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  • 251.
    Carlson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bellani, Gabriele
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Measuring contact line dissipation in dynamic wetting2011Report (Other academic)
    Abstract [en]

    Dynamic wetting of a solid surface is a process that is ubiquitous in nature, and also of increasing technological importance. The underlying dissipative mechanisms are however still unclear. We present here dynamic wetting experiments of a droplet on a dry surface, showing that an important part of the dissipation may arise from a friction related to the motion of the contact line itself, and that this may be dominating the viscous friction in the flow adjacent to the contact line. By a combination of simulations and experiments, values of a corresponding friction factor are obtained. By this procedure the contact line friction factor can be distinguished and quantified, also in room temperature where other sources of dissipation are present. Water and glycerin-water mixtures on various surfaces have been investigated. We show the dependency of the friction factor on the nature of the surface, and the viscosity of the liquid.

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  • 252.
    Carlson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bellani, Gabriele
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Universality in dynamic wetting dominated by contact-line friction2012In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 85, no 4, p. 045302-Article in journal (Refereed)
    Abstract [en]

    We report experiments on the rapid contact-line motion present in the early stages of capillary-driven spreading of drops on dry solid substrates. The spreading data fail to follow a conventional viscous or inertial scaling. By integrating experiments and simulations, we quantify a contact-line friction mu(f) which is seen to limit the speed of the rapid dynamic wetting. A scaling based on this contact-line friction is shown to yield a universal curve for the evolution of the contact-line radius as a function of time, for a range of fluid viscosities, drop sizes, and surface wettabilities.

  • 253.
    Carlson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dissipation in rapid dynamic wetting2011In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 682, p. 213-240Article in journal (Refereed)
    Abstract [en]

    In this article, we present a modelling approach for rapid dynamic wetting based on the phase field theory. We show that in order to model this accurately, it is important to allow for a non-equilibrium wetting boundary condition. Using a condition of this type, we obtain a direct match with experimental results reported in the literature for rapid spreading of liquid droplets on dry surfaces. By extracting the dissipation of energy and the rate of change of kinetic energy in the flow simulation, we identify a new wetting regime during the rapid phase of spreading. This is characterized by the main dissipation to be due to a re-organization of molecules at the contact line, in a diffusive or active process. This regime serves as an addition to the other wetting regimes that have previously been reported in the literature.

  • 254.
    Carlson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Droplet dynamics in a bifurcating channel2010In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 36, no 5, p. 397-405Article in journal (Refereed)
    Abstract [en]

    In the present paper we present a phenomenological description of droplet dynamics in a bifurcating channel that is based on three-dimensional numerical experiments using the Phase Field theory. Droplet dynamics is investigated in a junction, which has symmetric outflow conditions in its daughter branches. We identify two different flow regimes as the droplets interact with the tip of the bifurcation, splitting and non-splitting. A distinct criterion for the flow regime transition is found based on the initial droplet volume and the Capillary (Ca) number. The Rayleigh Plateau instability is identified as a driving mechanism for the droplet breakup close to the threshold between the splitting and non-splitting regime.

  • 255.
    Carlson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Modeling of dynamic wetting far from equilibrium2009In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 12Article in journal (Refereed)
    Abstract [en]

    In this paper we present simulations of dynamic wetting far from equilibrium based on phase field theory. In direct simulations of recent experiments [J. C. Bird, S. Mandre, and H. A. Stone, Phys. Rev. Lett. 100, 234501 (2008)], we show that in order to correctly capture the dynamics of rapid wetting, it is crucial to account for nonequilibrium at the contact line, where the gas, liquid, and solid meet. A term in the boundary condition at the solid surface that naturally arises in the phase field theory is interpreted as allowing for the establishment of a local structure in the immediate vicinity of the contact line. A direct qualitative and quantitative match with experimental data of spontaneously wetting liquid droplets is shown.

  • 256.
    Carlson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kim, P.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stone, H. A.
    Short and long time drop dynamics on lubricated substrates2013In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 104, no 3, p. 34008-Article in journal (Refereed)
    Abstract [en]

    Liquid infiltrated solids have been proposed as functional solvent-phobic surfaces for handling single and multiphase flows. Implementation of such surfaces alters the interfacial transport phenomenon as compared to a dry substrate. To better understand the interface characteristics in such systems we study experimentally the dynamics of a pendant water drop in air that contacts a substrate coated by thin oil films. At short times the water drop is deformed by the oil that spreads onto the water-air interface, and the dynamics are characterized by inertial and viscous regimes. At late times, the the oil film under the drop relaxes either to a stable thin film or ruptures. In the thin film rupture regime, we measure the waiting time for the rupture as a function of the drop equilibrium contact angle on a dry substrate and the initial film height. The waiting time is rationalized by lubrication theory, which indicates that long-range intermolecular forces destabilize the oil-water interface and is the primary mechanism for the film drainage.

  • 257.
    Carlsson, Allan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Near wall fibre orientation in flowing suspensions2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis deals with fibre orientation in wall-bounded shear flows. The primary application in mind is papermaking. The study is mainly experimental,but is complemented with theoretical considerations.The main part of the thesis concerns the orientation of slowly settlingfibres in a wall-bounded viscous shear flow. This is a flow case not dealt withpreviously even at small Reynolds numbers. Experiments were conducted usingdilute suspensions with fibres having aspect ratios of rp ≈ 7 and 30. It is foundthat the wall effect on the orientation is small for distances from the wall wherethe fibre centre is located farther than half a fibre length from the wall. Farfrom the wall most fibres were oriented close to the flow direction. Closer tothe wall than half a fibre length the orientation distribution first shifted to bemore isotropic and in the very proximity of the wall the fibres were orientedclose to perpendicular to the flow direction, nearly aligned with the vorticityaxis. This was most evident for the shorter fibres with rp ≈ 7.Due to the density difference between the fibres and the fluid there is anincreased concentration near the wall. Still, a physical mechanism is requiredin order for a fibre initially oriented close to the flow direction at about half afibre length from the wall to change its orientation to aligned with the vorticityaxis once it has settled down to the wall. A slender body approach is usedin order to estimate the effect of wall reflection and repeated wall contacts onthe fibre rotation. It is found that the both a wall reflection, due to settlingtowards the wall, and contact between the fibre end and the wall are expectedto rotate the fibre closer to the vorticity axis. A qualitative agreement withthe experimental results is found in a numerical study based on the theoreticalestimation.In addition an experimental study on fibre orientation in the boundarylayers of a headbox is reported. The orientation distribution in planes parallelto the wall is studied. The distribution is found to be more anisotropic closerto the wall, i.e. the fibres tend to be oriented closer to the flow direction nearthe wall. This trend is observed sufficiently far upstream in the headbox.Farther downstream no significant change in the orientation distribution couldbe detected for different distances from the wall.

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  • 258.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Koch, Donald L.
    Orbit drift of a slowly settling fibre in a wall-bounded shear flowManuscript (Other academic)
  • 259.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fibre orientation control related to papermaking2007In: Journal of Fluids Engineering, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 129, no 4, p. 457-465Article in journal (Refereed)
    Abstract [en]

    The orientation of fibers suspended in a shear flow flowing over a solid wall has been studied experimentally. The possibility to control this orientation with physical surface modifications, ridges, has also been studied. The fiber suspension was driven by gravity down a slightly inclined glass plate and a CCD-camera was used to capture images of the fibers in the flow. Image analysis based on the concept of steerable filters extracted the position and orientation of the fibers in the plane of the image. From these data, the velocity of the fibers was determined. When viewing the flow from the side, the velocity of the fibers at different heights was measured and found to agree with the theoretical solution for Newtonian flow down an inclined plate. Moving the camera so that the flow was filmed from below, the orientation and velocity of fibers in the plane parallel to the solid surface was determined. The known relationship between the velocity and the wall normal position of the fibers made it possible to determine the height above the plate for each identified fiber. Far away from the wall, the fibers were aligned with the flow direction in both cases. In a region close to the smooth plate surface the fibers oriented themselves perpendicular to the flow direction. This change in orientation did not occur when the surface structure was modified with ridges.

