An investigation of the effect of surface roughness on the mean flow properties of “tornado-like” vortices using large eddy simulations

Abstract

The effects of surface roughness on the target flow properties of tornadoes, like the maximum tangential velocity and core size, are not well understood and widely debated due to the discrepancies reported by previous studies. This study presents the results of large eddy simulations of “tornado-like” vortices (TLVs) of swirl ratios in the range of 0.22–1.00 over five ground roughness scenarios. The results indicate that roughness enhances flow convergence and reduces the core radius near the ground, except at the transition swirl ratio. Our study supports the widely reported claim that surface roughness has an effect similar to a reduction in swirl ratio. A reduction in core radius is accompanied by a speed-up in the tangential velocity due to the principle of conservation of angular momentum. The trends in maximum tangential velocity are, however, more height-sensitive due to the competing effects of depleting angular momentum in the surface layer and speed-up due to enhanced flow convergence. The results indicate that the effect of surface roughness on TLV characteristics is strongly dependent on the external swirl ratio. The lack of consensus in the literature is due to the limited range of swirl ratios and roughness considered in those studies.