Numerical Assessment and Flow Behavior in Blow-down Supersonic Wind Tunnels in Starting Stage

In order to perform experiments in wind tunnels it is needed to arrive in the “run” stage, and knowing that the “run” stage is arrived after passing the triggering and “start” stage, it is necessary to study the tunnel flow in the start stage. In the start stage, which is occurring in a very short time interval in an unsteady and transient mode, some strong shock waves are formed who should rapidly pass the test section to be swallowed by the second throat of the supersonic diffuser. Otherwise, the normal shock waves formed in the test section will remain and in addition to causing severe pressure drop, will hinder the flow in the tunnel. Such a phenomenon will practically make the tunnel unusable. In this paper, the “start” stage of a blow-down supersonic wind tunnel with a Mach number of 2.8 and a nuzzle geometry designed with “method of characteristics”, has been studied under unsteady and transient conditions. Mach counters and their diagrams were plotted to assess the propagation of shock waves and achieve goal Mach number. y+ diagram was used to cross examine the modeling of the flow turbulence done in Spalart – Allmaras method. The results confirmed the accuracy of the model.