Browsing by Author "Zhu, Xiaowei"
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
Item Numerical Study on Uniform-Shear Flow over a Circular Disk at Low Reynolds Numbers(American Institute of Physics Inc.) Yang, J.; Liu, M.; Wang, C.; Zhu, Xiaowei; Zhang, A.; Zhu, XiaoweiUniform shear flows over a circular disk of aspect ratio 10 (thickness/diameter) at low Reynolds numbers are numerically investigated with the main focus on the effect of inlet shear on the wake evolutions. The Reynolds numbers considered are Re = 140, 160, and 180 based on the inlet center velocity uc and disk diameter d. The non-dimensional shear rate k(= |V̅u|d/u_c) is varied from 0 to 0.09. The bifurcations leading to unsteady states with hairpin vortex shedding occur much earlier in uniform shear. In most cases, wake evolutions occurring as the shear rate increases in uniform shear are similar to those as the Reynolds number increases in uniform flow. A new wake mode termed as dragonfly-wings (DW) mode is captured at Re = 180 and k = 0.01 and 0.03. At DW mode, hairpin vortex structures are shed from diametrically opposite orientations, but with irregularity in strength and shape, i.e., three different vortex loops are observed in the wake, and produce three peaks at low frequencies in the frequency spectrum of the drag. The planar-symmetry plane for standing-wave and zig-zig modes is determined by both the initial conditions and the direction of the uniform shear. It is found that with increasing inlet shear rate, the non-dimensional shedding frequency remains nearly constant for the low shear rates (k < 0.1). Time-averaged drag and lift coefficients slightly increase with increasing inlet shear rate. Finally, the hysteretic property of the DW mode transition is examined and further investigated using the Landau mode, indicating that DW mode transition is non-hysteretic (supercritical). ©2018 Author(s).Item Wall Modeling for Turbulent Flow over Complex Roughness(2018-08) Zhu, Xiaowei; Anderson, William; Leonardi, Stefano; Lu, Hongbing; Minkoff, Susan E.; Qin, ZhenpengTurbulent flow over complex rough surfaces is crucial in both engineering and boundary layer meteorology science. The surface morphology has significant effects on the flow, but it is computationally expensive to solve all the turbulent scales especially when the air flows over complex roughness. Large-eddy simulation (LES) with wall model is therefore employed in this situation. This dissertation focuses on the wall modeling of turbulent flow over complex roughness such as urban topography. As the wall effects can be accurately represented by the equilibrium logarithmic law via roughness length, z0, this dissertation aims to parameterize z0 over complex roughness. In this dissertation, two types of complex roughness are discussed: spatially heterogenous urban-like topography (Chapter 3) and multiscale fractal urban-like topography (Chapter 4). For the spatially heterogeneous urban-like topography, a priori prediction method for z0 based on the statistical moments of surface height is proposed especially for the boundarylayer turbulent flow. Using a posteriori LES results, we demonstrate that the skewness of surface height (as measures of the presence of the extreme value, or the “heavy tail” events) has non-negligible effects, which received less attention as topographic parameters in the past. This finding is reconciled with a model recently proposed by Flack and Schultz (2010) who demonstrate that z0 can be modeled with standard deviation and skewness, and two empirical coefficients (one for each moment). We find that the empirical coefficient related to skewness is not constant but exhibits a dependence on standard deviation over certain ranges. For idealized, quasi-uniform cubic topographies and complex, fully random urbanlike topographies, we demonstrate robust performance of the generalized Flack and Schultz model against contemporary roughness correlations. The multiscale fractal-like topographies pose a particular challenge to numerical simulation schemes since the large-scale elements are resolved, but the small-scale descendant elements cannot be resolved on the computational mesh grid. A local wall model representing the effects of unresolved sub-generation roughness is needed in such scenario. By virtue of selfsimilarity among scales, we develop a methodology and a roughness model for the unresolved scales by learning from the large-scale momentum fluxes. And then the roughness model for the unresolved scales via the equilibrium logarithmic law is established. The research shows that aerodynamic stress associated with descendant, sub-generation scale elements can be parameterized, thus that the turbulent flow over fractal-like geometry can be simulated with only the large generations resolved on the computational mesh grid. The key questions we ask in this dissertation are: How does the spatial heterogeneity affect the transport of the turbulent flow? How to model the sub-generation scales which are smaller than the mesh grid for a fractal topography? The results, and the modeling framework developed herein, have practical implications for the operation of numerical weather prediction models and the initialization of high-resolution solutions.