Wind Mill Reliability: Determining Tornado-Wind Force Coefficients for Thin-Cylinder Structures with Computer Modeling

Quentin S. Ragan, R. Panneer Selvam

Abstract: A two-dimensional (2D), Computational Fluid Dynamics (CFD) model was used to study tornado-wind force interaction with a smooth, thin-cylinder structure having a radius less than the tornado radius and little roof area. They are widely used as wind turbine support towers in tornado-prone areas. The Navier-Stokes equations were approximated with a Finite-Difference method. Simulations were performed to determine the minimum required grid resolution to prevent divergence of the program, replicate known straight-wind force coefficients, and determine tornado-wind force coefficients. The ratio of the core, forced-vortex tornado radius to the cylinder diameter (r_core/D) was increased with constant tornado and translation velocities to determine the tornado drag and lift force coefficients (C_d and C_l). The rotational velocity strength (V_tan) and r_core were then simultaneously increased to determine their effects on C_d and C_l. Tornado-wind force coefficients C_d and C_l were corrected for velocity and combined to determine a single tornado-wind force coefficient (C). The combined coefficient was compared to published force coefficients, including ASCE 7, for straight-wind. Force coefficients for tornados approached constant values when r_core exceeded 15D. Tornado-wind force coefficients were 2-3 times the published values for straight-wind.