Micro-Air Vehicles

Designers of Micro Air Vehicles (MAVs) face a huge challenge.  These small remotely (or autonomously) operated vehicles fly at very low Reynolds number, so laminar separation is common.  Predicting free shear layer tripping and reattachment is essential.  Lift and propulsion are often generated using flapping wings, so the resulting flows are highly unsteady.  Forces are sensitive to flapping amplitude, frequency, and phase.  Methodologies for predicting stability or developing control algorithms are not well understood. 

AFT uses unsteady moving-grid RANS simulations to analyze flapping wing aerodynamics.  The picture at right shows one tested MAV design consisting of a fixed lift-generating wing followed by two flapping propulsion wings.  Trailing wing motion amplitude, frequency, and phase are varied to identify the most efficient mode for straight-line flight.  A large number of flapping motions were simulated, and models developed to relate amplitude, frequency, and phase to lift and thrust.  These models can be used to populate stability and control algorithms, or built into flight motion simulators for devising methods to maneuver and hover.

AFT’s RANS codes are well suited to these types of problems.  An initial grid is prepared, and then perturbed in time using motions prescribed by the designer.  Once statistically stationary force histories are reached, model development begins. 

The two embedded movies show how AFT’s moving grid capabilities can be used to simulate MAV aerodynamics.   The first shows a centerline cut of how the overset method resolves relative body motion, and the second shows the resulting vortex wake pattern.