Near-field flow features of underexpanded microjets emerging from a rectangular convergent nozzle with a high-aspect-ratio

Underexpanded microjets emerging from a rectangular convergent nozzle with an aspect ratio of 10 at the exit face are quantitatively visualized via Mach–Zehnder tomography (MZT) with the finite-fringe setting. Multiview interferograms around the central axis of the microjet are taken at a nozzle pressure ratio (NPR) of 4.0. The Reynolds number based upon the height in the minor-axis plane and flow properties at the nozzle exit is 1.74 × 10 4 . Fourier fringe analysis is utilized to extract the phase shifts between the background fringe and deformed fringe patterns. The density fields of the microjets are subsequently reconstructed via the convolution back-projection (CBP) method. Near-field flow features of the microjets are demonstrated with the isopycnic surface as well as the density contour plots in the minor-axis and major-axis planes. In addition, microjets issuing from the nozzle with the same geometry as in the experiment are solved via the Reynolds-averaged Navier–Stokes (RANS) equations with Menter’s shear stress transport (SST) k − ω turbulence model for mutual comparison with the experimental results. The streamwise density profiles along the jet centerline as well as the liplines in the minor-axis and major-axis planes are quantitatively compared between the experimental and simulation results. Shock structures and topology showing the streamwise evolution of a shock system in a high-aspect-ratio rectangular microjet are experimentally clarified for the first time with a high spatial resolution of approximately 5 . 0 μ m . • Three-dimensional density field of shock-containing rectangular microjets is measured at high-spatial resolution using Mach–Zehnder tomography. • Shock structure and flow topology of underexpanded rectangular microjets are experimentally clarified for the first time. • RANS predictions show good quantitative agreement with experimental results in the near-field region.