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  • 201. Dankowicz, Harry
    et al.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Untitled2014Inngår i: Applied Mechanics Review, ISSN 0003-6900, E-ISSN 1088-8535, Vol. 66, nr 2, s. 020201-Artikkel i tidsskrift (Fagfellevurdert)
  • 202. De Aguiar Quintanilha Junior, H. R.
    et al.
    Kataras, P. B.
    Theofilis, V.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Marcus Wallenberg Laboratoriet.
    Nonmodal stability analysis of the HIFiRE-5 elliptic cone model flow in different flight altitudes2018Inngår i: 58th Israel Annual Conference on Aerospace Sciences, IACAS 2018, Israel Annual Conference on Aerospace Sciences , 2018, s. 1543-1555Konferansepaper (Fagfellevurdert)
    Abstract [en]

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

  • 203. de Lange, H. C.
    et al.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Drag reduction on external surfaces induced by wall waves2011Inngår i: ERCOFTAC Series, Springer Netherlands, 2011, Vol. 15, s. 437-442Konferansepaper (Fagfellevurdert)
  • 204. de Luzan, Charles Farbos
    et al.
    Chen, Jie
    Mihaescu, Mihai
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Khosla, Sid M.
    Gutmark, Ephraim
    Computational study of false vocal folds effects on unsteady airflows through static models of the human larynx2015Inngår i: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 48, nr 7, s. 1248-1257Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 205.
    de Stadler, Matthew B.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Rapaka, Narsimha R.
    Sarkar, Sutanu
    Large eddy simulation of the near to intermediate wake of a heated sphere at Re=10,0002014Inngår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 49, s. 2-10Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 206.
    De Vita, Francesco
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Rosti, Marco E.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Izbassarov, Daulet
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Duffo, L.
    Tammisola, Outi
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Hormozi, S.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Elastoviscoplastic flows in porous media2018Inngår i: Journal of Non-Newtonian Fluid Mechanics, ISSN 0377-0257, E-ISSN 1873-2631, Vol. 258, s. 10-21Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 207.
    Deusebio, Enrico
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Numerical Investigation of Rotating and Stratified Turbulence2012Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

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

  • 208.
    Deusebio, Enrico
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Numerical studies in rotating and stratified turbulence2013Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

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

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

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

  • 209.
    Deusebio, Enrico
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    The open-channel version of SIMSON2010Rapport (Annet vitenskapelig)
  • 210.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Augier, Pierre
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Third order structure function in rotating and stratified turbulence: analytical and numerical results compared with data from the stratosphereManuskript (preprint) (Annet vitenskapelig)
  • 211.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Augier, Pierre
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Third-order structure functions in rotating and stratified turbulence: a comparison between numerical, analytical and observational results2014Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, s. 294-313Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 212.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. DAMTP, England.
    Boffetta, Guido
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Musacchio, Stefano
    Dimensional transition in rotating turbulence2014Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 90, nr 2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 213.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Centre for Mathematical Sciences, Cambridge, England.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A numerical study of the unstratified and stratified Ekman layer2014Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, s. 672-704Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 214.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Centre for Mathematical Sciences, Cambridge, England.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Helicity in the Ekman boundary layer2014Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, s. 654-671Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 215.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Direct numerical simulations of stratified open channel flows2011Inngår i: 13th European Turbulence Conference (ETC13): Wall-Bounded Flows And Control Of Turbulence, 2011, s. 022009-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We carry out numerical simulations of wall-bounded stably stratified flows. We mainly focus on how stratification affects the near-wall turbulence at moderate Reynolds numbers, i.e. Re-tau = 360. A set of fully-resolved open channel flow simulations is performed, where a stable stratification has been introduced through a negative heat flux at the lower wall. In agreement with previous studies, it is found that turbulence cannot be sustained for h/L values higher than 1.2, where L is the so-called Monin-Obukhov length and h is the height of the open channel. For smaller values, buoyancy does not re-laminarize the flow, but nevertheless affects the wall turbulence. Near-wall streaks are weakly affected by stratification, whereas the outer modes are increasingly damped as we move away from the wall. A decomposition of the wall-normal velocity is proposed in order to separate the gravity wave and turbulent flow fields. This method has been tested both for open channel and full channel flows. Gravity waves are likely to develop and to dominate close to the upper boundary (centerline for full channel). However, their intensity is weaker in the open channel, possibly due to the upper boundary condition. Moreover, the presence of internal gravity waves can also be deduced from a correlation analysis, which reveals (together with spanwise spectra) a narrowing of the outer structures as the stratification is increased.

