Design, implementation and validation of a flow management device for fan-array wind tunnels

Although fan-array wind tunnels can generate a wide range of flow configurations, from steady uniform to unsteady shear flows, their control is currently based on iterative procedures and require extensive flow characterization. To simplify the control problem, a novel flow management device architecture is presented. The device preserves the independence of the jets issuing from each square-section inlet fan through the use of individual channels up to the outlet plane. Existing analytical models of mean flow and mesh screens interaction are used to design the device. Moreover, it is shown using literature data and experimental measurements that the downstream flow quality depends on the momentum thickness of the wake issuing from the channel walls inside the device. Momentum thickness is predicted using a boundary layer model, modified to include the presence of mesh screens, and validated through experimental measurements. A configuration with three honeycombs (thickness 15mm, cell size 3.2mm), 4 stainless steel mesh screens (wire diameter, d=0.14mm, and mesh size, M=0.4mm) was retained to achieve the flow quality (i.e. mean flow uniformity and turbulence level) typically required for general aerodynamic testing. Experimental characterization of the flow downstream of the device through hot wire anemometry showed, outside of the wake regions, non-uniformities <3% and unfiltered turbulence levels between 0.45%-0.7% for mean flow velocities of 1.4 and 10 m/s respectively measured at a downstream distance x/H=7.5. The evolution of velocity non-uniformities and turbulence inside the wake issuing from the device walls was shown to follow existing scaling laws. The flow uniformity and boundary layer models validated in the present study are expected to guide the design of flow management devices for fan-array wind tunnels of any size.