The effect of swirl ratio and surface roughness on the boundary layer of “tornado-like” vortices

Abstract

Large eddy simulations (LES) of “tornado-like” vortices (TLVs) for a range of swirl ratios and surface roughness were conducted. The mean axial velocity distribution was used to identify a region of predominantly horizontal flow. An examination of the boundary layer within this inflow region was conducted with the primary objective of applying the traditional understanding of atmospheric boundary layer (ABL) to TLV boundary layer. A comparison with the boundary layer development over a flat plate revealed that the tornado boundary layer can be divided into a region of zero and favourable pressure gradient. Furthermore, the mean measures of the boundary layer in TLVs like the BL depth, displacement thickness, and momentum thickness were observed to be dependent on the external swirl ratio, ground roughness, and streamwise distance. The turbulent stresses, friction velocity and the aerodynamic roughness length in the TLV boundary layer were also observed to depend on the streamwise distance, indicating a lack of equilibrium due to a short fetch. ESDU predicted turbulence intensities, based on a crude comparison, were observed to be conservative for the lateral and vertical directions but under-conservative for the longitudinal component in regions marked by high aerodynamic roughness length.