Biomechanics and Medicine in Swimming XI

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Aquatic Space Activities – Practice Needs Theory ungerechts, B., Klauck, J². University of Bielefeld, Germany 2 German Sport University Cologne, Germany Aquatic Space Activities (ASA) is a collective term for activities in water such as: Competitive – Recreational – Masters swimming, Finswimming, Triathlon, Scuba-diving or Water-exercises. ASA must obey flow physics. Traditionally, flow physics in aquatic sports is confined to steady conditions assuming that the flow velocity is constant and the body is rigid. It is not new information that this is not and was never the case in human swimming. The fact is that the limbs change motion in all ASA which causes unsteady flow conditions. Unsteady flow demands closer consideration of water-motion induced by motion of the limbs’. The purpose of this paper is to introduce some challenging information concerning unsteady flow conditions and consequences for practise. Key words: unsteady flow, momentum-induced propulsion, jet-flow IntroductIon Aquatic Space Activities (ASA) focuses attention on the interaction between the actively moving body (or its parts) and water-mass set in motion. ASA also deals with the mutual effects of buoyancy and the reaction to induced water-motion. Buoyancy is a natural reaction due to displacement of water-mass of any body, submerged in water. Induced water-motion or water flow is a physical reaction due to displacement of water-mass of any submerged body moving relative to the surrounding water. Reactions to induced water-motion have some contra-intuitive features. A simple drag approach suggested by most books on swimming does not explain swimming speed sufficiently. The hands’ actions are not most effective if water resistance is at a maximum (the term effective focuses on the result of an action). Intuition may tell us that e.g. a hand is moving the same block of water either back or moving new blocks by changing hand motion. This is not the complete story. What really happens is (completely) different. According to the law of conservation of mass, in a flow, water cannot be pushed away in relation to the surrounding water-mass. Nevertheless it is true that, where there is one solid body (like a swimmer’s hand) there cannot be another body. However, where does mass of displaced water go? The aim of this paper is to introduce some facts of flow physics, beyond our understanding of the existing biomechanical aspects, and relevant for a better understanding of aspects of aquatic space activities and the reaction of water-set-inmotion. Methods Swimming text books mostly provide a chapter related to biomechanical background of activities in aquatic space. It is curious that over the decades, the content of this chapter, including pictures has not really changed from the very first publication of the author and coach “Doc” Counsilman. Obviously these text books were so convincing worldwide that later authors had no reason to change this. In nearly the same decades numerous studies have been presented in the congress series of biomechanics and medicine in swimming, it seems as if their outcome is of minor relevance. During the same time period in parallel, much research was done in the field of swimming animals. Researchers in this field, starting with similar assumptions to their colleagues in human swimming, soon became aware that the steady flow approach was not helpful in answering their questions. The alternative approach, mostly based on concepts published by Lighthill, opened a quite new route to structuring the research and to study unknown hydrodynamic features related to self-induced chaPter2.Biomechanics propulsion. Some of these results have held and can act as a guideline in swimming literature, because the myths and the impact on swimming technique are considerable. Coaches and researchers may mislead their swimmers when still following the ancient concepts. results Let’s consider the properties of water mass. It has long been known that aquatic space is characterized, like any body, by mass and other features. Firstly, water has the property to change shape. Any body, e.g. the hands or feet, even when moving relative to water, displaces some water-mass. This means the motion of the water (in the vicinity to the moving body) is changed and flow is created: if water is at rest (before it is displaced) this is also true: what counts is the relative change of motion. Since the relative motion between the body and water is essential, research on aquatic activities executed in a flume also applies to activities in a pool. Pressure is an important feature of water at rest and in motion: the weight of the water mass displaced by a body submerged in water exerts a force due to water pressure in an upward direction, called buoyancy. A body, submerged and moving relative to water produces a change of pressure and due to that change “some” particles are set into motion. Based on the continuity-pressure-concept, the pressure causes and maintains a flow, a continuous movement of the fluid from one place to another. According to Euler (1707 - 1783) particles-in-motion (i) possess mass, weight, acceleration, velocity, etc. and (ii) occupy some space in the vicinity of the moving body (whereas the rest of the water-mass remains undisturbed). This space of particles-in-motion is full of vectors and scalars, forming vector fields. Fig 1 Flowing water: sketch by Leonardo Da Vinci, and today: cm.math.uniuc.edu/242syl.php Vector fields allow for transitional and rotational motion of the particles (the particle itself does not rotate, but follows a circular path), indicating that vorteses form and the effects induced by vortex forms are likely to occur. By independent variations in time and space, the flow is to a certain extent “unpredictable” (also when the displacing action causes “un-steady” flow conditions). From practical experience it is known that swimmers claim they sometimes feel a “different slip” although using the same biomechanical actions. Internal friction exists between particles, measured as viscosity, which is the cause of pressure change. The change of pressure is mediating between any displacing action of any body and particles-in-motion; hence, the interaction of the limbs and water-mass means pressurechange. In addition, viscosity causes repulsive actions. The reaction to this repulsive action is mostly measured by the established u²-law of resistance – provided that the flow remains unaffected by any body (called a steady situation). Momentum-induced propulsion in aquatic space ie the act of displacing a whole body in aquatic space, is related to a change of motion of water-mass. The change of motion of any mass is called momentum. Galileo (1565-1642) already pointed out that momentum is the property by which the motive agency moves and the body resists. Newton (1642-1727) claimed that displacement means imparting momentum to 175
