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Experimental Investigation of Boundary Layer Tripping on Oscillating Foils
Dylan Iverson, Matthieu Boudreau, Guy Dumas, Peter Oshkai

Last modified: 2018-06-25

Abstract


A foil that oscillates in pitch and heave can, under proper kinematics, be used to extract energy from a flow. Existing numerical and experimental research campaigns have spanned laminar, transitional, and turbulent regimes in attempts to quantify the unsteady coupled fluid-structure interaction. Laminar numerical studies indicated a coherent leading edge vortex to be the dominant energetic source in the flow. However, this mechanism can be suppressed at higher Reynolds numbers (O(10^5-10^6)). In transitional regimes (Re O(10^4)), which are encountered in many model-scale experiments, the flow is less predictable. We performed an experimental study to develop an approach for mitigation of the uncertainties associated with the transitional flow regime by applying surface roughness to promote a controlled bypass to turbulence. The experiments were conducted in a recirculating water channel with an oscillating NACA 0015 hydrofoil in the range of Reynolds numbers of 20,000 ≤ Re ≤ 40,000. We employed a combination of direct force measurements and particle image velocimetry (PIV) to quantify the hydrodynamic performance of the foil and provide quantitative insight into the structure of its near-wake. The experiments involved a smooth foil and a foil with a strip of roughness applied at the position of its maximum thickness. Under oscillating conditions, the roughened foils exhibited flow structures and performance that approximated known baseline conditions corresponding to Re = 500,000 for smooth foils.


Keywords


Oscillating foil; transition to turbulence; energy harvesting

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