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Old 07-11-2012, 10:37 PM   #1 (permalink)
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Question Does skin friction scale up on small cars ?

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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Old 07-11-2012, 10:48 PM   #2 (permalink)
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Re: difference in boundary between car and HPVs: I don't think it's the size of the machine so much; it's in how small an aero improvement the two camps decide is worth going after. HPVs have only so many watts for their power source so if they can save a watt, they will. Cars have so many excess watts for motive power that saving a watt there will never show up in the gas logs or on a scangauge.

Perhaps an even bigger factor is the basic shapes involved. Many HPVs are designed with forms that aim for laminar flow, if not all the way back, at least 2/3s of the way back. Plus, HPVs are rarely 4-wheelers which helps aero. Car shapes are so far away from achieving laminar that "true laminar" flow isn't even an issue, cuz it ain't gonna happen. Cars do have thicker boundary layers and those boundary layers thicken as they go back too. As such I don't think surface finish is as critical.

Re: body gaps: I think each gap is a YMMV thing- is it parallel or perpendicular to the wind? Is the leading edge from the "back" of the gap standing proud of the trailing edge or is it flush or is it set in a bit? Is the cavity behind the gap such that flow isn't really happening to any degree... or is there a lot of flow through the gap? I'd surmise if there's flow then that could be a draggy gap.
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Old 07-12-2012, 02:17 AM   #3 (permalink)
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Good answer Frank! I'd add to it, but dam....ya hit all the points.

I think the only thing to mention further is that due to the thick boundary layer, whether the car is waxed or not, it will not reduce "Skin Friction" because the the air is all stirred up right on the surface of the car any way.
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Old 07-12-2012, 01:23 PM   #4 (permalink)
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So cars don't get laminar flow because of the forms themselves, or because of some other difference from HPV's?

I'm also still wondering how thick the boundary layer would be. Is it 7mm or 70mm?
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Old 07-12-2012, 02:02 PM   #5 (permalink)
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I think we can learn a lot from the progress made in electric vehicle aerodynamics, its not all for not.
I bet if Basjoos civic had a smooth irregularity free body it would improve the mpg, i must admit though it does superb with all the silicone and dents.
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Old 07-12-2012, 02:28 PM   #6 (permalink)
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I am going to test the boundary layer out today.
My thought is to stick my hand out the window and put it on the door panel and upper right fender near the hood and test for wind resistance, the boundary layer may be moving as well.
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Old 07-12-2012, 10:45 PM   #7 (permalink)
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form drag VASTLY overshadows skin drag.

so, benefits are MUCH more likely to be seen with changes in shape and size than skin friction.

THink golf balls.The skin friction is used to reduce form drag, resulting in much better aerodynamics.
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Old 07-13-2012, 06:31 PM   #8 (permalink)
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skin/gaps

Quote:
Originally Posted by Cd View Post
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.
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.
------------------------------------------------------------------------
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.
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Old 07-13-2012, 06:37 PM   #9 (permalink)
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because

Quote:
Originally Posted by Sven7 View Post
So cars don't get laminar flow because of the forms themselves, or because of some other difference from HPV's?

I'm also still wondering how thick the boundary layer would be. Is it 7mm or 70mm?
The air itself is turbulent so there's virtually zero chance for a laminar profile to work.This is something Abbott and Von Doenhoff emphasize in their airfoil work.
Can't tell you anything about the boundary layer thickness.
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Old 07-14-2012, 03:58 AM   #10 (permalink)
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With my hand against my car door while driving, i noticed no airflow pushing my hand, but only when my hand was against the door panel. Could be the air is passing over the back of my hand with little disturbance at 50 - 60 kmh, i 'll try higher speeds when i need to use the highway.

Once the form is good having a polished smooth surface would benefit the cd, no ?

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Last edited by ecomodded; 07-14-2012 at 04:04 AM..
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