Unsteady Structure of Compressor Tip Leakage Flows

Direct numerical simulations (DNS) are performed of a cantilevered stator blade to identify the unsteady and turbulent flow structure within compressor tip flows. The simulations were performed with clearances of 1.6% and 3.2% of chord. The results show that the flow both within the gap and at the exit on the suction side highly unsteady phenomena controlled by fine-scale turbulent structures. The signature of the classical tip-leakage vortex is a consequence of time-averaging and does not exist in the true unsteady flow. Despite the complexity, we are able to replicate the flow within the tip gap using a validated quasi-three-dimensional (Q3D) model. This enables a wide range of Q3D DNS simulations to study the effects of blade tip corner radius and Reynolds number. Tip corner radius is found to radically alter the unsteady flow in the tip; it affects both separation bubble size and shape, as well as transition mechanisms in the tip flow. These effects can lead to variations in tip mass flow of up to 10% and a factor of 2 variation in dissipation within the tip gap.