A wall has been placed at portion(s) of an OUTLET
 

Dear friends, I know what it means, the problem is very interesting.
I am simulating a sedimentation pool, in the first step of the project, I am doing some mesh sensibilities analysis. When I am using a course mesh the problem converge smoothly without problems, but when I refine the mesh, during the convergence procedure, a backflow problem starts at the outlet.
WHY IS THIS HAPPENING ?
Why does it happen when the mesh is refined?
:confused:

A coarse mesh tends to have a diffusive effect on the calculated flow field.
This could mean for example that vortices near the outlet, which are resolved on finer meshes, are dampened out by the coarse mesh.

In your case, an "opening" boundary condition would allow backflow into the domain at the outlet. This would prevent the warning message you get.
Nevertheless, your problem is ill-posed if there are vortices at the position of the outlet, both with outlet and opening BoCo.
So keep the outlet condition, but move the outlet further downstream until no warning messages concerning backflow appear on the finest mesh.

Now you can conduct a proper sensitivity analysis with the mesh size.

As said above, try to move it away from the inlet, or from areas you know the flow will cause backflow.

Try reading the help about Inlets, Outlets and Openings, they have schematics that will help you understand what may be happening.

We know opening is not what you want, so try messing around with the geometry first, put several outlets and open/close with "wall" boundary conditions until you solve this problem.

It happens a lot with me too, I always wonder why...

Dear all, I love when people get involver easy with our post!!.. that is why this exists.
I know very well why this happen, my main question was, why is this happening after I refine the mesh?
This was my main question, because it is very weird, but the answer of flotus1 was perfect!!!
greetings colleagues!!