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Old   July 30, 2003, 12:55
Default Angular momentum
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I have done some steady viscous calculations of a turbine stage with a cell centered multigrid code using Jameson scheme and Baldwin Lomax turbulence model. After doing these calculations, I have found the power produced by the rotor with an enthalpy balance, that is:

Power=(ro cx h)_in - (ro cx h)_out

Then I have calculated it using the angular momentum, that is:

Power=(ro cx r cth w)_in -(ro cx r cth w)_out

with :

ro = density

cx = axial velocity

h = total enthalpy

r = radius

cth = tangential velocity

w = angular speed of the rotor

_in = inlet of the rotor

_out= outlet of the rotor

I have obtained different values (differences are greater than 1%); then I have calculated, and included in the balance, the torque due to the endwall boundaries and I have also changed the grid but ... nothing.

Can you help me?

Best regards,

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Old   July 30, 2003, 13:28
Default Re: Angular momentum
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Try repeating your prediction with a coarser grid and/or a finer grid. I suspect you will find that you have simply derived an indirect way of demonstrating numerical error in you solution (probably not particually useful one because of postive errors cancelling negative ones). A code can only strictly conserve a small number of physical quantities, the rest have to suffer the effects of truncation errors.

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Old   July 31, 2003, 11:09
Default Re: Angular momentum
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Since you have a mass flow term in your calculations, the level of mass flow convergence enters into this. How well is your solution converged? Also, as you have a density term, you have to consider how accurately density is calculated. Your final comment regarding torque brings in another source of error.
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Old   July 31, 2003, 12:31
Default Re: Angular momentum
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Mass flow error is 0.4% between inlet and outlet sections and I think solution is converged.

>Your final comment regarding torque brings in another source of error

Can you explain?

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