Numerical Investigation of Boundary Effects on Flapping Wing Study

Abstract

It was experimentally observed that a rectangular SD7003 wing, with an aspect ratio of 3, mounted wall to wall in a water channel, causes strong spanwise velocity variation when the wing undergoes highfrequency pitching or plunging motions. This observation demonstrates a drastic departure from what we observe when the wing is stationary, in which flow is nearly two-dimensional across the span except near the walls. Because the experiment was designed to approximate a two-dimensional flapping wing, the strong spanwise variation imposes two challenges: first, is the experiment still a valid approximation to the two-dimensional case? Second, which cross section is a good approximation to the two-dimensional case? In this paper we aim to answer the two questions using numerical simulation. The flow field is obtained by solving the three-dimensional Navier-Stokes equations in curvilinear coordinates. The flapping wing motion is handled using an algebraic moving grid technique. We compare the flow field and force histories between a three-dimensional wing and a two-dimensional airfoil undergoing the same flapping motions. In this paper we also investigate the tunnel blockage/computational domain size effect on flow field and aerodynamic forces. Our simulation indicates that the blockage/computational domain size plays an important role in both flow field and aerodynamic forces. It also shows that the strong spanwise variation is caused by the sidewalls but extenuated by the leading edge vortex.