Physics holds the secret to volleyball’s highly unpredictable “float serve”

A good float serve at just the right moment in volleyball can make or break a tight game, since the ball’s trajectory is so tough to predict. It’s the surface panels on conventional volleyballs that give rise to these unpredictable trajectories, and modifying the surface patterns could make for a more consistent flight, according to a recent paper in Applied Sciences.

It all comes down to gravity and aerodynamics. Any moving ball leaves a wake of air that trails behind it as it flies through the air. The inevitable drag slows the ball down. The trajectories of various sports balls are affected not just by their diameter and speed but also by any tiny irregularities on their surface. Golf balls have dimples, for example, while baseballs have stitching in a figure-eight pattern—both sufficiently bumpy to affect the airflow around the ball.

It’s well known that the movement of a baseball creates a whirlpool of air around it, commonly known as the Magnus effect. The raised seams churn the air around the ball, creating high-pressure zones in various locations that (depending on the type of pitch) can cause deviations in its trajectory. Golf ball dimples reduce the drag flow by creating a turbulent boundary layer of air, while the ball’s spin generates lift by creating a higher air pressure area on the bottom of the ball than on the top.

The surface patterns on volleyballs can also affect their trajectories. Conventional volleyballs have six panels, but more recent designs have eight panels, a hexagonal honeycomb pattern, or dimples.

There have been numerous past studies examining the aerodynamics of sports balls: golf, cricket, tennis, baseball, rugby, and soccer balls. But for some reason, there has been a dearth of research focusing on the physics of volleyballs. Back in 2010, Takeshi Asai of the University of Tsukuba and several Japanese colleagues decided to remedy that by conducting a series of wind tunnel experiments with three ball types with distinctly different surface patterning: a conventional Molten ball with six panels; a newer Molten ball with a honeycomb pattern; and a Mikasa dimpled ball. They used a robotic device to “serve” the balls to ensure consistency, then measured the drag coefficients for each ball.

The drag coefficient describes how much the flowing air “sticks” to the ball’s surface. The faster the ball is moving, the less “sticky” the ball becomes. Typically, wakes are larger, and drags are higher, at slow speeds, but if the ball hits a critical speed threshold, it experiences a so-called “drag crisis“: the wake shrinks suddenly, and the drag plummets. It’s basically the point where the air flow abruptly switches from laminar (smooth) to turbulent. That critical speed threshold—the velocity at which the air flow becomes truly turbulent—can vary significantly just among volleyballs.

In the 2010 study, each ball was served 20 times with three different panel orientations. The authors found that for perfectly smooth spheres, the critical speed is around 25 meters per second, or about 56 miles per hour. All the volleyballs they tested showed lower critical speeds than the smooth sphere. The traditional Molten ball had similar low drag, while the honeycomb-patterned Molten ball had a higher final drag. Asai and his co-authors suggested that this might be because the honeycomb pattern increased the ball’s surface roughness, while the surface panel orientation (in transverse or diagonal directions) on the traditional ball as it is served changes how the air flows around the ball mid-flight, affecting its trajectory.

For this latest study, Asai and several colleagues used four different types of volleyballs—two with panels, one with the honeycomb pattern, and one dimpled ball—to study the aerodynamics of the float serve. Unlike a fast top-spin serve or jump serve—both of which follow fairly predictable flight paths—a float serve has no spin. That makes it tough to predict the ball’s trajectory; it can swerve unexpectedly, giving the server a competitive advantage.

From a physics standpoint, the float serve is similar to throwing a knuckleball in baseball, which is largely unaffected by the Magnus force, because it has no spin. Its trajectory is determined entirely by how the seams affect the turbulent airflow around the baseball. The seams of a baseball can change the speed (velocity) of the air near the ball’s surface, speeding the ball up or slowing it down, depending on whether said seams are on the top or the bottom. The panels on conventional volleyballs have a similar effect.

In this latest study, each ball was tested 30 times, for a total of 240 tests. The results: the balls with panels had the highest critical speed threshold, leading to unpredictable flight patterns. The honeycomb-patterned ball had a much lower critical threshold, while the dimpled pattern increased the threshold. Both the honeycomb and dimpled balls also had less difference in the drag crisis, regardless of panel orientation. So the authors surmise that it should be possible to control when the drag crisis occurs just by changing the ball’s surface design.

“The most commonly used volleyballs have six panels with three parallel rectangular strips,” said Asai of these results. “Using a hexagonal or dimpled pattern instead could significantly increase the consistency of its flight. This research may have important implications not only within sports, but also for developing more efficient and stable drones.”

DOI: , 2019. 10.3390/app9194007 (About DOIs).

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