Instability and Transition in a Separation Bubble Under a Three-Dimensional Freestream Pressure Distribution

This paper presents measurements of the instability and transition processes in separation bubbles under a three-dimensional freestream pressure distribution. The measurements are performed on a flat plate on which a pressure distribution is imposed by a contoured surface facing the flat test-surface. The three-dimensional pressure distribution that is established on the test-surface approximates the pressure distributions encountered on swept blades. This type of pressure field produces crossflows in the laminar boundary layer upstream of the separation and within the separation bubble. The effects of these crossflows on the instability of the upstream boundary layer and on the instability, transition onset, and transition rate within the separated shear-layer are examined. The measurements are performed at two flow-Reynolds numbers and relatively low level of freestream turbulence. The results of this experimental study show that the three-dimensional freestream pressure field and the corresponding redistribution of the freestream flow can cause significant spanwise variation in the separation-bubble structure. It is demonstrated that the instability and transition processes in the modified separation bubble develop on the basis of the same fundamentals as in two-dimensional separation bubbles and can be predicted with the same level of accuracy using models that have been developed for two-dimensional separation bubbles.