Aerodynamic drag spinning disc

[Designmechman]

Hi guys, im looking for a simple formula to calculate the aerodynmaic drag on a spinning disc. The disc is basically the rotor for a mechanical centrifuge where the test tubes are hanging downwards and experience drag at 3000RPM.

Any help would be very nice thanks

 

[40818]

You need to be more specific with your post, which way is the disc inclined to the flow etc.

 

[IRstuff]

I think he means drag from the rotation itself

TTFN

FAQ731-376

 

[Designmechman]

Think of it like a centrifuge. It has a central shaft vertically downwards connected to some gears. The disc is spinning at 3000RPM, with test tubes, say four of them hanging downwards. Now when these tubes spin fast, due to centrifugal force they stretch outwards almost 90 degrees. I already know how much energy my device is giving which is 840J. Through gearing 560J are left which is to overcome drag and rotor inertia.

is this clear??

 

[Compositepro]

At 3000 rpm unshrouded tube holders and hinges would act like a blower rotor and experience serious drag. All the high speed centrifuges I see have an aerodynamic shroud that spins with the tubes. Inertia is only a factor while accelerating the rotor up to speed. You would need some pretty sophisticated software calculate drag with reasonable accuracy. And in the end you still have to validate with testing.

 

[GregLocock]

Incidentally you should be working out a power (W), not energy (J)

Worst case - assume area is the length times the diameter of each tube, * no of tubes. Assume Cd=1

assume v= tip speed of tube

power=v^3*1/2*Cd*A*rho

If that is absurdly high (it should be) then find an equation for a fan.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[btrueblood]

see thread378-196286

 

[Designmechman]

Sorry didnt get what aerodynamic shorouds are??

Secondly I understand what your saying about the sofisticated software to study drag. Could I use Solidworks CosmosFlow works and tr maybe a CFD analysis??

To GregLocock, thanks for that formula but how can i get power when i know the torque and energy??

 

[btrueblood]

Follow the link posted above, and click on the link to the NASA paper there. The paper gives values for the moment coefficients for rotors (discs), cylinders (shafts) and propellors; not theoretical values, but experimental ones.

Forget about energy. The moment, or torque if you'd rather, to keep the rotor spinning at a given speed (against the resistance of air) is in units of N-m. You'll find equations to get a good estimate of the aero torque, for a shrouded rotor, from the link posted. A shroud is a smooth cover (think fairing, or cowling) surrounding your centrifuge wheel, to make it look more aerodynamic, more like a wheel and less like a fan.

In calculating the torque, you had to know the speed of your wheel. The rotational speed is in revolutions per minute, which you then convert to units of radians/sec, or 1/sec, by multiplying rpm by (2*pi/60).

Multiply torque by rotational speed, and the units are
N-m/sec, which is watts, i.e. Power = Torque x Rot. Speed