hi, i study on a 3d car muffler design. and i gave 20kpa pressure inlet. 1-)but sometimes i read "turbulent viscosity limited to viscosity ratio in 512312 cells." and "reverse flow". what does it mean?

2-)and how can i see the velocity profile at inlet and outlet boundaries.

3-)what is the different between second upwind and first upwind

dear sir, please send me the design of muffler that bring saving energy and increse power. thanks