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  • 301.
    Costa, Pedro
    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.
    Brandt, L.
    Picano, F.
    Erratum: Interface-resolved simulations of small inertial particles in turbulent channel flow (Journal of Fluid Mechanics (2020)883 (A54) DOI: 10.1017/jfm.2019.918)2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 891, article id E2Article in journal (Refereed)
    Abstract [en]

    Equation (2.10) in Costa, Brandt & Picano (2020) for the lift force model used in the point-particle direct numerical simulations (DNS), and which is derived from the classical lift force of Saffman (1965), (Equation presented) does not correspond to the force model actually used in the point-particle DNS with lift force presented in the manuscript. Instead, the following equation was used: (Equation presented) which replaces the first occurrence of the term |Us| on the right-hand-side of (1) with |ω|D. We recall that two cases were considered in the manuscript depending on the value of J in the lift force equation: J = 1 in the case denoted PP-Saffman; and J given by (Equation presented) with ϵ = √|ω|ν/|Us|, in the case denoted PP-McLaughlin. Also, equation (2.13) of the manuscript - describing the perfectly elastic hard-sphere rebound - is incorrect; the term D/2 should be D: (Equation presented) Despite the lapse in the manuscript, equation (4) was implemented correctly (Costa et al. 2020). The results from the point-particle DNS with the model reported in (2.10) of Costa et al. (2020) ((1) above) differ from those reported in the manuscript, and are shown (Figure presented) in figure 1 (cf. figures 7 and 8 of Costa et al. (2020)). The statistics presented here have been collected in the fully developed state from 600 samples over a time interval of 250h/Ub, which ensured statistical convergence of the results. The results from the point-particle cases presented in the original manuscript are also reproduced here with this (higher) statistical sampling, and show very minor differences with respect to figures 7 and 8 of Costa et al. (2020). In light of these results, the conclusions drawn from the results in the last section of § 3 of the manuscript must be therefore reformulated: (i) The Saffman lift model does not correctly predict the near-wall statistics of the interface-resolved DNS very close to the wall, including the near-wall concentration peak. (ii) The equation proposed by Mei (1992) that fits the model of McLaughlin (1991) shows results similar to those reported in the original manuscript for this model. That is, it predicts well the near-wall concentration peak, and fails to predict the other observables near the wall. (iii) Equation (2) for Fl presented above, with J = 1, predicts very well all the observables in figure 1. We have therefore accidentally discovered that the expression (2) for Fl predicts the observed particle statistics very well. Still, the reason for the strikingly good agreement remains elusive to us. We hope that this result can be further exploited for the improvement lift force models for point-particle simulations of wall-bounded turbulent flows. 

  • 302.
    Costa, Pedro
    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. Univ Iceland, Fac Ind Engn Mech Engn & Comp Sci, Hjardarhagi 2-6, IS-107 Reykjavik, Iceland..
    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.
    Picano, Francesco
    Univ Padua, Dept Ind Engn, Via Venezia 1, I-35131 Padua, Italy..
    Interface-resolved simulations of small inertial particles in turbulent channel flow2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, article id A54Article in journal (Refereed)
    Abstract [en]

    We present a direct comparison between interface-resolved and one-way-coupled point-particle direct numerical simulations (DNS) of gravity-free turbulent channel flow laden with small inertial particles, with high particle-to-fluid density ratio and diameter of approximately three viscous units. The most dilute flow considered, solid volume fraction O(10(-5)), shows the particle feedback on the flow to be negligible, whereas differences with respect to the unladen case, notably a drag increase of approximately 10 %, are found for a volume fraction O(10(-4)). This is attributed to a dense layer of particles at the wall, caused by turbophoresis, flowing with large particle-to-fluid apparent slip velocity. The most dilute case is therefore taken as the benchmark for assessing the validity of a widely used point-particle model, where the particle dynamics results only from inertial and nonlinear drag forces. In the bulk of the channel, the first- and second-order moments of the particle velocity from the point-particle DNS agree well with those from the interface-resolved DNS. Close to the wall, however, most of the statistics show major qualitative differences. We show that this difference originates from the strong shear-induced lift force acting on the particles in the near-wall region. This mechanism is well captured by the lift force model due to Saffman (J. Fluid Mech., vol. 22 (2), 1965, pp. 385-400), while other widely used, more elaborate, approaches aiming at extending the lift model for a wider range of particle Reynolds numbers can actually underpredict the magnitude of the near-wall particle velocity fluctuations for the cases analysed here.

  • 303.
    Costa, Pedro
    et al.
    Univ Iceland, Mech Engn & Comp Sci, Fac Ind Engn, Hjardarhagi 2-6, IS-107 Reykjavik, Iceland..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Linne FLOW Ctr & SeRC, Dept Engn Mech, KTH, SE-10044 Stockholm, Sweden..
    Picano, Francesco
    Univ Padua, Dept Ind Engn, Via Venezia,1, I-35131 Padua, Italy..
    Near-wall turbulence modulation by small inertial particles2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 922, article id A9Article in journal (Refereed)
    Abstract [en]

    We use interface-resolved simulations to study near-wall turbulence modulation by small inertial particles, much denser than the fluid, in dilute/semi-dilute conditions. We considered three bulk solid mass fractions, , and , with only the latter two showing turbulence modulation. The increase of the drag is strong at , but mild in the densest case. Two distinct regimes of turbulence modulation emerge: for smaller mass fractions, the turbulence statistics are weakly affected and the near-wall particle accumulation increases the drag so the flow appears as a single-phase flow at slightly higher Reynolds number. Conversely, at higher mass fractions, the particles modulate the turbulent dynamics over the entire flow, and the interphase coupling becomes more complex. In this case, fluid Reynolds stresses are attenuated, but the inertial particle dynamics near the wall increases the drag via correlated velocity fluctuations, leading to an overall drag increase. Hence, we conclude that, although particles at high mass fractions reduce the fluid turbulent drag, the solid phase inertial dynamics still increases the overall drag. However, inspection of the streamwise momentum budget in the two-way coupling limit of vanishing volume fraction, but finite mass fraction, indicates that this trend could reverse at even higher particle load.

  • 304.
    Costa, Pedro
    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. Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, University of Iceland, Hjardarhagi 2-6, 107 Reykjavik, Iceland.
    Phillips, Everett
    NVIDIA Corporation, Santa Clara CA 95050, United States of America.
    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.
    Fatica, Massimo
    NVIDIA Corporation, Santa Clara CA 95050, United States of America.
    GPU acceleration of CaNS for massively-parallel direct numerical simulations of canonical fluid flows2020In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668Article in journal (Refereed)
    Abstract [en]

    This work presents the GPU acceleration of the open-source code CaNS for very fast massively-parallel simulations of canonical fluid flows. The distinct feature of the many-CPU Navier–Stokes solver in CaNS is its fast direct solver for the second-order finite-difference Poisson equation, based on the method of eigenfunction expansions. The solver implements all the boundary conditions valid for this type of problems in a unified framework. Here, we extend the solver for GPU-accelerated clusters using CUDA Fortran. The porting makes extensive use of CUF kernels and has been greatly simplified by the unified memory feature of CUDA Fortran, which handles the data migration between host (CPU) and device (GPU) without defining new arrays in the source code. The overall implementation has been validated against benchmark data for turbulent channel flow and its performance assessed on a NVIDIA DGX-2 system (16 T V100 32Gb, connected with NVLink via NVSwitch). The wall-clock time per time step of the GPU-accelerated implementation is impressively small when compared to its CPU implementation on state-of-the-art many-CPU clusters, as long as the domain partitioning is sufficiently small that the data resides mostly on the GPUs. The implementation has been made freely available and open source under the terms of an MIT license.

