Endre søk
Begrens søket
12 1 - 50 of 94
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Treff pr side
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
Merk
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1.
    Ahlman, Daniel
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    A numerical method for simulation of turbulence and mixing in a compressible wall-jet2007Rapport (Annet vitenskapelig)
  • 2.
    Ahlman, Daniel
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Direct numerical simulation of a plane turbulent wall-jet including scalar mixing2007Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, nr 6, s. 065102-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulation is used to study a turbulent plane wall-jet including the mixing of a passive scalar. The Reynolds and Mach numbers at the inlet are Re=2000 and M=0.5, respectively, and a constant coflow of 10% of the inlet jet velocity is used. The passive scalar is added at the inlet enabling an investigation of the wall-jet mixing. The self-similarity of the inner and outer shear layers is studied by applying inner and outer scaling. The characteristics of the wall-jet are compared to what is reported for other canonical shear flows. In the inner part, the wall-jet is found to closely resemble a zero pressure gradient boundary layer, and the outer layer is found to resemble a free plane jet. The downstream growth rate of the scalar is approximately equal to that of the streamwise velocity in terms of the growth rate of the half-widths. The scalar fluxes in the streamwise and wall-normal direction are found to be of comparable magnitude. The scalar mixing situation is further studied by evaluating the scalar dissipation rate and the mechanical to mixing time scale ratio.

  • 3.
    Ahlman, Daniel
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Direct numerical simulation of a reacting turbulent wall-jet2007Rapport (Annet vitenskapelig)
  • 4.
    Ahlman, Daniel
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Direct numerical simulation of non-isothermal turbulent wall-jets2009Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, nr 3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulations of plane turbulent nonisothermal wall jets are performed and compared to the isothermal case. This study concerns a cold jet in a warm coflow with an ambient to jet density ratio of ρa/ρj = 0.4, and a warm jet in a cold coflow with a density ratio of ρa/ρj = 1.7. The coflow and wall temperature are equal and a temperature dependent viscosity according to Sutherland’s law is used. The inlet Reynolds and Mach numbers are equal in all these cases. The influence of the varying temperature on the development and jet growth is studied as well as turbulence and scalar statistics. The varying density affects the turbulence structures of the jets. Smaller turbulence scales are present in the warm jet than in the isothermal and cold jet and consequently the scale separation between the inner and outer shear layer is larger. In addition, a cold jet in a warm coflow at a higher inlet Reynolds number was also simulated. Although the domain length is somewhat limited, the growth rate and the turbulence statistics indicate approximate self-similarity in the fully turbulent region. The use of van Driest scaling leads to a collapse of all mean velocity profiles in the near-wall region. Taking into account the varying density by using semilocal scaling of turbulent stresses and fluctuations does not completely eliminate differences, indicating the influence of mean density variations on normalized turbulence statistics. Temperature and passive scalar dissipation rates and time scales have been computed since these are important for combustion models. Except for very near the wall, the dissipation time scales are rather similar in all cases and fairly constant in the outer region.

  • 5.
    Ahlman, Daniel
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Direct numerical simulation of mixing in a plane compressible and turbulent wall jet2005Inngår i: 4th International Symposium on Turbulence and Shear Flow Phenomena, 2005, s. 1131-1136Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulation (DNS) is used to simulate the mixing of a passive scalar in a plane compressible and turbulent wall jet. The Mach number of the jet is M = 0.5 at the inlet. The downstream development of the jet is studied and compared to experimental data. Mixing in the inner and outer shear layers of the wall jet is investigated through scalar fluxes, the probability density function of the scalar concentration and the joint probability density function of the wall normal velocity fluctuation and the scalar concentration

  • 6.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    The effect of rotation on rapidly sheared homogeneous turbulence and passive scalar transport. Linear theory and direct numerical simulation2005Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 542, s. 305-342Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of rotation on a homogeneous turbulent shear flow has been studied by means of a series of direct numerical simulations with different rotation numbers. The evolution of passive scalar fields with mean gradients in each of the three orthogonal directions in the flow was investigated in order to elucidate the effect of rotation on turbulent scalar transport. Conditions of the near-wall region of a boundary layer were approached by using a rapid shear and therefore, comparisons could be made with rapid distortion theory based on the linearized equations of the flow and scalar transport. Reynolds stresses, pressure-strain correlations and two-point velocity correlations were computed and turbulent structures were visualized. It is shown that rotation has a strong influence on the time development of the turbulent kinetic energy, the anisotropy of the flow and on the turbulent structures. Furthermore, rotation significantly affects turbulent scalar transport. The transport rate of the scalar and the direction of the scalar flux vector show large variations with different rotation numbers, and a strong alignment was observed between the scalar flux and the principal axes of the Reynolds stress tensor. The ratio of the turbulent and scalar time scales is influenced by rotation as well. The predictions of the linear theory of the turbulent one-point statistics and the scalar flux agreed fairly well with direct numerical simulation (DNS) results based on the full nonlinear governing equations. Nonetheless, some clear and strong nonlinear effects are observed in a couple of cases which significantly influence the development of the turbulence and scalar transport.

  • 7.
    Brethouwer, Geert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Billant, P.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Simulation of strongly stratified fluids2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Stably and strongly stratified turbulent flows have been studied by employing scaling analysis of the governing equations along the lines of [1], [2] and [3]. The scaling analysis suggests the existence of two different dynamical states. The parameter determining the state is R = ReF h 2, where Re and Fh are the Reynolds number and horizontal Froude number, respectively. If R≫1, viscous forces are negligible and the turbulence is strongly anisotropic but three-dimensional and causes a forward energy cascade. The vertical length scale lv scales as l v ∼ U/N (U is a horizontal velocity scale and N is the Brunt-Väisälä frequency). If R≪1, horizontal inertial forces are balanced by vertical viscous shearing and lv ∼ l hRe -1/2 (l h is a horizontal length scale). The scaling analysis has been confirmed by direct numerical simulations of homogeneous stratified turbulence. Spectra have been studied as well.

