Influence of downstream unsteadiness on shock pattern in separated nozzle flows

Abstract

The flow pattern of supersonic nozzles has been investigated numerically. The objective of the study is to identify the origin of low-frequency oscillations and side-loads generation in over-expanded nozzles. Examples of a thrust-optimized contour (TOC) nozzle with free shock separation (FSS) and restricted shock separation (RSS) are considered to study the effect of the downstream flow conditions on nozzle pressure fluctuations and flow asymmetry. A complex flow structure with a massive jet separation and a complex shock pattern is observed. Special attention is paid to the behavior of the flow near the nozzle lip. Three different dynamic structures with two vortex rings and large scale jet motion are observed. The interaction between these turbulent structures and their influence on the flow dynamics is analyzed. The computational results are compared with the experimental data for validation. The flow evolution is proved to be complex due to the presence of large scale turbulent motion, jet unsteadiness and flow recirculation. In particular, the flapping of the supersonic jet associated with the breakdown of the coherence of large structures and its interaction with the nozzle-lip vortex strongly influences the flow recirculation and is characterized by low frequencies. In case of transient simulation (start-up), similar phenomena occur and strongly influence the flow topology.