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Old 06-28-2014, 02:30 PM   #1 (permalink)
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Aerodynamic Streamlining Mini-Course

*With sharp leading edges separation is immediate,leading to high drag.

*With an 'optimum' nose,separation is eliminated and kinetic energy is conserved for downstream conversion to pressure rise

*With subtle softening of the leading edges,the same flow attachment is achieved as with the 'optimum' nose.

*With minimum softening,flow attachment is achieved as long as the roof,sides,and underfloor respect certain contours.



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*The challenge for streamlining,is to reduce,or eliminate the low base pressure turbulent wake,who's eddies and turbulence cannot be converted to a pressure increase.

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*Observation of the wake region itself suggests a flow contour which would support attached flow.

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*With 'boat-tailing' we achieve the elongation necessary to reduce,or eliminate the separated wake.









*This is a 2D flow image,however you can see how the flow comes apart when the aft-body contour is too 'fast.'

*The action is at the rear.This is prime real estate for drag reduction.
*Any automobile manufactured after the mid-1980s is a good candidate for the elongation and aft-body mods necessary for significant drag reduction.
*Here is the 'golf ball' effect of the turbulent boundary layer's ability to 'hold' the air against the aft-body,even though it would like to flow backwards,upstream,towards the low pressure region preceding the position of max camber as shown with the laminar boundary layer.The contour,between the 12:00 o'clock position and 4-seconds after,or 6:00 o'clock and 4-seconds before,is essentially the aerodynamic streamlining template.

Here is the same thing happening underwater with a bowling ball.US NAVY researchers have glued sand to the forward stagnation area to induce an immediate transition to TBL,and as you can see,the separation point is moved back 4-seconds of clock sweep.

*Any analog watch or clock face could be used to pattern a successful 'template.' Compare this to the 2014 VW XL1.
*Here is Mair's 1969 'Template' for a 'best' boat tail.He never goes beyond 22-degrees,and he takes awhile to get there.If you remove the constant-velocity portion of Mair's 'Gavre artillery projectile',simply combining the nose and tail,you get a pretty fine 'Template.' It's already in mirror-image ground reflection.Just cut away for ground clearance and add wheels.
*Estimating Mair's 'Template' as a simple car model,we'd have Cd 0.448 with no boat-tailing,Cd 0.284 at Mair's recommended truncation (the hashed vertical line),and Cd 0.166 with full tail.

Here is a drag breakdown provided by HOT ROD Magazine in 1962.The 'Form' drag component is what streamlining is about

Here is a aero power relationship table from Walter Korff in 1963,denoting the cube function of aero power requirements as a function of velocity,for a car of 25-sq-ft frontal area

Here is a HOT ROD table depicting the relationship of Cd,velocity,as it pertains to aero hp load.

Here is R.G.S.White's drag table for a vehicle of 22-sq-ft frontal area

Here is a comparative chart illustrating the low-drag nature of the streamlined body of revolution.All 4 bodies have identical drag.When messengers commented in the 1970s,that we could have large vehicles with less drag than some tiny econoboxes,this is what they were referring to.

In this table,I've taken the 'golf ball' from above,and added aft-bodies of increasing radius to the forebody.In every case,flow always separates at 4-seconds seconds after 12:00 on top and 4-seconds before 6:00 on the bottom.At some radius,the separation point is lost at the longitudinal bifurcation line,leaving only the drag do to the boundary layer torus which cannot be recovered.This is the reason that streamline bodies cannot have a lower Cd than 0.04.It's all skin friction.

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Last edited by aerohead; 07-19-2014 at 03:47 PM.. Reason: add additional data
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Old 06-28-2014, 09:44 PM   #2 (permalink)
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I could use some tips and info on the finer aspects of aeromodding , Its easy to miss little details that matter.

