Oscillating fin flow has been a subject of study due to its simplicity and similarity to aquatic propulsion methods found in nature. At intermediate Reynolds numbers, in the range of 10 to 1000, drag tends to have a larger impact than at higher Reynolds numbers and the boundary layer thickness on the fin surface tends to be larger, which can alter the vortices shed during oscillation. To attempt to reduce these effects, an apparent slip condition may be placed on the surface of the fin. In this investigation, entrapment of pneumatically supplied air at a perforated fin surface was used to supply the slip condition. The expectation was that the apparent slip condition would reduce the boundary layer thickness on the fin, with a corresponding effect on the generated wake and generated forces. To observe the the flow behavior, a rectangular oscillating fin (pitching only motion) with air entrapment was placed in a flow tunnel. The fin ratio (length:height) was 2:1 Experiments were performed at a range of reduced frequencies and amplitudes such that the Strouhal number varied between 0.07 and 0.9. The Reynolds number was approximately 700. The flow was measured empirically using defocusing digital particle tracking velocimetry (DDPTV). The flow field data collected was processed with in house code to determine the thrust produced and circulation of the vortices. No strong effects on the thrust or circulation were observed. However, in some cases, differences in vortex geometry and orientation were observed. To further study this effect, a heaving motion could be added to the oscillations. Additionally, the Reynolds number could be varied.
Dr. Paul S. Krueger
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Davis, Nicholas, "The Effect of Surface Air Entrapment on Oscillating Fin Flow at Intermediate Reynolds Number" (2020). Mechanical Engineering Research Theses and Dissertations. 32.
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