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13:30   (Soccer) ball studies Chair: Lionel Manin 13:30 20 mins VALIDATION OF FOOTBALL’S VELOCITY PROVIDED BY A RADIO-BASED TRACKING SYSTEM Thomas Seidl, Titus Czyz, Dominik Spandler, Norbert Franke, Matthias Lochmann Abstract: Nowadays, many tracking systems in football provide positional data of players but only a few systems provide reliable data of the ball itself. The tracking quality of many available systems suffers from high ball velocities of up to 34 ms-1 and from the occlusion of both the players and the ball. Knowledge about the position and velocity of the football can yield valuable information for players, coaches and the media. To assess the accuracy of the football’s velocity provided by the radio-based tracking system RedFIR, we used a ball shooting machine to repeatedly simulate realistic situations at different velocities ranging from 7.9 ms-1 to 22.3 ms-1 in an indoor environment. We then compared velocity estimates for 50 shots at five speed levels with ground truth values derived from light gates by way of mean percentage error (MPE) and Bland-Altman analysis. The speed values had an MPE of 2.6% (-0.49 ms-1). These results suggest that RedFIR is capable of providing accurate information about the kinematics of a football. 13:50 20 mins EFFECT OF BALL SPIN RATE ON DISTANCE ACHIEVED IN A LONG SOCCER THROW-IN Nicholas Linthorne, Jamie Thomas Abstract: The long throw-in can be a potent form of attacking play, particularly when used near to the goal mouth of the opposing team. In this study, we investigated the effect of the rate of backspin on the distance achieved in a soccer throw-in. A skilled male player performed 55 throws for maximum distance while deliberately manipulating the rate of backspin applied to the ball. A wide range of spin rates was achieved, from almost no spin up to about 2.6 rev/s. A two-dimensional video analysis was used to obtain measures of the flight distance, flight time, release velocity, release angle, release height, and spin rate of the ball. A mathematical model of the flight of a soccer ball was used to calculate the distance achieved as a function of the release conditions. The model included the influence of the rate of spin on the aerodynamic drag and lift on the ball, and included the player’s relationship between release velocity and release angle. We found that putting greater backspin on the ball did not adversely affect the player’s ability to produce a high release velocity. Release velocity, release angle, and release height remained almost constant across the whole range of ball spin rates. The throw distance achieved increased slightly as spin increased, at a rate of about 0.6 m per 1 rev/s increase in spin. The experimental data was in good agreement with the predictions of the mathematical model. Investigations with the mathematical model revealed the inter-relationships between throw distance, spin rate, and optimum release angle over a wide range of parameter values. We recommend that players should aim for high backspin when performing a long throw-in. Even though the increase in flight distance achieved with high backspin is relatively modest, any increase in flight distance could lead to an increase in the team’s goal-scoring opportunities. However, we caution against re-designing the aerodynamic properties of the ball to improve throw-in performance because any changes are likely to produce ball behaviour that is unacceptable in other aspects of play. 14:10 20 mins COMPARISON OF TURBULENCE MODELLING APPROACHES TO THE SIMULATION OF A DIMPLED SPHERE John Hart Abstract: Use of computational fluid dynamics (CFD) in the aerodynamic simulation of sports projectiles has always been a challenge. The majority of these are spherical, classic bluff bodies, which typically experience flow transition during flight, and large flow separations. Current research of such flows is predominantly concentrated on the use of computationally intensive large eddy scale (LES) simulation methods, and even direct numerical simulation (DNS). Use of such approaches requires careful application of the models, and significant computational resource. The alternative is the use of unsteady Reynolds-averaged Navier Stokes (URANS) turbulence models, which are typically known to struggle in such flow scenarios. URANS however are, in comparison to LES, computationally economical and as such these models find significant use amongst both industry and academia alike, and their development still continues. In recent years transitional URANS models based on the calculation of intermittency, and hybrid scale resolving simulation approaches (SRS), have started to appear in proprietary CFD codes. Hybrid SRS models such as scale adaptive simulation (SAS) and detached eddy simulation (DES), combine LES with the use of economical well tuned URANS in the simulation of near wall flows. However to date the use of such models in the simulation of sports projectiles has been extremely limited. This paper provides a CFD comparison of these turbulence modelling approaches, with application to the simulation of a dimpled sphere, a golf ball. The study investigates and compares the suitability of low Re URANS, transitional URANS, and SRS models. Simulations are run between 25,000 < Re < 115,000, from subcritical through transition to supercritical. Comparisons are drawn between predictions of drag coefficient, flow separation angle and skin friction coefficient, with URANS being shown to be in reasonable agreement with SRS. However as may be expected although URANS predicted a comparable size of wake to SRS, no small scale structure was observed. Indeed it is shown how URANS in subcritical simulation of the dimpled sphere fail to demonstrate any time periodic shedding phenomena, instead becoming essentially steady state.