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flow in pipe with momentum and energy source

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Old   March 27, 2005, 12:43
Default flow in pipe with momentum and energy source
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Atit Koonsrisuk
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I try to model the thing so-called actuator disc. It is the approximation of turbine. The actuator disc is very thin such that flow field properties change discontinuously across it. Then I test the concept by model the flow of water in circular pipe. I set the boundary condition at pipe inlet as 'inlet' with velocity specified, and at pipe exit as 'outlet' with zero relative static pressure. In addition, I create the fluid sub-domain within the pipe, this sub-domain is the actuator disc. The ratio of thinkness to diameter of sub-domain is 0.0125, and I use mesh control with 1.2 expansion factor to make the finer grid in sub-domain. I set this sub-domain as the energy source and momentum source simultaneously. The energy source is (-dW/volume) and the energy source is (-dW/(volume x velocity)), where dW is the power that I want the actuator disc to extract from the flow, and 'volume' is the volume of sub-domain. The result seems reasonable. The difference between velocity before actuator disc and velocity after actuator disc is only 0.04%. In addition, the difference of total enthalpy before and after actuator disc is about 'dW'. So I'm pretty sure that I understand how to model actuator disc in CFX. However one thing that I do not understand from the result is that the change of velocity inside the sub-domain. The velocity inside the actuator disc is fluctuated, and the maximum value is 163% of the velocity before actuator disc, whereas the pressure is not fluctuated. Do anyone have idea why the velocity inside sub-domain is fluctuated with a large value? I think it might be about the finer grid, however I do not know how to mitigate this problem. Thank you very much for your kind consideration.
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Old   March 28, 2005, 13:06
Default Re: flow in pipe with momentum and energy source
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Phil
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Try setting the CCL parameter 'Redistribute in Rhie Chow=t' under the GENERAL MOMENTUM SOURCE object. (You'll have to do this by changing the CCL rather; it is not currently in the GUI.) This changes some numerics details of momentum sources and often gives better results when the momentum source balances a pressure rise.
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Old   March 29, 2005, 06:16
Default Re: flow in pipe with momentum and energy source
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Atit Koonsrisuk
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Dear Phil, Thank you very much for your kind suggestion. However I am not familiar with 'CCL', could you please say it in more details?

Atit
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Old   March 29, 2005, 16:58
Default Re: flow in pipe with momentum and energy source
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Phil
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In Pre, right-click on your subdomain option and choose 'Edit in Command Editor'. Then type in the line 'Redistribute in Rhie Chow = t' inside the GENERAL MOMENTUM SOURCE object and hit 'Process'. Pre will give a red error message saying that this is an unrecognized option (because it is an expert option); just ignore the error and run in the solver.
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