  • 305.
    Costa, Pedro
    et al.
    Delft Univ Technol, Proc & Energy Dept Multiphase Syst, Leeghwaterstr 21, NL-2621 CA Delft, Netherlands..
    Picano, Francesco
    Univ Padua, Dept Ind Engn, Via Venezia 1, I-35131 Padua, Italy..
    Brandt, L.uca
    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.
    Breugem, Wim-Paul
    Delft Univ Technol, Proc & Energy Dept Multiphase Syst, Leeghwaterstr 21, NL-2621 CA Delft, Netherlands..
    Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions2018In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 843, p. 450-478Article in journal (Refereed)
    Abstract [en]

    We use interface-resolved numerical simulations to study finite-size effects in turbulent channel flow of neutrally buoyant spheres. Two cases with particle sizes differing by a factor of two, at the same solid volume fraction of 20% and bulk Reynolds number are considered. These are complemented with two reference single-phase flows: the unladen case, and the flow of a Newtonian fluid with the effective suspension viscosity of the same mixture in the laminar regime. As recently highlighted in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501), a particle-wall layer is responsible for deviations of the mesoscale-averaged statistics from what is observed in the continuum limit where the suspension is modelled as a Newtonian fluid with (higher) effective viscosity. Here we investigate in detail the fluid and particle dynamics inside this layer and in the bulk. In the particle-wall layer, the near-wall inhomogeneity has an influence on the suspension microstructure over a distance proportional to the particle size. In this layer, particles have a significant (apparent) slip velocity that is reflected in the distribution of wall shear stresses. This is characterized by extreme events (both much higher and much lower than the mean). Based on these observations we provide a scaling for the particle-to-fluid apparent slip velocity as a function of the flow parameters. We also extend the scaling laws in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501) to second-order Eulerian statistics in the homogeneous suspension region away from the wall. The results show that finite-size effects in the bulk of the channel become important for larger particles, while negligible for lower-order statistics and smaller particles. Finally, we study the particle dynamics along the wall-normal direction. Our results suggest that single-point dispersion is dominated by particle-turbulence (and not particle-particle) interactions, while differences in two-point dispersion and collisional dynamics are consistent with a picture of shear-driven interactions.

  • 306. Costa, Pedro
    et al.
    Picano, Francesco
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Breugem, Wim-Paul
    Universal Scaling Laws for Dense Particle Suspensions in Turbulent Wall-Bounded Flows2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, no 13, article id 134501Article in journal (Refereed)
    Abstract [en]

    The macroscopic behavior of dense suspensions of neutrally buoyant spheres in turbulent plane channel flow is examined. We show that particles larger than the smallest turbulence scales cause the suspension to deviate from the continuum limit in which its dynamics is well described by an effective suspension viscosity. This deviation is caused by the formation of a particle layer close to the wall with significant slip velocity. By assuming two distinct transport mechanisms in the near-wall layer and the turbulence in the bulk, we define an effective wall location such that the flow in the bulk can still be accurately described by an effective suspension viscosity. We thus propose scaling laws for the mean velocity profile of the suspension flow, together with a master equation able to predict the increase in drag as a function of the particle size and volume fraction.

  • 307.
    Cremades, Andrés
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. CMT-Motores Térmicos, Universitat Politècnica de València, Camino de Vera s/n, 46022, Valencia, Spain, Camino de Vera s/n.
    Hoyas, Sergio
    Instituto Universitario de Matemática Pura y Aplicada, Universitat Politècnica de València, 46022, Valencia, Spain.
    Deshpande, Rahul
    Department of Mechanical Engineering, University of Melbourne, 3010, Parkville, VIC, Australia.
    Quintero, Pedro
    CMT-Motores Térmicos, Universitat Politècnica de València, Camino de Vera s/n, 46022, Valencia, Spain, Camino de Vera s/n.
    Lellep, Martin
    SUPA, School of Physics and Astronomy, The University of Edinburgh, James Clerk Maxwell Building, Peter Guthrie Tait Road, EH9 3FD, Edinburgh, UK, James Clerk Maxwell Building, Peter Guthrie Tait Road.
    Lee, Will Junghoon
    Department of Mechanical Engineering, University of Melbourne, 3010, Parkville, VIC, Australia.
    Monty, Jason P.
    Department of Mechanical Engineering, University of Melbourne, 3010, Parkville, VIC, Australia.
    Hutchins, Nicholas
    Department of Mechanical Engineering, University of Melbourne, 3010, Parkville, VIC, Australia.
    Linkmann, Moritz
    School of Mathematics and Maxwell Institute for Mathematical Sciences, University of Edinburgh, EH9 3FD, Edinburgh, UK.
    Marusic, Ivan
    Department of Mechanical Engineering, University of Melbourne, 3010, Parkville, VIC, Australia.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Identifying regions of importance in wall-bounded turbulence through explainable deep learning2024In: Nature Communications, E-ISSN 2041-1723, Vol. 15, no 1, article id 3864Article in journal (Refereed)
    Abstract [en]

    Despite its great scientific and technological importance, wall-bounded turbulence is an unresolved problem in classical physics that requires new perspectives to be tackled. One of the key strategies has been to study interactions among the energy-containing coherent structures in the flow. Such interactions are explored in this study using an explainable deep-learning method. The instantaneous velocity field obtained from a turbulent channel flow simulation is used to predict the velocity field in time through a U-net architecture. Based on the predicted flow, we assess the importance of each structure for this prediction using the game-theoretic algorithm of SHapley Additive exPlanations (SHAP). This work provides results in agreement with previous observations in the literature and extends them by revealing that the most important structures in the flow are not necessarily the ones with the highest contribution to the Reynolds shear stress. We also apply the method to an experimental database, where we can identify structures based on their importance score. This framework has the potential to shed light on numerous fundamental phenomena of wall-bounded turbulence, including novel strategies for flow control.

  • 308.
    Crialesi-Esposito, M.
    et al.
    INFN, Sez Torino, via Pietro Giuria 1, I-10125 Turin, Italy..
    Boffetta, G.
    Univ Torino, Dipartimento Fis, via P Giuria 1, I-10125 Turin, Italy..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Norwegian Univ Sci & Technol NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
    Chibbaro, S.
    Univ Paris Saclay, CNRS, LISN, Saclay F-91400, France..
    Musacchio, S.
    Univ Torino, Dipartimento Fis, via P Giuria 1, I-10125 Turin, Italy..
    Intermittency in turbulent emulsions2023In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 972, article id A37Article in journal (Refereed)
    Abstract [en]

    We investigate the statistics of turbulence in emulsions of two immiscible fluids of the same density. We compute velocity increments between points conditioned to be located in the same phase or in different phases, and examine their probability density functions (PDFs) and the associated structure functions (SFs). This enables us to demonstrate that the presence of the interface reduces the skewness of the PDF at small scales and therefore the magnitude of the energy flux towards the dissipative scales, which is quantified by the third-order SF. The analysis of the higher-order SFs shows that multiphase turbulence is more intermittent than single-phase turbulence. In particular, the local scaling exponents of the SFs display a saturation below the Kolmogorov-Hinze scale, which indicates the presence of large velocity gradients across the interface. Interestingly, the statistics of the velocity differences in the carrier phase recovers that of single-phase turbulence when the viscosity of the dispersed phase is high.

  • 309.
    Crialesi-Esposito, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. INFN, Sez Torino, Via Pietro Giuria 1, I-10125 Turin, Italy..
    Chibbaro, Sergio
    Univ Paris Saclay, CNRS, LISN, F-91400 Orsay, France.;Univ Paris Saclay, SPEC, CEA, CNRS,UMR 3680,CEA Saclay, Gif Sur Yvette, France..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Norwegian Univ Sci & Technol NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
    The interaction of droplet dynamics and turbulence cascade2023In: Communications Physics, E-ISSN 2399-3650, Vol. 6, no 1, article id 5Article in journal (Refereed)
    Abstract [en]

    The dynamics of droplet fragmentation in turbulence is described by the Kolmogorov-Hinze framework. Yet, a quantitative theory is lacking at higher concentrations when strong interactions between the phases and coalescence become relevant, which is common in most flows. Here, we address this issue through a fully-coupled numerical study of the droplet dynamics in a turbulent flow at R-lambda & AP; 140, the highest attained up to now. By means of time-space spectral statistics, not currently accessible to experiments, we demonstrate that the characteristic scale of the process, the Hinze scale, can be precisely identified as the scale at which the net energy exchange due to capillarity is zero. Droplets larger than this scale preferentially break up absorbing energy from the flow; smaller droplets, instead, undergo rapid oscillations and tend to coalesce releasing energy to the flow. Further, we link the droplet-size distribution with the probability distribution of the turbulent dissipation. This shows that key in the fragmentation process is the local flux of energy which dominates the process at large scales, vindicating its locality.

