On the Detectability of the Turbulent/Non-Turbulent Interface in Free-Shear Flow Using Combined PIV and Mie Scattering

The turbulent/non-turbulent interface (TNTI) plays a central role in turbulent entrainment and mixing processes by separating turbulent fluid from the surrounding ambient environment. Although the TNTI has been extensively investigated through direct numerical simulations, experimental studies remain comparatively scarce, particularly in high-Reynolds-number compressible flows. This limitation is largely due to the difficulty of simultaneously acquiring velocity and scalar information with sufficient spatial and temporal correspondence to accurately identify the interface. The present work introduces a unified experimental methodology for detecting and characterizing TNTI in a weakly underexpanded supersonic jet. Time-resolved particle image velocimetry (TR-PIV) measurements are combined with Mie-scattering imaging, exploiting the same image acquisition to obtain both velocity and scalar information. The recorded image intensity is used as a scalar proxy for particle concentration, enabling extraction of a TNTI candidate without additional optical instrumentation. The flow under investigation consists of a supersonic jet issuing from a circular nozzle of diameter D = 5 mm at a nozzle pressure ratio of NPR = 5, corresponding to a Reynolds number of approximately 4 × 105. Time-resolved measurements are performed in the self-similar region of the jet x/D ≈100 using a high-speed camera and continuous laser illumination. A dedicated image-processing framework based on intensity thresholding, connected-component analysis, and contour extraction is developed to identify instantaneous interface realizations. Subsequently, a signed-distance formulation is introduced to establish a local coordinate system centered on the TNTI, enabling conditional statistical analysis of scalar and velocity quantities. The proposed methodology can extract continuous TNTI realizations across the time-resolved sequence of frames. Conditional profiles of scalar intensity, velocity magnitude, and velocity components normal and tangential to the interface reveal clear transitions between the turbulent jet and the surrounding ambient fluid. The results demonstrate the feasibility of combining PIV and Mie-scattering measurements for TNTI detection in high-Reynolds-number supersonic flows and lay the foundation for future investigations of interface dynamics, intermittency, and entrainment mechanisms in compressible turbulent jets.