plaque. We show that the combined effect of stenosis severity and the stiffness of the lipid core can drastically change the axial stress distribution, strongly affecting the potential sites of plaque rupture. The core stiffness is also an important factor when assessing plaque vulnerability, where a mild stenosis with a lipid-filled core presents higher stress levels than a severe stenosis with a calcified plaque. A shorter lipid core gives rise to an increase in the stress levels. However, the fibrous cap stiffness does not influence the stress distributions for the range of values considered in this work. Based on these mechanical analyses, we identify potential sites of rupture in the axial direction for each case: the midpoints of the upstream and downstream regions of the stenosis (for severe, lipid-filled plaques), the ends of the lipid core (for short cores), and the middle of the stenosis (for mild stenoses with positive remodelling of the arterial wall). 

  • 6.
    Amini, Kasra
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mishra, Ases Akas
    Department of Industrial and Materials Science, Chalmers University of Technology, 412 96 Gothenburg, Sweden.
    Sivakumar, Amit Kumar
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden.
    Arlov, Dragana
    Tetra Pak Processing Systems, 221 86 Lund, Sweden.
    Innings, Fredrik
    Tetra Pak Processing Systems, 221 86 Lund, Sweden.
    Kádár, Roland
    Department of Industrial and Materials Science, Chalmers University of Technology, 412 96 Gothenburg, Sweden.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Scaling laws for near-wall flows of thixo-elasto-viscoplastic fluids in a millifluidic channel2024In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 36, no 2, article id 023107Article in journal (Refereed)
    Abstract [en]

    Thixo-elasto-viscoplastic (TEVP) fluids are very complex fluids. In addition to elasticity and viscoplasticity, they exhibit thixotropy, i.e., time-dependent rheology due to breakdown and recovery of internal structures at different length- and timescales. General and consistent methods for a priori flow prediction of TEVP fluids based on rheological characteristics are yet to be developed. We report a combined study of the rheology and flow of 18 samples of different TEVP fluids (three yogurts and three concentrations of Laponite and Carbopol, respectively, in water in both the unstirred and a stirred state). The rheology is determined both with standard protocols and with an ex situ protocol aiming at reproducing the shear history of the fluid in the flow. Micrometer resolution flow measurements in a millimeter scale rectangular duct are performed with Doppler Optical Coherence Tomography (D-OCT). As expected, the results show the existence of a plug flow region for samples with sufficiently high yield stress. At low flow rates, the plug extends almost all the way to the wall and the extent of the plug decreases not only with increased flow rate but also with increased thixotropy. The ex situ rheology protocol enables estimation of the shear rate and shear stress close to the wall, making it possible to identify two scaling laws that relates four different non-dimensional groups quantifying the key properties wall-shear stress and slip velocity. The scaling laws are suggested as an ansatz for a priori prediction of the near-wall flow of TEVP fluids based on shear flow-curves obtained with a rheometer.

  • 7.
    Bazesefidpar, Kazem
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    A dual resolution phase-field solver for wetting of viscoelastic droplets2022In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363, Vol. 94, no 9, p. 1517-1541Article in journal (Refereed)
    Abstract [en]

    We present a new and efficient phase-field solver for viscoelastic fluids with moving contact line based on a dual-resolution strategy. The interface between two immiscible fluids is tracked by using the Cahn-Hilliard phase-field model, and the viscoelasticity incorporated into the phase-field framework. The main challenge of this approach is to have enough resolution at the interface to approach the sharp-interface methods. The method presented here addresses this problem by solving the phase field variable on a mesh twice as fine as that used for the velocities, pressure, and polymer-stress constitutive equations. The method is based on second-order finite differences for the discretization of the fully coupled Navier–Stokes, polymeric constitutive, and Cahn–Hilliard equations, and it is implemented in a 2D pencil-like domain decomposition to benefit from existing highly scalable parallel algorithms. An FFT-based solver is used for the Helmholtz and Poisson equations with different global sizes. A splitting method is used to impose the dynamic contact angle boundary conditions in the case of large density and viscosity ratios. The implementation is validated against experimental data and previous numerical studies in 2D and 3D. The results indicate that the dual-resolution approach produces nearly identical results while saving computational time for both Newtonian and viscoelastic flows in 3D. 

  • 8.
    Bazesefidpar, Kazem
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Norwegian Univ Sci & Technol NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
    Tammisola, Outi
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical simulation of the coalescence-induced polymeric droplet jumping on superhydrophobic surfaces2022In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 307, article id 104872Article in journal (Refereed)
    Abstract [en]

    Self-propelled jumping of two polymeric droplets on superhydrophobic surfaces is investigated by three-dimensional direct numerical simulations. Two identical droplets of a viscoelastic fluid slide, meet and coalesce on a surface with contact angle 180 degrees. The droplets are modelled by the Giesekus constitutive equation, introducing both viscoelasticity and a shear-thinning effects. The Cahn-Hilliard Phase-Field method is used to capture the droplet interface. The simulations capture the spontaneous coalescence and jumping of the droplets. The effect of elasticity and shear-thinning on the coalescence and jumping is investigated at capillary-inertial and viscous regimes. The results reveal that the elasticity of the droplet changes the known capillary-inertial velocity scaling of the Newtonian drops at large Ohnesorge numbers; the resulting viscoelastic droplet jumps from the surface at larger Ohnesorge numbers than a Newtonian drop, when elasticity amplifies visible shape oscillations of the merged droplet. The numerical results show that polymer chains are stretched during the coalescence and prior to the departure of two drops, and the resulting elastic stresses at the interface induce the jumping of the liquid out of the surface. This study shows that viscoelasticity, typical of many biological and industrial applications, affects the droplet behaviour on superhydrophobic and self-cleaning surfaces.

  • 9.
    Bazesefidpar, Kazem
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    The effect of contact angle hysteresis on a droplet in a viscoelastic two-phase system2024In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 36, no 3, article id 033119Article in journal (Refereed)
    Abstract [en]

    We investigate the dynamic behavior of a two-dimensional droplet adhering to a wall in Poiseuille flow at low Reynolds numbers, in a system where one of the phases is viscoelastic represented by a Giesekus model. The Cahn-Hilliard Phase-Field method is used to capture the interface between the two phases. The presence of polymeric molecules alters the viscoelastic drop's deformation over time, categorizing it into two stages before contact line depinning. In the first stage, the viscoelastic droplet deforms faster, while in the second stage, the Newtonian counterpart accelerates and its deformation outpaces the viscoelastic droplet. The deformation of viscoelastic drop is retarded significantly in the second stage with increasing Deborah number De. The viscous bending of viscoelastic drop is enhanced on the receding side for small De, but it is weakened by further increase in De. On the advancing side, the viscous bending is decreased monotonically for Ca<0.25 with a non-monotonic behavior for Ca=0.25. The non-monotonic behavior on the receding side is attributed to the emergence of outward pulling stresses in the vicinity of the receding contact line and the inception of strain-hardening at higher De, while the reduction in the viscous bending at the advancing side is the result of just strain-hardening. Finally, when the medium is viscoelastic, the viscoelasticity suppresses the droplet deformation on both receding and advancing sides, and this effect becomes more pronounced with increasing De. Increasing the Giesekus mobility parameter enhances the weakening effect of viscous bending on the advancing side.

  • 10.
    Bazesefidpar, Kazem
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    The effect of contact angle hysteresis on adroplet in a viscoelastic two-phase systemManuscript (preprint) (Other academic)
    Abstract [en]

    We investigate the dynamic behaviour of a two-dimensional (2D) droplet adhering to a wall in Poiseuille flow at low Reynolds numbers, in a system where either the droplet is viscoelastic (V/N) or the surrounding medium (N/V). The fluid viscoelasticity has been modeled by the Giesekus constitutive equation, and the Cahn–Hilliard Phase-Field method is used to capture the interface between two phases. The contact angle hysteresis is represented by an advancing contact angle and a receding contact angle . The results reveal that the deformation of the viscoelastic drop over time is changed due to the presence of polymeric molecules, and it can be categorized in two stages prior to depinning of the contact lines. In the first stage, the viscoelastic droplet speeds up and deforms faster, while in the second stage, the Newtonian counterpart accelerates and its deformation outpaces the viscoelastic droplet. The deformation of viscoelastic drop is retarded significantly in the second stage with increasing Deborah number De. In the V/N case, the viscous bending is enhanced on the receding side for small De, but it is weakened by further increase in De, and this non-monotonic behavior brings about an increase in the receding contact line velocity at small De and a decrease at large De. On the advancing side, the viscous bending is decreased monotonically, and hence the advancing contact line velocity is decreased with increasing De. The non-monotonic behavior on the receding side is attributed to the emergence of outward pulling stresses in the vicinity of the receding contact line and the inception of strain-hardening at higher De, while the reduction in the viscous bending at the advancing side is the result of just strain-hardening due to the presence of dominant extensional flow on the advancing side. Finally, in the N/V system, the viscoelasticity of the medium suppresses the droplet deformation on both receding and advancing sides, and this effect is more pronounced with increasing De; the weakening effect of viscous bending is enhanced significantly at the advancing side by increasing the Giesekus mobility parameter in the N/V system. These results give a thorough understanding of viscoelastic effect on both drop deformation and depinning of both contact lines over a surface with contact angle hysteresis.

