Golf ball vs. race car
Golf ball vs. race car
(OP)
I am a materials engineer, my aerodynamic background is limited. I have always wondered why you would want to "trip" the boundary layer on a golf ball to develop turbulent flow and reduce drag, but then race car design encourages laminar flow.
Can anyone explain this too me?
I started thinking about this because living in the usa i have seen the lexus commercial where they claim that using dimples on the bottom of the car reduces drag, but dimples are used to encourage turbulent flow while race cars are designed for laminar. I was also under the impression in my fluids class that dimples cause turbulance when the ball is spinning. Hopefully your lexus won't be spinning like a golf ball.
Can anyone explain this too me?
I started thinking about this because living in the usa i have seen the lexus commercial where they claim that using dimples on the bottom of the car reduces drag, but dimples are used to encourage turbulent flow while race cars are designed for laminar. I was also under the impression in my fluids class that dimples cause turbulance when the ball is spinning. Hopefully your lexus won't be spinning like a golf ball.





RE: Golf ball vs. race car
Maybe an aero expert could explain to us why dimples can only apply to golf balls. Perhaps it works because the body has to be spinning...
RE: Golf ball vs. race car
A car has an elongated surface, and the trailing edges are pinched together. The faster they go, the sharper they get.
The wake becomes less important, so keeping the air flowing smoothly is the goal.
Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout
RE: Golf ball vs. race car
RE: Golf ball vs. race car
Speed skiers and cyclists employ an efficient shape for their helmets to enough smooth laminar flow aswell. Motorcycle helmets have to perform in impact situations and a pointy shape could break your neck! I would like to try dimples on my bike helmet thank you, hehe.
RE: Golf ball vs. race car
RE: Golf ball vs. race car
RE: Golf ball vs. race car
As for the dimples under the car, they may reduce the shear force by inducing turbulent flow. Is an analogy of rolling over 'marbles of air' appropriate?
RE: Golf ball vs. race car
The bottom line is whether moving the point of flow separation gives a valuable drag reduction, and whether making the boundary layer turbulent will move the location of the separation. On a sphere, both are true. On many cars, neither is true. For example, consider a car with a more or less sharp corner somewhere near the back, like where the roof line meets the rear window. The flow is very likely to separate there, whether it is laminar or turbulent, so forcing it to be turbulent cannot reduce drag. This is only an illustrative example, and the shape of an F1 car is much different. There could well be regions on an F1 car where the separation point could be moved further back through the use of dimples or some other kind of turbulator, but the drag reduction would certainly not be as dramatic as it is for a sphere.
AJW308, the 'marbles of air' analogy is really not appropriate. Shear stress is higher in turbulent flow than in laminar flow. This fact is why it does not make sense to put dimples on every moving body to reduce its drag. Unless you can benefit by moving the separation point back, the dimples cause a drag increase, rather than a decrease.
vortexman
RE: Golf ball vs. race car
Supercavitation is another example where deliberately causing flow separation signficantly reduces drag in high-velocity underwater motion.
TTFN
RE: Golf ball vs. race car
Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout
RE: Golf ball vs. race car
Regards
Dave
RE: Golf ball vs. race car
m*V2/(2m) = 1/2 V2
RE: Golf ball vs. race car
Hope I remembered correctly...
RE: Golf ball vs. race car
The spin of the golf ball is using the Magnus effect to keep the ball in the air (backspin) retards the air below the ball and speeds up the air up top...this creates the pressure difference. Have you ever tried driving a ball without dimples? It won't go very far!!!
RE: Golf ball vs. race car