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 cfx_user March 6, 2013 06:01

energy eqn + constant fluid properties

Hello to all,
I'm bit confused about the impact of energy equation on the obtained results, namely: I have launched 2 analyses, one with total energy equation and second one without energy equation.
In both cases I'm simulating the flow of incompressible fluid (water) with constant fluid properties (viscosity/conductivity, density).
Finally i'm obtaining two different pressure drops.
My question is why? Since the fluid properties in both cases are the same (are not temperature depended) I would expect the same results...
Or it does impact on the turbulence ?
Thanks in advance for your suggestions.
Cheers, bob

 andy_ March 6, 2013 08:13

Quote:
 Originally Posted by cfx_user (Post 411892) Hello to all, I'm bit confused about the impact of energy equation on the obtained results, namely: I have launched 2 analyses, one with total energy equation and second one without energy equation. In both cases I'm simulating the flow of incompressible fluid (water) with constant fluid properties (viscosity/conductivity, density). Finally i'm obtaining two different pressure drops. My question is why? Since the fluid properties in both cases are the same (are not temperature depended) I would expect the same results... Or it does impact on the turbulence ? Thanks in advance for your suggestions. Cheers, bob
Discounting the trivial case, if you solve an energy related equation then the density would normally change. For example, incompressible combustion problems will have a strongly varying density.

If the code lets you hold the density constant then the "thermodynamic pressure" would have to change but I am not sure the physical meaning would be sensible. An incompressible flow usually has a constant "thermodynamic pressure" throughout the solution region and then the "pressure" variations simply follow the mechanical acceleration/deceleration of the fluid. This knocks out sound waves which is usually the main objective. The constant background thermodynamic pressure may change through time (e.g. a piston chamber) but remains constant in space. Varying this background thermodynamic component while hold density constant seems odd but there may be some physical cases where the assumption is reasonable. Anyone?

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