  • 8.
    Brethouwer, Geert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Billant, P.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Chomaz, J. M.
    Scaling analysis and simulation of strongly stratified turbulent flows2007Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 585, s. 343-368Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulations of stably and strongly stratified turbulent flows with Reynolds number Re >> 1 and horizontal Froude number F-h << 1 are presented. The results are interpreted on the basis of a scaling analysis of the governing equations. The analysis suggests that there are two different strongly stratified regimes according to the parameter R = ReFh2. When R >> 1, viscous forces are unimportant and l(v) scales as l(v) similar to U/N (U is a characteristic horizontal velocity and N is the Brunt-Vaisala frequency) so that the dynamics of the flow is inherently three-dimensional but strongly anisotropic. When R << 1, vertical viscous shearing is important so that l(v) similar to l(h)/Re-1/2 (l(h) is a characteristic horizontal length scale). The parameter R is further shown to be related to the buoyancy Reynolds number and proportional to (l(O)/eta)(4/3), where l(O) is the Ozmidov length scale and eta the Kolmogorov length scale. This implies that there are simultaneously two distinct ranges in strongly stratified turbulence when R >> 1: the scales larger than l(O) are strongly influenced by the stratification while those between l(O) and eta are weakly affected by stratification. The direct numerical simulations with forced large-scale horizontal two-dimensional motions and uniform stratification cover a wide Re and F-h, range and support the main parameter controlling strongly stratified turbulence being R. The numerical results are in good agreement with the scaling laws for the vertical length scale. Thin horizontal layers are observed independently of the value of R but they tend to be smooth for R < 1, while for R > 1 small-scale three-dimensional turbulent disturbances are increasingly superimposed. The dissipation of kinetic energy is mostly due to vertical shearing for R < 1 but tends to isotropy as R increases above unity. When R < 1, the horizontal and vertical energy spectra are very steep while, when R > 1, the horizontal spectra of kinetic and potential energy exhibit an approximate k(h)(-5/3)-power-law range and a clear forward energy cascade is observed.

  • 9.
    Brethouwer, Geert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Duguet, Yohann
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Numerical study of turbulent-laminar patterns in MHD, rotating and stratified shear flows2011Inngår i: Direct and Large-Eddy Simulation VIII, 2011, s. 125-130Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Coexisting laminar and turbulent regions have been observed in several types of wall bounded flows. In Taylor Couette flow, for example, alternating helical shaped laminar and turbulent regions have been observed within a limited Reynolds number range (Prigent et al., 2002) and oblique laminar and turbulent bands have been seen in experiments (Prigent et al., 2002) and simulations (Barkley and Tuckerman, 2005), (Duguet et al., 2010) of plane Couette flow for Reynolds numbers Re=U w h/ν between about 320 and 380. Here ±U w is the velocity of the two walls, h is the half width of the wall gap and ν is the viscosity. In this Reynolds number range the turbulent-laminar patterns seem to sustain while at lower Re the flow becomes fully laminar and at higher Re no clear laminar patterns can be distinguished and the flow eventually becomes fully turbulent. Similar oblique laminar-turbulent bands appeared as well in direct numerical simulations (DNS) of plane channel flow for friction Reynolds numbers Re τ =u τ h/ν=60 and 80 (Fukudome et al., 2009), (Tsukahara, 2010), where u τ is the friction velocity and h is again the gap half width.

  • 10. Brethouwer, Geert
    et al.
    Hunt, J. C. R.
    Nieuwstadt, F. T. M.
    Micro-structure and Lagrangian statistics of the scalar field with a mean gradient in isotropic turbulence2003Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 474, s. 193-225Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents an analysis and numerical study of the relations between the small-scale velocity and scalar fields in fully developed isotropic turbulence with random forcing of the large scales and with an imposed constant mean scalar gradient. Simulations have been performed for a range of Reynolds numbers from Re-lambda = 22 to 130 and Schmidt numbers from Sc = 1/25 to 144. The simulations show that for all values of Sc > 0.1 steep scalar gradients are concentrated in intermittently distributed sheet-like structures with a thickness approximately equal to the Batchelor length scale eta/Sc-1/2 with eta the Kolmogorov length scale. We observe that these sheets or cliffs are preferentially aligned perpendicular to the direction of the mean scalar gradient. Due to this preferential orientation of the cliffs the small-scale scalar field is anisotropic and this is an example of direct coupling between the large- and small-scale fluctuations in a turbulent field. The numerical simulations also show that the steep cliffs are formed by straining motions that compress the scalar field along the imposed mean scalar gradient in a very short time period, proportional to the Kolmogorov time scale. This is valid for the whole range of Sc. The generation of these concentration gradients is amplified by rotation of the scalar gradient in the direction of compressive strain. The combination of high strain rate and the alignment results in a large increase of the scalar gradient and therefore in a large scalar dissipation rate. These results of our numerical study are discussed in the context of experimental results (Warhaft 2000) and kinematic simulations (Holzer & Siggia 1994). The theoretical arguments developed here follow from earlier work of Batchelor & Townsend (1956), Betchov (1956) and Dresselhaus Tabor (1991).

  • 11.
    Brethouwer, Geert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Divergent and rotational modes in stratified flows2007Inngår i: ADVANCES IN TURBULENCE XI / [ed] Palma, JMLM; Lopes, AS, SPRINGER-VERLAG BERLIN: BERLIN , 2007, Vol. 117, s. 720-720Konferansepaper (Fagfellevurdert)
  • 12.
    Brethouwer, Geert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Numerical study of vertical dispersion by stratified turbulence2009Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 631, s. 149-163Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Numerical simulations are carried Out to investigate vertical fluid particle dispersion in uniformly stratified stationary turbulent flows. The results are compared with the analysis of Lindborg & Brethouwer (J. Fluid Mech., vol. 614, 2008, pp. 303-314), who derived long- and short-time relations for the mean square vertical displacement of fluid particles. Several direct numerical simulations (DNSs) with different degrees of stratification and different buoyancy Reynolds numbers are carried out to test the long-time relation = 2 epsilon(P)t/N-2. Here, epsilon(P) is the mean dissipation of turbulent potential energy; N is the Brunt-Vaisala frequency; and t is time. The DNSs show good agreement with this relation, with a weak dependence on the buoyancy Reynolds number. Simulations with hyperviscosity are carried out to test the relation = (1 + pi C-PL)2 epsilon(P)t/N-2, which should be valid for shorter time scales in the range N-1 << t << T, where T is the turbulent eddy turnover time. The results of the hyperviscosity simulations come closer to this prediction with C-PL about 3 with increasing stratification. However, even in the simulation with the strongest stratification the growth of is somewhat slower than linear in this regime. Based on the simulation results it is argued that the time scale determining the evolution Of is the eddy turnover time, T, rather than the buoyancy time scale N-1, as suggested in previous studies. The simulation results are also consistent with the prediction of Lindborg & Brethouwer (2008) that the nearly flat plateau Of observed at t similar to T should scale as 4E(P)/N-2, where E-P is the mean turbulent potential energy.

