Takashi Aoyama

Acoustic environment at a launch site is affected by jets from rocket engines, as well as by geometric features of a launch pad. Then, spacecraft are also exposed to acoustic pressure with the wide frequency range transmitted through a payload fairing. In the case that vibro-acoustic response is predicted using steady-state vibroacoustic analysis, there actually exists the mid-frequency range in which there are no mature numerical methods. This report deals with the novel Wave Based Method (WBM), a deterministic approach for steady-state vibroacoustic analysis in the wide frequency range. Then the two-dimensional (2D) WBM code developed by the authors is applied to some sound transmission problems and exterior problems. In the sound transmission analysis, it is found that discontinuous impedance boundary conditions (BCs) must be fi tted by using continuous functions to obtain more accurate results using both WBM and FEM. After smoothing the BCs, numerical prediction results of our sound transmission models by the 2D WBM and FEM are compared, and the 2D WBM are verifi ed. Furthermore, in order to verify the 2D code extended for the exterior problems, it is applied to simple launch p

The mechanical vibration is applied to a spacecraft via the interface to a launch vehicle at lift-off. The spacecraft is also exposed to acoustic pressure with wide frequency range. Lightweight and large area structures, such as solar paddles, antennas, and components with relatively high natural frequencies, are sensitive to acoustic load. Although the acoustic tests are usually conducted for components and system of the spacecraft, acoustic tests and analysis of the spacecraft mounted in a fairing have not been sufficiently done. It is expected that numerical analysis can be applied to the prediction of acoustic environment inside of fairings. Japan Aerospace Exploration Agency (JAXA) has been developing a vibroacoustic prediction tool by the wave based method (WBM), which is one of the deterministic approaches and is proposed for numerical prediction up to the mid-frequency range. The existing deterministic approaches can not accurately solve vibroacoustic problems in this range. In this paper, numerical prediction results of sound transmission by the 3D WBM and FEM are compared with those by an experiment, and verification of the 3D WBM is examined.

18 June-19 June, 2009, Japan Aerospace Exploration AgencyFlapping motion consists of combined motions of wing reciprocation and rotation, and thus generates complex flow field and pressure change around the wing, which produce typical sound sources such as dipole or quadrupole sources. This implies the control of flapping sound is an important issue to realize practical flapping wing air vehicles. For this purpose, we developed a numerical scheme to analyze sound generation and transmission of flapping wing by combining CFD and acoustic analyses. This scheme is applied to two types of motions, i.e. hovering and forward moving motion of insect (hawkmoth). Conspicuous similarity is found in both flying types, i.e. the dominance of fundamental frequency wave in front and the second or higher harmonic wave in side or bottom. Another similarity is found in the sound directivity, i.e. the strong sound transmission in the direction perpendicular to the flapping plane and the weak transmission in the direction parallel to the flapping plane. These results clearly represent the relationship between wing motion and sound characteristics, and thus providing us with insights on the control methods of flapping sound.Physical characteristics: Original contains color illustrations

For a two dimensional analysis of sound transmission to structural model, it is considered that one sine sound pulse incidents on a curved plate in the Cartesian coordinate. Because of using the square mesh, the curved surfaces of plate are modeled by steps, which are small enough to the wave length of the incident sound wave. From the result of sound transmission calculation using the FDTD (Finite Difference Time Domain) method, it can be seen that the transmitted waves from the curved plate are generated by lateral and bending waves in solid. The results of FFT analyses of transmitted waves make the characteristic frequencies clear. These frequencies almost coincide with the symmetrical mode frequencies obtained by MSC NASTRAN. Therefore, the FDTD analysis can simulate that the transmission waves through the curved plate are produced by its mode shapes.

Digital Wind Tunnel (DWT) is being developed in a project entitled Digital/Analog Hybrid Wind Tunnel in JAXA. The main objective of this project is to sophisticate the transonic wind tunnel in JAXA by applying IT technologies. We have several hurdles to establish DWT such as the developments of an automatic grid generation tool and a high performance CFD solver. In this paper, the prediction accuracy of aircraft loads by using the automatic grid generation tool and the modeling of porous wall for the CFD calculations of whole wind tunnel including an aircraft model, support instruments, and walls are discussed.

A simulation method for full helicopter configuration is constructed by combining an unsteady Euler code and an aeroacoustic code based on the Ffowcs-Williams and Hawkings formulation. The flow field and helicopter noise are calculated using a moving overlapped grid system, and the mutual effect of main rotor and tail rotor are studied for the helicopter in hover or forward flight. In the hovering flight calculation, the tip vortex of the tail rotor is dragged by the induced flow of the main rotor, and the detailed phenomena of the flow pattern are captured well. In a forward-flight calculation, noises from the main rotor and tail rotor are predicted to understand the tail rotor noise for both self noise and the interaction noise with the main-rotor wake. Comparison of noise magnitude shows that the relative importance of tail rotor noise with respect to the main rotor noise according to the flight conditions.