  • 216.
    Deusebio, Enrico
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Vallgren, Andreas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    The route to dissipation in strongly stratified and rotating flows2013Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 720, s. 66-103Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate the route to dissipation in strongly stratified and rotating systems through high-resolution numerical simulations of the Boussinesq equations (BQs) and the primitive equations (PEs) in a triply periodic domain forced at large scales. By applying geostrophic scaling to the BQs and using the same horizontal length scale in defining the Rossby and the Froude numbers, R0 and Fr, we show that the PEs can be obtained from the BQs by taking the limit Fr-2/R0(2)-> 0. When Fr-2/R0(2) is small the difference between the results from the BQ and the PE simulations is shown to be small. For large rotation rates, quasi-geostrophic dynamics are recovered with a forward enstrophy cascade and an inverse energy cascade. As the rotation rate is reduced, a fraction of the energy starts to cascade towards smaller scales, leading to a shallowing of the horizontal spectra from k(h)(-3) to k(h)(-5/3) h at the small-scale end. The vertical spectra show a similar transition as the horizontal spectra and we find that Charney isotropy is approximately valid also at larger wavenumbers than the transition wavenumber. The high resolutions employed allow us to capture both ranges within the same simulation. At the transition scale, kinetic energy in the rotational and in the horizontally divergent modes attain comparable values. The divergent energy is several orders of magnitude larger than the quasi-geostrophic divergent energy given by the Omega-equation. The amount of energy cascading downscale is mainly controlled by the rotation rate, with a weaker dependence on the stratification. A larger degree of stratification favours a downscale energy cascade. For intermediate degrees of rotation and stratification, a constant energy flux and a constant enstrophy flux coexist within the same range of scales. In this range, the enstrophy flux is a result of triad interactions involving three geostrophic modes, while the energy flux is a result of triad interactions involving at least one ageostrophic mode, with a dominant contribution from interactions involving two ageostrophic and one geostrophic mode. Dividing the ageostrophic motions into two classes depending on the sign of the linear wave frequency, we show that the energy transfer is for the largest part supported by interactions within the same class, ruling out the wave-wave-vortex resonant triad interaction as a mean of the downscale energy transfer. The role of inertia-gravity waves is studied through analyses of time-frequency spectra of single Fourier modes. At large scales, distinct peaks at frequencies predicted for linear waves are observed, whereas at small scales no clear wave activity is observed. Triad interactions show a behaviour which is consistent with turbulent dynamics, with a large exchange of energy in triads with one small and two large comparable wavenumbers. The exchange of energy is mainly between the modes with two comparable wavenumbers.

  • 217. Discetti, S.
    et al.
    Bellani, G.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Serpieri, J.
    Sanmiguel Vila, C.
    Raiola, M.
    Zheng, X.
    Mascotelli, L.
    Talamelli, Alessandro
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Ianiro, A.
    Characterization of very-large-scale motions in high-Re pipe flows2019Inngår i: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, s. 1-8Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Very-large-scale structures in pipe flows are characterized using an extended Proper Orthogonal Decomposition (POD)-based estimation. Synchronized non-time-resolved Particle Image Velocimetry (PIV) and time-resolved, multi-point hot-wire measurements are integrated for the estimation of turbulent structures in a pipe flow at friction Reynolds numbers of 9500 and 20000. This technique enhances the temporal resolution of PIV, thus providing a time-resolved description of the dynamics of the large-scale motions. The experiments are carried out in the CICLoPE facility. A novel criterion for the statistical characterization of the large-scale motions is introduced, based on the time-resolved dynamically-estimated POD time coefficients. It is shown that high-momentum events are less persistent than low-momentum events, and tend to occur closer to the wall. These differences are further enhanced with increasing Reynolds number.

  • 218.
    Dogan, Eda
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hearst, R. Jason
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England.;Norwegian Univ Sci & Technol, Dept Energy & Proc Engn, NO-7491 Trondheim, Norway..
    Hanson, Ronald E.
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England.;York Univ, Dept Mech Engn, Toronto, ON M3J 1P3, Canada..
    Ganapathisubramani, Bharathram
    Univ Southampton, Engn & Environm, Southampton SO17 1BJ, Hants, England..
    Spatial characteristics of a zero-pressure-gradient turbulent boundary layer in the presence of free-stream turbulence2019Inngår i: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, nr 8, artikkel-id 084601Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Particle image velocimetry (PIV) measurements are performed to examine the structural organization inside a turbulent boundary layer under the influence of free-stream turbulence (FST). In particular, streamwise-wall-normal plane PIV measurements are presented for two cases at two different turbulent intensity levels (about 13% and 8%). The free-stream turbulence is generated using an active grid in a wind tunnel. The statistical information of the flow regarding the wall-normal velocity and Reynolds shear stress are presented. The effect of increasing the turbulence level in the free stream for these flows has been found to have similarities with increasing Reynolds number for high-Reynolds-number canonical flows. Quadrant analysis is performed to determine the contributions of different Reynolds-stress-producing events. In this regard, the distribution of momentum transport events shows some similarity with channel flows, which can be justified by comparison of similar intermittency characteristics of both flows. In addition, the coherent structures found inside the boundary layer have inclined features that are consistent with the previous studies for canonical flows. The fact that the external disturbance, such as FST in this study, does not alter the organization of the structures inside the boundary layer supports the growing evidence for a universal structure for wall-bounded flows.

  • 219.
    Dogan, Eda
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Gatti, Davide
    Karlsruhe Inst Technol, Inst Fluid Mech, Kaiserstr 10, D-76131 Karlsruhe, Germany..
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Quantification of amplitude modulation in wall-bounded turbulence2019Inngår i: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 51, nr 1, artikkel-id 011408Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Many recent investigations on the scale interactions in wall-bounded turbulent flows focus on describing so-called amplitude modulation, the phenomenon that deals with the influence of large scales in the outer region on the amplitude of the small-scale fluctuations in the near-wall region. The present study revisits this phenomenon regarding two aspects, namely the method for decomposing the scales and the quantification of the modulation. First, the paper presents a summary of the literature that has dealt with either or both aspects. Second, for decomposing the scales, different spectral filters (temporal, spatial or both) and empirical mode decomposition (EMD) are evaluated and compared. The common data set is a well-resolved large-eddy simulation that offers a wide range of Reynolds numbers spanning Re-theta = 880-8200. The quantification of the amplitude modulation is discussed for the resulting scale components. Particular focus is given to evaluate the efficacy of the various filters to separate scales for the range of Reynolds numbers of interest. Different to previous studies, the different methods have been evaluated using the same data set, thereby allowing a fair comparison between the various approaches. It is observed that using a spectral filter in the spanwise direction is an effective approach to separate the small and large scales in the flow, even at comparably low Reynolds numbers, whereas filtering in time should be approached with caution in the low-to-moderate Re range. Additionally, using filters in both spanwise and time directions, which would separate both wide and long-living structures from the small and fast scales, gives a cleaner image for the small-scales although the contribution to the scales interaction from that filter implementation has been found negligible. Applying EMD to decompose the scales gives similar results to Fourier filters for the energy content of the scales and thereby for the quantification of the amplitude modulation using the decomposed scales. No direct advantage of EMD over classical Fourier filters could be seen. Potential issues regarding different decomposition methods and different definitions of the amplitude modulation are also discussed.

