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Join Date: Oct 2006
Location: Princeton, NJ
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# Aero drag on cylinder shape

01/15/2009 5:16 PM

I am trying to estimate airload on a cylindrical door panel using the Drag Equation (D = q * Cd * A). I am designing a drive system to move the door forward and aft (in direction of airflow) and need to account for the force exerted by the airstream.

The door axis is at an angle to the airstream (angle of attack). My references give a Cd of 1.17 for airflow normal to a cylinder but I'm not sure how to account for an angle other than normal. Can I use 1.17 and the frontal area (projected) for the cylinder?

Haven't calculated Re, but airspeed = 300 kt. Thanks for any advice.

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Guru

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#1

### Re: Aero drag on cylinder shape

01/16/2009 4:02 AM

If I understand your sketch the right way you cannot use the data for a cylinder since the door is only part of a cylinder. I would approximate the force with a plane surface placed at same angle with respect to flow direction. The best way to take would be to make a flow simulation with a CFD programme.

In your equation I do not know what "q" is, usually for drag is used an other equation F=Cf*A*v²*ρ/2 in consistent units.

If you consider the plate approximation it would be recommendable to compute as well the force due to the flow direction change on the plate.

Hope it will help.

Guru

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#2

### Re: Aero drag on cylinder shape

01/17/2009 2:20 AM

"The door axis is at an angle to the airstream (angle of attack)".

Whatever the the angle attack, the area under calculations is that area perpendicular to the air stream, and in design we have to take the worst case.

Design Wind Force, F = qz * Gh * Cf * Af , lb, and for nomenclature and more details please see the following CR4 Thread Calculating Wind Resistance which is very interesting in calculating the force due to wind.

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Guru

Join Date: Mar 2007
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#3
In reply to #2

### Re: Aero drag on cylinder shape

01/17/2009 7:28 AM

very good answer as usual! I think that due to the angle also the jet deviation force should be considered. What do you think, since the equation considers only the drag?

Anonymous Poster
#4

### Re: Aero drag on cylinder shape

01/17/2009 11:37 AM

From your wording, I'm guessing that his is not an outdoor system, so the air-stream is not due to wind.

The other posts imply the following but I don't think they are as explicit as they might be:

There will likely be very significant wind force in other directions as well (commonly called "lift") - so take care in your design of restraints as well as on the force for the drive system.

And as indicated by Abdel Halim Galala, as a general case the forces you need to account for need to be sufficient to redirect all the air that would otherwise pass the full area of the door; then you should at least double that to account for local turbulence (this is in addtion to gusting if the source is wind - but in that case you can use the standard formulas) and vibration - and finally add the usual engineering margins. (only if you can model the situation accurately or measure the actual forces should you consider doing anything less than this - and in either case be certain your system is rapid enough to detact (or model) the peaks))

Guru

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#5

### Re: Aero drag on cylinder shape

01/17/2009 1:29 PM

I'm afraid that this is an example of a situation in which CFD analysis is almost essential (depending upon the size and cost of the door and whatever is attached to the door, the safety hazards involved, etc.)

Presumably, the door seals against a surface (also locally cylindrical). The flow, when the door is closed, could be very nearly laminar across the outer surface, and when the door just begins to open there could be considerable lift generated on the door, (which would be dramaticaly affected by the things behind the door.) As the door opens further, eventually, virtually all the flow behind the door will be fully turbulent (assuming that the angle of attack is over about 10 degrees or so) and drag (very roughly) could be estimated as that of a flat plate with equivalent projected area, and lift could be (very, very roughly) estimated from impact on a flat plate.

If you are in the conceptual stage, and wanting to know if a really big cylinder or a really small cylinder would be needed, then using 1 as a coefficient of drag (using the forward projected area) and perhaps . 7 as a coefficient of lift (for when the door is in a position, relative to the surrounding stuff, to best act as a lifting surface, and using the area perpendicular to the flow direction) would get you close enough for a rough budget. But if you are really designing the structure and mechanisms involved, then I think you either need a good 3D CFD program, or wind tunnel tests, or if the nature of the project allows, real world tests, which are by far the best -- just measure the forces at lower speeds, and calculate them at higher speeds, allowing for reynolds number effects.

A much simpler set of calculations is involved in determining the lift and drag of a planing hull across water. Savitsky did a good job of putting together the calculations required to get pretty close without having to use a towing tank. The paper describing the calculations is 24 pages long, and is based on a number of simplifying assumptions.

One of my competitors, the Aptera people, claimed a (CFD-determined) Cd of .06 for their three-wheel car. From 3000 miles away, I said, in this forum, that .10 would be a reasonable lower limit. Many months later, they claimed a revised CD of .11 (although now they are claiming .15... but they have changed to front wheel drive with exposed axles shafts, which has a large negative effect on drag). After seeing that I could do a better job, by eye, from 3000 miles away, than they could using CFD, I declared myself a human CFD machine. But I'm afraid I can't get you very close on this one, and that real CFD, with a skilled operator, would be the way to go.

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Anonymous Poster
#6

### Re: Aero drag on cylinder shape

01/18/2009 5:26 AM

More important than the drag on the door is the turbulence at the opening and the effects of that turbulence on the area downstream. Is the door to be located upstream? or downstream from the opening? The size of the cavity and the size of the opening will both have significant impact on the aerodynamics of the door at various points along its intended path. Boundary layer thickness and turbulence upstream of the door location will have to be taken into consideration for any reasonably accurate calculation of forces. Wind tunnel or CFD time would be a wise investment............................A curious pile of atoms, animated and self aware, looks upon itself and wonders why it wonders.

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