Experimental and Numerical Evaluation of Non-Reflecting Porous Tailboards for Transonic Cascade Wind Tunnels

A linear cascade of turbine blades in a transonic wind tunnel can be affected by spurious wave reflections at the open flow boundaries. This phenomenon causes a loss of pitchwise periodicity, affecting measurements of blade surface pressure, isentropic Mach number, blade turning angle, and stage loss coefficient. A joint experimental and numerical approach is followed to investigate the mechanism of wave generation and propagation and to test a perforated tailboard which reduces its negative effects. Measurements and visualisations of supersonic discharge flow are analysed and compared to numerical simulations of the periodic flow around a blade and to a numerical model of the complete test section. This comparative analysis has shown that (i) the effects of non-periodic boundary conditions are fundamentally different when the discharge flow is supersonic instead of subsonic; (ii) a perforated tailboard approximately aligned with the discharge flow shear layer reduces these disturbances.