Nicolas Lupoglazoff

Instabilities and pressure oscillations in solid rocket motors

The purpose of this paper is to give an overview of the main results obtained on instabilities and pressure oscillations in segmented solid rocket motors. A major part of this work was carried out in the framework of the ASSM and POP R&T programs supported by the French national space agency CNES during the last decade. ASSM is related to Aerodynamics of Segmented Solid Motors and POP for Pressure Oscillations Program for the Ariane 5 solid booster (P230). Due to the use of segmented technology for the P230 motor and the possible acoustic oscillations inside the motor chamber, anticipated at the beginning of the programs and confirmed later on static firing tests, the main scientific objective of the ASSM program was oriented towards the comprehension and the modeling of the vortex shedding phenomena that are supposed to be responsible of the pressure and thrust oscillations observed in the P230. POP program was started in order to obtain an experimental and numerical data base using subscale tests of 1/15th of the P230. After the description of the instabilities observed in the P230 solid rocket booster, the scientific approach of the ASSM program is detailed insisting on the validation of numerical tools in order to predict oscillation frequencies and amplitudes. The logic of work regarding POP program is also presented. The main section of this paper provides an overview of different results obtained in ASSM and POP programs to understand the mechanisms driving to the instabilities in solid rocket chamber. The most important recent result, inside ASSM and POP programs, was the discovery of the parietal vortex shedding and the role of aluminum combustion on instabilities. Together, these two mechanisms seem to be an important potential source of instabilities and provide a new vision of the P230 stability.

Numerical simulation of vortex shedding phenomenon in 2D test case solid rocket motors

Segmented Solid Rocket Motors tend to develop unpredicted thrust and uressure oscillations. linked to a criodic vortex shcdding phcnomenon. Aspart of Arianc

2D NAVIER-STOKES STABILITY COMPUTATIONS FOR SOLID ROCKET MOTORS : ROTATIONAL, COMBUSTION AND TWO-PHASE FLOW EFFECTS.

PUO MPS solid motor stability assessmcnt. a research effort was initiated and aimed at il full numerical simulation of the unsteady, compressible internal flow. The objective of the present work is to demonstrate the feasibility of a direct numerical simulation of the vortex shedding henomenon on test case motors. The computer equations, by means of an expicit predictor/corrector Mc Cormack scheme with an improved (Jameson type) artificial viscosity algorithm. A r p e r test ,case motor was devised by means of a hydro ynamic stability analy+ of mean flow shear layer. Computations done wth optimized artificial viscosity terms gave rise to a marked self excited vortex shedding phenomenon of constant am litude close to the second axial mode frequency, in v excited phenomenon was observefwith both the Euler and Navler-Stokes codes. Moreover planar com utations artificial viscosity added, and gave rise to identical vortex shedding henomenon. First comparisons with Flandro’s codes, so P ve the 2D unstead , Euler or Navier-Stokes bot R axisymmetrie and planar confi urations. Similar self have been performed with the Euler code, w ~ t R out any linearize 2 approach are also presented. INTRODUCTION-OBJECTIVES This work is part of the overall combustion stability assessment of the Ariane 5 P230 MPS solid ro ellant motor and has been supported b CNES and E P J . Due to the segmented design of the P

Aeroacoustic Numerical Method Assessment for a Double Stream Nozzle

motor, it is believed that there exists a potentially severe risk for motor instabiMy, although the moFor is predicted stable by conventional linear acoustic balance computations. Indeed, a number of US works, related to segmented motors (Space Shuttle and Titan solid boosters) 11-51 have riported significant and un redicted chamber frequencies. Further studies [4,6] have shown that a periodic vortex sheddin phenomenon is likely to be at the source of these oscifations. First suggestion of acoustic mode excitation by vortex shedding in solid rocket motors was made by Flandro and ressure and thrust oscillations at t f e first axial mode 1) Jacobs [7l. The eriodic shedding of vortices is the result of a strong cou Ping between the instabilit of mean flow sense, vortex shedding can be viewed as a, by-product of grain segmentation which produces regons of highly sheared mean flow, due to port area discontinuities and protrusions of inhibitor rings. Several related cold flow e eriments [4, 6, 8-12 have these experiments assume that a pair of diaphragms, at least, are required to give rise to vortex sheddin vortices are enerated at the first diaphragm and provife However, this is not the ony,mechanism for vortex shedding acoustic driving: one diaphragm or one region of sheared flow can be sufficient, the feedback mechanism being provided by the exhaust nozzle. This was the case in the ex eriment of reference [7], and could be the case in the & motor (Fig.1). Among, possible nozzle feedback mechanisms are direct vortex impingement on the nozzle wall or modulation of !he exiting mass flow rate by inhomogeneous flow, associated with one vortex. Flandro [13], extending the work of reference [7] has pro osed a linearized approach to evaluate the vortex shesding risk and to incorporate a corresponding driving term in linear acoustic balance codes. A lication of Flandro’s linearized model to Ariane 5 P a m o t o r has been carried out 141 and concluded of a possible vortex Recognizing the possible failure of classical linear acoustic balance computations even improved by Flandro’s mqdel, to achieve reliable stability predictions in complex internal eometries, such as in the P230 numerical simulation of the unsteady, comprcssiblc, internal flow field. Such a simulation would naturally couple mean flow shear layer instabilities and acoustic motions, including nonlinear effects and nozzle res once to vortices. The present work is the first part, of that effort and is devoted to demonstrating the easibility of a direct numerical capture of the periodic vortex shcdding phenomenon in test case solid propellant motors. This work was made possible by recent rogresses at unsteady state flow fields [15,16]. shear layers an B acoustic motions in the clamber. In this documented the vortex shed 3 ing phenomenon. Mlost of the acoustic B eedback when im inging on the second onc. P shedding driving 1 or Ariane 5 P230. motor,, a research ef B ort was initiated, aiming at a full ONERA in numerical simulations of bot g steady and * Research scientist .. Research scientist, Member AIAA Copyright

