Far-Field Sound-Source Behavior in an Idealized Tip Clearance Flow

The present study investigates the wall pressure fluctuations and the velocity field in a tip clearance flow formed by placing a stationary, cambered airfoil next to a wall with the aim to understand the far-field sound behavior. The tip clearance heights considered range between 14 and 30% of the incoming, undisturbed boundary-layer thickness; and the clearance height-based Reynolds numbers were between 2600 and 16,000. The results show that, besides the tip-leakage vortex, large-scale rib vortices that reside near the trailing edge are a major source of low-frequency unsteadiness that produces correlated pressure fluctuations across the tip and the wall surfaces, resulting in a strong low-frequency noise radiation from this region. A reduction in clearance height suppresses these vortices, leading to a reduction in both near- and far-field pressure fluctuations at low frequencies. The turbulence levels and wall pressure fluctuations further upstream in the midchord region of the clearance are weaker, and reducing the clearance height decreases the low-frequency pressure fluctuation levels but increases the levels in the mid- to high-frequency range. For the smallest clearance height in the present work, a significant flowfield unsteadiness near the leading edge that results in intense high-frequency near- and far-field pressure fluctuations is observed.