In this study, the relationship between the characteristics of sound transmission and acoustically induced vibration has been experimentally investigated. It was found that sound transmission is affected by the proper oscillation of a material. In particular, at the natural frequencies with axisymmetircal modes, lower transmission peaks have been observed.

Aerodynamics around unsteady flapping wings is analyzed by using CFD (Computational Fluid Dynamics) techniques. Two types of flapping wing are modeled and analyzed referring to the hovering motion of hawkmoth and honeybee. Multi-block technique is used to make the suitable calculation grids for wing and body, and the grid-overset technique is used for the interpolation of physical values between those grids. CFD results show several vortices are generated at the leading edge, tip, and the trailing edge of flapping wings which comprise the complex flow fields around the wings and a body. The analysis also clarified the leading edge vortex significantly contributes to the generation of lift. Subsequent acoustic analysis is conducted referring to the CFD results to simulate flapping sound of honeybee. Directivity of sound propagation is recognized showing strong sound propagation in the front direction. Consequently, many features of complex flow around the flapping wing and its sound generation are clarified both qualitatively and quantitatively by using CFD and acoustic techniques.

Spacecraft are placed at the top of launch vehicles, and are excited with mechanical vibrations via interfaces between spacecraft and launch vehicles during launch. In addition to such mechanical vibrations, the spacecraft are also exposed to acoustic pressure with wide frequency range. Lightweight and large area structures, such as solar arrays and antenna dishes, and components with relatively high natural frequencies are sensitive to acoustic loads. We have studied on a series of numerical simulation techniques for searching main acoustic sources at lift-off, and analyzing acoustic wave propagation, transmission through fairing and spacecraft acoustic vibrations. This paper focuses on numerical prediction approaches for steady-state coupled interior vibro-acoustic problems, especially for spacecraft structural vibrations by acoustic loads with the wide frequency range acted during the lift-off. Numerical prediction of vibro-acoustic responses might enable us to cover the ground acoustic tests, and is therefore quite important to design and develop reliable spacecraft. For spacecraft vibro-acoustic simulations, there are deterministic prediction techniques such as finite element method (FEM) and boundary element method (BEM) applicable in the low frequency range, and statistical ones such as statistical energy analysis (SEA) in the high frequency range. However, there generally exists mid-frequency range where no mature numerical methods are applicable. In this paper, some FE analyses are firstly investigated using some simple models to check FE modeling techniques and responses by structural analysis with random acoustic loads. In addition, the simple primary structure with some stiffeners of a large spacecraft is modeled, and is used to solve the eigen value problem, and transient and steady-state analysis with random acoustic loads. Some results show that FEM is limited to be applied in low frequency range. Then, a novel deterministic prediction approach called the wave based method (WBM) is considered to be applied to vibro-acoustic analysis with higher frequency range. Using a 2-dimensional WBM code developed in this study, some coupled vibro-acoustic problems are solved to verify the code and to understand some fundamental features of WBM. Moreover, an uncoupled acoustic problem and a spacecraft vibro-acoustic problem inside a payload fairing are modeled, and solved using the WBM code and commercial FEM software. The results show that WBM is quite practical without using meshes, and has high potential for the vibroacoustic analysis in the wide frequency range due to no numerical dispersion errors.

A three-dimensional unsteady Euler code for the analysis of active flap control (AFC) has been developed in this study based on an advanced CFD code for the full configuration of helicopters. The effect of phase angle and geometric position of AFC on blade-vortex interaction (BVI) noise is analyzed by combining the new CFD code with an acoustic code based on the Ffowcs Williams and Hawkings (FW-H) equation. As a result, a simple model is proposed to understand the effect of flap phase angle on BVI noise and 3D calculations by the present method confirm that the model properly works. The prediction of the effect of flap phase angle by the present method is remarkably improved by applying the understandings derived from the simple model. A quantitative noise reduction of 5.62dB is obtained at the flap phase angle of 60° and noise signal strongly propagates almost downward in the present condition. The prediction of flap position effect shows that more effective BVI noise reduction can be achieved by the flap at the outer position with well-adjusted phase. When the flap is located near the blade tip, the merger between outer flap vortex and tip vortex can change the tip vortex strength and the blade surface pressure in the BVI condition. As the flap location moves inward, the sudden change of BVI noise reduction mechanism happens, which needs more discussion for the understandings.

