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• Last Post 13 November 2018
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RajuMath posted this 13 November 2018

I am facing some troubles to run compressible flow in a nozzle.

1. I am using mass flow rate boundary condition at inlet and providing static pressure 8.56 atm. But from contour, the static pressure showing    4.86 atm. How FLUENT calculating this pressure? Also static pressure showing negative, how to solve the issue?

2.   If I change the mass flow rate or pressure, the inlet and outlet Mach number after converged remains same for all cases. This is not expected. Why this is happening?

Raju.

Kremella posted this 13 November 2018

Hello,

Mass flow rate inlet boundary condition allows for specification of mass flow rate. Here, the total pressure is not fixed and will rise to whatever value is necessary based on the computed static pressure. This value of static pressure is dependent on the mass flow rate specified. In the boundary condition panel. The static pressure (you specified) will be used only when your flow is supersonic or if you are initializing the solution from your mass flow inlet boundary condition. This value will be ignored if your flow is subsonic.

Static pressure is always specified relative to the operating pressure. Unless you make a change, operating pressure is always set to 1 atm. Negative values of static pressure, if your operating conditions are set to 1 atm, are not an issue. It just means that your absolute pressure is lower than 1 atm.

Please have a look at sections 6.3.5 - Mass-flow inlet boundary conditions, 6.3.4.1.7 - defining static pressure, and 7.14.1 The significance of operating pressure from the Fluent Users Guide. This will provide you with a better understanding.

I hope this helps.

Best Regards,

Karthik

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RajuMath posted this 13 November 2018

I am initializing the solution from your mass flow inlet boundary condition. Is there any option to see the specified static pressure at the contour?

Thanks.

abenhadj posted this 13 November 2018

Please read the detailed answer of my colleague more thoroughly. The specified pressure is only used for supersonic cases or is parsed into initialization if you say "initialize" from that "inlet". You won't see it in any contour plot (only by chance)

Best regards,

Amine