  • 13.
    Brethouwer, Geert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. 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.
    Particle Diffusion in Stably Stratified Flows2010Inngår i: PROGRESS IN TURBULENCE III / [ed] Peinke, J.; Oberlack, M.; Talamelli, A., 2010, Vol. 131, s. 163-166Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Numerical simulations are used to study the vertical dispersion of fluid particles in homogeneous turbulent flows with a stable stratification. The results of direct numerical simulations are in good agreement with the relation for the long time fluid particle dispersion, = 2 epsilon(P)t / N-2, derived by [6], though with a small dependence on the buoyancy Reynolds number. Here, is the mean square vertical particle displacement, epsilon p is the dissipation of potential energy, t is time and N is the Brunt-Vaisala frequency. A simulation with hyperviscosicity is performed to verify the relation = (1 + pi C-PL)2 epsilon(P)t / N-2 for shorter times, also derived by [6]. The agreement is reasonable and we find that C-PL similar to 3. The onset of a plateau in is observed in the simulations at t similar to E-P / epsilon(P) which scales as 4E(P) / N-2, where E-P is the potential energy.

  • 14.
    Brethouwer, Geert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Lindborg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Passive scalars in stratified turbulence2008Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, nr 6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Statistics of a passive scalar (or tracer) with a horizontal mean gradient in randomly forced and strongly stratified turbulence are investigated by numerical simulations. We observe that horizontal isotropy of the passive scalar spectrum is satisfied in the inertial range. The spectrum has the form E-theta(k(h)) = C-theta epsilon theta epsilon(-1/3)(K) k(h)(-5/3), where epsilon(theta), epsilon(K) are the dissipation of scalar variance and kinetic energy respectively, and C-theta similar or equal to 0.5 is a constant. This spectrum is consistent with atmospheric measurements in the mesoscale range with wavelengths 1 - 500 km. We also calculate the fourth-order passive scalar structure function and show that intermittency effects are present in stratified turbulence.

  • 15. Brethouwer, Geert
    et al.
    Nieuwstadt, F. T. M.
    DNS of mixing and reaction of two species in a turbulent channel flow: A validation of the conditional moment closure2001Inngår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 66, nr 3, s. 209-239Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider the chemical reaction in a turbulent flow for the case that the time scale of turbulence and the time scale of the reaction are comparable. This process is complicated by the fact that the reaction takes place intermittently at those locations where the species are adequately mixed. This is known as spatial segregation. Several turbulence models have been proposed to take the effect of spatial segregation into account. Examples are the probability density function (PDF) and the conditional moment closure (CMC) models. The main advantage of these models is that they are able to parameterize the effects of turbulent mixing on the chemical reaction rate. As a price several new unknown terms appear in these models for which closure hypothesis must be supplied. Examples are the conditional dissipation < chi \ phi >, the conditional diffusion < kappa del (2) phi \ u, phi > and the conditional velocity < u \ phi >. In the present study we investigate these unknown terms that appear in the PDF and CMC model by means of a direct numerical simulation (DNS) of a fully developed turbulent flow in a channel geometry. We present the results of two simulations in which a scalar is released from a continuous line source. In the first we consider turbulent mixing without chemical reaction and in the second we add a binary reaction. The results of our simulations agree very well with experimental data for the quantities on which information is available. Several closure hypotheses that have been proposed in the literature, are considered and validated with help of our simulation results.

  • 16.
    Brethouwer, Geert
    et al.
    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.
    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.
    Duguet, Yohann
    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.
    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.
    Recurrent Bursts via Linear Processes in Turbulent Environments2014Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 14, s. 144502-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Large-scale instabilities occurring in the presence of small-scale turbulent fluctuations are frequently observed in geophysical or astrophysical contexts but are difficult to reproduce in the laboratory. Using extensive numerical simulations, we report here on intense recurrent bursts of turbulence in plane Poiseuille flow rotating about a spanwise axis. A simple model based on the linear instability of the mean flow can predict the structure and time scale of the nearly periodic and self-sustained burst cycles. Poiseuille flow is suggested as a prototype for future studies of low-dimensional dynamics embedded in strongly turbulent environments.

  • 17.
    Brethouwer, Geert
    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.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Effects of rapid spanwise rotation on turbulent channel flow with a passive scalar2011Inngår i: Proc. 7th International Symposium on Turbulence and Shear Flow Phenomena, 2011Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulations of fully developed turbulentchannel flow including a passive scalar rotating about thespanwise axis have been performed. The mean bulk Reynoldsnumber, Reb = Ubh/n ≥ 20000, where Ub is the bulk meanvelocity and h the channel half width, is higher than in previoussimulations and the rotation rate covers a wide range.At moderate rotation rates, turbulence on the stable channelside is significantly less damped than in DNS at lower Reb. Athigh rotation rates we observe re-occurring, quasi-periodic instabilitieson the stable channel side. Between these events theturbulence is weak, but during the instability events the wallshear stress and turbulence intensity are much stronger. Theinstabilities are caused by structures resembling Tollmien-Schlichting (TS) waves that at some instant rapidly grow, thenbecome unstable and finally break down into intense turbulence.After some time the TS waves form again and the processrepeats itself in a periodic-like manner.Mean scalar profiles are also strongly affected by rotationand large scalar fluctuations are found on the border of the stableand unstable channel side. The turbulent Prandtl/Schmidtnumber of the scalar is much less than unity if there is rotation.Predicting scalar transport in rotating channel flow willtherefore pose a challenge to turbulence models.

