Transient force augmentation due to counter-rotating vortex ring pairs

Abstract

Of particular significance to biological locomotion is vortex ring interaction. In the wakes of animals, this unsteady process determines the changes in the impulse of counter-rotating vortex ring pairs (VRPs), which consist of two vortex rings with opposite sense of rotation. In this paper, these VRPs are proposed to be of particular importance to unsteady force generation. We carry out numerical computations, simulating the piston-cylinder apparatus, to study the transient changes in the impulse of counter-rotating VRPs composed of a positive and a negative vortex ring. We model the negative vortex ring (NeVR) of a VRP by making reasonable assumptions about their vorticity distributions and spatial locations, which are initially prescribed. The result of modelling is superimposed on the flow, which has a pre-existing positive vortex ring (PoVR), leading to a VRP. The simulation quantitatively demonstrates that the unsteady force resulting from a VRP is significantly larger compared with an isolated PoVR (without an NeVR). The force enhancement is also correlated to vortex configurations. A normalised force coefficient characterising force augmentation over the entire stroke is given. The force augmentation coefficient grows significantly and then reaches a plateau as the momentum input increases. The results are consistent with those in fully unsteady vortex interaction, which involves the generation of an NeVR. It is suggested that counter-rotating VRPs could offer a new perspective to explain unconventional force generation for biological swimming and flying.

How the bending kinematics of swimming lampreys build negative pressure fields for suction thrust

“…Recently, it has been shown that these interactions at the interface of vortices can enhance thrust (Fu and Liu, 2015). However, this effect is highly dependent on vortex spacing (Fu and Liu, 2015) and is controlled by the local vorticity conditions (Stanaway et al, 1988).…”

“…However, this effect is highly dependent on vortex spacing (Fu and Liu, 2015) and is controlled by the local vorticity conditions (Stanaway et al, 1988). We argue that, by maintaining a constant distance between adjacent vortices, the bending kinematics of the control lampreys controlled the interaction of the adjacent vortices, whose circulation grew as they traveled toward the tail but whose spacing was kept constant.…”

Measuring the 3D wake of swimming dice snakes using volumetric velocimetry

“…Although this was not measured in our study, the bending of the snake’s body was observed in our video recordings suggesting that the angular velocity of the snake’s body plays a role in the creation of body vortices in addition to the change of direction of its oscillation. The production of counter-rotating vortex tubes along the body has been suggested to generate additional trust (Fu and Liu 2015) during swimming.…”