[jinggca]

I am learning about the source term in the momentum equation, and I got to the point where the momentum equation looks like this:

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However, from this point onwards every source I came about provides the expression for without providing a derivation. I understand that constitutive laws are involved here that states the shear stress is proportional to the spatial velocity gradient, but there's a lot more to it. Could someone please point me to a source where a detailed derivation, or at least an explanation of where each term comes from is given? Thanks a lot in advance!