We study the use of small counter-rotating cylinders to control the streaming flow past a larger main cylinder for drag reduction. In a water tunnel experiment at a Reynolds number of 47 000 with a three-dimensional and turbulent wake, particle image velocimetry (PIV) measurements show that rotating cylinders narrow the mean wake and shorten the recirculation length. The drag of the main cylinder was measured to reduce by up to 45 %. To examine the physical mechanism of the flow control in detail, a series of two-dimensional numerical simulations at a Reynolds number equal to 500 were conducted. These simulations investigated a range of control cylinder diameters in addition to rotation rates and gaps to the main cylinder. Effectively controlled simulated flows present a streamline that separates from the main cylinder, passes around the control cylinder, and reattaches to the main cylinder at a higher pressure. The computed pressure recovery from the separation to reattachment points collapses with respect to a new scaling, which indicates that the control mechanism is viscous.

Dynamic shape change of the octopus mantle during fast jet escape manoeuvres results in added mass energy recovery to the energetic advantage of the octopus, giving escape thrust and speed additional to that due to jetting alone. We show through numerical simulations and experimental validation of overall wake behaviour, that the success of the energy recovery is highly dependent on shrinking speed and Reynolds number, with secondary dependence on shape considerations and shrinking amplitude. The added mass energy recovery ratio η_ma, which measures momentum recovery in relation to the maximum momentum recovery possible in an ideal flow, increases with increasing the non-dimensional shrinking parameter σ^∗ = ȧ_max/URe₀, where ȧ_max is the maximum shrinking speed, U is the characteristic flow velocity and Re₀ is the Reynolds number at the beginning of the shrinking motion. An estimated threshold σ^∗∼10 determines whether or not enough energy is recovered to the body to produce net thrust. Since there is a region of high transition for 10 < σ^∗ < 30 where the recovery performance varies widely and for σ^∗ > 100 added mass energy is recovered at diminishing returns, we propose a design criterion for shrinking bodies to be in the range of 50 < σ^∗ < 100, resulting in 61-82% energy recovery.

The flow mechanisms of shape-changing moving bodies are investigated through the simple model of a foil that is rapidly retracted over a spanwise distance as it is towed at constant angle of attack. It is shown experimentally and through simulation that by altering the shape of the tip of the retracting foil, different shape-changing conditions may be reproduced, corresponding to: (i) a vanishing body, (ii) a deflating body and (iii) a melting body. A sharp-edge, 'vanishing-like' foil manifests strong energy release to the fluid; however, it is accompanied by an additional release of energy, resulting in the formation of a strong ring vortex at the sharp tip edges of the foil during the retracting motion. This additional energy release introduces complex and quickly evolving vortex structures. By contrast, a streamlined, 'shrinking-like' foil avoids generating the ring vortex, leaving a structurally simpler wake. The 'shrinking' foil also recovers a large part of the initial energy from the fluid, resulting in much weaker wake structures. Finally, a sharp edged but hollow, 'melting-like' foil provides an energetic wake while avoiding the generation of a vortex ring. As a result, a melting-like body forms a simple and highly energetic and stable wake, that entrains all of the original added mass fluid energy. The three conditions studied correspond to different modes of flow control employed by aquatic animals and birds, and encountered in disappearing bodies, such as rising bubbles undergoing phase change to fluid.

For a body moving within a fluid, its shape and the manner in which it morphs greatly impact the energy transfer between it and the flow. In vanishing bodies, vorticity is globally shed, while added mass-related energy is released into the fluid. We investigate square-tipped, streamlined-tipped, and hollow foils towed at 10◦ angle of attack and quickly retracted in the span-wise direction, as generic models of bodies of different form undergoing rapid shape and volume change. Particle image velocimetry shows that large differences exist in their globally shed wakes. The retracting square-tipped foil forms a wake with energy in excess of the potential flow estimate before retraction starts; the extra energy results in the formation of an additional vortex ring that adds unsteadiness and complexity to the form of the wake. The streamlined-tipped foil avoids creating such ring vortices, but sheds a much less energetic wake: numerical simulation shows that energy is transferred back to the foil during the retraction phase through a thrust force. Circulation calculations show that energy transfer is enabled by the gradual shape change in this foil and is associated with simultaneous pressure gradient-induced and vorticity tilting-induced vorticity annihilation. Finally, the hollow foil combines the advantages of near-complete transfer of the original added mass-related energy to the wake and absence of a vortex ring formation, resulting in an energetic and also cleanly-evolving, stable wake. Hence, modest differences in morphing body shape are shown to result in significantly different flow patterns.

The fluid mechanics employed by aquatic animals in their escape or attack maneuvers, what we call survival hydrodynamics, are fascinating because the recorded performance in animals is truly impressive. Such performance forces us to pose some basic questions on the underlying flow mechanisms that are not yet in use in engineered vehicles. A closely related issue is the ability of animals to sense the flow velocity and pressure field around them in order to detect and discriminate threats in environments where vision or other sensing is of limited or no use. We review work on animal flow sensing and actuation as a source of inspiration and as a way to formulate a number of basic problems and investigate the flow mechanisms that enable animals to perform these remarkable maneuvers. We also describe some intriguing mechanisms of actuation and sensing.

