CFD Theory confusion
 

Hello,

I am trying to learn the theory behind CFD and have been watching the following lectures:

http://www.youtube.com/watch?v=nPSJxmICStA

The lecturer is deriving the Navier Stokes Equations and I have gotten confused with one of his methods.

Could somebody please explain to me what is happening at 44:30 when he rearranges the variables for the isotropic conditions.

Thank you.

The topic is very standard in the basic course of fluid mechanics you can find all details in many textbook starting from the hystorical book of Batchelor up to more recent of Kundu. Then, isotropy is useful to further link the stress tensor to the velocity gradient by means of the linear Newtonian assumption.
I suggest to study first details on a book ;)