  • 310.
    Crialesi-Esposito, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Gonzalez-Montero, L. A.
    Univ Politecn Valencia, CMT Motores Term, Valencia, Spain..
    Salvador, F. J.
    Univ Politecn Valencia, CMT Motores Term, Valencia, Spain..
    Effects of isotropic and anisotropic turbulent structures over spray atomization in the near field2022In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 150, article id 103891Article in journal (Refereed)
    Abstract [en]

    Sprays and atomization processes are extremely diffused both in nature and in industrial applications. In this paper we analyze the influence of the nozzle turbulence on primary atomization, focusing on the resulting turbulent field and atomization patterns in the Near Field (NF). In order to do so, a Synthetic Boundary Condition (SBC) and a Mapped Boundary Condition (MBC), producing respectively isotropic and anisotropic turbulent fields, have been generated as inflow conditions for the spray Direct Numerical Simulations (DNS). We present a specific methodology to ensure consistency on turbulence intensity and integral lengthscale between the two inflows. The analysis performed on the turbulent field (using one-point statistics and spectrum analysis) reveals a significantly stronger turbulent field generated by the inflow boundary conditions with anisotropic structures. While the increased turbulence field generated in the MBC case results in a higher number of droplets generated, the probability functions of both cases are extremely similar, leading to the non-obvious conclusion that the atomization patterns are only slightly affected by the inflow condition. These considerations are supported by the analysis of droplet size distributions, radial distribution functions, axial and radial distributions, highlighting extremely similar behaviors between the MBC and the SBC cases. Finally, these analyses and their computations are presented in detail, underlining how this type of point-process characterization shows interesting potential in future studies on sprays.

  • 311.
    Crialesi-Esposito, Marco
    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.
    Pal, Sagar
    Institut Jean le Rond ∂’Alembert, Sorbonne Université and CNRS, Paris, France.
    Zaleski, Stephane
    Institut Jean le Rond ∂’Alembert, Sorbonne Université and CNRS, Paris, France.
    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. Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, Norway.
    Statistical dispersion of flapping events in air-assisted atomization2021In: ICLASS 2021 - 15th Triennial International Conference on Liquid Atomization and Spray Systems, ILASS - Europe, Institute for Liquid Atomization and Spray Systems , 2021Conference paper (Refereed)
    Abstract [en]

    Mixing layers between air and liquid are ubiquitous in nature and extremely common in industrial applications. Among the latest, air-assisted atomization is one of the predominant forcing used to induce breakup in liquid jets. In these flows, a low-momentum liquid jet and a high-momentum air stream are injected simultaneously along the same direction, separated only by a thin solid plate. The interaction between the two phases generates an instability that manifests in the form of flapping events, during which most of the droplets are created owing to the breakup of these liquid sheets. In this study, we discuss the statistical relevance of flapping events and how they contribute to determine the final droplet-size-distribution. We present an algorithm to isolate each event in Direct Numerical Simulations and discuss the dispersion between each event. Finally, we demonstrate the importance of performing this type of analysis to reveal of statistical properties of the droplet size distribution.

  • 312.
    Crialesi-Esposito, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rosti, Marco Edoardo
    Okinawa Inst Sci & Technol Grad Univ, Complex Fluids & Flows Unit, 1919-1 Tancha, Onna, Okinawa 9040495, Japan..
    Chibbaro, Sergio
    Sorbonne Univ, Inst Jean Rond Alembert, Paris, France..
    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. Norwegian Univ Sci & Technol NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
    Modulation of homogeneous and isotropic turbulence in emulsions2022In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 940, article id A19Article in journal (Refereed)
    Abstract [en]

    We present a numerical study of emulsions in homogeneous and isotropic turbulence (HIT) at Re-lambda = 137. The problem is addressed via direct numerical simulations, where the volume of fluid is used to represent the complex features of the liquid-liquid interface. We consider a mixture of two iso-density fluids, where fluid properties are varied with the goal of understanding their role in turbulence modulation, in particular the volume fraction (0.03 < alpha < 0.5), viscosity ratio (0.01 < mu(d)/mu

  • 313.
    Crialesi-Esposito, Marco
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Scapin, Nicolo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Demou, Andreas
    Rosti, Marco Edoardo
    Costa, Pedro
    Spiga, Filippo
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    FluTAS: A GPU-accelerated finite difference code for multiphase flowsIn: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944Article in journal (Refereed)
    Abstract [en]

    We present the Fluid Transport Accelerated Solver, FluTAS, a scalable GPU code for multiphase flows with thermal effects. The code solves the incompressible Navier-Stokes equation for two-fluid systems, with a direct FFT-based Poisson solver for the pressure equation. The interface between the two fluids is represented with the Volume of Fluid (VoF) method, which is mass conserving and well suited for complex flows thanks to its capacity of handling topological changes. The energy equation is explicitly solved and coupled with the momentum equation through the Boussinesq approximation. The code is conceived in a modular fashion so that different numerical methods can be used independently, the existing routines can be modified, and new ones can be included in a straightforward and sustainable manner. FluTAS is written in modern Fortran and parallelized using hybrid MPI/OpenMP in the CPU-only version and accelerated with OpenACC directives in the GPU implementation. We present different benchmarks to validate the code, and two large-scale simulations of fundamental interest in turbulent multiphase flows: isothermal emulsions in HIT and two-layer Rayleigh-Bénard convection. FluTAS is distributed through a MIT license and arises from a collaborative effort of several scientists, aiming to become a flexible tool to study complex multiphase flows.

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  • 314. Croze, Ottavio A.
    et al.
    Sardina, Gaetano
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ahmed, Mansoor
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bees, Martin A.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dispersion of swimming algae in laminar and turbulent channel flows: consequences for photobioreactors2013In: Journal of the Royal Society Interface, ISSN 1742-5689, E-ISSN 1742-5662, Vol. 10, no 81, p. 20121041-Article in journal (Refereed)
    Abstract [en]

    Shear flow significantly affects the transport of swimming algae in suspension. For example, viscous and gravitational torques bias bottom-heavy cells to swim towards regions of downwelling fluid (gyrotaxis). It is necessary to understand how such biases affect algal dispersion in natural and industrial flows, especially in view of growing interest in algal photobioreactors. Motivated by this, we here study the dispersion of gyrotactic algae in laminar and turbulent channel flows using direct numerical simulation (DNS) and a previously published analytical swimming dispersion theory. Time-resolved dispersion measures are evaluated as functions of the Peclet and Reynolds numbers in upwelling and downwelling flows. For laminar flows, DNS results are compared with theory using competing descriptions of biased swimming cells in shear flow. Excellent agreement is found for predictions that employ generalized Taylor dispersion. The results highlight peculiarities of gyrotactic swimmer dispersion relative to passive tracers. In laminar downwelling flow the cell distribution drifts in excess of the mean flow, increasing in magnitude with Peclet number. The cell effective axial diffusivity increases and decreases with Peclet number (for tracers it merely increases). In turbulent flows, gyrotactic effects are weaker, but discernable and manifested as non-zero drift. These results should have a significant impact on photobioreactor design.

  • 315. Cunha, F. R.
    et al.
    Lima Albernaz, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Brasília.
    Oscillatory motion of a spherical bubble in a non-Newtonian fluid2013In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 191, p. 35-44Article in journal (Refereed)
    Abstract [en]

    The motion of a spherical bubble in a nonlinear viscoelastic media subjected to an acoustic pressure field is considered. The ambient fluid is composed of a Newtonian liquid in which additives at small volume fraction are diluted. The contribution of the additives with high aspect ratio brings strong anisotropy and is described by an extensional viscosity. The elastic effect is presented by the relaxation time of the additives. A lower convected Maxwell model is adopted to describe the viscoelastic properties, resulting in a modified Rayleigh-Plesset equation. The set of governing equations does not require a numerical solution for the space domain. Non-linear radial oscillations of a single bubble are obtained numerically using a fifth order Runge-Kutta scheme with adaptive time step. The results predict an extra anisotropy for a Deborah number regime De∼. 1, due to stretched additives, which contributes to bubble motion stabilization. Under this condition, the relaxation time is greater than the time scale of the flow, where no interaction between the elastic effect of the additives and the motion of the bubble is found. However, for De∼. 0.1 we observe an increase of vibrational modes on the frequency domain and higher bubble internal pressure, which may lead to collapse occurrence. The decrease in the volume fraction of the additives also shows significant variation of bubble oscillations as the elastic effect has a proportionally larger contribution than the anisotropic effect. Other results and considerations regarding relevant parameters are also discussed.

  • 316.
    Dadfar, Reza
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fabbiane, Nicolo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bagheri, Shervin
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dans S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Centralised Versus Decentralised Active Control of Boundary Layer Instabilities2014In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 93, no 4, p. 537-553Article in journal (Refereed)
    Abstract [en]

    We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude three-dimensional Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer. A three-dimensional viscous, incompressible flow developing on a zero-pressure gradient boundary layer in a low Reynolds number environment is analyzed using direct numerical simulations. In this configuration, we distribute evenly in the spanwise direction up to 72 localised objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration, the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanwise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. If n and m are the number of sensor-actuator pairs for the whole system and for a single control unit, respectively, then in a decentralised strategy, the number of interconnections deceases mn compared to a centralized strategy, which has n (2) interconnections. We find that using a semi-decentralized approach - where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction - results only in a 11 % reduction of control performance. We explain how "wide" in the spanwise direction a control unit should be for a satisfactory control performance. Moreover, the control unit should be designed to account for the perturbations that are coming from the lateral sides (crosstalk) of the estimation sensors. We have also found that the influence of crosstalk is not as essential as the spreading effect.

