drag estimation with only geometric parameters

[Taffqm]

I need to estimate the drag on a bobsleigh cowling, however the only information avaliable to me is the geometric sizes, i.e. its height and its length (which are variable).

The formula or method used will be most helpful and should only include the parameters which are functions of height and length. A 2-D model will suffice, however a 3-d model which would then include a varying width would be fantastic.
I must state that a computational model is not required, only a theoretical application.

Thank you, even a little help will be appriciated.

Taff

 

[i278]

Well, it looks like you're only going to get my opinion, take it for what it's worth:

I don't like being a party-pooper, but I don't think you'll find even an empirical relationship for what you're looking for. Even a simple ballpark drag estimate requires a much more defined system than length, height, and width. To take an extreme example: compare a rectangular block with an ellipsoid of equal length, width, and height.

Typically, estimating total parasitic drag coefficient of an arbitrary form is done by performing a 'drag build-up', which goes something like this:

-figure out the total 'wetted' surface area of the object. This is all the surface area that has air flowing over it. Then, assuming a turbulent boundary layer, you multiply your wetted area by a flat plate drag coefficient (~.005). This gives skin friction drag.
-to the skin friction value you add individual drag increments for every feature that sticks out, sinks in, or otherwise interferes with smooth flow. These are all approximate values taken from various sources of aerodynamic drag information. (Fluid-Dynamic Drag by S. Hoerner is the classic text.)
-you then add any additional drag increments caused by interference effects of features that stick out from the otherwise smooth form. (lifted from the same sources)

Note that there's very little direct dependance on the overall dimensions. However, having said that, you could try to ballpark some sort of relationship by estimating total drag with total skin friction, a simple drag increment for the forebody (nose), and another simple increment for the bluff after body. Of course, that'll most likely just tell you that drag is lowest with the least wetted surface area, which may or may not be all the story that needs to be told. I don't know.

Regards

 

[GregLocock]

That's a great approach i278, roughly that's how we do it for solar cars.

From my experience with them a laminar drag coefficient of 0.007 is a better bet for this surface type/size/speed regime.

You also need to add the drag due to lift/downforce. I'd use an L/D ratio of 1 to start with, perhaps 2 if you can get some data. I guess you try and get neutral to downforce on a bobsleigh. We always trimmed solar cars to zero lift for maximum speed, I expect bobsleighs do the same.

You also need to decide where you transition to turbulent, and where you are going to get separation. This is a detail that is not even slightly containable within your desire for an estimate based on LWH, of course.

/Cynical mode on/
Given the shape of a bob sleigh you might be better off just saying Cd=0.8 on a frontal area basis and have done with it!

Cheers

Greg Locock

 

[matt00000]

Didn't the DERA do some wind tunnel tests on a bobsleigh for the british team?
Email them and ask them what they know!!