- #1
Silviu
- 624
- 11
Hello! I am reading about stress energy tensor of a perfect fluid and I don't understand the ##T^{ij}## terms. They are defined to be the flux of i-th momentum through the j-th surface. Now you take a fluid element and in its momentary comoving reference frame (MCRF) you calculate these ##T^{ij}##. In the book they look at the case where fluid elements have just perpendicular forces on each other. And from here they get that ##T^{ij}## is equal to the pressure, if i=j and 0 otherwise. Although it makes sense unit-wise, I am not sure I understand physically. In MCRF, the fluid element doesn't move in space, so how can you have a flux of momentum? Can someone clarify all this stuff for me? I am sure I am missing something important but I don't know what. Thank you!