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Showing papers on "Turbulence published in 2008"


Journal ArticleDOI
TL;DR: In this article, a CFD strategy is proposed that combines delayed detached-eddy simulation (DDES) with an improved RANS-LES hybrid model aimed at wall modelling in LES (WMLES).

1,543 citations


Journal ArticleDOI
TL;DR: In this article, a reformulated version of the author's k-ω model of turbulence has been presented, which has been applied to both boundary layers and free shear flows and has little sensitivity to finite freestream boundary conditions on turbulence properties.
Abstract: This paper presents a reformulated version of the author'sk-ω model of turbulence. Revisions include the addition of just one new closure coefficient and an adjustment to the dependence of eddy viscosity on turbulence properties. The result is a significantly improved model that applies to both boundary layers and free shear flows and that has very little sensitivity to finite freestream boundary conditions on turbulence properties. The improvements to the k-ω model facilitate a significant expansion of its range of applicability. The new model, like preceding versions, provides accurate solutions for mildly separated flows and simple geometries such as that of a backward-facing step. The model's improvement over earlier versions lies in its accuracy for even more complicated separated flows. This paper demonstrates the enhanced capability for supersonic flow into compression corners and a hypersonic shock-wave/ boundary-layer interaction. The excellent agreement is achieved without introducing any compressibility modifications to the turbulence model.

882 citations


Journal ArticleDOI
TL;DR: In this article, the authors used direct numerical simulation (DNS) to predict the flamelet structure and the burning velocity of premixed turbulent combustion and showed that the results were valid even for highly turbulent flames.

703 citations


Book
16 Sep 2008
TL;DR: In this paper, the Equations of Motion, Mean Flow and Turbulence Behavior of boundary layers have been studied in the context of boundary layer mean flow behavior in two-dimensional interactions.
Abstract: Contents: 1. Introduction. 2. The Equations of Motion. 3. The Equations for Turbulent Flow. 4. Fundamental Concepts. 5. Boundary Layer Mean Flow Behavior. 6. Boundary Layer Turbulence Behavior. 7. Mixing Layers. 8. Perturbed Boundary Layers. 9. Two-Dimensional Interactions. 10. Three-Dimensional Interactions.

578 citations


Journal ArticleDOI
TL;DR: The overall numerical scheme obtained is highly suitable for the simulation of reactive turbulent flows in realistic geometries, for it combines arbitrarily high order of accuracy, discrete conservation of mass, momentum, and energy with consistent boundary conditions.

573 citations


Journal ArticleDOI
TL;DR: In this article, the authors compare the density statistics of compressible turbulence driven by the usually adopted solenoidal forcing (divergence-free) and by compressive forcing (curl-free).
Abstract: The probability density function (PDF) of the gas density in turbulent supersonic flows is investigated with high-resolution numerical simulations. In a systematic study, we compare the density statistics of compressible turbulence driven by the usually adopted solenoidal forcing (divergence-free) and by compressive forcing (curl-free). Our results are in agreement with studies using solenoidal forcing. However, compressive forcing yields a significantly broader density distribution with standard deviation ~3 times larger at the same rms Mach number. The standard deviation-Mach number relation used in analytical models of star formation is reviewed and a modification of the existing expression is proposed, which takes into account the ratio of solenoidal and compressive modes of the turbulence forcing.

557 citations


Journal ArticleDOI
TL;DR: In this article, an eddy-viscosity turbulence model employing three additional transport equations is presented and applied to a number of transitional flow test cases, which is based on the k- framework and represents a substantial refinement to a transition-sensitive model that has been previously documented in the open literature.
Abstract: An eddy-viscosity turbulence model employing three additional transport equations is presented and applied to a number of transitional flow test cases. The model is based on the k- framework and represents a substantial refinement to a transition-sensitive model that has been previously documented in the open literature. The third transport equation is included to predict the magnitude of low-frequency velocity fluctuations in the pretransitional boundary layer that have been identified as the precursors to transition. The closure of model terms is based on a phenomenological (i.e., physics-based) rather than a purely empirical approach and the rationale for the forms of these terms is discussed. The model has been implemented into a commercial computational fluid dynamics code and applied to a number of relevant test cases, including flat plate boundary layers with and without applied pressure gradients, as well as a variety of airfoil test cases with different geometries, Reynolds numbers, freestream turbulence conditions, and angles of attack. The test cases demonstrate the ability of the model to successfully reproduce transitional flow behavior with a reasonable degree of accuracy, particularly in comparison with commonly used models that exhibit no capability of predicting laminar-toturbulent boundary layer development. While it is impossible to resolve all of the complex features of transitional and turbulent flows with a relatively simple Reynolds-averaged modeling approach, the results shown here demonstrate that the new model can provide a useful and practical tool for engineers addressing the simulation and prediction of transitional flow behavior in fluid systems. DOI: 10.1115/1.2979230

