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2005, Applied Mechanics Reviews
https://doi.org/10.1115/1.1894402…
31 pages
1 file
The past decade has seen a qualitative advancement of our understanding of physical phenomena involved in flow separation in supersonic nozzles; in particular, the problem of side loads due to asymmetrical pressure loads, which constitutes a major restraint in the design of nozzles for satellite launchers. The development in this field is to a large extent motivated by the demand for high-performance nozzles in rocket engineering. The present paper begins with an introduction to the physical background of shock-boundary-layer interactions in basic 2D configurations, and then proceeds to internal axisymmetric nozzle flow. Special attention is given to past and recent efforts in modeling and prediction, turning physical insight into applied engineering tools. Finally, an overview is given on different technical solutions to the problem if separation and side loads, discussed in the context of rocket technology.
2002
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 In addition to results available in the literature, it also contains previously unpublished material based on this author's work, whose main contributions are (i) discovery the role of transition between different separation patterns for side-load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.
A numerical study, using a mixed finite element/finite volume method on unstructured meshes adapted for compressible flows, is conducted to investigate turbulent boundary-layer separation in overexpanded subscale supersonic nozzles including shock/shock and shock/boundary layer interactions. Two test-cases are investigated, namely a TIC (Truncated Ideal Contour) nozzle and a TOP (Thrust Optimized Parabolic contour) nozzle with a secondary jet injection. Particular attention is paid to the appearance of a recirculation region downstream of the Mach disc as well as in the vicinity of the secondary nozzle exit. The results so obtained are analyzed and compared with the experimental data. The results suggest that very different shock structures and flow separation may appear depending on the nozzle contour as well as on the operating pressure and temperature ratios. In the case of TOP nozzle, the simulations reveal the existence of a small recirculation bubble at the vicinity of the secondary injection, due to a shock/boundary layer interaction. In addition, it has been shown that, at high temperature ratios, compressibility effects on the growth rate of the mixing layer, which develops between the main stream and the secondary jet injection, become significant and cannot be neglected in the computation.
European Journal of Mechanics - B/Fluids, 2007
Time average shock-induced boundary layer separation is investigated using scale analyses, analytical modeling, and experiments. While the study focuses on turbulent boundary layer separation in overexpanded rocket nozzles, many of the analyses presented apply to the broad family of free interaction, shock-separated flows in which the structure of the boundary layer-shock interaction zone is self-similar and independent of the shock generator. The scale analyses lead to two approximate expressions for the wall pressure ratio at separation; over a range of separation Mach numbers, both models provide reasonable predictions of observed separation pressure ratios. The second model, representing a refinement of the first, appears to provide a fairly general description of free interaction separation: the model approximately captures separation pressure ratios observed in supersonic flow over backward facing steps and in the case of overexpanded nozzle flow, provides predictions that are consistent with the free interaction model. Experiments are carried out in a sub-scale nozzle under overexpanded, cold-flow conditions. The principal observations are as follows: (i) For the range of separation Mach numbers investigated (5.0 M i 5.4), nominal separation line locations can be predicted with reasonable accuracy using the classical generalized quasi-one-dimensional compressible flow model and an appropriate separation criterion. (ii) Over the same range of overexpanded flow conditions, the time-average pressure rise over the shock interaction zone can be accurately fit by the free interaction model.
Open Physics
This article aims to conduct a numerical investigation of phenomena induced by gas expansion in chemical propulsion nozzles. A numerical simulation of full-scale flat convergent-divergent nozzle geometry using the finite volume method on structured meshes is performed to predict the change in the convergent geometry on the boundary layer separation resulting from a shock/shock and shock/boundary layer. Two turbulence models are tested, namely, the k−ε and k−ω shear-stress transport (SST) models. Three steps are considered to achieve this work. First, 10 numerical schemes are tested to select the accurate one. The findings of the first step are used to predict the boundary layer separation in a supersonic overexpanded nozzle. The available experimental data from the NASA Langley Research Center are used to validate the results. The third step concerns investigating the impact of the convergent geometric profile on the downstream flow of the nozzle. The obtained results are analyzed a...
