dimples

[klaas2]

does any one knows or dimples work in round moving object like a wheel

 

[Onemorechance]

Dimples work like vortex generators on a wing. They cause the boundary layer to become turbulent and therefore enable it to stay better attached to the wheel. Overall they will reduce the aerodynamic drag of the wheel.

 

[klaas2]

any formula to back this up
and at wich speed does it works then
and when wheel comes in a yaw angle? does it still work

if the layer becomes more turbulent it create more force right or am i wrong
since like a cyclist want the air not to be turbulant

so what you are saying turbulant air make you faster
i thought it make's you slower??

 

[rb1957]

dimples, like in a golf ball, work by tripping the boundary layer to become turbulent, rather than laminar. this keeps the flow attached (as there is more energy in the boundary layer) and this reduces Cd. i've noticed that BMW have added some dimples on the stalk for the rear view mirror, probably to reduce noise (from separation of the airflow). as far as applying this to bicycle wheels, i think there are many factors to consider ... reynold's number, etc. you mention yaw effects ... bicycle aerodynamic improvements seem to be very sensitive to these effects (sometimes in cross winds they'll use solid wheel discs, sometimes not ... it seems to be more a personal chioce of the cyclist, than a demonstratable advantage. most likely there are many conflicting issues ... a slight redcution in drag countered by more work to spin the disc offset by more inertia in the disc (which can be good on a straight course and bad on a twisty, turny one).

 

[klaas2]

yes for a car i can see the use but on wheels i just wonder wich formula is just to calc this? i only can see more problems then improvements so really like to solve this with calc

 

[Onemorechance]

Yes a turbulent boundary layer creates more friction-drag than a laminar one. But the drag of the wheel (same for a wing) is not only due to the friction in the boundary layer, but also of PRESSURE-drag caused by the pressure distribution in the flow-field outside the boundary layer. A turbulent boundary layer can stay attached to the wheel for a greater distance what makes the pressure distribution in the flow-field outside the boundary layer more like a potential flow-field. The pressure drop (pressure-drag) behind the wheel is less so the result of a higher friction-drag but a lower pressure-drag will still lead to a lower OVERALL drag.
I don't have any formula to back it up, maybe somebody else has more time to look something up for you?

Somewhere cd-values must be available for wheels with or without dimples.

 

[Onemorechance]

Well, I forgot to tell, I do have a formula (but incomplete because you still need a cd-value).
Once you have a cd-value for the wheel with dimple that you want to study, you can find the total drag with the following formula:

Drag = cd * S * .5 * ? * V^2

S = wheel reference area
? = air density
V = airspeed
But I suppose you knew this already.

 

[klaas2]

yes i kow this formula
the problem that is till have that the dimples move or up or down depends where you look on a wheel
i my mind it only create's more drag force
so that why i look at the formula not only as cd figures that meaby a compagny published to promoto there dimples wheel

 

[Onemorechance]

Look at this: maybe you'll learn what you need:
Some more on dimple aerodynamics

 

[klaas2]

well in my mind thats marketing and not science's
to promoto their own product