Characterization of Side Load Phenomena Using Measurement of Fluid/Structure Interaction (original) (raw)
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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.
Side loads and thermal loads in rocket nozzles. Overview of the CNES-ONERA ATAC programme
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.
Transient Three-Dimensional Analysis of Nozzle Side Load in Regeneratively Cooled Engines
41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2005
Three-dimensional numerical investigations on the start-up side load physics for a regeneratively cooled, high-aspect-ratio nozzle were performed. The objectives of this study are to identify the three-dimensional side load physics and to compute the associated aerodynamic side load using an anchored computational methodology. The computational methodology is based on an unstructured-grid, pressure-based computational fluid dynamics formulation, and a transient inlet condition based on an engine system simulation. Computations were performed for both the adiabatic and cooled walls in order to understand the effect of boundary conditions. Finite-rate chemistry was used throughout the study so that combustion effect is always included. The results show that three types of shock evolution are responsible for side loads: generation of combustion wave; transitions among free-shock separation, restricted-shock separation, and simultaneous free-shock and restricted shock separations; along with oscillation of shocks across the lip. Wall boundary conditions drastically affect the computed side load physics: the adiabatic nozzle prefers free-shock separation while the cooled nozzle favors restricted-shock separation, resulting in higher peak side load for the cooled nozzle than that of the adiabatic nozzle. By comparing the computed physics with those of test observations, it is concluded that cooled wall is a more realistic boundary condition, and the oscillation of the restricted-shock separation flow pattern across the lip along with its associated tangential shock motion are the dominant side load physics for a regeneratively cooled, high aspect-ratio rocket engine. *Technical Assistant, Thermal and Combustion Analysis Branch, ER43, Senior Member AIAA. 1 American Institute of Aeronautics and Astronautics separation (RSS) and medium side load for LE-7A, and transition from FSS to RSS, but missed three high side load peaks for the CTP50-R5-L nozzles. The authors stated that the grid used was probably too coarse, resulting in overestimated flow fluctuation and smearing of distinctive peaks. After the works of Yonezawa, et ak', Wang* performed a series of 2-D and axisymmetric numerical studies on the Block-I SSME start-up side load physics, using hybrid grids that were anchored for axial force and wall heat fluxesg The side load physics captured in that study include the Coanda effect, combustion wave, transitions between FSS and RSS, and shock oscillation across the lip. More importantly, it is found that inlet ramp rate and combustion drastically affect the computed side load physics. Note that Yonezawa, et a t v 7 ignored combustion for LE-7 and LE-7A engines and assumed constant ramp rates. The lesson learned sets up the computational strategy for the 3-D investigation of the regeneratively cooled, Block-I SSME start-up side load physics in this study: a transient inlet condition based on an engine system simulation and the reacting flow calculation using finite-rate chemistry. Furthermore, the effect of wall cooling is investigated to see if wall boundary condition plays a role in side load physics. 11. Computational Methodology A. Computational Fluid Dynamics The CFD methodology is based on a multi-dimensional, finite-volume, viscous, chemically reacting, unstructured grid, and pressure-based formulation. Time-varying transport equations of continuity, momentum, total enthalpy, turbulence, and species continuity were solved using a time-marching sub-iteration scheme. A predictor and corrector solution algorithm was employed to provide coupling of the governing equations. A second-order central-difference scheme was employed to discretize the diffusion fluxes and source terms. For the convective terms, a second-order upwind total variation diminishing difference scheme was used. To enhance the temporal accuracy, a second-order backward difference scheme was employed to discretize the temporal terms. Details of the numerical algorithm can be found in R e f s 9-12. An extended k-& turbulence modelI3 was used to describe the turbulence. A modified wall function approach was employed to provide wall boundary layer solutions that are less sensitive to the near-wall grid spacing. Consequently, the model has combined the advantages of both the integrated-to-the-wall approach and the conventional law-of-the-wall approach by incorporatin a complete velocity profile and a universal temperature profile14. A 7-species, 9-reaction detailed mechanismF4 was used to describe the finite-rate, hydrogedoxygen afterburning chemical kinetics. The seven species are Hz. 0 2 , HzO, 0, H, OH, and N2.
Influence of nozzle random side loads on launch vehicle dynamics
Journal of Applied Physics, 2010
It is well known that the dynamic performance of a rocket or launch vehicle is enhanced when the length of the divergent section of its nozzle is reduced or the nozzle exit area ratio is increased. However, there exists a significant performance trade-off in such rocket nozzle designs due to the presence of random side loads under overexpanded nozzle operating conditions. Flow separation and the associated side-load phenomena have been extensively investigated over the past five decades; however, not much has been reported on the effect of side loads on the attitude dynamics of rocket or launch vehicle. This paper presents a quantitative investigation on the influence of in-nozzle random side loads on the attitude dynamics of a launch vehicle. The attitude dynamics of launch vehicle motion is captured using variable-mass control-volume formulation on a cylindrical rigid sounding rocket model. A novel physics-based stochastic model of nozzle side-load force is developed and embedded in the rigid-body model of rocket. The mathematical model, computational scheme, and results corresponding to side loading scenario are subsequently discussed. The results highlight the influence of in-nozzle random side loads on the roll, pitch, yaw, and translational dynamics of a rigid-body rocket model.
