Underwater glider performance at model-scale and full-scale Reynolds numbers

Underwater gliders are used for autonomous collection of oceanographic data. These vehicles glide through the water column in a vertical saw-tooth trajectory by alternating their net buoyancy. It is important to achieve high Lift to Drag if long range and endurance are sought. However, such gliders are likely to have significant areas of laminar flow over their hull. This may be critical when predicting the performance of the full-scale glider from the results of physical model-scale testing.

In this study, computational fluid dynamics (CFD) and model-scale testing was used forpredicting the Lift to Drag ratio and assessing the static stability of a blended wing body glider in pitch and yaw. Both fully turbulent and transitional flows were studied. The glider was simulated for a range of Angle-of-Attack at Reynolds numbers ranging from 1.0 x 105 – 2.4 x 106, based on the length of the glider.

As the Reynolds number was increased there was a significant increase in Lift to Drag ratio and improved pitch stability. The glider was unconditionally stable in yaw for all cases modelled. Reynolds number and physical scale therefore has a significant effect on the performance of underwater gliders. This implies that the performance of the full-scale glider cannot be simply correlated with model-scale results, but requires model testing or numerical prediction at full-scale Reynolds numbers.