Normal velocity (vector) Dirichlet boundary condition used for a surface of 3d mesh

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@xirancao1

Thank you for your reply! If the order of assigning boundary conditions between the wall and the inlet is changed, there will be no gradual decrease in the circular inlet surface？ I will try to verify the exchange order.

If you change the order of application of your bcs, you should see no gradual decrease at the inlet, but you would see an instantaneous narrowing of the flow after the first element.

@xirancao1

I also have another question, it is that does the grid drawn by Gmsh have no units when the. msh file is opened? If the coordinate units of grid points do not match the input parameter units in the equation, do we need to change the weak solution form of the equation while changing the boundary conditions? For example, if the coordinates in the NS equation are changed to mm, do the corresponding velocity and pressure boundary conditions need to be converted, and do the weak solution forms need to be changed?

DOLFIN/DOLFINx does not assume any specific units. If you read in a mesh defined in millimeter, your should ensure that the units of your problem makes sense.