  • 11.
    Bellani, Gabriele
    et al.
    Graduate School of Engineering, Tohoku University, Sendai, Japan.
    Imagawa, K.
    Tammisola, Outi
    KTH.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Higuchi, H.
    Hayase, T.
    Measurement-Integrated simulations and Kalman filter applied to a turbulent co-flowing jet2010Report (Other academic)
    Abstract [en]

    This paper deals with the experimental evaluation of a flow analysis system based on the integration between an under-resolved Navier-Stokes simulation and experimental measurements with the mechanism of feedback (referred to as Measurement-Integrated simulation), applied to the case of a planar turbulent co-flowing jet. The experiments are performed with inner-to-outer-jet velocity ratio around 2 and the Reynolds number based on the inner-jet heights about 10000. The measurement system is a high-speed PIV, which provides timeresolved data of the flow-field, on a field of view which extends to 20 jet heights downstream the jet outlet. The experimental data can thus be used both for providing the feedback data for the simulations and for validation of the M-Isimulations over a wide region. The effect of reduced data-rate and spatial extent of the feedback was investigated. Then, to deal with the reduced data different feedback strategies were tested. It was found that for small data-rate reduction the results are basically equivalent to the case of full-information feedback but as the feedback data-rate is reduced further the error increases and tend to be localized in regions of high turbulent activity. Moreover, it is found that the spatial distribution of the error looks qualitatively different for different feedback strategies. Feedback gain distributions calculated by optimal control theory are presented and proposed as a mean to make it possible t operform MI-simulations based on localized measurements only. So far, we have not been able to low error between measurements and simulations by using these gain distributions.

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  • 12.
    Cannon, Ianto
    et al.
    Okinawa Inst Sci & Technol Grad Univ, Complex Fluids & Flows Unit, 1919-1 Tancha, Onna Son, Okinawa 9040495, Japan..
    Izbassarov, Daulet
    Aalto Univ, Dept Mech Engn, FI-00076 Aalto, Finland..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rosti, Marco E.
    Okinawa Inst Sci & Technol Grad Univ, Complex Fluids & Flows Unit, 1919-1 Tancha, Onna Son, Okinawa 9040495, Japan..
    The effect of droplet coalescence on drag in turbulent channel flows2021In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 33, no 8, article id 085112Article in journal (Refereed)
    Abstract [en]

    We study the effect of droplet coalescence on turbulent wall-bounded flows by means of direct numerical simulations. In particular, the volume-of-fluid and front-tracking methods are used to simulate turbulent channel flows containing coalescing and non-coalescing droplets, respectively. We find that coalescing droplets have a negligible effect on the drag, whereas the non-coalescing ones steadily increase drag as the volume fraction of the dispersed phase increases: indeed, at 10% volume fraction, the non-coalescing droplets show a 30% increase in drag, whereas the coalescing droplets show less than 4% increase. We explain this by looking at the wall-normal location of droplets in the channel and show that non-coalescing droplets enter the viscous sublayer, generating an interfacial shear stress, which reduces the budget for viscous stress in the channel. On the other hand, coalescing droplets migrate toward the bulk of the channel forming large aggregates, which hardly affect the viscous shear stress while damping the Reynolds shear stress. We prove this by relating the mean viscous shear stress integrated in the wall-normal direction to the centerline velocity.

  • 13.
    Chaparian, Emad
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    De Vita, Francesco
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Royal Inst Technol KTH, Mech Dept, SERC, Stockholm, Sweden..
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Royal Inst Technol KTH, Mech Dept, SERC, Stockholm, Sweden..
    Tammisola, Outi
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Yield-stress fluids in porous media: a comparison of viscoplastic and elastoviscoplastic flows2020In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 55, no 2, p. 331-342Article in journal (Refereed)
    Abstract [en]

    A numerical and theoretical study of yield-stress fluid flows in two types of model porous media is presented. We focus on viscoplastic and elastoviscoplastic flows to reveal some differences and similarities between these two classes of flows. Small elastic effects increase the pressure drop and also the size of unyielded regions in the flow which is the consequence of different stress solutions compare to viscoplastic flows. Yet, the velocity fields in the viscoplastic and elastoviscoplastic flows are comparable for small elastic effects. By increasing the yield stress, the difference in the pressure drops between the two classes of flows becomes smaller and smaller for both considered geometries. When the elastic effects increase, the elastoviscoplastic flow becomes time-dependent and some oscillations in the flow can be observed. Focusing on the regime of very large yield stress effects in the viscoplastic flow, we address in detail the interesting limit of 'flow/no flow': yield-stress fluids can resist small imposed pressure gradients and remain quiescent. The critical pressure gradient which should be exceeded to guarantee a continuous flow in the porous media will be reported. Finally, we propose a theoretical framework for studying the 'yield limit' in the porous media.

  • 14.
    Chaparian, Emad
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Niazi Ardekani, Mehdi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Particle migration in channel flow of an elastoviscoplastic fluid2020In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 284, article id 104376Article in journal (Refereed)
    Abstract [en]

    We study the dynamics of a neutrally buoyant rigid sphere carried by an elastoviscoplastic fluid in a pressure-driven channel flow numerically. The yielding to flow is marked by the yield stress which splits the flow into two main regions: the core unyielded region and two sheared yielded regions close to the walls. The particles which are initially in the plug region are observed to translate with the same velocity as the plug without any rotation/migration. Keeping the Reynolds number fixed, we study the effect of elasticity (Weissenberg number) and plasticity (Bingham number) of the fluid on the particle migration inside the sheared regions. In the viscoelastic limit, in the range of studied parameters (low elasticity), inertia is dominant and the particle finds its equilibrium position between the centreline and the wall. The same happens in the viscoplastic limit, yet the yield surface plays the role of centreline. However, the combination of elasticity and plasticity of the suspending fluid (elastoviscoplasticity) trigger particle-focusing: in the elastoviscoplastic flow, for a certain range of Weissenberg numbers (≈0.5), isolated particles migrate all the way to the centreline by entering into the core plug region. This behaviour suggests a particle-focusing process for inertial regimes which was not previously found in a viscoelastic or viscoplastic carrying fluid. 

  • 15.
    Chaparian, Emad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    An adaptive finite element method for elastoviscoplastic fluid flows2019In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 271, p. 104148-Article in journal (Refereed)
    Abstract [en]

    Elastoviscoplastic fluids are a class of yield-stress fluids that behave like neoHookean (or viscoelastic) solids when the imposed stress is less than the yield stress whereas after yielding, their behaviour is described by a viscoplastic fluid with an additional elastic history. This exceptional behaviour has been recently observed by many yield stress fluids in rheometric tests such as waxy crude oil, Carbopol gel, etc. Moreover, interesting phenomena have been evidenced experimentally such as the presence of a negative wake and a loss of fore-aft symmetry about a settling particle which are predominantly related to the elastic behaviour of yield-stress fluids (i.e., coupling of elasticity and plasticity). Here, we present a numerical scheme based on the so-called augmented Lagrangian method for numerical simulation of elastoviscoplastic fluid flows. The method is benchmarked by two rheometric flows: Poiseuille and circular Couette flows for which analytical solutions are derived. Moreover, anisotropic adaptive mesh procedure (which was previously introduced for viscoplastic fluid flows by Saramito and Roquet, Comput. Meth. Appl. Mech. Eng., vol. 190, 2001, pp. 5391-5412) is coupled to obtain a fine resolution of the yield surfaces. Finally, the presented method is applied to study more complex flows: elastoviscoplastic fluid flow in a wavy channel.

