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A jet is said to be over expanded when the nozzle exit pressure (pe) is lower than the ambient pressure (pa) to which it is discharging. Oblique shocks will be formed to increase pe attempting to reach equilibrium with the ambient pressure pa. These shockwaves of opposite families starting at the nozzle trailing edge, cross each other at the jet axis and travel to the jet boundary and are reflected as expansion waves. These shock and expansion waves cause a periodic shock-cell structure in the jet flow field. Figure 1.1 schematically shows the shock-cell structure in an over expanded jet.