[zacko]

Hello there,

I asked myself, because I worked before on Autodesk CFD and there is an option about "Free spinning", whether in CFX is a similar option. For example I want to simulate a water turbine and want to know which rotation speed the turbine gets through the flow, which option can I use?
In Autodesk you can give the inertia and then the rotation speed will be one of your output values.

When I want to simulate a turbine and I give an angular velocity I thought it will work as a pump!

Maybe someone can explain whether I am wrong and whats the best way to simulate such things.

Thank you !

[ghorrocks]

I do not know what Autodesk means when it says free spinning. I would guess it is the zero net torque point. In that case the inertia is irrelevant so that does not seem to match your comments.

[zacko]

When I give an angular velocity[rev/min] it is like a motor which gives a torque to rotate the turbine, but "Free spinning" would be, that the turbine only rotates through the flow and therefore the program needs the inertia which the flow has to find the strength for...

understandable?

[ghorrocks]

Understandable? No, not yet.

A rotating machine has a performance curve, so for any speed it delivers a torque. When the speed is grossly too fast or slow the torque will be negative, but at the design torque a turbine should generate positive torque. There is obviously a point where the positive torque transitions to negative torque as it crosses zero, and at that point the turbine will be free spinning.

Am I on the right track or have I missed the point?

[zacko]

Okay maybe is it possible that you explain me your thoughts about what free spinning is for you ? I thought when the torque is zero the machine would stand still, because there is no rotation...

[ghorrocks]

No, zero net torque means there is no acceleration. If it is already spinning it just keeps spinning at the same velocity. Anything operating at steady state is running at zero net torque.

[zacko]

Ah Okay now i got it.
So zero net torque means the rotation is constant with no acceleration ok good.

Then back to my new problem. How is it possible to find the best angular velocity for the turbine, where it would achieve the most pressure drop... I mean i can't start 50 Simulations with 50 different angular velocity's.

[ghorrocks]

To optimise a parameter it is best to run a few simulation and use numerical techniques to estimate the maximum point, then do a few more simulations and continue optimising until you are close enough to the maximum point. You can normally do this in a lot less than 50 runs, but you will need to do several iterations.

[zacko]

Okay thank you very much for your help

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