508 citations


Journal ArticleDOI
16 May 2008-Science
TL;DR: This model presents a physical mechanism that transfers the gravitational energy to the turbulence and magnetic field energies in the large-scale structure of the universe.
Abstract: The nature and origin of turbulence and magnetic fields in the intergalactic space are important problems that are yet to be understood. We propose a scenario in which turbulent-flow motions are induced via the cascade of the vorticity generated at cosmological shocks during the formation of the large-scale structure. The turbulence in turn amplifies weak seed magnetic fields of any origin. Supercomputer simulations show that the turbulence is subsonic inside clusters and groups of galaxies, whereas it is transonic or mildly supersonic in filaments. Based on a turbulence dynamo model, we then estimated that the average magnetic field strength would be a few microgauss (μG) inside clusters and groups, approximately 0.1 μG around clusters and groups, and approximately 10 nanogauss in filaments. Our model presents a physical mechanism that transfers the gravitational energy to the turbulence and magnetic field energies in the large-scale structure of the universe.

482 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the flow over a circular cylinder at Reynolds number Re=3900 with a large eddy simulation and experimentally with hot-wire anemometry and particle image velocimetry.
Abstract: This work contributes to the study of flow over a circular cylinder at Reynolds number Re=3900. Although this classical flow is widely documented in the literature, especially for this precise Reynolds number that leads to a subcritical flow regime, there is no consensus about the turbulence statistics immediately just behind the obstacle. Here, the flow is investigated both numerically with large eddy simulation and experimentally with hot-wire anemometry and particle image velocimetry. The numerical simulation is performed using high-order schemes and a specific immersed boundary method. The present study focuses on turbulence statistics and power spectra in the near wake up to ten diameters. Statistical estimation is shown to need large integration times increasing the computational cost and leading to an uncertainty of about 10% for most flow characteristics considered in this study. The present numerical and experimental results are found to be in good agreement with previous large eddy simulation da...

441 citations


Journal ArticleDOI
TL;DR: In this paper, a force-balance model was used to investigate the relationship between the critical Shields stress and the relative roughness of the sediment in laboratory flumes and natural streams.
Abstract: Data from laboratory flumes and natural streams show that the critical Shields stress for initial sediment motion increases with channel slope, which indicates that particles of the same size are more stable on steeper slopes. This observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force. Processes that might explain this discrepancy are explored using a simple force-balance model, including increased drag from channel walls and bed morphology, variable friction angles, grain emergence, flow aeration, and changes to the local flow velocity and turbulent fluctuations. Surprisingly, increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical Shields stress are similar for flume experiments and natural streams, and significant variations in bed morphology in flumes is unlikely. Instead, grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed-roughness scale to flow depth (i.e., relative roughness). A model for the local velocity within the grain-roughness layer is proposed based on a 1-D eddy viscosity with wake mixing. In addition, the magnitude of near-bed turbulent fluctuations is shown to depend on the depth-averaged flow velocity and the relative roughness. Extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes.

437 citations


Journal ArticleDOI
TL;DR: A quantitative estimate of the anisotropic power and scaling of magnetic field fluctuations in inertial range magnetohydrodynamic turbulence, using a novel wavelet technique applied to spacecraft measurements in the solar wind, shows for the first time that the spacecraft-frame spectrum has a spectral index near 2.
Abstract: We present a quantitative estimate of the anisotropic power and scaling of magnetic field fluctuations in inertial range magnetohydrodynamic turbulence, using a novel wavelet technique applied to spacecraft measurements in the solar wind. We show for the first time that, when the local magnetic field direction is parallel to the flow, the spacecraft-frame spectrum has a spectral index near 2. This can be interpreted as the signature of a population of fluctuations in field-parallel wavenumbers with a k −2 k spectrum but is also consistent with the presence of a “critical balance” style turbulent cascade. We also find, in common with previous studies, that most of the power is contained in wavevectors at large angles to the local magnetic field and that this component of the turbulence has a spectral index of 5/3. Magnetised plasmas fill most of the Universe and in many regions, turbulence plays important roles in the transport of energy and momentum and the acceleration and scattering of charged particles. Many aspects of plasma turbulence remain poorly understood, however. Here we present results on one of these, the anisotropy of the energy spectrum of magnetohydrodynamic (MHD) turbulence with respect to the magnetic field. In classical hydrodynamics, velocity fluctuations δuk with a wavenumber k decay and transfer energy to smaller scales on the shear timescale, τS ≈ 1/(kδuk). Within the steady inertial range, far from the energy input (“outer”) and dissipation scales, this leads to the dimensional result (δuk) 3 ∝ ǫ/k, where ǫ is the energy dissipation rate per unit mass. This gives the familiar Kolmogorov energy spectrum P(k) ∝ k −5/3 , widely observed in hydrodynamic turbulence. In a plasma, fluc⊥ ! .

