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Old   March 27, 2005, 13:11
Default Don't know where energy lost from my domain
Atit Koonsrisuk
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I model the flow in solar chimney. The flow gains thermal energy from energy source, afterward the energy is extracted by the model of turbine. I use the actuator disc as the turbine approximation. The actuator disc is very thin such that flow field properties change discontinuously across it. I create the fluid sub-domain, this sub-domain is the actuator disc representation. 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 momentum 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 which I input as the number. So only 'velocity that I input as 'v' variable. After CFX converge, I found that the difference of total enthalpy before and after actuator disc doesn't equal 'dW'. For example,

- dW=165; mdot x ( static enthalpy + 0.5x velocity^2)=158

- dW=320; mdot x ( static enthalpy + 0.5x velocity^2)=304

- dW=590; mdot x ( static enthalpy + 0.5x velocity^2)=560 where I get 'mdot x static enthalpy' from the integration of CFX.

I do not understand why they don't equal. I set the wall as the adiabatic free-slip wall. So no other way for energy to be lost, isn't it? Then I specify that when the flow past this sub-domain, the power must be extracted 'dW', the total enthalpy must be lost 'dW'. Do anyone have idea why it doesn't? Thank you very much for your kind consideration.
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Old   March 28, 2005, 18:35
Default Re: Don't know where energy lost from my domain
Glenn Horrocks
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Have you checked the imbalances? Your simulation might not have converged. Converging on imbalances and residuals is often a more reliable convergence measure than residuals alone when you have user sources.

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Old   March 29, 2005, 06:30
Default Re: Don't know where energy lost from my domain
Atit Koonsrisuk
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Dear Glenn, Actually I never check the imbalance, thank you that you introduce me to know about 'imbalance'. After reading your suggestion, I found that the maximum global imbalance occur for H-Energy. It is 0.0036%. So we can be sure that it already converged. What do you think?

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