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Werner J A Dahm - One of the best experts on this subject based on the ideXlab platform.

  • direct assessment of vorticity alignment with local and nonlocal Strain Rates in turbulent flows
    Physics of Fluids, 2008
    Co-Authors: Peter E Hamlington, Jorg Schumacher, Werner J A Dahm
    Abstract:

    A direct Biot-Savart integration is used to decompose the Strain Rate into its local and nonlocal constituents, allowing the vorticity alignment with the local and nonlocal Strain Rate eigenvectors to be investigated. These Strain Rate Tensor constituents are evaluated in a turbulent flow using data from highly resolved direct numerical simulations. While the vorticity aligns preferentially with the intermediate eigenvector of the combined Strain Rate, as has been observed previously, the present results, for the first time, clearly show that the vorticity aligns with the most extensional eigenvector of the nonlocal Strain Rate. This, in turn, reveals a significant linear contribution to the vortex stretching dynamics in turbulent flows.

  • local and nonlocal Strain Rate fields and vorticity alignment in turbulent flows
    Physical Review E, 2008
    Co-Authors: Peter E Hamlington, Jorg Schumacher, Werner J A Dahm
    Abstract:

    Local and nonlocal contributions to the total Strain Rate Tensor S(ij) at any point x in a flow are formulated from an expansion of the vorticity field in a local spherical neighborhood of radius R centered on x. The resulting exact expression allows the nonlocal (background) Strain Rate Tensor S(ij)(B)(x) to be obtained from S(ij)(x). In turbulent flows, where the vorticity naturally concentRates into relatively compact structures, this allows the local alignment of vorticity with the most extensional principal axis of the background Strain Rate Tensor to be evaluated. In the vicinity of any vortical structure, the required radius R and corresponding order n to which the expansion must be carried are determined by the viscous length scale lambda(nu). We demonstRate the convergence to the background Strain Rate field with increasing R and n for an equilibrium Burgers vortex, and show that this resolves the anomalous alignment of vorticity with the intermediate eigenvector of the total Strain Rate Tensor. We then evaluate the background Strain field S(ij)(B)(x) in direct numerical simulations of homogeneous isotropic turbulence where, even for the limited R and n corresponding to the truncated series expansion, the results show an increase in the expected equilibrium alignment of vorticity with the most extensional principal axis of the background Strain Rate Tensor.

  • Reynolds stress closure for nonequilibrium effects in turbulent flows
    Physics of Fluids, 2008
    Co-Authors: Peter E Hamlington, Werner J A Dahm
    Abstract:

    From consideration of turbulence anisotropy dynamics due to spatial or temporal variations in the mean Strain Rate, a new Reynolds stress closure for nonequilibrium effects in turbulent flows has been developed. This closure, formally derived from the Reynolds stress anisotropy transport equation, results in an effective Strain Rate Tensor that accounts for the Strain Rate history to which the turbulence has been subjected. In contrast to prior nonequilibrium models that have sought to address nonequilibrium effects via changes in the eddy viscosity, the present approach accounts for nonequilibrium effects in the fundamental relation between the anisotropy Tensor and the Strain Rate Tensor. The time-local form of the nonequilibrium closure can be readily implemented in place of the classical equilibrium Boussinesq closure on which most existing computational frameworks are currently based. This new closure is applied here to four substantially different classes of nonequilibrium test problems. Results sho...

Tianshou Zhao - One of the best experts on this subject based on the ideXlab platform.

Zhenhua Chai - One of the best experts on this subject based on the ideXlab platform.

  • the computation of Strain Rate Tensor in multiple relaxation time lattice boltzmann model
    Computers & Mathematics With Applications, 2018
    Co-Authors: Wenhuan Zhang, Changsheng Huang, Yihang Wang, Baochang Shi, Shibo Kuang, Zhenhua Chai
    Abstract:

    Abstract The multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is an important class of LB model with lots of advantages over the traditional single-relaxation-time (SRT) LB model. Generally, the computation of Strain Rate Tensor is crucial for the MRT-LB simulations of some complex flows. At present, only two formulae are available to compute the Strain Rate Tensor in the MRT LB model. One is to compute the Strain Rate Tensor using the non-equilibrium parts of macroscopic moments (Yu formula). The other is to compute the Strain Rate Tensor using the non-equilibrium parts of density distribution functions (Chai formula). The mathematical expressions of these two formulae are so different that we do not know which formula to choose for computing the Strain Rate Tensor in the MRT LB model. To overcome this problem, this paper presents a theoretical study of the relationship between Chai and Yu formulae. The results show that the Yu formula can be deduced from the Chai formula, although they have their own advantages and disadvantages. In particular, the Yu formula is computationally more efficient, while the Chai formula is applicable to more lattice patterns of the MRT LB models. Furthermore, the derivation of the Yu formula in a particular lattice pattern from the Chai formula is more convenient than that proposed by Yu et al.

  • effect of the forcing term in the multiple relaxation time lattice boltzmann equation on the shear stress or the Strain Rate Tensor
    Physical Review E, 2012
    Co-Authors: Zhenhua Chai, Tianshou Zhao
    Abstract:

    In this work, the effect of the forcing term (or external force) in the multiple-relaxation-time lattice Boltzmann equation (MRTLBE) on the shear stress or the Strain Rate Tensor is studied theoretically and numerically. Through a Chapman-Enskog analysis and numerical simulations, we show that the shear stress (or the Strain Rate Tensor) derived from the MRTLBE is second-order accuRate in space. We then examine the influence of the forcing term on the shear stress or the Strain Rate Tensor, and demonstRate that the forcing term effect must be included when the shear stress or the Strain Rate Tensor is computed with the nonequilibrium part of the distribution function.

