Performance of slotted end wall linear cascade tunnels at off-design conditions

Linear cascade transonic wind tunnels with an open jet test section can suffer from pitchwise end-wall interference. This causes a loss of pitchwise periodicity in the cascade, increasing the uncertainty in the measurements and producing less accurate estimates of turbine stage performance, flow exit angle and loss coefficient. To reduce the end-wall interference, a slotted tailboard is tested in a transonic cascade run off-design, in a regime at which the profile trailing edge shocks produce substantial reflections in the absence of a suitable end-wall treatment. The tailboard is optimised by numerical modelling for an isentropic discharge Mach number of 1.27. Tests over the wider isentropic Mach number range 1:20 ≤ Mi ≤ 1:32 quantify the restored periodicity gained by the use of this tailboard. When the tailboard is used away from its design point of Mi = 1:27, the discharge remains more periodic than with an open jet test section. The tailboard performance varies nonmonotonically away from its design point, driven by complex changes in the discharge wave pattern. Some of these changes are identified by schlieren flow visualisation.