Effect of chevrons on double stream jet noise from hybrid CAA computations

D Navier-Stokes stability computations are performed on a simple cylindrical port motor. The 2D results, in terms of motor frequency and damping, as well as in terms of full acoustic field, are compared to classical ID linear acoustic balance. The 2D computations are performed with models representing the propellant combustion response and two-phase flow behavior. The models are used separately and then together. The results show that the overall tendencies are correctly obtained over a large range of model parameter settings. Surprisingly it is found that the 2D results do not depend on the grid spatial resolution and that the details of the so-called acoustic boundary layer (ABL) need not to be resolved. This finding is also true for the two-phase flow damping. Analysis of the computed 2D acoustic field show that the ABL displacement effect is an effective damping source (even for an ABL penetrating into the core of the flow) and that an extra damping exists which is not incorporated into the classical acoustic balance.

Recent progress in numerical simulations for jet noise computation using LES on fully unstructured meshes

Industrial results obtained from different steady and unsteady numerical and analytical methods for a double stream jet in confluent flow configuration are presented. These results are compared with measurements from anechoic wind tunnel tests of a model nozzle. The test campaign was conducted in the ONERA CEPRA19 facility, and exhaust parameters are representative of take-off, with and without simulated flight effects. During this campaign, in addition to acoustic measurements, LDV (Laser Doppler Velocimetry) technique was used in order to measure steady and unsteady aerodynamic data in several planes downstream the exhaust plane. Those data are used to assess several RANS methods (Reynolds Averaged Navier Stokes - steady calculation) on structured and unstructured 2D meshes using k-e turbulence model. Particularly, the effects of the refinement and of the structure of the mesh, as well as flight effects are investigated. Those data are also used to assess LES method (Large Eddy Simulations - 3D unsteady calculation) performed for the same model nozzle in confluent flow configuration, but without plug. Because results of both steady and unsteady aerodynamic calculations are usually used as inputs of acoustic prediction methods, an assessment of two different aeroacoustic prediction processes is also achieved, with different RANS calculations as input of an improvement of MGB (Mani Gliebe Balsa), and LES calculations as input of an acoustic integral method in the time domain (FfowcsWilliams and Hawkings surface integration). Particularly, flight effects and the ability of the whole aeroacoustic chain to predict them are studied.

High-order computation of burning propellant surface and simulation of fluid flow in solid rocket chamber

Double stream nozzle configurations are computed to evaluate the noise radiated to the far field. The method combines a LES simulation with an acoustic solver based on surface integral method. The presented works have been carried out in the framework of the EXEJET French national program and focus on the effects of chevrons which are added to an axisymmetric reference configuration. The LES computations rely on hybrid meshes, which mix structured and unstructured grids, combining tetrahedra, pyramids and hexahedra. This approach has been developed in the past years at Onera. The computations are performed on both the axisymmetric and the chevron configurations. The LES results are used as inputs to a Ffowcs Williams and Hawkings acoustic surface integral method to reconstruct the far field noise. Several hybrid grids are considered and the grid effects are analyzed. In particular, the appearance of a vortex shedding phenomenon is observed for one of the grids family and is found to depend on the ability of the grid to resolve the nacelle external boundary layer. The computations are run in advance of the test campaigns to be carried out at the Onera Cepra19 facility. Comparisons with experimental measurement will be performed as they become available in the course of the EXEJET project.

Investigation of integral surface formulations for acoustic post-processing of unsteady aerodynamic jet simulations

Numerical simulations of a hot turbulent jet are performed on full unstructured grids with the aim to compute the noise generated in the far field. The jet flow is computed with the Onera CEDRE code using the LES (Large Eddy Simulation) approach and provide the unsteady inputs to the Onera KIM acoustic solver, based on the integral surface FW-H (Ffowcs Williams and Hawkings) approach which is used to reconstruct the acoustic far field. Several unstructured grids of increasing resolution are considered and the results obtained for both the computed jet flow and far field acoustics are compared to measurements carried out in the Onera CEPRA19 anechoic wind tunnel and to previous computations with the same codes but based on structured grids. Results show that unstructured grids allow a better grid tailoring work with an evident ease of construction, compared to the structured grid approach. Very good results are obtained and compare well with available measurements with an unstructured grid of roughly 130 millions cells. These results open the way to the use of unstructured grids in more complex geometries such as those encountered in real life configurations dealing with noise reduction devices or installation effects.

Investigation of Integral Surface Formulations for Acoustic Predictions of Hot Jets Starting from Unsteady Aerodynamic Simulations

In this paper, we present a numerical approach for predicting fluid flows in solid rocket motor (SRM) chambers. We use a novel high-order technique to track the burning grain surface. Spectral convergence toward the exact burning surface is achieved thanks to Fourier differentiation. In addition, we make use of a body-fitted mesh deforming with the burning surface and present a method to avoid manual remeshing. We describe several methods to deform the volume mesh and to keep good mesh element quality during the computation. We then couple the surface and volume approaches. The resulting coupled method is able to handle the formation of geometric singularities on the burning surface while keeping constant surface and volume mesh topology. This geometrical approach is integrated into a complex code for compressible, multi-species, turbulent flow simulations. Applications to the simulation of the internal flow in realistic solid rocket motors with complex grain geometry are then presented.