  • 18.
    Brethouwer, Geert
    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.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulence instabilities and passive scalars in rotating channel flow2011Inngår i: 13th European Turbulence Conference (ETC13): Instability, Transition, Grid Turbulence And Jets / [ed] K. Bajer, Institute of Physics Publishing (IOPP), 2011, s. 032025-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Fully developed channel flow with a passive scalar rotating about the spanwise axis is studied by direct numerical simulations. The Reynolds number based on the bulk mean velocity Re-b is up to 30000, substantially higher than in previous studies, and the rotation rates cover a broad range. Turbulence on the stable channel side is less strongly damped at moderate rotation rates than in channel flow at lower Re-b. At high rotation rates and sufficiently high Re-b, intermittent strong instabilities occur on the stable side caused by rapidly growing modes resembling two-dimensional Tollmien-Schlichting waves which at some instant become unstable and break down into intense turbulence. The turbulence decays and after some time the waves form again and the process is repeated in a cyclic manner. Rotation also strongly affects the mean passive scalar profiles and turbulent scalar fluxes. Large scalar fluctuations are observed on the border between the stable and unstable channel sides. While in non-rotating channel flow the turbulent Prandtl number of the passive scalar is about one like in other shear flows, it is much smaller in the rotating cases.

  • 19.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Passive scalar transport in rotating turbulent channel flow2018Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 844, s. 297-322Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Passive scalar transport in turbulent channel flow subject to spanwise system rotation is studied by direct numerical simulations. The Reynolds number R-e= U(b)h/nu is fixed at 20 000 and the rotation number R-o= 2 Omega h/U-b is varied from 0 to 1.2, where U-b is the bulk mean velocity, h the half channel gap width and Omega the rotation rate. The scalar is constant but different at the two walls, leading to steady scalar transport across the channel. The rotation causes an unstable channel side with relatively strong turbulence and turbulent scalar transport, and a stable channel side with relatively weak turbulence or laminar-like flow, weak turbulent scalar transport but large scalar fluctuations and steep mean scalar gradients. The distinct turbulent-laminar patterns observed at certain Ro on the stable channel side induce similar patterns in the scalar field. The main conclusions of the study are that rotation reduces the similarity between the scalar and velocity field and that the Reynolds analogy for scalar-momentum transport does not hold for rotating turbulent channel flow. This is shown by a reduced correlation between velocity and scalar fluctuations, and a strongly reduced turbulent Prandtl number of less than 0.2 on the unstable channel side away from the wall at higher Ro. On the unstable channel side, scalar scales become larger than turbulence scales according to spectra and the turbulent scalar flux vector becomes more aligned with the mean scalar gradient owing to rotation. Budgets in the governing equations of the scalar energy and scalar fluxes are presented and discussed as well as other statistics relevant for turbulence modelling.

  • 20.
    Brethouwer, Gert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Duguet, Y.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulent-laminar coexistence in wall flows with Coriolis, buoyancy or Lorentz forces2012Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 704, s. 137-172Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulations of subcritical rotating, stratified and magnetohydrodynamic wall-bounded flows are performed in large computational domains, focusing on parameters where laminar and turbulent flow can stably coexist. In most cases, a regime of large-scale oblique laminar-turbulent patterns is identified at the onset of transition, as in the case of pure shear flows. The current study indicates that this oblique regime can be shifted up to large values of the Reynolds number R e by increasing the damping by the Coriolis, buoyancy or Lorentz force. We show evidence for this phenomenon in three distinct flow cases: plane Couette flow with spanwise cyclonic rotation, plane magnetohydrodynamic channel flow with a spanwise or wall-normal magnetic field, and open channel flow under stable stratification. Near-wall turbulence structures inside the turbulent patterns are invariably found to scale in terms of viscous wall units as in the fully turbulent case, while the patterns themselves remain large-scale with a trend towards shorter wavelength for increasing Re. Two distinct regimes are identified: at low Reynolds numbers the patterns extend from one wall to the other, while at large Reynolds number they are confined to the near-wall regions and the patterns on both channel sides are uncorrelated, the core of the flow being highly turbulent without any dominant large-scale structure.

  • 21.
    Brethouwer, Gert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Lindborg, Anders V.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Investigation of fluid particle dispersion in stably stratified turbulence2009Inngår i: 6th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2009, International Symposium on Turbulence and Shear Flow Phenomena, TSFP , 2009, s. 1160-1163Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Numerical simulations are used to study vertical dispersion of fluid particles in homogeneous turbulent flows with a stable stratification (Brethouwer and Lindborg, 2009). The results of direct numerical simulations are in good agreement with the relation for the long time fluid particle dispersion, δz2 = 2εP t/N2, derived by Lindborg and Brethouwer (2008), though with a small dependence on the buoyancy Reynolds number. Here, δz2 is the mean square vertical particle displacement, εP is the dissipation of potential energy, t is time and N is the Brunt-Väisälä frequency. Simulations with hyperviscosicity are performed to verify the relation δz2 = (1 + πCP L)2εP t/N2 for N−1 t T, where N is the Brunt-Väisälä frequency and T is the turbulent eddy turnover time. The simulation results approach the relation for increasing stratification and we find that CP L is about 3 in strongly stratified fluids. The onset of a plateau in δz2 is observed in the simulations at t ∼ T . 

  • 22.
    Brethouwer, Gert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. 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.
    Numerical simulations of particle dispersion in stratified flows2009Inngår i: ADVANCES IN TURBULENCE XII: PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE / [ed] Eckhardt, B., 2009, Vol. 132, s. 51-55Konferansepaper (Fagfellevurdert)
  • 23.
    Brethouwer, Gert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Matsuo, Y.
    A numerical study of homogeneous turbulence and passive scalar transport in rotating shear flow2005Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Direct numerical simulations of homogeneous turbulent shear flow subject to spanwise rotation have been carried out. A passive scalar field with an imposed mean gradient was also included in the simulations. The flow reached a state close to the equilibrium structure with a slowly varying turbulence anisotropy and nondimensional shear number SK/ε. Different rotation numbers have been used in the simulations and the rotation either accelerated the growth of kinetic energy or slowed it down. The growth was approximately exponential in a few cases at intermediate shear times. At longer shear times the kinetic energy was growing linearly in most of the cases. The rotation affected considerably the anisotropy of the flow and the velocity correlations. The scalar-velocity fluctuation correlations and the direction of the turbulent scalar flux vector were according to the simulations also strongly influenced by rotation, even as the mechanical to scalar time scale ratio.