  • 260.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Evaluation of a steerable filter for detection of fibres in flowing suspensionsManuscript (Other academic)
    Abstract [en]

    Steerable filters are concluded to be useful in order to determine the orientation of fibres captured in digital images. The fibre orientation is a key variable in the study of flowing fibre suspensions. Here digital image analysis based on a filter within the class of steerable filters is evaluated for suitability of finding the position and orientation of fibres suspended in flowing suspensions. In sharp images with small noise levels the steerable filter succeeds in determining the orientation of artificially generated fibres with well-defined angles. The influence of reduced image quality on the orientation has been quantified. The effect of unsharpness and noise is studied and the results show that the error in orientation is less than 1◦ for moderate levels. A set of images with fibres suspended in a shear flow is also analyzed. The fibre orientation distribution is determined in the flow-vorticity plane. In this analysis a comparison is also made to a robust, but computationally more expensive, method involving convolutions with an oriented elliptic filter. A good agreement is found when comparing the resulting fibre orientation distributions obtained with the two methods.

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  • 261.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Orientation of slowly sedimenting fibers in a flowing suspensionnear a plane wall2007In: Svenska Mekanikdagarna, 2007Conference paper (Refereed)
    Abstract [en]

    The effect of a wall on the orientation of slowly sedimenting fibers suspended in a shear flow has been studied experimentally. Experiments were performed at two concentrations with two aspect ratios, rp ≈ 7 and rp ≈ 30, where rp is defined as the fiber length divided by the diameter. For all cases the majority of the fibers were oriented close to parallel to the flow direction for distances farther away from the wall than half a fiber length. As the distance from the wall decreased a change in orientation was observed. At distances from the wall closer than about an eighth of a fiber length a significant amount of the fibers were oriented close to perpendicular to the flow. This was particularly clear for the shorter fibers. Due to the density difference between the fibers and the surrounding fluid the fiber concentration was increased in the near wall region. An increased concentration was found in a limited region close to half a fiber length from the wall. For the shorter fibers a large number of fibers was also detected in the very proximity of the wall.

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  • 262.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fibre Orientation Control Related To Papermaking2006In: PROCEEDINGS OF THE ASME FLUIDS ENGINEERING DIVISION SUMMER CONFERENCE, VOL 1, PTS A AND B, 2006, p. 1501-1509Conference paper (Refereed)
    Abstract [en]

    The wall effect on the orientation of fibres suspended in a shear flow has been studied experimentally. A fibre suspension, driven by gravity down an inclined glass plate, constitutes the shear flow field. A CCD-camera was mounted underneath the flow in order to visualize the flow. The orientation of fibres in the plane perpendicular to the plate was determined, by using the concept of steerable filters. In a region close to the smooth plate surface the fibres oriented themselves perpendicular to the flow direction. This did not occur when the surface structure was modified with ridges.

  • 263.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    Innventia AB, Box 5604, SE–114 86 Stockholm.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fibre orientation measurements near a headbox wall2010In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 25, no 2, p. 204-212Article in journal (Refereed)
    Abstract [en]

    Experimental results on the fibre orientation in a laboratory scale headbox are reported. Images containing fibres in approximately 1 mm thick slices parallel to the wall were captured at different wall distances. A steerable filter was used to determine the orientation of bleached and unbeaten birch fibres, suspended in water, at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only dilute suspensions were studied. It is shown that the fibre orientation distribution varies with the distance from the wall. Sufficiently far upstream in the headbox a more anisotropic distribution is found closer to the wall.

  • 264.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. STFI-Packforsk AB, Sweden.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fibre orientation in the boundary layers of a planar converging channel2008In: TAPPI Press - Paper Conference and Trade Show, PaperCon '08, 2008, p. 384-408Conference paper (Refereed)
    Abstract [en]

    Experimental results on the fibre orientation in a laboratory scale headbox are reported. A steerable filter was used to determine the orientation of bleached and unbeaten birch fibres at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only low concentrations were studied. It is shown that the orientation varies with the distance from the wall. For most studied cases a more anisotropic profile was found closer to the wall.

  • 265.
    Carlsson, Allan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. STFI-Packforsk AB, SE - 114 86 Stockholm, Sweden.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fibre orientation near a wall of a headbox.2010Conference paper (Refereed)
    Abstract [en]

    Experimental results on the fibre orientation in a laboratory scale headbox are reported. A steerable filter was used to determine the orientation of bleached unbeaten birch fibres at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only dilute suspensions were studied. It is shown that the fibre orientation distribution varies with the distance from the wall. Sufficiently far upstream in the headbox a more anisotropic distribution is found closer to the wall as compared to farther away from the wall.

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  • 266. Castellani, F.
    et al.
    Astolfi, D.
    Mana, M.
    Becchetti, M.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wake losses from averaged and time-resolved power measurements at full scale wind turbines2017In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 854, no 1, article id 012006Article in journal (Refereed)
    Abstract [en]

    This work deals with the experimental analysis of wake losses fluctuations at full-scale wind turbines. The test case is a wind farm sited on a moderately complex terrain: 4 turbines are installed, having 2 MW of rated power each. The sources of information are the time-resolved data, as collected from the OPC server, and the 10-minutes averaged SCADA data. The objective is to compare the statistical distributions of wake losses for far and middle wakes, as can be observed through the "fast" lens of time-resolved data, for certain selected test-case time series, and through the "slow" lens of SCADA data, on a much longer time basis that allow to set the standards of the mean wake losses along the wind farm. Further, time-resolved data are used for an insight into the spectral properties of wake fluctuations, highlighting the role of the wind turbine as low-pass filter. Summarizing, the wind rose, the layout of the site and the structure of the data sets at disposal allow to study middle and far wake behavior, with a "slow" and "fast" perspective.

  • 267.
    Castellani, Francesco
    et al.
    Univ Perugia, Dept Engn, Via G Durand 93, I-06125 Perugia, Italy..
    Eltayesh, Abdelgalil
    Benha Univ, Benha Fac Engn, Mech Engn Dept, Banha 13512, Egypt..
    Becchetti, Matteo
    Univ Perugia, Dept Engn, Via G Durand 93, I-06125 Perugia, Italy..
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Aerodynamic Analysis of a Wind-Turbine Rotor Affected by Pitch Unbalance2021In: Energies, E-ISSN 1996-1073, Vol. 14, no 3, article id 745Article in journal (Refereed)
    Abstract [en]

    The aerodynamics of a rotor with pitch imbalance has been investigated experimentally and numerically in the present work. The comparison of mean velocity and turbulence intensity in the balanced and unbalanced cases indicated that a pitch imbalance modifies both the mean velocity and the turbulent activity; the latter is weakly increased by the imbalance. Spectral analysis indicated that the dynamics of the wake is also affected by the pitch imbalance since the tip vortices loose strength and disorganise more quickly than in the balanced case. The pitch imbalance has, however, a detrimental effect on the power coefficient and it affects the thrust coefficient as well. Only the blade affected by the imbalance shows significant modifications of the applied load, while the other blades operate with the same loading conditions.