Peter E Hamlington - One of the best experts on this subject based on the ideXlab platform.

  • direct assessment of vorticity alignment with local and nonlocal Strain Rates in turbulent flows
    Physics of Fluids, 2008
    Co-Authors: Peter E Hamlington, Jorg Schumacher, Werner J A Dahm
    Abstract:

    A direct Biot-Savart integration is used to decompose the Strain Rate into its local and nonlocal constituents, allowing the vorticity alignment with the local and nonlocal Strain Rate eigenvectors to be investigated. These Strain Rate Tensor constituents are evaluated in a turbulent flow using data from highly resolved direct numerical simulations. While the vorticity aligns preferentially with the intermediate eigenvector of the combined Strain Rate, as has been observed previously, the present results, for the first time, clearly show that the vorticity aligns with the most extensional eigenvector of the nonlocal Strain Rate. This, in turn, reveals a significant linear contribution to the vortex stretching dynamics in turbulent flows.

  • local and nonlocal Strain Rate fields and vorticity alignment in turbulent flows
    Physical Review E, 2008
    Co-Authors: Peter E Hamlington, Jorg Schumacher, Werner J A Dahm
    Abstract:

    Local and nonlocal contributions to the total Strain Rate Tensor S(ij) at any point x in a flow are formulated from an expansion of the vorticity field in a local spherical neighborhood of radius R centered on x. The resulting exact expression allows the nonlocal (background) Strain Rate Tensor S(ij)(B)(x) to be obtained from S(ij)(x). In turbulent flows, where the vorticity naturally concentRates into relatively compact structures, this allows the local alignment of vorticity with the most extensional principal axis of the background Strain Rate Tensor to be evaluated. In the vicinity of any vortical structure, the required radius R and corresponding order n to which the expansion must be carried are determined by the viscous length scale lambda(nu). We demonstRate the convergence to the background Strain Rate field with increasing R and n for an equilibrium Burgers vortex, and show that this resolves the anomalous alignment of vorticity with the intermediate eigenvector of the total Strain Rate Tensor. We then evaluate the background Strain field S(ij)(B)(x) in direct numerical simulations of homogeneous isotropic turbulence where, even for the limited R and n corresponding to the truncated series expansion, the results show an increase in the expected equilibrium alignment of vorticity with the most extensional principal axis of the background Strain Rate Tensor.

  • Reynolds stress closure for nonequilibrium effects in turbulent flows
    Physics of Fluids, 2008
    Co-Authors: Peter E Hamlington, Werner J A Dahm
    Abstract:

    From consideration of turbulence anisotropy dynamics due to spatial or temporal variations in the mean Strain Rate, a new Reynolds stress closure for nonequilibrium effects in turbulent flows has been developed. This closure, formally derived from the Reynolds stress anisotropy transport equation, results in an effective Strain Rate Tensor that accounts for the Strain Rate history to which the turbulence has been subjected. In contrast to prior nonequilibrium models that have sought to address nonequilibrium effects via changes in the eddy viscosity, the present approach accounts for nonequilibrium effects in the fundamental relation between the anisotropy Tensor and the Strain Rate Tensor. The time-local form of the nonequilibrium closure can be readily implemented in place of the classical equilibrium Boussinesq closure on which most existing computational frameworks are currently based. This new closure is applied here to four substantially different classes of nonequilibrium test problems. Results sho...

Lennart Lofdahl - One of the best experts on this subject based on the ideXlab platform.

  • an estimate of the pressure Strain Rate Tensor in a plane cylinder wake
    Fifth European Turbulence Conference in Siena Italy July 5-8 1994, 1995
    Co-Authors: Dag Aronson, Lennart Lofdahl
    Abstract:

    The far wake of a cylinder has been studied in order to provide accuRate experimental information on the normal component of the pressure-Strain Rate Tensor by balancing the turbulent kinetic energy budget and Reynolds stress transport (RST) equations. A non-isotropic dissipation Rate Tensor was found, thus fulfilling the basic physical integral conStraint of the diffusion and indicating an energy redistribution as described by the pressure-Strain Rate correlations.

  • the plane wake of a cylinder an estimate of the pressure Strain Rate Tensor
    Physics of Fluids, 1994
    Co-Authors: Dag Aronson, Lennart Lofdahl
    Abstract:

    The modelling of the pressure Strain Rate terms is an important issue in the improvement of the generality of closure models for the Reynolds stress transport (RST) equations. A part of these efforts is to provide accuRate experimental information on the pressure-Strain Rate Tensor, which in turn requires equally accuRate information on the dissipation and diffusion Tensors. Here the far wake of a cylinder was studied in order to enable the required balances of the RST equations. The experimental results indicate a nonisotropic dissipation Tensor, and show the energy redistribution between the different components as described by the pressure-Strain Rate correlations. Comparisons are made between the experimentally determined pressure-Strain Rate distributions and the corresponding distributions predicted by closure models.