Journal ArticleDOI
TL;DR: In this paper, the influence of tree planting with different tree crown porosity was investigated and the results obtained in this work by combining wind tunnel experiments and CFD based simulations suggest ways to obtain quantitative information for assessment, planning and implementation of exposure mitigation using trees in urban street canyons.

Journal ArticleDOI
TL;DR: In this paper, a cascade model is presented, based on the assumptions of local nonlinear energy transfer in Wavenumber space, critical balance between linear propagation and nonlinear interaction times, and the applicability of linear dissipation rates for the nonlinearly turbulent plasma.
Abstract: This paper studies the turbulent cascade of magnetic energy in weakly col- lisional magnetized plasmas. A cascade model is presented, based on the assumptions of local nonlinear energy transfer in wavenumber space, critical balance between linear propagation and nonlinear interaction times, and the applicability of linear dissipation rates for the nonlinearly turbulent plasma. The model follows the nonlinear cascade of energy from the driving scale in the MHD regime, through the transition at the ion Lar- mor radius into the kinetic Alfven wave regime, in which the turbulence is dissipated by kinetic processes. The turbulent fluctuations remain at frequencies below the ion cy- clotron frequency due to the strong anisotropy of the turbulent fluctuations, kk ≪ k⊥ (implied by critical balance). In this limit, the turbulence is optimally described by gy- rokinetics; it is shown that the gyrokinetic approximation is well satisfied for typical slow solar wind parameters. Wave phase velocity measurements are consistent with a kinetic Alfven wave cascade and not the onset of ion cyclotron damping. The conditions under which the gyrokinetic cascade reaches the ion cyclotron frequency are established. Cas- cade model solutions imply that collisionless damping provides a natural explanation for the observed range of spectral indices in the dissipation range of the solar wind. The dis- sipation range spectrum is predicted to be an exponential fall off; the power-law behav- ior apparent in observations may be an artifact of limited instrumental sensitivity. The cascade model is motivated by a programme of gyrokinetic simulations of turbulence and particle heating in the solar wind.

Journal ArticleDOI
TL;DR: In this paper, an incompressible turbulent pipe flow at bulk-velocity and pipe-diameter-based Reynolds number ReD=44000 was simulated with second-order finite-difference methods on 630 million grid points.
Abstract: Fully developed incompressible turbulent pipe flow at bulk-velocity- and pipe-diameter-based Reynolds number ReD=44000 was simulated with second-order finite-difference methods on 630 million grid points. The corresponding Karman number R+, based on pipe radius R, is 1142, and the computational domain length is 15R. The computed mean flow statistics agree well with Princeton Superpipe data at ReD=41727 and at ReD=74000. Second-order turbulence statistics show good agreement with experimental data at ReD=38000. Near the wall the gradient of with respect to ln(1−r)+ varies with radius except for a narrow region, 70 0.4. For 5300 0.4. A rationale based on the curvature of mean velocity gradient profile is proposed to understand the perplexing existence of logarithmic mean velocity profile in very-low-Reynolds-number pipe flows. Beyond ReD=44000, axial turbulence intensity varies linearly with radius within the range of 0.15 < 1−r < 0.7. Flow visualizations and two-point correlations reveal large-scale structures with comparable near-wall azimuthal dimensions at ReD=44000 and 5300 when measured in wall units. When normalized in outer units, streamwise coherence and azimuthal dimension of the large-scale structures in the pipe core away from the wall are also comparable at these two Reynolds numbers.