  • 317.
    Dadfar, Reza
    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.
    Hanifi, Ardeshir
    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.
    Henningson, Dan S.
    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.
    Control of instabilities in an unswept wing boundary layer2018In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 5, p. 1750-1759Article in journal (Refereed)
    Abstract [en]

    Linear control theory is used to construct an output feedback controller to attenuate the amplitude of the Tollmien–Schlichting waves inside the boundary layer developing over an unswept wing. The analysis is based on direct numerical simulations. The studied scenario includes the impulse response of the system to a generic disturbance in the freestream, which triggers a Tollmien–Schlichting wave packet inside the boundary layer. The performance of a linear quadratic Gaussian controller is analyzed to suppress the amplitude of the Tollmien–Schlichting wave packet using a row of sensors and plasma actuators localized at the wall. The target of the controller is chosen as a subset of proper orthogonal decomposition modes describing the dynamics of the unstable disturbances. The plasma actuators are implemented as volume forcing. To account for the limitations of the plasma actuators concerning a unidirectional forcing, several strategies are implemented in the linear quadratic Gaussian framework. Their performances are compared with that for classical linear quadratic Gaussian controller. These controllers successfully reduced the amplitude of the wave packet.

  • 318.
    Dadfar, Reza
    et al.
    KTH.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    Feedback control for laminarization of flow over wings2013In: International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2013, TSFP-8 , 2013Conference paper (Refereed)
  • 319.
    Dadfar, Reza
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dans S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Control of instabilities in boundary layer of unswept wingManuscript (preprint) (Other academic)
  • 320.
    Dadfar, Reza
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dans S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Feedback Control for Laminarization of flow over Wings2015In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 94, no 1, p. 43-62Article in journal (Refereed)
    Abstract [en]

    An active control strategy is implemented to attenuate the amplitude of the Tollmien-Schlichting (TS) waves inside the boundary layer of an airfoil. The dynamics of the system are modelled by the linearised Navier-Stokes equations. The impulse response to an initial disturbance, initially located outside of the boundary layer and in front of the airfoil is considered. The perturbation evolves and penetrates inside the boundary layer and triggers the TS waves. Different control strategies including the linear quadratic Gaussian (LQG) and model predictive control (MPC) are designed based on a reduced order model where the sensors and actuators are localised near the wall. An output projection is used to identify the unstable disturbances; the objective function of the controller is selected as a set of proper orthogonal decomposition (POD) modes; to isolate the dynamics of the TS waves, the modes with high energy contents in the TS wave frequency band are considered as the objective of the controller. A plasma actuator is modelled and implemented as an external forcing on the flow. To account for the limitations of the plasma actuator several strategies are examined and the results are compared with a classical LQG controller. The outcomes reveal successful performance in mitigating the amplitude of the wavepacket developing inside the boundary layer.

  • 321.
    Dadfar, Reza
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Semeraro, Onofrio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Output Feedback Control of Blasius Flow with Leading Edge Using Plasma Actuator2013In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 51, no 9, p. 2192-2207Article in journal (Refereed)
    Abstract [en]

    The evolution and control of a two-dimensional wave packet developing on a flat plate with a leading edge is investigated by means of direct numerical simulation. The aim is to identify and suppress the wave packets generated by freestream perturbations. A sensor is placed close to the wall to detect the upcoming wave packet, while an actuator is placed further downstream to control it. A plasma actuator is modeled as an external forcing on the flow using a model based and validated on experimental investigations. A linear quadratic Gaussian controller is designed, and an output projection is used to build the objective function. Moreover, by appropriate selection of the proper orthogonal decomposition modes, we identify the disturbances to be damped. A reduced-order model of the input-output system is constructed by using system identification via the eigensystem realization algorithm. A limitation of the plasma actuators is the unidirectional forcing of the generated wall jet, which is predetermined by the electrodes' location. In this paper, we address this limitation by proposing and comparing two different solutions: 1) introducing an offset in the control signal such that the resulting total forcing is oriented along one direction, and 2) using two plasma actuators acting in opposite directions. The results are compared with the ideal case where constraints are not accounted for the control design. We show that the resulting controllers based on plasma actuators can successfully attenuate the amplitude of the wave packet developing inside the boundary layer.

  • 322.
    Dahan, Jeremy
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aerodynamics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Futrzynski, Romain
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aerodynamics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    O'Reilly, Ciarán
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Efraimsson, Gunilla
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Aerodynamics.
    Aero-acoustic source analysis of landing gear noise via dynamic mode decomposition2014In: 21st International Congress on Sound and Vibration, ICSV21: / [ed] Malcolm J. Crocker, Marek Pawelczyk, Jing Tian, 2014, Vol. 2, p. 1245-1252Conference paper (Refereed)
    Abstract [en]

    In this paper, we apply dynamic mode decomposition (DMD) on time accurate simulationsof the pressure distribution on a realistic full-scale noselanding gear configuration in order toidentify noise generating structures on landing gear surfaces. The simulated pressure data isobtained from DES simulations using the commercial software STAR-CCM+ by CD-adapco.The dynamics of the surface pressure on a tyre are discussed and the DMD modes are com-puted from instantaneous pressure snapshots. The far-fieldnoise is determined via the FfowcsWilliams-Hawkings analogy, where a given frequency band source term can be reconstructedby choosing an appropriate number of DMD modes.

  • 323.
    Dahlkild, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Finite wavelength selection for the linear instability of a suspension of settling spheroids2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 689, p. 183-202Article in journal (Refereed)
    Abstract [en]

    The instability of an initially homogeneous suspension of spheroids, settling due to gravity, is reconsidered. For non-spherical particles, previous studies in the literature report that normal-mode density perturbations of maximum growth rate are those of arbitrarily large, horizontal wavelength. Using the 'mixture theory' for two-phase flow we show that the maximum growth rate for horizontal density perturbations is obtained for a finite wavelength if the inertia of the bulk motion associated with the normal-mode density perturbation is accounted for. We find that for long wavelengths, lambda -> infinity, the growth rate approaches zero as lambda(-2/3). The maximum growth rate is obtained for lambda similar to d/root alpha(0)Re(L)(1/2), where d is the axis perpendicular to the axis of rotational symmetry of the spheroid, alpha(0) is the undisturbed volume fraction of particles and Re(L), typically << 1, is a Reynolds number of the bulk motion on a typical length scale L similar to d/p root alpha(0) and a velocity scale on the order of the undisturbed settling speed. The theoretical results for the wavelength selection agree qualitatively well with previous experimental results in the literature of measured correlation lengths of vertical streamers in settling fibre suspensions.

  • 324.
    Dahlkild, Anders
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Particle orientation distribution in a rotating, dilute suspension of rod-shaped particles2023In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 954, article id A49Article in journal (Refereed)
    Abstract [en]

    We consider a theoretical model for the settling of rod-shaped particles of a dilute, initially homogeneous, suspension in rapid rotation. The particle Reynolds number and the particle Taylor number of the detailed flow around the particles are assumed small, representing a relevant limit for an industrial centrifugal separation process. By applying a statistical approach using the Fokker-Planck equation, and neglecting particle-particle interactions, we obtain an explicit, analytical solution for the time dependent, spatially uniform particle orientation distribution function. Not only does the volume fraction in the bulk of the suspension decrease with time due to the divergent centrifugal field, as similarly described in the literature for suspensions of spherical particles, the orientation of the rod particles also changes with time from an initially uniform distribution to one where the particles tend to align with a plane perpendicular to the axis of rotation. The corresponding particle trajectories, as also influenced by first-order effects from the Coriolis acceleration and gyroscopic effects, are obtained numerically for different initial particle orientation angles.

