The Magnus / Venturi Effect
Q - “In the Ball Behaviour chapter, you’ve used the term “Magnus Effect”, when in The Golfing Machine it says “Venturi Effect”. Was Homer Kelley wrong?”

Yes, but only very strictly speaking. The Venturi and Magnus effects describe the same phenomenon- the change in a fluid’s pressure is inversely proportional to a fluid’s velocity, and vice versa. For example, if we take a golf ball moving through the air, spinning backwards on a horizontal axis. The top of the golf ball will be rotating away from the target and so the surrounding air will be moving with the ball. The bottom of the ball will be rotating towards the target, so the surrounding air will be moving against the ball. The Magnus and Venturi effects both state that because the air flow above the ball moves quicker than the air flow below it, there will be a decrease in air pressure above the ball and an increase of air pressure below it. It’s this imbalance of air pressure which forces the ball upwards in this example.

In that respect both Magnus and Venturi effects are the same. However, the Magnus Effect specifically describes that phenomenon in “real life” examples, such as with golf. The Venturi Effect on the other hand is a derivative of what’s known as the Bernoulli Principle. The Bernoulli Principle explains fluid pressure, velocity, kinetic and potential energies, but only with a non-viscous, incompressible fluid. The problem with applying the Venturi Effect to golf balls is that the fluid it passes through, the air, is viscous and is compressed by the travelling ball. The Venturi Effect, because it’s based on the Bernoulli Principle, doesn’t take into account this aspect of the air’s behaviour.

Of course this is all academic and only of real consequence to those who are using the corresponding mathematical equations that go with the Venturi Effect when calculating ball behaviour. So again, strictly speaking, Mr Kelley had used the wrong term, but as the Venturi Effect describes the exact same phenomenon in ball flight as the Magnus Effect, it’s not really an issue.

Mr Kelley discussed the Venturi Effect in chapter 2-B of The Golfing Machine.

Bernoulli’s Principle - http://en.wikipedia.org/wiki/Bernoulli's_principle
Venturi Effect - http://en.wikipedia.org/wiki/Venturi_effect
Magnus Effect - http://en.wikipedia.org/wiki/Magnus_effect

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