Numerical study of the propulsive performance of two-dimensional pitching foils at very high frequencies: scaling laws and turbulence effects

Purpose - This paper aims to analyze the propulsive performance of small-amplitude pitching foils at very high frequencies with double objectives: to find out scaling laws for the time-averaged thrust and propulsive efficiency at very high frequencies; and to characterize the Strouhal number above which the effect of turbulence on the mean values cannot be neglected. Design/methodology/approach - The thrust force and propulsive efficiency of a pitching NACA0012 foil at high reduced frequencies (k) and a Reynolds number Re = 16 000 are analyzed using accurate numerical simulations, both assuming laminar flow and using a transition turbulence model. The time-averaged results are validated with available experimental data for k up to about 12 (Strouhal number, St, up to 0.6). This study also compares the present numerical results with the predictions of theoretical models and existing numerical results. For a foil pitching about its quarter chord with amplitude alpha(0) = 8 degrees, the reduced frequency is varied here up to k = 30 (St up to 2), much higher than in any previous numerical or experimental work. Findings - For this pitch amplitude, turbulence effects are found negligible for St less than or similar to 0.8, and affecting less than 10% to the time-averaged thrust coefficient (C-T) over bar ( )for larger St Linear potential theory fails for very large k, even for the small pitch amplitude considered, particularly for the power coefficient, and therefore for the propulsive efficiency. It is found that (C-r) over bar similar to St(2) for large St, in agreement with recent models, and the propulsive efficiency decays as 1/k, in disagreement with the linear potential theory. Originality/value - Pitching foils are increasingly studied as efficient propellers and energy harvesting devices. Their performance at very high reduced frequencies has not been sufficiently analyzed before. The authors provide accurate numerical simulations to discern when turbulence is relevant for the computation of the time-averaged thrust and efficiency and how their scaling with the reduced frequency is affected in relation to the laminar-flow predictions. This is relevant because some small-amplitude theoretical models predict high propulsive efficiency of pitching foils at very high frequencies over certain ranges of the structural parameters, and only very accurate numerical simulations may decide on these predictions.

Automated summary

This study examines the propulsive performance of small-amplitude pitching foils at very high frequencies, finding that turbulence effects are negligible for Strouhal numbers less than or similar to 0.8, and that efficiency decreases as 1/k for larger St. The research provides valuable insights into the scaling laws for time-averaged thrust and efficiency of pitching foils under different conditions, highlighting the importance of accurate numerical simulations in predicting propulsive efficiency at high frequencies.