The importance of accurate and fast interpolation algorithm is growing up for multi-body configuration with arbitrary overlap. One of the critical cases can be a helicopter simulation because of the complex relative movement of rotor-rotation and body motion. In this paper, new searching algorithms are implemented for the interpolation between two different grids, Cartesian grid and curvilinear grid, of which the overlapped grid system consists for the massive computation of the full helicopter configuration. These searching algorithms are proposed to make full use of (1) the characteristics of Cartesian grid, (2) special geometric configuration of helicopter, and (3) load balancing in parallel computation. In the first stage, Alternating Index Searching (AIS) algorithm, which alternates a searching direction by jumping the grid index to the searching point, is constructed to compare the iteration speed with a conventional Linear Searching (LS) algorithm. Simple two dimensional problems and three dimensional helicopter simulations are conducted to compare the efficiency of these searching algorithms. The result shows a considerable enhancement in computing time for whole computation domain. In the second stage, Reverse Index Searching (RIS) algorithm, which is developed to consider the load balance among each processing element (PE) during parallel computation, is proposed. By applying these searching algorithms, efficient massive computation can be achieved for the helicopter configuration.

The importance of accurate and fast interpolation algorithm is growing up for multi-body configuration with arbitrary overlap. One of the critical cases can be a helicopter simulation because of the complex relative movement of rotor-rotation and body motion. In this paper, new searching algorithms are implemented for the interpolation between two different grids, Cartesian grid and curvilinear grid, of which the overlapped grid system consists for the massive computation of the full helicopter configuration. These searching algorithms are proposed to make full use of (1) the characteristics of Cartesian grid, (2) special geometric configuration of helicopter, and (3) load balancing in parallel computation. In the first stage, Alternating Index Searching (AIS) algorithm, which alternates a searching direction by jumping the grid index to the searching point, is constructed to compare the iteration speed with a conventional Linear Searching (LS) algorithm. Simple two dimensional problems and three dimensional helicopter simulations are conducted to compare the efficiency of these searching algorithms. The result shows a considerable enhancement in computing time for whole computation domain. In the second stage, Reverse Index Searching (RIS) algorithm, which is developed to consider the load balance among each processing element (PE) during parallel computation, is proposed. By applying these searching algorithms, efficient massive computation can be achieved for the helicopter configuration.

A study to establish a numerical prediction method for vibro-acoustics during rocket launch has been conducted. This method consists of some analysis elements: numerical analysis of sound generation, propagation, transmission, and vibro-acoustics of payload. In this paper, sound pressure distributions obtained by these analyses are visualized. At first, flow and acoustic fields of modeled H-IIA launch pad are shown, which is obtained by a hybrid methodology of the Euler and Linearized Euler Equations (LEE) solvers. Then, an arch-shaped transmission wave is observed from a sound transmission analysis of hollow wall model using the Finite Difference Time Domain (FDTD) method. Finally, it is clearly shown that the acoustic field is affected by structural vibrations and the coupled vibro-acoustic problem of flexible satellite model can be solved by the Wave Based Method (WBM).

Inviscid numerical simulations of two-dimensional parallel blade-vortex interaction are carried out in the framework of Task 1 (1st year) of the cooperative research activity between JAXA and ONERA on the "Comparison of CFD and BVI noise prediction methods for realistic rotors". The capability of respective JAXA and ONERA Euler methods for an accurate capture of main features of parallel Blade-Vortex Interaction is evaluated. The results from each side are compared quite well to each other and with NASA experiments with the main features well captured. But some discrepancies are found mainly due to different numerical schemes and computation approaches adapted by each organization.

Inviscid numerical simulations of two-dimensional parallel blade-vortex interaction are carried out in the framework of Task 1 (1st year) of the cooperative research activity between JAXA and ONERA on the "Comparison of CFD and BVI noise prediction methods for realistic rotors". The capability of respective JAXA and ONERA Euler methods for an accurate capture of main features of parallel Blade-Vortex Interaction is evaluated. The results from each side are compared quite well to each other and with NASA experiments with the main features well captured. But some discrepancies are found mainly due to different numerical schemes and computation approaches adapted by each organization.

