The Development and Control of Axial Vortices over Swept Wings

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Virginia Tech


The natural unsteadiness in the post-breakdown flowfield of a 75° sweep delta wing at 40° angle of attack was studied with dual and single point hot-wire anemometry in the Engineering Science and Mechanics (ESM) Wind Tunnel at a Reynolds number Re = 210,000. Data were taken in five crossflow planes surrounding the wing's trailing edge. Results showed a dominant narrowband Strouhal frequency of St = 1.5 covering approximately 80% of the area with lower-intensity broadband secondary frequencies over 15% of that region. Cross-correlations between a fixed and traversing wire were calculated and phase and coherences mapped to determine the convection speed and trajectory of the helical mode instability. High-speed Particle Image Velocimetry (PIV) was conducted over a 75° sweep delta wing at 40° angle of attack in the ESM Water Tunnel II at Re = 45,000. Data were taken along the axis of the vortex in the breakdown flowfield at a speed of 0.1% of the convective time scale of the flow. Animations of instantaneous streamlines and velocity vectors revealed the impression of a helically spiralling vortex core on the measurement plane. Spectral analysis of the PIV data showed reduced frequencies which confirmed those found with the single-point measurements made in the ESM Wind Tunnel. The effect of four novel control surfaces on the breakdown flowfield of the delta wing was studied with surface pressure measurements along the axis of the vortex in the ESM Wind Tunnel. The apex flap was found effective and delayed vortex breakdown by 8° for a fixed wing. The flowfield was characterized over the delta wing executing a pitch-up maneuver at a reduced frequency of 0.06. Surface pressure measurements were taken in the ESM Wind Tunnel and Laser Doppler Velocimetry (LDV) was employed in the ESM Water Tunnel I as both the unmodified wing and then the wing with an apex flap deployed at an optimal angle b = 15° executed the pitch-up. Both sets of data confirmed the hysteresis of the flowfield. The LDV data, taken in two crossflow planes throughout the maneuver, showed an asymmetric breakdown development. As a practical extension of the study of the breakdown wake flowfield, hot-wire measurements were made over an F/A-18 model to determine the spectral characteristics of the flowfield. Three-dimensional vortex interactions were investigated with helium bubble flow visualization in the VPI Stability Tunnel.



Particle Image Velocimetry, control surfaces, forebody vortices, F/A-18 vortex interaction, delta wing, vortex breakdown, flow periodicity