Journal ArticleDOI
TL;DR: In this article, the scaling of the nonzero components of the Reynolds-stress tensor is presented for numerical channels with Reynolds numbers in the range Reτ=180-2000. But the authors do not consider the effect of the wall-parallel large-scale motions.
Abstract: Budgets for the nonzero components of the Reynolds-stress tensor are presented for numerical channels with Reynolds numbers in the range Reτ=180–2000. The scaling of the different terms is discussed, both above and within the buffer and viscous layers. Above x2+≈150, most budget components scale reasonably well with uτ3/h, but the scaling with uτ4/ν is generally poor below that level. That is especially true for the dissipations and for the pressure-related terms. The former is traced to the effect of the wall-parallel large-scale motions, and the latter to the scaling of the pressure itself. It is also found that the pressure terms scale better near the wall when they are not separated into their diffusion and deviatoric components, but mostly only because the two terms tend to cancel each other in the viscous sublayer. The budgets, together with their statistical uncertainties, are available electronically from http://torroja.dmt.upm.es/channels.

Journal ArticleDOI
TL;DR: In this paper, the physical processes responsible for the transport and deposition of particles and their theoretical modeling are discussed, including stochastic Lagrangian particle tracking and a unified Eulerian advection diffusion approach.
Abstract: This article reviews the physical processes responsible for the transport and deposition of particles and their theoretical modeling. Both laminar and turbulent processes are considered, emphasizing the physical understanding of the various transport mechanisms. State-of-the-art computational methods for determining particle motion and deposition are discussed, including stochastic Lagrangian particle tracking and a unified Eulerian advection-diffusion approach. The theory presented includes Brownian and turbulent diffusion, turbophoresis, thermophoresis, inertial impaction, gravitational settling, electrical forces, and the effects of surface roughness and particle interception. The article describes two example applications: the deposition of particles in the human respiratory tract and deposition in gas and steam turbines.

Journal ArticleDOI
TL;DR: In this paper, the authors present a direct numerical simulation of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees.
Abstract: Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.

Journal ArticleDOI
TL;DR: In this article, the overlap parameters for the logarithmic law are obtained for available turbulent pipe and channel flow data using composite profiles fitted to the mean velocity, and their resulting behavior with Reynolds number is examined for these flows and compared to results from boundary layers.
Abstract: The overlap parameters for the logarithmic law are obtained for available turbulent pipe and channel flow data using composite profiles fitted to the mean velocity. The composite profile incorporates κ, B, and Π as the varying parameters and their resulting behavior with Reynolds number is examined for these flows and compared to results from boundary layers. The von Karman coefficient in channel flow is smaller than the well-established value for zero pressure gradient turbulent boundary layers of 0.384, while in pipe flows it is consistently higher. In contrast, the estimates of the wake parameter Π are the smallest for channel flows and largest for boundary layers. Further, the Superpipe data are reanalyzed to reveal that κ=0.41 is a better value for the von Karman constant in pipe flow. The collective behavior of κ in boundary layers, pipes, and channels reveals that the von Karman coefficient is not universal and exhibits dependence not only on the pressure gradient but also on the flow geometry.

Journal ArticleDOI
TL;DR: A multichannel free-space optical (FSO) communication system based on orbital angular momentum (OAM)-carrying beams is studied and it is found that turbulence induces attenuation and crosstalk among channels.
Abstract: A multichannel free-space optical (FSO) communication system based on orbital angular momentum (OAM)-carrying beams is studied. We numerically analyze the effects of atmospheric turbulence on the system and find that turbulence induces attenuation and crosstalk among channels. Based on a model in which the constituent channels are binary symmetric and crosstalk is a Gaussian noise source, we find optimal sets of OAM states at each turbulence condition studied and determine the aggregate capacity of the multichannel system at those conditions. OAM-multiplexed FSO systems that operate in the weak turbulence regime are found to offer good performance. We verify that the aggregate capacity decreases as the turbulence increases. A per-channel bit-error rate evaluation is presented to show the uneven effects of crosstalk on the constituent channels.

Journal ArticleDOI
TL;DR: In this paper, a laboratory scale bubble column with three different spargers (perforated plate, sintered plate and single hole) has been simulated using three different turbulence closure (k-ɛ, RSM and LES) models, with the purpose of critically comparing their predictions with experimental data.

