University:
North Idaho College
Course:
PHYS-211 | Engineering Physics I
Academic year:

2023
Views:

197

Pages:

5
Author:

Hani T.

Gas viscosity. gas molecules move in Layers In doing so, one layer of flowing gas exerts a viscous drag on an adjacent layer moving at a higher velocity. Consider figure shown which depicts two Layers of gas, Known as laminar flow - flowing parallel to the y-axis The velocity increases from zero at the plane x=0 to longer values of M with increasing X / Laminar flow The two layers are at a distance of λ from each other. The velocities are u and u + du/dx If molecule crosses from layer P → Q in one of its free path between Collisions. It will bring to Q on the average, an amount of momentum which is lower than that for molecules in the layer P because of the distance along the x-axis. Conversely, if a molecule crosses from Q to P, the momentum added will be higher than that molecules in Layer P. The net result of such random thermal motions of the molecules is to reduce the average velocities of molecules in layer Q and increase the same for the layer P. This transfer of momentum tends to counter the velocity gradient. The length of the mean free path λ, may be taken as the average distance over which momentum is transferred Thus we have: velocity gradient = λ [du/dx] Momentum transferred by molecular mass m = m λ [du/dx]