Unsteady Low-Speed Wind Tunnels

Abstract

A theory based upon linearized governing equations is developed that describes the operation principles and design parameters for low-speed wind tunnels with longitudinal freestream oscillations. Existing measurements made in unsteady wind tunnels are shown to be consistent with the theory and targeted validation experiments performed in a variable-geometry blowdown-type wind tunnel revealed excellent correspondence with the theoretical results. In particular, the tunnel frequency bandwidth is proportional to the mean tunnel freestream velocity and inversely proportional to the test-section length and the square of the exit area to test-section area ratio. The acoustics equations reveal a “Helmholtz damping ratio” that is not only dependent on the tunnel geometry and exit area but also proportional to the freestream Mach number. At appreciable reduced frequencies, dynamic stall on the louver vanes increases pressure losses, thereby reducing the mean flow speeds. Varying the exit area results in louver-vane vortex shedding that can excite resonances, resulting in an apparent increase in the turbulence level. To achieve large freestream amplitudes and high-frequency bandwidths, substantial blockage, and therefore pressure losses, will be incurred; thus, large increases in the tunnel power factor should be anticipated.