Wedesign and test an octopus-inspired flexible hull robot that demonstrates outstanding fast-startingperformance. The robot is hyper-inflated with water, and then rapidly deflates to expel the fluid so asto power the escape maneuver. Using this robot we verify for the first time in laboratory testing thatrapid size-change can substantially reduce separation in bluff bodies traveling several body lengths,and recover fluid energy which can be employed to improve the propulsive performance. The robot isfound to experience speeds over ten body lengths per second, exceeding that of a similarly propelledoptimally streamlined rigid rocket. The peak net thrust force on the robot is more than 2.6 times thaton an optimal rigid body performing the same maneuver, experimentally demonstrating large energyrecovery and enabling acceleration greater than 14 body lengths per second squared. Finally, over 53%of the available energy is converted into payload kinetic energy, a performance that exceeds the estimatedenergy conversion efficiency of fast-starting fish. The Reynolds number based on final speedand robot length isRe ≈ 700 000.We use the experimental data to establish a fundamental deflationscaling parameter ?^∗ which characterizes the mechanisms of flow control via shape change. Based onthis scaling parameter, we find that the fast-starting performance improves with increasing size.

This paper studies the effects of geometrical features on harbor seal whiskers on its force reduction ability. Each feature in the seal whisker is sequentially stripped from the original whisker shape. Four whisker-like geometries are created from such methodology. 3D simulations of flow around the structures are performed and the resulting forces on the structures are non-dimensionalized. The lift and drag coefficients of these structures are compared to the real whisker case. The undulations on minor and major axes are found to be necessary in reducing the lift forces. Existence of only one of the undulations fails to weaken the flow and break the vortex tubes and braids in the wake. The offset angle between leading edge and trailing edge are found to have slight effects on the lift and drag coefficients. The reduction in drag coefficient is found to be dependent on the existence of undulations in minor axis. The force responses of whisker-like geometries with undulations on one of their axes are found to be periodic. When the force response on whisker-like geometries with both undulations are found to be chaotic. Vortex shedding frequency of the structure is observed to decrease considerably when no offset angle is introduced into the geometry. This may result in longer lifespan of the structure.

In this work a cephalopod-like deformable body that fills an internal cavity with fluid and expels it to propel an escape manoeuvre, while undergoing a drastic external shape change through shrinking, is shown to employ viscous as well as mainly inviscid hydrodynamic mechanisms to power an impressively fast start. First, we show that recovery of added-mass energy enables a shrinking rocket in a dense inviscid flow to achieve greater escape speed than an identical rocket in a vacuum. Next, we extend the shrinking body results of Weymouth & Triantafyllou (J. Fluid Mech., vol. 702, 2012, pp. 470-487) to three-dimensional bodies and show that three hydrodynamic mechanisms must be combined to achieve rapid escape performance in a viscous fluid: added-mass energy recovery; flow separation elimination; and an optimized energy storage and recovery. In particular, we show that the mechanism of separation elimination achieved through rapid body shrinking, coordinated with the mechanism of recovering the initially imparted added-mass energy, is critical to achieving a high escape speed. Hence a flexible, collapsing body can be vastly superior to a rigid-shell jet-propelled body.

In this paper, inline and transverse forces on elliptical cylinder immersed in the wake of circular cylinder are carefully evaluated. The effects of variations on characteristic dimension, inline and transverse position of the elliptical cylinder are determined through numerical simulations at Re =1300 based on the diameter of circular cylinder. The results indicate minute effects due to variation on inline positioning. However, variations on transverse positioning and characteristic dimension have profound effects on the amplitude of lift coefficient as well as frequency peaks in Power Spectral Density (PSD). 1:1 frequency lock-in is observed for all simulation cases.

If a moving body were made to vanish within a fluid, its boundary-layer vorticity would be released into the fluid at all locations simultaneously, a phenomenon we call global vorticity shedding. We approximate this process by studying the related problem of rapid vorticity transfer from the boundary layer of a body undergoing a quick change of cross-sectional and surface area. A surface-piercing foil is first towed through water at constant speed, U, and constant angle of attack, then rapidly pulled out of the fluid in the spanwise direction. Viewed within a fixed plane perpendicular to the span, the cross-sectional area of the foil seemingly disappears. The rapid spanwise motion results in the nearly instantaneous shedding of the boundary layer into the surrounding fluid. Particle image velocimetry measurements show that the shed layers quickly transition from free shear layers to form two strong, unequal-strength vortices, formed within non-dimensional time t* = 0:03, based on the foil chord and forward velocity. These vortices are connected to, and interact with, the foil's tip vortex through additional streamwise vorticity formed during the rapid pulling of the foil. Numerical simulations show that two strong spanwise vortices form from the shed vorticity of the boundary layer. The three-dimensional effects of the foil removal process are restricted to the tip of the foil. This method of vorticity transfer may be used for quickly introducing circulation to a fluid to provide forcing for biologically inspired flow control.

We study numerically the viscous flow around a steadily moving two-dimensional cylinder undergoing a rapid reduction in its diameter as a model problem for force production through shape change which is encountered in the locomotion of certain animals. We consider first the case of a rapidly collapsing circular cylinder in steady translation, starting from an original diameter and reaching a final, smaller diameter under prescribed kinematics. We show that the difference in added mass energy is recovered by the body, and the boundary layer vorticity is reduced through annihilation with opposite-sign vorticity generated during the reduction phase. Next we consider a steadily moving circular cylinder which undergoes rapid but orderly melting, resulting in the same reduction of its diameter but which exhibits radically different flow patterns compared to the collapsing cylinder. The original vorticity in the boundary layer is shed instantaneously and globally in the fluid at the start of the melting phase, and then rapidly rolls up to form a pair of strong vortices, which contain the energy difference between the original and final cylinder states. The formation of the vortices in the melting cylinder takes less than a third of the time required by a rigid translating cylinder to form such vortices.