Ansys Fluent offers a very simplified version of boundary conditions. So if I know the velocity at the inlet, I need not care about pressure and vice versa. What I am having trouble with is that I want to fix both velocity and pressure at the inlet of my domain! But doesn't seem possible. So what I am looking for is that if I define an inlet velocity to the domain, how is the pressure decided at that point by ANSYS?

  • $\begingroup$ Don’t use the simplified conditions version. $\endgroup$
    – Solar Mike
    Sep 17, 2020 at 6:10
  • $\begingroup$ very simplified version of BCs? How did you come to that conclusion?. Actually, from your question it can be seen that you are not familiar with the basic differences between velocity inlet and pressure inlet BCs, so don't blame FLUENT. Anyhow, assuming you're simulating an incompressible flow you can use velocity inlet boundary condition and set the pressure through operating conditions tab not through inlet conditions. $\endgroup$
    – Algo
    Sep 17, 2020 at 17:20

1 Answer 1


I am not familiar with Fluent too much (I use CFX), but you can specify inlet pressure and velocity unit vectors or angles. This way the direction of the flow and pressure are set. For example, in CFX for some simulations I specify inlet total pressure, inlet velocity unit vectors, and exit static pressure. This is one of the more "streamlined" boundary condition options you can choose. Does this make sense?

EDIT: I should add this explanation. If you specify the pressure at the inlet, pressure at the outlet, and velocity at the inlet, you've over-constrained the problem. This very well could set two different mass flow rates; one from the pressure ratio and another from the velocity. This is why the unit vectors/angles are used: to only specify a velocity direction and not a magnitude.

  • $\begingroup$ But what if I want to specify the magnitude of velocity along with that of pressure? $\endgroup$ Sep 19, 2020 at 22:59
  • $\begingroup$ That can lead to non-sensible results, especially if this flow is compressible. Starting on side 7 (bakker.org/dartmouth06/engs150/06-bound.pdf). Velocity inlets are intended for incompressible flows. Any time you specify an inlet pressure, the exit pressure must be given to define a pressure gradient. You can't specify inlet pressure, exit static pressure, and velocity since this will over constrain the model. $\endgroup$
    – mechcad
    Sep 21, 2020 at 13:24

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