Vortical structures responsible for delayed stall in an idealized humpback whale flipper model*

In this study, we investigate how the tubercles on the leading edge of an idealized humpback whale flipper model delay the stall. Oil-surface visualization is performed to see the surface flow pattern on the suction surface, and PIV is conducted in several streamwise and crossflow planes at different attack angles ($\alpha$). Without tubercles, leading edge separation first occurs near the tip region and progresses inboard with increasing $\alpha$. With tubercles, however, two types of vortical motions are observed at the mid-span. The first is streamwise vortex arrays which are dominant at $\alpha \leq 9^\circ$, and they are observed downstream of small separation bubbles near the leading edge. The second is asymmetric counter-rotating streamwise vortex pairs that become dominant at $\alpha > 9^\circ$, and these structures appear near the trailing edge. These two types of vortical motions delay flow separation at the peak regions of the mid-span, eliminating the spanwise stall progression and resulting in delayed stall. At $\alpha$ = $16^\circ$ at which the tubercle model stalls, a large-scale streamwise vortex is originated from flow separation near the root region. This structure delays flow separation at the mid-span, leading to higher lift coefficient.