  • 16.
    Chaparian, Emad
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Sliding flows of yield-stress fluids2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 911, article id A17Article in journal (Refereed)
    Abstract [en]

    A theoretical and numerical study of complex sliding flows of yield-stress fluids is presented. Yield-stress fluids are known to slide over solid surfaces if the tangential stress exceeds the sliding yield stress. The sliding may occur due to various microscopic phenomena such as the formation of an infinitesimal lubrication layer of the solvent and/or elastic deformation of the suspended soft particles in the vicinity of the solid surfaces. This leads to a 'stick-slip' law which complicates the modelling and analysis of the hydrodynamic characteristics of the yield-stress fluid flow. In the present study, we formulate the problem of sliding flow beyond one-dimensional rheometric flows. Then, a numerical scheme based on the augmented Lagrangian method is presented to attack these kind of problems. Theoretical tools are developed for analysing the flow/no-flow limit. The whole framework is benchmarked in planar Poiseuille flow and validated against analytical solutions. Then two more complex physical problems are investigated: slippery particle sedimentation and pressure-driven sliding flow in porous media. The yield limit is addressed in detail for both flow cases. In the particle sedimentation problem, method of characteristics - slipline method - in the presence of slip is revisited from the perfectly plastic mechanics and used as a helpful tool in addressing the yield limit. Finally, flows through model and randomized porous media are studied. The randomized configuration is chosen to capture more sophisticated aspects of the yield-stress fluid flows in porous media at the yield limit - channelization. 

  • 17.
    Chaparian, Emad
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability of particles inside yield-stress fluid Poiseuille flows2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 885, article id A45Article in journal (Refereed)
    Abstract [en]

    The stability of neutrally and non-neutrally buoyant particles immersed in a plane Poiseuille flow of a yield-stress fluid (Bingham fluid) is addressed numerically. Particles being carried by the yield-stress fluid can behave in different ways: they might (i) migrate inside the yielded regions or (ii) be transported without any relative motion inside the unyielded region if the yield stress is large enough compared to the buoyancy stress and the other stresses acting on the particles. Knowing the static stability of particles inside a bath of quiescent yield-stress fluid (Chaparian & Frigaard, J. Fluid Mech., vol. 819, 2017, pp. 311-351), we analyse the latter behaviour when the yield-stress fluid Poiseuille flow is host to two-dimensional particles. Numerical experiments reveal that particles lose their stability (i.e. break the unyielded plug and sediment/migrate) with smaller buoyancy compared to the sedimentation inside a bath of quiescent yield-stress fluid, because of the inherent shear stress in the Poiseuille flow. The key parameter in interpreting the present results is the position of the particle relative to the position of the yield surface in the undisturbed flow (in the absence of any particle): the larger the portion of a particle located inside the undisturbed sheared regions, the more likely is the particle to be unstable. Yet, we find that the core unyielded plug can grow locally to some extent to contain the particles. This picture holds even for neutrally buoyant particles, although they are strictly stable when they are located wholly inside the undisturbed plug. We propose scalings for all cases.

  • 18.
    De Vita, Francesco
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Duffo, L.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hormozi, S.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Elastoviscoplastic flows in porous media2018In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 258, p. 10-21Article in journal (Refereed)
    Abstract [en]

    We investigate the elastoviscoplastic flow through porous media by numerical simulations. We solve the Navier–Stokes equations combined with the elastoviscoplastic model proposed by Saramito for the stress tensor evolution [1]. In this model, the material behaves as a viscoelastic solid when unyielded, and as a viscoelastic Oldroyd-B fluid for stresses higher than the yield stress. The porous media is made of a symmetric array of cylinders, and we solve the flow in one periodic cell. We find that the solution is time-dependent even at low Reynolds numbers as we observe oscillations in time of the unyielded region especially at high Bingham numbers. The volume of the unyielded region slightly decreases with the Reynolds number and strongly increases with the Bingham number; up to 70% of the total volume is unyielded for the highest Bingham numbers considered here. The flow is mainly shear dominated in the yielded region, while shear and elongational flow are equally distributed in the unyielded region. We compute the relation between the pressure drop and the flow rate in the porous medium and present an empirical closure as function of the Bingham and Reynolds numbers. The apparent permeability, normalized with the case of Newtonian fluids, is shown to be greater than 1 at low Bingham numbers, corresponding to lower pressure drops due to the flow elasticity, and smaller than 1 for high Bingham numbers, indicating larger dissipation in the flow owing to the presence of the yielded regions. Finally we investigate the effect of the Weissenberg number on the distribution of the unyielded regions and on the pressure gradient.

  • 19.
    Feneuil, Blandine
    et al.
    Univ Oslo, Dept Math, Oslo, Norway.;SINTEF Ind Petr, Trondheim, Norway..
    Iqbal, Kazi Tassawar
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Tillämpad strömningsmekanik. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Jensen, Atle
    Univ Oslo, Dept Math, Oslo, Norway..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Carlson, Andreas
    Univ Oslo, Dept Math, Oslo, Norway..
    Experimental and numerical investigation of bubble migration in shear flow: Deformability-driven chaining and repulsion2023In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, no 6, article id 063602Article in journal (Refereed)
    Abstract [en]

    We study the interaction-induced migration of bubbles in shear flow and observe that bubbles suspended in elastoviscoplastic emulsions organize into chains aligned in the flow direction, similarly to particles in viscoelastic fluids. To investigate the driving mechanism, we perform experiments and simulations on bubble pairs, using suspending fluids with different rheological properties. First, we notice that, for all fluids, the interaction type depends on the relative position of the bubbles. If they are aligned in the vorticity direction, then they repel, if not, then they attract each other. The simulations show a similar behavior in Newtonian fluids as in viscoelastic and elastoviscoplastic fluids, as long as the capillary number is sufficiently large. This shows that the interaction-related migration of the bubbles is strongly affected by the bubble deformation. We suggest that the cause of migration is the interaction between the heterogeneous pressure fields around the deformed bubbles, due to capillary pressure.

  • 20.
    Fraggedakis, Dimitrios
    et al.
    MIT, Dept Chem Engn, Cambridge, MA 02139 USA..
    Chaparian, Emad
    Univ British Columbia, Math Dept, Vancouver, BC V6T 1Z2, Canada..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    The first open channel for yield-stress fluids in porous media2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 911, article id A58Article in journal (Refereed)
    Abstract [en]

    The prediction of the first fluidized path of yield-stress fluids in complex porous media is a challenging yet important task to understand the fundamentals of fluid flow in several industrial and biological processes. In most cases, the conditions that open this first path are known either through experiments or expensive computations. Here, we present a simple network model to predict the first open channel for a yield-stress fluid in a porous medium. For porous media made of non-overlapping discs, we find that the pressure drop Delta P-c required to open the first channel for a given yield stress tau(y) depends on both the relative discs size R-s to the macroscopic length L of the system and the packing fraction phi. The non-dimensional pressure gradient Delta PcRs/tau L-y (i.e. the critical yield number), however, depends on the packing fraction phi only, leading to a mastercurve for all examined ratios of R-s/L. In the case of non-overlapping discs, we find Delta PcRs/tau L-y similar to phi/(1 - phi). We also report the statistics on the arclength of the first open path. Finally, we discuss the implication of our results for the design of porous media used in energy storage applications.

  • 21.
    Ge, Zhouyang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Loiseau, Jean Christophe
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Arts et Métiers ParisTech, France.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    An efficient mass-preserving interface-correction level set/ghost fluid method for droplet suspensions under depletion forces2018In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 353, p. 435-459Article in journal (Refereed)
    Abstract [en]

    Aiming for the simulation of colloidal droplets in microfluidic devices, we present here a numerical method for two-fluid systems subject to surface tension and depletion forces among the suspended droplets. The algorithm is based on an efficient solver for the incompressible two-phase Navier–Stokes equations, and uses a mass-conserving level set method to capture the fluid interface. The four novel ingredients proposed here are, firstly, an interface-correction level set (ICLS) method; global mass conservation is achieved by performing an additional advection near the interface, with a correction velocity obtained by locally solving an algebraic equation, which is easy to implement in both 2D and 3D. Secondly, we report a second-order accurate geometric estimation of the curvature at the interface and, thirdly, the combination of the ghost fluid method with the fast pressure-correction approach enabling an accurate and fast computation even for large density contrasts. Finally, we derive a hydrodynamic model for the interaction forces induced by depletion of surfactant micelles and combine it with a multiple level set approach to study short-range interactions among droplets in the presence of attracting forces.

