Early observations by Ernst Mach used grooves in the wall of a duct to produce Mach waves in a duct to obtain data about the flow in nozzles and ducts .
A duct were then photographed by the schlieren method .
Early observations by Ernst Mach used grooves in the wall of a duct to produce Mach waves in a duct , which were then photographed by the schlieren method , to obtain data about the flow in nozzles and ducts .