Zixuan Yang

Large eddy simulation of rotating turbulent channel flow with a new dynamic global-coefficient nonlinear subgrid stress model

In this paper, a new dynamic global-coefficient nonlinear subgrid scale (SGS) model is proposed for large eddy simulation (LES) of rotating turbulent channel flow. The basic model is a nonlinear model with a tensorial polynomial relation between the SGS stress and the resolved strain rate tensor. A new dynamic procedure is proposed to determine the model coefficients of the nonlinear model. The new dynamic method is derived from the globally averaged transport equation of the Reynolds shear stress, on which the rotation has strong and direct effects. The new dynamic nonlinear SGS model is examined in rotating turbulent channel at Re=umh/ν=7000, Ro=2Ωh/um =0.3 and 0.6, where Reynolds number Re and Rotation number Ro are defined by bulk mean velocity um , half channel width h, kinematic viscosity ν and angular velocity of spanwise rotation Ω. The statistical results obtained from the new model agree well with those from direct numerical simulation (DNS). The new model also successfully predicts the major st...

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.

A Sharp-Interface Immersed Boundary Method for Simulating Incompressible Flows with Arbitrarily Deforming Smooth Boundaries

We develop a sharp interface immersed boundary (IB) method to simulate the interactions between fluid flows and deformable moving bodies. Fluid–solid interfaces are captured using a level-set (LS) function, which is updated at every time step by a reinitialization procedure. Motions of solid bodies are dynamically coupled with fluid flows by calculating the fluid forces exerted on solid bodies. The accuracy and robustness of the LS-based IB method are tested systematically in the context of several benchmark cases and self-propelled fish swimming. The effects of computational parameters on the accuracy of deformable body capturing are analyzed. It is found that the algorithm performs well in simulating the flow motions surrounding the deforming and moving bodies.

A modified nonlinear sub-grid scale model for large eddy simulation with application to rotating turbulent channel flows

In this paper, a dynamic subgrid scale (SGS) stress model is proposed by modifying the existing quadratic nonlinear model. Unlike the conventional eddy viscosity models, the nonlinear model suggests an anisotropic tensorial polynomial relation between the SGS stress and the resolved strain rate tensor. Examined in the rotating turbulent channel flow, the previous nonlinear model is found unable to successfully predict the turbulent kinetic energy and the Reynolds shear stress. In the previous nonlinear model, an excessive backward energy transfer from the SGS to the resolved scale is found, so it is removed in the new model to provide sufficient SGS dissipation. The dynamic method is reconsidered, based on the analysis of the transport equation of the resolved Reynolds shear stress. The new dynamic nonlinear model is examined in the rotating turbulent channel flow at Re = 7000 and various rotation numbers (Ro), ranging from 0.0 to 0.6. The new dynamic procedure determines a more proper model coefficient f...

Study on the analogy between velocity and temperature fluctuations in the turbulent rotating channel flows

Turbulent rotating channel flow is of great interest in engineering practice, such as the cooling of the blades in turbo machinery. This paper investigates the turbulence characteristics of temperature fluctuations in rotating turbulent channel flow by direct numerical simulation. It is known that the analogy exists between temperature and velocity fluctuations in non-rotating channel flows. In this paper the analogous property is investigated between temperature and streamwise velocity fluctuations in rotating turbulent channel flows and it has been found that the analogy exists at pressures side in near wall region but the analogy is violated at the suction wall. The influence of the large scale flow structure in rotating channel, known as the Taylor-Gortler vortex, on the characteristics of temperature fluctuations are revealed.