  • 22.
    Ge, Zhouyang
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH Mech, INTERFACE Ctr, SE-10044 Stockholm, Sweden..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH Mech, INTERFACE Ctr, SE-10044 Stockholm, Sweden..
    Flow-assisted droplet assembly in a 3D microfluidic channel2019In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 15, no 16, p. 3451-3460Article in journal (Refereed)
    Abstract [en]

    Self-assembly of soft matter, such as droplets or colloids, has become a promising scheme to engineer novel materials, model living matter, and explore non-equilibrium statistical mechanics. In this article, we present detailed numerical simulations of few non-Brownian droplets in various flow conditions, specifically, focusing on their self-assembly within a short distance in a three-dimensional (3D) microfluidic channel, cf. [Shen et al., Adv. Sci., 2016, 3(6), 1600012]. Contrary to quasi two-dimensional (q2D) systems, where dipolar interaction is the key mechanism for droplet rearrangement, droplets in 3D confinement produce much less disturbance to the underlying flow, thus experiencing weaker dipolar interactions. Using confined simple shear and Poiseuille flows as reference flows, we show that the droplet dynamics is mostly affected by the shear-induced cross-stream migration, which favors chain structures if the droplets are under an attractive depletion force. For more compact clusters, such as three droplets in a triangular shape, our results suggest that an inhomogeneous cross-sectional inflow profile is further required. Overall, the accelerated self-assembly of a small-size droplet cluster results from the combined effects of strong depletion forces, confinement-mediated shear alignments, and fine-tuned inflow conditions. The deterministic nature of the flow-assisted self-assembly implies the possibility of large throughputs, though calibration of all different effects to directly produce large droplet crystals is generally difficult.

  • 23.
    Geetha Balasubramanian, Arivazhagan
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Prediction of wall-bounded turbulence in a viscoelastic channel flow using convolutional neural networks2022In: Prediction of wall-bounded turbulence in a viscoelastic channel flow using convolutional neural networks, 2022Conference paper (Other academic)
    Abstract [en]

    Turbulent flow of purely viscoelastic fluids has gained attention in the drag-reduction and flow control communities since a tiny amount of polymer has proven efficient in reducing friction drag in pipe flows. Drag reduction by polymers (elasticity) is related to their ability to modify coherent structures in wall-bounded turbulence. When it comes to practical flows of interest, numerical simulations of such flows become challenging due to the associated computational cost of capturing the multiple physical mechanisms that drive the flow. On the other hand, experimental investigations of drag reduction in viscoelastic flows are limited by the near-wall measurements and the capability of the experimental techniques to accurately quantify the flow, without disturbing it. A complete description of viscoelastic turbulence would require the characterization of both velocity and polymeric stresses. However, the polymer deformation cannot be accessed directly from the experiments. Hence, in the objective of the present study, the idea of non-intrusive sensing has been applied to viscoelastic channel flow to predict the velocity fluctuations and polymeric stress components near the wall using the quantities measured at the wall. To this aim, the convolutional neural network (CNN) models are trained to predict the two-dimensional velocity fluctuation and polymeric shear stress fluctuation and elongation fields at different wall-normal distances in a viscoelastic channel flow. The present work would highlight the capability of a data-driven approach to model turbulence in complex fluid flows and in addition also finds useful applications in experimental settings.

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  • 24. Ghosh, Sukhendu
    et al.
    Ranganathan, Usha
    Govindarajan, Rama
    Tammisola, Outi
    The University of Nottingham, UK.
    Inviscid instability of two-fluid free surface flow down an incline2015In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, p. 1-18Article in journal (Refereed)
    Abstract [en]

    The inviscid temporal stability analysis of two-fluid parallel shear flow with a free surface, down an incline, is studied. The velocity profiles are chosen as piecewise-linear with two limbs. The analysis reveals the existence of unstable inviscid modes, arising due to wave interaction between the free surface and the shear-jump interface. Surface tension decreases the maximum growth rate of the dominant disturbance. Interestingly, in some limits, surface tension destabilises extremely short waves in this flow. This can happen because of the interaction with the shear-jump interface. This flow may be compared with a corresponding viscous two-fluid flow. Though viscosity modifies the stability properties of the flow system both qualitatively and quantitatively, there is qualitative agreement between the viscous and inviscid stability analysis when the less viscous fluid is closer to the free surface.

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    fulltext
  • 25. Imagawa, K.
    et al.
    Bellani, Gabriele
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Higuchi, H.
    Hayase, T.
    Measurement-Integrated simulations and Kalman filter applied to a co-flowing jet2010In: 5th Flow Control Conference, 2010, p. 2010-4420-Conference paper (Refereed)
    Abstract [en]

    This paper deals with the experimental evaluation of a flow analysis system based on the integration between an under-resolved Navier-Stokes simulation and experimental measurements with the mechanism of feedback (referred to as Measurement-Integrated simulation), applied to the case of a planar turbulent co-flowing jet. The experiments are performed with inner-to-outer-jet velocity ratio around 2 and the Reynolds number based on the inner-jet heights about 10000. The measurement system is a high-speed PIV, which provides time-resolved data of the flow-field, on a field of view which extends to 20 jet heights downstream the jet outlet. The experimental data can thus be used both for providing the feedback data for the simulations and for validation of the MI-simulations over a wide region. The effect of reduced data-rate and spatial extent of the feedback (i.e. measurements are not available at each simulation time-step or discretization point) was investigated. At first simulations were run with full information in order to obtain an upper limit of the MI-simulations performance. The results show the potential of this methodology of reproducing first and second order statistics of the turbulent flow with good accuracy. Then, to deal with the reduced data different feedback strategies were tested. It was found that for small data-rate reduction the results are basically equivalent to the case of full-information feedback but as the feedback data-rate is reduced further the error increases and tend to be localized in regions of high turbulent activity. Moreover, it is found that the spatial distribution of the error looks qualitatively different for different feedback strategies. Feedback gain distributions calculated by optimal control theory are presented and proposed as a mean to make it possible to perform MI-simulations based on localized measurements only. So far, we have not been able to low error between measurements and simulations by using these gain distributions.

  • 26.
    Izbassarov, Daulet
    et al.
    Aalto Univ, Dept Mech Engn, FI-00076 Aalto, Finland..
    Ahmed, Zaheer
    Mehran Univ Engn & Technol, Dept Mech Engn, SZAB Campus, Khairpur Mirs 66020, Sindh, Pakistan..
    Costa, Pedro
    Univ Iceland, Fac Ind Engn Mech Engn & Comp Sci, Hjardarhagi 2-6, IS-107 Reykjavik, Iceland..
    Vuorinen, Ville
    Aalto Univ, Dept Mech Engn, FI-00076 Aalto, Finland..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Muradoglu, Metin
    Koc Univ, Dept Mech Engn, TR-34450 Istanbul, Turkey..
    Polymer drag reduction in surfactant-contaminated turbulent bubbly channel flows2021In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, no 10, article id 104302Article in journal (Refereed)
    Abstract [en]

    Polymer additives are commonly utilized to manipulate bubbly flows in various applications. Here we investigate the effects of clean and contaminated bubbles driven upward (upflow) in Newtonian and viscoelastic turbulent channel flows. Interface-resolved direct numerical simulations are performed to examine sole and combined effects of soluble surfactant and viscoelasticity using an efficient three-dimensional finite-difference-front-tracking method. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is accomplished by a nonlinear equation of state that relates the surface tension to the surfactant concentration. For Newtonian turbulent bubbly flows, the effects of Triton X-100 and 1-pentanol surfactant are examined. It is observed that the sorption kinetics highly affect the dynamics of bubbly flow. A minute amount of Triton X-100 is found to be sufficient to prevent the formation of bubble clusters restoring the single-phase behavior while even two orders of magnitude more 1-pentanol surfactant is not adequate to prevent the formation of layers. For viscoelastic turbulent flows, it is found that the viscoelasticity promotes formation of the bubble wall-layers and thus the polymer drag reduction is completely lost for the surfactant-free bubbly flows, while the addition of small amount of surfactant (Triton X-100) in this system restores the polymer drag reduction resulting in 25% drag reduction for the Wi = 4 case.

