Biomechanics and Medicine in Swimming XI

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A Full Body Computational Fluid Dynamic Analysis of the Freestyle Stroke of a Previous Sprint Freestyle World Record Holder Keys, M. 1 ; lyttle, A. 2 ; Blanksby, B.A. 1 & cheng, l. 1 1 The University of Western Australia, Australia 2 Western Australian Institute of Sport, Australia Computational Fluid Dynamics (CFD) allows simulation of complex fluid flow regimes and geometry. A case-study examined propulsive and drag forces experienced across the body during full-body freestyle swimming using CFD. An elite male swimmer was scanned using a whole body 3D scanner and manual digitising provided the 3D freestyle kinematics to animate the model. A Finite Volume Method of CFD modelling was used which incorporated a realisable K-ε turbulence model. The CFD analysis enabled an examination of the forces distributed across the body throughout the full freestyle stroke. This project increases the level of foundational knowledge and presents practical points that may improve swimming performance. Key words: computational Fluid dynamics; Freestyle; drag; Propulsion IntroductIon Typically, current techniques of elite swimmers are derived from a mix of natural genetics, feel for the water, knowledge of experienced coaches, and trial and error methods. Although this is considered to be effective, little is known of the hydrodynamic factors making one technique faster than another. It is widely accepted that an increased understanding of fluid flow patterns in swimming should enhance performance. However, it is very difficult to quantify the relative effects of these flow patterns experimentally when swimming with, at best, only approximations of total body effects were being provided. To date, research has incorporated one or more of the following methods to estimate the drag/propulsion effects and flow patterns: • Physical testing using either force plates, drag lines or towing devices. • Analysis and numerical modelling of recorded flow lines and vortex patterns measured by injecting dye or Particle Image Velocimetry (PIV) methods, based on swimmers in a test pool or swimming flume. • Entirely numerical modelling which use estimations of drag and inertia effects on shapes similar to those of human limbs. Each method provided valuable information and some empirical data concerning a few of the questions raised. However, inherent limitations exist and fluid flows around an irregularly shaped human form that is always changing in shape and position, are highly complex. Hence, none of these techniques can provide a full understanding of what is actually occurring throughout a full swimming stroke cycle. Computational Fluid Dynamics (CFD) can be used to model and solve complex problems of fluid flow and is ideally suited to analysing drag and propulsion across the body when swimming. Based on fundamental fluid mechanics principles, CFD allows complex fluid flow regimes and geometry to be simulated, providing visualization of the resulting variables across the entire solution domain. This can provide insights into problems thus far unobtainable via known physical testing techniques. To date, CFD predictions of forces acting on a swimmer have been limited to passive drag studies (Bixler et al., 2007), hand motion through the water (Bixler & Riewald, 2001; Sato & Hino, 2002) and underwater kicking (Lyttle et al., 2006; Von Loebbecke et al., 2009). The current study was the culmination of several years spent devel- chaPter2.Biomechanics oping a full body stroking CFD model. The objective was to examine the distribution of forces across the human body for an elite sprint freestyler. A better understanding of the interaction of propulsive and drag forces across the body will increase the foundational knowledge of swimming hydrodynamics. Methods A CFD case-study examined the propulsion and drag forces across the body during a full freestyle swimming stroke. At the time of the kinematic recordings, the swimmer used held both the 50m and 100m freestyle World Records. Therefore the technique examined was highly evolved. The 3D kinematics recorded one full stroke cycle with a separate above- and below-water camera used on each side of the swimmer. Each camera was pointed at 45-60° to the horizontal plane. The swimmer performed regular freestyle at race pace and a full 3D kinematic analysis was performed using manual video digitizing. Medial and lateral body landmarks, rather than joint centres, were digitized for each body segment to allow for calculating full segment rotations. Cartesian coordinates for the derived joint centres were then converted to a polar coordinate system. A full 3D surface scan of the swimmer provided accurate 3D geometry for the CFD simulations. The laser scanning of the swimmer was performed using a Cyberware WBX whole body laser scanner, with a density of one point every 4mm. Higher resolution scans were also conducted of the hands, head and feet (density of one point every 0.67mm). The higher resolutions acknowledged the importance of these areas in setting the initial flow conditions and in developing thrust. The higher resolution scans were then aligned and merged seamlessly into the full body scan to provide more accuracy at these locations (see Figure 1). The 3D model was then processed to extract 288 non-uniform rational b-splines (NURBS), curved surfaces forming a 3D solid model of the swimmers. The computer simulation was performed using the CFD software package “FLUENT” (version 6.3.26; Fluent Inc., Lebanon, NH) which utilizes the Finite Volume Method of CFD modelling. A realizable K-ε turbulence model, together with a multi-phase fluid domain, was used with standard wall functions to simulate the boundary layer. This was combined with the use of prism cells near the wall boundaries and tetrahedral cells in the main fluid domain. The domain surfaces contained varying mesh densities to define the detail around highly curved areas while still maintaining a workable mesh size. The surface mesh on the swimmer comprised approximately 100000 triangular surface elements with the total simulation consisting of close to 5 million cells (see Figure 1 for sample triangulated meshing surrounding the hand). Figure 1. 3D laser scanned image of the subject (top) and sample triangulated mesh surrounding the hand (bottom). 105
