Coherent structures in the wake of a stalled rectangular wing

Experimental and computational techniques are employed in an investigation of the evolution and behavior of distinct large scale structures in the separated flow field around a finite rectangular wing in high angle of attack orientations. The experimental program includes model surface flow visualization, mean loads, and acquisition of both mean and unsteady static pressure data from the model surface and wake. The computational simulation employs a two-dimensional full Navier-Stokes flow solver (INS2D, developed in the NAS Branch of NASA Ames) in combination with the Baldwin-Barth turbulence modeL Time accurate solution of the governing equations allows investigation of the unsteady structures within the separated flow field.;The mean and time dependent characteristics of the separated flow field suggest the existence of two distinct angle of attack regimes. The shallow stall regime occurs over a narrow range of incidence angles (2-3 deg.) immediately following the inception of leading edge separation. In this regime, the principal mean flow structures, termed stall cells, are manifested as a distinct spanwise periodicity in the chordwise extent of the separated region on the model surface. The cells are found to have a preferred lateral extent, consistent with previous investigations, and a distinct lateral mobility not previously reported. The effects of cellular separation on the lift and drag characteristics of the wing are minimal. The mean characteristics of the flow field associated with cellular separation are shown to be inconsistent with current models, and a new model is proposed Within the stall cells, large amplitude pressure fluctuations occur with a frequency much lower than anticipated for bluff body shedding. These fluctuations are not associated with discrete spanwise vortices but appear to be related to a convective instability in the upper surface shear layer. The low frequency pressure fluctuations observed in the vicinity of the model surface do not extend into the far wake. Both the stall cell and low frequency phenomena in the shallow stall regime may result from instabilities in the turbulent separated shear layer. In the deep stall regime, stall cells are not observed and the separated region near the model is relatively free of large amplitude pressure disturbances. The numerical simulation suggests that the two regimes are distinguished by the presence (in the shallow stall regime) or absence (in the deep stall regime) of strong convecting vortex structures resulting from periodic accumulation of lower surface vorticity in the vicinity of the trailing edge.