Abstract
We experimentally investigate the effects of multiscale rough patches on the drag and flow structure of a fully rough turbulent boundary layer in a wind tunnel. Several patches containing both organized and randomized arrangements of cubes of multiple sizes are tested in order to study the dependence of drag on the frontal solidity of the patch. The drag of each patch is measured with a drag balance for a range of Reynolds numbers, indicating a dependence of the drag on the frontal solidity following the trend predicted by Macdonald et al. (Atmos Environ 32(11):1857–1864, 1998). One of the patches is also replicated with the smallest scales removed and measurements show that the smaller scales have negligible impact on the overall drag. Flow fields in several cross-sections are captured using particle image velocimetry, and maps of the velocity deficit and increased turbulence activity in the wake of the patches are determined and used to define the extent of the internal boundary layer formed by each patch.
Highlights
Modelling of atmospheric flow over complex terrain is important for understanding the flow dynamics and corresponding dispersion in urban areas, as well as for the siting of wind farms in various onshore and offshore locations
The results indicate increased turbulent activity in the wake of each patch compared with having no patch, the extent of which is consistent with the edge of the internal boundary layer identified by the velocity deficit
Measurements of the drag and flow structure above finite multiscale patches showed that, in all cases, the internal boundary layers created by the patches were much shallower than the boundary-layer thickness, indicating that the patches did not affect outer-layer similarity
Summary
Modelling of atmospheric flow over complex terrain is important for understanding the flow dynamics and corresponding dispersion in urban areas, as well as for the siting of wind farms in various onshore and offshore locations. Atmospheric boundary-layer (ABL) flow over complex terrain exhibits a range of characteristics with differing vertical profiles and high turbulence intensities. These variations are intimately related to the details of the surface terrain and the drag at the surface. It is important to be able to predict the flow features over such complex terrain for a range of applications; it is important to obtain experimental data (either in the field or through controlled wind-tunnel experiments) that can be used to validate the variety of modelling strategies. This paper presents results from a systematic series of experiments
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