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. |
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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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. |
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 :D 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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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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HEY !!! I found out how to make the smiley faces !!! Now .. what about the other faces ? Perhaps I should create a crude picture and upload it later as an example. I get started on that now. EDIT : SCRATCH THAT .... I'm going outside now. I'll work on this later . :) EDIT #2 HEY !!! I found out how to make the smiley faces !!! Now .. what about the other faces ? |
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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? |
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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I've always wondered if drag factors for various vehicles were constants. To me it seems possible that as speeds get higher, certain laminar flows that don't interfere with each other at say 60 mph could possibly create havoc with one another at say 80 mph.
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Would it be? I thought the purpose of a kamback or boat tail was to keep the flow attached as the projected area was reduced (approx a 5:1 or 7% slope), the reduction of the low pressure system statisticly diminishing when the reduction reached 50%, hence the abrupt chop off in a true kamback. :confused: |
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.... gives me a whole new perspective on wheel skirts :-) |
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From the little reading I did on aerodynamics (wikipedia mostly), I managed to learn that the formulas and calculations are slightly different for subsonic vs. supersonic speeds. Probably different for automotive speeds vs. small prop plane speeds. If you're going to talk 60 mph vs. 80 mph, I don't know. I don't think the order of magnitude change is big enough to make much difference in the basic calculations and formulas used. But the simple fact of increasing from 60-80 mph alone is enough to increase the total drag considerably. |
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