Characterizing Hydrodynamic Interactions of Underwater Vehicles in Close Proximity Using an Identical Ellipse Pair

Abstract

The hydrodynamic interactions between two identical 6:1 ellipses in close proximity were investigated using a 2D immersed interface method simulator in a viscous, rotational flow at Re=1500. Interactions in tandem, side-by-side, and staggered arrangements were characterized based on changes to the drag, lift, and yaw moment coefficients experienced by the ellipses. The drag and lift results agreed with existing studies of 2D cylinders performed in subcritical flow regimes. The drag interactions were divided into five regions based on changes to the individual ellipses and the overall system. The lift was repulsive and, for the closest parallel configurations, up to four times the value of drag. An overtaking maneuver was investigated by introducing a relative velocity between the ellipses. When both ellipses were moving, the lift was repulsive throughout the maneuver. The mean drag of the slower ellipse was mostly unaffected; although the largest instantaneous drag increase reached 2.5 times that of an isolated ellipse at the highest relative velocity, this was matched by a similar drag decrease in the second half of the maneuver. The drag of the faster ellipse was relatively unaffected by the overtaking maneuver. When one ellipse was stationary, the lift transitioned from repulsive to attractive as the moving ellipse passed the stationary ellipse. The stationary ellipse experienced a significant increase in mean drag at higher overtaking speeds, reaching more than half the value of an isolated ellipse moving at Re=1500. Its lift also changed significantly and was similar in magnitude to the drag. The overtaking ellipse experienced a three-to-four-fold increase in mean drag at all speeds, a thirty-fold increase in peak drag at the highest speed, and a mean lift similar in magnitude to the mean drag. The findings of this study can be used to inform fuel-efficient swimming configurations for underwater vehicles traveling in formation, as well as to increase safety when maneuvering in close proximity.