Coupled CFD/CSD-Based Aeroelastic Analysis with Flowfield Measurements of Avian-Based Flexible Flapping Wings for MAV Applications

Experiments were systematically executed in conjunction with a coupled CFD-CSD-based aeroelastic analysis for a MAV-scale flexible flapping-wing in forward flight. 2-D time-resolved particle image velocimetry (PIV) and force measurements were performed in a wind tunnel at a flow speed of 3 m/s. The flexible wing undergoes pure flapping kinematics held at a fixed wing-pitch angle at the root. Chordwise velocity fields were obtained at equally spaced spanwise sections along the wing (30% to 90% span) at two instants during the flap cycle (mid-downstroke and mid-upstroke) for the reference Reynolds numbers of 15,000. The flowfield measurements and averaged force measurements were used for the validation of the 3-D aeroelastic model. The objectives of the combined efforts were to understand the unsteady aerodynamic mechanisms and their relation to force production on a flexible wing undergoing an avian-type flapping motion. The coupled CFD-CSD analysis combined a compressible Reynolds Averaged Navier Stokes (RANS) solver (OVERTURNS) with a multi-body structural solver (MBDyn) to resolve the complex, highly vortical, three-dimensional flow on a flexible wing. The coupled CFDCSD predicted and experiment flowfields showed comparable results. A vorticity summation approach was used to calculate the circulation of the leading edge vortex (LEV) from the PIV measurements and the numerical simulations to further validate the CFD-CSD code. The time-averaged vertical and horizontal aerodynamic forces measured from the experiment using a miniature force balance were also used to validate the force predictions from the CFD-CSD model. Pertaining to the flow physics, the flapping motion induces large angles of attack along the wing span causing the outboard sections to stall during downstroke. During the upstroke, the outboard sections operate at very low or even negative angles of attack. The present study showed that the dynamic twisting produced by the flexible wing helped in decreasing the effective angle of attack during the downstroke and upstroke. This temporal and spanwise variation of wing pitch angle affected both lift and drag, and primarily helped the wing produce positive thrust during both upstroke and downstroke which is not possible with a rigid wing undergoing pure flap at a constant pitch angle. Both PIV and CFD-CSD studies showed that the LEV stayed attached for a longer duration of the flap cycle during downstroke for the flexible wing compared to the rigid wing, especially towards the outboard sections. Also, during the upstroke, the LEV strength for the flexible wing was significantly higher than that for the rigid wing.