[alimokh91]

In a pipe reaching a bifurcation simulation, with first-order upwind I have no problems with residuals, and they decrease normally, then after convergence, I switch from first-order to second-order upwind, the residuals remain constants and oscillating at 10e-2.
I refined my mesh, I used different schemes (simple,...), but nothing changed.

Why? What I have to do to solve this problem

Just as a short description:
Geometry is shown in the attached picture
For the mesh:
in the root, the element size is: 0.6 mm
in the red sphere zone, the element size is: 0.3 mm
for the boundary layer, the first layer thickness is 0.08, number of layers: 5, growth ratio: 1.2
the minimum orthogonal quality is 0.15 and the highest skewness is 0.84!

inlet B.C: parabolic Velocity profile: 2*V_mean*(1-(r*r/(R*R)))
outlet B.C: pressure outlet: 13332 pa
Scheme: coupled
Steady-state

Thanks!