[ad6556s]

Hey Friends!

My task is to simulate a flow through an outlet valve of an combustion engine. The pressure in cylinder is 13bar and the pressure in the exhaust tube is 3bar. The reference pressure is set up to 1.01325bar. Now I get in some reagions negative values for pressure. They are smaller than 1.01325bar.

So I think, that starccm+ uses the reference pressure, which I am able to set up before simulation, and calculates the pressure distribution in the flow field. In this case do I have to calculate the initial conditions? To get 13bar in the cylinder, I use 13bar minus 1.01325? And to get 3bar in exhaust tube I habe to subtract 1.01325 from 3bar?

Thank you!

[Deranda]

Hi,
pressure in Star CCM is a bit difficult. First of all, have you chosen constant density or ideal/real gas in the physic continua?

If you use constant density, the reference doesn't really matter instead of an offset value.

The initial pressure, is the pressure, star ccm starts with. If the initial pressure is close to the actual pressure, the solver will converge faster.

Here you have some explenations for different types of pressure:

AbsolutePressure: The AbsolutePressure is the absolute value of the pressure, regarding 0Pa (Vacuum/AbsoluteZeroPressure)

ReferencePressure: The AbsolutePressure can reach enormeous high values. Due to avoid numeric rounding errors, the AbsolutePressure is devided into the ReferencePressure(which is constant) and the StaticPressure.

StaticPressure: Depending on your physics, the staticPressure is devided into different other Pressures. The most important part is the workingPressure (in Star CCM: Pressure) which is responsible for the dynamics of the fluidflow.

Be carefull, if you have a turbulence model, the pressure will be computed with a turbulent part, which is just a model and can not be measured. To differentiate this "normal Stress Term" you have activate the Option in:
Continua -> Physics -> SST(Menter)k-Omega / k-Epsilon -> Expert properties ->NormalStressTerm

hope I can help you, thanks to my prof who taught me that complex structure

[ad6556s]

Thank you for your reply, Deranda!

I chose in the physic continua the ideal gas model.

So starccm+ displays not the absolut pressure in scalar visualization...you allways have to add the reference pressure?

Thanks your Porf!

[Deranda]

it depends on what you display in your scalaScene. You can chose the AbsolutePressure as the ScalaFieldFunction as well ;-)

[ad6556s]

Thanks a lot! ))