On the generation of single and multiple plane compound shear layers with multi-fan wind facility*

Although multi-fan wind facilities can generate complex spatial flows, due to their large number of degrees of freedom, finding the control laws has been elusive so far. This study demonstrates that plane compound shear flows can be generated using a simple control law. An existing solution to the two-dimensional incompressible Navier-Stokes equations in the absence of pressure gradients, i.e. Goertler, predicts the normalized time averaged axial velocity profile inside a compound shear layer and adding an empirically determined parameter allows predicting the dimensional velocity profiles. The model combines previous literature experiments with additional PIV 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 main additional variable that determines the downstream velocity field. The boundaries of applicability of the simple control law, predicted by the model, were validated experimentally through PIV measurements on a multi-fan wind facility. The applicability of the simple control law to multiple adjacent compound shear layers was also demonstrated experimentally. Turbulence profiles for different shear ratios were measured through hot wire anemometry to characterize the time scales of the produced shear flows, with quasi-steady applications in mind. Although a small-scale wind facility was used for model validation, the results are applicable to the control of multi-fan facilities of any size.