An expansion fan is the "sound" disturbance of a supersonic fluid turning around a convex corner, such that it is forced to expand. It is a result of the motion of the sound's medium relative to the disturbance.
It is analogous to a shock wave. It propagates away from the corner, carried away by the fluid (if the fluid is moving past the object) or left behind by the object (if the object is moving through the fluid). It consists (in effect) of infinitely many "low pressure" waves fanning propagating from the corner, their directions fanned out, the initial wave propagation-direction angled (Mach angle) according to the fluid's relative speed before it reached the corner, and the final based upon its relative speed after it has turned the corner.