Don't be a Drag: Investigating Fluid Forces on Manta Ray Models Near a Free Surface

The shape of a manta ray body allows them to be one of the most efficient aquatic creatures. A project to investigate how a rigid manta ray model performs while gliding near the free surface was performed in two facilities: a water tunnel and a tow tank. Two identically shaped, but facility-scaled, manta ray models were subjected to various flow speeds, angles of attack, and submergences from the free surface to examine the fluid force on the body. To ensure accurate comparison between the facilities, the flow speed and submergence were scaled on similitude. Results from both facilities show the lift experienced increases with flow speed near the free surface, but at a critical depth, is constant. Similarly, the measured lift nearly doubled over the range of angle of attack analyzed. The drag force, however, varied between facilities. The tow tank experiments showed very little change at faster flow speeds, while the water tunnel experiments showed a clear increase in drag. The next phase of experiments is to examine the behavior of a flexible model in the same flow conditions. The results from this experiment can assist in the design and implementation of bio-inspired aquatic vehicles operating near the free surface.