  • 325.
    Dalla Barba, Federico
    et al.
    CISAS, University of Padova, Padova, Italy.
    Scapin, Nicolo
    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.
    Demou, Andreas
    KTH, School of Engineering Sciences (SCI). demou@kth.se.
    Rosti, Marco E.
    Complex Fluids and Flows Unit, Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Okinawa 904-0495, Japan.
    Picano, Francesco
    Department of Industrial Engineering & CISAS, University of Padova, Padova, Italy.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, Norway.
    An interface capturing method for liquid-gas flows at low-Mach number2021In: Computers & Fluids, ISSN 0045-7930, E-ISSN 1879-0747, Vol. 216, article id 104789Article in journal (Refereed)
    Abstract [en]

    Multiphase, compressible and viscous flows are of crucial importance in a wide range of scientific and engineering problems. Despite the large effort paid in the last decades to develop accurate and efficient numerical techniques to address this kind of problems, current models need to be further improved to address realistic applications. In this context, we propose a numerical approach to the simulation of multiphase, viscous flows where a compressible and an incompressible phase interact in the low-Mach number regime. In this frame, acoustics are neglected but large density variations of the compressible phase can be accounted for as well as heat transfer, convection and diffusion processes. The problem is addressed in a fully Eulerian framework exploiting a low-Mach number asymptotic expansion of the Navier-Stokes equations. A Volume of Fluid approach (VOF) is used to capture the liquid-gas interface, built on top of a massive parallel solver, second order accurate both in time and space. The second-order-pressure term is treated implicitly and the resulting pressure equation is solved with the eigenexpansion method employing a robust and novel formulation. We provide a detailed and complete description of the theoretical approach together with information about the numerical technique and implementation details. Results of benchmarking tests are provided for five different test cases. 

  • 326. Daly, C. A.
    et al.
    Schneider, Tobias M.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Peake, N.
    Secondary instability and tertiary states in rotating plane Couette flow2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 761, p. 27-61Article in journal (Refereed)
    Abstract [en]

    Recent experimental studies have shown rich transition behaviour in rotating plane Couette flow (RPCF). In this paper we study the transition in supercritical RPCF theoretically by determination of equilibrium and periodic orbit tertiary states via Floquet analysis on secondary Taylor vortex solutions. Two new tertiary states are discovered which we name oscillatory wavy vortex flow (oWVF) and skewed vortex flow (SVF). We present the bifurcation routes and stability properties of these new tertiary states and, in addition, we describe a bifurcation procedure whereby a set of defected wavy twist vortices is approached. Further to this, transition scenarios at flow parameters relevant to experimental works are investigated by computation of the set of stable attractors which exist on a large domain. The physically observed flow states are shown to share features with states in our set of attractors.

  • 327. Dangendorf, Soenke
    et al.
    Müller-Navarra, Sylvin
    Jensen, Juergen
    Schenk, Frederik
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wahl, Thomas
    Weisse, Ralf
    North Sea Storminess from a Novel Storm Surge Record since AD 18432014In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, no 10, p. 3582-3595Article in journal (Refereed)
    Abstract [en]

    The detection of potential long-term changes in historical storm statistics and storm surges plays a vitally important role for protecting coastal communities. In the absence of long homogeneous wind records, the authors present a novel, independent, and homogeneous storm surge record based on water level observations in the North Sea since 1843. Storm surges are characterized by considerable interannual-to-decadal variability linked to large-scale atmospheric circulation patterns. Time periods of increased storm surge levels prevailed in the late nineteenth and twentieth centuries without any evidence for significant long-term trends. This contradicts with recent findings based on reanalysis data, which suggest increasing storminess in the region since the late nineteenth century. The authors compare the wind and pressure fields from the Twentieth-Century Reanalysis (20CRv2) with the storm surge record by applying state-of-the-art empirical wind surge formulas. The comparison reveals that the reanalysis is a valuable tool that leads to good results over the past 100 yr; previously the statistical relationship fails, leaving significantly lower values in the upper percentiles of the predicted surge time series. These low values lead to significant upward trends over the entire investigation period, which are in turn supported by neither the storm surge record nor an independent circulation index based on homogeneous pressure readings. The authors therefore suggest that these differences are related to higher uncertainties in the earlier years of the 20CRv2 over the North Sea region.

  • 328. Dankowicz, Harry
    et al.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Untitled2014In: Applied Mechanics Review, ISSN 0003-6900, E-ISSN 1088-8535, Vol. 66, no 2, p. 020201-Article in journal (Refereed)
  • 329. De Aguiar Quintanilha Junior, H. R.
    et al.
    Kataras, P. B.
    Theofilis, V.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.
    Nonmodal stability analysis of the HIFiRE-5 elliptic cone model flow in different flight altitudes2018In: 58th Israel Annual Conference on Aerospace Sciences, IACAS 2018, Israel Annual Conference on Aerospace Sciences , 2018, p. 1543-1555Conference paper (Refereed)
    Abstract [en]

    Nonmodal instability analysis is carried out for a 2:1 elliptic cone with base flow conditions selected for a Ma=7 and two different ight altitudes, namely 33km and 21km with unit Reynolds number Re′ = 1.89 x 106 m-1 and Re′ = 1.015 x 107 m-1, respectively. The aim is to analyze the effects of transiently growing optimal disturbances and their possible relation to instability mechanisms that have been confirmed to exist in previous modal crossow. Local linear stability results obtained at several streamwise locations on the cone surface indicate that transient growth in the crossow region may be correlated to streamwise oriented structures having spanwise spacing of the same order of magnitude as which have long been known to exist in this flow.

  • 330. de Lange, H. C.
    et al.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Drag reduction on external surfaces induced by wall waves2011In: ERCOFTAC Series, Springer Netherlands, 2011, Vol. 15, p. 437-442Conference paper (Refereed)
  • 331. de Luzan, Charles Farbos
    et al.
    Chen, Jie
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Khosla, Sid M.
    Gutmark, Ephraim
    Computational study of false vocal folds effects on unsteady airflows through static models of the human larynx2015In: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 48, no 7, p. 1248-1257Article in journal (Refereed)
    Abstract [en]

    Compressible large eddy simulation is employed to numerically investigate the laryngeal flow. Symmetric static models of the human larynx with a divergent glottis are considered, with the presence of false vocal folds (FVFs). The compressible study agrees well with that of the incompressible study. Due to the high enough Reynolds number, the flow is unsteady and develops asymmetric states downstream of the glottis. The glottal jet curvature decreases with the presence of FVFs or the ventricular folds. The gap between the FVFs stretches the flow structure and reduces the jet curvature. The presence of FVFs has a significant effect on the laryngeal flow resistance. The intra-glottal vortex structures are formed on the divergent wall of the glottis, immediately downstream of the separation point. The vortices are then convected downstream and characterized by a significant negative static pressure. The FVFs are a main factor in the generation of stronger vortices, and thus on the closure of the TVFs. The direct link between the FVFs geometry and the motion of the TVFs, and by extension to the voice production, is of interest for medical applications as well as future research works. The presence of the FVFs also changes the dominant frequencies in the velocity and pressure spectra.

  • 332.
    de Stadler, Matthew B.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rapaka, Narsimha R.
    Sarkar, Sutanu
    Large eddy simulation of the near to intermediate wake of a heated sphere at Re=10,0002014In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 49, p. 2-10Article in journal (Refereed)
    Abstract [en]

    Large eddy simulation is used to numerically simulate flow past a heated sphere at Re = 10,000. A second order accurate in space and time, semi-implicit finite difference code is used with the immersed boundary to represent the sphere in a Cartesian domain. Visualizations of the vorticity field and temperature field are provided together with profiles of the temperature and velocity fields at various locations in the wake. The laminar separated shear layer was found to efficiently transport heat from the hot sphere surface to the cold fluid in the wake. The thin separated shear layers are susceptible to Kelvin-Helmholtz instability and the pronounced rollers that subsequently form promote entrainment of both cold free-stream fluid and warmer fluid near the back of the sphere. Breakdown of the shear layer into turbulence and subsequent interaction with the recirculation zone results in rapid mixing of the temperature field in the lee of the sphere. The wake dimensions of the velocity field and the temperature field were found to be comparable in the developed flow behind the re-circulating region. Profiles of the mean and fluctuating temperature and velocity in the near wake are provided together with profiles of the Reynolds stresses and thermal fluxes. Similarity was observed for the mean temperature, rms temperature, rms velocity, and the Reynolds stress component < u(x)'u(r)'>, and the thermal fluxes < T'u(x)'> and < T'u(r)'>.

  • 333.
    De Vincentiis, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Durovic, Kristina
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Lengani, Davide
    Univ Genoa, DIME, Via Opera Pia 15, I-16145 Genoa, Italy..
    Simoni, Daniele
    Univ Genoa, DIME, Via Opera Pia 15, I-16145 Genoa, Italy..
    Pralits, Jan
    Univ Genoa, DICCA, Viale Cambiaso 6, I-16145 Genoa, Italy..
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade2023In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 145, no 5, article id 051011Article in journal (Refereed)
    Abstract [en]

    In the present work, the evolution of the boundary layer over a low-pressure turbine blade is studied using direct numerical simulations, with the aim of investigating the unsteady flow field induced by the rotor-stator interaction. The freestream flow is characterized by the high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream of the turbine blades. To include the presence of the wake without employing an ad-hoc model, we simulate both the moving bars and the stationary blades in their respective frames of reference and the coupling of the two domains is done through appropriate boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part of the blade is suppressed. Moreover, the presence of the wake introduces alternating regions in the streamwise direction of high- and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress. The analysis of the velocity fields allows the characterization of the streaky structures forced in the boundary layer by turbulence carried by upstream wakes. The breakdown events are observed once positive streamwise velocity fluctuations reach the end of the blade. Both the fluctuations induced by the migration of the wake in the blade passage and the presence of the streaks contribute to high values of the disturbance velocity inside the boundary layer with respect to a steady inflow case. The amplification of the boundary layer disturbances associated with different spanwise wavenumbers has been computed. It was found that the migration of the wake in the blade passage stands for the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the low- and high-speed regions associated with the wakes.

