Dimple effect (Golf balls vs. Ping Pong balls)

Submitted by yaroub789 on April 7th, 2010 at 3:53:18 AM

<?xml:namespace prefix = o />http://vimeo.com/10736951



Introduction:

Ever since sports have become more competitive and technology more apt in helping athletes achieve peak performances, engineering has found a home as the driving force behind these results and astounding leaps and bounds in apparatus used. Whether it is by enhancing swimmers’ suits, aerodynamics on a bobsleigh or designing the top driving golf clubs, it is undoubtedly engineering at its peak, pushing the limits of what we know and applying it to shave off seconds, add metres, increase velocity and thus, break records and earn golds.

           

            The same can be said for golf balls. The shape, the size, the weight are all variables that are played with, tested in order to achieve peak results. This experiment tackles the advantage of dimples in golf balls versus a smooth surface and more importantly, its effects on results through a scientific explanation pertaining to fluid dynamics. Such terms as pressure, boundary layers, drag, lift and Reynolds number will be related to the issue at hand.

 

Theory:

            There are two factors that affect the path of a ball when it is hit: the momentum, both direction and amount, transferred to the ball, in this case by a golf club, as well as backspin that was given to the ball. These two internal factors affect directly the trajectory, length and height travelled.

 

            For a golf ball, the dimples create a turbulent boundary layer. For a smooth sphere such as a ping pong ball, there is a laminar boundary layer instead. Firstly, the turbulence created by the turbulent boundary layer reenergizes the flow allowing it to remain attached to the surface. Due to this, the wake behind the ball is much narrower and of a lower pressure behind the ball reducing drag. This reduction helps the golf ball travel faster.  However, for a ping pong ball, there is flow separation, due to the laminar boundary layer that happens as the ball travels through a medium. This separation leads to an unsteady flow with vortices behind the ball creating a much larger wake.

 

            Moreover, with the backspin, the golf ball has a higher pressure at the bottom of the sphere and a lower pressure at the top and behind the sphere. This phenomenon creates thrust and lift by reducing drag. For a ping pong ball, the laminar boundary layer permits the molecules of the air to stick to the surface colliding with incoming molecules only creating more drag and slowing down the ball, minimizing distance travelled. This worsens separation of the flow, resulting in the loss of the ideal pressure distribution creating a drag force on the sphere.

 

  

 
Turbulent layer vs. laminar layer on a sphere

 

Experimental procedure:

            A ping pong ball and a golf ball were chosen for the experiment. Red food dye was chosen to highlight streamlines around the balls. The ping pong ball was filled with water in order to make it sink in the water as its light weight made it hard to do so. A string was attached to both balls in order to place them and keep them in place in the bathtub. Water from a garden hose was used as the medium, maintaining steady flow throughout the experiment. The red dye was injected in the flow and a SONY HD-SR10 was used to capture the footage. Many tries were made, some more successful than others, which were kept for the final cut. At first, the golf ball was tested and then the ping pong ball was put in the water with the stream and the dye as seen on the video.

 

Results and discussion:

            It would be very complicated to create a mathematical model which predicts the optimal amount of dimples on a golf ball. However, Reynolds number is a pretty good tool in analyzing the turbulent or laminar flow around a sphere. For a smooth sphere, Re is much larger than the average Reynolds number experienced by a golf ball. However, as the Reynolds number continues to increase, the drag increases, therefore there is an optimal point. The dimpled ball has a fairly constant Reynolds numbers after 105. Here is a diagram illustrating how the drag, calculated by using the pressure coefficient, of a sphere varies with the reynolds number.

 

 

Re = (1000)(5 m/s)(0.02) / (8.93 x 10-4) = 111 982

 

With ρ = 1000 kg/m3

         V = 5 m/s (water flow velocity)

         D = 0.02 m

         T = 25 degrees Celsius

        μ = 8.93 x 10-2 Ns/m2

 

Graph 1:

 
sphere-drag.gif

 

            In the video, although it is hard to see, we can notice flow separation on the back of the ping pong ball and a larger wake. However, the golf ball has a more turbulent streamline and a smaller wake at the back. There are many limitations to the experiment due to the apparatus used but it pertains to the theory pretty well for the scope of this project/course.

 

Conclusion:

 

            Once again, the dimples on golf balls help optimize results in flight, lift and lessen drag making it a lot more useful for the golfers and providing competitive results that can make the difference between a birdie and a double bogey.

 

Reference:

 

·        Golf ball - en.wikipedia.org/wiki/Golf_ball




 


 


 


 


Comments

Submitted by umsidh56 on April 7th, 2010 at 4:06:28 AM

Your video is full of detail and explains the process well

 

Good Work!

Jap Jyotan Sidhu

Submitted by umlessap_6855397 on April 7th, 2010 at 5:25:28 AM

The narration was killer man. Lots of info in the video. I had to watch it twice to get everything

Submitted by umbiggsj on April 7th, 2010 at 3:40:46 PM

Really good work on comparing the two kinds of balls, it made for a much more convinciang argument. Lots of detail as well!

Submitted by umwan279 on April 8th, 2010 at 1:53:57 AM

Video is well done!

Submitted by GS on April 8th, 2010 at 10:06:30 PM

It was hard to pay attention to video while paying attention to narration. but its not something watching over cant fix. in the second time, having both at once actually works its benefits since i know exactly what "flow seperation", "following the ball" and all the stuff explained since its is mentioned right when the phenomena is happening (ie. easy reference) .

 

Good job. so is that your radio voice youssef? lol

Submitted by umbourgh1 on April 10th, 2010 at 5:48:57 PM

You were very clear in you explanations.

Submitted by Mohammad7603980 on April 17th, 2010 at 8:13:22 PM

A good video.

Submitted by Nelson D Sprout on April 23rd, 2010 at 8:36:07 AM

wow lots of meticulous analysis and detail! i had trouble seeing the difference between the two streamlines around the balls, maybe because they werent side by side. but once i checked out the picture i could see the idea. Maybe not the most original idea but easily the best explanation and report!

Submitted by ummcalli on April 25th, 2010 at 9:51:21 PM

Thanks for the detailed written explaination! You explained the phenomena really well, and gave understanding as to why the balls travel differently in the air.

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