  • 27.
    Izbassarov, Daulet
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Aalto Univ, Dept Mech Engn, FI-00076 Aalto, Finland..
    Rosti, Marco E.
    Okinawa Inst Sci & Technol Grad Univ, Complex Fluids & Flows Unit, 1919-1 Tancha, Onna Son, Okinawa 9040495, Japan..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Effect of finite Weissenberg number on turbulent channel flows of an elastoviscoplastic fluid2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 927, article id A45Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations are carried out to study the effect of finite Weissenberg number up to Wi = 16 on laminar and turbulent channel flows of an elastoviscoplastic (EVP) fluid, at a fixed bulk Reynolds number of 2800. The incompressible flow equations are coupled with the evolution equation for the EVP stress tensor by a modified Saramito model that extends both the Bingham viscoplastic and the finite extensible nonlinear elastic-Peterlin (FENE-P) viscoelastic models. In turbulent flow, we find that drag decreases with both the Bingham and Weissenberg numbers, until the flow laminarises at high enough elastic and yield stresses. Hence, a higher drag reduction is achieved than in the viscoelastic flow at the same Weissenberg number. The drag reduction persists at Bingham numbers up to 20, in contrast to viscoplastic flow, where the drag increases in the laminar regime compared with a Newtonian flow. Moreover, elasticity affects the laminarisation of an EVP flow in a non-monotonic fashion, delaying it at lower and promoting it at higher Weissenberg numbers. A hibernation phenomenon is observed in the EVP flow, leading to large changes in the unyielded regions. Finally, plasticity is observed to affect both low- and high-speed streaks equally, attenuating the turbulent dissipation and the fragmentation of turbulent structures.

  • 28.
    Izbassarov, Daulet
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Niazi Ardekani, Mehdi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Sarabian, Mohammad
    Hormozi, Sarah
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Computational modeling of multiphase viscoelastic and elastoviscoplastic flows2018In: International Journal for Numerical Methods in Fluids, ISSN 0271-2091, E-ISSN 1097-0363, Vol. 88, no 12, p. 521-543Article in journal (Refereed)
  • 29.
    Izbassarov, Daulet
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. Aalto Univ, Dept Mech Engn, FI-00076 Espoo, Finland..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Dynamics of an elastoviscoplastic droplet in a Newtonian medium under shear flow2020In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 5, no 11, article id 113301Article in journal (Refereed)
    Abstract [en]

    The dynamics of a single elastoviscoplastic drop immersed in plane shear flow of a Newtonian fluid is studied by three-dimensional direct numerical simulations using a finite-difference and level-set method combined with the Saramito model for the elastoviscoplastic fluid. This model gives rise to a yield stress behavior, where the unyielded state of the material is described as a Kelvin-Voigt viscoelastic solid and the yielded state as a viscoelastic Oldroyd-B fluid. Yielding of an initially solid drop of Carbopol is simulated under successively increasing shear rates. We proceed to examine the roles of nondimensional parameters on the yielding process; in particular, the Bingham number, the capillary number, the Weissenberg number, and the ratio of solvent and total drop viscosity are varied. We find that all of these parameters, and not only the Bingham number, have a significant influence on the drop dynamics. Numerical simulations predict that the volume of the unyielded region inside the droplet increases with the Bingham number and the Weissenberg number, while it decreases with the capillary number at low Weissenberg and Bingham numbers. A new regime map is obtained for the prediction of the yielded, unyielded, and partly yielded modes as a function of the Bingham and Weissenberg numbers. The drop deformation is studied and explained by examining the stresses in the vicinity of the drop interface. The deformation has a complex dependence on the Bingham andWeissenberg numbers. At low Bingham numbers, the droplet deformation shows a nonmonotonic behavior with an increasing drop viscoelasticity. In contrast, at moderate and high Bingham numbers, droplet deformation always increases with drop viscoelasticity. Moreover, it is found that the deformation increases with the capillary number and with the solvent to total drop viscosity ratio. A simple ordinary differential equation model is developed to explain the various behaviours observed numerically. The presented results are in contrast with the heuristic idea that viscoelasticity in the dispersed phase always inhibits deformation.

  • 30.
    Juniper, Matthew
    et al.
    Department of Engineering, Cambridge University, Cambridge.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    The local and global stability of confined planar wakes at intermediate Reynolds number2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 686, p. 218-238Article in journal (Refereed)
    Abstract [en]

    At high Reynolds numbers, wake flows become more globally unstable whenconfined within a duct or between two flat plates. At Reynolds numbers around100, however, global analyses suggest that such flows become more stable whenconfined, while local analyses suggest that they become more unstable. Theaim of this paper is to resolve this apparent contradiction. In this theoreticaland numerical study, we combine global and local stability analyses of planarwake flows at Re = 100 to determine the effect of confinement. We find thatconfinement acts in three ways: it modifies the length of the recirculation zoneif one exists, it brings the boundary layers closer to the shear layers, and itcan make the flow more locally absolutely unstable. Depending on the flowparameters, these effects work with or against each other to destabilize orstabilize the flow. In wake flows at Re = 100 with free slip boundaries, flowsare most globally unstable when the outer flows are 50% wider than the halfwidthof the inner flow because the first and third effects work together. Inwake flows at Re = 100 with no slip boundaries, confinement has little overalleffect when the flows are weakly confined because the first two effects workagainst the third. Confinement has a strong stabilizing effect, however, whenthe flows are strongly confined because all three effects work together. Bycombining local and global analyses, we have been able to isolate these threeeffects and resolve the apparent contradictions in previous work.1.

  • 31.
    Lashgari, Iman
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tammisola, Outi
    Department of Engineering, University of Cambridge, Cambridge, UK.
    Citro, Vincenzo
    DIIN, University of Salerno, Fisciano, Italy.
    Juniper, Matthew P.
    Department of Engineering, Univerisyt of Cambridge, Cambridge, UK.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    The planar X-junction flow: stability analysis and control2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 753, p. 1-28Article in journal (Refereed)
    Abstract [en]

    The bifurcations and control of the flow in a planar X-junction are studied via linear stability analysis and direct numerical simulations. This study reveals the instability mechanisms in a symmetric channel junction and shows how these can be stabilized or destabilized by boundary modification. We observe two bifurcations as the Reynolds number increases. They both scale with the inlet speed of the two side channels and are almost independent of the inlet speed of the main channel. Equivalently, both bifurcations appear when the recirculation zones reach a critical length. A two-dimensional stationary global mode becomes unstable first, changing the flow from a steady symmetric state to a steady asymmetric state via a pitchfork bifurcation. The core of this instability, whether defined by the structural sensitivity or by the disturbance energy production, is at the edges of the recirculation bubbles, which are located symmetrically along the walls of the downstream channel. The energy analysis shows that the first bifurcation is due to a lift-up mechanism. We develop an adjustable control strategy for the first bifurcation with distributed suction or blowing at the walls. The linearly optimal wall-normal velocity distribution is computed through a sensitivity analysis and is shown to delay the first bifurcation from Re = 82.5 to Re = 150. This stabilizing effect arises because blowing at the walls weakens the wall-normal gradient of the streamwise velocity around the recirculation zone and hinders the lift-up. At the second bifurcation, a three-dimensional stationary global mode with a spanwise wavenumber of order unity becomes unstable around the asymmetric steady state. Nonlinear three-dimensional simulations at the second bifurcation display transition to a nonlinear cycle involving growth of a three-dimensional steady structure, time-periodic secondary instability and nonlinear breakdown restoring a two-dimensional flow. Finally, we show that the sensitivity to wall suction at the second bifurcation is as large as it is at the first bifurcation, providing a possible mechanism for destabilization.

