Wind Shadow Effects on a Sonic Anemometer in Low Reynolds Number Flow

Abstract A sonic anemometer is being developed to measure wind velocity on the surface of Mars and has also been deployed for stratospheric wind measurements on Earth. Wind velocity is measured through the difference in acoustic time of flight or acoustic phase lag along three orthogonal paths. However, the structure of the sensor head itself disrupts the flow and creates an internal wake. The slower flow in the wake reduces measured wind speed compared to the free stream, an effect dependent on orientation and speed. To understand these effects and generate quantitative correction factors, computational fluid dynamics (CFD) simulations were conducted under Mars surface conditions. We investigate the impact of flow speed, incident angle, and anemometer structural components (fairings and equatorial bars). Quantitative wake-correction factors are provided, and a method for their automated application is demonstrated on experimental data collected in the Aarhus Mars Simulation Wind Tunnel. At intermediate flow speeds (near 5 m s −1 ) under Martian surface conditions (CO 2 at 600 Pa and 0°C), correction factors are on the order of 20%. However, for the worst-case directions, when the transducer wake is directly in line with the acoustic path, or at low speeds, corrections may be as large as 84%. Applying the correction factors computed from CFD substantially reduced error in the experiments from 18% to 45% RMS apparent error with no wake correction to 5.4%–8.9% apparent error with the correction applied, although some of this apparent error could be experimental uncertainty in the flow conditions and not due to the instrument.