Pulsatory phenomenon in a thrust optimized contour nozzle

A numerical study of the flow in an axisymmetric overexpanded thrust optimized contour nozzle is presented. The separation flow structures at different pressure ratios are investigated. The start-up process exhibits two different shock structures. For a range of pressure ratios, hysteresis phenomenon occurs between these two separation patterns. For a larger pressure ratio, where the principal separation point is always inside the nozzle, another phenomenon appears. This phenomenon results in an oscillatory longitudinal quasi periodic movement of the separation structure. The computed nozzle wall pressures show a correct agreement with the experimental measurements and the pulsations frequency of the oscillatory phenomenon is also well predicted.