Div is the flux density or divergence of velocity field, that is, the sum of partial derivatives of velocity field components.

Grad is a gradient vector, which refers to the direction in which the directional derivative changes fastest.

From the above, div (gradu) refers to the divergence when the gradient vector of U is used as the velocity field.

Divergence can be used to describe the divergence of vector field at various points in space, and the physical meaning of divergence is the activity of the field. When div F>0 indicates that there is a positive source (divergent source) of emission flux at this time, when div f