  • 334.
    De Vincentiis, Luca
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Durovic, Kristina
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Simoni, Daniele
    DIME—Università di Genova, Via Montallegro 1, 16145 Genoa, Italy..
    Lengani, Davide
    DIME—Università di Genova, Via Montallegro 1, 16145 Genoa, Italy..
    Pralits, Jan O.
    DICCA—Università di Genova, Via Montallegro 1, 16145 Genoa, Italy..
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of upstream wakes on the boundary layer over a low-pressure turbine bladeManuscript (preprint) (Other academic)
    Abstract [en]

    In the present work the evolution of the boundary layer over a low-pressureturbine blade is studied by means of direct numerical simulations. The set-upof the simulations follows the experiments by Lengani et al. (2017), aimingto investigate the unsteady flow field induced by the rotor-stator interaction.The free-stream flow is characterized by high level of free-stream turbulenceand periodically impinging wakes. As in the experiments, the wakes are shedby moving bars modeling the rotor blades and placed upstream of the turbineblades. To include the presence of the wake without employing an ad-hoc model,we simulate both the moving bars and the stationary blades in their respectiveframes of reference and the coupling of the two domains is done throughappropriate boundary conditions. The presence of the wake mainly affects thedevelopment of the boundary layer on the suction side of the blade. In particular,the flow separation in the rear part of the blade is suppressed. Moreover, thepresence of the wake introduces alternating regions in the streamwise direction ofhigh- and low-velocity fluctuations inside the boundary layer. These fluctuationsare responsible for significant variations of the shear stress. The analysis of thevelocity fields allows the characterization of the streaky structures forced inthe boundary layer by turbulence carried by upstream wakes. The breakdownevents are observed once positive streamwise velocity fluctuations reach theend of the blade. Both the fluctuations induced by the migration of the wakein the blade passage and the presence of the streaks contribute to high valuesof the disturbance velocity inside the boundary layer with respect to a steadyinflow case. The amplification of the boundary layer disturbances associatedwith different spanwise wavenumbers has been computed. It was found thatthe migration of the wake in the blade passage stands for the most part of theperturbations with zero spanwise wavenumber. The non-zero wavenumbers arefound to be amplified in the rear part of the blade at the boundary betweenthe low and high speed regions associated with the wakes.

  • 335.
    De Vincentiis, Luca
    et al.
    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.
    Henningson, Dan S.
    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.
    Hanifi, Ardeshir
    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.
    Transition in an infinite swept-wing boundary layer subject to surface roughness and free-stream turbulence2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 931, article id A24Article in journal (Refereed)
    Abstract [en]

    The instability of an incompressible boundary-layer flow over an infinite swept wing in the presence of disc-type roughness elements and free-stream turbulence (FST) has been investigated by means of direct numerical simulations. Our study corresponds to the experiments by Orlu et al. (Tech. Rep., KTH Royal Institute of Technology, 2021, http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-291874). Here, different dimensions of the roughness elements and levels of FST have been considered. The aim of the present work is to investigate the experimentally observed sensitivity of the transition to the FST intensity. In the absence of FST, flow behind the roughness elements with a height above a certain value immediately undergoes transition to turbulence. Impulse-response analyses of the steady flow have been performed to identify the mechanism behind the observed flow instability. For subcritical roughness, the generated wave packet experiences a weak transient growth behind the roughness and then its amplitude decays as it is advected out of the computational domain. In the supercritical case, in which the flow transitions to turbulence, flow as expected exhibits an absolute instability. The presence of FST is found to have a significant impact on the transition behind the roughness, in particular in the case of a subcritical roughness height. For a height corresponding to a roughness Reynolds number Re-hh = 461, in the absence of FST the flow reaches a steady laminar state, while a very low FST intensity of Tu = 0.03% causes the appearance of turbulence spots in the wake of the roughness. These randomly generated spots are advected out of the computational domain. For a higher FST level of Tu = 0.3%, a turbulent wake is clearly visible behind the element, similar to that for the globally unstable case. The presented results confirm the experimental observations and explain the mechanisms behind the observed laminar-turbulent transition and its sensitivity to FST.

  • 336.
    De Vita, Francesco
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rosti, Marco E.
    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. Okinawa Inst Sci & Technol Grad Univ, Complex Fluids & Flaws Unit, 1919-1 Tancha, Onna Son, Okinawa 9040495, Japan..
    Caserta, Sergio
    Univ Naples Federico II, Dept Chem Mat & Ind Prod Engn, Ple Tecchio 80, I-80125 Naples, 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), Mechanics.
    Numerical simulations of vorticity banding of emulsions in shear flows2020In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 16, no 11, p. 2854-2863Article in journal (Refereed)
    Abstract [en]

    Multiphase shear flows often show banded structures that affect the global behavior of complex fluids e.g. in microdevices. Here we investigate numerically the banding of emulsions, i.e. the formation of regions of high and low volume fractions, alternated in the vorticity direction and aligned with the flow (shear bands). These bands are associated with a decrease of the effective viscosity of the system. To understand the mechanism of experimentally observed banding, we have performed interface-resolved simulations of the two-fluid system. The experiments were performed starting with a random distribution of droplets, which under the applied shear, evolve in time resulting in a phase separation. To numerically reproduce this process, the banded structures are initialized in a narrow channel confined by two walls moving in opposite directions. We find that the initial banded distribution is stable when droplets are free to merge and unstable when coalescence is prevented. In this case, additionally, the effective viscosity of the system increases, resembling the rheological behavior of suspensions of deformable particles. Droplet coalescence, on the other hand, allows emulsions to reduce the total surface of the system and, hence, the energy dissipation associated with the deformation, which in turn reduces the effective viscosity.

  • 337.
    De Vita, Francesco
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rosti, Marco E.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Caserta, Sergio
    Univ Naples Federico II, Dept Chem Mat & Ind Prod Engn, Piazzale V Tecchio 80, I-80125 Naples, Italy..
    Brandt, Luca
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    On the effect of coalescence on the rheology of emulsions2019In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 880, p. 969-991Article in journal (Refereed)
    Abstract [en]

    We present a numerical study of the rheology of a two-fluid emulsion in dilute and semidilute conditions. The analysis is performed for different capillary numbers, volume fractions and viscosity ratios under the assumption of negligible inertia and zero buoyancy force. The effective viscosity of the system increases for low values of the volume fraction and decreases for higher values, with a maximum for approximately 20% concentration of the disperse phase. When the dispersed fluid has lower viscosity, the normalised effective viscosity becomes smaller than 1 for high enough volume fractions. To single out the effect of droplet coalescence on the rheology of the emulsion we introduce an Eulerian force which prevents merging, effectively modelling the presence of surfactants in the system. When the coalescence is inhibited the effective viscosity is always greater than 1 and the curvature of the function representing the emulsion effective viscosity versus the volume fraction becomes positive, resembling the behaviour of suspensions of deformable particles. The reduction of the effective viscosity in the presence of coalescence is associated with the reduction of the total surface of the disperse phase when the droplets merge, which leads to a reduction of the interface tension contribution to the total shear stress. The probability density function of the flow topology parameter shows that the flow is mostly a shear flow in the matrix phase, with regions of extensional flow when the coalescence is prohibited. The flow in the disperse phase, instead, always shows rotational components. The first normal stress difference is positive, except for the smallest viscosity ratio considered, whereas the second normal difference is negative, with their ratio being constant with the volume fraction. Our results clearly show that the coalescence efficiency strongly affects the system rheology and that neglecting droplet merging can lead to erroneous predictions.

  • 338.
    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.

