## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Time dependent operating pressure with incompressible ideal gas

• Francisco Acosta
Subscriber

Hello,

Im working with the simulation of an internal flow of a gas at low Mach number. There are large temperature changes within the domain, which is why the temeprature dependence of the gas physical properties are taken into account using polynomials. In steady state simulations, the density is determined using the incompressible ideal gas model, defining an adequate operating pressure. I would like to run a transient simulation in which the average pressure in the domain varies slowly. This could be done by defining a floating (time depentent) operating pressure without modifying any other setting/model. However, Fluent's user guide explicitely says that the floating operating pressure should not be used for incompressible flows:

Important:  The floating operating pressure option should not be used for transonic flows or for incompressible flows. It is meaningful only for slow subsonic flows of ideal gases, when the characteristic time scale is much larger than the sonic time scale.

Could someone explain why this is?

To achieve my goal, an alternative would be to define a time dependent inlet or outlet pressure for the domain and define the density as an UDF which depends on pressure and temperature. Given the small pressure drop within the domain, I would expect this approach to lead to similar results than just changing the operating pressure (i.e. p(x,y,z)~p_operating, for all x,y,z), which is why I don't anderstand the note from Fluent's users guide. I would appreciate any clarification on the matter.

• C N
Ansys Employee

Hello Francisco,

The reason for the floating point pressure option being limited to subsonic flow is because the pressure rise calculation includes only integral mass equation. In high speed flows there are possibilities of shock waves and high flow velocity present, these factors are not considered by the floating point pressure option as it does not include pressure correction factors which is coupled to velocity , pressure , density and energy for high speed compressible flows. I hope now you have a clarity on the limitation of the this option. I have also attached a simulation example for best practice to be followed to while dealing with high speed compressible flows . Hope this video is useful

https://courses.ansys.com/index.php/courses/shock-expansion-theory/lessons/simulation-examples-homework-and-quizzes-11/topic/homework-supersonic-flow-over-an-expansion-corner/

Thanks

Chaitanya Natraj