LARGE-SCALE STRUCTURE OF A LEADING-EDGE SEPARATION BUBBLE WITH LOCAL FORCING

The Karhunen-Loeve (K-L) expansion is used to extract coherent structu res from a leading-edge separation bubble with local forcing. A leadin g-edge separation bubble is simulated using the discrete vortex method , where a time-dependent source forcing is perturbed near the separati on point. Based on the wealth of numerical data, the K-L procedure is applied in a range of the forcing amplitude (A(0) = 0, 0.5, 1.0 and 1. 5) and forcing frequency (0 less than or equal to f(F)H/U-infinity les s than or equal to 0.3). Application of K-L procedure reveals that the eigenstructures are changed noticeably by local forcings. In an effor t to investigate the mechanism of decreasing reattachment length (x(R) ), dynamic behaviors of the expansion coefficients and contributions o f the eigenfunctions are scrutinized. As the forcing amplitude increas es, large-scale vortex structures are formed near the separation point . Furthermore, the flow becomes more organized, which results in the r eduction of x(R). Two distinctive regimes are classified: the regime o f decreasing x(R) and the regime of Increasing x(R). The K-L global en tropy indicates that x(R) is closely linked to the organization of the flow structure.