  • 339.
    Dellacasagrande, M.
    et al.
    DIME—Università di Genova, Via Montallegro 1, 16145 Genoa, Italy.
    Lengani, D.
    DIME—Università di Genova, Via Montallegro 1, 16145 Genoa, Italy.
    Simoni, D.
    DIME—Università di Genova, Via Montallegro 1, 16145 Genoa, Italy.
    Pralits, J. O.
    DICCA—Università di Genova, Via Montallegro 1, 16145 Genoa, Italy.
    Durovic, Kristina
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Turbulence.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Turbulence.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Turbulence.
    Statistical characterization of free-stream turbulence induced transition under variable Reynolds number, free-stream turbulence, and pressure gradient2021In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 33, no 9, p. 094115-094115Article in journal (Refereed)
    Abstract [en]

    In this work, the free-stream turbulence (FST) induced transition of a flat plate boundary layer is studied using particle image velocimetry (PIV) under variable Reynolds number (Re), FST intensity, and adverse pressure gradient (APG). Overall, 10 different flow conditions were tested concerning the variation of these parameters. The streak spacing and the probability density function (PDF) of turbulent spot nucleation are computed for all cases. The streak spacing is shown to be constant in the transition region once scaled with the turbulent displacement and momentum thickness, with resulting values of around 3 and 5, respectively. Nucleation events are shown to occur near the position where the dimensionless streak spacing reaches such constant values. The streamwise position where most turbulent spots are formed is strongly influenced by the FST intensity level. Additionally, the PDF of spot nucleation becomes narrower with increase in the APG, while FST has the opposite effect. A common distribution of all the PDFs is provided as a function of a similarity variable accounting for the streak spacing, the shape factor of the boundary layer, and the FST intensity.

  • 340.
    Dellacasagrande, Matteo
    et al.
    Univ Genoa, DIME, Via Montallegro 1, I-16145 Genoa, Italy..
    Lengani, Davide
    Univ Genoa, DIME, Via Montallegro 1, I-16145 Genoa, Italy..
    Simoni, Daniele
    Univ Genoa, DIME, Via Montallegro 1, I-16145 Genoa, Italy..
    Pralits, Jan
    Univ Genoa, DICCA, Via Montallegro 1, I-16145 Genoa, Italy..
    Durovic, Kristina
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A procedure for computing the spot production rate in transitional boundary layers2022In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 63, no 8, article id 136Article in journal (Refereed)
    Abstract [en]

    The present work describes a method for the computation of the nucleation rate of turbulent spots in transitional boundary layers from particle image velocimetry (PIV) measurements. Different detection functions for turbulent events recognition were first tested and validated using data from direct numerical simulation, and this latter describes a flat-plate boundary layer under zero pressure gradient. The comparison with a previously defined function adopted in the literature, which is based on the local spanwise wall-shear stress, clearly highlights the possibility of accurately predicting the statistical evolution of transition even when the near-wall velocity field is not directly available from the measurements. The present procedure was systematically applied to PIV data collected in a wall-parallel measuring plane located inside a flat plate boundary layer evolving under variable Reynolds number, adverse pressure gradient (APG) and free-stream turbulence. The results presented in this work show that the present method allows capturing the statistical response of the transition process to the modification of the inlet flow conditions. The location of the maximum spot nucleation is shown to move upstream when increasing all the main flow parameters. Additionally, the transition region becomes shorter for higher Re and APG, whereas the turbulence level variation gives the opposite trend. The effects of the main flow parameters on the coefficients defining the analytic distribution of the nucleation rate and their link to the momentum thickness Reynolds number at the point of transition are discussed in the paper. [GRAPHICS] .

  • 341.
    Demou, Andreas D.
    et al.
    Cyprus Inst, Computat Based Sci & Technol Res Ctr, 20 Kavafi St, CY-2121 Nicosia, Cyprus..
    Niazi Ardekani, Mehdi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Mirbod, Parisa
    Univ Illinois, Dept Mech & Ind Engn, Chicago, IL 60607 USA..
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. Norwegian Univ Sci & Technol NTNU, Dept Energy & Proc Engn, NO-7491 Trondheim, Norway..
    Turbulent Rayleigh-Benard convection in non-colloidal suspensions2022In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 945, article id A6Article in journal (Refereed)
    Abstract [en]

    This study presents direct numerical simulations of turbulent Rayleigh-Benard convection in non-colloidal suspensions, with special focus on the heat transfer modifications in the flow. Adopting a Rayleigh number of 10(8) and Prandtl number of 7, parametric investigations of the particle volume fraction 0 <= Phi <= 40% and particle diameter 1/20 <= d(p)* <= 1/10 with respect to the cavity height, are carried out. The particles are neutrally buoyant, rigid spheres with physical properties that match the fluid phase. Up to Phi = 25 %, the Nusselt number increases weakly but steadily, mainly due to the increased thermal agitation that overcomes the decreased kinetic energy of the flow. Beyond Phi = 30 %, the Nusselt number exhibits a substantial drop, down to approximately 1/3 of the single-phase value. This decrease is attributed to the dense particle layering in the near-wall region, confirmed by the time-averaged local volume fraction. The dense particle layer reduces the convection in the near-wall region and negates the formation of any coherent structures within one particle diameter from the wall. Significant differences between Phi <= 30% and 40% are observed in all statistical quantities, including heat transfer and turbulent kinetic energy budgets, and two-point correlations. Special attention is also given to the role of particle rotation, which is shown to contribute to maintaining high heat transfer rates in moderate volume fractions. Furthermore, decreasing the particle size promotes the particle layering next to the wall, inducing a similar heat transfer reduction as in the highest particle volume fraction case.

  • 342.
    Deshpande, Rahul
    et al.
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia.
    Van Den Bogaard, Aron
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia; Physics of Fluids Group, University of Twente, P.O. Box 217, 7500AE Enschede, Netherlands.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Lindić, Luka
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia.
    Marusic, Ivan
    Department of Mechanical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia.
    Reynolds-number effects on the outer region of adverse-pressure-gradient turbulent boundary layers2023In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 8, no 12, article id 124604Article in journal (Refereed)
    Abstract [en]

    We study the Reynolds-number effects on the outer region of moderate adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) and find that their small-scale (viscous) energy reduces with increasing friction Reynolds number (Reτ). The trend is based on analyzing APG TBL data across 600≲Reτ≲7000 and contrasts with the negligible variation in small viscous-scaled energy noted for canonical wall flows. The data sets considered include those from a well-resolved numerical simulation [Pozuelo, J. Fluid Mech. 939, A34 (2022)0022-112010.1017/jfm.2022.221], which provides access to an APG TBL maintained at near-equilibrium conditions across 1000≲Reτ≲ 2000, with a well-defined flow history, and a new high-Reτ (∼7000) experimental study from the large Melbourne wind tunnel, with its long test section modified to permit development of an APG TBL from a "canonical"upstream condition. The decrease in small-scale energy with Reτ is revealed via decomposing the streamwise normal stresses into small- and large-scale contributions, based on a sharp spectral cutoff. The origin for this trend is traced back to the production of turbulent kinetic energy in an APG TBL, the small-scale contribution to which is also found to decrease with Reτ in the outer region. The conclusion is reaffirmed by investigating attenuation of streamwise normal stresses due to changing spatial resolutions of the numerical grid or hotwire sensors, which reduces with increasing Reτ and is found to be negligible at Reτ∼7000 in this study. The results emphasize that new scaling arguments and spatial-resolution corrections should be tested rigorously across a broad Reτ range, particularly for pressure gradient TBLs.

  • 343.
    Deusebio, Enrico
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical Investigation of Rotating and Stratified Turbulence2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Atmospheric and oceanic flows are strongly affected by rotation and stratification. Rotation is exerted through Coriolis forces which mainly act in horizontal planes whereas stratification largely affects the motion along the vertical direction through buoyancy forces, the latters related to the vertical variation of the fluid density. Aiming at a better understanding of atmospheric and oceanic processes, in this thesis the properties of turbulence in rotating and stably stratified flows are studied by means of numerical simulations, with and without the presence of solid walls.                                                                                                                                                                                           A new code is developed in order to carry out high-resolution numerical simulations of geostrophic turbulence forced at large scales. The code was heavily parallelized with MPI (Message Passing Interface) in order to be run on massively parallel computers. The main problem which has been investigated is how the turbulent cascade is affected by the presence of strong but finite rotation and stratification. As opposed to the early theories in the field of geostrophic turbulence, we show that there is a forward energy cascade which is initiated at large scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales where leads to a shallowing of the energy spectrum. Despite the idealized set-up of the simulations, two-point statistics show remarkable agreement with measurements in the atmosphere, suggesting that this process may be an important mechanism for energy transfer in the atmosphere.                                                                                                                                                                                                                                                                               The effect of stratification in wall-bounded turbulence is investigated by means of direct numerical simulations of open-channel flows. An existing full-channel code was modified in order to optimize the grid in the vertical direction and avoid the clustering of grid points at the upper boundary, where the solid wall is replaced by a free-shear condition. The stable stratification which results from a cooling applied at the solid wall largely affects the outer structures of the boundary layer, whereas the near-wall structures appear to be mostly unchanged. The effect of gravity waves is also studied, and a new decomposition is introduced in order to separate the gravity wave field from the turbulent field.

