Passive Control of Suddenly Expanded Flow and Temperature Distribution along the Duct at Critical Mach Number

Abrupt relief increases at the blunt base are common in aerospace due to a recirculation zone that decreases pressure and raises drag. This study investigates the control of base pressure at Mach 1.0 by using quarter-ribs placed at various points within an axisymmetric duct. CFD simulations with ANSYS Fluent identified the optimal rib size, position, and nozzle pressure ratio (NPR) for either maximizing or minimizing base pressure. Air flows from a converging nozzle into a 16 mm diameter duct, with lengths ranging from L/D = 1 to 6, rib radii from 1 mm to 2.5 mm, and placements from L/D = 0.5 to 2.0. Results demonstrate that ribs have a notable impact on base pressure by interacting with the shear layer and duct wall, thereby affecting flow reattachment and pressure differences, which depend on rib size and location. Ribs at 0.5D and 1D slightly increase base pressure, while those at 1.5D and 2D are more effective. Larger rib radii enhance shear layer reattachment, further boosting pressure. Temperature analysis reveals a rise immediately behind the rib, attributed to the increased flow area, followed by a subsequent drop. These findings enable the optimization of passive control strategies in high-speed aerodynamics, thereby improving system reliability, reducing costs, increasing fuel efficiency, and supporting environmental goals.