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TurbulentFluid June 30, 2010 08:52

Inlet / Outlet boundary condition? (CFX, but also general CFD)
I have a situation where water flows into a pipe for a given period of time, and then flows out of it for a different period of time (a "can" is first filled, then emptied through the same pipe). The flow regime can be:
- governed by a function, let's say f(t) = sin(0.3*t).
- governed by an if statement, eg. if(t<2[s],1[m/s],-1[m/s])

I have tried setting this function at an inlet boundary condition.
I do get the inflow for the inflow part of the interval correctly, however the water does not exit through the inlet BC during the outflow interval. The inlet BC seems to treat all the negative velocity or mass flow values as 0, whether I define them as "Normal to boundary" or by the coordinates.

How do I solve this problem? Is there a possibility to change the BC during the simulation time by the means of an expression (eg, if(t<2[s],INLET,OUTLET)? Or is there a workaround that will allow me to "pour out" the fluid through the inlet?

I am using Ansys CFX, but I also have access to other softwares (FLUENT, OpenFlow, Comsol...). Any advice would be much appreciated! :)

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