Project IV – Sports Balls Aerodynamics

Project Aim

For this project after semester one I aim to study the different characteristics of various footballs and how it changes their flight. I will try to investigate to find out what the ideal football is and perform practical testing to investigate further.

 

 

Objectives

• I will start by looking into previous research and literature along a similar path of study to learn what has already been done to improve my understanding and help inspire myself.
• I will then undertake practical tests to gain some direct understanding and experience in the field.
• I will finally produce a small report into relevant technological evolutions that could be helpful towards this project, if I was to continue.

 

 

 

Dynamics of a Football

The travel path of the ball in football is one of the most crucial aspects within the game, balls must travel true with predictability to aid the goalkeeper. An example where this was not true would be the famous ‘jabulani’ ball used in the 2010 world cup. Famed for its unpredictability through the air this tournament made for some of the most iconic long range goals in international football history, all thanks to aerodynamics. I want to discover what made this ball so unpredictable and how it differs from most other footballs through history. The flight of the ball is one of the most important aspects to modern ball sports. A 2010 report acknowledged this stating that:

 

‘Aerodynamics plays a prominent role in defining the flight trajectory of all high speed ball sports. Depending on aerodynamic behaviour, the ball can be deviated from its anticipated flight path significantly resulting in a curved and unpredictable flight trajectory.’ (Firoz Alam 2010: p1)

 

I will begin by investigating the variables that are at play in affecting the path of travel for a football, and how a variation of football design manipulates its aerodynamics. Factors that affect the flight of a football could include the number of panels, with ‘modern’ footballs (standard black and white hexagon ball) being made from 32 panels and newer balls varying in panel number, dropping to as low as 12. This affects the behaviour when travelling through the air as fewer panels result in fewer seams and therefore in a less turbulent flight. A comparative study on the aerodynamics of sports balls states, ‘a cricket balls seam introduces different separation locations, hence causing a lateral force, colloquially known as swing’. (Mehta 1985 151-189). Studies of this ‘swing’ or ‘Magnus effect’ can date back further with even Isaac Newton commenting on the swing of a tennis ball:

 

‘For, a circular as well as a progressive motion…, its parts on that side, where the motions conspire, must press and beat the contiguous air more violently than on the other, and there excite a reluctance and react of the air proportionally greater.’ (Newton – 1672)

 

However, when discussing swing and manipulation of passing air it is important to discuss the impact of surface finish, whether that is smooth or grooved both will directly impact the air passing around it. Whilst modern footballs have become predominantly bonded this change allows for more freedom in design, as you are not confined to work within the seams. This has allowed for manufacturers to implement divots and dimples in various configurations in order to achieve predictable and consistent behaviour. These advancements come with a price and often lead to a match ball costing in excess of £100, therefore the cheap alternative machine stitched balls are often used in training due to their durability and relative low price. The traditional hand stitched balls are still made today however for the technology is dated for common usage and high price often means they are bought to look at rather than to use.

Previous Literature and Research

Here you can see a the results from a wind tunnel test, (Alam et al, 2014), primarily researching the drag coefficient (Cd) across 9 different footballs. They also aimed to study predictability and performance of the balls with professional footballers undertaking specific challenges; penalty, free kick and shoot for consistency (aim for a target). Dissecting the results from the study will provide great information as the all the balls vary in; number of panels, panel shape, surface finish and panel joint type. Wind speed was converted to Reynolds Number (Re) and all the balls in this study were tested from 20km/h to 120km/h with increments of 10km/h. The repeatability of measured forces within this study are, +/- 0.01 N, meaning that if the test was redone the results would fall +/- of 0.01 of the results in this experiment.

 

 

Before dissecting these results in is important to mention that the Reynolds number (Re) and drag coefficient (Cd) are defined as:

1.                                                                                           2. 

 

 

This graph reads from Re = 1.00 x 10^5 as this was when the smooth spheres flow transition occurred. This graph visually outlines the various behaviour of each differing ball and its transition from laminar to turbulent flow. It helps to outline the effect of; panel shape, number of panels, surface finish and panel join type. Through the visual representation of various flow states; critical, supercritical and transcritical.