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  • 32.
    Le Clainche, S.
    et al.
    Univ Politecn Madrid, Sch Aerosp Engn, E-28040 Madrid, Spain..
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rosti, Marco E.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Coherent structures in the turbulent channel flow of an elastoviscoplastic fluid2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 888, article id A5Article in journal (Refereed)
    Abstract [en]

    In this numerical and theoretical work, we study the turbulent channel flow of Newtonian and elastoviscoplastic fluids. The coherent structures in these flows are identified by means of higher order dynamic mode decomposition (HODMD), applied to a set of data non-equidistant in time, to reveal the role of the near-wall streaks and their breakdown, and the interplay between turbulent dynamics and non-Newtonian effects. HODMD identifies six different high-amplitude modes, which either describe the yielded flow or the yielded-unyielded flow interaction. The structure of the low- and high-frequency modes suggests that the interaction between high- and low-speed streamwise velocity structures is one of the mechanisms triggering the streak breakdown, dominant in Newtonian turbulence where we observe shorter near-wall streaks and a more chaotic dynamics. As the influence of elasticity and plasticity increases, the flow becomes more correlated in the streamwise direction, with long streaks disrupted for short times by localised perturbations, reflected in reduced drag. Finally, we present streamwise-periodic dynamic mode decomposition modes as a viable tool to describe the highly complex turbulent flows, and identify simple well-organised groups of travelling waves.

  • 33.
    Magri, Luca
    et al.
    Shinshu University, Nagano, Japan.
    See, Y.-C.
    Tammisola, Outi
    University of Nottingham.
    Ihme, Mattias
    Juniper, Matthew
    Department of Engineering, Cambridge University, Cambridge.
    Multiple-scale thermoacoustic stability analysis of a coaxial jet combustor2017In: Proceedings of the Combustion Institute, ISSN 1540-7489, E-ISSN 1873-2704, Vol. 36, no 3, p. 3863-3871Article in journal (Refereed)
    Abstract [en]

    In this paper, asymptotic multiple-scale methods are used to formulate a mathematically consistent set of thermo-acoustic equations in the low-Mach number limit for linear stability analysis. The resulting sets of nonlinear equations for hydrodynamics and acoustics are two-way coupled. The coupling strength depends on which multiple scales are used. The double-time-double-space (2T-2S), double-time-single-space (2T-1S) and single-time-double-space (1T-2S) limits are revisited, derived and linearized. It is shown that only the 1T-2S limit produces a two-way coupled linearized system. Therefore this limit is adopted and implemented in a finite-element solver. The methodology is applied to a coaxial jet combustor. By using an adjoint method and introducing the intrinsic sensitivity, (i) the interaction between the acoustic and hydrodynamic subsystems is calculated and (ii) the role of the global acceleration term, which is the coupling term from the acoustics to the hydrodynamics, is analyzed. For the confined coaxial jet diffusion flame studied here, (i) the growth rate of the thermo-acoustic oscillations is found to be more sensitive to small changes in the hydrodynamic field around the flame and (ii) increasing the global acceleration term is found to be stabilizing in agreement with the Rayleigh Criterion.

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  • 34.
    Nicholson, J. M. P.
    et al.
    Univ Nottingham, Dept Mech Mat & Mfg Engn, Nottingham NG7 2RD, England..
    Power, H.
    Univ Nottingham, Dept Mech Mat & Mfg Engn, Nottingham NG7 2RD, England..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hibberd, S.
    Univ Nottingham, Sch Math Sci, Nottingham NG7 2RD, England..
    Kay, E. D.
    Norton Straw Consultants, Darley Abbey Mills, Steam Engine House, Derby DE22 1DZ, England..
    Fluid dynamics of the slip boundary condition for isothermal rimming flow with moderate inertial effects2019In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, no 3, article id 033602Article in journal (Refereed)
    Abstract [en]

    Motivated by evaluating coating oil films within bearing chambers in an aero-engine application, an analysis is presented for the fluid dynamics relevant in their dual capacity as both the coolant and lubricant in highly sheared flows that may approach microscale thickness. An extended model is developed for isothermal rimming flow driven by substantial surface shear within a stationary cylinder. In particular, a partial slip condition replaces the no-slip condition at the wall whilst retaining inertial effects relevant to an intrinsic high speed operation. A depth-averaged formulation is presented that includes appropriate inertial effects at leading-order within a thin film approximation that encompasses a more general model of assessing the impact of surface slip. Non-dimensional mass and momentum equations are integrated across the film depth yielding a one dimensional problem with the a priori assumption of local velocity profiles. The film flow solutions for rimming flow with wall slip are modeled to a higher order than classical lubrication theory. We investigate the impact of wall slip on the transition from pooling to uniform films. Numerical solutions of film profiles are provided for the progressively increased Reynolds number, within a moderate inertia regime, offering evaluation into the effect of film slippage on the dynamics of rimming flow. We find that slip allows non-unique solution regions and existence of multiple possible steady state solutions evaluated in transforming from smooth to pooling film solutions. Additionally, boundary slip is shown to enhance the development of recirculation regions within the film which are detrimental to bearing chamber flows. 

  • 35.
    Pellegrino, Michele
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics, Biophysics. KTH, Centres, Science for Life Laboratory, SciLifeLab.
    Kunchi Kannan, Parvathy
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics.
    Hess, Berk
    KTH, Centres, Science for Life Laboratory, SciLifeLab. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics, Biophysics.
    Cahn-Hilliard phase-field modeling captures the nanoscale hydrodynamics of contact lines on high-friction surfacesManuscript (preprint) (Other academic)
    Abstract [en]

    Incorporating molecular scale effects in the description of contact lines is difficult but necessary in order to accurately account for all sources of energy dissipation in wetting dynamics. This  holds particularly true in the cases where contact line friction determines wetting dynamics and hydrodynamics models struggle to find a regularisation due to the negligible slip of the wetting liquid over the solid surface. We perform Molecular Dynamics simulations of water/hexane biphasic systems, in the two-phase Couette flow configuration. Wetting occurs over a no-slip silica-like surface with variable wettability. The simulation results are reproduced by a Cahn-Hilliard Navier-Stokes model, which includes localised contact line slip and contact angle dynamics. The continuous equations are directly parametrized from Molecular Dynamics simulation results, under the assumption of the numerical sharp interface limit. The reconfiguration of the liquid/liquid interface and the flow structure are found to be in good quantitative agreement. In particular, interface bending due to viscous flow and contact line friction is fully reproduced. Navier slip is calibrated to ensure numerical stability. The viable combinations of Navier slip and Cahn-Hilliard mobility parameters that agree with Molecular Dynamics simulations in the sharp interface limit are reported and discussed. The results presented in this article indicate that Phase Field modeling can capture the effects of molecular processes on the mobility of contact lines and that an accurate determination of contact line friction is key to fully reproduce Molecular Dynamics simulations.

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  • 36.
    Picano, Francesco
    et al.
    Univ Padua, Dept Ind Engn, Via Venezia 1, I-35131 Padua, Italy.;Univ Padua, Ctr Studies & Activ Space CISAS, Via Venezia 1, I-35131 Padua, Italy..
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Editorial2020In: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 55, no 2, p. 295-297Article in journal (Other academic)
  • 37.
    Rosti, Marco E.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Izbassarov, Daulet
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Hormozi, Sarah
    Ohio Univ, Dept Mech Engn, Athens, OH 45701 USA..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Turbulent channel flow of an elastoviscoplastic fluid2018In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 853, p. 488-514Article in journal (Refereed)
    Abstract [en]

    We present numerical simulations of laminar and turbulent channel flow of an elastoviscoplastic fluid. The non-Newtonian flow is simulated by solving the full incompressible Navier-Stokes equations coupled with the evolution equation for the elastoviscoplastic stress tensor. The laminar simulations are carried out for a wide range of Reynolds numbers, Bingham numbers and ratios of the fluid and total viscosity, while the turbulent flow simulations are performed at a fixed bulk Reynolds number equal to 2800 and weak elasticity. We show that in the laminar flow regime the friction factor increases monotonically with the Bingham number (yield stress) and decreases with the viscosity ratio, while in the turbulent regime the friction factor is almost independent of the viscosity ratio and decreases with the Bingham number, until the flow eventually returns to a fully laminar condition for large enough yield stresses. Three main regimes are found in the turbulent case, depending on the Bingham number: for low values, the friction Reynolds number and the turbulent flow statistics only slightly differ from those of a Newtonian fluid; for intermediate values of the Bingham number, the fluctuations increase and the inertial equilibrium range is lost. Finally, for higher values the flow completely laminarizes. These different behaviours are associated with a progressive increases of the volume where the fluid is not yielded, growing from the centreline towards the walls as the Bingham number increases. The unyielded region interacts with the near-wall structures, forming preferentially above the high-speed streaks. In particular, the near-wall streaks and the associated quasi-streamwise vortices are strongly enhanced in an highly elastoviscoplastic fluid and the flow becomes more correlated in the streamwise direction.

