The answer lies in the dimples. By roughening the surface, the critical Reynolds number is reduced to about 50 000, with speeds within the golfer’s range. The dimples cause a transition to turbulence at a lower Reynolds number.
Full Answer
What are the advantages of turbulent boundary layers?
Turbulent boundary layers adhere longer to the surfaces of the body around which flow occurs. The (pressure) drag is significantly lower. If there is a risk of flow separation, turbulent boundary layers should be aimed for. This fact is taken advantage of, for example, with golf balls.
Is the wake behind a dimpled golf ball laminar or turbulent?
Comparison of the turbulent wake behind a dimpled golf ball (top), which originates from a turbulent boundary layer separation, and a smooth ball (bottom), which originates from a laminar boundary layer separation. Note that the separation point for the dimpled ball is located much further downstream and the wake is smaller.
How does turbulence affect the flight of a golf ball?
This turbulence helps the flow remain attached to the surface of the ball and reduces the size of the separated wake so as to reduce the drag it generates in flight. When the drag is reduced, the ball flies farther.
What is the boundary layer of a turbulent flow?
For higher Reynolds numbers, the boundary layer is turbulent and the streamwise velocity is characterized by unsteady (changing with time) swirling flows inside the boundary layer. The external flow reacts to the edge of the boundary layer just as it would to the physical surface of an object.
How fast does a golf ball fly off the tee?
"A golf ball leaves the tee with a speed of about 70 m/s and a backspin of at least 50 rev/s."
How dimples on a golf ball affect its flight and aerodynamics?
Dimples on a golf ball create a thin turbulent boundary layer of air that clings to the ball's surface. This allows the smoothly flowing air to follow the ball's surface a little farther around the back side of the ball, thereby decreasing the size of the wake.
Why are there 336 dimples on a golf ball?
Essentially, golfers would have little hope controlling a smooth golf ball. When dimples are added to a golf ball, it creates a tiny layer of air around the golf ball that significantly cuts down drag. This forces the air to flow over a larger portion of the ball, which results in a much smoother ball flight.
What is the Magnus effect on a golf ball?
They wouldn't refer to it, but it is all thanks to what is called the Magnus Effect, which effects all rotating balls or cylinders when they fly. As a ball picks up speed, air on the front side of the ball is going in the same direction as it's spin. Therefore, it gets dragged along with the ball and deflected back.
How many dimples does a Titleist Pro V1 have?
388 dimpleNew Spherically-tiled Tetrahedral Dimple Designs– featuring a 388 dimple layout for Pro V1, and 348 for Pro V1x – are optimized to each model to maximize distance and flight consistency.
Which golf ball has 500 dimples?
GOLF AERODYNAMICS. The Ultra (Registered TradeMark) 500 Series golf ball (above), introduced in 1995 by Wilson Sporting Goods Company, Humboldt, Tennessee, has 500 dimples arranged in a pattern of 60 spherical triangles.
Why do golf balls fly so far?
0:544:54Science of Golf: Why Golf Balls Have Dimples - YouTubeYouTubeStart of suggested clipEnd of suggested clipIn as a golf ball flies through the air the airflow interacts with the surface of the ball. And canMoreIn as a golf ball flies through the air the airflow interacts with the surface of the ball. And can greatly affect the amount of drag.
Which golf ball has the most dimples?
As for the highest number of dimples, that would be Ultra 500 Series Golf Balls (500 dimples). Then you also have Nike and Maxfli golf balls with +400 dimples.
How many dimples does a Callaway SuperSoft golf ball have?
Both the Callaway SuperSoft and Pinnacle Soft have 332 dimples. The SuperSoft's have a hexagonal dimple design. The Pinnacle soft's have an icosahedral shape. Both balls amplify your distance.
Why is a golf ball white?
It was a bright color and was a good alternative compared to the natural brown of the feathery balls. It is not as if high-intensity yellows and neons were available options, so for hundreds of years, white was the standard color.
How do you calculate trajectory on a golf ball?
Fzw = Fw cos y = -sin y (Cwvw) If there is any side spin on the ball - and there usually is - then the ball will curve in the direction of the spin. Side wind will, of course, cause the ball to curve in its direction and head or tail wind will affect the ball's carry.
What affects the trajectory of a golf ball?
A golf balls flight pattern is effected directly by the presence of air resistance and the rotation of the ball. Looking a both air resistance and rotation flight paths can be predicted.
What is the effect of external flow on the boundary layer?
So the boundary layer gives any object an "effective" shape which is usually slightly different from the physical shape.
Why is the boundary layer called the boundary layer?
Engineers call this layer the boundary layer because it occurs on the boundary of the fluid.
What happens when an object moves through a fluid?
As an object moves through a fluid, or as a fluid moves past an object, the molecules of the fluid near the object are disturbed and move around the object. Aerodynamic forces are generated between the fluid and the object.
How do aerodynamic forces work?
As the fluid moves past the object, the molecules right next to the surface stick to the surface. The molecules just above the surface are slowed down in their collisions with the molecules sticking to the surface. These molecules in turn slow down the flow just above them.
Why is the boundary layer important?
Engineers call this layer the boundary layer because it occurs on the boundary of the fluid. The details of the flow within the boundary layer are very important for many problems in aerodynamics, including wing stall , the skin friction drag on an object, and the heat transfer that occurs in high speed flight.
What are the two properties of a fluid?
The magnitude of these forces depend on the shape of the object, the speed of the object, the mass of the fluid going by the object and on two other important properties of the fluid; the viscosity, or stickiness, and the compressibility, or springiness, of the fluid.
Is a boundary layer turbulent?