The conclusions drawn from this test can include the Adidas Cafusa as being one of the best performing balls as a result of its low coefficient of drag at high speeds due to its thermally bonded panels creating little surface disturbance, however with its grooved surface finish it creates a consistent predictable flight. Although the transition of flow occurred at the same velocity for both the Nike Maxim and Adidas Cafusa the latter possessed a lower drag coefficient at the transcritical stage during turbulent flow.

The Mitre Ultimax pre-supercritical transition possessed the lowest drag coefficient due to its complex panel shape and surface roughness creating turbulence and delaying separation. The Mitre also performs similarly in supercritical and transcritical transitions to the Adidas Cafusa. The Jubalani possesses a higher drag coefficient than that of the Teamgeist II at higher speeds (transcritical), this is due to it recording more surface disturbances. The Umbro Neo reaches its flow transition at higher speeds than all the other balls in the test this is because it has the smoothest surface, when compared with the smooth sphere you can see it behave similarly.

Adverse characteristics for a football, such as knuckling, occurs after flow separation of the boundary layer creates a wake of recirculating flow. Within the separation region at certain velocities, vortex shedding can occur which is a big contributing factor into knuckling. Flow separation occurs as fluid passing over the surface accelerates, leading to a decrease in pressure in the direction of flow known as ‘favourable pressure gradient’. As the fluid travels further along the surface it begins to slow increasing the pressure, known as adverse pressure gradient. The change in pressure leads to a change in the direction of flow but due to the oncoming fluid it cannot travel backwards and instead detaches from the surface leading to flow separation.

This flow separation is what leads to an increased pressure drag, if the balls boundary layer is in turbulent flow the fluid finds it harder to separate meaning the separation occurs later down the surface of the ball creating a smaller wake resulting in less pressure drag forces. This is because turbulence creates a mixing of the various layers of flow and the momentum transfer can sustain larger adverse pressure gradient without separation. This is why golf balls have dimples, as the creation of turbulence delays flow separation and therefore reduces drag allowing the ball to travel further.

 

 

Practical Laboratory Work

In order to study the aerodynamics of footballs I must understand the theory but also learn the process of conducting tests, especially when involving a windtunnel. I undertook a lab workshop (see workshop induction page) to enable me to carry out further test with various ball shapes. To begin the test I wanted to compare a smooth sphere with a golf ball, to study the effect these dimples have on flight of the ball. To more thoroughly demonstrate this I used string to show the turbulent/laminar air around the ball in both instances. This test involved a number of pieces of apparatus, namely; a closed circuit wind tunnel, anemometer, wire lead load cell with power supply, multimeter, thermometer and a barometer.

To start with it was imperative that we measured the ambient temperature and air pressure since these affect the properties of the air, higher temperatures lead to lower air density and higher pressures lead to a higher air density, known as the combined gas law. To ensure our results possessed strong validity and repeatability, it was essential that we carried out calibration tests not just to ensure that the system was in working order, but to also get a benchmark of data from which we can compare and contrast the following results. Using the load cell to calculate the drag force expressed onto the test shape, I rearranged the drag force equation to solve for the drag coefficient. This is defined as:

I also needed to find out the reynolds number for the experiment and this could be achieved through conversion of the given flow velocity into ‘Re’. This equation is defined as:

 

Using string I was able to perform a rudimentary flow visualisation technique. These photos that were taken during the highest wind velocity (20.1m/s) of the test, at this stage the air circulating the sphere (fig.5.) has begun to separate leaving a turbulent wake behind it. Comparatively, the golf ball (fig.4.) with its dimples has delayed the boundary separation and this has created a smaller low pressure area with a less turbulent wake behind it. This means that the normal stress drag or pressure drag of the golf ball will be less than that of the smooth sphere. The above culminates to suggest that when hit, the golf ball will travel further.

Here is a table of results after calculating the drag coefficient and reynolds number for both the smooth sphere and golf ball to quantitatively confirm the previously stated behaviour.