  • 38. Samuelsson, John
    et al.
    Tammisola, Outi
    Juniper, Matthew
    Department of Engineering, Cambridge University, Cambridge.
    Breaking axi-symmetry in stenotic flow lowers the critical transition Reynolds number2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27Article in journal (Refereed)
    Abstract [en]

    Flow through a sinuous stenosis with varying degrees of non-axisymmetric shape variations and at Reynolds numberranging from 250 to 750 is investigated using direct numerical simulation (DNS) and global linear stability analysis. At low Reynolds numbers (Re < 390), the flow is always steady and symmetric for an axisymmetric geometry. Two steady state solutions are obtained when the Reynolds number is increased: a symmetric steady state and an eccentric, non-axisymmetric steady state. Either one can be obtained in the DNS depending on the initial condition. A linear global stability analysis around the symmetric and non-axisymmetric steady state reveals that both flows are linearly stable for the same Reynolds number, showing that the first bifurcation from symmetry to antisymmetry is subcritical. When the Reynolds number is increased further, the symmetric state becomes linearly unstable to an eigenmode, which drives the flow towards the non-axisymmetric state. The symmetric state remains steady up to Re = 713, while the non-axisymmetric state displays regimes of periodic oscillations for Re ≥ 417 and intermittency for Re ≳ 525. Further, an offset of the stenosis throat is introduced through the eccentricity parameter E. When eccentricity is increased from zero to only 0.3% of the pipe diameter, the bifurcation Reynolds number decreases by more than 50%, showing that it is highly sensitive to non-axisymmetric shape variations. Based on the resulting bifurcation map and its dependency on E, we resolve the discrepancies between previous experimental and computational studies. We also present excellent agreement between our numerical results and previous experimental results.

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  • 39. Schmidt, Simon
    et al.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Lesshafft, Lutz
    Oberleithner, Kilian
    Global stability and nonlinear dynamics of wake flows with a two-fluid interface2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 915, article id A96Article in journal (Refereed)
    Abstract [en]

    A framework for the computation of linear global modes, based on time stepping of a linearised Navier-Stokes solver with an Eulerian interface representation, is presented. The method is derived by linearising the nonlinear solver BASILISK, capable of computing immiscible two-phase flows, and offers several advantages over previous, matrix-based, multi-domain approaches to linear global stability analysis of interfacial flows. Using our linear solver, we revisit the study of Tammisola et al. (J. Fluid Mech., vol. 713, 2012, pp. 632-658), who found a counter-intuitive, destabilising effect of surface tension in planar wakes. Since their original study does not provide any validation, we further compute nonlinear results for the studied flows. We show that a surface-tension-induced destabilisation of plane wakes is observable which leads to periodic, quasiperiodic or chaotic oscillations depending on the Weber number of the flow. The predicted frequencies of the linear global modes, computed in the present study, are in good agreement with the nonlinear results, and the growth rates are comparable to the disturbance growth in the nonlinear flow before saturation. The bifurcation points of the nonlinear flow are captured accurately by the linear solver and the present results are as well in correspondence with the study of Tammisola et al. (J. Fluid Mech., vol. 713,2012, pp. 632-658).

  • 40.
    Shahmardi, Armin
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Rosti, Marco Edoardo
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Protein Science.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A fully Eulerian hybrid immersed boundary-phase field model for contact line dynamics on complex geometries2021In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 443, p. 110468-110468, article id 110468Article in journal (Refereed)
    Abstract [en]

    We present a fully Eulerian hybrid immersed-boundary/phase-field model to simulate wetting and contact line motion over any arbitrary geometry. The solid wall is described with a volume-penalisation ghost-cell immersed boundary whereas the interface between the two fluids by a diffuse-interface method. The contact line motion on the complex wall is prescribed via slip velocity in the momentum equation and static/dynamic contact angle condition for the order parameter of the Cahn-Hilliard model. This combination requires accurate computations of the normal and tangential gradients of the scalar order parameter and of the components of the velocity. However, the present algorithm requires the computation of averaging weights and other geometrical variables as a preprocessing step. Several validation tests are reported in the manuscript, together with 2D simulations of a droplet spreading over a sinusoidal wall with different contact angles and slip length and a spherical droplet spreading over a sphere, showing that the proposed algorithm is capable to deal with the three-phase contact line motion over any complex wall. The Eulerian feature of the algorithm facilitates the implementation and provides a straight-forward and potentially highly scalable parallelisation. The employed parallelisation of the underlying Navier-Stokes solver can be efficiently used for the multiphase part as well. The procedure proposed here can be directly employed to impose any types of boundary conditions (Neumann, Dirichlet and mixed) for any field variable evolving over a complex geometry, modelled with an immersed-boundary approach (for instance, modelling deformable biological membranes, red blood cells, solidification, evaporation and boiling, to name a few). 

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  • 41.
    Shahmardi, Armin
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tammisola, Outi
    KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Edoardo Rosti, Marco
    The role of wetting on the flow of two immiscible fluids in porous mediaManuscript (preprint) (Other academic)
  • 42.
    Shahmardi, Armin
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Chinappi, Mauro
    Univ Roma Tor Vergata, Dipartimento Ingn Ind, Via Politecn 1, I-00133 Rome, Italy..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Effects of surface nanostructure and wettability on pool boiling: A molecular dynamics study2021In: International journal of thermal sciences, ISSN 1290-0729, E-ISSN 1778-4166, Vol. 167, article id 106980Article in journal (Refereed)
    Abstract [en]

    We study the role of surface topology, surface chemistry, and wall superheat temperature on the onset of boiling, bubble nucleation and growth, and the possible formation of an insulating vapour film by means of a novel setup for large-scale MD simulations. To minimise the effects of the system size on the bubble growth and the formation of the vapour film, we perform simulations in a box larger than those previously considered. The effect of the system pressure on bubble nucleation and growth is isolated by imposing a constant force on a moving piston and mechanically controlling the pressure. The simulations reveal that the presence of a nanostructure determines the nucleation site and facilitates the energy transfer from the hot substrate to the water. The surface chemistry, on the other hand, governs the shape of the formed bubble. A hydrophilic surface accelerates the bubble nucleation, however, decelerates the bubble expansion, thus postponing the formation of the film of vapour. Hence, a hydrophilic surface provides better energy transfer from the hot wall to the water. By analysing the system energy, we show that irrespective of wall topology and chemistry, there is a wall temperature for which the amount of transferred energy from the wall is maximum.

  • 43.
    Söderberg, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. STFI-Packforsk, Sweden.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The fundamental mechanism behind headbox jet break-up2008In: TAPPI Press - Paper Conference and Trade Show, PaperCon '08, 2008, p. 3564-3596Conference paper (Refereed)
    Abstract [en]

    It has previously been shown that MD streaks are created in the headbox jet, which is closely connected to the appearance of waves on the jet surface. The fundamental mechanism behind this break-up is presented. This has been achieved by implementing state-of-the-art methods for determining the characteristics and evolution of hydrody-namic instabilities. The methodology also allows the headbox slice to be designed in order to minimise jet break-up. This possibility has been evaluated in pilot-scale.

