Boxfish swimming paradox resolved: forces by the flow of water around the body promote manoeuvrability

The shape of the carapace protecting the body of boxfishes has been attributed an important hydrodynamic role in drag reduction and in providing automatic, flow-direction realignment and is therefore used in bioinspired design of cars. However, tight swimming-course stabilization is paradoxical given the frequent, high-performance manoeuvring that boxfishes display in their spatially complex, coral reef territories. Here, by performing flow-tank measurements of hydrodynamic drag and yaw moments together with computational fluid dynamics simulations, we reverse several assumptions about the hydrodynamic role of the boxfish carapace. Firstly, despite serving as a model system in aerodynamic design, drag-reduction performance was relatively low compared with more generalized fish morphologies. Secondly, the current theory of course stabilization owing to flow over the boxfish carapace was rejected, as destabilizing moments were found consistently. This solves the boxfish swimming paradox: destabilizing moments enhance manoeuvrability, which is in accordance with the ecological demands for efficient turning and tilting.     

Hydrodynamics of the double-wave structure of insect spermatozoa flagella

In addition to conventional planar and helical flagellar waves, insect sperm flagella have also been observed to display a double-wave structure characterized by the presence of two superimposed helical waves. In this paper, we present a hydrodynamic investigation of the locomotion of insect spermatozoa exhibiting the double-wave structure, idealized here as superhelical waves. Resolving the hydrodynamic interactions with a non-local slender body theory, we predict the swimming kinematics of these superhelical swimmers based on experimentally collected geometric and kinematic data. Our consideration provides insight into the relative contributions of the major and minor helical waves to swimming; namely, propulsion is owing primarily to the minor wave, with negligible contribution from the major wave. We also explore the dependence of the propulsion speed on geometric and kinematic parameters, revealing counterintuitive results, particularly for the case when the minor and major helical structures are of opposite chirality.     

The fluid dynamics of swimming by jumping in copepods

Copepods swim either continuously by vibrating their feeding appendages or erratically by repeatedly beating their swimming legs, resulting in a series of small jumps. The two swimming modes generate different hydrodynamic disturbances and therefore expose the swimmers differently to rheotactic predators. We developed an impulsive stresslet model to quantify the jump-imposed flow disturbance. The predicted flow consists of two counter-rotating viscous vortex rings of similar intensity, one in the wake and one around the body of the copepod. We showed that the entire jumping flow is spatially limited and temporally ephemeral owing to jump-impulsiveness and viscous decay. In contrast, continuous steady swimming generates two well-extended long-lasting momentum jets both in front of and behind the swimmer, as suggested by the well-known steady stresslet model. Based on the observed jump-swimming kinematics of a small copepod Oithona davisae, we further showed that jump-swimming produces a hydrodynamic disturbance with much smaller spatial extension and shorter temporal duration than that produced by a same-size copepod cruising steadily at the same average translating velocity. Hence, small copepods in jump-swimming are in general much less detectable by rheotactic predators. The present impulsive stresslet model improves a previously published impulsive Stokeslet model that applies only to the wake vortex.     

Generalized squirming motion of a sphere

A number of swimming microorganisms, such as ciliates (Opalina) and multicellular colonies of flagellates (Volvox), are approximately spherical in shape and swim using beating arrays of cilia or short flagella covering their surfaces. Their physical actuation on the fluid may be mathematically modeled as the generation of surface velocities on a continuous spherical surface—a model known in the literature as squirming, which has been used to address various aspects of the biological physics of locomotion. Previous analyses of squirming assumed axisymmetric fluid motion and hence required all swimming kinematics to take place along a line. In this paper we generalize squirming to three spatial dimensions. We derive analytically the flow field surrounding a spherical squirmer with arbitrary surface motion and use it to derive its three-dimensional translational and rotational swimming kinematics. We then use our results to physically interpret the flow field induced by the swimmer in terms of fundamental flow singularities up to terms decaying spatially as \({\sim } 1/r^3\) . Our results will make it possible to develop new models in biological physics, in particular in the area of hydrodynamic interactions and collective locomotion.     

