Periodically plunging foil near a free surface

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Abstract

Experiments were performed to investigate the effects of amplitude and depth on the drag reduction of a NACA 0012 airfoil plunging near a free surface for a range of frequencies. Beyond the effect of the free surface, at low Strouhal numbers based on amplitude, SrA, the drag reduction follows a parabolic trend with greater effect for greater amplitude, similar to the Garrick predictions. At SrA ≈ 0.08, larger amplitudes break from this trend due to leading-edge vortex formation. As a result, smaller amplitudes become preferable for SrA > 0.12. In addition, for the first time, vortex lock-in is documented experimentally. The effect of depth is twofold; firstly with decreasing depth, there is a general departure from the Garrick trends. Secondly, a reduction in thrust is observed around a constant unsteady parameter of τ = U∞2 πf/g ≈ 0.25; around this value significant free-surface waves form that detract from thrust creation. For depths greater than two chord lengths, there is negligible free-surface effect.
LanguageEnglish
Article number1491
Number of pages18
JournalExperiments in Fluids
Volume54
Issue number3
DOIs
StatusPublished - Mar 2013

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Metal foil
foils
Drag reduction
Vortex flow
drag reduction
Strouhal number
trends
thrust
Airfoils
Surface waves
vortices
airfoils
leading edges
surface waves
Experiments
predictions

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Periodically plunging foil near a free surface. / Cleaver, David; Calderon, Dario E; Wang, Zhijin; Gursul, Ismet.

In: Experiments in Fluids, Vol. 54, No. 3, 1491, 03.2013.

Research output: Contribution to journalArticle

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