The influence of a rigid boundary and a free boundary on the motion of singly flagellated bacteria is experimentally investigated. The speed of backward swimming cells is faster near the rigid or free boundary than in the free space without boundary. It is also found that backward swimming speed is faster than forward near the rigid or free boundary. The trajectory of the cells swimming backward near a rigid or free boundary comprises circular parts, while most of forward swimming cells have straight trajectories. Backward swimming cells tend to gather on a rigid or free boundary rather than forward swimming cells. These asymmetric characteristics between forward and backward motions close to a rigid boundary has been predicted by a fluid dynamic simulation.

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The application of dynamics in organisms to the field of engineering is very instructive. From this point of view, we aimed at examining the micropropulsion mechanism modeled on aquatic microorganisms, and proposed a micropropulsion mechanism modeled on a flagellum with projecting mastigonemes that propels the directions of the wave propagation of the flagellum in water. We examined the effects of the number and dimensions of projecting protrusions on the thrust force and flow of water using computational fluid dynamics.

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This paper is concerned with the swimming characteristics of small aquatic creatures. The swimming behavior of small aquatic creatures was analyzed by a digital high speed video camera system. The test aquatic creatures were zooplankton and opossum shrimp. Both free swimming and tethered swimming of these aquatic creatures were analyzed. It was found that the average thrust force of aquatic creatures arises from the drag difference between the power stroke and the recovery stroke in beating movement of their swimming legs. Two swimming modes of these aquatic creatures were found out, that is, cruise swimming and sudden swimming were observed. In sudden swimming of the opossum shrimp, thrust force was generated by the water jet. The swimming mechanisms of two kinds of small aquatic creatures were revealed experimentally.

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In this study, we explore mechanical constraints on the swimming performance of zebrafish larvae (Danio rerio) that might explain why larvae switch from sustained swimming to the more efficient burst & coast as they grow. Two hypotheses have been proposed to explain why young fish larvae perform poorly at burst & coast. First, their initial momentum might be low; second, their drag coefficient might be high. To test the two hypotheses, this study makes a quantitative comparison between experimental observations of swimming fish larvae and a CFD model of a self-propelled fish. The study focuses on larvae of the crucial age and size range in which zebrafish switch swimming style. Our studies show that hatchlings perform poorly not only because they cannot accelerate to a high initial coasting speed and hence do not gain enough initial momentum. But they also suffer higher decelerations while coasting due to a high drag coefficient. Overall, the fivefold difference in coasting distance between hatchlings and older larvae corresponds closely to a threefold difference in the time constant of the speed decay and a threefold difference in initial momentum. Our data also show that swimming speed does not decay exponentially, as predicted by the drag-speed relationship in the viscous flow regime, but hyperbolically, due to flow phenomena developing in the boundary layer during the coast.

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A numerical study of undulatory locomotion is presented. Unsteady hydrodynamics around an undulatory swimming body is solved using an integrated modeling method combining a 3D Computational Fluid Dynamics (CFD) method and a Computational Swimming Dynamics (CSD) method. A larva fish, zebrafish, Danio rerio, is modeled, which “swims” by sending a laterally compressed, sinusoidal wave down its body. Hydrodynamics of the three-dimensional larva fish model in terms of the burst, the continuous and the coast swimming modes were then analyzed and compared with conventional hydrodynamic theories, which provide a general understanding of the relationship between the dynamics of vortex flow and the jet-stream propulsion associated with the undulatory locomotion of vertebrates. As a result this analysis demonstrates a detailed picture of the structure of vortex wake behind a zebrafish larva and its correlation with force-generation.

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This paper investigates the flow physics of Koi carp's routine turns with a novel CFD method solving the body-fluid interaction problem, which consists of deforming body dynamics and unsteady fluid dynamics. Firstly, the dynamical equations of the deforming body are presented. Secondly, the coupled equations of body dynamics and fluid dynamics are solved together. The numerical simulation is based on the kinematics data from a video tracking measurement system and the predicted body kinematics and flow visualization are well agreed with the experimental results. Single-beat turn and cruising turn are the basic two turning modes of Koi carp, and the former is the mostly observed. Through comparative studies of the turning maneuverability performance and energetics of these two kinds of turns, their common flow control mechanisms and different features are discovered, such as (1) agility (defined as turning rate) is correlated positively with maneuverability (defined as the reciprocal of the turning radius), (2) the total power appears good linear relation with the turning rate, and (3) single-beat turns are more efficient than cruising turns.

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Since the propulsion mechanism in fluid using an elastic fin, such as the caudal fin or the pectoral fin of fish, is effective, a number of studies have examined the use of elastic fins for propulsion in water and the development of fish robots using elastic fins. However, the optimum elasticity of the fin is not constant and changes with the movement task and environment, such as the swimming speed and the oscillating frequency. It is very difficult to exchange fins of different stiffness while the robot is swimming. Thus, we attempt to develop a variable-stiffness fin with a variable-effective-length spring. The apparent stiffness of this spring can be changed dynamically. We constructed a water tunnel to investigate the characteristics of the fin in a uniform flow. The present paper describes the thrust force, thrust efficiency, and flow velocity corresponding to the self propelled speed of the fin in a uniform flow. Furthermore, we developed a flow visualization system and discussed the flow-field around the fin in a uniform flow.

