EXPERIMENTAL CHARACTERIZATION AND POTENTIAL CORE TOPOLOGY OF A SUBSONIC FREE JET FOR AERODYNAMIC PROBE CALIBRATION

Accurate aerodynamic probe calibration requires a flow environment with high mean flow uniformity. This study presents the experimental characterization and topological mapping of the potential core in a 70 mm diameter subsonic free jet facility developed for multi-hole probe calibration. Using a custom-designed automated traverse system with a positioning repeatability of <0.10 mm, the flow field was mapped at Mach numbers of 0.3, 0.5, and 0.8 across axial distances ranging from Z\/D=0.1 to 3.0. Pressure scans with an average spatial resolution of approximately 5.1 mm (0.073D) revealed that the facility produces a highly symmetric top-hat total pressure profile. A key finding is the self-similarity of the flow; the normalized centerline pressure profiles collapsed onto a single curve independent of the Mach number in the near field at Z/D = 0.5 and 1.0. However, three-dimensional topological reconstruction of the potential core (P_t \/P_0 ≥0.99) demonstrated that the core length is inversely related to flow speed, exhibiting significant contraction at Mach 0.8 due to enhanced shear layer mixing. The results indicate that while the facility is valid for calibration across the subsonic regime, probes must be positioned within Z\/D ≤1.5 to ensure placement within the uniform potential core at high subsonic speeds.