- Abstract This paper presents numerical and experimental findings regarding flow separation phenomenon in a high Mach number over expanded planar nozzle. The experimental work is done using a tapered nozzle with a variable area ratio that can produce separation Mach numbers in the range 2.6-3.5. Shadowgraph visualization reveals that depending on the nozzle pressure ratio and the area ratio, steady symmetric, unsteady symmetric and steady asymmetric separations can occur. These separation types are reproduced by numerical simulations. The unsteady separation produces a sharp tone, which does not match the characteristics of familiar tone producing mechanisms. The transition to asymmetric separation results in attachment of the jet to the nozzle wall, and the reverse transition is accompanied by a hysteresis effect.