The Effect of Adverse Pressure Gradient on Tip Clearance Noise

A test-rig and an airfoil blade have been designed for the UNSW Anechoic Tunnel (UAT) to measure the flow and noise statistics in tip leakage flows under adverse pressure gradient. The baseline flow (without blade) in the rig has been fully characterised through mean wall pressure and boundary layer velocity and turbulence measurements. The test rig was then used to study the tip clearance noise radiation and mean wall pressure under zero and adverse pressure gradient conditions. Flow-field simulations using RANS approach that show good agreement with the measurements are also presented. The baseline measurements show that the test rig is capable of generating pressure gradient parameter up to approximately 1.11 on the endwall. The mean wall pressure and noise radiation from a tip leakage flow were measured for three pressure gradient parameters (0, 0.51 and 1.11), three geometric angles of attack (0◦, 6◦ and 12◦) and chord-based Reynolds number of approximately 350,000. For each measurement, 53 clearance heights between 1 mm and 101 mm were considered. It is shown that the adverse pressure gradient increases the overall wall pressure under the tip without significantly modifying the overall pressure distribution, except for very small clearances at the largest angle of attack configuration where a low pressure zone forms near the leading-edge for adverse pressure gradient inflow conditions. The effect of pressure gradient on the tip clearance noise is a strong function of tip clearance height. For small clearances, adverse pressure gradient results in lower noise levels, while for larger clearances it results in an increase in noise levels due to a stronger tip vortex associated noise source.