Golf ball vs. race car

[49078]

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.

 

[Rufus182]

I had the same thought about "dimple technology" in the bath the other day. Maybe there's some active patent/copyright on golf balls which is why this method has not emerged in other products, like motorcycle helmets for example?

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...

 

[Sparweb]

The two objects are very different. The ball is a sphere. Flow separation (turbulent vortices) behind the ball causes more drag than the boundary layer friction. By upsetting the BL, the designers are encouraging a turbulent flow, that actually weakens the effect of flow separation. The wake shrinks, thus C[sub]D[/sub] drops. I don't know what effect spin has.

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

 

[49078]

Why don't F1 cars have dimpled surfaces if it's an advantage?

 

[Rufus182]

I think the point is that aeroplanes/F1 cars have an efficient shape and hence getting laminar flow with min separation over the body is the goal. Golf balls are spheres and have a unefficient shape. Dimples can make a golf ball travel upto twice the distance!

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.

 

[rnordquest]

That smooth shape on an airplane improves the lift to drag ratio. I would think a smooth golf ball would generate too much lift from spin and make it way too hard to control.

 

[sreid]

From racing days many years ago I recall that at the Renold's number for air at 150 MPH caused turbulent flow to begin a few inches from the leading edge. A smooth golf ball at the same speed would be in laminar flow contitions hence the dimples to create turbulent flow.

 

[AJW308]

I'm a mechanical engineer with an interest in aerodynamics.

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?

 

[vortexman]

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

 

[IRstuff]

And everything is heavily contextual.

Supercavitation is another example where deliberately causing flow separation signficantly reduces drag in high-velocity underwater motion.

TTFN

 

[Sparweb]

And I forgot to mention that golf balls and cars operate at Reynold's numbers two orders of magnitude different.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[CESSNA1]

49078: Two different things. The golf ball dimples are turbulators. Without them the air would loose energy and actually separate from the golf ball and increase drag dramatically. The dimples add energy to the air and it stays attached to the golf ball all the way around. This actually reduces the drag. Look at the inboard wings on commercial jets (707, 747, etc) and you will see little pieces of metal sticking up from the wing skin. These are vortex generators. They add energy to the air by spinning it so that it will stay attached to the wing skin all the way to the trailing edge. If they were not there the air would separate and the wing would lose quite a bit of lift (crash!!)

Regards
Dave

 

[btrueblood]

The kinectic energy to mass ratio? That would be
m*V[sup]2[/sup]/(2m) = 1/2 V[sup]2[/sup]

 

[scotty7]

I was always given to understand that when the golf ball is hit, it has significant backspin. Then the dimples on the top of the ball have a low relative velocity to the local airflow, whereas the dimples on the bottom have a high relative velocity. The high velocity over the dimples causes turbulence, hence a higher static pressure. Over the top of the ball, the flow is smoother (less turbulent) hence lower static pressure. End result: lift. This is why the dimples improve the golf ball flight and give it extra range. If you watch a ball carefully, you can see it rise above the expected ballisitic trajectory shortly after being hit as the effect cuts in. The detail design of the dimple patterns can change the lift significantly.
Hope I remembered correctly...

 

[Spaghettiohead]

The spin of a golf ball has nothing to do with drag. The dimples do disturb the boundary layer so that airflow seperation happens further down the road (the turbulent air has more energy) and the wake is smaller.

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!!!

 

[davidjh]

Just a clarification on Cessna1's comments. The "VGs" seen on commercial jets are intended to impart higher energy air over the upper wing surface at the higher angles of attack (AOA) near/at stall. This is to either attain a slightly slower stall sped, or create acceptable handling qualities at the stall AOAs. They do increase drag a very small amount at normal flight AOAs, but this is accepted due to their "high dollar" benefit at those higher AOAs.