The results, although not tested to a very high reynolds number, show an early increase in effectiveness of flow dynamics for the dimpled golf ball. The test range that you have been shown is situated at the reynolds number between 10^3 and 10^4, as shown in fig.6. below. The results show how the dimples allow the golf ball to delay boundary separation and maintain a lower level of drag early on and as the flow velocity rises the golf ball continuously maintains better aerodynamic efficiency.

In football this technological advancement has been incorporated into the most recent 2022 World Cup ball the ‘Al Rihla’. Using divots and dimples to get a truer more consistent flight, it is the first certified ball to have these changes and could be a turning point in football design so long is this ball is a success at the World Cup.

Below is a CFD analysis of a smooth sphere to more accurately analyse and depict the characteristics and prevalent with this shape and surface finish, it will also help to outline the poor aerodynamic qualities of this structure and outline opportunities for improvement. A detailed excel file holding all the results of the CFD test can be found here: Smooth sphere CFD

 

 

Technological Evolution for Improved Data Collection

Particle Image Velocimetry

Advancements in technology are constant and relentless and it is this development that aids scientists, researchers and engineerings in creating more efficient and better equipped products. Relating to this project to better understand the aerodynamic behaviour of sports balls, new techniques have been devised that result in a more expansive collection of detailed data.

Particle Image Velocimetry (PIV), defined by Atkins (2016) VIC “is an optical measurement technique where the velocity field of an entire region within the flow is measured simultaneously”. Using seeding particles and ultra high-speed cameras along with a 400-500 nm wavelength laser, particles within the fluid are photographed as they flow across an entity frame by frame. This then enables comparison of images that can show not just fluid behaviour, but also precise velocities of flow as displacement vectors for specific particles can be drawn and converted to velocity using the time between the two photos (delta t) and change in position of tracked particles (delta x/y).

Individually, the almost instantaneous availability of fluid flow visualisation and collection of detailed quantitative data are extremely beneficial in analysis and research. However, this technique combines the both to create a productive and convenient optical method for flow visualisation and analysis.

 

Magnetic Suspension for Wind Tunnels

When conducting wind tunnel tests, calculations must be done to account for the mountings affecting the shape and aerodynamic profile of the relating component. This negatively impacts the accuracy of the results as it is impossible to know the exact uninterrupted behaviour of the test piece in relation to the fluid.

Japanese aerospace agency, JAXA, have developed new magnetic suspension technology allowing for a one-of-one practical testing facility without interference from supporting devices. Due to the departure of any supporting devices the testing of previously scarcely researched axisymmetric shapes (bluff bodies such as; spheres, disks and cylinders) can now be researched further. It enables insightful research into atmospheric re-entry of JAXA’s capsules from outer space, as it is known that when entering high subsonic and transonic speeds theses capsules will begin to diverge due to dynamic instability. When testing this in a windtunnel due to the ultra-high speeds the supporting devices impact negatively on the accuracy of the results, this is where the new technology enables a greater validity for the findings.

The difficulty with this new technology is that when the test piece is under load from the fluid, forces expressed onto the piece in terms of lift and drag need to be accounted for by the strength of the magnetic field. The test piece must stay static in the tunnel and a complex mechanism that compensates the change in the forces to change the magnetic field is necessary.

 

Conclusion

In terms of the strive for true and predictable flight the most important factor for footballs, aerodynamically speaking, is surface finish. As this is the most prevalent characteristic in direct contact with the air it therefore has the biggest impact on the balls flight. Now that almost all modern footballs use bonded joins there are less prominent seams meaning the impact from the number of panels and seams is greatly reduced. The change to bonded seams is mainly a manufacturing improvement lowering the cost for higher quality balls.

With so many eyes on football design processes can be lead by commercial and financial factors with the World Cup ball needing to be memorable, panel shape and styling can be purely aesthetic. However, due to past shortcomings with the ‘Jabulani’, football producers are slowly incorporating more and more aerodynamic efficiencies to benefit both forwards and goalkeepers. If I was to take this further I would like to investigate the aerodynamics behind swing and curve, and also try to utilize any new technologies that could improve the data collection.

 

 

Bibliography