Drag Force on Hollow Rectangular Shape

[AEng123]

Hello,

Does anyone know the drag force calculation and drag coefficient of a hollow rectangular shape?

Thanks

 

[rb1957]

I wouldn't've described your sketch as a "hollow rectangle", as it is closed with a web.

The question (as I understand it) is does it matter if the web is upstream or downstream face ?

I'd expect a very minor difference. It seems to me that the dominate effect is the web; having the sides ahead or behind the web would be a 2nd order effect.

another day in paradise, or is paradise one day closer ?

 

[berkshire]

I would expect that shape to very closely follow the drag curves for a flat plate.
B.E.

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[GregLocock]

Incidentally, there doesn't seem to be an analytical (as opposed to simulation based) approach for calculating drag coefficients, if that's what the OP is after.

https://www.grc.nasa.gov/

http://www/k-12/airplane/shaped.html

for the basics, I'd guess that 1.28 was a good place to start, plus a bit for the drag on the sides, minus a bit because it's a bit like a prism (but only a bit). Most of the drag will be pressure recovery so the 1.28 is appropriate.





Cheers

Greg Locock


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[verymadmac]

Hoerner's "fluid dynamic drag" would be the first place one would look.

Without checking on hoerner, I would expect the drag be slightly lower than a flat plate flat. With the hollow on the aft side the drag will be lower than with it forward facing.

 

[IRstuff]

Drag is also dependent on velocity, low Reynolds' numbers result in higher drag, and may change the desired orientation of the object.

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[WKTaylor]

Be carful comparing this square 'open-box' to a flat plate.

Speculation...

As a practical matter I would expect some odd/unstable 'spill-over' and vorticity formations/shedding off the up-stream-facing 'box-edges'... which would make for hard-to-predict rumbling and vibration modes... and a tendency to have a pitch-over moment.

IF the 'box-edges' were facing opposite [down-stream] the cavity-edges might generate vorticity roll-up into the low pressure aft-facing cavity increasing relative Cd. This might be a more stable shape.

Of course, anything on a rigid mast mounting and of reasonably small size might survive in open-air flow... at relatively low angles of attack [from perpendicular flow].



Regards, Wil Taylor

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[GregLocock]

I ran the 2d code microcfd

and

The second created more drag

Please note, this is exactly the second time I've run this software. So the answers may be rubbish



Cheers

Greg Locock


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[AEng123]

Thank you everyone for your replies!

 

[Coughar]

Although the results of the two simulations seem logical to us, I thought the drag coefficient a bit higher than 1.6179 related to the first condition-value of 1.5409., So we have only a drag increase of (+) more 5%.