Transient simulation (Problem with massflow and Pressure)

ChristianF · August 22, 2011, 10:58

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.

ChristianF · August 22, 2011, 11:00

I am sorry i forgot to upload the picture

monkey1 · August 22, 2011, 11:40

How big is the time step used and how did you set up your inlet boundary(pressure and mass flow)?

ChristianF · August 22, 2011, 12:46

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°

ghorrocks · August 22, 2011, 22:42

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

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

August 22, 2011, 10:58	Transient simulation (Problem with massflow and Pressure)	#1
ChristianF New Member Christian F Join Date: Mar 2011 Posts: 8 Rep Power: 16	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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August 22, 2011, 11:40		#3
monkey1 Senior Member Join Date: Jul 2011 Location: Berlin, Germany Posts: 173 Rep Power: 16	How big is the time step used and how did you set up your inlet boundary(pressure and mass flow)?

August 22, 2011, 12:46		#4
ChristianF New Member Christian F Join Date: Mar 2011 Posts: 8 Rep Power: 16	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°

August 22, 2011, 22:42		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 18,017 Rep Power: 146	http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F You would need to include more details if you want us to be more specific.