Research into vortex breakdown control

*† ‡ Vortex control concepts employed for slender and nonslender delta wings were reviewed. Important aspects of flow control include flow separation, vortex formation, flow reattachment, vortex breakdown, and vortex instabilities. The occurrence and relative importance of these phenomena strongly depend on the wing sweep angle. Various flow control methods were discussed: multiple vortices, control surfaces, blowing and suction, low-frequency and high-frequency excitation, feedback control, passive control with wing flexibility, and plasma actuators. For slender delta wings, control of vortex breakdown is achieved by modifications to swirl level and external pressure gradient acting on the vortex core. Effects of flow control methods on these two parameters were discussed, and their effectiveness was compared whenever possible. With the high-frequency excitation of the separated shear layer, reattachment and lift enhancement in the post-stall region is observed, which is orders of magnitude more effective than steady blowing. This effect is more pronounced for nonslender wings. Re-formation of vortices is possible with sufficient amplitude of forcing at the optimum frequency. Passive lift enhancement on flexible wings is due to the self-excited wing vibrations, which occur when the frequency of wing vibrations is close to the frequency of the shear layer instabilities, and promote flow reattachment.

Review of flow control mechanisms of leading-edge vortices

The separated flow around a balance-mounted, 60-deg sweptback, semispan delta wing with a sharp leading edge was controlled using zero-mass-flux periodic excitation from a segmented leading-edge slot. Excitation was generated by cavity-installed piezoelectric actuators operating at resonance with amplitude modulation (AM) and burst mode (BM) signals being used to achieve reduced frequencies (scaled with the freestream velocity and the root chord) in the range from O(1) to O(10). Results of a parametric investigation, studying the effects of AM frequency, BM duty cycle and frequency, excitation amplitude, location of the actuation along the leading edge, and optimal phase difference between the actuators, as well as the Reynolds number, are reported and discussed

Delta wing stall and roll control using segmented piezoelectric fluidic actuators

: 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.

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

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.

Vortical Substructures in the Shear Layers Forming Leading-Edge Vortices

Injection of jets from nozzles embedded in a delta wing were used to add momentum into the core of the leading edge vortices and thus delay their breakdown. The tests were performed on a delta wing with a 60° sweep angle at a Reynolds number of 260,000 and angles of attack varying from 12 to 20°. At a typical angle of attack of 15°, the vortex breakdown location was moved from its natural location at 55% of the wing chord to 90% near the trailing edge, using a blowing coefficient of Cu = 0.007. Though optimized for a = 15°, the jet blowing was effective in the entire range of angles of attack studied. Flow visualizations, which were performed by illuminating smoke with a laser sheet, showed the effect of the jet blowing on the vortex core and were used to determine the vortex trajectory and breakdown location. The mean axial velocity and turbulence intensity were mapped across the vortex at several axial locations using Laser Doppler Velocimetry. Vortex breakdown resulted in strong deceleration of the mean velocity in the core region, the appearance of negative flow and increased turbulence intensity. Fluctuations in the vortex breakdown location at a Strouhal number of 0.13 were recorded. The control jet injection accelerated the core velocity and stabilized the vortex as evidenced by a reduction in the turbulence intensity and a substantial shift of the vortex breakdown location towards the trailing edge. Nomenclature

Delay of Vortex Breakdown over a Delta Wing via Near-Core Blowing

Large amplitude pressure oscillations are sometimes encountered in the combustion chamber of solid rocket motors. These oscillations can be excited by the acoustic modes of the chamber, flow turning, flame instabilities or, if inhibitors are present, by their interaction with the mean flow. A cold air facility was built to study these interactions in a solid rocket motor simulator. Several membranes were tested and two different resonance conditions were found. One was caused by the natural response of the membrane, while the other corresponded to the first axial mode of the pipe. A finite element analysis was performed to study theoretically the behavior of the membrane, and good consistence was found with the experimental results. Flow visualization and mean velocity measurements were done to examine the structure of the jet. Flow behavior depended on the excitation of the preferred mode and/or the initial shear layer instability.

T. J. Garrison

Papers

Delay of Vortex Breakdown over a Delta Wing via Near-Core Blowing

Interaction of Flow and Structure Instabilities in a Jet Flowing through a Flexible Membrane