Vortex burst behavior of a dynamically pitched delta wing under the influence of a von Kàrmàn vortex street and unsteady freestream
An experimental investigation was undertaken at Wichita State University in order to quantify the vortex burst behavior of a pitching 70-degree sweep delta wing subjected to a variable freestream velocity (accelerating or decelerating flow), as well as to an impinging von Kàrmàn vortex street generated by a cylinder placed ahead of the apex. The experiments were inspired by flow features present in the flow field of an aircraft executing a “Cobra” maneuver. A total of 222 test runs were conducted which resulted in the analysis of 6481 video frames for the von Kàrmàn experiments and 8566 video frames for the variable velocity experiments. It was found that at different a−ranges and velocity ratios, accelerating the flow produced a mild to strong negative effect (i.e., an acceleration of the forward propagation velocity) on the burst location. This negative effect was almost independent of the actual acceleration or range of a over which it occurred. Deceleration, on the other hand, was found to delay the forward burst movement along the vortex core. This was consistent for the pitch rates tested, and in all cases resulted in a momentary reduction of the forward propagation of the burst location. In the more extreme cases where the velocity ratio was large (Vstart/Vfinal =2) a complete stop to the forward burst movement was possible. The most important result from the von Kàrmàn experiments was that the burst could be observed by the “jumping” of the burst location forward towards the wing’s apex in response to the convection of the von Kàrmàn wake filament. This was accomplished at both a regular frequency of approximately 3 Hz, and at higher non-uniform frequency centered around 5 Hz, the only difference between the two being the degree of change in the burst location. The experiments performed here represent an initial step towards a more complex Cobra maneuver experiment.
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