I am curious of the cars side aerodynamics at the moment , I am assuming they should get the same attention as the roof , by adding a camber point like it does for the roof in the boat tail template ?
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Old 06-29-2014, 06:51 AM   #3 (permalink)
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The last image in Phil's post deals with side taper - and yes I think the sides are similar to the top. But since they are adjacent to the bottom - and the ground, they are a bit different than the top. If they slope "too fast" (to use Phil's term) then they will pull air down from the top more than up from the bottom and change the air flow over the top, and possibly cause swirling at the back.

If the sides are not tapered enough, then the air lifts up to the top, causing a swirl in the other direction. (On the image of Phil's boattailed truck, you can see this slightly.)

I think the ideal is to have as even air pressure as possible all around the rear of the vehicle.
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Old 06-29-2014, 10:20 AM   #4 (permalink)
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Aerodynamic Streamlining Mini-Course

You will be graded at the pump.
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Old 06-29-2014, 12:42 PM   #5 (permalink)
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My thought is , the front wheel flares are the camber point , if I trim the rear fenders it would provide a smoother transition from front to rear.


The Beetle has these pronounced side skirts that could be exaggerated on , to give the car more side air stability , I think ?

I could use these $175 aftermarket Xenon side skirts help keep the side air cleaner ... its more of a guess at this point.



Opinions on if the aftermarket side skirts are a good addition or a waste of material / mpg ? for a vehicle modded with mpg in mind.
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Old 06-30-2014, 01:47 PM   #6 (permalink)
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sides

Quote:
Originally Posted by ecomodded View Post
I could use some tips and info on the finer aspects of aeromodding , Its easy to miss little details that matter.

I am curious of the cars side aerodynamics at the moment , I am assuming they should get the same attention as the roof , by adding a camber point like it does for the roof in the boat tail template ?
Here is a plan-view of the 'template.'

It's based upon a symmetrical wing/strut section of minimum drag (approx.4:1 length/thickness).

Here is the 1957 MG EX 181 land speed record car,with Cd 0.12.It has a similar plan-view profile for the rear.

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Since the side flow is closer to road speed,it does not have as much kinetic energy to impart into the turbulent boundary layer,it cannot 'carry' as fast a curvature as over the roof.Many low-drag cars have used this more relaxed profile and I believe that,short of having a wind tunnel to experiment with,that this profile is a sure-thing.
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Old 06-30-2014, 05:35 PM   #7 (permalink)
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Quote:
Opinions on if the aftermarket side skirts are a good addition or a waste of material / mpg ? for a vehicle modded with mpg in mind.
Two schools of thought:
Keep the air topside at all costs, which the body kit tries to do, vs the 'banana car' which is low front to back, but arched to allow crosswinds underneath.
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Old 07-01-2014, 12:55 AM   #8 (permalink)
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Click image for larger version

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Here it is in stock form

They are actually pretty beefy.
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Old 07-01-2014, 02:44 AM   #9 (permalink)
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Where the stock runningboard leaves 2-3" of fender exposed, the body kit exends all the way to the fender opening and is still 1-1 1/2" wider there. It establishes a line that is picked up by the airdam and carried across the car.



The other strategy shows in the MG EX 181 aerohead posted. The large-radiused rocker combines with the tight-fitted spats.

Which is best? :shrug: The MG is likely better in a cross-wind but the design isn't really compatible with doors. Consider a rocker that leans out 30 vs one that leans in 30 (similar to your body kit). Intuitively, the 'fillet' shape would offer less interference drag between the ground and the car's side.
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Old 07-01-2014, 11:29 PM   #10 (permalink)
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I see 24 different complete body kits for the New beetle advertised , which includes front & rear spoiler /bumper covers and side skirts. VW Beetle Body Kits I can purchase pieces of the kits separately.
Although I think I will be staying with home fabricated parts at the moment , I should be able to make what I need.

I ordered a anemometer / wind gauge today from china.
I want to use it to do a small scale wind tunnel for the Beetle 1/32 kammback.

I need to know the wind speed, for scale model smoke testing.

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