Magnus effect

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The Magnus effect is the name given to the physical phenomenon whereby an object's rotation affects its path through a fluid, in particular, air. It is a product of various phenomena including the Bernoulli effect and the formation of boundary layers in the medium around moving objects.

A spinning object creates a kind of whirlpool of rotating air about itself. On one side of the object, the motion of the whirlpool will be in the same direction as the windstream that the object is exposed to. On this side the velocity will be increased. On the other side, the motion of the whirlpool is in the opposite direction of the windstream and the velocity will be decreased. The pressure in the air is reduced from atmospheric pressure by an amount proportional to the square of the velocity, so the pressure will be lower on one side than the other causing an unbalanced force at right angles to the wind.

This is not the only thing causing the deflection of the object. In addition to the Magnus force, the boundary layer of the flow is delayed on the side that is moving in the same direction as the free stream flow, and is advanced on the side moving against the flow. The flow is deflected away from the side moving against the flow, and this momentum change in the flow is balanced by a momentum change in the object in the opposite direction. Anything that disrupts the boundary layer will therefore tend to straighten out the trajectory. This is the reason for dimples on a golf ball: They disrupt the flow around the ball and tend to lessen the effects of spin, as well as helping to reduce pressure drag due to early flow detachment (see drag).

It is often referred to in the context of explaining otherwise mysterious but commonly observed movements of spinning balls in sport, especially golf, baseball, football and cricket. Another sport in which the effect is starkly observed is Table Tennis. An experienced player can place a wide array of spins on the ball, the effects of which are an integral part of the sport itself. Table Tennis rackets often have outer layers made of rubber to give the racket maximum grip against the ball to facilitate spinning.

It is worth noting, however, that the Magnus effect is not responsible for the movement of a cricket ball seen in swing bowling.

German physicist Heinrich Magnus first described the effect in 1853.

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The Magnus effect in external ballistics, also known as 'spin drift'

Another context where the Magnus effect can be found is advanced external ballistics. A spinning bullet in flight is often subject to a sideways wind. In the simple case of horizontal wind, depending on the direction of rotation, the Magnus effect causes an upward or downward force to act on the projectile, affecting its point of impact. Even in a complete calm, with no sideways wind movement at all, a real bullet will still experience a small sideways wind component. This is due to the fact that real bullets have a yaw motion that causes the nose of the bullet to point in a slightly different direction that the bullet is actually travelling in. This means that the bullet is "skidding" sideways at any given moment, and thus experiences a small sideways wind component.[1] All in all, the effect of the Magnus force on a bullet is not significant when compared to other forces like drag. However, the Magnus effect has a significant role in bullet stability due to the fact that the Magnus force does not act upon the bullet's center of gravity, but the center of pressure. This point is located either behind or in front of the center of gravity, depending on the flowfield structure, in other words, depending on whether the bullet is in super-sonic or sub-sonic flight. The Magnus force thus affects stability because it tries to "twist" the bullet along its flight.

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