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Nov 2007

Volume 19, Issue 11, Articles (11xxxx)

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Phys. Fluids 19, 114108 (2007); http://dx.doi.org/10.1063/1.2800371 (11 pages)

J. P. Kubitschek and P. D. Weidman
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Self-consistent high-Reynolds-number asymptotics for zero-pressure-gradient turbulent boundary layers

Peter A. Monkewitz, Kapil A. Chauhan, and Hassan M. Nagib

Phys. Fluids 19, 115101 (2007); http://dx.doi.org/10.1063/1.2780196 (12 pages) | Cited 30 times

Online Publication Date: 5 November 2007

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The asymptotic behavior of mean velocity and integral parameters in flat plate turbulent boundary layers under zero pressure gradient are studied for Reynolds numbers approaching infinity. Using the classical two-layer approach of Millikan, Rotta, and Clauser with a logarithmic velocity profile in the overlap region between “inner” and “outer” layers, a fully self-consistent leading-order description of the mean velocity profile and all integral parameters is developed. It is shown that this description fits most high Reynolds number data, and in particular their Reynolds number dependence, exceedingly well; i.e., within experimental errors.
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47.27.nb Boundary layer turbulence
46.70.De Beams, plates, and shells

Estimating intermittency exponent in neutrally stratified atmospheric surface layer flows: A robust framework based on magnitude cumulant and surrogate analyses

Sukanta Basu, Efi Foufoula-Georgiou, Bruno Lashermes, and Alain Arnéodo

Phys. Fluids 19, 115102 (2007); http://dx.doi.org/10.1063/1.2786001 (11 pages) | Cited 5 times

Online Publication Date: 7 November 2007

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This study proposes a novel framework based on magnitude cumulant and surrogate analyses to reliably detect the presence of intermittency and estimate the intermittency coefficient from short-length coarse-resolution turbulent time series. Intermittency coefficients estimated from a large number of neutrally stratified atmospheric surface layer turbulent series from various field campaigns are shown to remarkably concur with well-known laboratory experimental results. In addition, surrogate-based hypothesis testing significantly reduces the likelihood of detecting a spurious nonzero intermittency coefficient from nonintermittent series. The discriminatory power of the proposed framework is promising for addressing the unresolved question of how atmospheric stability affects the intermittency properties of boundary layer turbulence.
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47.27.nb Boundary layer turbulence
47.20.Ib Instability of boundary layers; separation
47.55.Hd Stratified flows

Conditional source-term estimation with laminar flamelet decomposition in large eddy simulation of a turbulent nonpremixed flame

M. Wang and W. K. Bushe

Phys. Fluids 19, 115103 (2007); http://dx.doi.org/10.1063/1.2795210 (12 pages) | Cited 1 time

Online Publication Date: 8 November 2007

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Conditional source-term estimation (CSE) is a method to close the mean chemical reaction source term based on the conditional moment closure hypothesis. It has been shown previously to be successful in a priori tests against direct numerical simulation data and in the large eddy simulation (LES) of a nonpremixed flame using reduced chemistry. Laminar flamelet decomposition (LFD) is a method to incorporate more complex chemistry into CSE by using laminar flamelets as basis functions and inverting an integral equation for a basis function coefficient vector which describes the linear combination of flamelets that best approximates the conditional average of certain scalar fields within an ensemble of points in a flow field. This coefficient vector is used to obtain the conditional average of the chemical source term for this ensemble of points which can then be transformed into the unconditional average chemical source term in the transport equations of reactive scalars in the flow. This study focuses on the application of CSE with LFD in LES of the Sandia D-flame. The simulation results show that LFD is able to predict temperature and major species well with both steady and unsteady flamelet libraries. However, in order to predict NO well, it is necessary to use a mixed library that includes both steady and unsteady flamelets. The computational cost of this method is low because very few transport equations need to be solved (specifically, equations for the filtered continuity, momentum, mixture fraction and its variance, temperature and the mass fraction of CO are solved) while other species mass fractions can be obtained directly from the flamelet library.
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47.70.Pq Flames; combustion
47.27.E- Turbulence simulation and modeling
47.15.-x Laminar flows

Scalar gradient and small-scale structure in turbulent premixed combustion

Seung Hyun Kim and Heinz Pitsch

Phys. Fluids 19, 115104 (2007); http://dx.doi.org/10.1063/1.2784943 (14 pages) | Cited 11 times