  • 220.
    Donelli, Raffaele
    et al.
    CIRA.
    de Rosa, D.
    Hein, Stefan
    DLR.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mughal, Shahid
    Imperial College.
    Perraud, Jean
    ONERA.
    Schrauf, Geza
    Airbus.
    STABILITY ANALYSIS OF THE PATHFINDER WINDTUNNEL MODEL FOR THE CALIBRATION OF TRANSITION PREDICTION IN ETW2009Rapport (Annet vitenskapelig)
  • 221.
    Do-Quang, Minh
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.
    Modelling the influence of wetting properties on the solid liquid impact2008Inngår i: PROCEEDINGS OF THE 6TH INTERNATIONAL CONFERENCE ON NANOCHANNELS, MICROCHANNELS, AND MINICHANNELS, PTS A AND B, NEW YORK: AMER SOC MECHANICAL ENGINEERS , 2008, s. 1915-1917Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The impact of a solid object on a free liquid surface is quite complex. This problem has challenged researchers for centuries and remains of interest today. Recently Duez et al. [1] published experimental results on the splash when a solid sphere enters a liquid Surprisingly, a small change in the surface chemistry of the object can turn a big splash into an inconspicuous disappearance and vice versa. We study this problem by solving the Navier-Stokes together with the Cahn-Hilliard equations, [2, 3], which allows us to simulate the motion of a free air-water surface in detail, in the presence of surface tension and dynamic wetting. Quantitative computational modeling of dynamic wetting is difficult in itself, but here the use of this tool allows us to study in detail how the wetting properties determine whether a splash appears or not. Our simulated results are compared with the experiments of Duez et al.

  • 222.
    Do-Quang, Minh
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    The splash of a solid sphere impacting on a liquid surface: Numerical simulation of the influence of wetting2009Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, nr 2Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The impact of a solid sphere on a liquid surface has challenged researchers for centuries and remains of interest today. Recently, Duez [Nat. Phys. 3, 180 (2007)] published experimental results of the splash generated when a solid sphere enters water. Interestingly, the microscopic properties of the solid surface control the nature of the macroscopic behavior of the splash. So by a change in the surface chemistry of the solid sphere, a big splash can be turned into an inconspicuous disappearance and vice versa. This problem was investigated by numerical simulations based on the Navier-Stokes equations coupled with the Cahn-Hilliard equations. This system allows us to simulate the motion of an air-water interface as a solid sphere impacts the liquid pond. The inclusion of the surface energies of the solid surface in the formulation gives a reasonably quantitative description of the dynamic wetting. Numerical results with different wetting properties and impact speed are presented and directly compared with the recent experimental results from Duez.

  • 223.
    Do-Quang, Minh
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Simulation of finite-size fibers in turbulent channel flows2014Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 89, nr 1, s. 013006-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The dynamical behavior of almost neutrally buoyant finite-size rigid fibers or rods in turbulent channel flow is studied by direct numerical simulations. The time evolution of the fiber orientation and translational and rotational motions in a statistically steady channel flow is obtained for three different fiber lengths. The turbulent flow is modeled by an entropy lattice Boltzmann method, and the interaction between fibers and carrier fluid is modeled through an external boundary force method. Direct contact and lubrication force models for fiber-fiber interactions and fiber-wall interaction are taken into account to allow for a full four-way interaction. The density ratio is chosen to mimic cellulose fibers in water. It is shown that the finite size leads to fiber-turbulence interactions that are significantly different from earlier reported results for point like particles (e.g., elongated ellipsoids smaller than the Kolmogorov scale). An effect that becomes increasingly accentuated with fiber length is an accumulation in high-speed regions near the wall, resulting in a mean fiber velocity that is higher than the mean fluid velocity. The simulation results indicate that the finite-size fibers tend to stay in the high-speed streaks due to collisions with the wall. In the central region of the channel, long fibers tend to align in the spanwise direction. Closer to the wall the long fibers instead tend to toward to a rotation in the shear plane, while very close to the wall they become predominantly aligned in the streamwise direction.

  • 224.
    Do-Quang, Minh
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Villanueva, Walter
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Singer-Loginova, Irina
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Parallel adaptive computation of some time-dependent materials-related microstructural problems2007Inngår i: Bulletin of the Polish Academy of Sciences: Technical Sciences, ISSN 0239-7528, Vol. 55, nr 2, s. 229-237Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Some materials-related microstructural problems calculated using the phase-field method are presented. It is well known that the phase field method requires mesh resolution of a diffuse interface. This makes the use of mesh adaptivity essential especially for fast evolving interfaces and other transient problems. Complex problems in 3D are also computationally challenging so that parallel computations are considered necessary. In this paper, a parallel adaptive finite element scheme is proposed. The scheme keeps the level of node and edge for 2D and level of node and face for 3D instead of the complete history of refinements to facilitate derefinement. The information is local and exchange of information is minimized and also less memory is used. The parallel adaptive algorithms that run on distributed memory machines are implemented in the numerical simulation of dendritic growth and capillary-driven flows.

