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

Detecting video phenomena, such as fire in an aircraft cargo bay, includes receiving a plurality of video images from a plurality of sources, compensating the images to provide enhanced images, extracting features from the enhanced images, and combining the features from the plurality of sources to detect the video phenomena. The plurality of sources may include cameras having a sensitivity of between 400 nm and 1000 nm and/or may include cameras having a sensitivity of between 7 and 14 micrometers. Extracting features may include determining an energy indicator for each of a subset of the plurality of frames. Detecting video phenomena may also include comparing energy indicators for each of the subset of the plurality of frames to a reference frame. The reference frame corresponds to a video frame taken when no fire is present, video frame immediately preceding each of the subset of the plurality of frames, or a video frame immediately preceding a frame that is immediately preceding each of the subset of the plurality of frames.

Video detection/verification system

: 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

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

Control of Vortex Breakdown over Highly Swept Wings

Vortical lift over a highly swept wing can be controlled by stabilizing or strengthening the major vortex by a small continuous jet of air, or a pulsating jet, near the core of the vortex. This method relies on the natural inductive action of the vortex swirling flow to entrain the injected air into the core of the vortex. When the additional momentum from the blowing air becomes aligned with the vortex mean flow direction, the vortex is stabilized and vortex breakdown is delayed.

Stephen A. Guillot

Papers

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

Control of Vortex Breakdown over Highly Swept Wings

Vortical lift control over a highly swept wing

Lift contol of a delta wing by jet injection