CFD Online Discussion Forums

CFD Online Discussion Forums (
-   CFX (
-   -   very confused problem,Please help me (

jiaye gan March 24, 2004 11:39

very confused problem,Please help me
Hi everyone: (1)I got a difficult problem while learning CFX5.5 tutorial12.The problem as follows: For a rotor/stator (such as hydraulic turbine) problem,the rotation velocity is not known first.Why in the example gave a known velocity? (2)For a hydraulic turbine problem,We all know that the rotor is rotated by the water.Then the velocity is not known first.Could the CFX5.5 solve this problem? How do we treat the rotation reference frame without specified a rotated velocity?

All your returns would be appreciated.

Glenn Horrocks March 24, 2004 17:45

Re: very confused problem,Please help me
Hi Jiaye,

It is possible to link the rotational speed to the solution. You would use a CEL expression to get the net torque on the blades and would adjust the rotational speed to get zero net torque. This would be quite tricky to get running reliably so is not a simple thing to do.

If you run a series of fixed speeds with given inlet conditions there will be some with negative torque and some with positive torque. From there you should be abel to interpolate to get the steady state zero net torque speed.

Regards, Glenn

jiaye gan March 25, 2004 07:10

Re: very confused problem,Please help me
Hi Glenn Horrocks :Thanks for your returns indeed. (1)I'm a beginner in learning CFX5.5. I got confused for your returns at that why we need to get the zero net torque. Could it explain that we can specified a rotational speed first in hydraulic problem? (2)Could you tell me how can I use CEL to get the net torque?

Thanks a lot.

BAK_FLOW March 25, 2004 11:25

Re: very confused problem,Please help me

yes it looks like you need to take a step back to the basics. The reason we have to specify the rotational speed first is because we solve the rotor flow field in a coordinate frame which rotates at the rotor rotational speed. This greatly simplifies the problem because (in the limit of being far away from any stationary components) the problem becomes steady in that frame. Since this is a non-inertial frame there must be some pseudo-forces added (in the steady rotating case they are Coriolis and Centripetal forces).

The only way to solve a problem in this frame (to date anyhow) is to guess the rotational rate, set up the equations in that frame, do the calculation to determine the forces or flow rates. From this calulation we can then adjust the rotational rate, do the calucation again, and so on....see.

For some of the fundamentals a good place to start is some of the papers written by Bill Dawes or John Dneton in the 1980's in ASME Journal of Turbomachinery. I don't have the exact reference handy, however. Maybe someone else on here has the detials.

I hope this is a good starting point.



Glenn Horrocks March 25, 2004 17:23

Re: very confused problem,Please help me
Hi Jiaye,

When the impeller has zero net torque it will not accelerate or decelerate but rotate at constant velocity. Hence this will be the steady state speed.

If you are a beginner at CFD then I would have a look at BAK_FLOW's posting as he summaries why it is far easier to use a constant speed (even if guessed).

A question for the CFX gurus out there: If you wanted to run this simulation where it found the steady state speed by converging to zero net torque on the impeller, how could it be done? You could do it in fortran, but can anybody think of a way of doing it in CEL? You would need the rotational speed in the last iteration so you could apply a correction to it and converge to the steady state value. Any ideas how you would get this variable from the last iteration in CEL?

Regards, Glenn

jiaye gan March 26, 2004 12:02

Re: very confused problem,Please help me
Hi Glenn I also have no idea about questions. I hope somebody can help us.

jiaye gan March 26, 2004 12:02

Re: very confused problem,Please help me
Hi BAK_FLOW: Thanks for your information.Now I get some rough idea that how to solve my problem.And I also know that it is a difficult job.

gg March 27, 2004 09:54

Re: very confused problem,Please help me
Hi Glen,

I think this is not right way because turbine power equals to P=T*omega (or torque times rotational speed). So if torque goes to zero then power also goes to zero and hydraulic machines are not able anymore to produce electricity.

Right way is to determine total pressure at the runner inlet or even better at the spiral-casing inlet and define static pressure at the draft tube outlet.

Regards, gg

jiaye gan March 28, 2004 11:06

Re: very confused problem,Please help me

Here I am again.When I got the forces or flow rate form the calculation,how to ajust the the rotational rate according to it.Could you explain it detailly.Should I follow Glenn Horrocks' introduction?

Glenn Horrocks March 29, 2004 00:42

Re: very confused problem,Please help me
Hi gg,

Of course you have to include the shaft torque in summing the torques. Then the shaft power is included. When there is zero net torque on the impeller then the impeller will not change velocity.


Robin May 7, 2004 16:07

Re: very confused problem,Please help me
Actually, in a hydraulic turbine the speed is NOT determined by the flow, but rather by the generator. A three-phase motor will operate at a fixed speed based on the number of poles and the frequency of the grid. What will vary is the flow rate and power output.

Regards, Robin

Mangnan May 8, 2004 03:06

Re: very confused problem,Please help me
Hi Robin:

My model is similar to the hydraulic turbine,and it has no generator to output.So the constant speed cann't be got.Is there another way to get it?It might be get from torque calculation.But to get the zero net torque is not an easy job.It comes back to my problem again.I think the design of hydraulic turbine has been developed for decades.There might be some ways to solve this problem.

Regards, Mangnan

Robin May 11, 2004 11:10

Re: very confused problem,Please help me
Hi Mangnan,

You could add expressions for torque to your solver run and compute the rotation rate accordingly. However, what one would typically do is run a speed line and determine where the operating point falls.

Regards, Robin

All times are GMT -4. The time now is 12:34.