Development and Commissioning of a Supersonic Blow Down Wind Tunnel for Educational Purposes

A cost-effective test rig is presented that allows for the experimental investigation of supersonic flows for educational purposes. The individual units for the test rig were designed and built by students as part of their degrees. The test rig allows for operating times up to 10 seconds and features a nozzle test section, that can house different test objects. The divergent part of the de Laval nozzle geometry is designed using the method of characteristics for planar two-dimensional supersonic flow. State of the art 3D printing technology has been utilized to manufacture the nozzle geometry. Both optical and pneumatic measurement techniques have been adopted for the current setup. A z-type schlieren setup with two parabolic mirrors is used to perform flow visualization. The entire run can be recorded with a digital high speed camera. Stagnation pressure and temperature are measured in the pressure reservoir. Measurements are used to demonstrate basic thermodynamic effects such as the depressurization of gas-filled pressure vessels. Schlieren photography is used to graphically derive the Mach number and some aspects of Mach waves, oblique shock waves, and expansion waves are discussed. Finally, some effects of surface roughness on the flow field are addressed in this paper. Initial tests with the untreated nozzle geometry led to a fine pattern of very weak oblique shock waves in the supersonic part of the nozzle, that were caused by the finite layer thickness of the printer.