Anthony Mitchell

Research into vortex breakdown control

Abstract Vortex breakdown remains a significant and intriguing phenomenon that can have detrimental or beneficial effects, depending on the application. Thus there is a strong need to both better understand the phenomenon and to control it, either to prevent breakdown or to promote it. For the past 50 years, multiple flow control techniques have demonstrated the ability to manipulate the vortex breakdown location over slender delta wings at high angles of attack. An extensive historical review of these diverse control methods, mechanical and pneumatic, steady or periodic, is presented and discussed; however, none of these techniques has clearly demonstrated a superior efficiency or effectiveness in controlling either the vortical flow structure or the vortex breakdown location. Each technique, does, on the other hand, provide a unique approach to the control of the vortex breakdown depending on the desired outcome. There are still major obstacles to overcome before the control of vortex breakdown is implemented in flight. For example, oscillations of the vortex breakdown locations are difficult to quantify and to identify. The often poor effectiveness of control techniques can be in great part attributed to insufficient knowledge of breakdown and in an inability to accurately predict breakdown. When considering the large quantity of studies aimed at vortex breakdown control and their relative success, it is clear that decisive progress in this domain will require further basic investigations to clearly elucidate the physics of the phenomenon and to improve the predictive capability.

Vortical Substructures in the Shear Layers Forming Leading-Edge Vortices

Control of leading-edge vortex breakdown over sharpedged, slender, delta wings at high angles of attack is highly dependent on the knowledge of and the ability to detect or observe the principle characteristics of the phenomenon. Substantial theoretical, experimental and numerical research has focused on the characteristics of leading-edge vortices and vortex breakdown. However limited efforts have sought to understand the separating shear layers which roll up to form the leading-edge vortices. Increased interest in the role of vorticity in the vortex breakdown phenomena and recent advances in experimental measurement techniques have enabled more detailed analysis of the vortical flow field and the separating shear layer. The objective of this study is to characterize the vortical substructures in the separating shear layers forming the leading-edge vortices around a delta wing. Threedimensional Laser Doppler Velocimetry flow field measurements are realized in an ONERA subsonic wind tunnel around a sharp-edged, delta wing model with a 70°sweep angle and a root chord of 950mm at a chord based Reynolds number of 1.56xl0 6. The results provide a description of the structure and path of these vortical substructures around the leading-edge vortex cores. Additionally, information is obtained on the structure and path of the vortical substructures under the influence of along-the-core blowing which is used to manipulate the vortex breakdown location. This research is the next step towards a better understanding of the origin of the substructures and their influence on the leading-edge vortices and vortex breakdown.

Oscillation of Vortex Breakdown Location and Blowing Control of Time-Averaged Location

Abstract : The goal of this research is the control of leading-edge vortex breakdown location utilizing along-the-core blowing near the apex on the leeward surface of sharp-edged, slender, delta wings at high angles of attack. In the S2Ch subsonic wind tunnel at ONERA Chalais Meudon, two delta wing models with 70-deg sweep angles and root chords of 950 mm have been configured to collect qualitative and quantitative surface and flowfield data. First, an examination of the streamwise, time-dependent oscillation of the leading-edge vortex breakdown locations without active flow control is presented. These results further the understanding of the vortex breakdown phenomena and provide a more precise basis for evaluating the effectiveness of various flow control methods. Second, open-loop blowing along one of the vortex cores on the leeward surface of the delta wing demonstrates the ability to displace downstream the controlled, time-averaged, vortex breakdown location by 20% of the root chord.

Control of vortex breakdown location by symmetric and asymmetric blowing

The goal of this research is the control of leadingedge vortex breakdown location over sharp-edged, slender, delta wings at high angles of attack. A delta wing model with a 70” sweep angle and a root chord of 950 mm was used during these tests. Experiments utilizing open-loop blowing near the wing apex and along the vortex cores tangential to the leeward surface of the delta wing were accomplished in two subsonic wind tunnels at Onera. Asymmetric flow control along one vortex core demonstrates the ability to displace the controlled vortex breakdown location downstream. Asymmetric blowing, however, has no significant influence on the uncontrolled vortex breakdown location. Symmetric flow control along both vortex cores displaces both vortex breakdown locations downstream. The effectiveness of the flow control improves as the blowing mass flow rate is increased.