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Inlet/Outlet Boundary Conditions

Submitted by sdup on April 18, 2011 - 09:18

Two questions:

1. Is it possible, instead of assigning a velocity to fluid, to assign an inlet and outlet pressures? I realise this will imply some transient flow solutions.

2. How can one easily determine the stagnation pressure of the fluid at any position? One can get P (I assume static pressure) and U results. It should be relatively easy to combine those to give the stagnation pressure change. I’m thinking specifically in terms of determining pressure losses over industrial ventilation and pollution control equipment.

‹ Can Caedium do this? Probe Result ›

Boundary Condition Types and use of Reference option

Submitted by symscape on April 18, 2011 - 09:48.

In answer to your questions:

  1. The Substance->Properties->Reference values, such as U (velocity) are reference values that you can refer to in boundary conditions. Then each quantity (e.g., U) within a boundary condition has various options on how you can specify the required values - one being Reference. For an outlet boundary condition you can change the p/rho option from Reference to Pressure. After you assign a boundary condition there are often different types of that boundary condition available, e.g., set Inlet->Type to Pressure. Note all pressures for incompressible solvers are gauge pressure (relative). Setting pressure conditions does not force you to use transient solvers.
  2. You can create new variables using File->New->Results, for an example review the tutorial "Fluid Flow Around a Sphere: Theory Comparison". In your case you'll want to create p + 0.5 x rho (use a constant) x U.Mag x U.Mag

Still seem to require a reference velocity

Submitted by sdup on June 1, 2011 - 10:56.

Managed easily to create new outputs. Thanks

However. I still seem to have to assign a velocity to the fluid else the analysis falls over easily.

Reference Velocity is Necessary

Submitted by symscape on June 1, 2011 - 13:00.

You'll always need to assign a Substance->Properties->Reference->U (Velocity), it's used by turbulent variables and by default (with options to override) it's used to initialize the velocity field. The reference velocity doesn't have to be exact - just in the right ballpark.

I've often seen that using a pressure inlet boundary condition will cause a simulation to converge much slower than the equivalent velocity inlet.

Another think to watch is to make sure you are using relative pressures (not absolute) for incompressible flows.

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