Journal ArticleDOI
31 Oct 2008-Science
TL;DR: It is demonstrated that in addition to the magnitude of the instantaneous turbulent forces applied on a sediment grain, the duration of these turbulent forces is also important in determining the sediment grain's threshold of motion, and that their product is better suited for specifying such conditions.
Abstract: Fundamental to our understanding of erosional and transport phenomena in earth-surface dynamics and engineering is knowledge of the conditions under which sediment motion will begin when subjected to turbulent flow. The onset criterion currently in use emphasizes the time-averaged boundary shear stress and therefore is incapable of accounting for the fluctuating forces encountered in turbulent flows. We have validated through laboratory experiments and analytical formulation of the problem a criterion based upon the impulse imparted to a sediment grain. We demonstrate that in addition to the magnitude of the instantaneous turbulent forces applied on a sediment grain, the duration of these turbulent forces is also important in determining the sediment grain's threshold of motion, and that their product, or impulse, is better suited for specifying such conditions.

Journal ArticleDOI
TL;DR: In this article, the authors compared computational fluid dynamics (CFD) results using various revised k-e models and large eddy simulation (LES) applied to flow around a high-rise building model with 1:1:2 shape placed within the surface boundary layer.

Journal ArticleDOI
TL;DR: In this paper, a digital filter-based generation of turbulent inflow conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing flows.
Abstract: Using a numerical weather forecasting code to provide the dynamic large-scale inlet boundary conditions for the computation of small-scale urban canopy flows requires a continuous specification of appropriate inlet turbulence. For such computations to be practical, a very efficient method of generating such turbulence is needed. Correlation functions of typical turbulent shear flows have forms not too dissimilar to decaying exponentials. A digital-filter-based generation of turbulent inflow conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing flows. The artificially generated turbulent inflows satisfy the prescribed integral length scales and Reynolds-stress-tensor. The method is much more efficient than, for example, Klein’s (J Comp Phys 186:652–665, 2003) or Kempf et al.’s (Flow Turbulence Combust, 74:67–84, 2005) methods because at every time step only one set of two-dimensional (rather than three-dimensional) random data is filtered to generate a set of two-dimensional data with the appropriate spatial correlations. These data are correlated with the data from the previous time step by using an exponential function based on two weight factors. The method is validated by simulating plane channel flows with smooth walls and flows over arrays of staggered cubes (a generic urban-type flow). Mean velocities, the Reynolds-stress-tensor and spectra are all shown to be comparable with those obtained using classical inlet-outlet periodic boundary conditions. Confidence has been gained in using this method to couple weather scale flows and street scale computations.


Journal ArticleDOI
TL;DR: In this article, the authors reviewed recent work on flow and transport in channels with submerged vegetation, including discussions of turbulence structure, mean velocity profiles, and dispersion. And they showed that the dominant characteristic of the flow is the generation of a shear-layer at the top of the canopy.
Abstract: This paper reviews recent work on flow and transport in channels with submerged vegetation, including discussions of turbulence structure, mean velocity profiles, and dispersion. For submerged canopies of sufficient density, the dominant characteristic of the flow is the generation of a shear-layer at the top of the canopy. The shear-layer generates coherent vortices by Kelvin-Helmholtz (KH) instability. These vortices control the vertical exchange of mass and momentum, influencing both the mean velocity profile, as well as the turbulent diffusivity. For flexible canopies, the passage of the KH vortices generates a progressive wave along the canopy interface, termed monami. The KH vortices formed at the top of the canopy penetrate a distance δe into the canopy. This penetration scale segregates the canopy into an upper layer of rapid transport and a lower layer of slow transport. Flushing of the upper canopy is enhanced by the energetic shear-scale vortices. In the lower layer turbulence is limited to length-scales set by the stem geometry, and the resulting transport is significantly slower than that of the upper layer.

Journal ArticleDOI
TL;DR: In this article, the authors performed large-eddy simulations of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter.
Abstract: In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.

