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Old 04-19-2012, 08:22 AM   #351 (permalink)
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Quote:
Originally Posted by aerohead View Post
I'll try and get a couple of images up which illustrate the impact 'gaps' can have with respect to drag coefficient.
For the stock cars drafting,the cars in solo have Cd 0.315.
For the buses,they have a solo Cd 0.50.
I'm still working on the railroad materials but have been swamped with other duties.








link

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Old 04-19-2012, 04:47 PM   #352 (permalink)
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pressure

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Originally Posted by NeilBlanchard View Post
I'd say that the first one is lower drag, and here's why: the shape of the front accelerates the air out and around the high point on the roof, and this creates a localized high pressure, which then help the air "spring" back and it helps maintain attached flow. So, it adds to the atmospheric pressure and this improves the air flow a bit.

If the rear of the shape is most important, then the front is next most important -- it matters less, but it matters. At the very least the second one has more surface drag.
A Fluid Dynamics professor would say that the forebody takes air at rest,and high static pressure,and accelerates it to its highest velocity pressure and lowest static pressure.Then the aft-body provides a gentle deceleration ramp in which the kinetic energy of the velocity pressure can be converted to the original ambient atmospheric pressure of the original air at rest.
The air always wants to be at a peaceful equilibrium and uniform pressure.
Streamlining allows to car to transit through a given volume of air,gently displacing it,then allowing it to gather itself back together as if you were never there.
Walter Lay's research proved that a perfect boat tail is useless if the nose isn't good enough,so the nose is not unimportant.But no amount of perfect nose can lead to low drag if the tail is wrong.
Reading last night about the EZ-HOOK 295 mph motorcycle streamliner of Cd 0.103,the Cal Tech guys reiterated that a round front is all that's necessary.The Devil's at the back.
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Old 04-19-2012, 05:02 PM   #353 (permalink)
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slide

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Originally Posted by cfg83 View Post
Frank -



I want aerohead to chime in. Years ago he told me that I couldn't slide the template back on my wagon to "fit the template".



I knew someone would point that out. I think that the spoiler is deliberately sticking out to "trip the flow" (not the right term?).

CarloSW2
There must have been a specific context for telling you not to move the template.Don't remember,sorry!
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Old 04-19-2012, 05:21 PM   #354 (permalink)
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L/D

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Originally Posted by euromodder View Post
But its length/diameter ratio is better, which should be lowering drag again.
It's true that the fineness ratio is increased, which is good, but when Hucho and others speak of only being able to lower a cars drag by further lengthening,I interpret that to mean that as the body is extended,the cross-section continues to reduce in the process.
This would be a good experiment,as I myself have remarked at times that for utility,a body might be stretched in constant section to guarantee some interior headroom which is what airliners do and submarines.
The one bus model goes to Cd 0.13 even with all its length and the Cal Tech GALCIT E-Z HOOK streamliner kinda takes Sam Whittingham's VARNA Diablo and stretches it out beyond 13-feet in length,and still pulls off Cd 0.103.
It would be nice to have $2,500/day to test the theory in Cal Tech's tunnel.
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Old 04-19-2012, 05:28 PM   #355 (permalink)
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split

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Originally Posted by Frank Lee View Post
I'm glad we have more clarification on it. Looks to me like you have the top nailed.

Now I'll throw a small wrench in it and say it looks like the template could be split horizontally so that the belly line can be moved forward...
I think it's okay.The slide would allow a quick check of ground clearance for belly,fairings,diffuser.
And your point about shifting things for a best fit without chopping parts away but rather, adding to, is very important,as we want to make streamlining as painless as possible.
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Old 04-19-2012, 05:36 PM   #356 (permalink)
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Thanks for showing the pictures of the trucks and busses. It reminds me of Car and Driver's response to the challenge for magazine publishers to get the best gas mileage with the 1st Gen Insight. They modified an SUV and tailgated the SUV getting 150mpg at 55mph.

The part about trucks reminds me of Luigi Colani's truck designs. No info about cD and only focuses on front aero but it would be better than the bricks that drive down the road today.
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Old 04-19-2012, 06:12 PM   #357 (permalink)
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Cd

Colani hauled 7 of his creations to Bonneville for testing inside one of his rigs.In a photo of the tractor he had the Cd posted on its side.It was under 0.5.I'll look.
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Old 05-09-2012, 09:38 PM   #358 (permalink)
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Old 05-10-2012, 03:00 AM   #359 (permalink)
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I made this in SW, but I have no idea how to test it. It was for a recent competition on the GrabCAD site. I used the template for the most part of the roofline and the side intakes.
(The chassis and rollcage was given and participants had to design the body, so many dimensions can not be changed much.)

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Old 05-10-2012, 07:53 AM   #360 (permalink)
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Quote:
Originally Posted by aerohead View Post
A Fluid Dynamics professor would say that the forebody takes air at rest, and high static pressure, and accelerates it to its highest velocity pressure and lowest static pressure.
I'm having issues placing in my brain what you wrote.

Perhaps because it different than my typical thoughts which surrounds hovercraft lift and thrust systems.

In those systems I tend to think:

High air velocity = Low pressure

Low air velocity = High pressure

Still trying to get my head around the statement;
highest velocity pressure and lowest static pressure

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