Question

In a supersonic flow experiment, the supersonic flow is in a tube and is slowed down by friction with the tube wall, which eventually caused a normal shock to form and slow the gas back to subsonic range. But if no-slip condition applies to the gas, there would be a boundary layer on the wall of the tube where the gas has 0 velocity at the wall and slowly increase in speed as it gets close to the free stream velocity. But why aren't there shockwave along the wall where the gas slowly increase/decrease the speed?

Answer 1

Normal shockwaves do not form along the wall in a supersonic flow because the velocity changes within the boundary layer are smooth due to viscosity, unlike the abrupt jumps in flow properties across a shockwave.

Step-by-step explanation:

In supersonic flow experiments, the presence of a thin boundary layer where the gas velocity increases from zero at the wall to the free stream velocity is due to the viscosity of the fluid. This gradient in velocity is steady and smooth, resulting in a laminar flow, as opposed to the abrupt changes in flow properties like velocity, temperature, and pressure that occur across a shockwave. Thus, normal shockwaves do not form along the wall but rather within the flow where these abrupt changes from supersonic to subsonic speeds are necessitated by factors like changes in area or other flow restrictions.

In the scenario mentioned, the friction between the gas flow and the walls causes a gradual energy loss and thickening of the boundary layer, eventually leading to a normal shock somewhere in the flow, slowing the gas back to a subsonic range.