Suction effects on transitional bubbles

Abstract

The effects of suction on the structure of a transitional bubble forming on a low-Reynolds-number airfoil are examined using the Reynolds-averaged Navier–Stokes and k–kL–ω transition model. The suction effects on the laminar and turbulent portions of the separation bubble and the locations of the main points in the separation bubble are discussed in relation to the transition process of the bubble. A single suction distribution located in the region of the baseline transitional bubble is used with two suction rates. One suction rate is sufficiently strong to eliminate the bubble from its original location and a lower suction rate that is only sufficient to create shallower bubbles. Eliminating the bubble from its original location maintains a laminar boundary layer downstream of the baseline transition location until a shallower separation bubble forms near the trailing edge. The lower suction rate shortens the separation bubble and reduces its height while approximately maintaining its original location. Analyzing the lengths of different portions of the bubble suggests that suction affects the instability growth rate and the nonlinear interactions in the separated shear layer. The lower suction rate shortens the distance between the separation and transition onset suggesting a higher growth rate of the inviscid instability. The higher suction rate, on the other hand, increases the distance between the separation and transition onset indicating a stabilizing effect by slowing down the growth rate of the inviscid instability. However, the percentage of distance between transition and separation to the total length is only slightly affected by the suction and the angle of attack.