  • 225.
    Downs, Robert S. , I I I
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fallenius, Bengt E. G.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Mårtensson, H.
    Evaluation of miniature vortex generators for flow control in Falkner-Skan boundary layers2016Inngår i: 54th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics, 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress growth of unsteady disturbances which would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works have demonstrated the versatility of these miniature vortex generators (MVGs) in zero pressure gradient boundary layers. In this work, mean-flow disturbances developing from MVGs in adverse and favorable pressure gradient boundary layers are measured systemically to assess the possibility of extending MVG-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values and an empirical scaling is found in congruence with existing results. The effect of streaks on transition in an adverse pressure gradient boundary layer is also tested and moderate increases to laminar flow extents are observed.

  • 226.
    Downs, Robert S., III
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Fransson, Jens H. M.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Tollmien-Schlichting wave growth over spanwise-periodic surface patterns2014Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 754, s. 39-74Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A novel type of surface roughness is deployed in a zero-pressure-gradient boundary layer with the goal of delaying the onset of laminar-to-turbulent transition for drag reduction purposes. This proof-of-concept experiment relies on forcing phase-triggered Tollmien-Schlichting (TS) waves across a range of initial amplitudes to produce amplified boundary-layer disturbances in a controlled and repeatable manner. Building on earlier work demonstrating attenuation of forced disturbances and delay of transition with spanwise arrays of discrete roughness and miniature vortex generators (MVGs), the present work seeks a roughness shape which might find success in a wider range of flows. Toward that end, streamwise-elongated humps are regularly spaced in the spanwise direction to form a wavy wall. By direct modulation of the mean flow, growth rates of the forced disturbances are increased or decreased, depending on the roughness configuration. Boundary-layer velocity measurements with hot-wire probes have been performed in a parametric study of the effects of roughness-field geometry and forcing amplitude on TS-wave growth and transition. The roughness field proves detrimental to passive flow control efforts in some configurations, while a reduction in the TS-wave amplitudes compared with the smooth-wall reference case is observed at other conditions. Substantial delays in the onset of transition are demonstrated when TS waves are forced with large amplitudes.

  • 227.
    Du, Lin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx). Beijing University of Aeronautics and Astronautics, China.
    Holmberg, Andreas
    Karlsson, Mikael
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Åbom, Mats
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Sound amplification at a rectangular T-junction with merging mean flows2016Inngår i: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 367, s. 69-83Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper reports a numerical study on the aeroacoustic response of a rectangular T-junction with merging mean flows. The primary motivation of the work is to explain the high sound amplification, recently seen experimentally, when introducing a small merging bias flow. The acoustic results are found solving the compressible Linearized Navier-Stokes Equations (LNSEs) in the frequency domain, where the base flow is first obtained using RANS with a k-epsilon turbulence model. The model predicts the measured scattering data well, including the amplitude and Strouhal number for the peak amplification, if the effect of eddy viscosity damping is included. It is found that the base flow changes significantly with the presence of a small bias flow. Compared to pure grazing flow a strong unstable shear layer is created in the downstream main duct starting from the T-junction trailing edge. This means that the main region of vortex-sound interaction is moved away from the junction to a downstream region much larger than the junction width. To analyze the sound amplification in this region Howe's energy corollary and the growth of acoustic density are used.

  • 228. Duguet, Y.
    et al.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Oblique laminar-turbulent interfaces in plane shear flows2013Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 3, s. 034502-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Localized structures such as turbulent stripes and turbulent spots are typical features of transitional wall-bounded flows in the subcritical regime. Based on an assumption for scale separation between large and small scales, we show analytically that the corresponding laminar-turbulent interfaces are always oblique with respect to the mean direction of the flow. In the case of plane Couette flow, the mismatch between the streamwise flow rates near the boundaries of the turbulence patch generates a large-scale flow with a nonzero spanwise component. Advection of the small-scale turbulent fluctuations (streaks) by the corresponding large-scale flow distorts the shape of the turbulence patch and is responsible for its oblique growth. This mechanism can be easily extended to other subcritical flows such as plane Poiseuille flow or Taylor-Couette flow.

  • 229.
    Duguet, Yohann
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Larsson, B. Robin J.
    Towards minimal perturbations in transitional plane Couette flow2010Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 82, nr 2, s. 026316-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For parallel shear flows, transition to turbulence occurs only for perturbations of sufficiently large amplitude. It is therefore relevant to study the shape, amplitude, and dynamics of the least energetic initial disturbances leading to transition. We suggest a numerical approach to find such minimal perturbations, applied here to the case of plane Couette flow. The optimization method seeks such perturbations at initial time as a linear combination of a finite number of linear optimal modes. The energy threshold of the minimal perturbation for a Reynolds number Re=400 is only 2% less than for a pair of symmetric oblique waves. The associated transition scenario shows a long transient approach to a steady state solution with special symmetries. Modal analysis shows how the oblique-wave mechanism can be optimized by the addition of other oblique modes breaking the flow symmetry and whose nonlinear interaction generates spectral components of the edge state. The Re dependence of energy thresholds is revisited, with evidence for a O(Re(-2))-scaling for both oblique waves and streamwise vortices scenarios.

