Downforce estimate

greggy_thommo · Post 23 Nov 2005, 08:16

I was wondering if this shape would have any downforce, and if so, how much (i.e. a lift coefficient): an arc of a circle (i.e. a bowl right side up), 2300mm wide and 500mm deep, - I know that if it was enclosed at the top it would, but I am not sure what happens if it is open at the top.

Anyone have any ideas? It's for an assignment...

RH1300S · Post 23 Nov 2005, 14:36

People will need more information to give a considered answer.

If you look at the object three dimensionally, is it actually bowl shaped (i.e. a compound curve - the curve is visible from whichever direction you look at it). Or is it a curve formed by a flat plane bent around the curve in a single plane?

Have you looked up Bernoulli's theorem and how it relates to ground effect cars? I hazard a guess that you are thinking about the shape of the underside of the object in relation to the ground.

Rather than a curve - think about ground effect cars having three parts to the underbody - "throat" "nozzle" "diffuser". The throat section feeds the air into the nozzle section, which being narrower produces the low pressure by accelerating the air, the diffuser scavenges the air which (I think) helps prevent the air in the nozzle from stalling. You get all of these features with a simple curve, but the work better when each part is designed to do it's job. Preventing air from entering from the sides is important with a true "ground effect" car, which is why in the early 1980's they had side skirts. I placed "ground effect" in commas because you could agrue that all cars operate in ground effect (it's just that they don't all exploit the possibility of getting downforce).

greggy_thommo · Post 23 Nov 2005, 23:48

Yeah, I would post a picture of it but I don't know how to 'host' it on a website.

The best way I can describe it is like an f1 wing, but raked back so much that in cross section it looks like a bowl, or semi-circle.

manchild · Post 24 Nov 2005, 01:56

greggy_thommo wrote:Yeah, I would post a picture of it but I don't know how to 'host' it on a website.

Just upload it on some free web space like yahoo geocites. If you already have some yahoo e-mail than use same user and pass to access yahoo geocities and select easy upload, browse for picture and upload it.

http://geocities.yahoo.com/

greggy_thommo · Post 24 Nov 2005, 03:17

The image should be rotated right (clockwise) and then the airflow would be horizontal across the page.

greggy_thommo · Post 24 Nov 2005, 03:20

You should just copy and paste that URL.

Don't worry about all that superfluous "glass tube" and "absorber" etc. - you can see what I mean with the "open bowl" analogy. That part is all I'm interested in.

Thanks for your help!

Monstrobolaxa · Post 25 Nov 2005, 23:41

I can teach give you an idea of how to calculate it mathematicly....I don't fell like writting everything down....it's around 1,5 pages (A4).

To calculate the CL of the half bowl, you'l have to consider 3 things....the equation of a constante laminar flow, the equation of a doublet and the equation of a vortex!

Knowing that these equations respect Lagrange's theory, the sum of all of them also respects it...so it's a valid equation.....

Don't forget write the equations down in the integral form.....actually you only write the vortex equation in the integral form!

So you'll end up with a equation for the upper part of the sphere from the leading edge to the trailing edge....and a equation for the lower half....the lower half!

Now this is where I'm not 100% sure....if you consider the equation of the lower half as 0....you'll end up with only a equation of the top half....so it's a bowl turned upside down!

It isn't that hard....and I suspect it's what your Aero teacher really wants!

Just look it up in any good Aero book....the equations of the 3 elementary flows used in this exercice...."Introduction to Aerodynamics" - J. Andersson, is what my Aero teacher uses!