Appearance of Restricted Shock Separation in Rocket Nozzles

Abstract

Uncontrolled ow separation in nozzles of rocket engines is not desired because it can lead to dangerous lateral forces. Different origins for side loads were identi ed in the past. Meanwhile, it is proven that in thrust-optimized or parabolic nozzles, a major side load occurs as a result of the transition of separation pattern from free shock separation to restricted shock separation and vice versa. Reasons for the transition between the separation patterns are discussed, and the cap-shock pattern, which is identi ed to be the cause of this transition, is closely analyzed. It turns out that this pattern can be interpreted as an inverse Mach re ection of the internal shock at the nozzle axis. To prove the transition effect as main side-load driver, a subscale test campaign has been performed. Two different nozzle contours, a thrust-optimized and a truncated ideal nozzle with equal performance data, were tested. Highest side loads were measured in the thrust-optimized nozzle, when the separation pattern changes from free to restricted shock separation. Side loads measured in the truncated ideal nozzle were only about one-third as high as in the thrust-optimized nozzle.Nomenclature I sp = speci c impulse l = length p a = ambient pressure p c = chamber pressure p w = wall pressure r t = throat radius x = length°= ratio of speci c heats " = area ratio µ = ow de ection angle across oblique shock Á = ow angle

Wall Pressure Unsteadiness and Side Loads in Overexpanded Rocket Nozzles

“…These illustrations are based on computational simulations performed at NASA Marshall Space Flight Center (MSFC) for this particular TOP nozzle. [29] In general, these schematics are similar to the models presented by Frey & Hagemann (2000) [4], Hagemann et al (2002) [9] and Verma & Haidn (2009) [10]. However, deviations are attributed to the differences in the TOP nozzle contours, NPR values and test environments; the sub-scale nozzles used in experimental campaigns exhaust in a diffuser and ejector pipe that have different designs.…”

“…As an illustration, the Fourier modes at a single position x j =x and one time step t =t are presented in figure 11 according to the parameterization in polar coordinates as shown in Eq. (9). Only the first mode (m = 1 & m = −1) is responsible for side loads as indicated by the square which represents the peak of the resultant force.…”

Numerical study of shock/boundary layer interaction in supersonic overexpanded nozzles

“…Several experimental studies, performed on either subscale [17,26,27] or full-scale [25] optimized nozzles, corroborated by different numerical simulations [6,7,13], demonstrated the existence of two distinct separation processes, namely the Free Shock Separa- tion (FSS), in which the boundary layer separates from the nozzle wall and never reattaches, and the Restricted Shock Separation (RSS) characterized by a closed recirculation bubble, downstream of the separation point, with reattachment on the wall. These two type of flow separation occur during transient startup or shutdown even if the nozzle operates with full-flow at steady-state chamber pressure.…”

Numerical simulation of shock-induced separated flows in overexpanded rocket nozzles

“…Later on, Chen et al [3] performed numerical calculations for the J-S2 nozzle and were able to capture the hysteresis §ow ¦eld for a wide range of NPRs. Reasons for the transition between the separation patterns have been discussed by Hagemann et al [11] who identi¦ed the capshock pattern to be the cause of this transition.…”