Flow around oscillating airfoil

Underlying Flow Regime 2-03 © copyright ERCOFTAC 2004

Description

Preface

The flow around oscillating aerofoil is one of the fundamental and important problems investigated in aerospace for wings and helicopter blades. Up to date, studies, which provided quality data, were conducted mainly experimentally. During the last decade there appeared a good selection of papers that concentrated on different numerical techniques for resolving boundary movement in oscillating aerofoils. Majority of these papers show only inviscid representation of the flow. Literature review, using citation index, shows that there are only few papers, which present viscous, turbulent flows around oscillating aerofoils, and which make an attempt to compare results with experiments.

Introduction

Investigations involving oscillating aerofoils are usually performed to gain better understanding of unsteady flows with presence of dynamic stall and/or buffet.

Buffet can occur at transonic speeds. At moderate incidence angle, flow around aerofoils at transonic speeds shows a large supersonic region on the upper surface followed by an isentropic compression or a weak shock wave. Increasing the incidence angle results in stronger shocks, which initially thicken the upper surface boundary layer. Dependent on the magnitude of the rear adverse pressure gradient, a trailing edge separation may occur. A further increase in shock strength causes the boundary layer to separate at the foot of the shock and the development of a shock induced separation bubble. Continuation of increase of the incidence angle causes the shock-induced separation bubble to spread downstream while rear separation may slowly move upstream [1,2]. Joining of the two separated regions may lead to aerofoil buffet. The accurate prediction of buffet is important for both civil and military aircraft. It has been reported [3], that the buffet computations are more demanding than dynamic stall computations, due to the high resolution in time required to resolve the flow unsteadiness. In addition, to predict the buffet onset several computations need to be performed at different conditions to compare the predicted loads.

Dynamic stall is associated with low, transonic speeds and is of practical importance to aircraft manoevrability, helicopter rotors and wind turbines. It combines unsteady effects with flow non-linearity and strong viscous-inviscid interaction. Dynamic stall vortex is a characteristic energetic vortical structure which leads to temporary lift increase. The development of the dynamic stall vortex is accompanied by boundary-layer growth, separation, unsteadiness, shock–boundary, viscous-inviscid, vortex-aerofoil and vortex-vortex interactions. Satisfactory predictions had been reported in ref [8] for laminar flows at low Mach numbers, while the results for turbulent cases depended strongly on the turbulent closure. References [8] and [9] and literature therein provide a good selection of problems for investigation of dynamic stall.

Review of UFR studies and choice of the test case

There are only a limited number of journal papers that present viscous, turbulent flows around oscillating aerofoil, and compare results with experiments. The author can recommend references 3, 8, 9 and 12. Comments about the material in these references are made, as appropriate, in the other parts of this document.

Since an accurate prediction of buffet and dynamic stall is still challenging, a simpler test case illustrating shock movement during oscillatory pitching of the NACA 64A010 has been chosen. This test case had been identified in ref [6] as one of the most important preferred cases for investigation of oscillating aerofoil. Three key references are reviewed in detail in section 5.

Contributors: Joanna Szmelter - Cranfield University