# Help needed with boundary conditions

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 September 30, 2008, 17:39 Help needed with boundary conditions #1 JP Guest   Posts: n/a Hello All, Would you have an idea how should I set my boundary conditions in the following: - a fan is axially positioned inside a short cylinder (a cup) - the fan rotates inside the cup, airflow is subsonic and ***compressible*** - bottom of the cylinder is defined as a wall - the round wall of the cylinder is defined as a wall - the remaining base of the cylinder (the mouth of the cup) is not a wall; it is open. What is a proper boundary condition for this "wall"? Air flows both in and out, so it is not just an inlet or just an outlet, but both. It is also not far from the fan. Any ideas are appreciated.

 October 2, 2008, 02:30 Re: Help needed with boundary conditions #2 chandra manik Guest   Posts: n/a First of all you have to defined your system. - is the fan convert the mechanical energy to the fluid or opossite condition ( like turbine for example)? - is the condititon under steady state system or transient? For example : your system is the fan that generate the mechanical energy to the fluid and the system is analized in steady state condition. Boundary condition for this are : - you have to spesified the inlet boundary that can be as mass flow inlet for the case compresiblle flow and the outlet can be as the outflow where the pressure and velocity are unknown. - the wall of cylinder can be spesified as the rotating wall with the center and the axis same with the wall of the fan blade, this roating wall spesifed as freference with rotating speed is zero. - next the wall of the fan blade can be spesifed as rotating wall with same center and axis with cylinder wall where the speed is not zero but depend to your fan speed. now you can run the analysis, you can find this model sample in the FLUENT help with topic is rotating reference wall. I have performed the computation for the pump, compressor and turbine before. But for fan, since in typical design the fan can only rised the pressure of the air to 4-7%, you can modeled this as incompressible gas, but for gas turbine and the sentrifugal compressor it should be compressible.

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