  • 44.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Global stability of X-junction flow. Part 2: Sensitivity and control2012In: Proceedings of the Global Flow Instability and Control Symposium 2012, 2012Conference paper (Refereed)
  • 45.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Linear stability of plane wakes and liquid jets: global and local approach2009Licentiate thesis, comprehensive summary (Other academic)
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  • 46.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical stability studies of one-phase and immiscible two-phase jets and wakes2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The initial linear instability growth of two-dimensional plane wakes and jetsis investigated, by temporal two-dimensional global modes, and local spatialstability analysis. Comparisons are also made to experiments, direct numericalsimulations, and methods designed for weakly-non-parallel flows. The studiesproceed through three different flow setups with increasing complexity.The first flow analysed is a convectively unstable liquid sheet surroundedby a stagnant or co-flowing gas. The experimentally measured growth rates arefound to be in excellent agreement with spatial stability calculations, if the airboundary layer is taken into account, and not otherwise. The stabilizing effectof moderate air co-flow is quantified in the numerical study, and the governingparameters found to be the speed difference between water and air, and theshear from air at the water surface (inversely proportional to the air boundarylayer thickness).The second flow case is a one-phase confined wake, i.e. a wake in a channel.The effect of confinement (wall distance) on the global stability of wakes isanalysed by linear global modes, and compared to the results from DNS andweakly-non-parallel theory. At Re = 100, confinement is globally stabilizing,mostly due to a faster development towards a parabolic profile for confinedflows. The stabilizing effect of confinement almost disappears at Re ≈ 400.However, when the structural sensitivity of the wakes is analysed by an adjointbasedapproach, fundamental differences are seen in the global wavemakers ofconfined and unconfined wakes at Re ≈ 400.The third and most complex flow case is immiscible two-fluid wakes andjets. A parallel multi-domain spectral code is developed, where the kinematicand dynamic conditions on the interface are imposed as coupling conditions. Itis shown that intermediate values of surface tension can destabilize stable wakesand jets. In addition, surface tension has a considerable influence on the globaloscillation frequency and spatial shape of the global mode for unstable wakes.The character of the mode is gradually changed from a wake instability to aglobal shear layer instability. Both symmetric and antisymmetric modes areencountered for both wakes and jets, depending on the strength of the surfacetension (value of the Weber number) and the flow case.iii

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  • 47.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Optimal wavy surface to suppress vortex shedding using second-order sensitivity to shape changes2016In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, article id 10.1016/j.euromechflu.2016.12.006Article in journal (Refereed)
    Abstract [en]

    A method to find optimal 2nd-order perturbations is presented, and applied to find the optimal spanwise-wavy surface for the suppression of cylinder wake instability. As shown in recent studies (Hwang et al., 2013, Tammisola et al., 2014, Del Guercio et al., 2014), 2nd-order perturbations are required to capture the stabilizing effect of spanwise waviness, which is ignored by standard adjoint-based sensitivity analyses. Here, previous methods are extended so that (i) 2nd-order sensitivity is formulated for base flow changes satisfying the linearised Navier–Stokes, and (ii) the resulting method is applicable to a 2D global instability problem. This makes it possible to formulate the 2nd-order sensitivity to shape modifications. This formulation is used to find the optimal shape to suppress the a cylinder wake instability. The optimal shape is then perturbed by random distributions in full 3D stability analysis to confirm that it is a local optimal at the given amplitude and wavelength. At Re=100, surface waviness of maximum height 1% of the cylinder diameter is sufficient to stabilize the flow. The optimal surface creates streaks passively by extracting energy from the base flow derivatives and altering the tangential velocity component at the wall. This paper extends previous techniques to a fully two-dimensional method to find boundary perturbations which optimize the 2nd-order drift. The method should be applicable to generic flow instability problems, and to different types of control, such as boundary forcing, shape modulation or suction.

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  • 48.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Oscillatory sensitivity patterns for global modes in wakes2012In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 701, p. 251-277Article in journal (Refereed)
    Abstract [en]

    Globally unstable wakes with co-flow at intermediate Reynolds numbers are studied, to quantify important spatial regions for the development and control of the global instability. One region of high structural sensitivity is found close to the inlet for all wakes, in agreement with previous findings for cylinder wakes. A second, elongated region of high structural sensitivity is seen downstream of the first one for unconfined wakes at Re = 400. When base-flow modifications are considered, a spatially oscillating sensitivity pattern is found inside the downstream high-structural-sensitivity region. This implies that the same change in the base flow can either destabilize or stabilize the flow, depending on the exact position where it is applied. It is shown that the sensitivity pattern remains unchanged for different choices of streamwise boundary conditions and numerical resolution. The actual base-flow is modified in selected configurations, and the linear global modes recomputed. It is confirmed that the linear global eigenvalues move according to the predicted sensitivity pattern for small-amplitude base-flow modifications, for which the theory applies. We also look at the implications of a small control cylinder for the flow. Only the upstream high-sensitivity region proves to be robust in terms of control, but one should be careful not to disturb the flow in the downstream high-sensitivity region, in order to achieve control. The findings can have direct implications for the numerical resolution requirements for wakes at higher Reynolds numbers. Furthermore, they provide one more possible explanation for why confined wakes have a more narrow frequency spectrum than unconfined wakes.

  • 49.
    Tammisola, Outi
    et al.
    Cambridge.
    Giannetti, Flavio
    Citro, Vincenzo
    Juniper, Matthew
    Department of Engineering, Cambridge University, Cambridge.
    Second-order perturbation of global modes and implications for spanwise wavy actuation2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, p. 314-335Article in journal (Refereed)
    Abstract [en]

    Sensitivity analysis has successfully located the most efficient regions in which to apply passive control in many globally unstable flows. As is shown here and in previous studies, the standard sensitivity analysis, which is linear (first order) with respect to the actuation amplitude, predicts that steady spanwise wavy alternating actuation/modification has no effect on the stability of planar flows, because the eigenvalue change integrates to zero in the spanwise direction. In experiments, however, spanwise wavy modification has been shown to stabilize the flow behind a cylinder quite efficiently. In this paper, we generalize sensitivity analysis by examining the eigenvalue drift (including stabilization/destabilization) up to second order in the perturbation, and show how the second-order eigenvalue changes can be computed numerically by overlapping the adjoint eigenfunction with the first-order global eigenmode correction, shown here for the first time. We confirm the prediction against a direct computation, showing that the eigenvalue drift due to a spanwise wavy base flow modification is of second order. Further analysis reveals that the second-order change in the eigenvalue arises through a resonance of the original (2-D) eigenmode with other unperturbed eigenmodes that have the same spanwise wavelength as the base flow modification. The eigenvalue drift due to each mode interaction is inversely proportional to the distance between the eigenvalues of the modes (which is similar to resonance), but also depends on mutual overlap of direct and adjoint eigenfunctions (which is similar to pseudoresonance). By this argument, and by calculating the most sensitive regions identified by our analysis, we explain why an in-phase actuation/modification is better than an out-of-phase actuation for control of wake flows by spanwise wavy suction and blowing. We also explain why wavelengths several times longer than the wake thickness are more efficient than short wavelengths.

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  • 50.
    Tammisola, Outi
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Univ Nottingham, England.
    Juniper, M. P.
    Coherent structures in a swirl injector at Re=4800 by nonlinear simulations and linear global modes2016In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 792, p. 620-657Article in journal (Refereed)
    Abstract [en]

    The large-scale coherent motions in a realistic swirl fuel-injector geometry are analysed by direct numerical simulations (DNS), proper orthogonal decomposition (POD), and linear global modes. The aim is to identify the origin of instability in this turbulent flow in a complex internal geometry. The flow field in the nonlinear simulation is highly turbulent, but with a distinguishable coherent structure: the precessing vortex core (a spiralling mode). The most energetic POD mode pair is identified as the precessing vortex core. By analysing the fast Fourier transform (FFT) of the time coefficients of the POD modes, we conclude that the first four POD modes contain the coherent fluctuations. The remaining POD modes (incoherent fluctuations) are used to form a turbulent viscosity field, using the Newtonian eddy model. The turbulence sets in from convective shear layer instabilities even before the nonlinear flow reaches the other end of the domain, indicating that equilibrium solutions of the Navier-Stokes are never observed. Linear global modes are computed around the mean flow from DNS, applying the turbulent viscosity extracted from POD modes. A slightly stable discrete m = 1 eigenmode is found, well separated from the continuous spectrum, in very good agreement with the POD mode shape and frequency. The structural sensitivity of the precessing vortex core is located upstream of the central recirculation zone, identifying it as a spiral vortex breakdown instability in the nozzle. Furthermore, the structural sensitivity indicates that the dominant instability mechanism is the Kelvin-Helmholtz instability at the inflection point forming near vortex breakdown. Adjoint modes are strong in the shear layer along the whole extent of the nozzle, showing that the optimal initial condition for the global mode is localized in the shear layer. We analyse the qualitative influence of turbulent dissipation in the stability problem (eddy viscosity) on the eigenmodes by comparing them to eigenmodes computed without eddy viscosity. The results show that the eddy viscosity improves the complex frequency and shape of global modes around the fuel-injector mean flow, while a qualitative wavemaker position can be obtained with or without turbulent dissipation, in agreement with previous studies. This study shows how sensitivity analysis can identify which parts of the flow in a complex geometry need to be altered in order to change its hydrodynamic stability characteristics.

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