Force estimation from incompressible flow field data using a momentum balance approach

Abstract

This study investigates how aerodynamic forces on a body can be evaluated using only instantaneous or averaged velocity field data through a control volume approach in an incompressible flow. The method allows for velocity field data to be missing inside the control volume, but requires that the product of the acceleration, the volume of the solid body, and the fluid density are negligible compared to the forces of interest. Validation of the method for accuracy has been conducted using three-dimensional data from a numerical simulation of unsteady flow around a static airfoil and has shown good agreement between actual and estimated force. Additionally, the method is tested using phase-averaged PIV data from flexible flapping membrane wings to calculate flapping cycle averaged forces that are compared with directly measured time-averaged forces. The numerical validation suggests that the method is accurate, while it is only able to give a rough estimate for the noisier experimental data. The lower accuracy of the forces estimated from the PIV data can be explained through experimental constraints, such as too large grid spacing in one of the three dimensions (in one of the cases), having to interpolate data on the control volume surface especially where the PIV laser light is shaded by the wing, and not covering the entire span of the wing.