LES study on the turbulent flow fields over complex terrain covered by vegetation canopy

Abstract

In this study, (a) flat terrain covered by roughness blocks, (b) flat terrain covered by vegetation, (c) a 3-D hill covered by vegetation, and (d) a real forest covered complex terrain, were modeled step by step. For the flat terrain covered by roughness blocks, three different occupancy rates (25.0%, 12.5% and 5.6%) were examined. For each occupancy rate, three grid systems with different horizontal resolutions were examined. It was found that with intent to capture the turbulent characteristics accurately the horizontal grid size should be at least as large as the height of the roughness canopy. For the flow fields over vegetation covered flat terrain and a 3-D hill, the performance of the method modeling the vegetation used in this study was examined. The drag effects from the vegetation were modeled by adding a negative scour term in the momentum equation and satisfactory agreements with experiments were achieved. After sufficient validations, we reproduced the turbulent flow fields over a real forest covered complex terrain which is a terrain near Taikoyama hill, Kyoto, Japan, where extensive field measurements have been made available online by Kyoto Prefecture (2013). Two models for this real terrain with radius of the terrain equal to 4km and 8km in full scale were simulated under sixteen wind directions. Only the terrain model with large radius (8km) gave good results. It is believed to be due to the factor that the model of small region terrain smoothed some upstream tall hills whose wake effects could be transported for a long distance downstream.