  • 230. Duguet, Yohann
    et al.
    Le Maitre, Olivier
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Stochastic and deterministic motion of a laminar-turbulent front in a spanwisely extended Couette flow2011Inngår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 84, nr 6, s. 066315-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate numerically the dynamics of a laminar-turbulent interface in a spanwisely extended and streamwisely minimal plane Couette flow. The chosen geometry allows one to suppress the large-scale secondary flow and to focus on the nucleation of streaks near the interface. It is shown that the resulting spanwise motion of the interface is essentially stochastic and can be modeled as a continuous-time random walk. This model corresponds here to a Gaussian diffusion process. The average speed of the interface and the corresponding diffusion coefficient are determined as functions of the Reynolds number Re, as well as the threshold value above which turbulence contaminates the whole domain. For the lowest values of Re, the stochastic dynamics competes with another deterministic regime of growth of the localized perturbations. The latter is interpreted as a depinning process from the homoclinic snaking region of the system.

  • 231. Duguet, Yohann
    et al.
    Monokrousos, Antonios
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Brandt, Luca
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Minimal transition thresholds in plane Couette flow2013Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, nr 8, s. 084103-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Subcritical transition to turbulence requires finite-amplitude perturbations. Using a nonlinear optimisation technique in a periodic computational domain, we identify the perturbations of plane Couette flow transitioning with least initial kinetic energy for Re <= 3000. We suggest a new scaling law E-c = O(Re-2.7) for the energy threshold vs. the Reynolds number, in quantitative agreement with experimental estimates for pipe flow. The route to turbulence associated with such spatially localised perturbations is analysed in detail for Re = 1500. Several known mechanisms are found to occur one after the other: Orr mechanism, oblique wave interaction, lift-up, streak bending, streak breakdown, and spanwise spreading. The phenomenon of streak breakdown is analysed in terms of leading finite-time Lyapunov exponents of the associated edge trajectory.

  • 232.
    Duguet, Yohann
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philip
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Formation of turbulent patterns near the onset of transition in plane Couette flow2010Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 650, s. 119-129Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The formation of turbulent patterns in plane Couette flow is investigated near the onset of transition, using numerical simulation in a very large domain of size 800 h x 2h x 356 h. Based on a maximum observation time of 20 000 inertial units, the threshold for the appearance of sustained turbulent motion is Re-c = 324 +/- 1. For Re-c < Re <= 380, turbulent-banded patterns form, irrespective of whether the initial perturbation is a noise or localized disturbance. Measurements of the turbulent fraction versus Re show evidence for a discontinuous phase transition scenario where turbulent spots play the role of the nuclei. Using a smaller computational box, the angle selection of the turbulent bands in the early stages of their development is shown to be related to the amplitude of the initial perturbation.

  • 233.
    Duguet, Yohann
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philip
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Pattern formation in low Reynolds number plane Couette flow2009Inngår i: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, s. 93-96Konferansepaper (Fagfellevurdert)
  • 234.
    Duguet, Yohann
    et al.
    Universit´e Paris-Sud, Orsay, France.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Stochastic motion of a laminar/turbulent interface in a shear flow2011Inngår i: / [ed] K. Bajer, 2011Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Plane Couette flow is a classical prototype of a shear flow where transition toturbulence is subcritical, i.e. happens despite linear stability of the base flow. In this studywe are interested in the spatio-temporal competition between the (active) turbulent phase andthe (absorbing) laminar. Our three-dimensional numerical simulations show that the delimitinginterface, when parallel to the streamwise direction, moves in a stochastic manner which wemodel as a continuous-time random walk. Statistical analysis suggests a Gaussian diffusionprocess and allows us to determine the average speed of this interface as a function of theReynolds number Re, as well as the threshold in Re above which turbulence contaminatesthe whole domain. For the lowest value of Re, this stochastic motion competes with anotherdeterministic regime of growth of the localised perturbations. The latter, a rather unexpectedregime, is shown to be linked to the recently found localised snaking solutions of the Navier-Stokes equations. An extension of this thinking to more general orientations of the interfaceswill be proposed.

  • 235.
    Duguet, Yohann
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Localized edge states in plane Couette flow2009Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, nr 11Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The dynamics at the threshold of transition in plane Couette flow is Investigated numerically in a large spatial domain for a certain type of localized initial perturbation, for Re between 350 and 1000 The corresponding edge state is all unsteady spotlike Structure, localized in both streamwise and spanwise directions, which neither grows nor decays in size. We show that the localized nature of the edge state is numerically robust. and IS not Influenced by the size of the computational domain The edge trajectory appeals to transiently visit localized steady states This suggests that basic spatiotemporally intermittent features of transition to turbulence. such as the growth Of it turbulent spot, call be described as a dynamical system.

  • 236.
    Duguet, Yohann
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll.
    Stripy patterns in low-Re turbulent plane Couette flow2010Inngår i: SEVENTH IUTAM SYMPOSIUM ON LAMINAR-TURBULENT TRANSITION / [ed] Schlatter P; Henningson DS, 2010, Vol. 18, s. 159-164Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We present for the first time a complete bifurcation diagram of plane Couette flow based on direct numerical simulation of the full Navier-Stokes equations. The use of an unusually large computational domain (800h x 2h x 356h) is crucial for the determination of transition thresholds, because it allows to reproduce spatio-temporal intermittency structures such as transient spots, turbulent bands, and laminar holes. The threshold in Re (based on the half-gap) is found to he Re-c = 324 +/- 1 in very good agreement with available experimental data. This work points out that, at the onset of transition in Re, fragmented oblique patterns always emerge from the interaction of growing neighbouring spots. An analogy with thermodynamical phase transition seems relevant to describe the whole transition process.

