SimScale CAE Forum

Incompressible fluid analysis in CFD


Dear All,

i am new in SimScale and CFD Analysis.
i have a question regarding boundary condition, when user want to analyze the flow of incompressible fluid. if i have a water flow from one pump (with defined pressure and velocity) at the inlet side, and it will flow through pipe with branch, can i define the define both the parameter at the inlet side and just let the outlet side do nothing? therefore i could know the separation value whether its pressure and velocity at outlet side,

thanks for your answer and kind help, :slight_smile:


Hi @Aufar

Typically for incompressible flow you need to define atleast 1 variable (velocity or pressure) on the boundary to have a well defined system.

In you case of pipe flow, you can fix the velocity at the Inlet and set a reference pressure of 0 value at the outlet. Then the simulation would calculate the relative inlet pressure based on the reference outlet value (giving you the differential pressure or loss) and the velocity at the outlet.

I hope that helps



Hello @Aufar and welcome to SimScale!

Regarding the boundary conditions it is not possible to define pressure and velocity at one boundary (the same side) or the same boundary condition at both boundaries. So you can define velocity at the inlet and pressure at the outlet for instance. Below I have given you a small tutorial that covers your question I think.

For an easy tutorial of a water pipe junction, check out: Tutorial water pipe

You will see how the boundary conditions are applied in this case.

Hope that helps :slight_smile:




Hi Ali,

thanks for your clear explanation, i think i get it how to do my case after read your your explanation,

best regards,


Hello Jousef,

that’s lovely, sure i will try the tutorial,
thanks for your answer and kind help,

best regards,