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-   -   flow in pipe with momentum and energy source (http://www.cfd-online.com/Forums/cfx/21119-flow-pipe-momentum-energy-source.html)

 Atit Koonsrisuk March 27, 2005 12:43

flow in pipe with momentum and energy source

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.

 Phil March 28, 2005 13:06

Re: flow in pipe with momentum and energy source

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.

 Atit Koonsrisuk March 29, 2005 06:16

Re: flow in pipe with momentum and energy source

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

 Phil March 29, 2005 16:58

Re: flow in pipe with momentum and energy source

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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