  • 237. Duguet, Yohann
    et al.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Henningson, Dan S.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Eckhardt, Bruno
    Self-Sustained Localized Structures in a Boundary-Layer Flow2012Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, nr 4, s. 044501-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When a boundary layer starts to develop spatially over a flat plate, only disturbances of sufficiently large amplitude survive and trigger turbulence subcritically. Direct numerical simulation of the Blasius boundary-layer flow is carried out to track the dynamics in the region of phase space separating transitional from relaminarizing trajectories. In this intermediate regime, the corresponding disturbance is fully localized and spreads slowly in space. This structure is dominated by a robust pair of low-speed streaks, whose convective instabilities spawn hairpin vortices evolving downstream into transient disturbances. A quasicyclic mechanism for the generation of offspring is unfolded using dynamical rescaling with the local boundary-layer thickness.

  • 238.
    Duguet, Yohann
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Willis, A. P.
    Kerswell, R. R.
    Slug genesis in cylindrical pipe flow2010Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 663, s. 180-208Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Transition to uniform turbulence in cylindrical pipe flow occurs experimentally via the spatial expansion of isolated coherent structures called 'slugs', triggered by localized finite-amplitude disturbances. We study this process numerically by examining the preferred route in phase space through which a critical disturbance initiates a 'slug'. This entails first identifying the relative attractor - 'edge state' - on the laminar-turbulent boundary in a long pipe and then studying the dynamics along its low-dimensional unstable manifold, leading to the turbulent state. Even though the fully turbulent state delocalizes at Re approximate to 2300, the edge state is found to be localized over the range Re = 2000-6000, and progressively reduces in both energy and spatial extent as Re is increased. A key process in the genesis of a slug is found to be vortex shedding via a Kelvin-Helmholtz mechanism from wall-attached shear layers quickly formed at the edge state's upstream boundary. Whether these shedded vortices travel on average faster or slower downstream than the developing turbulence determines whether a puff or a slug (respectively) is formed. This observation suggests that slugs are out-of-equilibrium puffs which therefore do not co-exist with stable puffs.

  • 239. Duwig, C.
    et al.
    Fuchs, Laszlo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Lund University, Sweden .
    Numerical simulation of forced laminar flame2009Inngår i: 47th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2009, s. 2009-1186-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The simulation of laminar flames consists of capturing the evolution of a very large number of species that may react within a broad range of time scales. It therefore results in a highly non-linear stiff numerical problem that requires large computational resources. In the present paper, an alternative approach for the simulation of unsteady premixed flames is proposed and is applied for simulating an excited laminar flame. The approach is to using a single scalar (i.e. a progress variable) on relatively coarse grids. The unresolved details of the flame structure are expressed in terms of the spatial filtering. A model that has originally been developed for Large Eddy Simulation is applied for the simulation of unsteady laminar flames. The ability of the present approach to represent the filtered flame structure is compared qualitatively to experimental data.

  • 240. Dörr, P. C.
    et al.
    Kloker, M. J.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Effect of upstream flow deformation using plasma actuators on crossflow transition induced by unsteady vortical free-stream disturbances2017Inngår i: 47th AIAA Fluid Dynamics Conference, 2017, American Institute of Aeronautics and Astronautics, 2017Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Upstream flow deformation (UFD) has been shown to be an effective technique to delay roughness induced laminar-turbulent transition at low free-stream turbulence level in three-dimensional boundary-layer flows. Beneficial steady crossflow vortex (CFV) control modes are excited and the resulting nonlinear CFVs induce a useful mean-flow distortion. We recently showed by direct numerical simulations that plasma actuators, modeled by localized steady volume forcing, can be employed to excite the UFD control modes. In the current work we investigate the same actuator set-ups to control transition caused by traveling CFVs that are excited by single unsteady vortical free-stream disturbances (FSDs) impinging on the boundary layer. FSDs of various wavenumbers and frequencies are imposed either upstream or downstream, or at the position of the actuators to also scrutinize if the volume forcing has a direct unfavorable effect on the receptivity to the FSDs that adds to the stabilization by the UFD. For all investigated cases we show that a significant transition delay is achieved.

  • 241. Ebenhoch, Raphael
    et al.
    Muro, Blas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Dahlberg, Jan-Åke
    Berkesten Hägglund, Patrik
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A linearized numerical model of wind-farm flows2017Inngår i: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 20, nr 5, s. 859-875Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A fast and reasonably accurate numerical three-dimensional wake model able to predict the flow behaviour of a wind farm over a flat terrain has been developed. The model is based on the boundary-layer approximation of the Navier-Stokes equations, linearized around the incoming atmospheric boundary layer, with the assumption that the wind turbines provide a small perturbation to the velocity field. The linearization of the actuator-disc theory brought additional insights that could be used to understand the behaviour, as well as the limitations, of a flow model based on linear methods: for instance, it is shown that an adjustment of the turbine's thrust coefficient is necessary in order to obtain the same wake velocity field provided by the actuator disc theory within the used linear framework. The model is here validated against two independent wind-tunnel campaigns with a small and a large wind farm aimed at the characterization of the flow above and upstream of the farms, respectively. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region, thereby including more physical mechanisms than other simplified approaches. The conducted simulations (in agreement with the measurement results) show that the presence of a wind farm affects the approaching flow far more upstream than generally expected and definitely beyond the current industrial standards. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated providing an insight into the transfer of momentum inside the turbine rows.

