[ytyyutianyun]

I find when people did fluid computation such as flow around the cylinder, they will apply velocity boundary at inflow and pressure at the outflow.
Then the question is: Can we apply both velocity boundary conditions on inflow and outflow?
Thank you very much.

[piu58]

Originally Posted by ytyyutianyun

I find when people did fluid computation such as flow around the cylinder, they will apply velocity boundary at inflow and pressure at the outflow.
Then the question is: Can we apply both velocity boundary conditions on inflow and outflow?
Thank you very much.

In principle yes.

The advantage is, that you don't need to ensure that the flow by itself has a nearly zero gradient at the output. If your flow has changes / turbulence and you set the outflow to zero gradient, it may be you calculate false back flow.

Setting the output velocity however may lead to problems with the conservation of mass. It may be wise to set at least a part of the outflow to zero gradient to give the calculation a point where numerical difference may be balanced. That should be a region where the natural flow is most uniform.

[FMDenaro]

Originally Posted by ytyyutianyun

I find when people did fluid computation such as flow around the cylinder, they will apply velocity boundary at inflow and pressure at the outflow.
Then the question is: Can we apply both velocity boundary conditions on inflow and outflow?
Thank you very much.

First, I suppose you are talking only about incompressible flow problems as for compressible flows you have to set more conditions.
The problem is mathematically well posed by prescribing either velocity or pressure. The velocity must fulfill the continuity constraint. The problem is more of physical nature. While in inflow we can somehow suppose a certain shape of the velocity profile, in outflow the shape depends on the internal solution. Only if the outflow is far away and the flow can be supposed to restablish a certain regime (for example laminar and steady) we coudl set a velocity profile that does not perturb the solution. Often, to have a reduced extension of the computational domain we do not put the outflow section so far or the physical problem does not allow that. Therefore, the outflow conditions can be critical by a physical point of view, both using velocity and pressure.