Shock Waves, 2009
In the case of high overexpansion, the exhaust jet of the supersonic nozzle of rocket engines separates from nozzle wall because of the large adverse pressure gradient. Correspondingly, to match the pressure of the separated flow region, an oblique shock is generated which evolves through the supersonic jet starting approximately at the separation point. This shock reflects on the nozzle axis with a Mach reflection. Thus, a peculiar Mach reflection takes place whose features depend on the upstream flow conditions, which are usually not uniform. The expected features of Mach reflection may become much difficult to predict, depending on the nozzle shape and the position of the separation point along the divergent section of the nozzle.
International Journal of Engineering Research and, 2020
Flow separation in a supersonic contoured nozzle whilst operating under over expanded regime is an inevitable fluid dynamic phenomenon. The flow field comprise formation of complex shock structures and its interaction with the viscous boundary layer. Profound number of researches on various types of contoured nozzle profiles have been undertaken to better understand the said phenomenon. The present study is focused on further understanding the fundamentals associated with formation of shocks and its structural transformation under varying NPR for a defined area ratio. Flow visualization utilizing Schlieren photography has enabled to capture the details of shock and its structure along with other phenomenon viz boundary layer separation, shock boundary layer interaction, aftershocks etc. The locations of shock captured experimentally have also been ascertained with computational generated data for various NPR and the results have been found quite comparable. Keywords—Nozzle, superson...
IJRAME PUBLICATIONS, 2025
The convergent-divergent design of the nozzle of a rocket engine is the most defining part of it. The nature of the nozzle design enables it to convert the high pressure, temperature and relatively low velocity adiabatic expanding gaseous fluids into very high velocity and thrust gases. The rapidly exiting gas as a consequence of the rapid expansion process results in low pressure and temperature. It is critical in research to find out the influence of modification (grooves) would have in the application and design of a reaction device such as a rocket engine. The Computational Fluid Dynamics software, Analysis System (ANSYS) was employed together with necessary boundary conditions to design and simulate such a modified rocket engine nozzle. The interaction and relationship existing among the pressure, velocity, Mach number and temperature in the grooved divergent rocket nozzle was found and observed. From the plots and nozzle simulation contours, both velocity and Mach number increased rapidly along the nozzle area but decreased steadily. Pressure varied directly withtemperature and inversely with density. It is observed that divergent nozzle grooving influences the velocity, Mach number, pressure and temperature within the divergent nozzle area of a rocket nozzle.
Turbulence Models of Separated Flow in Shock Wave Thrust Vector Nozzle, 2013
In the present paper, based on typical jet flows in shock wave thrust vector nozzles, turbulence modeling in gas flow dynamics has been numerically explored in 2-D laval nozzle which has a secondary injection on one side of its divergent portion in order to simulate the complex strong pressure gradient and accurately capture flow separation point. Nine turbulence models have been adopted and assessed by comparing the obtained results which involve flow separation point and static wall pressure with the available experimental data. Another 3-D nozzle model is simulated using five different turbulence models and results are also compared with experimental data. The numerical results reveal that Goldberg's realizable k-epsilon model gives the best results compared with other models in predicting the shock wave position and separation point, while the SA model shows its advantage in predicting the static wall pressure under certain conditions compared to the Goldberg's realizable k-epsilon model. The computational results were analyzed and the change of the shock structure in different NPRs was discussed in laval nozzle with a secondary flow. Keywords: turbulence model, thrust vector nozzle, static wall pressure, shock wave
Shock Waves, 2005
The unsteady aspects of shock-inducedseparation patterns have been investigated inside a Mach 2 planar nozzle. The mean location of the shock can vary by changing, relatively to the nozzle throat, the height of the second throat which is positioned downstream of the square test section. This study focuses on the wall pressure fluctuations spectra and the unsteady behaviour of the shock. Symmetric shock configurations appear both for the largest openings of the second throat, and for the smallest openings. For an intermediate opening the shock system exhibits asymmetrical configurations. A coating with roughnesses sticked on the throat part of the nozzle in order to modify the state of the incoming boundary layers (from smooth to rought turbulent statement) is a driver for the asymmetry. The fluctuating displacements of the shock patterns were analysed by using an ultra fast shadowgraph visualization technique. A spectral analysis of the unsteady wall pressure measurements has revealed low frequency phenomena governed by large structure dynamics in the separated flows.
Flow Turbulence and Combustion, 2003
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.
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