  • 268. Castro, Ian P.
    et al.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Outer-layer turbulence intensities in smooth- and rough-wall boundary layers2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 727, p. 119-131Article in journal (Refereed)
    Abstract [en]

    Clear differences in turbulence intensity profiles in smooth, transitional and fully rough zero-pressure-gradient boundary layers are demonstrated, using the diagnostic plot introduced by Alfredsson, Segalini & Orlu (Phys. Fluids, vol. 23, 2011, p. 041702) u'/U versus U/U-e, where u' and U are the local (root mean square) fluctuating and mean velocities and U-e is the free stream velocity. A wide range of published data are considered and all zero-pressure-gradient boundary layers yield outer flow u'/U values that are roughly linearly related to U/U-e, just as for smooth walls, but with a significantly higher slope which is completely independent of the roughness morphology. The difference in slope is due largely to the influence of the roughness parameter (Delta U+ in the usual notation) and all the data can be fitted empirically by using a modified form of the scaling, dependent only on Delta U/U-e. The turbulence intensity, at a location in the outer layer where U/U-e is fixed, rises monotonically with increasing Delta U/U-e which, however, remains of O(1) for all possible zero-pressure-gradient rough-wall boundary layers even at the highest Reynolds numbers. A measurement of intensity at a point in the outer region of the boundary layer can provide an indication of whether the surface is aerodynamically fully rough, without having to determine the surface stress or effective roughness height. Discussion of the implication for smooth/rough flow universality of differences in outer-layer mean velocity wake strength is included.

  • 269.
    Ceci, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Gojon, Romain
    ISAE-SUPAERO, Université de Toulouse, France.
    Mihaescu, Mihai
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Computational analysis of the indirect combustion noise generation mechanism in a nozzle guided vane in transonic operating conditions2021In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 496Article in journal (Refereed)
    Abstract [en]

    The combustion noise in modern engines is mainly originating from two types of mechanisms. First, chemical reactions in the combustion chamber leads to an unsteady heat release which is responsible of the direct combustion noise. Second, hot and cold blobs of air coming from the combustion chamber are advected and accelerated through turbine stages, giving rise to entropy noise (or indirect combustion noise). In the present work, numerical characterization of indirect combustion noise of a Nozzle Guide Vane passage was assessed using three-dimensional Large Eddy Simulations. The present work offers an overview to the analytical, computational and experimental studies of the topic. Numerical simulations are conducted to reproduce the effects of incoming planar entropy waves from the combustion chamber and to characterize the generated acoustic power. The dynamic features of the flow are addressed by the means of frequency domain and modal analyses techniques such as Fourier Decomposition and Proper Orthogonal Decomposition. Finally, the predicted entropy noise from numerical calculations is compared with the analytical results of an actuator disk model for a stator stage. The present paper proves that the generated indirect combustion noise can be significant for transonic operating conditions. The blade acoustic response is characterized by the excitation of a latent dynamics at the forcing frequency of the planar entropy waves, and it increases as the amplitude of the incoming disturbances increases.

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  • 270.
    Chan, C. , I
    et al.
    School of Mechanical Engineering, University of Adelaide, South Australia 5005, Australi.
    Schlatter, Philipp
    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.
    Chin, R. C.
    Univ Adelaide, Sch Mech Engn, Adelaide, SA 5005, Australia..
    Interscale transport mechanisms in turbulent boundary layers2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 921, article id A13Article in journal (Refereed)
    Abstract [en]

    The flow physics of turbulent boundary layers is investigated using spectral analysis based on the spanwise scale decomposition of the Reynolds stress transport equation, with data obtained from a direct numerical simulation of the turbulent boundary layer at . Here, we extend the framework of Kawata & Alfredsson (Phys. Rev. Lett., vol. 120, 2018, p. 244501) for plane Couette flows to zero-pressure-gradient boundary layers. The equation contains three fundamental fluxes, which govern the Reynolds stress transport: (i) a scale flux of the interaction between small-scale and large-scale structures, and two spatial fluxes dominated by (ii) pressure and (iii) turbulent transport along the wall-normal direction. The scale flux reveals evidence of the inverse turbulent kinetic energy transfer, from small to large scales, occurring at the near-wall region, whereby for the scale flux of the Reynolds shear stress transport, the inverse transfer extends across the entire boundary layer. The wall-normal fluxes reveal the interactions occurring between scales at the buffer and logarithmic regions. In addition, there is interaction between the large-scale structures and the free stream flow occurring at the edge of the boundary layer, which was not observed in the Couette flow. Flow structures associated with inverse interscale transport of Reynolds shear stress are identified by applying conditional analysis to the spectrally decomposed velocity fields. While the inverse transport is interpreted as the net energy transfer from small-scale ejections (Q2) and sweeps (Q4) to the large-scale counterparts, conditional time estimates of the direct and inverse interscale transport reveal that both processes play a substantial role across a broad range of scales.

  • 271. Chan, C. , I
    et al.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Chin, R. C.
    Univ Adelaide, Sch Mech Engn, Adelaide, SA 5005, Australia..
    Large-scale and small-scale contribution to the skin friction reduction in a modified turbulent boundary layer by a large-eddy break-up device2022In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 7, no 3, article id 034601Article in journal (Refereed)
    Abstract [en]

    The role of streamwise length scales (lambda x) in turbulent skin friction generation is investigated using a direct numerical simulation data set of an incompressible zero pressure gradient turbulent boundary layer and the spectral analysis based on the FukagataL73 (2002)]. The total skin friction generation associated with motions scaled with local boundary layer thickness delta of lambda x 3 delta and lambda x 3 delta) contribute to a significant portion of turbulent skin friction. However, it is found that the large-scale ejection and sweep events with streamwise length scales at lambda x 3 delta are equally important. The turbulent skin friction reduction associated with the modification of largeand small-scale quadrant events is studied, using well-resolved simulation data sets of a large-eddy break-up (LEBU) device in a turbulent boundary layer. The results reveal that LEBUs modify both the large- and small-scale ejection and sweep events, yielding a substantial turbulent skin friction reduction.

  • 272.
    Chandramouli, Sathyanarayanan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gojon, Romain
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fridh, Jens
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical characterization of entropy noise with a density based solver2017In: 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, European Turbomachinery Society , 2017Conference paper (Refereed)
    Abstract [en]

    In this work, dbnsTurbFoam, a new coupled density based solver, written in the framework of FOAM-EXTEND, is considered. The solver is first assessed on two canonical compressible flow scenarios, namely the Sod's shock tube and the ONERA S8 transonic channel. Results are compared with analytical formulations and experiments, respectively. 2-D Unsteady Reynolds Averaged Navier-Stokes simulations and 3-D Large Eddy Simulations of the flow within the passages of a geometrically simplified High Pressure Turbine Nozzle Guide Vane are then performed. Results are compared against experimental data in order to justify the geometrical simplifications made. Finally, the case of a sinusoidal temperature forcing at the inlet is considered in order to study the phenomenon of indirect combustion noise. Notably, the impact of the forcing on the vortex shedding dynamics and on the losses is discussed.

