Abstract
The flow fields over a simplified 3D hill covered by vegetation have been examined by many researchers. However, there is scarce research giving the three-dimensional characteristics of the flow fields over a rough 3D hill. In this study, large eddy simulations were performed to examine the coherent turbulence structures of the flow fields over a vegetation-covered 3D hill. The numerical simulations were validated by the comparison with the wind-tunnel experiments. Besides, the flow fields were systematically investigated, including the examinations of the mean velocities and root means square of the fluctuating velocities. The distributions of the parameters are shown in a three-dimensional way, i.e., plotting the parameters on a series of spanwise slices. Some noteworthy three-dimensional features were found, and the mechanisms were further revealed by assessing the turbulence kinetic energy budget and the spectrum energy. Subsequently, the instantaneous flow fields were illustrated, from which the coherent turbulence structures were clearly identified. Ejection-sweep motion was intensified just behind the hill crest, leading to a spanwise rotation. A group of vertical rotations were generated by the shedding of the vortex from the lateral sides of the hill.
Highlights
As we all know, turbulence has a very important research position in many fields, and the turbulent flow field on the topography in different fields is closely watched by many researchers, including the wind turbine sittings [1], the pollution diffusions [2], estimation of aerodynamic loadings on structures [3], identifications of the tree damage with high risk [4], as well as the forest fire propagation [5]
Very little research is about the flow over topographies with the canopy covered, which can be divided into three groups, i.e., the flat topography, the 2D ridge, and the 3D hill
It was found that the mean velocity profiles were not sensitive to the density of the roughness and the roughness served as strong windbreaks, weakening the wind velocity near the Energies 2019, 12, 3624; doi:10.3390/en12193624
Summary
Turbulence has a very important research position in many fields, and the turbulent flow field on the topography in different fields is closely watched by many researchers, including the wind turbine sittings [1], the pollution diffusions [2], estimation of aerodynamic loadings on structures [3], identifications of the tree damage with high risk [4], as well as the forest fire propagation [5]. Dupont and Brunet examined the coherent structures in canopy edge flow over flat terrain by large eddy simulations (LES) [6]. Takahashi et al investigated the turbulence characteristics of the flow over a 2D ridge with a rough surface in wind tunnel experiments [7] and the effects of roughness density were studied. It was concluded that the velocity deficit varied with roughness conditions on the hill surface or inflow area, thereby creating a completely different turbulence structure. Dupont et al studied the turbulent flow over a 2D ridge with a vegetation canopy cover using LES [10], in which the coherent structures were identified. Ishihara et al conducted a study of the flow over a 2D ridge and a 3D hill under smooth and rough ground by wind tunnel experiment [11]. The three-dimensional features and the mechanisms were revealed by investigating the turbulence kinetic energy (TKE) budget and the spectrum energy
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