Development of an Aeroelastic Modeling Capability for Transient Nozzle Flow Analysis
Journal of Propulsion and Power, 2014
Lateral nozzle forces are known to cause severe structural damage to any new rocket engine in development during testing. Although three-dimensional, transient, turbulent, chemically reacting computational fluid dynamics methodology has been demonstrated to capture major side load physics with rigid nozzles, hot-fire tests often show nozzle structure deformation during major side load events, leading to structural damages if structural strengthening measures were not taken. The modeling picture is incomplete without the capability to address the two-way responses between the structure and fluid. The objective of this study is to develop a coupled aeroelastic modeling capability by implementing the necessary structural dynamics component into an anchored computational fluid dynamics methodology. The computational fluid dynamics component is based on an unstructured-grid pressure-based computational fluid dynamics formulation, whereas the computational structural dynamics component is developed under the framework of modal analysis. Transient aeroelastic nozzle startup analyses at sea level were performed to demonstrate the successful simulation of nozzle wall deformation with the proposed tightly coupled algorithm, and the computed results pertinent to fluid-structure interaction presented.
Annalen der Physik, 2011
A long-standing, though ill-understood problem in rocket dynamics, rocket response to random, altitude-dependent nozzle side-loads, is investigated. Side loads arise during low altitude flight due to random, asymmetric, shock-induced separation of in-nozzle boundary layers. In this paper, stochastic evolution of the in-nozzle boundary layer separation line, an essential feature underlying side load generation, is connected to random, altitude-dependent rotational and translational rocket response via a set of simple analytical models. Separation line motion, extant on a fast boundary layer time scale, is modeled as an Ornstein-Uhlenbeck process. Pitch and yaw responses, taking place on a long, rocket dynamics time scale, are shown to likewise evolve as OU processes. Stochastic, altitude-dependent rocket translational motion follows from linear, asymptotic versions of the full nonlinear equations of motion; the model is valid in the practical limit where random pitch, yaw, and roll rates all remain small. Computed altitudedependent rotational and translational velocity and displacement statistics are compared against those obtained using recently reported high fidelity simulations [Srivastava, Tkacik, and Keanini, J. Applied Phys., 108, 044911 (2010)]; in every case, reasonable agreement is observed. As an important prelude, evidence indicating the physical consistency of the model introduced in the above article is first presented: it is shown that the study's separation line model allows direct derivation of experimentally observed side load amplitude and direction densities. Finally, it is found that the analytical models proposed in this paper allow straightforward identification of practical approaches for: i) reducing pitch/yaw response to side loads, and ii) enhancing pitch/yaw damping once side loads cease.
This article presents a study of a testing bench structure for Rocket Engines, which is under development by the PUC-Minas Aerospace Research Group. The Bench is being built for civilian's liquid bipropellant rocket engines up to 5 kN of thrust. The purpose of this article is to evaluate the bench structure using the Finite Element Method (FEM), by structural linear static and dynamic analysis. Performed to predict the behavior of the structure to the requests of the tests. The virtual simulations were performed using a CAE software with the Nastran solver. The structure is 979 x 1638 mm by 2629 mm, consisting of folded-plates (¼ "x 3¼" x 8") and plates of 1/4" and 1/2 ", both SAE 1020 Steel .The rocket engine is fixed on the structure through a set called engine mount. It was included in the analysis clearances or misalignments that may occur during tests. As well as, the load applied was evaluated with components in varying orientations and directions. It was considered the maximum size of the engine mount and the maximum inclination angle of load. At the end of this article it was observed that the worst stress and displacement values obtained were for the hypothesis with the inclination of five-degrees with load components in the positive directions of the axes defined and it was also obtained the first twenty frequency modes of the structure.
Effects of Unsteady Pump Cavitation on Propulsion-Structure Interaction (Pogo) in Liquid Rockets
2004
The parameters that characterize the perturbational pressure and flow at the inlet and outlet of a pump are established through pogo stability analyses for a launch vehicle. Ground tests of the launch vehicle's engine indicate the presence of unsteady pump cavitation, and some flights of the launch vehicle exhibit frequency, amplitude, and phase locking between axial structural acceleration and engine chamber pressure-a condition emblematic of propulsion-structure interaction, or pogo. Models developed for several missions of the subject launch vehicle are used to establish the ranges of the pump parameters that yield instability during the flight pogo occurrences and stability at other times. The resulting nominal values of normalized pump cavitation stiffness and mass flow gain for the launch vehicle's engine fall in the range 0.62-0.86 and 0.31-0.59, respectively. These ranges account for sensitivity with respect to dynamic pump gain (1.0-2.7) and to structural damping (0.5%-1.0%) for the axial modes of the coupled launch vehicle-space vehicle system. The variation of cavitation stiffness with respect to cavitation number is also investigated. It is shown that cavitation stiffness is generally the predominate pump parameter when the feedline hydraulic and axial structural modes are separated in frequency. However, if the frequencies of these modes are in close proximity, mass flow gain has a strong destablizing effect; closed-loop damping markedly decreases as this pump parameter increases. The latter observation explains why unsteady pump cavitation can result in pogo instability for some missions of the subject launch vehicle.
Fluid structure interaction phenomena in highly over-expanded rocket nozzle
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.