Experimental Study on the Instability Characteristics of Free Jets with Varying Nozzle Wall Roughness

To explore the impact of nozzle inner wall roughness on the instability characteristics of free jets in spacecraft attitude and orbit control engines，a fluid polishing technique is implemented. This technique employs abrasives with varying grain sizes and polishing durations to precisely control the roughness of the nozzle inner walls. Experimental investigations on free jets are carried out under diverse jet velocity conditions. The findings reveal that fluid polishing markedly diminishes the roughness of the nozzle inner walls. Notably，the nozzle’s flow coefficient increases with prolonged polishing time and the use of finer abrasive grains，which concurrently leads to a decrease in the overall supply system pressure. During the progression of jet instability，two ineffective modes are identified：low-speed “oscillation” and high-speed “divergence”，both linked to the polishing process. Additionally，a method for extracting jet surface wave amplitude based on image processing technology is introduced and validated. The application of fluid polishing in nozzle processing and treatment is demonstrated to effectively enhance nozzle performance and stabilize jet behavior，offering substantial theoretical backing for optimizing nozzle processing techniques and improving engine performance.