How to use ∂ function in my code or how to compute the mixed second-order partial derivatives or first-order cross partial derivatives of the velocity field **u**?

[gdalle]

Where do you expect the \partial function to come from? It isn’t defined anywhere in your code. Usually, these computations are performed with an automatic differentiation library, so unless you have an explicit formula for ∂(i, j, I, u), you may need to pick one of these libraries in the Julia ecosystem.

Also, please try to format your posts using the correct backticks ```, so that the code is displayed properly, see Please read: make it easier to help you.