Mach Diamonds

[Ray]

Mach diamonds are the result of not returning the exhaust gasses to ambient pressures. A nozzle is a tranducer that takes low velocity, high pressure gas and converts it to high velocity, low pressure gas. During the process the acceleration is used to produce thrust in the opposite direction.

A well designed nozzle should return the exhaust gasses back to the same pressure as the surrounding environment, vacuum in space, atmospheric pressue in the atmosphere.

Mach diamonds represent energy loss.

You can create mach diamonds by under-expanding or over-expanding the nozzle.

You can read more here.

[Hank]

Hello- Mach Diamonds are a function of the boundary layer in the throat of the nozzle. As with any fluid moving through a containment the fluid in contact with the wall is going to move slower. This is due to the friction induced in the fluid by contact with the wall and stiction, the interaction of the fluids directly impinging on the wall with the gases farther towards the center of the flow. The fluids at the center of the flow are affected least by the forces mentioned, they move the farthest in the least time and form the point of the Diamond.
As Ray stated, these gasses will move towards equilibrium (ambient). Given the great speed and mass of the flow of a rocket engine it takes some time for this to happen. Hence we see nozzle throat conditions external to it.

Regards, Hank