Bing-Chen Wang

A dynamic nonlinear subgrid-scale stress model

In this paper, a dynamic subgrid scale (SGS) stress model based on Speziale’s quadratic nonlinear constitutive relation [C. G. Speziale, J. Fluid Mech. 178, 459 (1987); T. B. Gatski and C. G. Speziale, J. Fluid Mech. 254, 59 (1993)] is proposed, which includes the conventional dynamic SGS model as its first-order approximation. The closure method utilizes both the symmetric and antisymmetric parts of the resolved velocity gradient, and allows for a nonlinear anisotropic representation of the SGS stress tensor. Unlike the conventional Smagorinsky type modeling approaches, the proposed model does not require an alignment between the SGS stress tensor and the resolved strain rate tensor. It exhibits significant flexibility in self-calibration of the model coefficients, and local stability without the need for plane averaging to avoid excessive backscatter of SGS turbulence kinetic energy and potential modeling singularity problems. It also allows for variable tensorial geometric relations between the SGS str...

New dynamic subgrid-scale heat flux models for large-eddy simulation of thermal convection based on the general gradient diffusion hypothesis

Three new dynamic tensor thermal diffusivity subgrid-scale (SGS) heat flux (HF) models are proposed for large-eddy simulation of thermal convection. The constitutive relations for the proposed modelling approaches represent the most general explicit algebraic formulations possible for the family of SGS HF models constructed using the resolved temperature gradient and SGS stress tensor. As a result, these three new models include a number of previously proposed dynamic SGS HF models as special cases. In contrast to the classical dynamic eddy thermal diffusivity SGS HF model, which strictly requires the SGS heat flux be aligned with the negative of the resolved temperature gradient, the three new models proposed here admit more degrees of freedom, and consequently provide a more realistic geometrical and physical representation of the SGS HF vector. To validate the proposed models, numerical simulations have been performed based on two benchmark test cases of neutrally and unstably stratified horizontal channel flows.

Turbulence topologies predicted using large eddy simulations

In this paper, turbulence topologies related to the invariants of the resolved velocity gradient and strain rate tensors are studied based on large eddy simulation. The numerical results presented in the paper were obtained using two dynamic models, namely, the conventional dynamic model of Lilly and a recently developed dynamic nonlinear subgrid scale (SGS) model. In contrast to most of the previous research investigations which have mainly focused on isotropic turbulence, the present study examines the influence of near-wall anisotropy on the flow topologies. The SGS effect on the so-called SGS dissipation of the discriminant is examined and it is shown that the SGS stress contributes to the deviation of the flow topology of real turbulence from that of the ideal restricted Euler flow. The turbulence kinetic energy (TKE) transfer between the resolved and subgrid scales of motion is studied, and the forward and backward scatters of TKE are quantified in the invariant phase plane. Some interesting phenomenological results have also been obtained, including a wing-shaped contour pattern for the density of the resolved enstrophy generation and the near-wall dissipation shift of the peak location (mode) in the joint probability density function of the invariants of the resolved strain rate tensor. The newly observed turbulence phenomenologies are believed to be important and an effort has been made to explain them on an analytical basis.

A General Dynamic Linear Tensor-Diffusivity Subgrid-Scale Heat Flux Model for Large-Eddy Simulation of Turbulent Thermal Flows

In this article, a general dynamic linear tensor diffusivity model is proposed for representing the subgrid-scale (SGS) heat flux (HF). The tensor diffusivity for the model is an inhomogeneous linear function of the resolved strain and rotation rate tensors, and includes three conventional dynamic SGS HF modeling approaches as special cases. In contrast to the dynamic SGS eddy diffusivity modeling approach, the proposed model admits more degrees of freedom for representing the SGS thermal diffusivity, allows for nonalignment between the SGS HF and resolved temperature gradient, and consequently provides a more realistic geometric representation of the SGS heat flux. To validate the proposed modeling approach, numerical simulations have been performed based on a combined forced- and natural-convention flow in a vertical channel with a Reynolds number and a Grashof number Gr = 9.6 × 105. In comparison with the reported direct numerical simulation data and the results obtained using the conventional dynamic SGS eddy diffusivity model, it is shown that the proposed model is able to provide good predictions of various flow quantities at the resolved scale and, more important, offer new insights into near-wall flow physics at the subgrid scale.

Numerical study of pulsatile channel flows undergoing transition triggered by a modelled stenosis

In this research, we numerically investigate the physics of pulsatile flows confined within a 3-dimensional channel with a modelled stenosis formed eccentrically on the upper wall using the method of large-eddy simulation (LES). An advanced dynamic nonlinear subgrid-scale stress model was utilized to conduct numerical simulations and its predictive performance was examined in comparison with that of the conventional dynamic model. The Womersley number tested in the simulation was fixed at 10.5 and the Reynolds numbers tested were set to 750 and 2000, which are characteristics of human blood flows in large arteries. An in-house LES code, based on curvilinear Cartesian coordinates, has been developed to conduct the unsteady numerical simulations using three different grid systems. The physical characteristics of the flow field have been studied in terms of the resolved mean velocity, turbulence kinetic energy, viscous wall shear stress, resolved and subgrid-scale turbulent shear stresses, local kinetic ener...