Online Publication Date: 9 November 2007

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Scalar gradient and small-scale structure in turbulent premixed combustion are investigated with emphasis on flame thickening. A Lagrangian-type equation for the evolution of the scalar gradient following an isoscalar surface is presented, which is useful in studying physical mechanisms for the scalar gradient evolution in premixed flames. The terms in the Lagrangian-type form of the scalar gradient equation are analyzed using direct numerical simulation data for statistically one-dimensional planar flames with high intensity turbulence. Two flames with Da<1 and Ka>1 are investigated. Results show that the curvature plays an important role in the evolution of the scalar gradient in turbulent premixed flames. The tangential strain rate, which is the major term to steepen the scalar gradient, is shown to be negatively correlated with the curvature due to the relation between the dilatation and the displacement speed of isoscalar surfaces. This represents the effects of heat release on the scalar gradient evolution. The relation between the dilatation and the displacement speed of isoscalar surfaces is also related to the presence of negative dilatation in premixed flames. The alignment characteristics of the flame normal with the principal axis of the strain are also investigated in relation to the characteristics of the tangential strain rate. Variations of the curvature, weighted by the density and the diffusivity, along the normal to the isosurfaces are found to be the major sink term in the scalar gradient equation and to have a negative correlation with the magnitude of the curvature. This provides evidence that smaller-scale wrinkling is more responsible for flame thickening. In the preheat zone, the tangential strain rate is balanced with the variation of the mass flux normal to the isosurface, and the strength of the thickening process is determined by the curvature variation term. In the reaction zone, the evolution of the scalar gradient is determined by the balance of the tangential strain rate and the curvature variation term. It is also shown that the thickening process in the reaction zone is much weaker than that in the preheat zone. In one of the simulated flames, a sudden drop of the strength of flame thickening in the reaction zone is observed.
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82.33.Vx Reactions in flames, combustion, and explosions
47.70.Pq Flames; combustion
47.70.Fw Chemically reactive flows
47.27.-i Turbulent flows
02.60.-x Numerical approximation and analysis

A self-adapting turbulence model for flow simulation at any mesh resolution

J. Blair Perot and Jason Gadebusch

Phys. Fluids 19, 115105 (2007); http://dx.doi.org/10.1063/1.2780195 (11 pages) | Cited 2 times

Online Publication Date: 12 November 2007

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A two-equation transport model is used to model turbulence at any mesh resolution, from Reynolds-averaged Navier–Stokes (RANS), to large eddy simulations (LES), to direct numerical simulations. The two-equation model used is a slight modification of the standard k/ε model that allows the backscatter of energy to resolved scales. A mathematical explanation is provided for why RANS models (such as this two-equation model) are applicable to LES. The model automatically adapts to the mesh resolution provided and no interaction from the user is necessary. This approach is tested on the problem of moderately high Reynolds number isotropic decaying turbulence and gives good predictions at any mesh resolution and with different initial conditions. A detailed analysis shows that at LES resolutions the solution remains fully unsteady and three-dimensional and does not approach a RANS-like solution.
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47.27.-i Turbulent flows
47.10.A- Mathematical formulations
47.11.Fg Finite element methods
02.70.Dh Finite-element and Galerkin methods

Probability density function modeling of scalar mixing from concentrated sources in turbulent channel flow

J. Bakosi, P. Franzese, and Z. Boybeyi

Phys. Fluids 19, 115106 (2007); http://dx.doi.org/10.1063/1.2803348 (17 pages) | Cited 7 times

Online Publication Date: 14 November 2007

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Dispersion of a passive scalar from concentrated sources in fully developed turbulent channel flow is studied with the probability density function (PDF) method. The joint PDF of velocity, turbulent frequency and scalar concentration is represented by a large number of Lagrangian particles. A stochastic near-wall PDF model combines the generalized Langevin model of Haworth and Pope [Phys. Fluids 29, 387 (1986)] with Durbin's [J. Fluid Mech. 249, 465 (1993)] method of elliptic relaxation to provide a mathematically exact treatment of convective and viscous transport with a nonlocal representation of the near-wall Reynolds stress anisotropy. The presence of walls is incorporated through the imposition of no-slip and impermeability conditions on particles without the use of damping or wall-functions. Information on the turbulent time scale is supplied by the gamma-distribution model of van Slooten et al. [Phys. Fluids 10, 246 (1998)] . Two different micromixing models are compared that incorporate the effect of small scale mixing on the transported scalar: the widely used interaction by exchange with the mean and the interaction by exchange with the conditional mean model. Single-point velocity and concentration statistics are compared to direct numerical simulation and experimental data at Reτ = 1080 based on the friction velocity and the channel half width. The joint model accurately reproduces a wide variety of conditional and unconditional statistics in both physical and composition space.
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47.27.nd Channel flow
47.51.+a Mixing
47.61.Ne Micromixing
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.27.nb Boundary layer turbulence
47.27.te Turbulent convective heat transfer

The stress generated by non-Brownian fibers in turbulent channel flow simulations

J. J. J. Gillissen, B. J. Boersma, P. H. Mortensen, and H. I. Andersson

Phys. Fluids 19, 115107 (2007); http://dx.doi.org/10.1063/1.2800041 (8 pages) | Cited 13 times

