Shock wave visualization in a 3D-printed axisymmetric critical flow venturi nozzle using Background-oriented Schlieren technique

The shock wave visualization and its quantitative location measurement were performed with a 3D-printed axisymmetric critical flow venturi nozzle (CFVN). One of the challenges of this research is to develop visualization techniques for the conditions under which strong refraction occurs at the wall such as gas flow in an axisymmetric flow field. The axisymmetric CFVN made of transparent material is used to demonstrate the technique. CFVNs complying with ISO 9300 is widely used as flow meters and transfer of gas flow standard. Several experimental results indicate that the supersonic resonance between shock wave and boundary layer in CFVN slightly changes the flow rate．This phenomenon limit the CFVN operation condition and a major factor that undermines convenience of the CFVN. However, shock wave behavior in the axisymmetric CFVN is one of the unsolved issues. In this paper, the characteristics of the discharge coefficient of the 3D-printed axisymmetric CFVN for the supersonic flow fields were evaluated. Subsequently, the capability to visualize the shock wave in the CFVN was confirmed. As a result, the relationship between shock wave location and pressure ratio was clarified quantitatively.