Men’s World Cup Soccer Ball, the Al Rihla, Has the Aerodynamics of a Champion

Men’s World Cup Soccer Ball, the Al Rihla, Has the Aerodynamics of a Champion

The following essay is reprinted with permission from The ConversationThe Conversation, an online publication covering the latest research.

As with every World Cup, at the 2022 FIFA World Cup in Qatar the players will be using a new ball. Competitors don’t want the most important piece in the most popular sport to behave unexpectedly so it is important that every new World Cup ball feels familiar.

I am a physics professor at the University of Lynchburg who studies the physics of sports. Despite the controversy surrounding corruption and human rights concerns surrounding this year’s World Cup there is still beauty and skill in soccer science and skill. To understand the history behind the game’s most iconic piece, I analyze the World Cup ball every four years as part of my research.

The physics of drag

Between shots at goal, free kicks, and long passes, many of the most important moments in a soccer match occur when the ball is in the air. The ball’s ability to travel through air is one of its most important characteristics.

As the ball moves through air, a thin layer made up mostly of still air is called the boundary layer. It surrounds a portion of the ball. The boundary layer will cover only the front half the ball at low speeds before the air moves away from the surface. This is known as laminar flow.

When a ball moves quickly, however, the boundary layer wraps around the ball much further. The boundary layer forms when the air flows from the ball’s surface and separates into a series chaotic swirls. This is known as turbulent flow.

To calculate how much force moving air exerts on a moving object (called drag), physicists use the term drag coefficient. The drag coefficient determines how heavy an object feels at a given speed.

It turns out that a soccer ball’s drag coefficient is approximately 2.5 times larger for laminar flow than for turbulent flow. Although it may seem counterintuitive at first, roughening a soccer ball’s surface delays separation of the boundary layer. This allows a ball to remain in turbulent flow for longer. This is because dimpled golf balls fly farther than if they were smooth.

When it comes to making a soccer ball that is good, it is important to know the speed at which the airflow transitions from turbulent into laminar. The reason this is important is that the ball will slow down dramatically when it makes the transition from turbulent to laminar flow. If laminar flow begins at a high speed, the ball will slow down faster than if turbulent flow continues for longer.

Evolution of the World Cup ball

Adidas has supplied balls for the World Cup since 1970. Through 2002, each ball was made with the iconic 32-panel construction. The 20 hexagonal and 12 pentagonal panels were traditionally made of leather and stitched together.

A new era began with the 2006 World Cup in Germany. The 2006 ball, called the Teamgesit, consisted of 14 smooth, synthetic panels that were thermally bonded together instead of stitched. On rainy or humid days, the ball’s interior was kept dry by the tighter, glued seal.

Making balls out of new materials with new techniques and fewer panels can affect the ball’s flight through the air. Adidas has tried to balance panel number, seam properties, and surface texture over the past three World Cups to create balls with the perfect aerodynamics.

The eight-panel Jabulani ball in the 2010 South Africa World Cup had textured panels to make up for shorter seams and a fewer number of panels. Despite Adidas’ efforts, the Jabulani was a controversial ball, with many players complaining that it decelerated abruptly. When my colleagues and I analyzed the ball in a wind tunnel, we found that the Jabulani was too smooth overall and so had a higher drag coefficient than the 2006 Teamgesit ball.

The World Cup balls for Brazil in 2014–the Brazuca–and Russia in 2018–the Telstar 18–both had six oddly shaped panels. Though they had slightly different surface textures, they had generally the same overall surface roughness and, therefore, similar aerodynamic properties. Players generally liked the Brazuca and Telstar 18, but some complained about the tendency of the Telstar 18 to pop easily.

2022’s Al Rihla ball

The new Qatar World Cup soccer ball, the Al Rihla, is now available.

The Al Rihla is made with water-based inks and glues and contains 20 panels. Eight of these are small triangles with roughly equal sides, and 12 are larger and shaped sort of like an ice cream cone.

Instead of using raised textures to increase surface roughness like with previous balls, the Al Rihla is covered with dimplelike features that give its surface a relatively smooth feel compared to its predecessors.

To compensate for the rougher feel, the Al Rihla has wider and deeper seams. Perhaps they are learning from the mistakes made by the Jabulani, which had the shortest and shallowest seams in recent World Cup balls, and was felt to be slow in the air.

My colleagues in Japan tested the four most recent World Cup balls in a wind tunnel at the University of Tsukuba.

When air flow changes from turbulent to laminar, the drag coefficient increases rapidly. This happens when a ball is in flight. The ball will experience an abrupt increase in drag and slow down abruptly.

Most of the World Cup balls we tested made that transition at roughly 36 mph (58 kph). As expected, the Jubalani is the outlier, with a transition speed around 51 mph (82 kph). Considering that most free kicks start off traveling in excess of 60 mph (97 kph), it makes sense that players felt the Jabulani was slow and hard to predict. The Al Rihla’s aerodynamic characteristics are very similar to its predecessors and may move a bit more at lower speeds.

Every new ball is met by complaints from someone, but the science suggests that the Al Rihla should feel like a familiar ball to the players at this year’s World Cup.

This article was originally published on The Conversation. Read the original article.

ABOUT THE AUTHOR(S)

    John Eric Goff, Professor of Physics, University of Lynchburg.

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