  • 24.
    Brethouwer, Gert
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Matsuo, Y.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    DNS of rotating homogeneous shear flow and scalar mixing2006Inngår i: Direct and Large-Eddy Simulation VI / [ed] Lamballais, E; Friedrich, R; Geurts, BJ; Metais, O, DORDRECHT: SPRINGER , 2006, Vol. 10, s. 225-232Konferansepaper (Fagfellevurdert)
  • 25. Cimarelli, A.
    et al.
    De Angelis, E.
    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.
    Talamelli, A.
    Casciola, C. M.
    Sources and fluxes of scale energy in the overlap layer of wall turbulence2015Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 771, s. 407-423Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 26.
    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.

  • 27.
    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.

  • 28.
    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.

  • 29.
    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.

  • 30.
    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.

  • 31.
    Grigoriev, Igor A.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. 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.
    A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation2013Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, nr 10, s. 105112-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The explicit algebraic Reynolds stress model of Wallin and Johansson [J. Fluid Mech. 403, 89 (2000)] is extended to compressible and variable-density turbulent flows. This is achieved by correctly taking into account the influence of the mean dilatation on the rapid pressure-strain correlation. The resulting model is formally identical to the original model in the limit of constant density. For two-dimensional mean flows the model is analyzed and the physical root of the resulting quartic equation is identified. Using a fixed-point analysis of homogeneously sheared and strained compressible flows, we show that the new model is realizable, unlike the previous model. Application of the model together with a K - omega model to quasi one-dimensional plane nozzle flow, transcending from subsonic to supersonic regime, also demonstrates realizability. Negative "dilatational" production of turbulence kinetic energy competes with positive "incompressible" production, eventually making the total production negative during the spatial evolution of the nozzle flow. Finally, an approach to include the baroclinic effect into the dissipation equation is proposed and an algebraic model for density-velocity correlations is outlined to estimate the corrections associated with density fluctuations. All in all, the new model can become a significant tool for CFD (computational fluid dynamics) of compressible flows.

  • 32.
    Grigoriev, Igor A.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Swedish Defence Research Agency (FOI), Sweden.
    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.
    Capturing turbulent density flux effects in variable density flow by an explicit algebraic model2015Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, nr 4, artikkel-id 1.4917278Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The explicit algebraic Reynolds stress model of Grigoriev et al. ["A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation," Phys. Fluids 25, 105112 (2013)] is extended to account for the turbulent density flux in variable density flows. The influence of the mean dilatation and the variation of mean density on the rapid pressure-strain correlation are properly accounted for introducing terms balancing a so-called "baroclinic" production in the Reynolds stress tensor equation. Applying the weak-equilibrium assumption leads to a self-consistent formulation of the model. The model together with a K - ω model is applied to a quasi-one-dimensional plane nozzle flow transcending from subsonic to supersonic regimes. The model remains realizable with constraints put on the model parameters. When density fluxes are taken into account, the model is less likely to become unrealizable. The density variance coupled with a "local mean acceleration" also can influence the model acting to increase anisotropy. The general trends of the behaviour of the anisotropy and production components under the variation of model parameters are assessed. We show how the explicit model can be applied to two- and three-dimensional mean flows without previous knowledge of a tensor basis to obtain the general solution. Approaches are proposed in order to achieve an approximate solution to the consistency equation in cases when analytic solution is missing. In summary, the proposed model has the potential to significantly improve simulations of variable-density flows.

  • 33.
    Grigoriev, Igor
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. 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.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Swedish Defence Research Agency (FOI), Sweden.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Unified explicit algebraic Reynolds stress model for compressible, heat-releasing and supercritical flowswith large density variation2016Rapport (Annet vitenskapelig)
    Abstract [en]

    An explicit algebraic model (EARSM) for variable denstiy turbulent flow developed by Grigoriev et al. [Phys. Fluids (2015)] is revisited here. We apply it to a quasi one-dimensional nozzle flow, a wall-jet flow with combustion and large density variation and a supercritical flow of carbon dioxide with heat transfer and buoyancy. It is confirmed that the coupling between strong mean density gradient due to high speed, heat release or thermodynamic variations and the 'local mean acceleration' of the flow produces strong turbulent density and heat fluxes, which strongly affect the turbulence. The possible calibration branches are identified and analyzed. We show that a simple and unified calibration of the model gives good predictions for all cases considered. Therefore, the model is a reliable tool for the computation of compressible flows with large density variation.

  • 34.
    Grigoriev, Igor
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wallin, Stefan
    Swedish Defence Research Agency (FOI), Stockholm, Sweden.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Grundestam, Olof
    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.
    Algebraic Reynolds stress modeling of turbulence subject to rapid homogeneous and non-homogeneous compression or expansion2016Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, nr 2, s. 026101-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A recently developed explicit algebraic Reynolds stress model (EARSM) by Grigoriev et al. ["A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation," Phys. Fluids 25(10), 105112 (2013)] and the related differential Reynolds stress model (DRSM) are used to investigate the influence of homogeneous shear and compression on the evolution of turbulence in the limit of rapid distortion theory (RDT). The DRSM predictions of the turbulence kinetic energy evolution are in reasonable agreement with RDT while the evolution of diagonal components of anisotropy correctly captures the essential features, which is not the case for standard compressible extensions of DRSMs. The EARSM is shown to give a realizable anisotropy tensor and a correct trend of the growth of turbulence kinetic energy K, which saturates at a power law growth versus compression ratio, as well as retaining a normalized strain in the RDT regime. In contrast, an eddy-viscosity model results in a rapid exponential growth of K and excludes both realizability and high magnitude of the strain rate. We illustrate the importance of using a proper algebraic treatment of EARSM in systems with high values of dilatation and vorticity but low shear. A homogeneously compressed and rotating gas cloud with cylindrical symmetry, related to astrophysical flows and swirling supercritical flows, was investigated too. We also outline the extension of DRSM and EARSM to include the effect of non-homogeneous density coupled with "local mean acceleration" which can be important for, e.g., stratified flows or flows with heat release. A fixed-point analysis of direct numerical simulation data of combustion in a wall-jet flow demonstrates that our model gives quantitatively correct predictions of both streamwise and cross-stream components of turbulent density flux as well as their influence on the anisotropies. In summary, we believe that our approach, based on a proper formulation of the rapid pressure-strain correlation and accounting for the coupling with turbulent density flux, can be an important element in CFD tools for compressible flows.

