LES and RANS for Turbulent Flow over Arrays of Wall-Mounted Obstacles

Abstract

Abstract. Large-eddy simulation (LES) has been applied to calculate the turbulent flow over staggered wall-mounted cubes and staggered random arrays of obstacles with area density 25%, at Reynolds numbers between 5 × 10 3 and 5 × 10 6 , based on the free stream velocity and the obstacle height. Re = 5 × 10 3 data were intensively validated against direct numerical simulation (DNS) results at the same Re and experimental data obtained in a boundary layer developing over an identical roughness and at a rather higher Re. The results collectively confirm that Reynolds number dependency is very weak, principally because the surface drag is predominantly form drag and the turbulence production process is at scales comparable to the roughness element sizes. LES is thus able to simulate turbulent flow over the urbanlike obstacles at high Re with grids that would be far too coarse for adequate computation of corresponding smooth-wall flows. Comparison between LES and steady Reynolds-averaged Navier-Stokes (RANS) results are included, emphasising that the latter are inadequate, especially within the canopy region.

Unsteady dynamics and organized structures from DNS over an idealized building canopy

“…This grid resolution is sufficient to capture most of the dissipation at the simulated Reynolds number (Coceal et al, 2006). Moreover, as noted above, the Reynolds number dependence for this type of flow is weak (Xie and Castro, 2006). The adequacy of the grid was confirmed by a close comparison of the statistics with those for a test run at double the resolution but on a smaller domain of 4 h × 4 h × 4 h. Figure 2 demonstrates the good agreement obtained with the wind-tunnel data of Cheng and Castro (2002) and Castro et al (2006).…”

“…A Reynolds number of Re = 5800 is very small compared to typical Reynolds numbers encountered in real atmospheric flows, and the range of scales simulated here is correspondingly more limited. However, Xie and Castro (2006) have recently performed large eddy simulations (LES) over the same array of cubical buildings at Reynolds numbers of up to Re = 5 × 10 6 , and they concluded that Reynolds number dependency (if any) is very weak for these types of flows. This is in contrast to flows over smooth surfaces or around isolated bodies, and is because the turbulence is dominated by eddy shedding from the sharp edges of urban-like obstacles.…”

Influence of urban morphology on air flow over building arrays

“…Experimental studies on simplified and idealised geometries constitute the bulk of the experimental data used today as a basis for model development and validation. While some past efforts attempted to study the influence of geometrical parameters other than the building height and building density (Cheng and Castro, 2002;Xie and Castro, 2006;Xie et al, 2008), their conclusions, while useful, cannot be easily generalised for real urban settings.…”

“…This is in contrast to all the current approaches described above where z 0 is only a function of mean building height and density. Xie and Castro (2006) and Xie et al (2008) studied the same configurations by simulating the flow with both Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) CFD models. Their results showed that many features of the flow over the variable-height array are rather different from those in the flow over uniform roughness.…”

Large-Eddy Simulation of Dispersion from Surface Sources in Arrays of Obstacles

“…Kanda et al (2004), for example, used LES on simple cube arrays and obtained good agreement with the experiments and a detailed investigation of turbulent organized structures in aligned and staggered cube arrays using LES was subsequently reported by Kanda (2006). Xie and Castro (2006) compared LES and RANS for the flow past wall-mounted obstacles and observed that RANS performance is poor, especially within the canopy region. In contrast, they found satisfactory results using LES as compared to direct numerical simulations (DNS) by Coceal et al (2006) at a Reynolds number of Re = 5000 (based on building height and maximum mean velocity).…”