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 c0384 April 20, 2008 11:24

inlet total pressure

Hi

My model has an inlet boundary and an outlet pressure boundary. I assume constant velocity at inlet. May I know whether total pressure at inlet can be affected by the setting of my outlet pressure? or it is solely depends on the inlet velocity and my inlet elevation as Total Pressure = density*Velocity^2/2g + Static Pressure?

 sameer April 20, 2008 21:17

Re: inlet total pressure

You total pressure at the inlet has nothing to do with outlet pressure. You inlet toal pressure will depend upon what you specify as static pressure and dynamic pressure (this dynamic pressure is obtained from the inlet velocity , from the formula you mentioned above). I hope this helps

Sameer

 c0384 April 21, 2008 03:56

Re: inlet total pressure

Thanks for responding. As mentioned in my previous posting, my model has a single inlet and exit. I make simulation by assuming steady flow, using k-epsilon RNG turbulent model, assuming uniform inlet velocity. My model has a large exit area. This leads to reverse flow at outlet that caused residual hanging at high value. In order to overcome this problem, I carried out my simulation in 2 different ways as follow.

a) At pressure outlet boundary, I switch the Environment Pressure to ON. Other settings are pressure=0, piezo and zero gradient. b) I extended my pressure outlet boundary by coupling the original outlet boundary with a straight pipe. My new pressure outlet boundary has Environment Pressure OFF, and the rest of the setting are the same as in (a).

I found out that the different in Total Pressure at inlet for case (a) and (b) is about 3%. What other possible reasons that can cause this deviation?

 James April 21, 2008 07:50

Re: inlet total pressure

I believe environmental is only active for flow into the model - it uses bernoulli to drop the static pressure at the boundary which helps alleviate some of the problems associated with pressure boundaries on inlet plenums. It is incompressible and thus only appropriate for low speed flows.

The pressure reference cell is ignored if you have pressure boundaries. Thus if you change the model and keep the same value of pressure on the boundary then the inlet values will change as the loss in the system will change.

The 3% change is probably the loss associated with the extension.

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