  • 35.
    Lazeroms, Werner
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. 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.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    An explicit algebraic Reynolds-stress and scalar-flux model for stably stratified flows2013Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 723, s. 91-125Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work describes the derivation of an algebraic model for the Reynolds stresses and turbulent heat flux in stably stratified turbulent flows, which are mutually coupled for this type of flow. For general two-dimensional mean flows, we present a correct way of expressing the Reynolds-stress anisotropy and the (normalized) turbulent heat flux as tensorial combinations of the mean strain rate, the mean rotation rate, the mean temperature gradient and gravity. A system of linear equations is derived for the coefficients in these expansions, which can easily be solved with computer algebra software for a specific choice of the model constants. The general model is simplified in the case of parallel mean shear flows where the temperature gradient is aligned with gravity. For this case, fully explicit and coupled expressions for the Reynolds-stress tensor and heat-flux vector are given. A self-consistent derivation of this model would, however, require finding a root of a polynomial equation of sixth-order, for which no simple analytical expression exists. Therefore, the nonlinear part of the algebraic equations is modelled through an approximation that is close to the consistent formulation. By using the framework of a K-omega model (where K is turbulent kinetic energy and omega an inverse time scale) and, where needed, near-wall corrections, the model is applied to homogeneous shear flow and turbulent channel flow, both with stable stratification. For the case of homogeneous shear flow, the model predicts a critical Richardson number of 0.25 above which the turbulent kinetic energy decays to zero. The channel-flow results agree well with DNS data. Furthermore, the model is shown to be robust and approximately self-consistent. It also fulfils the requirements of realizability.

  • 36.
    Lazeroms, Werner
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Stockholm University, Sweden .
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Swedish Defence Research Agency (FOI), Sweden .
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Efficient treatment of the nonlinear features in algebraic Reynolds-stress and heat-flux models for stratified and convective flows2015Inngår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 53, s. 15-28Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work discusses a new and efficient method for treating the nonlinearity of algebraic turbulence models in the case of stratified and convective flows, for which the equations for the Reynolds stresses and turbulent heat flux are strongly coupled. In such cases, one finds a quasi-linear set of equations, which can be solved through an appropriate linear expansion in basis tensors and vectors, as discussed in earlier work. However, finding a consistent and truly explicit algebraic turbulence model requires solving an additional equation for the production-to-dissipation ratio (P+G)/ε of turbulent kinetic energy. Due to the nonlinear nature of the problem, the equation for (P+G)/ε is a higher-order polynomial equation for which no analytical solution can be found. Here we provide a new method to approximate the solution of this polynomial equation through an analysis of two special limits (shear-dominated and buoyancy-dominated), in which exact solutions are obtainable. The final result is a model that appropriately combines the two limits in more general cases. The method is tested for turbulent channel flow, both with stable and unstable stratification, and the atmospheric boundary layer with periodic and rapid changes between stable and unstable stratification. In all cases, the model is shown to give consistent results, close to the exact solution of (P+G)/ε. This new method greatly increases the range of applicability of explicit algebraic models, which otherwise would rely on the numerical solution of the polynomial equation.

  • 37.
    Lazeroms, Werner
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Explicit algebraic models for turbulent flows with buoyancy effects2013Rapport (Annet vitenskapelig)
    Abstract [en]

    An explicit algebraic model for the Reynolds stresses and turbulent heat fluxis presented for turbulent parallel shear flows in which buoyancy forces arepresent. The derivation is based on a model framework for two-dimensionalmean flows that was already presented in a previous work (Lazeroms et al.2013), and new test cases are investigated here. The model is formulated interms of the Reynolds-stress anisotropy and a normalized heat flux, which areexpanded as a linear combination of appropriate basis tensors. The coefficientsin this expansion can be found from a system of 18 linear equations. Thisformulation is complemented with a nonlinear equation for a quantity relatedto the total production-to-dissipation ratio, the solution of which is modelledwith an appropriate expression. Fully explicit model expressions are found fortwo fundamentally different flow geometries: a horizontal channel for whichthe temperature gradient is aligned with gravity, and a vertical channel wherethe temperature gradient and gravity are perpendicular. In the first case, weconsider both stable and unstable stratification, while in the second case, bothmixed and natural convection flows are investigated. Comparison with DNSdata shows that the model gives predictions that are substantially better thanstandard eddy-viscosity/eddy-diffusivity models.

  • 38.
    Lazeroms, Werner
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Wallin, Stefan
    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. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.
    Nonlinear features in explicit algebraic models for turbulent flows with active scalars2015Inngår i: 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, TSFP-9 , 2015Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A detailed discussion of explicit algebraic turbulence models in the case of active scalars is given. In particular, we discuss the appearance of nonlinearities in the models and the need for explicit solutions of the resulting nonlinear equations. Focussing on a recently published model for two-dimensional stratified flows, we present an intuitive way of approximating the solution of a sixth-order polynomial equation for the production-to-dissipation ratio (p + g)/e of turbulent kinetic energy K. This formulation is shown to be consistent for turbulent channel flow with stable and unstable stratification. The result is important for obtaining a robust model with a correct behaviour of the turbulence production in different limits of shear and buoyancy. The results have recently been published in Lazeroms et al. (2015).

  • 39.
    Lazeroms, Werner M. J.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Svensson, G.
    Bazile, E.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Wallin, Stefan
    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.
    Study of Transitions in the Atmospheric Boundary Layer Using Explicit Algebraic Turbulence Models2016Inngår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 161, nr 1, s. 19-47Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We test a recently developed engineering turbulence model, a so-called explicit algebraic Reynolds-stress (EARS) model, in the context of the atmospheric boundary layer. First of all, we consider a stable boundary layer used as the well-known first test case from the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study (GABLS1). The model is shown to agree well with data from large-eddy simulations (LES), and this agreement is significantly better than for a standard operational scheme with a prognostic equation for turbulent kinetic energy. Furthermore, we apply the model to a case with a (idealized) diurnal cycle and make a qualitative comparison with a simpler first-order model. Some interesting features of the model are highlighted, pertaining to its stronger foundation on physical principles. In particular, the use of more prognostic equations in the model is shown to give a more realistic dynamical behaviour. This qualitative study is the first step towards a more detailed comparison, for which additional LES data are needed.

