The onset of dynamic stall revisited

Abstract

Dynamic stall on a helicopter rotor blade comprises a series of complex aerodynamic phenomena in response to the unsteady change of the blade's angle of attack. It is accompanied by a lift overshoot and delayed massive flow separation with respect to static stall. The classical hallmark of the dynamic stall phenomenon is the dynamic stall vortex. The flow over an oscillating OA209 airfoil under dynamic stall conditions was investigated by means of unsteady surface pressure measurements and time-resolved particle image velocimetry. The characteristic features of the unsteady flow field were identified and analysed utilising different coherent structure identification methods. An Eulerian and a Lagrangian procedure were adopted to locate the axes of vortices and the edges of Lagrangian coherent structures, respectively; a proper orthogonal decomposition of the velocity field revealed the energetically dominant coherent flow patterns and their temporal evolution. Based on the complementary information obtained by these methods the dynamics and interaction of vortical structures were analysed within a single dynamic stall life cycle leading to a classification of the unsteady flow development into five successive stages: the attached flow stage; the stall development stage; stall onset; the stalled stage; and flow reattachment. The onset of dynamic stall was specified here based on a characteristic mode of the proper orthogonal decomposition of the velocity field. Variations in the flow field topology that accompany the stall onset were verified by the Lagrangian coherent structure analysis. The instantaneous effective unsteadiness was

Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

“…The experiments of Mulleners and Raffel [14,15] on a cambered airfoil pitching Δα 6-8 deg around a statically attached, but high, angle of attack, highlighted five stages of the flow corresponding to attached flow, stall development, stall onset, stall, and flow reattachment [14] and identified a "primary stall vortex" pinched off at the point of dynamic stall [15]. Furthermore, it was determined that light stall, characterized by a much smaller separation region, was caused when the primary stall vortex separated at maximum angle of attack due to the change in direction of the airfoil.…”

“…Furthermore, it was determined that light stall, characterized by a much smaller separation region, was caused when the primary stall vortex separated at maximum angle of attack due to the change in direction of the airfoil. Deep stall, which corresponds to a much larger separation region, occurred when the vortex separated before maximum angle of attack [14].…”

Critical evolution of leading edge suction during dynamic stall

“…A large range of dynamic stall cycles was obtained by varying the motion parameters α 0 , α 1 and f osc . The instantaneous eective unsteadiness was introduced previously by Mulleners and Rael [10] as a single representative parameter to describe the inuence of the motion parameters describing a sinusoidal pitching motion. The instantaneous eective unsteadinessα ss is dened as the rate of change of the angle of attack at the moment when the static stall angle is exceeded and it is non-dimensionalized by the convective time:α ss =α ss c/U 0 .…”

“…Stall is delayed under dynamic conditions to t ds . The timing of dynamic stall onset was determined directly from the velocity eld based on a characteristic mode of the proper orthogonal decomposition of the velocity eld [10]. The interval between the passage of the static stall angle t ss and the dynamic stall onset t ds is called the stall development stage.…”

Instability of the radial flow over a rotating disk in a separated edgewise stream

“…Studies of dynamic stall using pitching and plunging airfoils in nominally two-dimensional conditions 6,7 cannot capture the effects of reactive centrifugal forces that are present on a rotating blade. These effects become especially significant and apparent on the retreating blade side, through the stall and reattachment phases.…”