Periodically plunging foil near a free surface

D. J. Cleaver, D. E. Calderon, Z. Wang, I. Gursul*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

10 Citations (Scopus)

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, Sr-A, the drag reduction follows a parabolic trend with greater effect for greater amplitude, similar to the Garrick predictions. At Sr-A approximate to 0.08, larger amplitudes break from this trend due to leading-edge vortex formation. As a result, smaller amplitudes become preferable for Sr-A > 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 tau = U(infinity)2 pi f/g approximate to 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.

Original languageEnglish
Article number1491
Number of pages18
JournalExperiments in Fluids
Volume54
Issue number3
DOIs
Publication statusPublished - Mar 2013

Keywords

  • LOW-REYNOLDS-NUMBERS
  • FRICTION DRAG REDUCTION
  • PROPULSIVE EFFICIENCY
  • OSCILLATING AIRFOIL
  • WAKE
  • WAVE
  • FREQUENCY
  • FLOWS
  • GENERATION
  • AEROFOILS

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