Ready for the Women's World Cup?!?

The Women's World Cup begins on Friday, 7 June.  I'm excited to see the world's best go for glory in France!  My country will compete in Group F with Chile, Sweden, and Thailand.  The ball the players will use is the adidas Conext 19.  The panel design is exactly the same as the Telstar 18 ball that was used in the Men's World Cup last summer in Russia.  The color scheme, however, is quite different.  The color scheme pays homage to the ball that helped host France win the Men's World Cup in 1998.  That was the adidas Tricolore.

My colleagues at the University of Tsukuba in Japan, Takeshi Asai and Sungchan Hong, have been great collaborators for me.  We've published a few papers on soccer ball aerodynamics, including papers on the balls used in the past three men's Worlds Cup.  They sent me wind-tunnel data for the Conext 19.  The data look very similar to what we published for Telstar 18 (click here for our paper).  The graph below shows the drag coefficient as a function of ball speed (air speed in the wind tunnel).

I've labeled the so-called "critical speed" in green.  That's the speed at which air flow around the ball transitions from laminar below the critical speed to turbulent above the critical speed.  The critical speed above is at a great speed!  Most of the high-speed corner and free kicks will take place at speeds above that critical speed.  That means the players will deal with a ball in flight whose drag coefficient won't change much.

For balls hit with little to no spin, knuckling effects will play a role.  The graph below shows side and lift coefficients for the Conext 19.

Even though the ball wasn't spinning in my colleagues' wind tunnel, the boundary layer of air still separates asymmetrically off the back of the ball.  That's due to the fact that the seam pattern does not preserve spherical symmetry.  Air might separate off the ball near a seam on one side, but near a panel on the other side.  The large fluctuations at small speeds won't be noticed much by players because the side and lift forces are large compared to the ball's weight at those speeds.  For high speeds, kicked balls with little to no spin will wobble a little.  Lateral deflections could reach 6% of the ball's horizontal range on hard kicked balls.

My colleagues have wonderful facilities for testing soccer balls.  They sent me a video that I posted last year when media were contacting me about the Telstar 18 ball.  I'll show the video again below.

Ain't physics grand?!?  Now bring on the Women's World Cup!