I am looking at a solution of an engineering problem. There is a blast wave that moves through standard air conditions. The wave passes a stationary observer on the ground.
The problems asks to calculate mach number and direction of flow
It calculates Mach 1 = 10 , Mach 2 = 0.387
then finds u1 = 3557 , u2 = 0.38757
then it states : for stationary reference frame
V2 = u1 - u2 = 2935.1
calculates M2 = v2/a2 = 1.8270
thus V2 is directed to shock front
my question is : what is the reasoning behind this assumption that v2 is directed to shock front ?
Any input would be appreciated.