Pressure Center by diffuser alone

[Christopher Crowe]

Does anyone happen to know where the fore-aft pressure center falls on a typical F/C chassis from the diffuser alone? That is, if the wings were neutralized, where would the pressure center reside as generated by a legal F/C floor and diffuser alone?

Thanks,

Christopher Crowe

[asw]

The diffuser performance should always be accounted for when rear wings are in place.
The pressure signature under the floor and the diffuser region will change when rear wings are removed and depending on the design of the diffuser, the CoP will move forwards towards the tray inlet and the overall undertray/diffuser down-force would be less.

[Steve Demeter]

Mr. ASW, not to be an arse, but unless the rear wings have a lower element very close to the diffuser outlet, what affect would they have on the diffuser performance?

Educate me please as I am not following your logic.

thanks

[Rick Ross]

ASW is 100% correct. Both the front and rear wings have a huge effect upon the surface pressure distributions on the car bottom surface. A quick review of the published research literature will show this.....you may want to start by looking at the work of Joseph Katz and/or Lee Dykstra (look for their SAE and/or AIAA papers).

The wing/body interactions on a car are complex and non-linear. The total car downforce is not the sum of the downforce produced by the individual components in isolation. Underbody pressures are heavily influenced by both the front and rear wings, the front wing is heavily influenced by the front tires and ground effects, and the rear wing is influenced by the car body, rear tires, and the diffuser.

I personally doubt that a credible answer to Christopher's question even exists. FB/FC manufacturers and teams do not have the resources for the necessary CFD, wind tunnel, or track testing required to determine this. If you are designing, building, and racing a winged car.....why attempt to determing the aerodynamic characteristics of the car without the wings installed?

[Stan Clayton]

The total car downforce is not the sum of the downforce produced by the individual components in isolation. Underbody pressures are heavily influenced by both the front and rear wings, the front wing is heavily influenced by the front tires and ground effects, and the rear wing is influenced by the car body, rear tires, and the diffuser.

Don't try to get too technically accurate, Rick. The total downforce of the car most certainly IS the sum of the components. After all, no external factor is pressing down on the car independently of gravity and the downforce generated by the car's components.

I personally doubt that a credible answer to Christopher's question even exists.

I don't doubt one bit that a credible answer exists. We may not have an answer that'll stand up to a master's thesis defense, but we don't have to. We can still make generally accurate statements about what happens when you add a diffuser to a flat bottom car.

Christopher, if we are pretending the wings are not there and that we are dealing with a flat bottom car, then if there is any positive rake at all (or even if there isn't, so long as there's no significant negative rake), then there is a pulse of downforce at the leading edges of the undertray, followed by decreasing amounts further aft. Depending on the amount of rake and the height of the undertray above the ground, there is be a pressure distribution that has a cp that can be roughly estimated (typically a few inches behind the cg).

Add a diffuser that starts at the leading edge of the rear tires and a second, somewhat weaker pulse of downforce occurs just aft of the beginning of the diffuser. In the wind tunnel and in CFD this can be plotted as a pressure distribution curve with a sharp dip at the leading edge of the undertray, and a shallower dip just aft of the start of the diffuser.

It should be obvious, therefore, that adding a diffuser impacts the cp location, since it alters both the magnitude and the distribution of the downforce. In short, adding a diffuser will shift the cp aft, all other things being equal. In practice, it generally means you can take some rake out of the car and still generate the same or greater downforce as before, but with lower drag.

Hope that helps!

Stan

[Rick Ross]

Stan,

I stated that the total car downforce is not the sum of the downforce produced by the individual components in isolation. I stand by that statement, and I do not feel that it is "too accurate" for this discussion. What I meant was that the total car downforce will vary as you alter the relative position of the various components (wings, etc.). For example, if you test the rear wing all by itself in the wind tunnel, and then test it again while mounted on the car, the downforce produced by the wing will not be the same for the two cases. The same of course is true for all the other bits and pieces.

Christopher asked if anyone knew where the cp would be located on an FC car as "generated by a legal FC floor and diffuser alone". I just don't see why anyone would have bothered to determine this information, but perhaps I am mistaken. Who knows, perhaps someone out there doing a wind tunnel test may have made a few runs without the wings for some reason.

As for whether or not a credible answer exists to Christopher's question.....I suspect that may depend upon your definition of "credible".

[George Main]

Does anyone happen to know where the fore-aft pressure center falls on a typical F/C chassis from the diffuser alone? That is, if the wings were neutralized, where would the pressure center reside as generated by a legal F/C floor and diffuser alone?

Thanks,

Christopher Crowe

Chris, your thread title and post don't match. The title states "Pressure center by diffuser alone" and your post states "with wings neutralized". Those are two very different statements and will vary the results of your question.

And by "neutralized" wings do you mean balanced front and rear or lowest setting for DF?

[Rick Ross]

George makes a very good point. I had interpreted the question to be seeking data for a "no wings" scenario. But perhaps Chris is looking for data in which the wings are adjusted to a minimal downforce position? If so, I can imagine that there may be someone out there who has an idea of where the aero center is on their car in this particular configuration.

[Wren]

FB/FC manufacturers and teams do not have the resources for the necessary CFD, wind tunnel, or track testing required to determine this.

This.


F1 teams struggle with this. Whoever thinks they have it right, doesn't. Solidworks is just not going to give you this information.

You can get close with a rolling road wind tunnel, but that is for one specific case. Changing rake or anything else is going to screw that up completely.

We wouldn't pretend to know where the center of pressure is on Brandon's car. It hasn't really slowed him down.