[engineer.iman]

hi,'i have done a mesh Independence study for internal pipe flow in FLUENT.
the flow is turbulent water flow
i have three meshes, the fine and medium quality meshes returned the same y+ value (<20) when i generated the plot for y+
while the coarse mesh resulted in very high value as shown in the attached figure, how should i interpret these results and which one should be the best for k-e model

[davidwilcox]

Use the mesh-independent grid that saves you computational costs. The medium one that is. However, I would not look at y plus as a property for mesh independent studies. If you're using a scalable wall function with the k epsilon, y plus will remain at around 11 for fine meshes. It will not change if you were to refine further. Pick a velocity profile and see how that changes with the three meshes. OR... you could look at things like the Richardson extrapolation or GCI( grid convergence index) if you want a more quantitative reasoning.

[engineer.iman]

Originally Posted by davidwilcox

Use the mesh-independent grid that saves you computational costs. The medium one that is. However, I would not look at y plus as a property for mesh independent studies. If you're using a scalable wall function with the k epsilon, y plus will remain at around 11 for fine meshes. It will not change if you were to refine further. Pick a velocity profile and see how that changes with the three meshes. OR... you could look at things like the Richardson extrapolation or GCI( grid convergence index) if you want a more quantitative reasoning.

thank you very much for your reply.
i actually used the convergence time and the residuals as a basis for selecting the best mesh which in this case the medium one.
but i need to interpret the y+ for each mesh, the medium and fine meshes had the same plot of y+ values however, the coarse mesh was much higher!!!
but one thing i don't understand is why i got so many points for the plot of y+ values for the coarse mesh and should i conclude that a y+ value of less than 25, as in the medium and fine meshes, is considered good or not?