Steve Haake

The engineering of sport 4

The book provides an in-depth coverage of the state-of-the-art in sports technology and the latest contemporary developments in this area from the perspective of both industrial practice and academic research. It includes a wide range of sport disciplines and technologies, encompassed by topics such as design innovation and product development, mechanics, biomechanics, aerodynamics, materials and advanced analysis techniques.

The understanding and development of cycling aerodynamics

In elite cycling the resistive force is dominated by aerodynamics. Be it on the roads or in the velodrome, the sport has many examples where aerodynamics has won and lost races. Since the invention of the bicycle, engineers have strived to improve performance, often by reducing aerodynamic drag. Over the last 50 years a number of authors have presented their efforts in journals, books and magazines. This review summarises the publications that show the continued development in the aerodynamics of cycling. The review concludes by examining the shortcomings of the current understanding and making suggestions for future research and development.

Completely automated photoelastic fringe analysis

Abstract The integration of two automated systems of photoelastic analysis has been performed. This combines the results obtained with a full-field polariscope based on phase-stepping techniques with those from spectral contents analysis. The latter technique identifies the absolute value of the isochromatic parameter at a point. This information is used to calibrate maps of relative retardation produced by the phase-stepping method. Thus, completely automatic determination of calibrated isochromatic and isoclinic patterns in photoelastic specimens is achieved. The evaluation of the whole system, establishing limits of validity and influences of parameters, is performed. Results show that the analysis carried out with the combined system, incurred errors no longer than those of each system working independently.

Impact of a non-homogeneous sphere on a rigid surface

Abstract This paper examines the impact behaviour of a non-homogenous sphere (in this case a cricket ball with a rolled core construction) with a rigid surface. Experiments were carried out to measure the force-deflection behaviour of a cricket ball during a normal impact in two orientations (impacting on the seam and perpendicular to the seam). For the two orientations of impact, a disparity was found in terms of the force-deflection behaviour. Greater deformation was found for impacts landing on the seam, compared to those landing perpendicular to the seam. Comparisons with quasi-static test data suggested that only the bottom third of the ball may have been compressed during impact. The dynamic force-deflection behaviour was modelled using a mass attached to a Hertzian spring in parallel with a damper whose damping coefficient varied with the contact area. The coefficients in the model could be described using the velocity before impact alone. The model was found to be in good agreement with the experimental data. The model was then extended to predict oblique impacts by incorporating a measured coefficient of friction. This performed well in predicting the rebound velocity, angle and spin of a cricket ball after oblique impact with a cricket pitch. Inconsistencies in the results were attributed to deformation in the pitch surface.

Understanding the Effect of Seams on the Aerodynamics of an Association Football

Abstract The aerodynamic properties of an association football were measured using a wind tunnel arrangement. A third scale model of a generic football (with seams) was used in addition to a ‘mini-football’. As the wind speed was increased, the drag coefficient decreased from 0.5 to 0.2, suggesting a transition from laminar to turbulent behaviour in the boundary layer. For spinning footballs, the Magnus effect was observed and it was found that reverse Magnus effects were possible at low Reynolds numbers. Measurements on spinning smooth spheres found that laminar behaviour led to a high drag coefficient for a large range of Reynolds numbers, and Magnus effects were inconsistent, but generally showed reverse Magnus behaviour at high Reynolds number and spin parameter. Trajectory simulations of free kicks demonstrated that a football that is struck in the centre will follow a near straight trajectory, dipping slightly before reaching the goal, whereas a football that is struck off centre will bend before reaching the goal, but will have a significantly longer flight time. The curving kick simulation was repeated for a smooth ball, which resulted in a longer flight time, due to increased drag, and the ball curving in the opposite direction, due to reverse Magnus effects. The presence of seams was found to encourage turbulent behaviour, resulting in reduced drag and more predictable Magnus behaviour for a conventional football, compared with a smooth ball.