Excitation of Surface Plasma Waves in Microwave Radiation Sources by Electron Beam with Allowance for Thermal Velocity Spread

Based on the hydrodynamic model, in the linear approximation, the problem of excitation by an electron beam of surface waves in electrodynamic systems of plasma relativistic microwave electronics has been considered with consideration of electron velocity spread. Complex instability increments have been determined for complex parameters of the beam-plasma system. Two instability regimes differing in the dynamics of beam plasma oscillations during instability development have been observed: Compton (singleparticle) and Raman (collective) instabilities. The role of thermal effects in an electron beam and its density in the formation of a particular mode of beam instability has been analyzed.     

Analytical insights into optimality and resonance in fish swimming

This paper provides analytical insights into the hypothesis that fish exploit resonance to reduce the mechanical cost of swimming. A simple body–fluid fish model, representing carangiform locomotion, is developed. Steady swimming at various speeds is analysed using optimal gait theory by minimizing bending moment over tail movements and stiffness, and the results are shown to match with data from observed swimming. Our analysis indicates the following: thrust–drag balance leads to the Strouhal number being predetermined based on the drag coefficient and the ratio of wetted body area to cross-sectional area of accelerated fluid. Muscle tension is reduced when undulation frequency matches resonance frequency, which maximizes the ratio of tail-tip velocity to bending moment. Finally, hydrodynamic resonance determines tail-beat frequency, whereas muscle stiffness is actively adjusted, so that overall body–fluid resonance is exploited.     

Propulsive performance of a fish-like travelling wavy wall

Summary. A numerical simulation is performed to investigate the viscous flow over a smooth wavy wall undergoing transverse motion in the form of a streamwise travelling wave, which is similar to the backbone undulation of swimming fish. The objective of this study is to elucidate hydrodynamic features of the flow structure over the travelling wavy wall and to get physical insights to the understanding of fish-like swimming mechanisms in terms of drag reduction and optimal propulsive efficiency. The effect of phase speed, amplitude and Reynolds number on the flow structure over the wavy wall, the drag force acting on the wall, and the power consumption required for the propulsive motion of the wall is investigated. The phase speed and the amplitude, which are two important parameters in this problem, predicted based on the optimal propulsive efficiency agree well with the available data obtained for the wave-like swimming motion of live fish in nature.     

Geometrically designing the kinematic behavior of catalytic nanomotors

Catalytic nanomotors with silica microbead heads and TiO(2) arms are systematically designed by dynamic shadowing growth. The swimming trajectories are fine tuned by altering the arm length and orientation exploiting geometry-dependent hydrodynamic interactions at low Reynolds number. The curvature, angular frequency, and radius of curvature of the trajectories change as a function of arm length. Simulations based on the method of regularized Stokeslets are also described and correctly capture the trends observed in the experiments.     

On Helmholtz and higher-order resonance of twin floating bodies

The Helmholtz mode and other symmetric modes of resonance of a moonpool between two heaving rectangular floating cylinders are investigated. The hydrodynamic behavior around these resonant modes is examined together with the associated mode shapes in the moonpool region. It is observed that near each of the resonance frequencies, the damping coefficient can vanish. The Helmholtz mode is characterized by a region of modest variation of added-mass value from negative to positive near the Helmholtz frequency. The peaks are, however, bounded with the cross-over point in sign corresponding to a bounded spike in damping. The higher-order resonant modes are characterized by the presence of standing waves in the moonpool, which leads to large spikes in the hydrodynamic behavior near the resonance frequencies. The Helmholtz frequency has a distinct value, while the higher-order resonances occur at fairly regular intervals of the frequency parameter, σ²(w ? b)/g, where w ? b is the moonpool gap. The parametric dependence of the hydrodynamic behavior on frequency and geometry is discussed. With best wishes to my colleague and good friend, Nick Newman, on the occasion of his 70th birthday. A leader and staunch supporter of marine hydrodynamics, Nick has expanded the reach and influence of this field through his insights and publications. His contributions have been wide-ranged and his graciousness to young researchers is exemplary. May he enjoy the best of health in the years to come. R.W. Yeung      

Boundary element simulation of inviscid flow around an oscillatory foil with vortex sheet

