On the generation of plane compound shear flows with fan-array wind tunnels

Although fan-array wind tunnels (FAWT) can generate complex spatial and temporal flows through their high number of degrees of freedoms, finding the associated control laws has been elusive so far. This study demonstrates that plane compound shear flows with large shears can be generated with FAWT using a simple control law, given the addition of a carefully designed flow management device (FMD). The normalized time averaged axial velocity profile inside the compound shear layer is predicted using an existing solution to the two-dimensional incompressible Navier-Stokes equations in the absence of pressure gradients, i.e. Goertler solution. A semi-empirical model based on this solution was developed to predict the dimensional time averaged axial velocity profiles inside the shear layer. The model combines previous literature experiments with additional PIV and hot wire measurements and shows that when the downstream distance is normalized with a characteristic fan size, i.e. its height, the velocity ratio between adjacent fans is the only additional variable that determines the downstream velocity field. The model was experimentally validated through PIV measurements on a fan-array wind tunnel and allowed to find the boundaries of applicability of the simple control law. The applicability of the simple control law to multiple adjacent compound shear layers was also demonstrated experimentally. Turbulence profiles for different shear ratios are measured through hot wire anemometry to further characterize the shear flows. Although the model was validated using a small scale wind tunnel, the results are expected to be applicable to the control of fan-array wind tunnels of any size.