  • 40.
    Lazeroms, Werner
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Svensson, Gunilla
    Stockholm University, Department of Meteorology.
    Bazile, Eric
    Météo-France, CNRM-GAME.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Wallin, Stefan
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Study of transitions in the atmospheric boundary layer using explicit algebraic turbulence models.Manuskript (preprint) (Annet vitenskapelig)
  • 41.
    Lenaers, Peter
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    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.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Negative streamwise velocities and other rare events near the wall in turbulent flows2011Inngår i: 13th European Turbulence Conference (ETC13): Wall-Bounded Flows And Control Of Turbulence, Institute of Physics Publishing (IOPP), 2011, s. 022013-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Negative streamwise velocities, extreme wall-normal velocites and high flatness values for the wall-normal fluctuations near the wall are investigated for turbulent channel flow simulations at a series of Reynolds numbers up to Reτ = 1000 in this paper. Probability density functions of the wall-shear stress and velocity components are presented, as well as joint probability density functions of the velocity components and the pressure. Backflow occurs more often (0.06% at Reτ = 1000) and further away from the wall into the buffer layer for rising Reynolds number. An oblique vortex outside the viscous sublayer is found to cause this backflow. Extreme v events occur also more often for rising Rey nolds number. Positive and negative velocity spikes appear in pairs, located on the two edges of a strong streamwise vortex: the negative spike occurring in a high speed streak indicating a sweeping motion, while the positive spike is located between a high and low speed streak. These extreme v events cause high flatness values near the wall (F(v) = 43 at Reτ = 1000).

  • 42.
    Lenaers, Peter
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Li, Qiang
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    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.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Rare backflow and extreme wall-normal velocity fluctuations in near-wall turbulence2012Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 24, nr 3, s. 035110-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Rare negative streamwise velocities and extreme wall-normal velocity fluctuations near the wall are investigated for turbulent channel flow at a series of Reynolds numbers based on friction velocity up to Re-tau = 1000. Probability density functions of the wall-shear stress and velocity components are presented as well as joint probability density functions of the velocity components and the pressure. Backflow occurs more often (0.06% at the wall at Re-tau = 1000) and further away (up to y(+) = 8.5) from the wall for increasing Reynolds number. The regions of backflow are circular with an average diameter, based on ensemble averages, of approximately 20 viscous units independent of Reynolds number. A strong oblique vortex outside the viscous sublayer is found to cause this backflow. Extreme wall-normal velocity events occur also more often for increasing Reynolds number. These extreme fluctuations cause high flatness values near the wall (F(v) = 43 at Re-tau = 1000). Positive and negative velocity spikes appear in pairs, located on the two edges of a strong streamwise vortex as documented by Xu et al. [Phys. Fluids 8, 1938 (1996)] for Re-tau = 180. The spikes are elliptical and orientated in streamwise direction with a typical length of 25 and a typical width of 7.5 viscous units at y(+) approximate to 1. The negative spike occurs in a high-speed streak indicating a sweeping motion, while the positive spike is located in between a high and low-speed streak. The joint probability density functions of negative streamwise and extreme wall-normal velocity events show that these events are largely uncorrelated. The majority of both type of events can be found lying underneath a large-scale structure in the outer region with positive sign, which can be understood by considering the more intense velocity fluctuations due to amplitude modulation of the inner layer by the outer layer. Simulations performed at different resolutions give only minor differences. Results from experiments and recent turbulent boundary layer simulations show similar results indicating that these rare events are universal for wall-bounded flows. In order to detect these rare events in experiments, measurement techniques have to be specifically tuned.

  • 43.
    Lenaers, Peter
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik. 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
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A new high-order method for the simulation of incompressible wall-bounded turbulent flows2014Inngår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 272, s. 108-126Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A new high-order method for the accurate simulation of incompressible wall-bounded flows is presented. In the stream- and spanwise directions the discretisation is performed by standard Fourier series, while in the wall-normal direction the method combines high-order collocated compact finite differences with the influence matrix method to calculate the pressure boundary conditions that render the velocity field exactly divergence-free. The main advantage over Chebyshev collocation is that in wall-normal direction, the grid can be chosen freely and thus excessive clustering near the wall is avoided. This can be done while maintaining the high-order approximation as offered by compact finite differences. The discrete Poisson equation is solved in a novel way that avoids any full matrices and thus improves numerical efficiency. Both explicit and implicit discretisations of the viscous terms are described, with the implicit method being more complex, but also having a wider range of applications. The method is validated by simulating two-dimensional Tollmien-Schlichting waves, forced transition in turbulent channel flow, and fully turbulent channel flow at friction Reynolds number Re-tau = 395, and comparing our data with analytical and existing numerical results. In all cases, the results show excellent agreement showing that the method simulates all physical processes correctly.

  • 44.
    Lenaers, Peter
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    A new high-order method for the simulation of incompressible wall-bounded turbulent pipe flowManuskript (preprint) (Annet vitenskapelig)
  • 45.
    Lenaers, Peter
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    A new high-order method for the accurate simulation of incompressible wall-bounded flows2015Inngår i: 9th International Conference on Direct and Large-Eddy Simulation, 2013, Springer Publishing Company, 2015, s. 133-138Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A new high-order method for the accurate simulation of incompressible wall-bounded flows is presented. In stream- and spanwise direction the discretisation is performed by standard Fourier series, while in wall-normal direction the method combines high-order collocated compact finite differences with the influence matrix method to calculate the pressure boundary conditions that render the velocity field divergence-free. The main advantage over Chebyshev collocation is that in wall normal direction, the grid can be chosen freely and thus excessive clustering near the wall is avoided. Both explicit and implicit discretisations of the viscous terms are described, with the implicit method being more complex, but also having a wider range of applications. The method is validated by simulating fully turbulent channel flow at friction Reynolds number Reτ=395, and comparing our data with existing numerical results. The results show excellent agreement proving that the method simulates all physical processes correctly.

