Effect of horizontal airflow filtering using a porous cylindrical shroud on vertical turbulence characteristics

This paper presents the findings of a series of experimental studies to investigate the variation of vertical flow characteristics after filtering horizontal flow using porous cylindrical shrouds. Exploring this research question implies improving the existing method of observing horizontal wind speed and direction using Distributed Temperature Sensing (DTS) to develop it for the vertical direction to capture continuous and distributed turbulence. The experiments were performed using two sonic anemometers and two pressure ports in the open experimental area; one of each sensor is located inside the cylindrical shroud. The flow statistics were compared between different shroud configurations with different shapes, colors, rigidity, and porosity. Based on the coefficient of determination and mean error between shrouded and unshrouded data, the white insect screen shroud with a rigid structure and 60 cm diameter and 145 cm height is determined as the most conducive setup. The optimum shroud setup reduces the horizontal wind standard deviation by 35 percent, having a coefficient of determination of 0.972 between vertical wind standard deviations and RMSE less than 0.018 m/s between shrouded and unshrouded set up. However, the comparisons confirm that the vertical flow remains unaltered while reducing the horizontal flow, but the spectral energy ratio between the shrouded and unshrouded setup shows different responses. This ratio decreases exponentially in the high frequencies, which means the shroud damps the high-frequency eddies with a temporal scale of fewer than 6 seconds. Despite high frequencies, the ratio remains constant in the low frequencies for all energy spectrums, including temperature, wind components, momentum, and sensible heat flux.