Ryoji Takaki

Generation and Propagation of Pressure Waves from H-IIA Launch Vehicle at Lift-Off

Numerical simulation was carried out to investigate the generation mechanism of pressure waves radiated from the H-IIA launch vehicle at lift-off. It was revealed that the Mach wave due to the large-scale structure of the unsteady supersonic exhaust plumes is the dominant noise source. The Mach wave propagating obliquely downstream is reflected from the constructions of the launch-pad, and then, turns to reach the vehicles, resulting in the acoustic loading. It was also found that the fluctuating supersonic plume entering into the flame duct is the dominant noise source that appears in the flame duct. Then, the pressure wave propagates through the flame duct and is ejected outside to the vehicle.

Effect of Deflector Shape on Acoustic Field of Launch Vehicle at Lift-off

As part of the studies on the advanced solid rocket in JAXA, numerical simulations were carried out to investigate the effect of the flame deflector (FD) shape on the acoustic level of the launch vehicle at lift-off. The results indicate that there are mainly two types of acoustic sources; 1) “impingement noise” due to the interaction of the exhaust plume and FD, and 2) Mach wave generated by the flapping motion of the plume flowing over the FD. Since the impingement noise propagates directly to the vehicle, the acoustic level of the vehicle is dominated by the impingement noise. Based on the knowledge obtained here, it is found that the initial inclination angle of the FD should be steep to reduce the impingement noise. Besides, it also turns out that the FD is desirable to have a curved surface both for reducing the size of the FD and for preventing another impingement noise caused by the rapid change of the FD contour.

Numerical Study on Acoustic Radiation from a Supersonic Jet Impinging to an Inclined Plate

Acoustics radiated from a M=1.8 ideally-expanded jet impinging on a 45 degree-inclined flat plate is investigated numerically with the help of the experimental work. Validation of computational method and grid convergence study are conducted firstly. Result of the free jet is satisfactory, while the present study overestimates the noise level of the impinging jet by 5 dB in OASPL. Based on the numerical technique validated here, mechanism of acoustics is analyzed. In addition to the well-known free-jet noise sources such as the Mach wave and the fine-scale turbulent mixing noise, additional two noise sources are found; 1) interaction between the shock waves and the vortex of the shear layer, 2) the Mach wave radiated from the jet flowing on the inclined plate. The former is similar to the shockassociated noise, and the OASPL plot at far-filed shows omni-directional feature. These two waves are also observed in the present experiment.

Acoustic Generation Mechanism of a Supersonic Jet Impinging on Deflectors

Numerical simulation of a supersonic jet impinging to a 45-degree-inclined flat plate is conducted to reveal the correlation between hydrodynamic structure and the acoustic field. This type of acoustic generation is of interest for lift-off acoustics of launch vehicles. Through the conditional sampling, it is clarified that acoustic wave overarching the plate shock and the 1st tail shock is an onset of the acoustic wave generated near the impingement region. This result implies that the acoustic wave generated near the impingement region is formed by the interaction of turbulent structure of the jet shear layer with the plate and the tail shock waves. Preliminary numerical study is then conducted to analyze the effect of deflector shape on the plate and the tail shock waves and resulting acoustic level of launch vehicle. It turns out that magnitude of the plate shock and acoustic level around the vehicle are attenuated by employing steeply inclined flat plate. While, a tail-shock-free deflector with 45-degree initial inclination is designed to avoid formation of the tail shocks. It is found that the tail-shock-free deflector is compact in size to redirect the jet horizontally, but the acoustic level near the vehicle is almost the same with the 45-degree-inclined flat plate. The results obtained in this study shed light on deflector design criteria based on acoustic point of view.

A Three-Dimensional Particle-in-Cell Simulation of Quasi-Perpendicular Shock on Fujitsu FX1 Cluster

The high-specification computational power of Japan Aerospace Exploration Agencys new supercomputer system, called Fujitsu FX1 cluster, enables us to perform really macroscale 3-D situations with full particle plasma simulation [particle-in-cell (PIC) method]. A fully 3-D kinetic approach to collisionless shock problems, which is one of the most important problems in the space plasma science, is possible, and a challenging run is being executed for a pioneering study of the topic. About 0.4 billion grids are allocated for the electromagnetic fields, and about 0.1 trillion particles are loaded into the simulation run. The computational efficiency of the PIC code is about 8% of the peak performance (4.6 Tflops) using 5776 CPU cores (57 Tflops). The simulation parameters were selected to simulate ESAs Cluster-II spacecraft observational result reported by Seki (in 2009). The full mass ratio mi/me = 1840 was taken for this simulation, and almost one ion inertia length square could be allocated for the simulation. In this simulation, a quite complicated wave activity is found in the shock foot region. In this paper, comparing 3-D results with 2-D simulation results, a 3-D nature of shock transition region of quasi-perpendicular shock is reported.

