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Transient simulation (Problem with massflow and Pressure)

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Old   August 22, 2011, 09:58
Default Transient simulation (Problem with massflow and Pressure)
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Christian F
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Hello everybody,

i am simulating a rotary piston engine. The Simulation is working but the problem is the following:

At the beginning the pressure inside of the cylinder is 0 bar (reference pressure is 1 bar). At the inlet the pressure is 3 bar. While the piston is moving, air is flowing from the intlet in the cylinder and the pressure rises. When the pressure in the cylinder is reaching the 3 bars (The maximum pressure) the pressure an massflow increase from the inlet into the cylinder should go down, but i am reaching even 3.5 bars until the massflow goes down. As you can see in the diagram the pressure for a certain time step is not changing anymore after some iteration steps. So i am only reaching the 3 bars after some timesteps (which is of course different to the reality). In a steady state analysis for a certain Piston positon i am reaching the 3 bars after about 30 iteration steps.
So what do i have to do to get the simulation closer to the reality?

Thanks.
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Old   August 22, 2011, 10:00
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Christian F
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I am sorry i forgot to upload the picture
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Old   August 22, 2011, 10:40
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How big is the time step used and how did you set up your inlet boundary(pressure and mass flow)?
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Old   August 22, 2011, 11:46
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Christian F
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Inlet: -Total Pressure (3 bar)
- Total Temp 25C
Timestep: -1/60000 s (1.666e-5 s)
-> Engine is rotating with 10000 rpm so in one timestep the
piston is moving 1
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Old   August 22, 2011, 21:42
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http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

You would need to include more details if you want us to be more specific.
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