Letter: The effects of streamwise system rotation on pressure fluctuations in a turbulent channel flow

In this letter, we report the modulating effects of streamwise system rotation on both the amplitude and the wavenumber of pressure fluctuations in a plane channel flow. The analysis of the pressure field is conducted based on a set of comprehensive direct numerical simulation data of six rotation numbers. It is observed that high pressure fluctuation regions collocate with the Taylor–Gortler-like (TGL) vortex cores. By decomposing the pressure field into rotation-induced and convection-induced parts, it is observed that the rotation-induced part dominates the total pressure fluctuations and facilitates the growth of TGL vortices. Furthermore, through a spectral analysis, it is discovered that the system rotation acts as a “linear amplifier,” which converts high-wavenumber low-amplitude streamwise vorticity fluctuations into low-wavenumber high-amplitude pressure fluctuations.

Study of passive plume mixing due to two line source emission in isotropic turbulence

Direct numerical simulations are performed to investigate the mixing and dispersion of passive plumes emitted from two parallel line sources into a homogeneous isotropic turbulent flow. The focus of this study is on the turbulent convective regime of plume mixing, where the bulk meandering of the instantaneous plumes makes the primary contribution to the plume dispersion and concentration fluctuations. The quality of mixing and the interference between the two plumes have been studied in both physical and spectral spaces. It is found that the exceedance probability of high concentrations for the total plume released from dual sources is much smaller than that released from a single line source. The reduction in the exceedance probability across high concentration levels for the total plume is quantified using a reduction factor, whose value approaches unity as the cross correlation coefficient between the two concentration fields becomes increasingly positive. It is observed that the scatterplots of the normalized third- and fourth-order concentration moments against the normalized second-order concentration moment collapse onto a single curve, indicating that higher order concentration moments of the total plume can be determined effectively from the information on lower order concentration moments. Furthermore, it is demonstrated that the concentration probability density function for the total plume can be properly evaluated using a clipped-gamma model. In the spectral analysis, the results of the pre-multiplied co-spectra and coherency spectra reveal that the mixing process is the strongest and fastest at large scales for the turbulent convective regime of plume mixing.Direct numerical simulations are performed to investigate the mixing and dispersion of passive plumes emitted from two parallel line sources into a homogeneous isotropic turbulent flow. The focus of this study is on the turbulent convective regime of plume mixing, where the bulk meandering of the instantaneous plumes makes the primary contribution to the plume dispersion and concentration fluctuations. The quality of mixing and the interference between the two plumes have been studied in both physical and spectral spaces. It is found that the exceedance probability of high concentrations for the total plume released from dual sources is much smaller than that released from a single line source. The reduction in the exceedance probability across high concentration levels for the total plume is quantified using a reduction factor, whose value approaches unity as the cross correlation coefficient between the two concentration fields becomes increasingly positive. It is observed that the scatterplots of the n...

Euler Angle and Axis—“Fingerprints” of a Subgrid-Scale Stress Model

The concepts of Euler angle and axis are utilized to investigate the relative rotation between the eigenframes of the deviatoric subgrid-scale (SGS) stress tensor \(-\tau _{ij}^d\) and the resolved strain rate tensor \(\bar{S}_{ij}\). Both Euler angle and axis are “natural invariants” of fluid tensors, which uniquely describe the relative rotation between eigenframes of two tensors. The Euler angle and axis can be regarded as “fingerprints” of a SGS stress model and have a profound implication for structural modeling of the SGS stress tensor. As an application, three SGS models are tested in the context of turbulent channel flows. The proposed Euler angle and axis are proven to be effective for demonstrating geometrical properties of a SGS stress model.

Turbulence Properties of Passive Scalar in Rotating Turbulent Channel Flow by Direct Numerical Simulation

This paper investigates the turbulence properties of passive scalar in turbulent channel flows subject to spanwise rotation by direct numerical simulation. It has been found that the analogy exists between velocity and temperature fluctuations at pressures side in near wall region of the rotating channel flow but the analogy is violated near the suction wall. The influence of the large scale flow structure in rotating channel, known as the Taylor‐Gortler vortex, on the turbulence characteristics of temperature fluctuations are revealed and it is the main cause that the different behavior appears near the suction wall.