The increasing demand for higher performance in rocketlaunchers promotes the development of nozzles with higherperformance, which basically is achieved by increasing theexpansion ratio. However, th ...

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Supersonic Flow Separation with Application to Rocket Engine Nozzles

The increasing demand for higher performance in rocket launchers promotes the development of nozzles with higher performance, which is basically achieved by increasing the expansion ratio. However, this may lead to flow separation and ensuing unstationary, asymmetric forces, so-called side-loads, which may present life-limiting constraints on both the nozzle itself and other engine components. Substantial gains can be made in the engine performance if this problem can be overcome, and hence different methods of separation control have been suggested, however none has so far been implemented in full scale, due to the uncertainties involved in modelling and predicting the flow phenomena involved. The present thesis presents a comprehensive, up-to-date review of supersonic flow separation and side-loads in internal nozzle flows with ensuing side-loads. In addition to results available in the literature, it also contains previously unpublished material based on this author’s work, whose main contributions are (i) discovery the role of transition between different separation patterns for side-load generation, (ii) experimental verification of side-loads due to aeroelastic effects and (iii) contributions to the analysis and scaling of side-loads. A physical description of turbulent shock wave boundary layer interactions is given, based on theoretical concepts, computational results and experimental observation. This is followed by an in-depth discussion of different approaches for predicting the phenomena. This includes methods for predicting shock-induced separation, models for predicting side-load levels and aeroelastic coupling effects. Examples are presented to illustrate the status of various methods, and their advantages and shortcomings are discussed. The third part of the thesis focuses on how to design sub-scale models that are able to capture the relevant physics of the full-scale rocket engine nozzle. Scaling laws like those presented in here are indispensable for extracting side-load correlations from sub-scale tests and applying them to full-scale nozzles. The present work was performed at VAC's Space Propulsion Division within the framework of European space cooperation.

Flow Processes in Rocket Engine Nozzles with Focus on Flow Separation and Side-Loads

Separation of supersonic flow in a planar convergent–divergent nozzle with moderate expansion ratio is investigated by solving the Reynolds-averaged Navier–Stokes equations with a two-equation k-!turbulence model. The focus of the study is on the structure of the fluid and wave phenomena associated with the flow separation. Computations are conducted for an exit-to-throat area ratio of 1.5 and for a range of nozzle pressure ratios. The results are compared with available experimental data in a nozzle of the same geometry. The flow separates by the action of a lambda shock, followed by a succession of expansion and compression waves. For 1:5 < NPR < 2:4, the computation reveals the possibility of asymmetric flow structure. The computationally obtained asymmetric flow structuresareconsistentwithpreviousexperimental flowvisualizationsstudies.Inaddition,other flowfeaturessuch asshocklocationandwallpressuredistributionsarealsoingoodagreementwiththeexperimentaldata.Thepresent study provides new information that confirms earlier conjectures on the flow–wave structure relevant to the instability of the separated flow in convergent–divergent nozzles of moderate expansion ratio.

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Numerical investigation of supersonic nozzle flow separation

Turbulent flow separation in over-expanded rocket nozzles is investigated experimentally in a sub-scale model nozzle fed with cold air and having a thrust-optimized contour. Depending upon the pressure ratio either a free shock separation (FSS) or a restricted shock separation (RSS) is observed with a significant hysteresis between these two flow regimes. It is shown that the RSS configuration may involve several separated regions. Analysis of wall pressure fluctuations give quantitative information on the fluctuating pressure field directly connected with the occurrence of significant side loads. Direct measurements of the evolution of the side loads with respect to the pressure ratio show the occurrence of three distinct peaks which are explained by the wall pressure fluctuations measurements.

Unsteadiness of Flow Separation and End-Effects Regime in a Thrust-Optimized Contour Rocket Nozzle

Flow separation in supersonic convergent–divergent nozzles has been the subject of several experimental and numerical studies in the past. Today, with the renewed interest in supersonic flights and space vehicles, the subject has become increasingly important, especially for aerospace applications (rockets, missiles, supersonic aircrafts, etc.). Flow separation in supersonic nozzles is a basic fluid-dynamics phenomenon that occurs at a certain pressure ratio of chamber to ambient pressure, resulting in shock formation and shock/turbulent-boundary layer interaction inside the nozzle. From purely gas-dynamics point of view, this problem involves basic structure of shock interactions with separation shock, which consists of incident shock, Mach reflections, reflected shock, triple point and sliplines (see Figs. 1, 2). Several viscous phenomena, such as boundary layers with adverse pressure gradients, induced separation, recirculation bubbles, shear layers may additionally occur and can strongly affect the flow-field inside the nozzle (see Figs. 3, 4). Previous studies on supersonic nozzles [1,2] have shown that shock-wave/boundary layer interaction (SWBLI) occurring in highly overexpanded nozzles may exhibit strong unsteadiness that cause symmetrical or unsymmetrical flow separation. In rocket design community, shock-induced separation is considered undesirable because an asymmetry in the flow can yield dangerous lateral forces, the so-called

Nozzle flow separation

The operation of rocket engines in the overexpanded mode, that is, with the ambient pressure considerably higher than the nozzle exit wall pressure, can result in dangerous lateral loads acting on the nozzle. These loads occur as the boundary layer separates from the nozzle wall and the pressure distribution deviates from its usual axisymmetric shape. Different aerodynamic or even coupled aerodynamic/structural mechanic reasons can cause an asymmetric pressure distribution. A number of subscale tests have been performed, and three potential origins of side loads were observed and investigated, namely, the pressure fluctuations in the separation and recirculation zone due to the unsteadiness of the separation location, the transition of separation pattern between free-shock separation and restricted-shock separation, and aeroelastic coupling, which indeed cannot cause but do amply existing side loads to significant levels. All three mechanisms are described in detail, and methods are presented to calculate their magnitude and pressure ratio at which they occur.

Jan Östlund

Papers

Supersonic Flow Separation with Application to Rocket Engine Nozzles

Side-Load Phenomena in Highly Overexpanded Rocket Nozzles

Assessment of turbulence models in overexpanded rocket nozzle flow simulations