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search Unstart of axisymmetric inlet Unstarted shock structure
evident on left, Started inlet on right.  Unstart, is technical
term used in supersonic aerodynamics to refer to a generally
violent breakdown of the supersonic airflow.[1] Original use
came about as wind tunnels were designed that could drive the
airflow supersonic. 'Starting' the supersonic wind tunnel is
therefore the process in which the air becomes supersonic.
Unstart of the wind tunnel is the reverse process.[2] The shock
waves that develop during the starting or unstart process are
often visualized with schlieren or shadowgraph optical
techniques.  As airplanes began to achieve supersonic flight, an
inlet that delivers proper airflow to an engine became a major
complicating element to aircraft design. In a simple sense,
these supersonic inlets had to be carefully designed like a
supersonic wind tunnel. At high supersonic speeds, (usually
between Mach 2 to 3), the inlet is designed t