  • 273.
    Chaparian, Emad
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Niazi Ardekani, Mehdi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Particle migration in channel flow of an elastoviscoplastic fluid2020In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 284, article id 104376Article in journal (Refereed)
    Abstract [en]

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

  • 274.
    Chaparian, Emad
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    An adaptive finite element method for elastoviscoplastic fluid flows2019In: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 271, p. 104148-Article in journal (Refereed)
    Abstract [en]

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

  • 275.
    Chaparian, Emad
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Sliding flows of yield-stress fluids2021In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 911, article id A17Article in journal (Refereed)
    Abstract [en]

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

  • 276.
    Chaparian, Emad
    et al.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Tammisola, Outi
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability of particles inside yield-stress fluid Poiseuille flows2020In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 885, article id A45Article in journal (Refereed)
    Abstract [en]

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

  • 277.
    Chauvat, Guillaume
    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.
    Paredes, Pedro
    Leading-edge acoustic receptivity investigation through global modes analysis2020Report (Other academic)
    Abstract [en]

    The acoustic receptivity of the boundary-layer flow on a flat plate with an elliptic leading edge is determined using a global eigenmode analysis. Proper boundary conditions yielding physically relevant eigenmodes are fundamental to this approach. A branch of modes consisting of the incompressible limit of a plane acoustic wave interacting with the leading edge to form TS waves in the boundary-layer of the flat plate is found. The damping of this branch is sensitive to numerical details, and the modes can be interpreted as pseudoeigenvalues rather than pure eigenvalues of the system. The receptivity coefficient extracted from the spatial structure of eigenvectors has been corrected for the imaginary part of the eigenvalues in order to determine the response to a time-periodic perturbation. This correction is based on solving the parabolised stability equations (PSE) between the leading edge, the lower branch of neutral stabilityof the TS mode, and the outflow. The results are found to be in good agreement with recent direct numerical simulations (Shahriari et al. 2016). We have also obtained results for lower frequencies than those reported there.

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  • 278.
    Chauvat, Guillaume
    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.
    Tocci, Francesco
    Rius-Vidales, Alberto
    Kotsonis, Marios
    Hein, Stefan
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Direct numerical simulations of the effects of a forward-facing step on the instability of crossflow vortices2020Report (Refereed)
    Abstract [en]

    We reproduce three variants of an experimental setup consisting of a swept wing in a wind tunnel through direct numerical simulation (DNS). The effect of a forward-facing step of two different heights at location $x/c_x = 0.2$ on a steady crossflow instability is analysed. The effect of discrete roughness elements used to condition the instability modes in the experiments is accurately reproduced via perturbations computed through nonlinear parabolised stability equations. Our results demonstrate that the setup can be reproduced accurately numerically as far as the steady flows are concerned, with a good agreement between the experimental and numerical profiles upstream and downstream of the step. Our results shed light on the flow structure in the vicinity of the step.

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  • 279.
    Chen, Song
    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.
    Gojon, Romain
    ISAE-Supaero.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effect of an Adjacent Flat Plate on a Highly-Heated Rectangular Supersonic Jet2020In: AIAA Scitech 2020 Forum, AIAA, 2020, article id AIAA 2020-0746Conference paper (Refereed)
    Abstract [en]

    Solid surfaces located in the vicinity of a supersonic jet may affect its flow dynamics and greatly change the aeroacoustic characteristics. Large-eddy simulations (LES) are performed to investigate the plate effects on a highly-heated rectangular supersonic jet. The rectangular nozzle has an aspect ratio of 2.0 and is operated at the over-expanded condition with a nozzle pressure ratio of 3.0 and a nozzle temperature ratio of 7.0. Four cases, JetL0 to JetL3 with a plate-to-nozzle distance ranging from 0 to 3 times of the jet equivalent nozzle diameter are investigated. The large-scale implicit LES computations are performed by a well-validated in-house finite-volume based CFD code, which uses an artificial dissipation mechanism to represent the effect of small-scale turbulence and to damp the numerical oscillation near shocks. The temperature-dependent thermal properties of air in the highly-heated jets are considered by the chemical equilibrium assumption. Numerical results show that among the four cases, JetL0 with the plate directly attached at the nozzle lip shows significant different flow and acoustic fields from the others. It exhibits a longer jet potential core length but without forming a series of well-structured shock diamonds. The other cases show similar shock/expansion wave structures as observed in the free jet but their jet plumes bend towards the plate. This bending of jet leads to JetL1 scrubs over the plate in the downstream. The scrubbing effect, together with the unaffected shock-shear layer interactions and high plate pressure loading, makes JetL1 have a stronger OASPL in the near acoustic fields than the other cases. The spectrum analysis in the nozzle upstream direction shows that the plate removes or mitigates the screech tone observed in the free jet and slightly amplifies the acoustic amplitudes in the low-frequency range.

  • 280.
    Chen, Song
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
    Gojon, Romain
    ISAE-SUPAERO.
    Mihaescu, Mihai
    KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Flow and aeroacoustic attributes of highly-heated transitional rectangular supersonic jets2021In: Aerospace Science and Technology, ISSN 1270-9638, E-ISSN 1626-3219, Vol. 114, no 106747Article in journal (Refereed)
    Abstract [en]

    Heated transitional supersonic jets exhausting from a rectangular nozzle at over-expanded conditions are investigated by Large Eddy Simulations and Ffowcs-Williams and Hawkings acoustic analogy. Four cases with a fixed nozzle pressure ratio but different temperature ratios (TR) of 1.0, 2.0, 4.0, and 7.0 are analyzed. Numerical results show that with the increasing temperature the jet velocity significantly increases, whereas its Reynolds number decreases by about one order of magnitude, which leads to a 30% decrease in the jet potential core length and reduction in the number of shock cells. The increasing temperatures also result in supersonic shear layer convection Mach numbers and consequently Mach wave radiations in the acoustic fields. Pressure skewness and kurtosis factors indicate crackle noise and non-linear propagation effects in high temperatures. For the most heated jet TR 7.0, the Mach wave radiation is identified radiating noise at about 120 degrees, while the large turbulence structure noise at about 150 degrees. Furthermore, the vortex sheet model analysis and the LES data detect the existence of upstream-propagating neutral waves inside jet TR 7.0. The observed screech frequency falls within the range of antisymmetric mode indicating that the highly-heated jet is characterized by an antisymmetric oscillation mode at the screech frequency.

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  • 281.
    Chen, Song
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Nozzle Pressure Ratio Effects on Aerodynamics and Acoustics of a Highly-Heated Rectangular Supersonic Jet2019In: 25th AIAA/CEAS Aeroacoustics Conference, 2019, p. 16-, article id AIAA 2019-2753Conference paper (Refereed)
    Abstract [en]

    Implicit large-eddy simulations (LES) are performed in this work to study the flow-field and acoustic characteristics of a highly-heated rectangular supersonic jet. The focus is on the nozzle pressure ratio(NPR) effects. Three NPRs are investigated including 3.0, 3.67, and 4.0, which correspond to the nozzle over-expansion, perfect-expansion, and under-expansion conditions respectively. The current hot jet has a nozzle temperature ratio (NTR) of 7.0, corresponding to a total temperature of around 2100K. The rectangular nozzle has an aspect ratio of 2.0 and has been extensively tested at the Universityof Cincinnati. An in-house CFD code with an artificial dissipation mechanism is used to perform the large-scale implicit LES computations. By studying the pressure contours, density gradients and dilatation, it is found that the three-dimensional jet shock/expansion wave structure changes signifi-cantly when the jet NPR increases from an over-expanded to under-expanded condition. The length of the laminar shear layer right outside the nozzle is extended to the downstream before transitioning to be turbulent. The distance between the nozzle lip and the first shock cell is doubled while the total number of shock cells keeps the same, which results in a longer jet potential core. The increase of nozzle NPR also provides about an 11% increase in jet velocity and a 25% increase in shear layer convection Mach number, which leads to a stronger Mach wave radiation noise component in the acoustic fields. Pressure spectra in the near field reveal that screech only exists in the over-expansion case and the broadband shock-associated noise is enhanced in the perfect and under expansion cases.The far-field acoustics at 40Deq is characterized by about 4 dB increase of the overall sound pressure level in the Mach wave radiation direction and about 2 – 3 dB increase in all other directions. The far-field pressure spectra also confirm that the screech noise component vanishes when the nozzle NPR is increased to perfect- and under-expanded conditions.

