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Old 09-08-2007, 10:15 AM   #1
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Wake size and overall drag

Let's say you have two different cars - a 1990 Civic CRX and a 1990 Civic hatchback.
Both have the same front end, same dimesions etc., yet the CRX would have a smaller drag wake behind the car because its back glass slopes downward.

If you could clean up the underside of the hatchback and bring the .Cd figure down to that of the CRX, both would have the same .Cd figure, but the CRX would still have a smaller wake.
If both cars had exactly the same frontal area, the same weight, horsepower, and the exact same .Cd figure, which car would be have a lower drag and higher top speed ?

Think of the Mercedes Bionic car - it is really slick, but still has a large wake behind it.
If you could get a sedan that has a smaller wake behind it's trunk than the Bionic car has behind its hatch, at what point would the drag from the size of the cars wake overide the Bionic cars lower .Cd figure ?
For comparison, lets say both cars would have the same frontal area.
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Old 09-08-2007, 10:21 AM   #2
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um... correct me if i am wrong, but doesnt the wake contribute to the .cd?? i mean the definition is coefficient of drag right? so if the wake contributes to the drag then then it contributes to the .cd...

and as per your user name: hahahahahahaha. you are correct.
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Old 09-08-2007, 10:52 AM   #3
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Originally Posted by Nerds laugh at me View Post
If both cars had exactly the same frontal area, the same weight, horsepower, and the exact same .Cd figure, which car would be have a lower drag and higher top speed
If they have the same Cd x A, then by definition they have to overcome the same amount of drag & so would have the same top speed.
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Old 09-08-2007, 11:05 AM   #4
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and as per your user name: hahahahahahaha. you are correct.

Nerds laugh at me - meaning that I do things that are so strange that even a 'nerd' picks fun at how strange that I am.
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Old 09-08-2007, 11:09 AM   #5
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As metro said.... given everything equal - they will have equal top speeds.

Aero resistance of an object (that is, assume environmental variables such as fluid density constant) is a function of frontal area and coefficient of drag. Minimizing cD is great - but minimizing cDA wins

I'm curious, what makes you think that one would have a smaller wake than another? Perhaps one vehicle has one large perturbation -- but the other has several perturbations of smaller magnitude. Boats for example. Depending on hull design, you can have one large wake behind the boat, or several smaller wakes (originating from different locations of the hull).
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Old 09-08-2007, 11:10 AM   #6
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If they have the same Cd x A, then by definition they have to overcome the same amount of drag & so would have the same top speed.

I'm having a hard time understanding that.

EDIT : So where are the smiley face icons on this forum ?

I notice several of you have thought I had said something with a certain 'rude' tone to my post , but this was not the case at all.

I was looking at the above statement that I made and was thinking of how you guys might thibnk I had meant it.

The way it shopuld be read is "I'm having a hard time understanding that " ... with a puzzled tone to my post.
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Old 09-08-2007, 11:11 AM   #7
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As metro said.... given everything equal - they will have equal top speeds.

Aero resistance of an object (that is, assume environmental variables such as fluid density constant) is a function of frontal area and coefficient of drag. Minimizing cD is great - but minimizing cDA wins

I'm curious, what makes you think that one would have a smaller wake than another? Perhaps one vehicle has one large perturbation -- but the other has several perturbations of smaller magnitude. Boats for example. Depending on hull design, you can have one large wake behind the boat, or several smaller wakes (originating from different locations of the hull).
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Old 09-08-2007, 02:52 PM   #8
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I'm having a hard time understanding that...
Aerodynamic resistance is expressed in two parts, one part is the Cd, or Coefficient of drag, the other is A or area, as viewed from the front. The greater the CD or the A, the harder it is to push a vehicle through the air.

So two vehicles with the same CdA (the drag coefficient * the frontal area) should take the same amount of force to overcome the aerodynamic resistance at given speed.

Does that help?
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Old 09-09-2007, 08:43 PM   #9
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Cd is just a statistical measure, an after the fact way of describing the aerodynamic resistance of dis-similar shapes. Combining Cd with the area gives you a quantity that is comparable to other Cd*A results.

How a particular shape behaves to yield a particular Cd is infitely variable. Terms like wake are used to describe some of the more macroscopic aspects of the factors that contribute to aerodynamic resistance, but all these factors combine to yield a particular Cd result.
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Old 09-10-2007, 04:32 AM   #10
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the other is A or area, as viewed from the front.
Or as viewed from the rear, since it's projected area.
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