The starred conditions occur when the flow is choked and the Mach number is equal to one. Notice the important role that the Mach number plays in all the equations on the right side of this slide. If the Mach number of the flow is determined, all of the other flow relations can be determined.Similarly, determining any flow relation (pressure ratio for example) will fix the Mach number and set all the other flow conditions.e 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. The isentropic relations are no longer valid and the flow is governed by the oblique or normal shock relations.