The dynamic performance of a rigid foil with harmonic vertical and rotational motions in fluid flow has been studied through velocity potential theory. A boundary element based time stepping scheme is introduced to simulate the flow around the foil and the vortex wake. The body surface condition is satisfied on the exact foil surface and the motion and deformation of the wake sheet shed at the trailing edge is tracked. Kelvin condition is satisfied and a Kutta condition for the unsteady motion is proposed to circumvent the singularity at the trailing edge. Point vortex, which is reduced from wake vortex dipole, is introduced to approximate the vorticity. The performance of foil NACA0012 with harmonic vertical and rotational motions are studied extensively; the propulsion/swimming mode, energy harvesting mode and the flying mode are analysed in detail.     

Dynamics of the order parameter of superconducting aluminum films

The imaginary part of the pair-field susceptibility of dirty-limit, superconducting, aluminum films has been obtained both above and below the transition temperature by analyzing the I–V characteristic of asymmetric superconducting tunneling junctions. Results obtained above the transition temperature have been found to be consistent with a diffusive time-dependent Ginzburg-Landau equation. Below the transition temperature two modes have been found, one propagating and the other diffusing. The resonant frequency and damping constant of the propagating mode have been compared with the predictions of a phenomenological two-fluid hydrodynamic theory given by Bray and Schmidt. A somewhat more detailed comparison has been made with a microscopic theory given by Schön and Schmid in which the propagating and diffusing modes, respectively, are associated with transverse and longitudinal fluctuations of the order parameter. In both the microscopic and macroscopic theories the propagating mode is a high-frequency superconducting analog of second sound in that it consists of a counter motion of the normal and superfluid.Supported in part by the U.S. Energy Research and Development Administration under contract E(11-1) 1569 and by the Graduate School of the University of Minnesota.     

Turbulence triggers vigorous swimming but hinders motion strategy in planktonic copepods

Calanoid copepods represent a major component of the plankton community. These small animals reside in constantly flowing environments. Given the fundamental role of behaviour in their ecology, it is especially relevant to know how copepods perform in turbulent flows. By means of three-dimensional particle tracking velocimetry, we reconstructed the trajectories of hundreds of adult Eurytemora affinis swimming freely under realistic intensities of homogeneous turbulence. We demonstrate that swimming contributes substantially to the dynamics of copepods even when turbulence is significant. We show that the contribution of behaviour to the overall dynamics gradually reduces with turbulence intensity but regains significance at moderate intensity, allowing copepods to maintain a certain velocity relative to the flow. These results suggest that E. affinis has evolved an adaptive behavioural mechanism to retain swimming efficiency in turbulent flows. They suggest the ability of some copepods to respond to the hydrodynamic features of the surrounding flow. Such ability may improve survival and mating performance in complex and dynamic environments. However, moderate levels of turbulence cancelled gender-specific differences in the degree of space occupation and innate movement strategies. Our results suggest that the broadly accepted mate-searching strategies based on trajectory complexity and movement patterns are inefficient in energetic environments.     

输液管线中的地震动水压力及其影响因素

强烈地震作用可使地上输液管线中的液体产生较大的动水压力。基于模态叠加原理及地震反应谱理论，考虑流体可压缩性及管弹性，建立了输液管线地震动水压力的计算模型。根据流体所处边界条件的不同，给出两种典型工况下，管流地震动水压力的计算公式，并与依据附加质量模型计算所得的结果进行了比较。比较结果表明：管线长度对管流地震动水压力有显著的影响，对长管道而言，以附加质量模型求得的的地震动水压力高于考虑流体可压缩性时求得的地震动水压力，而对于短管道，情况刚好相反。另外，场地类型及管流所处工况对管流地震动水压力也有显著的影响，软土场地上的管理道。其内部流体地震动水压力明显高于硬土场地上的动水压力。     

Speed limits on swimming of fishes and cetaceans.

Physical limits on swimming speed of lunate tail propelled aquatic animals are proposed. A hydrodynamic analysis, applying experimental data wherever possible, is used to show that small swimmers (roughly less than a metre long) are limited by the available power, while larger swimmers at a few metres below the water surface are limited by cavitation. Depending on the caudal fin cross-section, 10-15 m s(-1) is shown to be the maximum cavitation-free velocity for all swimmers at a shallow depth.     