  • 282.
    Chevalier, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI).
    Hein, Stefan
    DLR.
    Sousa, Joao M.M.
    Technical University of Lisbon.
    Additional computationsfor the RWG results Final analysis2008Report (Other academic)
  • 283. Chiara, Luigi Filippo
    et al.
    Rosti, Marco Edoardo
    Picano, Francesco
    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.
    Suspensions of deformable particles in Poiseuille flows at finite inertia2020In: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 52, no 6, article id 065507Article in journal (Refereed)
    Abstract [en]

    We analyze a suspension of deformable particles in a pressure-driven flow. The suspension is composed of neutrally buoyant initially spherical particles and a Newtonian carrier fluid, and the flow is solved by means of direct numerical simulations, using a fully Eulerian method based on a one-continuum formulation. The solid phase is modeled with an incompressible viscous hyperelastic constitutive relation, and the flow is characterized by three main dimensionless parameters, namely the solid volume fraction, the Reynolds and capillary numbers. The dependency of the effective viscosity on these three quantities is investigated to study the inertial effects on a suspension of deformable particles. It can be observed that the suspension has a shear-thinning behavior, and the reduction in effective viscosity for high shear rates is emphasized in denser configurations. The separate analysis of the Reynolds and capillary numbers reveal that the effective viscosity depends more on the capillary than on the Reynolds number. In addition, our simulations exhibit a consistent tendency for deformable particles to move toward the center of the channel, where the shear rate is low. This phenomenon is particularly marked for very dilute suspensions, where a whole region near the wall is empty of particles. Furthermore, when the volume fraction is increased this near-wall region is gradually occupied, because of higher mutual particle interactions. Deformability also plays an important role in the process. Indeed, at high capillary numbers, particles are more sensitive to shear rate variations and can modify their shape more easily to accommodate a greater number of particles in the central region of the channel. Finally, the total stress budgets show that the relative particle-induced stress contribution increases with the volume fraction and Reynolds number, and decreases with the particle deformability.

  • 284.
    Chicchiero, Claudio
    et al.
    Univ Pisa, Dipartimento Ingn Aerosp, I-56122 Pisa, Italy..
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Camarri, Simone
    Univ Pisa, Dipartimento Ingn Aerosp, I-56122 Pisa, Italy..
    Triple-deck analysis of the steady flow over a rotating disk with surface roughness2021In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, no 1, article id 014103Article in journal (Refereed)
    Abstract [en]

    The effect of surface roughness on the steady laminar flow induced by a rotating disk submerged by fluid otherwise at rest is investigated here theoretically and numerically. A theory is proposed where a triple-deck analysis is applied leading to a fast evaluation of the steady-flow modification due to the rough surface. The theory assumes that the roughness is much smaller than the boundary-layer height and is characterized by a significantly longer length scale (slender roughness). Only the leading-order correction is developed here, corresponding to a velocity-field correction that is linear with the roughness height. The proposed theory neglects some curvature terms (here partially accounted by means of a stretching of the radial coordinate and of a scaling of the dependent variables). Numerical simulations performed with different roughness geometries (axisymmetric roughness, radial grooves, and localized bumps) have been used to validate the theory. Results indicate that the proposed theory leads to a good quantification of the flow modifications due to surface roughness at a very low computational cost. A demonstration of the capabilities of the theory is finally proposed where the statistical effects on the flow due to a random (but statistically known) roughness distributed on the surface of a rotating disk are characterized.

  • 285. Chin, C
    et al.
    Monty, J
    Hutchins, N
    Ooi, A
    Örlü, Ramis
    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.
    Simulation of a large-eddy-break-up device (LEBU) in a moderate Reynolds number turbulent boundary layer2015In: Proc 9th Turbulence and Shear Flow Phenomena Conference, 2015Conference paper (Refereed)
  • 286. Chin, C.
    et al.
    Vinuesa, Ricardo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Cardesa, J. I.
    Noorani, Azad
    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.
    Chong, M. S.
    Flow topology of rare back flow events and critical points in turbulent channels and toroidal pipes2018In: Journal of Physics: Conference Series, Institute of Physics Publishing (IOPP), 2018, Vol. 1001, no 1, article id 012002Conference paper (Refereed)
    Abstract [en]

    A study of the back flow events and critical points in the flow through a toroidal pipe at friction Reynolds number Reτ ≈ 650 is performed and compared with the results in a turbulent channel flow at Reτ ≈ 934. The statistics and topological properties of the back flow events are analysed and discussed. Conditionally-averaged flow fields in the vicinity of the back flow event are obtained, and the results for the torus show a similar streamwise wall-shear stress topology which varies considerably for the spanwise wall-shear stress when compared to the channel flow. The comparison between the toroidal pipe and channel flows also shows fewer back flow events and critical points in the torus. This cannot be solely attributed to differences in Reynolds number, but is a clear effect of the secondary flow present in the toroidal pipe. A possible mechanism is the effect of the secondary flow present in the torus, which convects momentum from the inner to the outer bend through the core of the pipe, and back from the outer to the inner bend through the pipe walls. In the region around the critical points, the skin-friction streamlines and vorticity lines exhibit similar flow characteristics with a node and saddle pair for both flows. These results indicate that back flow events and critical points are genuine features of wall-bounded turbulence, and are not artifacts of specific boundary or inflow conditions in simulations and/or measurement uncertainties in experiments.

  • 287. Chin, Cheng
    et al.
    Örlü, Ramis
    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.
    Monty, Jason
    Hutchins, Nicholas
    Influence of a Large-Eddy-Breakup-Device on the Turbulent Interface of Boundary Layers2017In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 99, no 3-4, p. 823-835Article in journal (Refereed)
    Abstract [en]

    The effects of implementing a large-eddy break-up device (LEBU) in a turbulent boundary layer on the interaction with the boundary layer is investigated with particular emphasis on the turbulent/non-turbulent interface (TNTI). The simulation data is taken from a recent well-resolved large eddy simulation (Chin et al. Flow Turb. Combust. 98, 445-460 2017), where the LEBU was implemented at a wall-normal distance of 0.8 delta (local boundary layer thickness) from the wall. A comparison of the TNTI statistics is performed between a zero-pressure-gradient boundary layer with and without the LEBU. The LEBU is found to delay the growth of the turbulent boundary layer and also attenuates the fluctuations of the TNTI. The LEBU appears to alter the structure size at the interface, resulting in a narrower and shorter dominant structure (in an average sense). Further analysis beneath the TNTI using two-point correlations shows that the LEBU affects the turbulent structures in excess of 100 delta downstream of the LEBU.

