Unstart

Unstart, is technical term used in supersonic aerodynamics to refer to a generally violent breakdown of the supersonic airflow. 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. The shock waves that develop during the starting or unstart process are often visualized with schlieren or shadowgraph optical techniques.