My PhD Student, Nazmus Sakib, just published our first paper together, which was on Magnus Force and Reverse Magnus on golf balls. You can download it here if you are interested.
While this site is about baseballs, Magnus is a lot easier to see on a golf ball since flow around a golf ball is much simpler than a baseball. The seams complicate things (in beautiful ways, don’t get me wrong).
The images below were acquired using Particle Image Velocimetry. If you are new to our measurements and how to interpret them, we have a primer here.
The movie below toggles between a golf ball moving 30 m/s without spinning to one at the same speed and spinning 3180 RPM. As always, interpret the red and blue the same way–they just mark where the wake begins to form. Note that the blue arrow marking the separation point on the top moves backward while the one on the bottom moves forward when the ball spins. Since the pressure in front of the separation point is changing along the surface of the ball, this means the pressure force on the top of the ball becomes different than the bottom of the ball. The spinning ball has an upward force.
David Kagan and I had an interesting discussion on how this is quite similar to the seam effects that I spend most of my time talking about, and I agree with him. Magnus changes the separation points, and so do seams. Magnus is more steady, especially on a golf ball. All you really need to know to understand what is happening to a ball are those separation points. It doesn’t even matter why they are where they are.
It is pretty academic to golf and to baseball, but it is possible for spin in the same direction to cause a force in the opposite direction, which is what Nazmus’s paper is about. This gets into some more narrow aerodynamics. Basically, if the ball speed is sufficiently slow, the flow on the front of the ball can be “laminar”. This is what laminar flow over a golf ball looks like:
Notice that the separation locations are much farther forward, near the fattest part of the ball and that the wake is much larger. This is what laminar flow does, and laminar flow is more likely if the relative speed of the air and ball is smaller. Now, if this ball spins, that relative speed on the top of the ball gets smaller while it gets larger on the bottom (for backspin). This can cause the flow on the top to be laminar while it is turbulent on the bottom, as in the image below.
This ball has backspin, but the wake is tilted up! This is because turbulent flow separates father back than laminar flow, the bottom is turbulent and the top is laminar. And, indeed, as shown by recent WSU lift results, the force is in the other direction.
While this is all pretty cool, baseballs and golfballs are seldom traveling at the right speeds and RPM to make this happen. See the paper for the details on that.