  • 288.
    Chomaz, Jean Marc
    et al.
    LadHyX, CNRS, Ecole Polytechnique, Palaiseau, France.
    Augier, Pierre
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics. LadHyX, CNRS, Ecole Polytechnique, Palaiseau, France.
    Billant, Paul
    LadHyX, CNRS, Ecole Polytechnique, Palaiseau, France.
    Spectral analysis of the transition to turbulence from a dipole in stratified fluid2013In: ETC 2013 - 14th European Turbulence Conference, Zakon Group LLC , 2013Conference paper (Refereed)
    Abstract [en]

    We investigate through numerical simulations the spectral properties of the turbulence generated during the nonlinear evolution of a Lamb-Chaplygin dipole in a stratified fluid for a high Reynolds number Re = 28000 and a wide range of horizontal Froude number Fh ∈ [0.0225 0.135] and buoyancy Reynolds number R = ReFh2 ∈ [14 510]. A spectral analysis shows that this transition is dominated by two kinds of transfers: first, the shear instability induces a direct non-local transfer toward horizontal wavelengths of the order of the buoyancy scale Lb = U/N, where U is the characteristic horizontal velocity of the dipole and N the Brunt-Väisälä frequency; second, the destabilization of the Kelvin-Helmholtz billows and the gravitational instability lead to small-scale weakly stratified turbulence. We show that the anisotropic spectra at the maximum of dissipation share many characteristics with those obtained from numerical simulations of forced stratified turbulence and from measurements in the atmosphere and in the ocean. The article presenting this study [2] is the subject of a Focus on Fluids article [10].

  • 289.
    Cifuentes, Luis
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Univ Duisburg Essen, Chair Fluid Dynam, Inst Combust & Gasdynam IVG, D-47057 Duisburg, Germany..
    Fooladgar, Ehsan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Duwig, Christophe
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Chemical Explosive Mode Analysis for a Jet-in-Hot-Coflow burner operating in MILD combustion2018In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 232, p. 712-723Article in journal (Refereed)
    Abstract [en]

    Large Eddy Simulations (LES) of Moderate and Intense Low oxygen Dilution (MILD) combustion of a Jet-in-HotCoflow (JHC) burner were performed using detailed chemistry. On the contrary to traditional flames, where heat release is occurring in very thin fronts, MILD combustion occurs in the distributed reaction regime where the reaction zone is broad, thus, this paper applies a direct Arrhenius closure with detailed chemistry to resolve important details of the fuel oxidation reactions. Comparisons of LES results are in good agreement with experiments, demonstrating that the simulations capture the intermediate species and finite reaction rate effects. A Chemical Explosive Mode Analysis (CEMA) was used to determine the flame structure and to detect the pre-and post-ignition regions, including the contributions to the CEMs analyzing the Explosion Index (EI) and Participation Index (PI). To the best of our knowledge, a detailed study of CEMA on MILD or flameless regime has never been reported. The flame structure was clearly visualized with CEMA, as well as the lean and the rich flame fronts. Different flame zones close to the anchoring points of these turbulent lifted flames were selected and the analysis demonstrates the contributions of dominant chemical species, such as HO2 and O. The reactions related to the dominant local CEM were obtained to highlight the nature of the stabilization in these highly diluted operating conditions.

  • 290. Cimarelli, A.
    et al.
    De Angelis, E.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Talamelli, A.
    Casciola, C. M.
    Sources and fluxes of scale energy in the overlap layer of wall turbulence2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 771, p. 407-423Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of turbulent channel flows at friction Reynolds numbers (Re) of 550, 1000 and 1500 are used to analyse the turbulent production, transfer and dissipation mechanisms in the compound space of scales and wall distances by means of the Kolmogorov equation generalized to inhomogeneous anisotropic flows. Two distinct peaks of scale-energy source are identified. The first, stronger one, belongs to the near-wall cycle. Its location in the space of scales and physical space is found to scale in viscous units, while its intensity grows slowly with Re, indicating a near-wall modulation. The second source peak is found further away from the wall in the putative overlap layer, and it is separated from the near-wall source by a layer of significant scale-energy sink. The dynamics of the second outer source appears to be strongly dependent on the Reynolds number. The detailed scale-by-scale analysis of this source highlights well-defined features that are used to make the properties of the outer turbulent source independent of Reynolds number and wall distance by rescaling the problem. Overall, the present results suggest a strong connection of the observed outer scale-energy source with the presence of an outer region of turbulence production whose mechanisms are well separated from the near-wall region and whose statistical features agree with the hypothesis of an overlap layer dominated by attached eddies. Inner-outer interactions between the near-wall and outer source region in terms of scale-energy fluxes are also analysed. It is conjectured that the near-wall modulation of the statistics at increasing Reynolds number can be related to a confinement of the near-wall turbulence production due to the presence of increasingly large production scales in the outer scale-energy source region.

  • 291.
    Cimarelli, Andrea
    et al.
    Dipartimento di Ingegneria Industriale, Università di Bologna, Forlì, 47121, Italy.
    de Angelis, Elisabetta
    Dipartimento di Ingegneria Industriale, Università di Bologna, Forlì, 47121, Italy.
    Schlatter, Philipp
    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.
    Brethouwer, Gert
    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.
    Talamelli, Alessandro
    Dipartimento di Ingegneria Industriale, Università di Bologna, Forlì, 47121, Italy.
    Casciola, Carlo Massimo
    Dipartimento di Ingegneria Meccanica e Aerospaziale, Università di Roma La Sapienza, 00185, Italy.
    Scalings of the outer energy source of wall-turbulence2020In: ETC 2013 - 14th European Turbulence Conference, Zakon Group LLC , 2020Conference paper (Refereed)
    Abstract [en]

    By means of the multidimensional description given by the Kolmogorov equation we study the energy transfer physics and the production mechanisms of wall-turbulent flows at moderately high Reynolds numbers. Two driving mechanisms are identified for the energy fluxes. The first stronger one, here called driving scale-range (DSR), belongs to the near-wall cycle. As expected, its topology remains unaltered with Reynolds number while its intensity is found to slightly increase with Re. The second mechanism, here called outer scale-range (OSR), takes place in the overlap layer and highlights different features in agreement with the attached eddies hypothesis usually considered to describe the overlap dynamics.

  • 292. Citro, Vincenzo
    et al.
    Giannetti, Flavio
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Luchini, Paolo
    Linear three-dimensional global and asymptotic stability analysis of incompressible open cavity flow2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 768Article in journal (Refereed)
    Abstract [en]

    The viscous and inviscid linear stability of the incompressible flow past a square open cavity is studied numerically. The analysis shows that the flow first undergoes a steady three-dimensional bifurcation at a critical Reynolds number of 1370. The critical mode is localized inside the cavity and has a flat roll structure with a spanwise wavelength of about 0.47 cavity depths. The adjoint global mode reveals that the instability is most efficiently triggered in the thin region close to the upstream tip of the cavity. The structural sensitivity analysis identifies the wavemaker as the region located inside the cavity and spatially concentrated around a closed orbit. As the flow outside the cavity plays no role in the generation mechanisms leading to the bifurcation, we confirm that an appropriate parameter to describe the critical conditions in open cavity flows is the Reynolds number based on the average velocity between the two upper edges. Stabilization is achieved by a decrease of the total momentum inside the shear layer that drives the core vortex within the cavity. The mechanism of instability is then studied by means of a short-wavelength approximation considering pressureless inviscid modes. The closed streamline related to the maximum inviscid growth rate is found to be the same as that around which the global wavemaker is concentrated. The structural sensitivity field based on direct and adjoint eigenmodes, computed at a Reynolds number far higher than that of the base flow, can predict the critical orbit on which the main instabilities inside the cavity arise. Further, we show that the sub-leading unstable time-dependent modes emerging at supercritical conditions are characterized by a period that is a multiple of the revolution time of Lagrangian particles along the orbit of maximum growth rate. The eigenfrequencies of these modes, computed by global stability analysis, are in very good agreement with the asymptotic results.

