Large Eddy Simulation of Turbulent Boundary Layers

K. RAJAGOPALAN, , G. PAWLAK, A. PATALANO, M. CANALS Y M. KOBAYASHI

Moscu

Conferencia; Fluxes and Structures in Fluids; 2009

Numerical modeling of turbulent boundary layers over regular roughness such as square ribs and monochromatic sinusoids has been an important focal area over the past decade. Extremely irregular roughness such as coral reefs in oceanic boundary layers and urban canopies in atmospheric boundary layers are only beginning to attract the attention of numerical and experimental researchers. The response of the boundary layer to a regular roughness can be parameterized in terms of the length scales defining the roughness [3]. The difficulty arises in the case of broad-banded and highly irregular roughness distributions where the length scale that determines the response of the boundary layer is not clear. One method to characterize the irregular nature of roughness is through the use of a spectral distribution [4]. In order to establish the relationship between the roughness spectrum and the hydrodynamic response of the boundary layer, we generate a variety of rough beds with different spectral slopes using 2D 10 % loading square waves as basis functions. These square waves can reproduce flow separation characteristics common to flow over rough beds. Large Eddy Simulation (LES) is then used to simulate the turbulent boundary layer over the rough beds. The LES solver is first validated with laboratory experiments on k type square ribs. A rigorous grid convergence study is undertaken to ascertain the accuracy of results from the irregular bed. We present first order statistics including mean velocity profiles and estimate the hydrodynamic length scale zo for spectral roughness distributions. We also present higher order statistics such as velocity fluctuations and Reynolds stress to shed light on the nature of the flow as the spectral slope of the bed changes from gentle to steep. Our results (see Figure 1) so far has shown a strong relationship between the slope of the roughness spectrum and flow over regular square ribs. For gentle slopes of the roughness spectrum the flow is dominated by regions of recirculation, similar to flow over d type roughness where as for large slopes the flow is dominated by flow separation and vortex shedding similar to flow over k type roughness.