Dissertation Defense: Capitalizing on Convective Instabilities in a Streamwise Vortex-Wall Interaction

Committee:

• Dr. Jeffrey P. Bons, Chair (AE)
• Dr. Mohammad Samimy (ME)
• Dr. James W. Gregory (AE)
• Dr. Jen-Ping Chen (AE)

Abstract:

Secondary flows in turbomachinery and similar engineering applications are often dominated by a single streamwise vortex structure which interacts strongly with the nearby casing, blade, and/or endwall surfaces. Investigations into the control of these flows using periodic forcing have shown a discrete range of forcing frequency at which the vortex is particularly receptive. Forcing in this frequency range results in increased movement of the vortex and decreased total pressure losses. Through a numerical stability analysis it is shown that the Crow instability and a range of elliptic instabilities exist in a similar form as to what has been studied in counter-rotating vortex pairs due to their kinematic equivalence in an inviscid flow. The Crow instability is particularly affected by the presence of a solid no-slip wall. Differences in the amplification rate, oscillation angle, Reynolds number sensitivity, and transient growth are each discussed. Despite these damping effects, it is still shown that amplitude growth on the order of 20% is possible and transient growth mechanisms might result in an order-of-magnitude growth if properly initiated. With these results in mind, an experiment is developed to isolate the streamwise vortex-wall interaction. Through the use of a vortex generating wing section and a suspended splitter plate, a stable interaction is created that agrees favorably in structure to the three-dimensional computations. A small synthetic jet actuator is mounted on the splitter plate below the vortex. Phase-locked stereo-PIV velocity data and surface pressure taps both show spatial amplification of the disturbance in a frequency range which agrees well with the prediction for the Crow instability. An analysis of the vortex response shows a primarily lateral oscillation of the vortex column which strongly interacts with the secondary vortex structure that develops in the boundary layer.