BiomechanicsandmedicineinswimmingXi all particles set in motion. Momentum is a vector; the magnitude is the product of mass (m) times its velocity (v): [m*v] = kgm/s = Ns and its direction coincides with the direction of the velocity. The momentum approach allows for application of the law of conservation of momentum (original enunciation: René Descartes (1596- 1650). According to this principle the total momentum is constant – provided a closed system is considered (which is not subject to external influence). If two (or more) masses belong to a closed system (Figure 2) and one mass changes velocity, simultaneously the other masses will adapt their momentum (in the opposite direction), hence, any interaction between body and surrounding water gives a reaction. The actions of (parts of ) any body in aquatic space transmit energy while changing the motion of water-mass over a period of time. The simultaneous reaction has an effect in and against the swimming direction at the same time. If increasing speed of a whole body is observed, there will be a zero “nett” effect in swimming direction. The principle of momentum claims that if the momentum before and after an interaction of limbs and water-mass differs this difference is induced by the interaction. This concept is elegantly applied to determine effects of a “displacing” rigid total body on a flow of constant speed, either in steady or unsteady conditions. Based on the law of conservation of momentum it is evident that momentum in front of the whole body must be equal to the momentum in the wake plus all momentum changes produced by the interaction between total body and particlesin-motion. The difference in momentum over time in steady flow is called drag or propulsion (Figure 3). Figure 2 Momentum change due to the total body action of the fish resulting in thrust Jet-propulsion in aquatic space: In context with induced water motion it was pointed out that body actions change the motion of water-mass due to interaction of the limbs and water-mass. The effect of the interaction of limbs and water-mass is a change of momentum. From this it follows that the change of momentum of particles (m’*v’) are combined with a change of momentum of the total body mass (m’’*v’’), in the opposite direction. From the swimmers view the reaction (m’’*v’’) provides components in and against the swimming direction (called drag or thrust). If the momentum of particles (m’*v’) is totally directed against the swimming direction of the total body, the reaction results in a 100 % effect. Water moving backwards is thrusting the whole body forward. However, this does not suggest that one should move the hand or leg backwards to gain an effective thrust. The most effective and efficient way to gain maximum thrust for a given energy-input is a jet-flow, used by squids or jelly fish (Fig . 3). Figure 3 Vortex induced jet by a jelly fish 176 Jet-flow is a powerful stream of mass carrying high momentum. Jetpropulsion exists also in the swimming of the skilled human. Matsuuchi et al. (2004) published research data for the first time based on the PIVmethod (Particle Image Velocimetry), demonstrating the existence of intermittent jet-flow. Close scrutiny revealed that first a vortex-ring had to be produced by the action of the swimmer’s hand and in its wake. The rotation of the vortex rings cause a particle drift in one direction. The shape and rotation of the vortex ring determines the jet length-todiameter-ratio and thus the momentum. Hence, the orientation of the jet-flow relative to swimming direction determines the contribution to thrusting the body. Figure 4 Vortex-induced jet-flow of a hand while swimming freestyle dIscussIon Concentration on the interaction of limbs and water-mass instead of limiting oneself to the mere action of limbs or bodies, augments the understanding of reality. The act of understanding is a dynamic process and prone to verification / falsification of theories. The saying, nothing is more practical than a good theory, is valid when theory mirrors reality. Sometimes a change in theory means that already existing, but not established concepts survive; e.g. Berger (1996, 95) found that high propulsive forces cannot be based solely on lift and drag of an acting hand and concluded “the key to producing effective propulsion at low energy cost may be the creation of vortices of a special form”. The research of jet-flow requires a water-related view of both the motion of the limbs and of the water-in-motion in aquatic space. Most of the existing studies on swimming use a body related approach focussing on the velocity of body actions. In most studies related to swimming the understanding of the movement of the surrounding water is limited to the lift and drag approach, established in 1970 by Counsilman. Already in 1986 Counsilman said in personal communication: “I can’t help it if people still refer to my publications of the end of the sixties”. He was aware that the drag and lift approach was not applicable to an unsteady swimming situation. In that time the vortex induced propulsion concept and influence of added mass to swimming bodies started to attract the interest of (a few) swimming researchers. Ungerechts (1988) published the idea that breaststrokers’ leg actions lead to rotating water forms by which “momentum is transferred to the fluid and in reaction, its “impulse” accelerates the body.” Van Tilborg et al. (1988) presented data of “impulses”, measured in Ns, resulting from the difference between propulsion and resistance for different phases in breaststroke. They pointed out that “impulses are more reliable than forces”. Ungerechts (1992), calculated momentum needed to change hip motion (Δv) during the arm stroke including an inertial term due to acceleration based on experimental data of eight breaststrokers and compared the data with forces due to the hands’ actions. It was shown that momentum =|16.5 – 45.9| Ns will explain (Δv) = |0.27 – 0.76| m/s with 93 % confidence. Ungerechts & Klauck (2006) presented the flow physical laws which are relevant for unsteady flow in conjunction with added mass effects. A change of paradigm from a body related to a water related view
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Pahlen Norge AS Pahlen Norge AS Cha
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Biomechanics and Medicine in Swimming XI

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