When analysing flow through a supersonic nozzle we draw the characteristics which are at an angle to the streamlines. Are these characteristics nothing but shock waves?

Thank you

No, they are not. A shock forms where the characteristics (from the same family) collide one another.

Characterstics are structure along which perturbation propagate (expansion, compression or contact discontinuity). Moreover it's not said that two impinging characteristic of the same family will generate a shock ... but more realistically you have to solve the so-called Riemann problem to obtain more information. E-mail me for further explanations. Bye

A characteristic is a line along which an infintely weak shock, called a Mach wave, would lie if you placed a very small perturbation in the flow at the point of origin of the characteristic. You can draw two characteristics (a left running one and a right running one)eminating from any point in a (supersonic) flow field. They are useful because the govering equations describing the flow reduce from partial diffs to ordinary diffs along the characteristic. Hence by drawing a mesh of intersecting characteristics you can propagate an flow field solution from upstream to downstream. This is known as the method of characteristics