Christopher Edward Holmes

The aerodynamic performance of a range of FIFA-approved footballs

Much discussion surrounds the flight of a football, especially that which is perceived as irregular, and is typically done so with little understanding of the aerodynamic effects or substantive evidence of the path taken. This work establishes that for a range of FIFA-approved balls there is a significant variation in aerodynamic performance. This paper describes the methods used for mounting stationary and spinning footballs in a wind tunnel enabling accurate force data to be obtained, and the analysis techniques used. The approach has been to investigate a number of scenarios: the non-spinning Reynolds sweep, unsteady loads, orientation sensitivity (yaw sweep), and the spinning Reynolds sweep. The techniques are applied to a number of footballs with differing constructions and the results reported. To put the aerodynamic data into context, the results are applied in a flight model to predict the potential differences in the behaviour of each ball in the air. This paper concludes that, although the drag characteristics are different for each different ball tested, the simulation suggests that this has only a limited effect on the flight of the ball. It is also shown that the unsteadiness of the aerodynamic loads is unlikely to be responsible for unpredictable behaviour. However, it is also shown that there are significant differences in the lateral aerodynamic forces for a range of FIFA-approved match balls, and that these aerodynamic differences have a significant effect on the flight path for both spinning and slowly rotating balls.

The application of simulation to the understanding of football flight

This paper demonstrates the value of using a flight model in the analysis of the flight of a football, and explores the complexity of the model required to produce useful results. Two specific aspects of the simulation are addressed: the need to include a model of spin decay and the requirement to include a full aerodynamic drag profile as a function of Reynolds number rather than a single indicative value. Both are aspects of the ball performance that are experimentally intensive to obtain. The simulated flights show that the inclusion of spin degradation is important if flight validation is the objective, but that it may be unnecessary in a comparative study. The simple analytical model of spin degradation is shown to over-estimate the reduction in lateral deviation when compared to experimentally acquired data. Therefore, the experimental method is preferred. The analysis of the shape of the drag profile (drag coefficient against Reynolds number) is explored, and it is shown from the simulated flights that post-critical coefficients of drag have the greatest effect on trajectories, and an average drag value is sufficient for most modelled scenarios.

Design of a force acquisition system for high-energy short-duration impacts

This paper describes a novel force acquisition system capable of measuring the force profiles of high-energy short-duration impacts. This force acquisition system was used to test dynamically a cricket leg guard and to create a contour map of the peak transmitted forces across the garments surface. The cricket leg guard was found to provide most protection in the central shin and knee regions, areas most likely to be impacted normally and so to receive the highest-energy impacts. The use of this system will enable a dynamic test procedure to be developed to mimic impact conditions encountered during a game, allowing optimization of cricket pad designs for specific impacts.

Tracking of foot movement during sprinting in studded footwear

Mechanical tests and finite element simulations are often used to assess the traction between a studded outsole and the surface. Appropriate boundary conditions are required in these experiments in order to replicate realistic loading scenarios. The result of using different boundary conditions has been well researched (Nigg 1990, Kuhlman et al. 2009. Kirk et al. (2007) used a high-speed video camera to investigate kinematic boundary conditions in 2D during the acceleration phase in sprinting. The study was limited as the angle of the shoe could only be calculated in one plane. Analysis of high-speed footage in 3D is possible using a two camera system, as per Choppin et al. (2007) who determined the orientation of a tennis racquet during play in three planes.

The Influence of Different Melt Temperatures on the Mechanical Properties of Injection Molded PA-12 and the Post Process Detection by Thermal Analysis

Abstract Polyamide 12 (PA-12) test plates were injection molded using different melt temperatures and the influence on mechanical properties was investigated using quasi-static tensile and instrumented impact behavior in two conditioned states: dried, and following accelerated moisture intake. Energy absorption in tension is strongly dependent on process temperature (variations up to 99%) and additional variation (around 18%) was evident when testing at different conditioning states. Under high-velocity loading, the total impact energy varied by up to 8.70% and 9.05%, when systematic changes were made to process melt temperature and at moisture content, respectively, with all samples failing ductile. Differential Scanning Calorimetry (DSC) was used to characterise the unique endothermic melting behavior of molded PA-12 samples, by linking different process histories to the respective mechanical properties. With focus on the first heating curve progression, significant changes within the endothermic melting region were pointed out and quantified by using MatLab (software), proving DSC as a reliable testing tool for post-production analysis with increased practical implications regarding quality control as well as failure analysis. Findings for the initial heating curve progression were explained by studying the re-crystallisation peak values during cooling phase and obtained data for the second heating.

Modelling Local Dynamic Pressure Within Inflatable Sports Balls

This study used a coupled Eulerian Lagrangian (CEL) approach to model the air within a football (soccer ball) during two types of impacts. Conventional modelling techniques (and those used in all previous football finite element models known by the author) utilize a uniform pressure method incapable of accounting for spatial pressure variation. Internal pressures and deformations for the CEL and uniform pressure models were within a few percent of each other, indicating good agreement between pressurization techniques. By necessity, the air was defined with different methods in each model and this may have contributed to a discrepancy in maximum internal pressure. Using the CEL model, the pressure wave generated at impact was observed to travel from one side of the ball to the other at the speed of sound. Though the CEL model helped illustrate the impact scenario, there were no clear distinctions that gave it an advantage over the uniform pressure method for simple impact analysis.Copyright