[flavia]

Goodmorning.

I'm trying to simulate a very very simple case: a verticale pipe with one enter and one exit, with a laminar flow.
The fluid is liquid water, in standard condition that comes from the down to the bottom.

I choosed, in the boundary condition's box, for:

1 enter, a velocity inlet of 0.01237 m/s
2 exit, a pressure outlet, constant normal to boundary, of 130000Pa.

In general's box i checked gravity with an acceleration magnitude of -9.81m/s² (on the z axis).
In operating condition i choosed an operating pressure of 101325 Pa and i checked specified operating density with a value of 0.

I choosed first order momentum end i made a standard initialization with the enter.

But i have a big value of residuals (in the orders of 1e-1).
If i don't check the specified operating density, residuals decreases.

I don't understand the reason;

Thank you in advanced for your answer

[JBurnham]

Compare the residuals to the convergence criteria epsi.

If resid < epsi, then the pressure solver has converged correctly. If you want to use a tighter convergence tolerance, use Model Setup > Numerics > Pressure Solver Options > Converge Controls > Convergence Crtieria Multiplier epsadj = 0.001 or 1e-6. The default is 1, and the number multiplies the dynamically-selected convergence criteria by that value.

If epsi > resid, then the pressure solution is not converging. Check: do you have fluid initialized in the domain? Do you have mesh smalls small enough that the pipe open area is resolved by at least 4 cells (and for an accurate answer, 10 - 20)? Do you have any other physics active besides the two or three you need: Gravity, Viscous Flow, and maybe Turbulence?

I don't understand what you mean about un-selecting 'specified operating density'. FLOW-3D requires fluid density RHOF to run.