Leading-edge flow criticality as a governing factor in leading-edge vortex initiation in unsteady airfoil flows

Abstract

A leading-edge suction parameter (LESP) that is derived from potential flow theory as a measure of suction at the airfoil leading edge is used to study initiation of leading-edge vortex (LEV) formation in this article. The LESP hypothesis is presented, which states that LEV formation in unsteady flows for specified airfoil shape and Reynolds number occurs at a critical constant value of LESP, regardless of motion kinematics. This hypothesis is tested and validated against a large set of data from CFD and experimental studies of flows with LEV formation. The hypothesis is seen to hold except in cases with slow-rate kinematics which evince significant trailing-edge separation (which refers here to separation leading to reversed flow on the aft portion of the upper surface), thereby establishing the envelope of validity. The implication is that the critical LESP value for an airfoil-Reynolds number combination may be calibrated using CFD or experiment for just one motion and then employed to predict LEV initiation for any other (fast-rate) motion. It is also shown that the LESP concept may be used in an inverse mode to generate motion kinematics that would either prevent LEV formation or trigger the same as per aerodynamic requirements.

Flow criticality governs leading-edge-vortex initiation on finite wings in unsteady flow

“…Further, the CFD-predicted time instants for LEV initiation for a large set of unsteady airfoil motions were also shown in Ramesh et al. (2017) to qualitatively agree with experimental results from dye-flow visualization of the corresponding unsteady motions in water-tunnel experiments. This experimental confirmation was achieved by showing that, for each motion, there was a formation of a distinct LEV structure in the dye-flow visualization just after the time instant at which LEV initiation was observed from the surface- signature in the RANS CFD result.…”

Variation of leading-edge suction during stall for unsteady aerofoil motions

“…As mentioned in the work of Ramesh et al (2014), an important limitation of the LESP hypothesis is that the in that approach is determined using inviscid/attached-flow theory (unsteady thin-aerofoil theory), in which the flow over the aerofoil surfaces is assumed to be attached. Thus, as discussed by Ramesh et al (2017), in situations characterized by trailing-edge flow separation, this ‘inviscid LESP’ is unlikely to be a true measure of the leading-edge suction. Furthermore, the value derived from unsteady inviscid theory for high pitch-rate motions with negligible trailing-edge boundary-layer separation is not applicable for much lower pitch-rate motions where trailing-edge flow separation is present.…”

“…Furthermore, the value derived from unsteady inviscid theory for high pitch-rate motions with negligible trailing-edge boundary-layer separation is not applicable for much lower pitch-rate motions where trailing-edge flow separation is present. The inability of the inviscid LESP to account for the effects of trailing-edge separation was postulated as the reason for the small variation of with pitch rate observed in Ramesh et al (2017). With a new approach to determine the ‘viscous LESP’ using RANS CFD solutions in the current work, an important aim is to study the viscous behaviour to examine if accounting for the effects of trailing-edge separation will collapse the viscous values for all motions into a small range, irrespective of whether or not boundary-layer separation is present during initiation of LEV formation.…”

Modeling the interplay between the shear layer and leading edge suction during dynamic stall

“…If this maximum limit is exceeded, vorticity is released from the leading edge, accumulates, and forms a leading edge or dynamic stall vortex. The critical value of the leading edge suction parameter depends on the airfoil shape and Reynolds number and is independent of the motion kinematics for scenarios where no trailing-edge separation is present 38 .…”

“…According to Ramesh & al. 38 , the critical value of the leading edge suction parameter depends mainly on the airfoil shape and the Reynolds number and is independent of the motion kinematics except when high degrees of trailing-edge flow separation occur during the motion. The ideal dynamic stall prediction parameter has a critical value at dynamic stall onset that is largely independent of the flow and motion conditions and can easily be determined or derived for various airfoil geometries.…”