The energy harvesting performance of a flapping airfoil is studied through discrete vortex model with shedding criterion at the leading edge. Results are obtained for a thin flat airfoil that undergoes a sinusoidal flapping motion for reduced frequencies of k = f C/U_inf = 0.06 - 0.16 where f is the heaving frequency of the foil, C is the chord length and U is the freestream velocity. The airfoil pitches about the mid-chord and the heaving and pitching amplitudes of the airfoil are h_0 = 0.5C and theta_0 = 70 degrees respectively, as these numbers have been shown to give optimal energy harvesting results for a rigid airfoil. The study applies a panel based discrete vortex model (A thin flat airfoil is modeled based on the surface distribution of singular vortex elements on panels with vortex shedding at the leading and trailing edges.) that incorporates a leading edge suction parameter criterion to understand the flow behavior around the airfoil. The leading edge suction parameter is found from 2D CFD simulations (Navier-Stokes equations solved in Fluent with a standard k-epsilon model under the same flow conditions) for all K values. A correlation between the critical leading edge suction parameter and reduced frequency is found from the identified critical LESP values. This correlation takes into account the length scale and velocity scale but does not consider the effect of leading edge geometry. This gives more insight to finding the critical LESP independent from CFD for future engineering purposes. An empirical trailing edge separation correction is also applied to the transient force results since flow separation at the trailing edge is anticipated. This correction calculates a fictitious separation point along the chord and adjusts the circulatory part of the force to account for the loss of bound circulation. This correction slightly corrects the overestimation of the the transient force results at some parts of the cycle. The parameters of interest from the model are transient distributions of force which is calculated with two methods (Bernoulli's and vortex impulse method), total power output which includes both heaving and pitching power calculated from Bernoulli's method, and overall energy harvesting efficiency. Model results are then validated against 2D CFD simulations. Results from both Bernoulli's and Impulse methods are in good agreement with 2D CFD results. Furthermore, The effect of reduced frequency on power production and overall energu harvesting efficiency is finally studied to identify the optimal reduced frequency for energy harvesting applications. The results from the presentstudy show that k=0.12 gives the maximum efficiency of all.


Discrete Vortex Modeling of a Flapping Foil Energy Harvester With LEV Shedding Criterion

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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