Learning Forum

[Gregory Harrington]

Former starccm+ user here where I only had the reference values input, not operating conditions.

The problem I am trying to model is a flowpath inside an engine at altitude. Knowing the ground static pressure and temperature I know my ambient conditions (pressure and temperature) at altitude.

The flowpath im modeling inside the control volume is pressurized and at higher temperature than ambient. It is a one inlet and 2 outlet system. The inlet is specified as a mass flow rate which I know so no issue there. The outlets are modeled as pressure outlets for which I know the absolute static pressure at each one. Using my ambient static pressure I therefore know the gauge static pressure to use as the boundary condition.

So I am confused about how to set the operating conditions and reference values. Coming from star-ccm+ I just set the reference values to be my ambient pressure and temperature and voila!

In fluent would I set the operating pressure to be my ambient pressure at altitude and leave the reference pressure as 0? Or would I set the both the operating pressure and the reference pressure as my ambient pressure? Or just set the operating pressure to be what I know the pressure is inside the domain and the reference pressure as the ambient pressure?

[Rob]

The reference values are used for various coefficients (lift, drag etc) so would be tuned to whatever datum is needed. Generally I don't use the coefficients so I leave the reference values alone. 

The operating conditions are there to set gauge (ambient) pressure, so I'd tend to set the operating pressure to ambient (the default is 101325Pa ie 1atm) and that allows you to set the gauge value on the boundaries. Or I'd set as zero depending on the application; density also needs some care in certain circumstances. So same principle but I suspect the terminology/labelling is switched.