Post by mikegarrison on Aug 31, 2011 11:25:06 GMT -5
Bringing this post from another thread and giving it its own topic.
What is going on here is a characteristic of spheres (or cylinders) in fluid flow. I'll simplify a bit.
Laminar (non-turbulent) flow around an object has less drag than turbulent flow. But turbulent flow has more energy. Because turbulent flow has more energy, it can follow "adverse pressure gradients" better. That means that turbulent flow remains attached to the surface of the ball on the back side of it, while laminar flow separates more easily.
The end result is that a smooth ball punches a bigger hole through the air than a rough ball, so the rough ball actually has less drag.
However, the actual amount of roughness required for optimum flight depends on the speed of the ball and the size of the ball (and also the altitude, the temperature, the humidity, and many other factors).
If a ball (or cylinder) has spin on it, that spin sets up a circulation in the air. Circulation is the mechanism that causes lift. So a top-spin ball generates negative lift and dives toward the ground faster than gravity alone would cause it to do. (Back-spin causes positive lift. Baseball home run hitters try to hit just under the center of the ball, giving it a back-spin that helps it carry over the fences.)
The "float" part of a float serve comes from the fact that the actual point of flow separation is dynamic. If the flow separates a little further along the surface of the ball on one side versus the other, that will cause an asymmetric force on the ball and it will be pushed in one direction. The more irregular the ball is, the more pronounced this will be, but even a perfectly smooth sphere will have some randomness in its flight path.
My guess (and this is only a guess) is that if these new balls exhibit more "float" that what is really happening is that the difference between the seams and the panels of the ball is bigger. If the separation is resisted on the panels but triggered on the seams, then the seams will work a bit like the stitches of a baseball. As the ball slowly rotates, first the seams will be prominent one one side and then they will be prominent on the other side. This will cause the ball to first dart one way, then to change and dart in the other direction.
If volleyballs did not have seams and panels, float serves would behave very differently than they do now. (Or so I guess, without actually doing any experimentation on the idea.)
What is going on here is a characteristic of spheres (or cylinders) in fluid flow. I'll simplify a bit.
Laminar (non-turbulent) flow around an object has less drag than turbulent flow. But turbulent flow has more energy. Because turbulent flow has more energy, it can follow "adverse pressure gradients" better. That means that turbulent flow remains attached to the surface of the ball on the back side of it, while laminar flow separates more easily.
The end result is that a smooth ball punches a bigger hole through the air than a rough ball, so the rough ball actually has less drag.
However, the actual amount of roughness required for optimum flight depends on the speed of the ball and the size of the ball (and also the altitude, the temperature, the humidity, and many other factors).
If a ball (or cylinder) has spin on it, that spin sets up a circulation in the air. Circulation is the mechanism that causes lift. So a top-spin ball generates negative lift and dives toward the ground faster than gravity alone would cause it to do. (Back-spin causes positive lift. Baseball home run hitters try to hit just under the center of the ball, giving it a back-spin that helps it carry over the fences.)
The "float" part of a float serve comes from the fact that the actual point of flow separation is dynamic. If the flow separates a little further along the surface of the ball on one side versus the other, that will cause an asymmetric force on the ball and it will be pushed in one direction. The more irregular the ball is, the more pronounced this will be, but even a perfectly smooth sphere will have some randomness in its flight path.
My guess (and this is only a guess) is that if these new balls exhibit more "float" that what is really happening is that the difference between the seams and the panels of the ball is bigger. If the separation is resisted on the panels but triggered on the seams, then the seams will work a bit like the stitches of a baseball. As the ball slowly rotates, first the seams will be prominent one one side and then they will be prominent on the other side. This will cause the ball to first dart one way, then to change and dart in the other direction.
If volleyballs did not have seams and panels, float serves would behave very differently than they do now. (Or so I guess, without actually doing any experimentation on the idea.)
