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The week in Biomechanics #CW35

Lasse Hansen 0

The weekly recap of what is going on in the world of biomechanics.

The reverse Magnus effect in golf balls

You might have heard about it: Golf balls are the fastest sport’s balls with the longest carry distance [1]. This fact makes the aerodynamics of the ball quite interesting since small changes in design can cause major differences in the ball’s flight behavior, and ultimately influence the outcome of the game. Earlier this year, a technical note was published in ‘Sports Engineering’ which reported interesting observations about the impacts of the golf ball design on its flight behavior.

The team took three commercially available golf balls with different dimple patterns and shot these through their lab. What sounds like a good time playing golf at first, was definitely a highly standardized experiment: The group used a pitching machine to accelerate the balls. In this machine, the pneumatic linear accelerator expedites the ball along with two opposing surfaces, one with high friction and the other one with low friction. By changing the distance the ball rolled along the two surfaces, the scientists were able to control the spin rate. This setup allowed led the team to reach spin rates of the ball of up to 3000 rpm!

Figure 1) Lyu, B., Kensrud, J. & Smith, L. The reverse Magnus effect in golf balls. Sports Eng 23, 3 (2020).

Ball speed, location, and rotation were measured using light gates and high-speed video cameras. The spin of the ball is of utter relevance since we know that the Magnus-Effect influences the flight path of spinning, round (or cylindric) objects. So let’s have a look at the aerodynamics: The airflow around a flying ball can either consist of laminar boundary layers (Fig.2a) or turbulent boundary layers (Fig.2b). It depends on the ball’s speed whether the flow is laminar or turbulent, as well as the size of the wake. Right here, it is most important to know that the turbulences behind the ball can slow it down.
But then a golf ball can also spin, and that changes the flow again: A golf ball with backspin, where the top of the ball is moving with the flow while the bottom moves against the flow, shows two different relative air-to-ball-surface speeds. This leads to the upward force known as the Magnus effect (Fig.2c).

Figure 2) Lyu, B., Kensrud, J. & Smith, L. The reverse Magnus effect in golf balls. Sports Eng 23, 3 (2020).

But then, what’s the reverse Magnus effect? The ball might travel near the speed where the laminar boundary layer shifts towards a turbulent one, and the right amount of backspin can cause a laminar flow on the top side, but a turbulent flow at the bottom. As a result, the Magnus effect changes its direction, pulling the ball downward (Fig.2d).

Interestingly, the critical speed is usually met during short chip shots and not in long drives. But this effect can occur with multiple sports balls traveling at their relative critical speed, as for example baseballs [2].

Lyu, B., Kensrud, J. & Smith, L. The reverse Magnus effect in golf balls. Sports Eng 23, 3 (2020).

Further References:
[1] Goff JE (2013) A review of recent research into aerodynamics of sport projectiles. Sports Eng 16(3):137–154
[2] Kensrud JR (2010) Determining aerodynamic properties of sports balls in situ, Washington State University

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