Flow memory effect on viscous hydrodynamic loads of a remotely operated vehicle undergoing drift motions

• Drift angle effect on the viscous hydrodynamic loads. • 3D hysteresis loops of viscous loads of an ROV in a two-DOF translational motion. • Hydrodynamic load responses of the unsteady viscous hydrodynamic loads. • Mechanism of flow memory effect revealed by a convolution-based method. The hydrodynamic loads on a remotely operated vehicle (ROV) undergoing oblique motions are studied experimentally and numerically. A novel convolution-based method for calculating the flow memory effect response function of the viscous hydrodynamic loads is presented. The nonlinear effect of the drift angle on the hydrodynamic loads is investigated through steady drift tests, and the flow separation that occurs around the ROV is measured using the Reynolds-averaged Navier–Stokes (RANS) turbulence model. The effect of coupling longitudinal and lateral velocities is examined using frequency analysis of the loads measured in impulse motion response tests. By analyzing the hysteresis loops of the hydrodynamic loads in one apparent period, the flow memory effect is identified, whereby the ROV has two different hydrodynamic force magnitudes when it reaches the same speed at different times in one period. The existence of the flow memory effect associated with hydrodynamic loads is explained by the response functions. All convolutions related to the longitudinal, lateral, and coupling velocity at the beginning of the motion contribute to the memory effect. When the ROV drifts stably, the memory effect is solely induced by the convolution related to the coupling velocity.