Boundary layers may be either laminar (layered), or turbulent (disordered) depending on the value of the Reynolds number. For lower Reynolds numbers, the boundary layer is laminar and the streamwise velocity changes uniformly as one moves away from the wall, as shown on the left side of the figure. For higher Reynolds numbers, ...
Why is the turbulent boundary layer steeper?
The reason for this is the increased transport of momentum between the fluid layers, which leads to a steeper velocity profile within the boundary layer.
How does a turbulent boundary layer affect the flow?
The turbulent wake becomes narrower with the shifting of the separation point. This reduces the flow losses and thus the pressure drag, which ultimately results in a reduction of the overall drag.
Why does skin friction drag increase?
At the same time, however, the pressure drops considerably due to the energy dissipation in the turbulent wake. This leads to a significant increase in pressure drag, which is far greater than the reduction in skin friction drag. In total, the parasitic drag ( air resistance) thus increases very strongly in the event of a flow separation.
What is boundary layer separation?
Animation: Boundary layer separation of a flow around on a cylinder. However, according to Bernoulli’s principle, a change in kinetic energy is directly linked to a change in static pressure. In this case, the increase in kinetic energy is at the expense of the pressure (energy).
What forces act when a fluid flows around a body?
When a fluid flows around a body, forces act between the fluid and the surface due to the viscosity . These intermolecular forces are the caue for the drag, among other things. However, due to the forces acting, the surface of the solid body also tries to bind the fluid to itself.
How does fluid flow around a cylinder?
In case of a cylinder, the fluid at first is forced upwards and downwards in order to flow around the cylinder. However, according to the continuity equation (conservation of mass), the same mass must still be moved around the cylinder, so that the flow velocity increases as a result.
Why can't the layer above the adherent fluid layer tear itself away from the surface?
The layer above this adherent fluid layer will not be able to simply tear itself away from it, because intermolecular attraction and pressure forces also act between the layers. As a result, any flow flowing around a body is tempted to follow the profile of the surface.
What happens when you move a ball away from the surface?
As we moved outward away from the surface of the ball, the air molecules would be feeling the effect of the friction and would be exhibiting a decrease in velocity. At some point, after moving far enough away from the surface, the molecules don’t even know that a ball is traveling through them.
What would happen if we zoomed in and saw the air molecules right at the surface of the ball?
If we could zoom in and see the air molecules right at the surface of the ball, we would see that they are getting stuck to the surface due to friction. In fact the velocity of the molecules right at the surface would be zero.
What happens to air molecules on a downswing?
On your downswing, air molecules are flowing around the head and there is the presence of skin friction and flow separation leading to skin drag & pressure drag respectively . The head of a driver is still considered a blunt body, such that pressure drag will dominate over skin drag on your downswing.
Which type of flow contains streamlines of molecules which are moving smoothly one behind the other in an orderly fashion?
Laminar flow contains streamlines of molecules which are moving smoothly one behind the other in an orderly fashion. Turbulent flow is chaotic; streamlines break up and molecule movement is random. Both types of boundary layers will eventually separate from the surface.
Do turbulent boundary layers stick to bodies moving through them?
Turbulent boundary layers want to stick to bodies moving through them. So as the molecules move to the back side of the ball , they stay closer to that back side of the ball more than a laminer boundary layer will. They wait to separate off the back.
What is the effect of air passing over the top of the ball on the ball?
This results – by way of Bernoulli’s principle – in lower pressure above and an upward force on the ball. Lift allows golfers to achieve greater distances. The character of fluid flow depends on a dimensionless quantity, the Reynolds number.
How is the aerodynamics of a ball determined?
The aerodynamics of a ball are determined by its mass and shape, in particular the nature of the surface. A complex pattern of dimples of varying sizes, shapes and depths influence the air flow around the ball. By roughening the surface, the critical Reynolds number is reduced to speeds within the golfer’s range.
How long does it take for a ball to bounce?
By elementary mechanics the vertical motion is decelerated by gravity, and the ball should bounce after about 200 metres and stop a few tens of metres further on.
How many dimples are in a golf ball?
Most balls have about 300 dimples. Manufacturers promise greater control, stability and velocity on longer shots. Hundreds of dimple patterns have been devised and patented but the optimal design remains a challenge.
How fast can a pro golfer drive?
The golfer controls the direction and spin of the ball by variations in his swing. A pro can swing his driver at up to 200 km/h, driving the ball 50% faster than this as it leaves the tee, on a trajectory about 10º above the horizon.
From Observation to Mathematical Model
As a child, I occasionally would walk around a nearby golf course with my family on rainy days; the only times when no golfers would dare to play. Our own game was to find lost balls from previous, unfortunate players. The person who found the most balls would win.
The Drag Crisis Observation
Throughout history, the flows around many different shapes have been studied by scientists. For example, vortex streets are generated by the flow around cylinders. Although a sphere does not generate this type of large alternating flow structure, the flow characteristics can also be linked to the Reynolds number.
The Reason for the Drag Crisis
First of all, let’s recall that the drag of an object is caused by two sources:
Modeling the Aerodynamic Forces of Golf Balls
Now we understand why golf balls have dimples in the first place. Let’s remember that the drag is lower and therefore the ball can go further. To find out how much further the ball can go, we first need to compute its trajectory.
Finding the Optimal Launch Angle
The effect of the dimples on a golf ball should now be clear: They make the ball go further. However, in practical terms, this does not say much about how I should hit the ball.
Conclusion
In today’s blog post, we answered a seemingly simple question about golf ball dimples, which has to do with the behavior of the turbulent boundary layer over a sphere at a specific range of Reynolds numbers. This also outlines a classical process in engineering.
Try It Yourself
Try computing the trajectory of a golf ball in COMSOL Multiphysics. Click the button below to access the model file featured in this blog post: