Pressure calculated by Fluent
 

Hello,
I have somme interrogations about pressure used and calculated by Fluent.
I'm simulating incompressible flow ans I used mass-flow for inlet conditions and outflow for oulet conditions.
In Fluent documentations I found that :
1) For incompressible flow, the Pressure Operating is not used (Pop)
2) Initiale gauge pressure (which is the Static pressure) is set to zero for the inlet condition (because this gauge pressure is specified just for pressure inlet or supersonic inlet).

1) In these conditions and when the Gravity is set off, how Fluent does calculate the static pressure (p') and other pressure fields) ?
2) Is the static pressure (p') given by Fluent correspond to (p-rho*g*r) ?

Thanks for your answers !

The operating pressure is always used.

There is a supersonic/initial gauge pressure which serves TWO purposes (hence the supersonic/initial in the name). This is the static pressure at the inlet. If the flow at the inlet is supersonic then static pressure must be specified explicitly. If subsonic then the static pressure may be computed based on the flow-solution.

The other purpose is for initialization of the fields for the initial condition if you use the "compute from" feature.

The modified pressure p'=p-rho*g*r is only w/ gravity.
With or without gravity the static pressure is NOT the variable that fluent operates on. Fluent operates only on the gauge pressure (to reduce truncation error).
The absolute static pressure is related to the gauge and reference pressures by:
p_abs = p_gauge - p_ref, where the p_ref is the user specified operating pressure.

Fluent solves for and plots p_gauge. You can recover the absolute static pressure by adding the operating pressure to the "static pressure" reported by Fluent.

There is only one physical pressure field and that is the static pressure (which may be referred to from either an absolute scale or gauge pressure). What "other" pressure field are you thinking of? If you mean total pressure then that is computed from the compressible flow relations.

Thanks for your reply, LuckyTran.

In Fluent documentation, the static pressure is referred as (termed as) : Initial Gauge Pressure. Does this mean that the Gauge pressure is a static pressure ? If we have : p_abs = p_gauge - p_ref (whats is really this gauge pressure, what represents ?)

2) If we disable gravity, how does Fluent get the static pressure (the static pressure is related normally to Rho*g*z), so in this case we don't activated gravity; Fluent still gives a static pressure, is it correct ? Is it a static pressure ?

p_abs = p_gauge - p_ref

This is the definition of the gauge pressure. You ask what this gauge pressure represents, well it is whatever this equation is telling you. The gauge pressure is a representation of the absolute pressure with respect to the reference pressure (the reference pressure is the datum).

The initial gauge pressure is (via circular reasoning) the gauge pressure initially. If you choose the "compute from" method and choose the inlet where the initial gauge pressure is specified, then your pressure field will be initialized based on this initial gauge pressure. It is a gauge pressure and not an absolute pressure (all the pressure inputs are gauge pressures).

The static pressure in terms of rho*g*z when gravity is disabled is simply via the same relationship via g=0. It is no different.

Thanks for your replies :)