14 th International Symposium on Particle Image Velocimetry – ISPIV 2021 August 1–5, 2021 Effect of Gust Wind on Flow over a Wall-Mounted Fence D. Bhamitipadi Suresh 1 , E. J. Aju 1 , M. J. Zaksek 1 , M. M. Leffingwell 1 , Y. Jin 1∗ 1 Department of Mechanical Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA ∗ yaqing.jin@utdallas.edu Abstract In this work, the characteristics of incoming and wake flows downstream of wall-mounted fences under wind gust were explored with wind tunnel experiments. A time-resolved particle image velocimetry was used to capture the flow dynamics across two different fence heights. The results show that during the gust period, the wake presents distinct meandering and strong flow mixing. The Probability Density Function distribution of flow velocities indicates that the mixing effect increases with the streamwise distances. Specifically, for locations above the fence top tip, the growth of streamwise distance decreases the footprint of wind gust. However, for locations lower than the fence top tip, the local wind flows exhibit stronger variations before and after wind gust with the growth of downstream distance. Overall, at the same relative streamwise and spanwise locations downstream of fences within the wake region, the higher fence better suppresses the influence of gust wind. 1 Introduction Wall Mounted Fences (WMF) have been studied for more than five decades (Plate, 1971) for their unique aerodynamic properties and applications. They have been shown to offer protection against damage caused by winds (Dong et al., 2010), reducing the wake flow and thus enhancing the deposition of air borne particles such as sand or snow (Alhajraf, 2004; Basnet et al., 2015), and accelerating wind speed at high altitude to increase the wind power production (Tobin et al., 2017), among others. In one of the earliest works on WMFs, Raine and Stevenson (1977) studied turbulent characteristics behind both solid and porous fences and they established empirical relationships between mean velocity and turbulence intensity in the wake. A wind tunnel study conducted by Judd et al. (1996) together with a large-eddy simulation by Patton et al. (1998) showed that the fluid flow accelerates in the region above the fence. Tobin et al. (2017) illustrated that this phenomenon can be effectively captured by installing WMFs upstream of wind turbines to increase power production by at least 10%. Kim and Lee (2001) deployed Particle Tracking Velocimetry to study turbulent shear flow upstream of WMF. Their results show that the separation point detaches and moves upstream of fence surface causing the streamline curvature to increase near the fence. More recent experimental investigation by Tobin and Chamorro (2018) found that wake- moment coefficient of WMF appeared to depend strongly on surface roughness and change with the cosine of incident angle. It, however, did not appear to change with aspect ratios (fence span length/height) of 10 or higher. Aside from normal flow conditions, sudden intermittent or persistent increases in velocities, also known as gusts, have long been known for altering the fluid wake and change aerodynamic loading of various structures (Hayashi, 1992; Gromke and Ruck, 2018; Letson et al., 2019). In natural environments, complex topographic terrain of earth acts as a precursor of local rapid increase in velocity in atmospheric boundary layer (ABL) (Wagenbrenner et al., 2016). Hayashi (1992) investigated turbulent wind flow in ABL and found that effective mixing of flow occurs at peak gust due to efficient transport of downward momentum and that momentum flux dictates gust development. Study by Gromke and Ruck (2018) highlights the effects of gust on damage patterns in forest and it illustrates that downstream vortex behind a row of trees (can be thought of as a WMF) deflects high momentum fluid from above which increases wind loading on subsequent rows. Recent experiments by Letson et al. (2019) analyzed properties of gust wind using Doppler lidar and sonic anemometer to best describe its behavior at wind turbine heights. Their probability distributions of gust