Experimental quantification of unsteady leading-edge flow separation

TL;DR

This paper introduces an experimental method to quantify unsteady flow separation at the leading edge of aerofoils using a leading-edge suction parameter derived from circulation measurements, validated under various flow conditions.

Contribution

It develops and validates a novel approach to measure unsteady leading-edge flow separation without assuming flow steady state or Kutta condition, applicable to different aerofoil thicknesses.

Findings

The leading-edge suction parameter accurately indicates flow separation degree.

Higher-order terms are necessary for thicker aerofoils.

The method works under unsteady flow conditions, including fluctuating velocities and pitching motions.

Abstract

We propose here a method to experimentally quantify unsteady leading-edge flow separation on aerofoils with finite thickness. The methodology relies on the computation of a leading-edge suction parameter based on measured values of the partial circulation around the leading-edge and the stagnation point location. We validate the computation of the leading-edge suction parameter for both numerical and experimental data under steady and unsteady flow conditions. The leading-order approximation of the definition of the leading-edge suction parameter is proven to be sufficiently accurate for the application to thin aerofoils such as the NACA0009 without a-priori knowledge of the stagnation point location. The higher-order terms including the stagnation point location are required to reliably compute the leading-edge suction parameter on thicker aerofoils such as the NACA0015. The computation of the leading-edge suction parameter from inviscid flow theory does not assume the Kutta condition to be valid at the trailing edge which allows us to compute its value for separated flows. The relation between the leading-edge suction parameter and the evolution of the shear layer height is studied in two different unsteady flow conditions, a fixed aerofoil in a fluctuating free-stream velocity and a pitching aerofoil in a steady free-stream. We demonstrate here that the instantaneous value of the leading-edge suction parameter based on the partial circulation around the leading edge is unambiguously defined for a given flow field and can serve as a directly quantitative measure of the degree of unsteady flow separation at the leading edge.