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-   -   pressure inlet in incompressible flow (http://www.cfd-online.com/Forums/fluent/37018-pressure-inlet-incompressible-flow.html)

 dirk June 22, 2005 09:31

pressure inlet in incompressible flow

Hello,

I have a simple 2D geometry with one inlet and one outlet, the flow is incompressible. I defined the inlet as a "pressure inlet" and the outlet as "outflow". I specified my operating pressure (around 100kPa) at the inlet and calculated the total pressure for the desired velocity with Bernoullis equation. So I had around 13Pa as (relative) total pressure and set the initial gauge pressure to zero. The velocity should have been around 4.7 m/s, but it became almost zero during calculation. Anyone knows, why this happens ?

Thanks, Dirk

 Jason June 22, 2005 12:57

Re: pressure inlet in incompressible flow

From the fluent manual:

<font color="blue">Note that outflow boundaries cannot be used in the following cases: </font>

<font color="red">if a problem includes pressure inlet boundaries; use pressure outlet boundary conditions (see Section 7.8) instead </font>

<font color="blue">if you are modeling compressible flow

if you are modeling unsteady flows with varying density, even if the flow is incompressible For an overview of flow boundaries, see Section 7.2. </font> So you can't combine pressure inlet and outflow BC. If you want to continue using outflow BC, switch the inlet to a velocity inlet with the 4.7m/s you described.

Also, the outflow is a sensitive BC. It assumes the diffusion flux for all variables is 0 (i.e. fully developed flow). The only place I've seen it used successfully is in pipe flow, several diameters downstream of any disturbance so that the fully developed flow assumption is valid. Just becareful where you use it.

Hope this helps, and good luck, Jason

 dirk June 22, 2005 13:27

Re: pressure inlet in incompressible flow

Thanks Jason,

I should have read this passage of the manual. I used velocity inlet before, but wanted to test other boundary conditions. When I did unsteady calculations I set a sinusoidal inlet velocity and got oscillating static and total pressure at the inlet. But I would like to have a constant total pressure. So I will have to use pressure inlet and pressure outlet, I guess.

Dirk

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