  • 242.
    Efraimsson, Gunilla
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Forsberg, Nicolas
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik.
    Nordström, Jan
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, Aeroakustik.
    Simulations of Acoustic Waves in a Turbo-Fan Engine Air Intake2010Inngår i: 16th AIAA/CEAS Aeroacoustics Conference (31st AIAA Aeroacoustics Conference), 2010Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    In this work, propagation of acoustic waves in the air intake of an turbo-fan engine are simulated using a commercial Navier-Stokes solver. Three different acoustic modes are studied, the first and second radial mode with no azimuthal variation and the first radial mode with four lobes in the azimuthal direction. From the results it can be concluded that the propagation of sound waves in a curved intake can indeed be simulated using a commercial CFD solver. Also, the acoustic source, when given as a boundary condition, should be set at the fan plane. A strong influence of the flow or the curved geometry is identified, yielding a focusing of sound waves to the middle part of the duct. A transmission loss of the acoustic power from the fan plane to the inlet plane of around 5 dB is identified for the first radial modes for acoustic powers in the interval [128 db, 158 dB]. Non-linear effects are identified for powers of 148 dB and higher, which seems reasonable. Finally, a shielding effect of supersonic regions is identified.

  • 243. Eitel-Amor, G
    et al.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulent boundary layers in long computational domains2015Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Wall-bounded turbulence emerges e.g. along the surface of moving ships and airplanes or in pipelines. The prediction of skin friction and drag is directly related to fuel consumption or the power needed to transport gases through pipelines, thereby emphasizing the practical relevance of wall turbulence

  • 244.
    Eitel-Amor, Georg
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Flores, O.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Hairpin vortices in turbulent boundary layers2014Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 506, nr 1, s. 012008-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present work addresses the question whether hairpin vortices are a dominant feature of near-wall turbulence and which role they play during transition. First, the parent-offspring mechanism is investigated in temporal simulations of a single hairpin vortex introduced in a mean shear flow corresponding to turbulent channels and boundary layers up to Re-tau = 590. Using an eddy viscosity computed from resolved simulations, the effect of a turbulent background is also considered. Tracking the vortical structure downstream, it is found that secondary hairpins are created shortly after initialization. Thereafter, all rotational structures decay, whereas this effect is enforced in the presence of an eddy viscosity. In a second approach, a laminar boundary layer is tripped to transition by insertion of a regular pattern of hairpins by means of defined volumetric forces representing an ejection event. The idea is to create a synthetic turbulent boundary layer dominated by hairpin-like vortices. The flow for Re-tau < 250 is analysed with respect to the lifetime of individual hairpin-like vortices. Both the temporal and spatial simulations demonstrate that the regeneration process is rather short-lived and may not sustain once a turbulent background has formed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former DNS studies is an outer layer phenomenon not being connected to the onset of near-wall turbulence.

  • 245.
    Eitel-Amor, Georg
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Simulation and validation of a spatially evolving turbulent boundary layer up to Reθ = 83002014Inngår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 47, s. 57-69Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Results of a finely resolved large-eddy simulation (LES) of a spatially developing zero-pressure-gradient turbulent boundary layer up to a Reynolds number of Reθ = 8300 are presented. The very long computational domain provides substantial assessment for suggested high Reynolds number (Re) trends. Statistics, integral quantities and spectral data are validated using high quality direct numerical simulation (DNS) ranging up to Reθ = 4300 and hot-wire measurements covering the remaining Re-range. The mean velocity, turbulent fluctuations, skin friction, and shape factor show excellent agreement with the reference data. Through utilisation of filtered DNS, subtle differences between the LES and DNS could to a large extent be explained by the reduced spanwise resolution of the LES. Spectra and correlations for the streamwise velocity and the wall-shear stress evidence a clear scale-separation and a footprint of large outer scales on the near-wall small scales. While the inner peak decreases in importance and reduces to 4% of the total energy at the end of the domain, the energy of the outer peak scales in outer units. In the near-wall region a clear k - 1 region emerges. Consideration of the two-dimensional spectra in time and spanwise space reveals that an outer time scale λt ≈ 10δ99 / U∞, with the boundary layer thickness δ99 and free-stream velocity U∞, is the correct scale throughout the boundary layer rather than the transformed streamwise wavelength multiplied by a (scale independent) convection velocity. Maps for the covariance of small scale energy and large scale motions exhibit a stronger linear Re dependence for the amplitude of the off-diagonal peak compared to the diagonal one, thereby indicating that the strength of the amplitude modulation can only qualitatively be assessed through the diagonal peak. In addition, the magnitude of the wall-pressure fluctuations confirms mixed scaling, and pressure spectra at the highest Re give a first indication of a -7/3 wave number dependence. © 2014 Elsevier Inc.