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The objectives of this study were to investigate the effect of trunk undulation on the swimming performance in underwater dolphin kick, and to clarify the ideal trunk undulation form. The reference swimming motion of an elite swimmer was firstly acquired from the video analysis, and input into the swimming human simulation model SWUM, which had been developed by the authors. The trunk motion was next optimized by the simulation for three objective functions: maximizing swimming speed, maximizing propulsive efficiency, and minimizing fluid force acting on hands. The following findings were obtained: In the case of maximizing swimming speed, the whole body forms a ‘C’ shape due to the in-phase trunk undulation. The swimming motion of maximizing propulsive efficiency and the reference swimming are considerably similar to each other. In both cases, the trunk moves as a seesaw with a node; whereas, the lower limbs form a traveling wave in the absolute space. The values of propulsive efficiency are around 0.2 in the cases of maximizing propulsive efficiency, minimizing fluid force on hands, and the reference swimming. The swimming motion in the case of minimizing fluid force on hands is almost the same as that of maximizing propulsive efficiency. The trunk undulation with the appropriate amplitudes and phases, especially bending at the chest, is important in realizing the swimming motion which maximizes propulsive efficiency.

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A numerical analysis of dynamic flight stability of a hovering hawkmoth is presented. A computational fluid dynamic (CFD) method is used to simulate the unsteady flow about a realistic hawkmoth model and to compute the aerodynamic derivatives of the aerodynamic forces and pitching moment in response with a series of small disturbances. With these parameters, the techniques of eigenvalue and eigenvector analysis is employed to investigate dynamic flight stability of the hawkmoth hovering. In the longitudinal disturbance motion, three natural modes are identified of a stable oscillatory mode, a stable fast subsidence mode and a stable slow subsidence mode, which indicate that the hawkmoth hovering flight is stable. In short, a hovering hawkmoth, if the body motion is dynamically stable and hence the disturbance dies out fast, might not need to make any adjustment with wing motions and could return to the equilibrium state ‘automatically’.

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It is well-known that the flapping sound of owl wings is much less than that of other birds. Fine serrations equally spaced apart is found at the leading edge of the primary feathers (remiges) of owl wings, and seems to produce the silencing effect. Paying attention to the owl's posture during capturing games, the author discusses that the effect of serrations brought changes of the aerodynamic characteristic of a wing besides damping sound. By attaching jigsaw blades with different numbers of cutting teeth imitating serrations at the leading edge of a laminar wing, the aerodynamic characteristics of an airfoil were measured and the flow field around the airfoil was also visualized. The author comes to conclusion that lift force is maintainable at larger angle of attack than the prototype wing in low Reynolds numbers.

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Molecular delivery using ultrasound (US) and nano/microbubbles (NBs), i.e., sonoporation, has applications in gene therapy and anticancer drug delivery. When NBs are destructed by ultrasound, the surrounding cells are exposed to mechanical impulsive forces generated by collapse of either the NBs or the cavitation bubbles created by the collapse of NBs. In the present study, experimental, theoretical and numerical analyses were performed to investigate cavitation bubbles mediated molecular delivery during sonoporation. Experimental observation using lipid NBs indicated that increasing US pressure increased uptake of fluorescent molecules, calcein (molecular weight: 622), into 293T human, and decreased survival fraction. Confocal microscopy revealed that calcein molecules were uniformly distributed throughout the some treated cells. Next, the cavitation bubble behavior was analyzed theoretically based on a spherical gas bubble dynamics. The impulse of the shock wave (i.e., the pressure integrated over time) generated by the collapse of a cavitation bubble was a dominant factor for exogenous molecules to enter into the cell membrane rather than bubble expansion. Molecular dynamics simulation revealed that the number of exogenous molecules delivered into the cell membrane increased with increasing the shock wave impulse. We concluded that the impulse of the shock wave generated by cavitation bubbles was one of important parameters for causing exogenous molecular uptake into living cells during sonoporation.

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Over the last few decades several methods have been used to compute the propagation coefficient, which is a complex number that provides information about the viscoelastic properties of blood vessels. Results from these methods show a considerable disparity between them and when they are compared to theoretical values. Moreover, the attenuation and phase velocity obtained by the three-point method shows more significant discrepancies than those obtained by the other methods. In order to clarify the source of the disparity of results carried out by various methods concerning the estimation of phase velocity and attenuation in the arterial network, we made investigations using numerical tool several methods. We studied, for each method, the effects of distance between measurement sites, the sampling interval and measurement errors on the determination of the propagation coefficient by each of these methods. The values of wave speed and attenuation computed by these methods were compared to the known input values. Our simulation demonstrates that the distance between measurement sites and the sampling interval may introduce significant errors when the noise becomes high. Moreover the error on the values of attenuation and phase velocity obtained by occlusion and three-point methods is significantly higher than the error on the values obtained by RV-I and RV-II methods for all experimental conditions studied within the examined frequency range. This result supports the idea that the discrepancy between studies reported in literature seems to be due to the inaccuracy of experimental measurement techniques and not associated with the methods themselves as concluded by some authors.

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The human forearm with elbow joint has two degrees of freedom of motion. Especially it is noticed that the wide range for the rotation of the forearm (pronation-supination) is attained according to the sophisticated complexity of the human forearm with elbow joint. The elucidation of its movement mechanism is useful for the functional evaluation for the medical treatment and application to the welfare devices for the upper limb. The purpose of this study is to develop the arm model that functionally mimics the musculoskeletal system of the human forearm with elbow joint. In this paper, we made a physical model and the computational models, which replicate the bionic function of the forearm with elbow joint. By estimating the moment arms in a physical model, the mobility of the simplified physical model was evaluated. In the three-dimensional computational forearm bone models different in the geometry, the beneficial property of the centroidal lines of the bones was confirmed to extend the range of motion for the pronation-supination.

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