BiomechanicsandmedicineinswimmingXi results Results below detail the forces on individual body segments throughout the full freestyle stroke. For a summary of momentum changes of the rigid (body segments) and flexible (joints) body parts, see Table 1. The full freestyle stroke was analysed over a cycle time of 1.04s. Momentum values were reported as a means of comparing between different cycle times and relative body weights. These momentum changes were then averaged to a per-second value to enable comparison with previous studies. Figure 2 displays the resultant forces over the entire body throughout the stroke cycle, and Table 2 outlines the temporal points associated with critical events in the stroke cycle. The swimmer’s velocity was between 1.9m.s -1 and 2.3m.s -1 , with the average over this cycle being 2.08m.s -1 . Table 1. The momentum (Ns) changes per second in the swimmer from the full freestyle stroke simulation over one full stroke cycle. Left Side Right Side Total Total per cycle 31.23 Total * 30.03 Hand * 12.21 11.59 23.80 Wrist * 4.65 6.47 11.12 Forearm * 3.89 6.03 9.92 Elbow * 2.35 4.21 6.56 Upper Arm * -0.50 0.27 -0.23 Shoulder * -9.17 -8.02 -17.20 Head * -10.18 Neck * -0.37 Upper Trunk * -37.94 Mid Trunk * -24.74 Pelvis * 3.18 Hips * -4.55 -2.85 -7.41 Thighs * 9.46 8.82 18.28 Knees * 4.18 5.23 9.41 Lower Leg * 14.81 12.57 27.39 Ankles * 0.38 -2.29 -1.91 Feet * 10.67 9.67 20.34 Combined Arms * 13.44 20.54 33.98 Combined Legs * 34.95 31.16 66.10 Trunk and Head * -70.05 * per second Force (N) 106 500 400 300 200 100 -100 -200 Overall Propulsion/Drag on Freestlye Swimmer 0 0.13 0.33 0.53 0.73 0.93 1.13 Time (sec) Figure 2. Overall propulsion/drag throughout a full stroke cycle (negative values indicate net drag force) Table 2. Critical temporal points through the total measured freestyle stroke cycle. Time (s) Description 0.19 Left foot reaches top as right foot reaches bottom of sweep 0.20 Right hand exits the water 0.37 Left foot reaches bottom as right foot reaches top of sweep 0.44 Left hand reaches the deepest point 0.56 Left foot reaches top as right foot reaches bottom of sweep 0.58 Right hand enters the water 0.64 Left forearm at closest point to vertical 0.70 Left hand exits the water 0.73 Left foot reaches bottom as right foot reaches top of sweep 0.90 Left foot reaches top as right foot reaches bottom of sweep 0.98 Right hand at deepest point 1.04 Right forearm at closest point to vertical 1.06 Left foot reaches bottom as right foot reaches top of sweep 1.08 Left hand enters the water dIscussIon The race analysis of the swimmer used in this study from his 50m Freestyle final at the 2008 Australian Olympic Trials (final time - 21.28sec) revealed that the free swimming component made up over 87% of total race time. Thus, major benefits will accrue if this swimmer can improve technique during the stroking phases. To date, no studies have successfully examined propulsive and drag forces across the total body while swimming. This information is crucial in order to optimise swimming technique. Examining the breakdown of force distributions revealed that the arms and legs create significant amounts of propulsion, with the trunk contributing the majority of the drag. The hands provided a total propulsive momentum of 23.8Ns; while the combined contributions of the wrist, forearm and elbow were 27.6Ns. This highlights that the forearm position during the underwater arm stroke was as critical as that of the hands. The head contributed less drag than the upper and lower trunk components. That could be because it is occasionally positioned in only a semi-submerged state, and that the lesser volume influences the potential amount of wave drag experienced. The thighs, knees and lower legs also contributed greater percentages of propulsion than the feet. This also reinforces the importance of entire leg movements and positioning, rather than just focusing on the feet positioning. However, this may be due to the feet coming out of the water regularly, and the possibility of wave assistance. The overall changes in forces throughout the stroke were characterized by six clear cycles, containing four small peaks and two large peaks. These peaks represent the six beat kick pattern that was adopted. The two large peaks correlated with the peak propulsion of the left and right arm strokes; and occurred simultaneously with two of the kick cycles. These peaks, particularly those associated with the arm stroke propulsion, were reflected by increases in the swimmer’s instantaneous velocity. The two highest velocity peaks occurred just after the peak propulsive forces, namely at 0.64s and 1.14s, when the swimmer’s velocity surged to 2.3m.s -1 . The initial review of the individual arm force profiles confirmed the observations detailed in the overall drag and propulsion review. There
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Pahlen Norge AS Pahlen Norge AS Cha
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