Numerical Study of Supersonic Exhaust Jet Flow Patterns

Kurzfassung

Traditional literature describes the fully adapted exhaust jet as the state where the flow is undisturbed and free of shocks or expansion fans. However, these definitions are contradicted by the observation that the separated flow in the nozzle always expands into an underpressure region. A cold flow test of a planar TIC nozzle conducted at the German Aerospace Centre (DLR) test facility P6.2 in Lampoldshausen showed that an oblique recompression shock wave forms at the nozzle's exit, preceded upstream by an expansion fan that further expands the flow at any full-flowing nozzle pressure ratio (NPR). The expansion fan-shock system grows larger as NPR increases. Similarly, the study did not identify shock-free exhaust jet NPR. The flow pattern was visualized using a classical b/w Schlieren system in Z-configuration across NPRs ranging from 5 to 35. The Schlieren images were validated through a CFD study employing RANS turbulence models. The CFD results showed good agreement with the Schlieren images regarding shock angle and symmetry but consistently overestimated shock intersection length at NPRs 10–17 and underestimated it at NPRs 20–35. The study highlights that there is no ideal expansion flow in practical rocket nozzle designs and underscores the inherent limitations of truncated nozzle geometries.