Wind shadowing of a sonic anemometer in low Reynolds number flows

Abstract

Computational Fluid Dynamics (CFD) simulations have been carried out to better understand the wind shadowing effects on a sonic anemometer system. The anemometer will measure wind speed on the surface of Mars and the stratosphere of Earth.The wind velocity vector is measured through the difference in acoustic time of flight in 3 directions. However, the structure itself disrupts the flow and creates an internal wake. To understand this and generate correction factors, CFD simulations were run under Mars surface conditions while varying incident angle and flow speed. Spalart-Allmaras turbulence models were employed in both steady and unsteady Reynolds Averaged Navier Stokes (RANS) frameworks and compared to a laminar model. Due to the low Reynold’s number (<150) we observe no shedding at speeds below 10 m/s. At speeds below 10 cm/s the flow diverts around the structure and sensitivity drops. At intermediate speeds near 5 m/s the wind shadow ratio (WSR) varies substantially with incident wind angle. Design changes to the structure allowed us to improve the worst WSR from 38% to 47%. We were also able to quantify correction factors. Validating comparisons to water tunnel and wind tunnel measurements will be presented.