Quasi-Periodicity of the Drag Coefficient and Nusselt Number Induced by Taylor-Görtler Vortices

In this article, we investigate the impact of Taylor-Görtler vortices on the drag coefficient and Nusselt number in a heated rotating channel flow using the method of large-eddy simulation (LES). We report the observation of quasi-periodicity of the drag coefficient and Nusselt number in the spanwise direction induced by Taylor-Görtler vortices. The physical conditions under which the extrema of the drag coefficient and Nusselt number occur are investigated. The turbulent flow field is characterized by a Reynolds number Re τ = 150 and various rotation numbers Ro τ ranging from 0 to 7.5. Numerical simulations are performed using two advanced dynamic subgrid-scale stress and heat flux models; namely, the dynamic nonlinear model (DNM) for closure of the filtered momentum equation and the dynamic full linear tensor thermal diffusivity model (DFLTDM) for closure of the filtered thermal energy equation.

Large-eddy simulation of turbulent flow and structures in a square duct roughened with perpendicular and V-shaped ribs

In this paper, highly disturbed turbulent flows confined within a square duct with uniformly spaced V-shaped ribs mounted on one wall are investigated using large-eddy simulation (LES). Two V-shaped (60° and 45°) rib cases are studied in comparison with the perpendicular (90°) rib case. The LES results are validated against a set of water-channel measurement data newly acquired by the authors. The effects of different shaped ribs on turbulent flow are systematically studied in terms of the mean velocity field, turbulence statistics, and coherent structures. The results show that strong secondary flows in the pattern of a pair of large symmetrical streamwise-elongated vortices exist in all three rib cases. The impacts of rib geometry on turbulent coherent structures are investigated using vortex identifiers, temporal autocorrections, spatial two-point autocorrelations, and velocity spectra.

Large-eddy simulation of turbulent flow and dispersion over a matrix of wall-mounted cubes

Turbulent flow over a matrix of wall-mounted cubic obstacles along with continuous release of a passive scalar from a ground-level point source has been investigated using wall-modeled large-eddy simulation (LES). The cubes are fully submerged in a modeled urban atmospheric boundary layer with high turbulence intensities. An inlet boundary condition has been proposed to reproduce the high turbulence level of the approaching flow based on generation of grid turbulence. Coherent flow structures induced by the cubes and their influences on dispersion of the concentration plume in the context of the highly disturbed flow are also investigated. The spatial evolution and temporal cascades of the kinetic and scalar energies have been examined in terms of their transport equations and resolved spectra. In order to validate the LES approach, numerical predictions of turbulence statistics for both velocity and concentration fields have been thoroughly validated against a set of comprehensive water-channel measureme...

An Integral-type Dynamic Localization Two-parameter Subgrid-scale Model: Formulation and Simulation

The dynamic two-parameter mixed model (DMM2) has been recently introduced into the large eddy simulation (LES) by Salvetti and Banerjee [“a priori Tests of A New Dynamic Subgrid-Scale Model for Finite-Difference Large-Eddy Simulations”, Phys. Fluids, 7 (1995) pp. 2831–2847], Horiuti [“A New Dynamic Two-Parameter Mixed Model for Large-Eddy Simulation”, Phys. Fluids, 9 (1997) pp. 3443–3464], Meneveau and Katz [“Dynamic Testing of Subgrid Models in Large Eddy Simulation Based on the Cermano Identity”, Phys. Fluids, 11 (1999) pp. 245–247], Sarghini et al. [“Scale-Similar Models for Large-eddy Simulations”, Phys. Fluids, 11 (1999) pp. 1596–1607], and Morinishi and Vasilyev [“A Recommended Modification to the Dynamic Two-Parameter Mixed Subgrid Scale Model for Large Eddy Simulation of Wall Bounded Turbulent Flow”, Phys. Fluids, 13 (2001) pp. 3400–3410]. However, current approaches in the literature are mathematically inconsistent. In this paper, the DMM2 has been optimized using the functional variational method. The mathematical inconsistency has been removed and a governing system of two integral equations for the model coefficients of the DMM2 has been obtained. Numerical simulation of turbulent Couette flow is used to validate the new optimal DMM2. The numerical results agree with the direct numerical simulation (DNS) data, the classical wall law and experimental correlations that are available in the literature.

Hybrid RANS/LES of turbulent flow in a rotating rib-roughened channel

In this paper, we investigate the effect of the Coriolis force on the flow field in a rib-roughened channel subjected to either clockwise or counter-clockwise system rotation using hybrid RANS/LES based on wall modelling. A simplified dynamic forcing scheme incorporating backscatter is proposed for the hybrid simulation approach. The flow is characterized by a Reynolds number of Re = 1.5 × 104 and a rotation number Ro ranging from −0.6 to 0.6. The mean flow speed and turbulence level near the roughened wall are enhanced under counter-clockwise rotation and suppressed under clockwise rotation. The Coriolis force significantly influences the stability of the wall shear layer and the free shear layers generated by the ribs. Consequently, it is interesting to observe that the classification of the roughness type relies not only on the pitch ratio, but also on the rotation number in the context of rotating rib-roughened flows. In order to validate the present hybrid approach, the first- and second-order statistical moments of the velocity field obtained from the simulations are thoroughly compared with the available laboratory measurement data.