Hydrodynamic drag constrains head enlargement for mouthbrooding in cichlids

Presumably as an adaptation for mouthbrooding, many cichlid fish species have evolved a prominent sexual dimorphism in the adult head. Since the head of fishes serves as a bow during locomotion, an evolutionary increase in head volume to brood more eggs can trade-off with the hydrodynamic efficiency of swimming. Here, the differences between males and females in three-dimensional shape and size of the external head surfaces and the effect thereof on drag force during locomotion was analysed for the Nile tilapia (Oreochromis niloticus), a maternal mouthbrooder. To do so, three-dimensional body surface reconstructions from laser scans and computational fluid dynamics simulations were performed. After scaling the scanned specimens to post-cranial body volume, in order to theoretically equalize propulsive power, the external volume of the head of females was 27% larger than that of males (head length + 14%; head width + 9%). These differences resulted in an approximate 15% increase in drag force. Yet, hydrodynamics imposed important constraints on the adaptation for mouthbrooding as a much more drastic drop in swimming efficiency seems avoided by mainly enlarging the head along the swimming direction.     

Swimming of the semi-infinite strip revisited

Idealized mathematical models have been devised over the years for study of the fundamentals of the swimming of fishes. The two-dimensional flexible strip propelled by execution of transverse traveling-wave undulation is one of the most well-studied of the simple models. This model is redeveloped here, with the finding that higher propulsive efficiencies are theoretically available within the undulatory swimming mode than have been previously exposed. This is by configuring the displacement wave-form for continuously zero circulation over the body length with time, and thereby avoiding the shedding of a vortex wake and its attendant induced drag. The thrust is reactive, via acceleration processes, rather than inductive via relative velocity and lift. As in most of the classical work on fish propulsion, the analysis assumes high Reynolds number and a thin boundary layer, which provides the use of ideal-flow theory. The advance speed is assumed constant and the analysis is initially linearized, but both nonlinear and linear transient analysis are provided in supporting the basic “wakeless swimming” possibility.     

Dynamics of a Rotor in Film Lubrication Bearings

A dynamic model of a rotor system with hydrodynamic plain bearings in which account has been taken of the actually existing phenomenon of the circular anisotropy of the rigidity of hydrodynamic plain bearings has been proposed. The operation of the rotor system has been modeled with the use of uniformity distributed sequences and the region of states of the system representing a set of instantaneous positions of the center of the end rotor surface by which the accuracy of its rotation is evaluated has been obtained.     

The use of optical tweezers to study sperm competition and motility in primates.

Optical trapping is a non-invasive biophysical tool which has been widely applied to study physiological and biomechanical properties of cells. Using laser 'tweezers' in combination with custom-designed computer tracking algorithms, the swimming speeds and the relative swimming forces of individual sperm can be measured in real time. This combination of physical and engineering tools has been used to examine the evolutionary effect of sperm competition in primates. The results demonstrate a correlation between mating type and sperm motility: sperm from polygamous (multi-partner) primate species swim faster and with greater force than sperm from polygynous (single partner) primate species. In addition, sperm swimming force linearly increases with swimming speed for each species, yet the regression relating the two parameters is species specific. These results demonstrate the feasibility of using these tools to study rapidly moving (microm s(-1)) biological cells.     

Hydrodynamic conditions in a fixed three-phase granular layer. Experimental investigation

An experimental investigation of the hydrodynamic conditions in a three-phase granular layer for direct upward flow of a gas and a liquid has been performed. The floating-up speeds of gas bubbles and pistons have been determined for different operating conditions A diagram of hydrodynamic conditions has been plotted.     

仿生机器鱼运动方向的模糊控制研究

结合仿生机器鱼运动的三种转弯模式,设计了一种用于机器鱼运动方向控制的模糊控制器,并阐述了具体算法的实现。实验结果表明,该方法是有效的,且具有较好实时性,能够满足多仿生机器鱼协调协作研究的需要,并为未来机器鱼的实际应用奠定了良好基础。