Analysis of Leading-Edge Separation Bubbles on Rotating Blades

Abstract

T HE regions of separated flow that form on blades govern the blade stall characteristics and the loading of wind turbine blades. The nature and extent of these regions are determined primarily (except for the airfoil shape, incidence, andReynolds number) by the two most important parameters that trigger three-dimensional (3-D) effects, and they are the radius by chord ratio r=c and the rotation parameter Vw= r defining the ratio of the wind velocity to the tangential velocity. The rotation parameter shows the interaction between the axial flow (wind) and the rotational flow induced by the rotor blades. The physical mechanism driving the 3-D effects is in conjunction with the interaction between the wind speed higher than the designvelocity and the flow induced by constant-rotational speed rotors. If the rotation parameter is less than unity along the entire span (Vw= r 1 at the inner part of the span producing both strong centrifugal andCoriolis forces that involve the separation of the boundary layer on the blade. Larger rotation parameters mean enhanced rotational flow that reduces the negative pressure peak from the leading edge and produces leading-edge separation bubbles (not leading-edge stall). On the other hand, the Coriolis forces contribute to stall delay by a favorable chordwise pressure gradient. The occurrence of what is termed inboard stall delay, characterized by an abrupt increase in lift and drag, can be attributed to the sudden suck of air from the separation bubble at the leading edge of the blade that is directed toward the radial direction. Of concern here are the small separation bubbles that form near the leading edge of the inboard sections of the blade, at r=c 1. At extreme inboard locations, separation bubble initiation and aft progression are closely associated with poststall lift force magnitudes that exceed those exhibited by nonrotating blades. The flow in the vicinity of the leading edge of a blade section subject to leading-edge stall is as sketched in Fig. 1. The laminar boundary layer, extending from the stagnation point over the leading edge, separates just downstream of the point of minimum pressure. Transition to turbulent flow occurs in the free-shear layer a short distance downstream of the separation point. The flow then reattaches to the section surface, with a turbulent boundary layer extending from the reattachment point to the trailing edge. If the r=c ratio is decreased, the bubble becomes slightly shorter and ultimately coalesces in a singular topological entity [1]. The interaction between the viscous and inviscid flows in the vicinity of the bubble is taken into account through a simplified procedure in the sequel.