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2002
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130 pages
1 file
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 In addition to results available in the literature, it also contains previously unpublished material based on this author's work, whose main contributions are (i) discovery the role of transition between different separation patterns for side-load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.
International Journal of Engineering Systems Modelling and Simulation, 2011
The ATAC group was created in the late 1990s on CNES and ONERA's initiative, and in cooperation with French laboratories and industrials, to investigate aerodynamics issues for space launchers nozzles and blunt bodies. This paper presents a synthesis of the research work done in the frame of the ATAC programme to better understand the flow separation phenomenon in over expanded nozzles.
Aerospace Science and Technology, 2006
An experimental test campaign has been carried out on a sub-scale thrust optimized parabolic (TOP) nozzle to study the relation between unsteady characteristics of separation and reattachment shocks and the origin of side-loads in rocket nozzles. The study was conducted using wall pressure measurements both in streamwise and circumferential directions, surface oil visualization technique and signals from strain gauges installed on the nozzle bending tube. It is observed that the nozzle pressure ratio at which peaks in maximum rms values occur, for separation and reattachment shocks, coincide with the nozzle pressure ratio at which peaks in strain gauge signal are observed. This clearly demonstrates the unsteady nature of separation and reattachment shocks to be directly related to origin of side-loads in rocket nozzles.
Applied Mathematical Modelling, 2011
In the present study, the turbulent gas flow dynamics in a two-dimensional convergentdivergent rocket nozzle is numerically predicted and the associated physical phenomena are investigated for various operating conditions. The nozzle is assumed to have impermeable and adiabatic walls with a flow straightener in the upstream side and is connected to a plenum surrounding the nozzle geometry and extended in the downstream direction. In this integrated component model, the inlet flow is assumed a two-dimensional, steady, compressible, turbulent and subsonic. The physics based mathematical model of the considered flow consists of conservation of mass, momentum and energy equations subject to appropriate boundary conditions as defined by the physical problem stated above. The system of the governing equations with turbulent effects is solved numerically using different turbulence models to demonstrate their numerical accuracy in predicting the characteristics of turbulent gas flow in such complex geometry. The performance of the different turbulence models adopted has been assessed by comparing the obtained results of the static wall pressure and the shock position with the available experimental and numerical data. The dimensionless shear stress at the nozzle wall and the separation point are also computed and the flow field is illustrated. The various implemented turbulence models have shown different behavior of the turbulent characteristics. However, the shear-stress transport (SST) k-x model exhibits the best overall agreement with the experimental measurements. In general, the proposed numerical procedure applied in the present paper shows good capability in predicting the physical phenomena and the flow characteristics encountered in such kinds of complex turbulent flow.
Applied Mathematical Modelling, 2017
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights Benchmark results for reactive and non-reactive compressible flows. Comparison of different physical models (one-species and multi-species) for compressible reactive flows. Numerical error evaluation for compressible reactive flows. Effects of physical models on global (thrust, specific impulse) and local (temperature, pressure) variables of interest. Validation and verification procedures for compressible reactive 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.
2019
The interaction of supersonic underexpanded exhaust jets for a four-nozzle clustered rocket configuration is studied using three-dimensional RANS simulations. Such interactions can lead to reversed flow of the nozzle exhaust gases towards the base, and result in high pressure and heat loads on the vehicle. Multi-jet interaction flow fields are characterised by a number of shocks and expansion waves, along with turbulent shear layers enclosing subsonic recirculating zones. The complexity of the flowfield is broken down by identifying various viscous/inviscid interactions, with particular focus on the interaction of turbulence with shockwaves. The shock-unsteadiness k-ω turbulence model is used to capture the physics of shock-turbulence interaction, and its effect on the multi-jet flow field.
Journal of Thermal Science, 2007
Gasdynamics in micro-rocket engines for satellites involves complex aspects regarding how to perform efficient simulations. It is well known that gas flow in the continuum regime -very low Knudsen (Kn) numbers-is accurately computed with Navier-Stokes solvers. Nevertheless, rarefaction due to the small geometrical scale may lead the continuum description to be inaccurate or even fail. This is the situation found in aerospace Micro-ElectroMechanical Systems (MEMS), where the characteristic length scale of the flow approaches the molecular mean free path, thus invalidating the continuum description and enforcing the use of particle methods, like the Direct Simulation Monte Carlo (DSMC), to deal with the non-equilibrium regions. Within the slip-regime (0.01<Kn<∼0.1) both approaches, continuum and particle-based, seem to behave well in terms of accuracy. The present study summarizes the implementation and results obtained with a 2 nd -order slip boundary condition in a Navier-Stokes solver to address the rarefaction near the nozzle walls. Its assessment and application to a cold-gas micro-scale conical nozzle of 300μm throat diameter, discharging into the low-pressure freestream, constitutes the major aim of the work. The slip-model incorporates the velocity slip with thermal creep and temperature jump, thus permitting to deal with non-isothermal flows as well. Results show that the gas experiences an intense rarefaction in the lip vicinity, pointing to the limits of model validity. Furthermore, a strong Mach deceleration is observed, attributed to the rather thick subsonic boundary layer and supersonic bulk heating caused by the viscous dissipation, in contrast with the expansion to occur in large rocket nozzles during underexpanded operation.
This project develops a computer code which uses the Method of Characteristics and the Stream Function to define high efficiency nozzle contours for isentropic, inviscid, irrotational supersonic flows of any working fluid for any user-defined exit Mach number. The contours are compared to theoretical isentropic area ratios for the selected fluid and desired exit Mach number. The accuracy of the nozzle to produce the desired exit Mach number is also checked. The flow field of the nozzles created by the code are independently checked with the commercial Computational Fluid Dynamics (CFD) code ANSYS-FLUENT. ANSYSFLUENT predictions are used to verify the isentropic flow assumption and that the working fluid reached the user-defined desired exit Mach number.
The aim of the present study is to analyze the aeroelastic stability of a supersonic nozzle in over-expanded conditions, by using an aeroelastic stability model. To reach this objective, a research software written in Fortran, has been developed for 2D and 3D nozzle configurations. The obtained results are compared and validated for the 2D and 3D cases with those of previously studies.
Graduate Research in Engineering and Technology
The reduction of Earth-to-orbit launch costs in conjunction with an increase in launcher reliability and operational Efficiency is the key demands on future space transportation systems, like single-stage-to-orbit vehicles (SSTO). The realization of these vehicles strongly depends on the performance of the engines, which should deliver high performance with low system complexity. Performance data for rocket engines are practically always lower than the theoretically attainable values because of imperfections in the mixing, combustion, and expansion of the propellants. The main part of the project addresses different nozzle concepts with improvements in performance as compared to conventional nozzles achieved by Different Mach numbers, thus, by minimizing losses caused by over- or under expansion. The design of different nozzle shapes and flow simulation is done in gambit and fluent software’s respectively for various parameters
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