Online Publication Date: 14 November 2007

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Turbulent fiber suspension channel flow is studied using direct numerical simulation. The effect of the fibers on the fluid mechanics is governed by a stress tensor, involving the distribution of fiber position and orientation. Properties of this function in channel flow are studied by computing the trajectories and orientations of individual particles, referred to as the particle method. It is shown that, due to computer restrictions, the instantaneous stress in channel flow cannot be simulated directly with the particle method. To approximate the stress we compute the second-order moment of the fiber distribution function. This method involves an unknown subgrid term, which is modeled as diffusion. The accuracy of the moment approximation is studied by comparing Reynolds averaged stress to results obtained from the particle method. It is observed that the errors are ∼ 1% for y+>20, and ∼ 20% for y+<20. The model is improved by applying a wall damping function to the diffusivity. The moment approximation is used to simulate drag-reduced channel flow. A simplified model for fiber stress is introduced as fiber viscosity times rate of strain, where fiber viscosity is defined as the ratio of Reynolds averaged dissipation due to fiber stress and Reynolds averaged dissipation due to Newtonian stress. Fluid velocity statistics predicted by the simple model compare very well to those obtained from the moment approximation. This means that the effect of fibers on turbulent channel flow is equivalent to an additional Reynolds averaged viscosity.
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47.27.-i Turbulent flows
47.11.-j Computational methods in fluid dynamics
47.27.E- Turbulence simulation and modeling
47.27.nd Channel flow
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.85.lb Drag reduction

A Reynolds stress closure description of separation control with vortex generators in a plane asymmetric diffuser

Olle Törnblom and Arne V. Johansson

Phys. Fluids 19, 115108 (2007); http://dx.doi.org/10.1063/1.2800877 (15 pages) | Cited 9 times

Online Publication Date: 16 November 2007

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A way to model the effects of streamwise vortices in a turbulent flow with one homogeneous direction is presented. The Reynolds averaged Navier-Stokes equations are solved with a differential Reynolds stress turbulence model. Assuming that the vortices can be approximated with the Lamb-Oseen model, wall-normal Reynolds stress distributions are calculated, corresponding to the spanwise variances of the estimated velocity distribution downstream of the vortex generators. The Reynolds stress contributions that are due to the vortex generators are added to the Reynolds stresses from the turbulence model so as to mimic the increased mixing due to the vortex generators. Volume forces are applied also in the mean momentum equations to account for the drag of the vortex generators. The model is tested and compared with experimental data from a plane asymmetric diffuser flow which is separating without vortex generators. The results indicate that the model is able to mimic the major features of vortex generator flow control and that the flow case in question is susceptible to separation control. The model results show that the pressure recovery of the diffuser could be increased by almost 10% by applying vortex generators and that, if keeping the shape of the vortex generators fixed, their optimal position is close to the diffuser inlet. Computations also indicated that the time to re-establish the separation zone when the control suddenly is turned off is substantially longer than the time it takes to remove the separation after the control is turned on again. Some work on adapting a differential Reynolds stress turbulence model was necessary in order to make it capable of realistic predictions of the asymmetric diffuser flow in which the vortex generator model is tested. However, the main focus of the article is on the modelling of vortex generator effects.
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47.10.ad Navier-Stokes equations
47.27.-i Turbulent flows
47.32.-y Vortex dynamics; rotating fluids
47.85.L- Flow control

Coherent vortices in high resolution direct numerical simulation of homogeneous isotropic turbulence: A wavelet viewpoint

Naoya Okamoto, Katsunori Yoshimatsu, Kai Schneider, Marie Farge, and Yukio Kaneda

Phys. Fluids 19, 115109 (2007); http://dx.doi.org/10.1063/1.2771661 (13 pages) | Cited 22 times

Online Publication Date: 19 November 2007

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Coherent vortices are extracted from data obtained by direct numerical simulation (DNS) of three-dimensional homogeneous isotropic turbulence performed for different Taylor microscale Reynolds numbers, ranging from Reλ = 167 to 732, in order to study their role with respect to the flow intermittency. The wavelet-based extraction method assumes that coherent vortices are what remains after denoising, without requiring any template of their shape. Hypotheses are only made on the noise that, as the simplest guess, is considered to be additive, Gaussian, and white. The vorticity vector field is projected onto an orthogonal wavelet basis, and the coefficients whose moduli are larger than a given threshold are reconstructed in physical space, the threshold value depending on the enstrophy and the resolution of the field, which are both known a priori. The DNS dataset, computed with a dealiased pseudospectral method at resolutions N = 2563, 5123, 10243, and 20483, is analyzed. It shows that, as the Reynolds number increases, the percentage of wavelet coefficients representing the coherent vortices decreases; i.e., flow intermittency increases. Although the number of degrees of freedom necessary to track the coherent vortices remains small (e.g., 2.6% of N = 20483 for Reλ = 732), it preserves the nonlinear dynamics of the flow. It is thus conjectured that using the wavelet representation the number of degrees of freedom to compute fully developed turbulent flows could be reduced in comparison to the standard estimation based on Kolmogorov’s theory.
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47.27.Gs Isotropic turbulence; homogeneous turbulence
47.32.cb Vortex interactions
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