  • 293.
    Coelho Leite Fava, Thales
    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.
    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.
    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.
    DIRECT NUMERICAL SIMULATIONS OF A THICK, ROTATING AIRFOIL AT A LOW REYNOLDS NUMBER2022In: 33rd Congress of the International Council of the Aeronautical Sciences, ICAS 2022, International Council of the Aeronautical Sciences , 2022, p. 3059-3072Conference paper (Refereed)
    Abstract [en]

    Direct numerical simulations of the flow around an FFA-W3 series airfoil at a chord Reynolds number of 100,000 are performed to study the effects of rotation on flow over a section of a rotating wing. In order to achieve this goal, three simulations with different rotation speeds (and corresponding angles of attack) are carried out. Three additional simulations with the same angles of attack of the former but without including Coriolis and centrifugal forces are also computed. It is shown that rotation moves the transition location upstream on the suction side for low angles of attack, and on the pressure side, due to the enhancement of the shear-layer instability and its spanwise modulation. Nevertheless, rotation delays transition on the suction side for larger angles of attack. The reason for this change is most likely the fact that the shear-layer instability is much stronger in this case, and it is not bypassed by instabilities generated by rotation such as that from the inflectional spanwise velocity profiles. However, the latter can reduce the shear in the separation bubble, mitigating the rapid growth of the former. The onset of separation is not changed by rotation, but the trailing edge of the separation bubble is displaced downstream because of an enhanced reverse flow. The lift is only significantly affected by rotation when there are large separation regions on both suction and pressure surfaces, promoting its reduction.

  • 294.
    Coelho Leite Fava, Thales
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lobo, B A
    Kiel University of Applied Sciences, Mechanical Engineering Dept..
    Nogueira, P A S
    Monash University, Aerospace Engineering.
    Schaffarczyk, A P
    Kiel University of Applied Sciences, Mechanical Engineering Dept..
    Breuer, M
    Helmut-Schmidt-Universitat Hamburg.
    Henningson, Dan S.
    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, Fluid Physics.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Influence of free-stream turbulence on the boundary layer stability of a wind turbine airfoil and near wake2023In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 2505, no 1, p. 012002-012002Article in journal (Refereed)
    Abstract [en]

    Free-stream turbulence (FST) alters the boundary layer of wind turbine blades, changing the hydrodynamic stability and near wake. Large-eddy simulations (LES) of a blade section with a laminar separation bubble for several turbulence intensities (TI) and a Reynolds number of 100,000 are performed. The effects of boundary-layer streaks generated by FST on Tollmien-Schlichting (TS) and Kelvin-Helmholtz (KH) instabilities are analyzed with a model based on the parabolized stability equations (PSE). Two competing effects on flow stability are identified. The spanwise-averaged mean-flow distortion stabilizes primary TS/KH modes for increasing TI. However, this contribution seems dominant only for TI ≥ 8.6%. For lower TI, the spanwise-oscillating distortion caused by streaks destabilizes the flow, and the growth rates of secondary modal instabilities increase with the streak amplitude. The destabilization occurs mainly at spanwise locations with negative streaks since the inflection point shifts away from the wall, enhancing inviscid instabilities. Inflection points in the spanwise direction formed by the streaks also contribute to the destabilization. The modal structures from PSE and LES agree. Finally, the trailing-edge near-wake coherent structures are more energetic for TI ≥ 8.6% due to the partial stabilization of modal instabilities, delaying the turbulent breakdown.

  • 295.
    Coelho Leite Fava, Thales
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lobo, B. A.
    Mechanical Engineering Department, Kiel University of Applied Sciences 2 , D-24149 Kiel, Germany.
    Nogueira, P. A. S.
    Aerospace Engineering, Monash University 3 , 3800 Clayton, Australia.
    Schaffarczyk, A. P.
    Mechanical Engineering Department, Kiel University of Applied Sciences 2 , D-24149 Kiel, Germany.
    Breuer, M.
    Professur für Strömungsmechanik, Helmut-Schmidt-Universität Hamburg 4 , D-22043 Hamburg, Germany.
    Henningson, Dan S.
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
    Hanifi, Ardeshir
    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.
    Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence2023In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 35, no 8Article in journal (Refereed)
    Abstract [en]

    The interaction of several instabilities and the influence of free-stream turbulence on laminar-turbulent transition on a 20% thick wind-turbine blade section with a laminar separation bubble (LSB) are investigated with wall-resolved large-eddy simulations (LES). Turbulence intensities (TI) of 0%, 2.2%, 4.5%, 8.6%, and 15.6% at chord Reynolds number 100,000 are considered. Linear receptivity occurs for the most energetic disturbances; high-frequency perturbations are excited via non-linear mechanisms for  TI≥8.6%⁠. Unstable Tollmien–Schlichting (TS) waves appear in the inflectional flow region for  TI≤4.5%⁠, shifting to inviscid Kelvin–Helmholtz (KH) modes upon separation and forming spanwise rolls. Sub-harmonic secondary instability occurs for  TI=0%⁠, with rolls intertwining before transition. Streaks spanwise modulate the rolls and increase their growth rates with TI for  TI≤4.5%⁠, reducing separation and shifting transition upstream. The  TI=4.5% case presents the highest perturbations, leading to the smallest LSB and most upstream transition. Earlier inception of TS/KH modes occurs on low-speed streaks, inducing premature transition. However, for  TI=8.6%⁠, the effect of the streaks is to stabilize the attached mean flow and front part of the LSB. This occurs due to the near-wall momentum deficit alleviation, leading to the transition delay and larger LSB than  TI=4.5%⁠. This also suppresses separation and completely stabilizes TS/KH modes for  TI=15.6%⁠. Linear stability theory predicts well the modal evolution for  TI≤8.6%⁠. Optimal perturbation analysis accurately computes the streak development upstream of the inflectional flow region but indicates higher amplification than LES downstream due to the capture of low-frequency, oblique modal instabilities from the LSB. Only low-amplitude [ O(1%)] streaks displayed exponential growth in the LES since non-linearity precludes the appearance of these modes.

  • 296.
    Coelho Leite Fava, Thales
    et al.
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Lokatt, Mikaela
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Sørensen, N
    Department of Wind Energy, Technical University of Denmark, Risø Campus, Roskilde, Denmark .
    Zahle, F
    Department of Wind Energy, Technical University of Denmark, Risø Campus, Roskilde, Denmark .
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. 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), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    A simplified model for transition prediction applicable to wind-turbine rotors2021In: Wind Energy Science, ISSN 2366-7443, E-ISSN 2366-7451, Vol. 6, no 3, p. 715-736Article in journal (Refereed)
    Abstract [en]

