Flow characteristics of overexpanded rocket nozzles

The current paper discusses the exhaust flow features and the various unsteady flow separation characteristics in subscale overexpanded nozzles that lead to generation of side-loads. While a DiMR type of Mach reflection features in the exhaust of a truncated ideal contour nozzle an InMR occurs in a thrust optimized parabolic nozzle. The main contributor towards generation of sideloads in a parabolic nozzle is known to be the flow transitions (free shock separation →restricted shock separation and vice-versa) and the shock unsteadiness preceding them. However, in a truncated ideal contour nozzle (that features only FSS condition) the strongest cause of side-load origin are seen to be (i) the asymmetry of separation front around the nozzle circumference, (ii) length of separation region and, (iii) length of back-flow region. The flow asymmetry causes variations in pressure rise at separation and hence, in the local value of rms fluctuations. The length of separation region also shows considerable variations with NPR and tends to peak at NPR similar to those at which peaks in (σ w/Pw)max value and strain-gauge signal occur. Data therefore suggests that that the necessary condition for side-load generation in a TIC nozzle is not just the flow asymmetry but also the length of separation region which shows significant variation as the separation front gradually approaches the nozzle exit. The length of back-flow region, on the other hand, also seems to control the large-scale fluctuations of the separation front. As the extent of back-flow region decreases, the oscillations of separation shock die down with a subsequent drop in the side-load signal even when the separation front exhibits circumferential asymmetry