Motomu Nakashima

Analysis of Breast, Back and Butterfly Strokes by the Swimming Human Simulation Model SWUM

In the preceding study, the swimming human simulation model “SWUM” has been developed by the authors, and the analysis of standard six beat crawl stroke has been conducted. In this paper, the analyses of three strokes, that is, breast, back and butterfly strokes were conducted. The swimming motion, velocity in the propulsive direction of the human body, and the fluid force in the propulsive direction acting on the upper and lower limbs were respectively shown for those three strokes. In addition, the stroke length and the propulsive efficiency of the four strokes were investigated by the simulation. It was found that the stroke lengths of the crawl, back and butterfly strokes in the simulation agree significantly with the actual values, and that of the breaststroke becomes relatively smaller. It was also found that the propulsive efficiency of the crawl, back and butterfly strokes becomes around 0.2, although that of the breaststroke becomes only 0.036.

Three-Dimensional Maneuverability of the Dolphin Robot (Roll Control and Loop-the-Loop Motion)

The final goal of this study is to realize three-dimensional maneuverability, a characteristic of fast swimming animals such as dolphins, with a biomimetic robot. As the first step towards this goal, a dolphin robot with a body length of 1 m was developed and loop-the-loop motion experiments were conducted. In order to stabilize the roll angle of the dolphin robot, the control algorithm for the joints of the dorsal and pectoral fins was examined by numerical simulation. From the simulation, it was found that the conventional PD control with regard to the roll angle is sufficient for this control. The dolphin robot could stabilize the roll motion in the experiment, too. Next, the motion of the dolphin robot was measured by a three-dimensional motion analysis system. The radius of the loop-the-loop motion was found to be 0.5m, which is half the body length of the dolphin robot.

Unsteady hydrodynamic forces acting on a robotic arm and its flow field: Application to the crawl stroke

This study aims to clarify the mechanisms by which unsteady hydrodynamic forces act on the hand of a swimmer during a crawl stroke. Measurements were performed for a hand attached to a robotic arm with five degrees of freedom independently controlled by a computer. The computer was programmed so the hand and arm mimicked a human performing the stroke. We directly measured forces on the hand and pressure distributions around it at 200 Hz; flow fields underwater near the hand were obtained via 2D particle image velocimetry (PIV). The data revealed two mechanisms that generate unsteady forces during a crawl stroke. One is the unsteady lift force generated when hand movement changes direction during the stroke, leading to vortex shedding and bound vortex created around it. This bound vortex circulation results in a lift that contributes to the thrust. The other occurs when the hand moves linearly with a large angle of attack, creating a Kármán vortex street. This street alternatively sheds clockwise and counterclockwise vortices, resulting in a quasi-steady drag contributing to the thrust. We presume that professional swimmers benefit from both mechanisms. Further studies are necessary in which 3D flow fields are measured using a 3D PIV system and a human swimmer.

Unsteady hydrodynamic forces acting on a robotic hand and its flow field.

This study aims to clarify the mechanism of generating unsteady hydrodynamic forces acting on a hand during swimming in order to directly measure the forces, pressure distribution, and flow field around the hand by using a robotic arm and particle image velocimetry (PIV). The robotic arm consisted of the trunk, shoulder, upper arm, forearm, and hand, and it was independently computer controllable in five degrees of freedom. The elbow-joint angle of the robotic arm was fixed at 90°, and the arm was moved in semicircles around the shoulder joint in a plane perpendicular to the water surface. Two-component PIV was used for flow visualization around the hand. The data of the forces and pressure acting on the hand were sampled at 200Hz and stored on a PC. When the maximum resultant force acting on the hand was observed, a pair of counter-rotating vortices appeared on the dorsal surface of the hand. A vortex attached to the hand increased the flow velocity, which led to decreased surface pressure, increasing the hydrodynamic forces. This phenomenon is known as the unsteady mechanism of force generation. We found that the drag force was 72% greater and the lift force was 4.8 times greater than the values estimated under steady flow conditions. Therefore, it is presumable that swimmers receive the benefits of this unsteady hydrodynamic force.

Numerical and experimental investigations of human swimming motions

ABSTRACT This paper reviews unsteady flow conditions in human swimming and identifies the limitations and future potential of the current methods of analysing unsteady flow. The capability of computational fluid dynamics (CFD) has been extended from approaches assuming steady-state conditions to consideration of unsteady/transient conditions associated with the body motion of a swimmer. However, to predict hydrodynamic forces and the swimmer’s potential speeds accurately, more robust and efficient numerical methods are necessary, coupled with validation procedures, requiring detailed experimental data reflecting local flow. Experimental data obtained by particle image velocimetry (PIV) in this area are limited, because at present observations are restricted to a two-dimensional 1.0 m2 area, though this could be improved if the output range of the associated laser sheet increased. Simulations of human swimming are expected to improve competitive swimming, and our review has identified two important advances relating to understanding the flow conditions affecting performance in front crawl swimming: one is a mechanism for generating unsteady fluid forces, and the other is a theory relating to increased speed and efficiency.