    This work aims to develop a simple framework for transition prediction over wind-turbine blades, including effects of the blade rotation and spanwise velocity without requiring fully three-dimensional simulations. The framework is based on a set of boundary-layer equations (BLEs) and parabolized stability equations (PSEs), including rotation effects. An important element of the developed BL method is the modeling of the spanwise velocity at the boundary-layer edge. The two analyzed wind-turbine geometries correspond to a constant airfoil and the DTU 10-MW Reference Wind Turbine blades. The BL model allows an accurate prediction of thechordwise velocity profiles. Further, for regions not too close to the stagnation point and root of the blade, profiles of the spanwise velocity agree with those from Reynolds-averaged Navier–Stokes (RANS) simulations.The model also allows predicting inflectional velocity profiles for lower radial positions, which may allow cross-flow transition. Transition prediction is performed at several radial positions through an “envelope-of-envelopes” methodology. The results are compared with the eN method of Drela and Giles, implemented in the EllipSys3D RANS code. The RANS transition locations closely agree with those from the PSE analysis of a 2D mean flow without rotation. These results also agree with those from the developed model for cases with low 3D and rota-tion effects, such as at higher radial positions and geometries with strong adverse pressure gradients where 2D Tollmien–Schlichting (TS) waves are dominant. However, the RANS and PSE 2D models predict a later transition in the regions where 3D and rotation effects are non-negligible. The developed method, which accounts for these effects, predicted earlier transition onsets in this region (e.g., 19 % earlier than RANS at 26 % of theradius for the constant-airfoil geometry) and shows that transition may occur via highly oblique modes. These modes differ from 2D TS waves and appear in locations with inflectional spanwise velocity. However, except close to the root of the blade, crossflow transition is unlikely since the crossflow velocity is too low. At higher radial positions, where 3D and rotation effects are weaker and the adverse pressure gradient is more significant, modes with small wave angles (close to 2D) are found to be dominant. Finally, it is observed that an increase in the rotation speed modifies the spanwise velocity and increases the Coriolis and centrifugal forces, shifting the transition location closer to the leading edge. This work highlights the importance of considering the blade rotation and the three-dimensional flow generated by that in transition prediction, especially in the inner part of the blade.

  • 297. Coppola, Gennaro
    et al.
    Semeraro, Onofrio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Interfacial instability of two rotating viscous immiscible fluids in a cylinder2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 6, p. 064105-Article in journal (Refereed)
    Abstract [en]

    A complete original study of the linear temporal instability analysis of two viscous and immiscible fluids enclosed in a rigid cylinder rotating about its axis and separated by a cylindrical interface is performed for the case of higher density fluid located in the annulus. The results of the present contribution fill the lack of an overall assessment of the system behavior due to the increase of both the analytical difficulties and the number of the governing parameters when the several physical effects are all included. The analysis is carried out numerically by discretizing the equations of the evolution of disturbances separately in the two phases formulated in a rotating reference frame. Normal mode analysis leads to a generalized eigenvalue problem which is solved by means of a Chebyshev collocation spectral method. The investigation of the preferred modes of instability is carried out over wide ranges of the parameters space. The behavior of the system is physically discussed and is compared to inviscid asymptotic limits and to viscous approximate solutions of the previous literature.

  • 298.
    Cosin, Renato
    et al.
    Embraer, Sao Jose Dos Campos, Brazil..
    Mendonca, Marcio T.
    Inst Aeronaut & Espaco, Sao Jose Dos Campos, Brazil..
    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.
    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.
    Becker, Gilberto G.
    Embraer, Sao Jose Dos Campos, Brazil..
    Deep Learning Based Transition Prediction for Aeronautical Applications2022In: Iutam Laminar-Turbulent Transition / [ed] Sherwin, S Schmid, P Wu, X, Springer Nature , 2022, Vol. 38, p. 725-735Conference paper (Refereed)
    Abstract [en]

    Drag reduction is vital for any new airplane design as the demand for a greener aviation is increasing. Wings and nacelles with laminar flow can reduce the total drag significantly and the backbone for designing laminar surfaces is the transition prediction. The highly complex physics of boundary-layer transition has led to the development of a wide range of prediction methods. Considering their good compromise between accuracy and computational cost, the standard prediction methods used by industry are based on the Linear Stability Theory (LST) and on the Parabolized Stability Equations (PSE). Although these methods have been successfully applied, it can be difficult to make them fully automated due to lack of robustness. Besides, tuning the model setups can be time-consuming. The inclusion of such methods in the aerodynamic design significantly increases the computational cost, especially in optimization loops. The present work proposes a solution for these disadvantages of LST and PSE using a metamodel based on deep learning. Complex Neural networks created using artificial intelligence concepts allows classification and regression of the large datasets necessary for transition analysis. The metamodel reproduces a local stability code and the results are promising both in terms of accuracy and processing speed.

  • 299. Cossu, C.
    et al.
    Chevalier, M.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Secondary optimal growth and subcritical transition in the plane Poiseuille flow2010In: 7th IUTAM Symposium on Laminar-Turbulent Transition, Springer Netherlands, 2010, p. 129-134Conference paper (Refereed)
    Abstract [en]

    Nonlinear optimal perturbations leading to subcritical transition with minimum threshold energy are searched in the plane Poiseuille flow at Re = 1500. To this end we proceed in two steps. First a family of optimally growing primary streaks U issued by the optimal vortices of the Poiseuille laminar solution is computed by direct numerical simulation for a set of finite amplitudes AI of the primary vortices. An adjoint technique is then used to compute the maximum growth and the finite time Lyapunov exponents of secondary perturbations growing on top of these primary base flows. The secondary optimals take into full account the non-normality and the local instabilities of the tangent operator all along the temporal evolution of the primary flows. The most amplified optimal perturbations are sinuous and realized in correspondence of streaks that are locally unstable. The combinations of primary and secondary perturbations optimal for transition are then explored using direct numerical simulations. It is shown that the minimum initial energy is realized by a large set of these combinations, revealing new paths to transition. Surprisingly we find that transition can be efficiently obtained even using secondary perturbations alone, in the absence of primary optimal vortices.

  • 300. Cossu, Carlo
    et al.
    Brandt, Luca
    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, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Secondary threshold amplitudes for sinuous streak breakdown2011In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 7, p. 074103-Article in journal (Refereed)
    Abstract [en]

    The nonlinear stability of laminar sinuously bent streaks is studied for the plane Couette flow at Re = 500 in a nearly minimal box and for the Blasius boundary layer at Re(delta)(*)=700. The initial perturbations are nonlinearly saturated streamwise streaks of amplitude A(U) perturbed with sinuous perturbations of amplitude A(W). The local boundary of the basin of attraction of the linearly stable laminar flow is computed by bisection and projected in the A(U) - A(W) plane providing a well defined critical curve. Different streak transition scenarios are seen to correspond to different regions of the critical curve. The modal instability of the streaks is responsible for transition for A(U) = 25%-27% for the considered flows, where sinuous perturbations of amplitude below A(W) approximate to 1%-2% are sufficient to counteract the streak viscous dissipation and induce breakdown. The critical amplitude of the sinuous perturbations increases when the streamwise streak amplitude is decreased. With secondary perturbations amplitude A(W) approximate to 4%, breakdown is induced on stable streamwise streaks with A(U) approximate to 13%, following the secondary transient growth scenario first examined by Schoppa and Hussain [J. Fluid Mech. 453, 57 (2002)]. A cross-over, where the critical amplitude of the sinuous perturbation becomes larger than the amplitude of streamwise streaks, is observed for streaks of small amplitude A(U) < 5%-6%. In this case, the transition is induced by an initial transient amplification of streamwise vortices, forced by the decaying sinuous mode. This is followed by the growth of the streaks and final breakdown. The shape of the critical A(U) - A(W) curve is very similar for Couette and boundary layer flows and seems to be relatively insensitive to the nature of the edge states on the basin boundary. The shape of this critical curve indicates that the stability of streamwise streaks should always be assessed in terms of both the streak amplitude and the amplitude of spanwise velocity perturbations.

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