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Originally Posted by Cd
I know we have discussed this in the past and I searched this forum using the keywords " skin friction " as well as " car wax " , but didnt get anything specific.
I have read conflicting articles regarding the importance of skin friction on an automobile.
Some experts claim that the boundary layer is actually pretty thick on an automobile, and therefore things such as hood pins have little to no effect on drag.
On the other hand, I have read about HPV bicycles where the surface is polished to a smooth finish. The article stressed that this was important in the reduction of drag, and this was from a scientific source - not just someone speculating.
( I'd have to search for the article )
So this is this due to the fact that the racer has such small dimensions right ?
With cars such as the AeroCivic, there are several dents and protrusions ( caulking ) on the car.
I'm confused because the car has its gaps filled in with caulking to reduce drag , but supposedly the boundary layer is thick enough that the surface irregularities go unnoticed by the airfow.
( Basjoos, the video of you talking about the sand on the car not blowing away due to the thick boundary layer comes to mind )
Enlighten me please folks . I'm clueless.
BTW, I have read conflicting statements about body gaps too . Both from aerodynamic experts .
Some say body gaps can contribute quite a bit of drag. Others say they have no effect on the Cd.
My guess is this is due to the tightening of the gaps on modern cars versus older cars.
I had a 280Z that had body gaps large enough to stick your index finger into. Not on ly this, but the fit was off as well and the body panels did not create a smooth body at all.
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Frank has pretty much nailed the answers.I'll throw in some minutia from old SAE papers.
*air coming at an automobile is already turbulent and with a perfectly polished paint job,at about 20 mph the boundary layer will transition to full turbulence.
*for the surface roughness of a polished smooth painted surface,this roughness is significant enough such that no further smoothing can produce a drag reduction.
*This skin friction is going to be around 7-9% of the vehicle's overall drag.
*The HPVs,as with solar racers and mpg champions are all considered to be forms of zero separation,hence,zero pressure drag.
*Their attempts at smoothing are an effort to delay the transition to turbulent boundary layer,which if it worked,would reduce the skin friction.
The big boys say the air is already turbulent due to viscous shearing forces of the air as you move from zero velocity at the road surface,to 'ground speed' at some distance above.This 'onset' or 'source' flow would render a 'laminar' form 'turbulent'.
A single racer,on a completely calm day,with no other 'traffic' around might see some laminar boundary layer for some distance down the body to the point of maximum thickness.This would be the 1st minimum pressure point and the boundary layer would be forced to transition over.
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As to gaps and protrusions I agree with Frank's appraisal,that if the gap is stagnant,the air will skip over.If the gap is aspirated,then we could see some drag there.When GM researchers were in the wind tunnel with their PNGV Precept models,the 'cut lines' for body openings added much lower drag than had been budgeted for.
*the caulk could actually behave as a trip-strip which unlike a large eddy breakup device behaves like a continuous turbulator,feeding energy into the boundary layer.It would depend on body location,but on AeroCivic,in the forebody,all these seams are in a favorable pressure regime and the source flow will tend to hold the boundary layer against the body.
You'd want to be more careful at the rear,but if the contours are gradual,you're still in a favorable pressure gradient and again,the ecrescences can act to strengthen the boundary layer.