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[MHDJ]

Hi,

I have the following question and I'd be thankful if you could help me with this regards,

Flow pattern in a cylindrical pipe was studied whereby there is flow into the pipe but no outflow from the other end of the pipe. The simulation was modeled with the following boundary conditions: Velocity inlet, and the outlet was modeled as wall (since there is no outflow). Fluent solved this scenario and gave the velocity profile and pressure profile result.

Since there is no outflow in the pipe, there should be no flow in the pipe, while the outflow is closed and the fluid is incompressible the change in the contour of velocity field appears. Also, the result obtained does not explains the mass conservation in the system and amazingly says the continuity is converged and the continuity residual reaches the below residual criteria 1e-6.

 

Thank you in advance.

[Rob]

It's a solver glitch, and should fail. If you plot the results they're usually rubbish. 

[MHDJ]

Dear rwoolhou,

Thank you for your reply, I have plotted the residuals and velocity contours for both closed and open outlet boundary conditions.

But they do not seems rubbish! because they satisfied the residuals criteria.

I attached the relevant report to this post.

 

 

Regards,

MJ

 

[Rob]

Staff are not permitted to open/download attachments. Please repost into the thread. 

[MHDJ]

Thank you for your reply,

I have plotted the residuals and velocity contours for both closed and open outlet boundary conditions. with a constant velocity inlet Boundary condition in a cylindrical pipe.

But they do not seem rubbish! because they have satisfied the residuals criteria!

In any iterative method (mathematical aspect of view) when the solution reaches the minimum criteria of residuals is called converged but here we expect to have divergence in closed outlet B.C. Since the continuity equation is not established!! and residual diagram in diverged results should have the oscillating or upward behavior as we expect in almost all diverged solutions.

The below pictures show the results of residuals diagram and velocity contour for opened and closed outlet boundary conditions respectively.

 

.

 

 

Thanks in advance.

[Rob]

So, you have a finite inlet velocity and the results are (near enough) zero?  What about the pressure field? 

[MHDJ]

Yes, and instead of diverged residuals I have satisfactory small amount of converged residuals.
Pressure field doesn't change.remains blue contour.

[Rob]

And is "blue contour" a physically meaningful value? 

For convergence we use a combination of monitors, fluxes and residuals to determine whether a solution is correct. 

[MHDJ]

Thank you for your reply, 

Lets back to my first question! I did not say the solution is converged, What I said was why the solution is not diverged. the results that I have sent, shows that the solution is not converged and I accept your statement.

but when it comes to the residual values it says the solution is not diverged.

Divergence is not equal to not converged!

In nut shell, I have problem why it is not diverged (the residuals of continuity) in mathematical aspect of view(numerical iterative Technics)? 

Regards.

[Rob]

Which goes back to my initial reply: it's a solver glitch.

It has been logged previously, and they're looking into a solution which doesn't also break when the same configuration is used with a compressible gas & transient solution. 

[MHDJ]

Thank you for your kind reply