  • 246.
    Eitel-Amor, Georg
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Flores, O.
    Hairpin vortices in turbulent boundary layers2015Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, nr 2, artikkel-id 025108Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present work presents a number of parallel and spatially developing simulations of boundary layers to address the question of whether hairpin vortices are a dominant feature of near-wall turbulence, and which role they play during transition. In the first part, the parent-offspring regeneration mechanism is investigated in parallel (temporal) simulations of a single hairpin vortex introduced in a mean shear flow corresponding to either turbulent channels or boundary layers (Re-tau less than or similar to 590). The effect of a turbulent background superimposed on the mean flow is considered by using an eddy viscosity computed from resolved simulations. Tracking the vortical structure downstream, it is found that secondary hairpins are only created shortly after initialization, with all rotational structures decaying for later times. For hairpins in a clean (laminar) environment, the decay is relatively slow, while hairpins in weak turbulent environments (10% of nu(t)) dissipate after a couple of eddy turnover times. In the second part, the role of hairpin vortices in laminar-turbulent transition is studied using simulations of spatial boundary layers tripped by hairpin vortices. These vortices are generated by means of specific volumetric forces representing an ejection event, creating a synthetic turbulent boundary layer initially dominated by hairpin-like vortices. These hairpins are advected towards the wake region of the boundary layer, while a sinusoidal instability of the streaks near the wall results in rapid development of a turbulent boundary layer. For Re-theta > 400, the boundary layer is fully developed, with no evidence of hairpin vortices reaching into the wall region. The results from both the parallel and spatial simulations strongly suggest that the regeneration process is rather short-lived and may not sustain once a turbulent background is developed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former direct numerical simulation studies is reminiscent of the transitional boundary layer and may not be connected to some aspects of the dynamics of the fully developed wall-bounded turbulence.

  • 247.
    El Khoury, George K.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulent pipe flow: Statistics, Re-dependence, structures and similarities with channel and boundary layer flows2014Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 506, nr 1, s. 012010-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulation data of fully developed turbulent pipe flow are extensively compared with those of turbulent channel flow and zero-pressure-gradient boundary layer flow for Re-tau up to 1 000. In the near-wall region, a high degree of similarity is observed in the three flow cases in terms of one-point statistics, probability density functions of the wall-shear stress and pressure, spectra, Reynolds stress budgets and advection velocity of the turbulent structures. This supports the notion that the near-wall region is universal for pipe and channel flow. Probability density functions of the wall shear stress, streamwise turbulence intensities, one-dimensional spanwise/azimuthal spectra of the streamwise velocity and Reynolds-stress budgets are very similar near the wall in the three flow cases, suggesting that the three cauonical wall-bounded flows share wally features. In the wake region, the wean streamwise velocity and Reynolds stress budgets show smile expected differences.

  • 248.
    El Khoury, George K.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Noorani, Azad
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Fischer, Paul F.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Direct Numerical Simulation of Turbulent Pipe Flow at Moderately High Reynolds Numbers2013Inngår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 91, nr 3, s. 475-495Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Fully resolved direct numerical simulations (DNSs) have been performed with a high-order spectral element method to study the flow of an incompressible viscous fluid in a smooth circular pipe of radius R and axial length 25R in the turbulent flow regime at four different friction Reynolds numbers Re (tau) = 180, 360, 550 and . The new set of data is put into perspective with other simulation data sets, obtained in pipe, channel and boundary layer geometry. In particular, differences between different pipe DNS are highlighted. It turns out that the pressure is the variable which differs the most between pipes, channels and boundary layers, leading to significantly different mean and pressure fluctuations, potentially linked to a stronger wake region. In the buffer layer, the variation with Reynolds number of the inner peak of axial velocity fluctuation intensity is similar between channel and boundary layer flows, but lower for the pipe, while the inner peak of the pressure fluctuations show negligible differences between pipe and channel flows but is clearly lower than that for the boundary layer, which is the same behaviour as for the fluctuating wall shear stress. Finally, turbulent kinetic energy budgets are almost indistinguishable between the canonical flows close to the wall (up to y (+) a parts per thousand aEuro parts per thousand 100), while substantial differences are observed in production and dissipation in the outer layer. A clear Reynolds number dependency is documented for the three flow configurations.

  • 249.
    Eliasson, Peter
    et al.
    Swedish Defence Research Agency, FOI.
    Hanifi, Ardeshir
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Peng, Shia-Hui
    Swedish Defence Research Agency, FOI.
    Influence of Transition on High-Lift Prediction with the NASA Trap Wing Model2011Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A computational analysis on the influence of the transition for the NASA Trap Wing Model has been carried out, which is an extension of the work presented for the 1st AIAA High Lift Prediction Workshop. The transition prediction is based on stability analyses with a database method in spanwise sections. Comparisons with experimental data are made to find appropriate N-factors for the eN method leading to the estimated interval 5< N <10. The computed transition locations are used to specify laminar and turbulent regions in the 3D calculations. Including transition improves the results, especially with locations from higher N-factors, and good agreement with experimental data for aerodynamic forces, moments and pressure distributions is obtained.

  • 250. Elnady, T.
    et al.
    Bodén, Hans
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strömningsakustik.
    Elhadidi, B.
    Validation of an Inverse Semi-Analytical Technique to Educe Liner Impedance2009Inngår i: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 47, nr 12, s. 2836-2844Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, the acoustic impedance of a liner is educed by a novel semi-analytical inverse technique. The liner sample is placed flush with the solid walls in a rectangular duct with grazing flow. The technique uses complex acoustic pressure measured at four positions at the wall of the duct, upstream and downstream of the lined section, and educes the impedance with a mode-matching method. Previous studies neglected grazing flow nonuniformity and the pressure discontinuity that appears at the liner-wall boundary caused by the discontinuity of the acoustic particle velocity into the wall. In the present paper, the mode-matching formulation is rederived in terms of pressure instead of velocity potential which is found to be more numerically stable. Moreover, the proposed methodology is validated with benchmark data from an experiment performed by NASA. First, the ability of the code to reproduce the pressure field for a given impedance is tested. Second, the ability to educe the correct impedance for a given pressure distribution is tested. The results of the mode-matching code are in very good agreement with the experimental data. The effect of shear flow is investigated and it can be concluded that the assumption of uniform flow is appropriate for the chosen liner, duct size, and frequency range of interest.

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