Blade vortex interaction (BVI) noise, which is generated by impulsive changea in the pressure over the interacting blade preceding the pressure jump, propagates sound to far field observers. In general, these interactions occur in forward-descent flight conditions, especially during a landing approach. The acoustic signal from BVI is generally in the frequency range to which the human subjective response is most sensitive. In order to reduce BVI noise, many researchers have been studying not only passive remedies such as rotor tip design and leading edge modification, but also active devices such as the higher harmonic control method, active tab, active flap, and tip-jet blowing. The National Aerospace Laboratory (NAL) in Japan and Pohang University of Science and Technology (POSTECH) in Korea have conducted a collaborative research program on the effects of lateral wing-tip blowing to reduce BVI noise from helicopter rotors. The lateral wing-tip method is one of the active control methods to control the generation and behavior of tip vortical flow by blowing a jet flow at the tip of the main rotor. In the first stage of the research, threedimensional compressible Euler/Navier-Stokes equations are solved to calculate the effect of blowing air from the blade tip on the tip vortex of a fixed single blade. In the next stage, predictions of BVI noise will be made by combining an unsteady Euler code with an aeroacoustic code based on the Ffowcs-Williams and Hawkings formulation.In the present report, predictions of BVI noise are performed for a two-blade rotor. A moving overlapped grid system with three types of grids including blade, inner and outer background grids is used to simulate the BVI of a helicopter with two blades in forward/ descending flight. The body-fitted blade grid moves with the blade motion, and the background grids in this Cartesian system are placed around the rotor disk in order to include the trace of the tipvortex. The effects of lateral blowing in the tip region to reduce BVI noise are examined using various jet blowing conditions including jet speed, jet slit area and injection direction, which are also used in the fixed rotor calculations. Calculations show that the reductions in BVI noise peak and the sound pressure level are more dependent on the jet velocity and flow rate than on jet directions, and the maximum decrease in BVI noise is 2.55 dB.

Blade vortex interaction (BVI) noise, which is generated by impulsive changea in the pressure over the interacting blade preceding the pressure jump, propagates sound to far field observers. In general, these interactions occur in forward-descent flight conditions, especially during a landing approach. The acoustic signal from BVI is generally in the frequency range to which the human subjective response is most sensitive. In order to reduce BVI noise, many researchers have been studying not only passive remedies such as rotor tip design and leading edge modification, but also active devices such as the higher harmonic control method, active tab, active flap, and tip-jet blowing. The National Aerospace Laboratory (NAL) in Japan and Pohang University of Science and Technology (POSTECH) in Korea have conducted a collaborative research program on the effects of lateral wing-tip blowing to reduce BVI noise from helicopter rotors. The lateral wing-tip method is one of the active control methods to control the generation and behavior of tip vortical flow by blowing a jet flow at the tip of the main rotor. In the first stage of the research, threedimensional compressible Euler/Navier-Stokes equations are solved to calculate the effect of blowing air from the blade tip on the tip vortex of a fixed single blade. In the next stage, predictions of BVI noise will be made by combining an unsteady Euler code with an aeroacoustic code based on the Ffowcs-Williams and Hawkings formulation.In the present report, predictions of BVI noise are performed for a two-blade rotor. A moving overlapped grid system with three types of grids including blade, inner and outer background grids is used to simulate the BVI of a helicopter with two blades in forward/ descending flight. The body-fitted blade grid moves with the blade motion, and the background grids in this Cartesian system are placed around the rotor disk in order to include the trace of the tipvortex. The effects of lateral blowing in the tip region to reduce BVI noise are examined using various jet blowing conditions including jet speed, jet slit area and injection direction, which are also used in the fixed rotor calculations. Calculations show that the reductions in BVI noise peak and the sound pressure level are more dependent on the jet velocity and flow rate than on jet directions, and the maximum decrease in BVI noise is 2.55 dB.

Blade vortex interaction noise (BVI), which is generated by an impulsive change in the pressure distribution over the interacting blade preceding the pressure jump, propagates sound to far field observers. In general, these interactions occur in forward-descent flight conditions, especially during a landing approach. The acoustic signal from BVI is generally in the frequency range to which the human subjective response is most sensitive. In order to reduce BVI noise, many researchers have been studying not only passive remedies such as rotor tip design and leading edge modification, but also active devices such as a higher harmonic control method, active tabs, flap and tip-jet blowing.The National Aerospace Laboratory (NAL) in Japan and Pohang University of Science and Technology (POSTECH) in Korea have conducted a collaborative research program on the effects of lateral wing-tip blowing to reduce BVI noise from helicopter rotors. The lateral wing-tip method is one of the active control methods to control the generation and behavior of tip vortical flow by blowing a jet flow at the tip of the main rotor. In the first stage of the research, three-dimensional compressible Euler/Navier-Stokes equations are solved to calculate the effect of blowing air from the blade tip on the tip vortex of a fixed single blade. In the next stage, predictions of BVI noise will be made by combining an unsteady Euler code with an aeroacoustic code based on the Ffowcs-Williams and Hawkings formulation. The present report corresponds to the first part of this effort: determining the effect of blowing air from the blade tip on the tip vortex of a fixed single blade. The computed circumferential velocity profiles of the tip vortex are compared with experimental results with a single fixed wing to validate the numerical technique. The numerical results include the position of the vortex center along the vortical flow, the size and strength of the rolled tip vortex, and the circulation and maximum tangential velocity of the tip vortex under various jet conditions. Jet flow from the wing tip can diffuse the tip vortex such as producing larger core sizes and lower velocity gradients, which can be effective ways to reduce BVI noise from a rotary wing.