Prediction of postural risk of fall initiation based on a two-variable description of body dynamics: Position and velocity of center of mass

This research addresses the question: what is the risk of fall initiation at a certain human posture? There are postures from which no one is able to keep their balance and a fall will surely initiate (risk=1), and others from which everyone may regain their stability (risk=0). In other postures, only a portion of people can control their stability. One may interpret risk to chance of a fall to be initiated, and based on the portion of fallers assign a risk value to a given human posture (postural risk). Human posture can be mapped to a point in a 2-dimensional space: the x-v plane, the axes of which are horizontal components of the position and velocity of the center of mass of the body. For every pair of (x, v), the outcome of the balance recovery problem defines whether a person with a given strength level is able to regain their stability when released from a posture corresponding to that point. Using strength distribution data, we estimated the portion of the population who will initiate a fall if starting at a certain posture. A fast calculation approach is also introduced to replace the time-consuming method of solving the recovery problem many times. Postural risk of fall initiation for situations expressed by (x, v) pairs for the entire x-v plane is calculated and shown in a color-map.

A new measure for upright stability

The control of balance is a primary objective in most human movements. In many cases, research or practice, it is essential to quantitatively know how good the balance is at a body posture or at every moment during a task. In this paper we suggest a new measure for postural upright stability which assigns a value to a body state based on the probability of avoiding a fall initiation from that state. The balance recovery problem is solved for a population sample using a strength database, and the probability of successfully maintaining the balance is found over the population and called the probability of recovery (PoR). It, therefore, describes an attribute of a body state: how possible the control of balance is, or how safe being at that state is. We also show the PoR calculated for a 3-link body model for all states on a plane, compare it to that found using a 2-link model, and compare it to a conventional metric: the margin of stability (MoS). It is shown, for example, that MoS may be very low at a state from which most of the people will be able to easily control their balance.

Numerical and Experimental Study of the Propulsive Speed of the Three Joint Bending Propulsion Mechanism

The three joint bending propulsion mechanism as a simplified model of fish propulsion is investigated. A numerical method for a coupled system of the bending propulsion mechanism and the fluid is developed, based on the two-dimensional discrete vortex method. The characteristics of the propulsive speed of the three joint bending propulsion mechanism are numerically and experimentally investigated. It is found that the propulsive speed has a maximum numerical and experimental value when all the phase differences of the joints are about zero for four amplitude patterns. The experimental propulsive speeds are found to be 65 ∼ 80 percent of the numerical ones.

Development of Swimming Prosthetic for Physically Disabled (Optimal Design for One Side of Above-Elbow Amputation)

Swimming is a suitable sport for the physically disabled, since the buoyancy provided by the water enables the whole body to be trained through it. However, physical disability may worsen the swimming form and affect body balance. In this paper, the effect of upper-extremity amputation on the crawl swimming was firstly analyzed by simulation, since crawl is the most common stroke, and its thrust is mainly generated by the upper limb motion. Next, a swimming prosthetic which compensates for the amputated limb was designed using an optimization method. Finally, a swimming prosthetic was manufactured based on the results of the optimization, and the prosthetic’ validity was confirmed by a preliminary evaluation.

On Flow Separation Control By Means of Flapping Wings

Experimental and computational results for two configurations which benefit from flow separation control are presented. Both configurations operate at low Reynolds numbers, on the order of 104, characterized by laminar flow, often where separation is unavoidable. The first, a flapping-wing propelled micro air vehicle (MAV), consists of a biplane pair of wings flapping in counterphase located downstream of a larger stationary wing, and it is shown that flow entrainment from the flapping wings suppresses stall over the stationary wing, greatly improving the MAV performance. This is experimentally substantiated both qualitatively and quantitatively. The second configuration, a flapping-wing hydropower generator, consists of two flapping wings arranged in a tandem configuration. Numerical results indicate that optimal performance of the device occurs in the presence of massive stall, as long as the flapping motion is properly matched to the convection of the dynamic stall vortices. For both configurations, simplified panel code ancl Navier-Stokes computations are presented to assist in the assessment of the major geometric and flow parameters affecting the operation of the devices.