The energy harvesting performance of an oscillating foil is studied by conducting a turbulent model simulation in ANSYS FLUENT. The simulation objective is to identify the key flow physics associated with the leading-edge separation of an airfoil with variable leading-edge thickness undergoing a sinusoidal oscillating motion at low freestream Reynolds numbers. Of the key characteristics are the wall shear stress, the vorticity, and the pressure field along the foil surface which all contribute to the spatial and temporal characteristics of the foil’s leading-edge flow separation. The leading-edge separation was detected from an abrupt drop in wall shear stress along the foil surface and is used as a prominent indicator of where and when vortices are being shed. These vortices have shown to be essential components of the oscillating foil’s energy harvesting performance. Similarly, the vorticity along the foil is also observed to evaluate the strength of the vortices. The pressure field is directly linked to the overall lift of the foil and impacts the rate at which the vortices separate from the foil. The results of varying geometric and motion parameters were evaluated to inform a panel-based discrete vortex model to evaluate the overall energy harvesting potential for a range of foil geometries.

The scope of the study was limited to the high-efficiency performance regime that has been shown for rigid foils operating at reduced frequencies ranging from k=fc/U=0.06-0.16, where f is heaving frequency, c is the chord length, and U is the freestream velocity. The prescribed motion of the foil was defined by a combination of sinusoidal heaving and pitching profiles where the pitching profile has a phase delay of pi/2 relative to the heaving profile. Geometric parameters were selected based on preexisting physical experimental setups for alignment and validation. Since the airfoil motion is sinusoidal, the airfoils tested are all symmetrical with no camber. The thickness of the airfoil leading edge tested ranges from 4% to 21% of the chord length and was constructed using the NACA00XX series as a reference. The key flow physics were analyzed based on various definitions of the foil’s angle of attack to define the specific conditions in which vortex shedding occurs. These definitions were selected based on some combination of the freestream velocity, heaving and pitching profiles, and tangent angles along the leading edge that defines the angle of attack. These results reveal a correlation that exists between leading-edge geometry and the effective angle of attack at which the vortex is shed.


Towards Understanding of Leading-Edge Separation for Energy Harvesting With an Oscillating Foil

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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