Internal Flow Measurements in Converging Ducts with Favorable and Localized Adverse Pressure Gradients

This paper presents internal flow measurements in two shape-transitioning nozzle ducts using planar Particle Image Velocimetry (PIV). Nozzle 1 is a converging nozzle transitioning from a square cross-section to a rectangular exit with an equivalent diameter (De) of 2.2 in, an exit aspect ratio of 6.68, and a length-to-diameter ratio (L/De) of 7.8. Nozzle 2 also converges globally but incorporates a diverging sidewall that introduces localized adverse pressure gradients. Both nozzles are tested at an exit Mach number of 0.2, corresponding to ReDe≈2.50×105. Wall-normal velocity profiles reveal boundary layer thinning under favorable pressure gradients followed by thickening in regions of streamwise curvature and local adverse pressure gradients. In nozzle 2, the adverse streamwise pressure gradient along the diverging wall produces thicker boundary layers than in nozzle 1, while a cross-stream pressure imbalance shifts the velocity peak toward the diverging wall. Complementary steady RANS simulations using the k–ω SST turbulence model yield wall-normal velocity profile agreement within 2% mean absolute error for both nozzles in the upstream and mid-duct regions, with errors increasing toward the exit. Discharge coefficients from CFD and experiment agree within approximately 1%, with nozzle 1 exhibiting greater integrated losses than nozzle 2 despite thinner boundary layers at the measured plane, indicating a three-dimensional loss distribution. Independent pitot probe measurements at the nozzle exit confirm the PIV trends over the CFD predictions in the near-exit region.