Richard Schwane

Validation of Unsteady Turbulent Flow Predictions for Over-expanded Rocket Nozzle

The present paper reports on a validation activity, where CFD solutions for the coupling of fluctuating ambient pressure with flow separation in over-expanded rocket nozzles are validated by means of a dedicated experiment in a hypersonic facility. The experimental and CFD methodology is discussed and the related data is compared. A simplified quasi-one-dimensional CFD model has been designed to identify the physical processes that govern the interaction. The methodology is then applied to the present experimental configuration and conclusions are drawn on the severity of the observed coupling for the design of space applications.

Simulation of Unsteady Three Dimensional Separation Behaviour in Truncated Ideal Contour Nozzles

The trend toward larger area ratio nozzle extensions for main-stage propulsion means that, whereas previously, nominal sea-level operation has been assuredly in the regime where incipient separation exists at the nozzle exit, new designs approach the boundary where a transition from incipient separation to free shock separation is seen in the nozzle extension. Such separations are not necessarily stable with respect to their location within the nozzle and, as a result of this, it is possible for large side loads to develop. Such behaviour is of immediate concern as the external pressure environment (including buffeting) is far from the steady, axisymmetric ideal that would be desirable and the sideloads generated may interact and excite nozzle structural modes. A clear understanding of the flow behaviour under such conditions is therefore deemed essential with respect to further nozzle extension development. This paper documents the results of an ongoing numerical investigation into the effect of an unsteady ambient pressure environment on the flow issuing from an over expanded separated nozzle. A discussion of the motivation for the research is given first, followed by a review of prior work. The effects of in-phase and outof-phase pressure variations in the far-field are then assessed with respect to conventional and dual bell nozzle configurations. Finally, the results from the numerical work are applied to the design of a new experimental program, the aim of which is to better quantify the effects of an unsteady pressure environment on separated nozzle flow-fields.