- #1

joshmccraney

Gold Member

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in reading a book on viscous stresses i found the following: [tex]\tau_{ij}=2\mu\Big(s_{ij}-\frac{1}{3}s_{kk}\delta_{ij}\Big)[/tex] where einstein summation is used. now we have [tex]s_{ij}=\frac{1}{2}\Big(\frac{\partial u_i}{\partial x_j}+\frac{\partial u_j}{\partial x_i}\Big)[/tex] and then the claim is incompressible flow implies [itex]s_{kk}=0[/itex]. can someone explain why this is so?

im using standard notation, but if i need to clarify let me know.

thaks!