SimScale CAE Forum

Boundary conditions


Hello, I am new to CFD and after watching some projects and tutorials I was wondering why do we almost always use a pressure outlet and not anything else ?


I think because it is the least temperamental to have ‘work as expected’ and generally works for almost all basic CFD projects.


Hi @WarChitect,

In most cases, the inlet is typically defined as a velocity inlet. In simulation, we need to ensure continuity is met, i.e whatever enters the domain must exit the domain. With the velocity inlet, we already imply a certain amount of input. If we define a rigid output (i.e a velocity outlet) we will risk running into continuity errors as any small turbulence, or flow deviation will not satisfy the outlet condition.

With a pressure outlet, we then able to negate this problem by letting the internal flow “solve itself” and adjust accordingly. The pressure outlet works like a non-rigid outlet. This will prevent our continuity problem while ensuring the flow can progress throughout the domain without the results being affected. At the same time, we will not also force the solution towards a certain value. After all, we want to know the fluid’s effects in the domain from the initial starting condition, not fix a defined solution.

Of course this is the most basic example. There other types of use cases but fundamentally, continuity must be maintained.

Hope this helps.



Thanks for expanding my short response…


Thank you very much i did understand that putting a velocity outlet could pose some issues with the velocity intlet and you confirmed it.