Passive control of base pressure and the flow in a duct with sudden expansion at sonic and supersonic mach numbers

Abstract

The pressure in the base area of the enlarged duct and the backward-facing step of any moving projectile is usually less than the ambient pressure. It is essential to decrease the base drag by raising the base pressure. The present investigation is concerned with controlling base pressure using an annular cavity in the enlarged duct as a passive control mechanism. This investigative work will be valuable for space & defense and automobile engineering, as a stream is split at the blunt base. The computational fluid dynamics (CFD) investigation has been carried out in this research to examine the effect of expansion level, cavity size, and location on controlling base pressure. The variable quantities for the analysis are the nozzle pressure ratio (NPR), Mach number (M), cavity location (C), area ratio, and length-to-diameter (L/D) ratio of the duct. The theoretical framework establishes three conditions for the stream through a convergent-divergent nozzle: ideal expansion, under-expansion, and over-expansion. These conditions are intricately linked to the behavior of the boundary layer, reattachment length, and the strength of the primary vortex, all of which influence base pressure values. The study reveals nuanced insights by applying this framework to three specific area ratios (2.89, 4, and 5.29). For an area ratio of 2.89, cavity placement at 0.5D proves highly effective in regulating base pressure, showcasing the significance of Mach number, L/D ratio, NPR, and cavity location. The cavity's efficiency is attributed to its ability to counteract over-expansion conditions, impacting reattachment length and the strength of the primary vortex. For an area ratio of 4.0, the cavity demonstrates viability for base flow control at NPR = 4, with negligible impact at other NPR values due to over-expansion or highly under-expanded conditions. The study underscores the substantial influence of duct diameter, Mach number, L/D ratio, NPR, and cavity location on base pressure control, as evidenced in Taguchi's main effect plots. For an area ratio of 5.29, the cavity's effectiveness in manipulating base pressure is highlighted at NPR values of 4 and 6 for Mach numbers 1.2 and 1.4. The research underscores the intricate interplay between cavity placement, NPR values, and Mach numbers, revealing the complex relationship governing base pressure manipulation. In conclusion, this research provides valuable insights into the intricate flow dynamics within an enlarged duct. It emphasizes the pivotal role of cavity placement, NPR values, and Mach numbers in influencing base pressure, offering practical guidance for optimizing duct designs in engineering applications. The findings acknowledge the nuanced interactions of various parameters in controlling base pressure, contributing to a deeper understanding of this complex fluid dynamics phenomenon.