In my course of Acoustics, in the chapter about fluid dynamics, we derive an expression for the stress tensor similar to this one, and then it is said: "It is convenient, to write this expression in a slightly different form, in which eta and lambda are replaced by other constants" The constants are similar to this. Why the replacement? What's so convenient about it? In further expansions, it appears that the bulk viscosity disappears for incompressible fluids, but if I'm not mistaken, you don't have to rewrite the equation to accomplish this, and in fact you make things more complicated. So why is it convenient? (I'd ask my prof but of course that would mean I'd have to wait for classes to start again - he's an engineer who doesn't care about pure physics and never answers email).