Numerical Investigation of Steady and Harmonic Vortex Generator Jets Flow Separation Control

Aria Alimi,Olaf Wünsch

doi:10.3390/fluids3040094

Abstract

Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient at reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures forming immediately downstream of the injection location led to the formation of hairpin-type vortices, causing an earlier transition to turbulence. Symmetric hairpin vortices were shown to develop downstream of the forcing location for the harmonic VGJs, as well. However, the increased control effectiveness for harmonic VGJs’ flow control strategy is attributed to the fact that the shear-layer instability mechanism was exploited. As a result, disturbances introduced by VGJs were strongly amplified, leading to the development of large-scale coherent structures, which are very effective at increasing the momentum exchange, thus limiting the separated region.

Highlights

A laminar boundary layer subject to an adverse pressure gradient (APG) is susceptible to separation.In the presence of strong APG, the boundary layer will detach from the solid surface
The amplified disturbances increase the exchange of momentum and eventually drive the flow to reattach as a turbulent boundary layer, leading to the so-called transitional laminar separation bubbles (LSBs) [1,2]
The main focus of this paper was to investigate the underlying flow physics of controlled LSBs using steady and harmonic vortex generator jets (VGJs). We found that both VGJs’ flow control strategies were successful at reducing the streamwise and wall-normal extent of the separation bubbles when the same blowing amplitude was used for the actuation

Summary

Introduction

A laminar boundary layer subject to an adverse pressure gradient (APG) is susceptible to separation.In the presence of strong APG, the boundary layer will detach from the solid surface. A laminar boundary layer subject to an adverse pressure gradient (APG) is susceptible to separation. A separated shear-layer exhibits inflectional velocity profiles, which support the amplification of small disturbances as a result of fluid dynamics instabilities. The amplified disturbances increase the exchange of momentum and eventually drive the flow to reattach as a turbulent boundary layer, leading to the so-called transitional laminar separation bubbles (LSBs) [1,2]. Laminar separation bubbles can occur in many important aerospace applications operating at low Reynolds numbers, such as low-pressure turbine blades, wings of small unmanned aerial vehicles (UAVs), wind turbine blades and laminar flow airfoils, to name a few. Flow separation can lead to massive degradation of aerodynamic performance characteristics such as loss in lift and a significant increase in drag [3]. Transitional LSBs have been the subject of numerous detailed experimental and numerical investigations in the past few decades [1,2,3,4,5,6]

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