[ringtail]

Hi all,
I have simulated a steady flow around a blunt body. Of couse, the really
flow is unsteady. When I simulated the flow with a coarse mesh, the result of
Cd convergented as well as residual.
However, when I refined the mesh, although the Cd convergented finally but it vibrated between a small range. So was the residual as shown in the pic.
Why? Is it because the fine mesh would catch more unsteady flow details?
And how I can improve the stability?

Thanks.

[cdegroot]

Likely the fine mesh is resolving the unsteadiness better. If it is a transient flow and you try solving as a steady flow the residuals will generally not converge and do what is shown in your plot.