Journal ArticleDOI
TL;DR: In this article, it was shown that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small scale inertial range, and that the time needed to establish this range is shorter than the eddy-turnover time, and is related to dispersive effects.
Abstract: Magnetic fluctuations in the solar wind are distributed according to Kolmogorov’s power law f −5/3 below the ion cyclotron frequency fci. Above this frequency, the observed steeper power law is usually interpreted in two different ways: a dissipative range of the solar wind turbulence or another turbulent cascade, the nature of which is still an open question. Using the Cluster magnetic data we show that after the spectral break the intermittency increases toward higher frequencies, indicating the presence of non-linear interactions inherent to a new inertial range and not to the dissipative range. At the same time the level of compressible fluctuations raises. We show that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small scale inertial range. We conjecture that the time needed to establish this inertial range is shorter than the eddy-turnover time, and is related to dispersive effects. A simple phenomenological model, based on the compressible Hall MHD, predicts the magnetic spectrum ∼ k −7/3+2α , which

Journal ArticleDOI
TL;DR: In this paper, the velocity and pressure fluctuations in the logarithmic and outer layers of turbulent flows are analyzed using spectral information and probability density functions from channel simulations at Reτ 2000.
Abstract: The behaviour of the velocity and pressure fluctuations in the logarithmic and outer layers of turbulent flows is analysed using spectral information and probability density functions from channel simulations at Reτ 2000. Comparisons are made with experimental data at higher Reynolds numbers. It is found, in agreement with previous investigations, that the intensity profiles of the streamwise and spanwise velocity components have logarithmic ranges that are traced to the widening spectral range of scales as the wall is approached. The same is true for the pressure, both theoretically and observationally, but not for the normal velocity or for the tangential stress cospectrum, although even those two quantities have structures with lengths of the order of several hundred times the wall distance. Because the logarithmic range grows longer as the Reynolds number increases, variables which are ‘attached’ in this sense scale in the buffer layer in mixed units. These results give strong support to the attached-eddy scenario proposed by Townsend (1976), but they are not linked to any particular eddy model. The scaling of the outer modes is also examined. The intensity of the streamwise velocity at fixed y/h increases with the Reynolds number. This is traced to the large-scale modes, and to an increased intensity of the ejections but not of the sweeps. Several differences are found between the outer structures of different flows. The outer modes of the spanwise and wall-normal velocities in boundary layers are stronger than in internal flows, and their streamwise velocities penetrate closer to the wall. As a consequence, their logarithmic layers are thinner, and some of their logarithmic slopes are different. The channel statistics are available electronically at http://torroja.dmt.upm.es/ftp/channels/.

Journal ArticleDOI
TL;DR: In this paper, the invariants of the velocity gradient R and Q, rate-of-strain RS and QS, and rate of rotation QW tensors are analyzed across the turbulent/nonturbulent T/NT interface by using a direct numerical simulation of a turbulent plane jet at Re120.
Abstract: The invariants of the velocity gradient R and Q, rate-of-strain RS and QS, and rate-of-rotation QW tensors are analyzed across the turbulent/nonturbulent T/NT interface by using a direct numerical simulation DNS of a turbulent plane jet at Re120. The invariants allow a detailed characterization of the dynamics, geometry and topology of the flow during the entrainment. The invariants Q and QS are almost equal and negative outside the turbulent region close to the T/NT interface, which shows the existence of high values of strain product hence viscous dissipation of kinetic energy at that location. Right at the T/NT interface, the invariants Q W and Q S show that virtually all flow points there are characterized by irrotational dissipation, with no discernible sign of the coherent structures which are known to exist deep inside the turbulent region. Moreover, the invariants of the velocity gradient tensor Q and R show that the classical “teardrop” shape of their associated phase map is not yet formed at the T/NT interface. All the invariants rapidly change after the T/NT interface is crossed into the turbulent region. For instance, the enstrophy density, proportional to QW, is zero in the irrotational flow region and high and more or less constant inside the turbulent region, after it undergoes a sharp jump near the T/NT interface. Inside the turbulent region, at a distance of only 1.7 from the T/NT interface, where is the Kolmogorov microscale, the invariants QW and QS suggest that large scale coherent vortices already exist in the flow. Furthermore, the joint probability density function of Q and R already displays its well known teardrop shape at that location. Moreover, the geometry of the straining or deformation of the fluid elements during the turbulent entrainment process is preferentially characterized by biaxial expansion with S:S:S=2:1:3,where S, S, and S are the eigenvalues of the rate-of-strain tensor arranged in descending order. Based on an analysis of the invariants, many aspects of the flow topology inside the turbulent region at a distance of only 1.7 from the T/NT interface are already similar to those observed deep inside the turbulent region. © 2008 American Institute of Physics. DOI: 10.1063/1.2912513

Journal ArticleDOI
TL;DR: This paper extends NSCBC to account for convection and pressure gradients in boundary planes, resulting in a 3D-NSCBC approach that brings a drastic reduction of flow distortion and numerical reflection, even in regions of strong transverse convection.