Computational Aeroacoustic Analysis of Overexpanded Supersonic Jet Impingement on a Flat Plate With/Without Hole

Aeroacoustic mechanisms of an axisymmetric over-expanded supersonic jet impinging on a flat plate with and without hole are numerically investigated. High-order weighted compact nonlinear scheme is used to simulate the unsteady flow including shock waves and sound radiation in the near field of the jet. Analyses of unsteady flowfield and related near-sound field reasonably identify three major noise generation mechanisms, that is, noises from Mach wave, shock cell-shear layer interaction and small fluctuations of jet shear layer. Especially, intense noise radiation in the form of Mach waves and its reflection at the plate predominates the noises from the other two finer sources. The simulated distributions of sound source power and its frequency along the jet axis qualitatively well coincide with typical experimental data used in NASA SP-8072. Similar sound pressure spectrum shape is obtained both the cases of flat plate with and without hole, but the case of without hole shows higher SPL by several dB than that of with hole due to the stronger Mach wave radiation. Aeroacoustic flowfield is drastically affected by the Reynolds number because the jet shear layer instability directly causes the strength of acoustic waves.Copyright

Large-Eddy Simulation of a Supersonic Jet using High-Order Flux Reconstruction Scheme

Large-eddy simulations of an ideally expanded supersonic jet at Mach number 1.8 are performed by a high-order unstructured grid solver for aiming at the validation for aeroacoustic applications. The developed solver is based on the flux reconstruction scheme to discretize unstructured hexahedral grids. The localized artificial diffusivity scheme is employed to capture shock waves robustly and to retain high-order accuracy away from the shocks. The turbulent jet flow is obtained by the FR-LAD solver using the degree of polynomial up to p=4. The computed results are in reasonable agreement with the experimental and numerical data in the literatures for time-averaged and fluctuating quantities.

Hybrid LES/RANS Simulations of Transonic Flowfield Around a Rocket Fairing

Transonic flowfield around a rocket fairing with flight Mach number of 0.8 is numerically investigated by the LES/RANS hybrid method using the Improved Delayed Detached Eddy Simulation (IDDES). It is found that sufficient grid resolution and turbulent content are important to obtain fully developed turbulent state of the separated shear layer. Otherwise, breakdown of the separated shear layer occurs with unphysical large scale vortex structure, which leads to spurious separation associated with large separation bubble. The numerical results are assessed by measured results taken in wind tunnel testing. Based on the obtained numerical results, it is clearly observed that the wall pressure fluctuation leading to the buffet load originates in the oscillation of the separated shock wave and the vortex structure of the separated shear layer. Correlation between feature of the wall pressure spectrum and the separated flowfield with shock wave is analyzed.

Development of a High-Order Flux Reconstruction Scheme for Body-Fitted Cartesian Unstructured Grids

A high-order flux reconstruction (FR) scheme on unstructured hexahedral grids is developed for aerospace flow simulation. In order to use computational grids generated by the body-fitted Cartesian (BFC) method, the grid which contains polyhedral cells with hanging nodes are subdivide into hexahedral cells first. Then, to perform computation by the FR scheme on the non-conformal hexahedral grid, the mortal element method (MEM) is employed to determine the numerical flux at cell interface. As a shock capturing scheme, the localized artificial diffusivity (LAD) method is developed for the FR scheme. The developed scheme is tested for typical benchmark problems including smooth and shocked flows. I. Introduction HE use of CFD in designing aerospace vehicles has been growing over the past decades. Commercial CFD software can be effectively used for early stage of the design process with the reasonable accuracy in quick turn around time. The finite volume methods (FVM) are widely used for the commercial and industrial CFD solvers, but the spatial order of accuracy is remained less than 2 nd -order due to several reasons mainly related to the robustness. In order to tackle more complicated flowfields and to improve the prediction accuracy by CFD, employing highorder methods is one of the prospective directions. It is well known that high-order methods can give the numerical solution more efficiently than the low-order methods with the same level of accuracy if the flowfield is sufficiently smooth. High-order methods were developed and investigated first in the context of the structured or multi-block structured grids. The formal order of accuracy can be retained with the relatively better quality mesh in contrast to the case with unstructured grids. A main drawback with structured grids is the difficulty in the mesh generation around complex geometries. Therefore, unstructured methods are often used in the production codes. Automated processes in the unstructured grid generation has facilitated the routinely works in the design cycle and has gained more users in the wide engineering areas. A criticism for this approach is related to the solution accuracy especially on the dirty grid around real complicated configurations. Recent research for the reconstruction of the solution gradient

Examination of Sound Suppression by Water Injection at Lift-off of Launch Vehicles

Analytical investigation of noise suppression effect of water injection to exhaust plume from rocket motors was carried out. The results showed acoustic absorption by water droplets, acoustic scatter by water droplets, absorption through air, and water curtain effect increase as frequency becomes high. It was also confirmed that acoustic absorption by water droplets has the most significant effects among the four effects. Furthermore, steady 2D-axisymmetric Reynolds-Averaged Navier-Stokes (RANS) simulations of a supersonic free jet were carried out in order to evaluate reduction of jet energy due to water injection. The reduction of the acoustic source strength along the jet axis was evaluated considering the difference of ρk value between with and without water injection. The far field sound power level (SPL) was analyzed using an empirical prediction method, NASA SP-8072 and compared to sub-scale motor test data. The strength of the acoustic source power along the jet axis was set based on the reduction rate of the ρk value due to water injection and the propagation to the far filed points was analyzed based on the NASA SP-8072. The results showed that the water injection effect can be reasonably evaluated by using both the analytical prediction methodology and the evaluation methodology of reduction of the jet energy based on the change of ρk .