The dynamic impact characteristics of tennis balls with tennis rackets

Abstract The dynamic properties of six types of tennis balls were measured using a force platform and high-speed digital video images of ball impacts on rigidly clamped tennis rackets. It was found that the coefficient of restitution reduced with velocity for impacts on a rigid surface or with a rigidly clamped tennis racket. Pressurized balls had the highest coefficient of restitution, which decreased by 20% when punctured. Pressureless balls had a coefficient of restitution approaching that of a punctured ball at high speeds. The dynamic stiffness of the ball or the ball-racket system increased with velocity and pressurized balls had the highest stiffness, which decreased by 35% when punctured. The characteristics of pressureless balls were shown to be similar to those of punctured balls at high velocity and it was found that lowering the string tension produced a smaller range of stiffness or coefficient of restitution. It was hypothesized that players might consider high ball stiffness to imply a high coefficient of restitution. Plots of coefficient of restitution versus stiffness confirmed the relationship and it was found that, generally, pressurized balls had a higher coefficient of restitution and stiffness than pressureless balls. The players might perceive these parameters through a combination of sound, vibration and perception of ball speed off the racket.

The impact of technology on sporting performance in Olympic sports

Abstract To assess the effect of technology on sport, the performance statistics for four disciplines were analysed: the 100-m sprint, pole vault, javelin, and cycling. The concept of a performance improvement index was developed to allow comparison between athletes and between sports with a higher index indicating a greater improvement in the sport. The following performance improvement indices were found: 100-m sprint, 24% over 108 years; pole vault, 86% over 94 years; javelin, 95% over 76 years; 4-km individual pursuit, 35% over 32 years; one-hour cycling record, 221% over 111 years. Around 4% of the index for the sprint was attributed to tighter, aerodynamic clothing, suggesting that general athletic improvement in sprint-type events has been around 20%. Technological developments in simple equipment such as the pole vault or javelin were seen to affect the index by around 30%, while the index associated with aerodynamic improvements in the one-hour record was around 100%. It is concluded that the performance improvement index could be extended to amateur as well as elite sport where distance or time is used as a measure of performance.

Photoelastic analysis of frozen stressed specimens using spectral-contents analysis

The complete birefringence, or isochromatic fringe order, in stress-frozen photoelastic models has been found by measuring the spectral contents of idividual points. A calibration procedure is porposed. The effects of nonuniform birefringence and dispersion of birefringence are considered. The results are presented from the analysis of models of a plate with a central hole and a disk subject to three radial loads.

Ball Spin Generation for Oblique Impacts with a Tennis Racket

In this paper, we describe an experimental investigation of the oblique impact between a tennis ball and head clamped tennis racket. It was found that the magnitude of the ball rebound spin was not a function of the material, gage or tension of the string used in the tennis racket. Furthermore, it was concluded that all strings exhibit a sufficiently large friction coefficient that the ball begins to roll during impact. There is anecdotal evidence from tennis players that suggests that a high string tension or a rough string surface enable them to impart more spin to the ball. For example, players have been quoted as saying that a high string tension makes the strings “bite” into the ball, giving more spin. The data reported in this study do not support these observations. Analysis of the experimental data has shown that the balls are rebounding from the surface with more spin than would typically be associated with rolling. A second experiment showed that the balls commenced rolling at the mid-point of the impact. This information was used in a theoretical model to show that the spin that acts on the ball during the impact can be higher than the value of the rolling spin at the end of the impact.

Experimental and finite element analysis of a tennis ball impact on a rigid surface

An explicit finite-element (FE) model of a pressurised tennis ball is presented. The FE model was used to model an oblique impact between a tennis ball and a rigid tennis surface, to further the understanding of this impact. Impacts were also conducted in the laboratory and the results from the FE model were in good agreement with this experimental data. The FE model was used to illustrate why a tennis ball rebounds with a higher vertical coefficient of restitution in an oblique impact compared to an equivalent impact perpendicular to the surface; this equivalent perpendicular impact has the same inbound velocity as the vertical component of the oblique impact. The FE model was also used to illustrate that the structural compliance of the felt covering on a tennis ball was a contributing factor to the ball attaining more spin in the impact than would have been calculated using a conventional analytical model. Also, the spin values calculated in the FE simulation were in good agreement with experimental data.