  • 46.
    Lenaers, Peter
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brethouwer, Gert
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Johansson, Arne V.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    A new high-order method for simulating turbulent pipe flow2016Inngår i: Springer Proceedings in Physics, Springer, 2016, s. 211-215Konferansepaper (Fagfellevurdert)
  • 47.
    Lindborg, Erik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Stratified turbulence forced in rotational and divergent modes2007Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 586, s. 83-108Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We perform numerical box simulations of strongly stratified turbulence. The equations solved are the Boussinesq equations with constant Brunt-Vaisala frequency and forcing either in rotational or divergent modes, or, with another terminology, in vortical or wave modes. In both cases, we observe a forward energy cascade and inertial-range scaling of the horizontal kinetic and potential energy spectra. With forcing in rotational modes, there is approximate equipartition of kinetic energy between rotational and divergent modes in the inertial range. With forcing in divergent modes the results are sensitive to the vertical forcing wavenumber K-v(f) If k(v)(f) is sufficiently large the dynamics is very similar to the dynamics of the V V simulations which are forced in rotational modes, with approximate equipartition of kinetic energy in rotational and divergent modes in the inertial range. Frequency spectra of rotational, divergent and potential energy are calculated for individual Fourier modes. Waves are present at low horizontal wavenumbers corresponding to the largest scales in the boxes. In the inertial range, the frequency spectra exhibit no distinctive peaks in the internal wave frequency. In modes for which the vertical wavenumber is considerably larger than the horizontal wavenumber, the frequency spectra of rotational and divergent modes fall on top of each other. The simulation results indicate that the dynamics of rotational and divergent modes develop on the same time scale in stratified turbulence. We discuss the relevance of our results to atmospheric and oceanic dynamics. In particular, we review a number of observational reports indicating that stratified turbulence may be a prevalent dynamic process in the ocean at horizontal scales of the order of 10 or 100m up to several kilometres.

  • 48.
    Lindborg, Erik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Brethouwer, Geert
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Vertical dispersion by stratified turbulence2008Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 614, s. 303-314Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We derive a relation for the growth of the mean square of vertical displacements, delta z, of fluid particles of stratified turbulence. In the case of freely decaying turbulence, we find that for large times (delta z(2)) goes to a constant value 2(E-P(0) + aE(0))/N-2, where E-P(0) and E(0) are the initial mean potential and total turbulent energy per unit mass, respectively, a < 1 and N is the Brunt-Vaisala frequency. In the case of stationary turbulence, we find that (delta z(2)) = /N-2 + 2 epsilon(P)t/N-2, where epsilon(P) is the mean dissipation of turbulent potential energy per unit mass and is the Lagrangian structure function of normalized buoyancy fluctuations. The first term is the same as that obtained in the case of adiabatic fluid particle dispersion. This term goes to the finite limit 4E(P)/N-2 as t -> infinity. Assuming that the second term represents irreversible mixing, we show that the Osborn & Cox model for vertical diffusion is retained. In the case where the motion is dominated by a turbulent cascade with an eddy turnover time T >> N-1, rather than linear gravity waves, we suggest that there is a range of time scales, t, between N-1 and T, where = 2 pi C-PL epsilon(P)t, where C-PL is a constant of the order of unity. This means that for such motion the ratio between the adiabatic and the diabatic mean-square displacement is universal and equal to pi C-PL in this range. Comparing this result with observations, we make the estimate C-PL approximate to 3.

  • 49. Lubbers, C. L.
    et al.
    Brethouwer, Geert
    Boersma, B. J.
    Simulation of the mixing of a passive scalar in a round turbulent jet2001Inngår i: Fluid Dynamics Research, ISSN 0169-5983, E-ISSN 1873-7005, Vol. 28, nr 3, s. 189-208Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper we present the results of the direct numerical simulation (DNS) of mixing of a passive scalar in a spatially developing free round turbulent jet. The Schmidt number used in the simulations is equal to 1.0 and the Reynolds number, based on the orifice diameter and velocity is equal to 2.0 x 10(3). The primary objective of this paper is to consider the self-similarity of the jot in the far field. Having considered the self-similarity of the velocity in a previous publication, we concentrate here on the self-similarity of the concentration of the passive scalar. To this end we have considered the profiles of the mean concentration and its fluctuations, together with the concentration probability density function distribution. The results have been compared with various experimental data that have been published in the literature. In general, the results agree very well with the experimental data. The conclusion is that the mean concentration is self-similar in the far field. The profiles of the root mean square of the concentration fluctuations are not self-similar. Furthermore, it is shown that the turbulent Schmidt number is equal to 0.74, which agrees very well with experimental values.

  • 50.
    Maffioli, Andrea
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Brethouwer, Geert
    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.
    Mixing efficiency in stratified turbulence2016Inngår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 794, artikkel-id R3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider mixing of the density field in stratified turbulence and argue that, at sufficiently high Reynolds numbers, stationary turbulence will have a mixing efficiency and closely related mixing coefficient described solely by the turbulent Froude number (Formula presented.), where (Formula presented.) is the kinetic energy dissipation, (Formula presented.) is a turbulent horizontal velocity scale and (Formula presented.) is the Brunt–Väisälä frequency. For (Formula presented.), in the limit of weakly stratified turbulence, we show through a simple scaling analysis that the mixing coefficient scales as (Formula presented.), where (Formula presented.) and (Formula presented.) is the potential energy dissipation. In the opposite limit of strongly stratified turbulence with (Formula presented.), we argue that (Formula presented.) should reach a constant value of order unity. We carry out direct numerical simulations of forced stratified turbulence across a range of (Formula presented.) and confirm that at high (Formula presented.), (Formula presented.), while at low (Formula presented.) it approaches a constant value close to (Formula presented.). The parametrization of (Formula presented.) based on (Formula presented.) due to Shih et al. (J. Fluid Mech., vol. 525, 2005, pp. 193–214) can be reinterpreted in this light because the observed variation of (Formula presented.) in their study as well as in datasets from recent oceanic and atmospheric measurements occurs at a Froude number of order unity, close to the transition value (Formula presented.) found in our simulations.

12 1 - 50 of 94
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf