Contents
How do the flow properties change across a shock wave?
Shock waves are very small regions in the gas where the gas properties change by a large amount. Across a shock wave, the static pressure, temperature, and gas density increases almost instantaneously. The Mach number and speed of the flow also decrease across a shock wave.
Why shock waves only occur for supersonic flow?
Since the flow across a shock wave is irreversible, the downstream state must have the higher entropy. The upstream state is supersonic and the downstream state is subsonic. Thus, shock waves can happen only in supersonic flow and the flow becomes subsonic once it crosses a shock wave.
Do shock waves travel faster than sound?
In particular, shock waves travel faster than sound, and their speed increases as the amplitude is raised; but the intensity of a shock wave also decreases faster than does that of a sound wave, because some of the energy of the shock wave is expended to heat the medium in which it travels.
How do you know if a flow is isentropic?
A supersonic flow that is turned while the flow area increases is also isentropic. We call this an isentropic expansion because of the area increase. If a supersonic flow is turned abruptly and the flow area decreases, shock waves are generated and the flow is irreversible.
Where do shock waves form on a wing?
Normal shock waves form on the wing’s upper surface and form an additional area of supersonic flow and a normal shock wave on the lower surface. As flight speed approaches the speed of sound, the areas of supersonic flow enlarge and the shock waves move nearer the trailing edge.
How are the properties of a shock wave changed?
The change in flow properties are then given by the isentropic relations Isentropic means “constant entropy”. But when an object moves faster than the speed of sound, and there is an abrupt decrease in the flow area, the flow process is irreversible and the entropy increases. Shock waves are generated.
When does a normal shock occur in a supersonic flow?
The equations have been further specialized for a one-dimensional flow without heat addition. A normal shock occurs in front of a supersonic object if the flow is turned by a large amount and the shock cannot remain attached to the body. The detached shock occurs for both wedges and cones.
Why are enthalpy and temperature constant in a shock wave?
Because a shock wave does no work, and there is no heat addition, the total enthalpy and the total temperature are constant. But because the flow is non-isentropic, the total pressure downstream of the shock is always less than the total pressure upstream of the shock.