  • 344.
    Deusebio, Enrico
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical studies in rotating and stratified turbulence2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Although turbulence has been studied for more than five hundred years, a thorough understanding of turbulent flows is still missing. Nowadays computing power can offer an alternative tool, besides measurements and experiments, to give some insights into turbulent dynamics. In this thesis, numerical simulations are employed to study homogeneous and wall-bounded turbulence in rotating and stably stratified conditions, as encountered in geophysical flows where the rotation of the Earth as well as the vertical density variation influence the dynamics.

    In the context of homogeneous turbulence, we investigate how the transfer of energy among scales is affected by the presence of strong but finite rotation and stratification. Unlike geostrophic turbulence, we show that there is a forward energy cascade towards small scales which is initiated at the forcing scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales where it leads to a shallowing of the energy spectrum, from k-3 to k-5/3. Two-point statistics show a good agreement with measurements in the atmosphere, suggesting that this process is an important mechanism for energy transfer in the atmosphere.

    Boundary layers subjected to system rotation around the wall-normal axis are usually referred to as Ekman layers and they can be seen as a model of the atmospheric and oceanic boundary layers developing at mid and high latitudes. We study the turbulent dynamics in Ekman layers by means of numerical simulations, focusing on the turbulent structures developing at moderately high Reynolds numbers. For neutrally stratified conditions, we show that there exists a turbulent helicity cascade in the logarithmic region. We focus on the effect of a stable stratification produced by a vertical positive temperature gradient. For moderate stratification, continuously turbulent regimes are produced which are in fair agreement with existing theories and models used in the context of atmospheric boundary layer dynamics. For larger degree of stratification, we show that laminar and turbulent motions coexist and displace along inclined patterns similar to what has been recently observed in other transitional flows.

    Download full text (pdf)
    EnricoDeusebio2013
  • 345.
    Deusebio, Enrico
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    The open-channel version of SIMSON2010Report (Other academic)
  • 346.
    Deusebio, Enrico
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Augier, Pierre
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Third order structure function in rotating and stratified turbulence: analytical and numerical results compared with data from the stratosphereManuscript (preprint) (Other academic)
  • 347.
    Deusebio, Enrico
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Augier, Pierre
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Third-order structure functions in rotating and stratified turbulence: a comparison between numerical, analytical and observational results2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, p. 294-313Article in journal (Refereed)
    Abstract [en]

    First, we review analytical and observational studies on third-order structure functions including velocity and buoyancy increments in rotating and stratified turbulence and discuss how these functions can be used in order to estimate the flux of energy through different scales in a turbulent cascade. In particular, we suggest that the negative third-order velocity-temperature-temperature structure function that was measured by Lindborg & Cho (Phys. Rev. Lett., vol. 85, 2000, p. 5663) using stratospheric aircraft data may be used in order to estimate the downscale flux of available potential energy (APE) through the mesoscales. Then, we calculate third-order structure functions from idealized simulations of forced stratified and rotating turbulence and compare with mesoscale results from the lower stratosphere. In the range of scales with a downscale energy cascade of kinetic energy (KE) and APE we find that the third-order structure functions display a negative linear dependence on separation distance r, in agreement with observation and supporting the interpretation of the stratospheric data as evidence of a downscale energy cascade. The spectral flux of APE can be estimated from the relevant third-order structure function. However, while the sign of the spectral flux of KE is correctly predicted by using the longitudinal third-order structure functions, its magnitude is overestimated by a factor of two. We also evaluate the third-order velocity structure functions that are not parity invariant and therefore display a cyclonic-anticyclonic asymmetry. In agreement with the results from the stratosphere, we find that these functions have an approximate r(2)-dependence, with strong dominance of cyclonic motions.

  • 348.
    Deusebio, Enrico
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. DAMTP, England.
    Boffetta, Guido
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Musacchio, Stefano
    Dimensional transition in rotating turbulence2014In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 90, no 2Article in journal (Refereed)
    Abstract [en]

    In this work we investigate, by means of direct numerical hyperviscous simulations, how rotation affects the bidimensionalization of a turbulent flow. We study a thin layer of fluid, forced by a two-dimensional forcing, within the framework of the "split cascade" in which the injected energy flows both to small scales (generating the direct cascade) and to large scale (to form the inverse cascade). It is shown that rotation reinforces the inverse cascade at the expense of the direct one, thus promoting bidimensionalization of the flow. This is achieved by a suppression of the enstrophy production at large scales. Nonetheless, we find that, in the range of rotation rates investigated, increasing the vertical size of the computational domain causes a reduction of the flux of the inverse cascade. Our results suggest that, even in rotating flows, the inverse cascade may eventually disappear when the vertical scale is sufficiently large with respect to the forcing scale. We also study how the split cascade and confinement influence the breaking of symmetry induced by rotation.

  • 349.
    Deusebio, Enrico
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Centre for Mathematical Sciences, Cambridge, England.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A numerical study of the unstratified and stratified Ekman layer2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, p. 672-704Article in journal (Refereed)
    Abstract [en]

    We study the turbulent Ekman layer at moderately high Reynolds number, 1600 < Re = delta(E)G/v < 3000, using direct numerical simulations (DNS). Here, delta(E) = root 2v/f is the laminar Ekman layer thickness, G the geostrophic wind, v the kinematic viscosity and f is the Coriolis parameter. We present results for both neutrally, moderately and strongly stably stratified conditions. For unstratified cases, large-scale roll-like structures extending from the outer region down to the wall are observed. These structures have a clear dominant frequency and could be related to periodic oscillations or instabilities developing near the low-level jet. We discuss the effect of stratification and Re on one-point and two-point statistics. In the strongly stratified Ekman layer we observe stable co-existing large-scale laminar and turbulent patches appearing in the form of inclined bands, similar to other wall-bounded flows. For weaker stratification, continuously sustained turbulence strongly affected by buoyancy is produced. We discuss the scaling of turbulent length scales, height of the Ekman layer, friction velocity, veering angle at the wall and heat flux. The boundary-layer thickness, the friction velocity and the veering angle depend on Lf/u(tau), where u(tau) is the friction velocity and L the Obukhov length scale, whereas the heat fluxes appear to scale with L+ = Lu-tau/v.

  • 350.
    Deusebio, Enrico
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Centre for Mathematical Sciences, Cambridge, England.
    Lindborg, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Helicity in the Ekman boundary layer2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, p. 654-671Article in journal (Refereed)
    Abstract [en]

    Helicity, which is defined as the scalar product of velocity and vorticity, H = u . omega, is an inviscidly conserved quantity in a barotropic fluid. Mean helicity is zero in flows that are parity invariant. System rotation breaks parity invariance and has therefore the potential of giving rise to non-zero mean helicity. In this paper we study the helicity dynamics in the incompressible Ekman boundary layer. Evolution equations for the mean field helicity and the mean turbulent helicity are derived and it is shown that pressure flux injects helicity at a rate 2 Omega G(2) over the total depth of the Ekman layer, where G is the geostrophic wind far from the wall and Omega = Omega e(y) is the rotation vector and e(y) is the wall-normal unit vector. Thus right-handed/left-handed helicity will be injected if Omega is positive/negative. We also show that in the uppermost part of the boundary layer there is a net helicity injection with opposite sign as compared with the totally integrated injection. Isotropic relations for the helicity dissipation and the helicity spectrum are derived and it is shown that it is sufficient to measure two transverse velocity components and use Taylor's hypothesis in the mean flow direction in order to measure the isotropic helicity spectrum. We compare the theoretical predictions with a direct numerical simulation of an Ekman boundary layer and confirm that there is a preference for right-handed helicity in the lower part of the Ekman layer and left-handed helicity in the uppermost part when Omega > 0. In the logarithmic range, the helicity dissipation conforms to isotropic relations. On the other hand, spectra show significant departures from isotropic conditions, suggesting that the Reynolds number considered in the study is not sufficiently large for isotropy to be valid in a wide range of scales. Our analytical and numerical results strongly suggest that there is a turbulent helicity cascade of right-handed helicity in the logarithmic range of the atmospheric boundary layer when Omega > 0, consistent with recent measurements by Koprov, Koprov, Ponomarev & Chkhetiani (Dokl. Phys., vol. 50, 2005, pp. 419-422). The isotropic relations which are derived may facilitate future measurements of the helicity spectrum in the atmospheric boundary layer as well as in controlled wind tunnel experiments.

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