A study on the effect of swirl ratio and surface roughness on the flow dynamics of tornadoes using LES
Abstract
Tornados are intense atmospheric vortices that result in high pressure-deficits and strong winds near the ground and pose significant damage threat. Experimental and numerical simulations have been conducted to characterize the flow dynamics of tornadic vortices. With the advancement and availability of high-performance computing, tornado-induced wind loads on buildings and other structures can be evaluated using computational fluid dynamics (CFD). For this, it is required to characterize near-ground surface winds resulting from CFD simulations of tornadoes as it has been practiced for wind tunnel simulations. Furthermore, it is necessary to identify and quantify the boundary conditions required for CFD models to achieve target tornado wind profiles.
Unlike the atmospheric boundary layer winds, surface winds from strong tornado vortices primarily depend on the swirl ratio – the strength of updraft relative to circulatory momentum - and ground roughness exposure. Large eddy simulations of TLVS were conducted such that the swirl ratio was controlled by specifying the inflow direction at the velocity inlet boundary, and surface drag was accounted for by explicitly modeling roughness elements on the ground. Velocity and pressure time histories from LES were analyzed to map flow structure as function of swirl ratio and ground roughness. Velocity profiles from LES were compared with experimental and full-scale data.
By examining the prevailing dominant flow structure across the flow domain, three flow regions were identified; inflow boundary layer, the corner flow region, and the aloft flow region. Generally, the introduction of ground roughness reduces the core size of the tornado vortex in the corner flow region. Unlike the classical boundary layer flows on rough flat plates, the pressure gradient is non-zero along the inflow surface winds towards the circulation axis. In this flow regime, both the mean and turbulent flow characteristics are quantified and discussed. With the increase in height from the ground, the ground roughness's drag effect and the radial inflow's convergence effect decrease. The asymptotic trends of flow profiles in this region allow for the simplification of the governing partial differential equations (PDEs) and the derivation of closed-form solutions for the tangential velocity profile.