Deriving Naviers Stokes Equations

Second question:

My question is about the stress component tau(xy). It is said that it is viscositu*(du/dy+dv/dx). I can see out of the definition that it is supposed to be viscosity*du/dy, but why also the dv/dx part?(v is the y-compononent of the velocity).

These questions have been nagging me for ours now, I would appreciate some help.

PS: All the deriviatives are supposed the be partial deriviatives offcourse.

Not going into the rest of it, but isn't there something pretty obvious going on with your Q2 and your post #2?

tau(xx) = viscosity*(du/dx+du/dx)

tau(xy) = viscosity*(dv/dx+du/dy).

Without any knowledge of this subject, would it be reasonable of me to suggest that

tau(xz) = viscosity*(dv/dx+dw/dz) ?

Feel free to kick me into touch - but if I'm wrong, could someone please explain?

(Symmetry rules, OK? )

The tau(xx) you have written I didnt write. You have written the right tau(xy), but not the right tau(xz), there you need w which is the z-component of the velocity.

I don't really understand what you are trying to say, because you are just repeating me, but you mixed something up.

Review the basic theory and it will all become crystal clear.

Say what!?? I've been reading this for hours now.