Takashi Aoyama

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.

The effect of blade-tip shape on high-speed impulsive (HSI) noise of helicopter rotors is numerically investigated using a Euler CFD code. Near-field acoustic pressure at 1.1 rotor radii is used for the evaluation of noise intensity, because it is found that the near-field acoustic pressure is in good correlation with the far-field HSI noise calculated by a method combining the CFD code with an acoustic code based on an extended Kirchhoff's formulation. Calculations are performed to analyze the effects of blade thickness and planform on the intensity of HSI noise under a non-lifting hover condition. As a result, the following three factors: 1) strength of shock wave on blade surface, 2) location of shock wave, and 3) area of supersonic region in the vicinity of blade tip, are found to be dominant for the intensity of HSI noise. Newly devised tip shapes that effectively reduce HSI noise are also proposed, based on this result.

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.

A low-noise helicopter blade, AT1, was designed with the concept of reducing noise without the drop of rotor performance. In the concept, High-Speed Impulsive (HSI) noise is reduced by applying a thin airfoil in the tip region and a dog-tooth like extension in the leading-edge of the tip region. Blade-Vortex Interaction (BVI) noise is reduced by applying the extension and a strong taper near the tip end. The stall angle of the blade is increased by the effect of the vortex generated from the leading-edge extension. As a result, the drop of rotor performance caused by the thin airfoil and the reduction of rotor rotational speed is recovered. The low-noise characteristics and the performance of AT1 were evaluated by a model rotor test conducted at Deutsch Niederländischer Windkanal (DNW). It is shown that AT1 reduces HSI noise and BVI noise and has good performance in forward flight conditions. However, the improvement of performance in high-lift conditions still remains as a future problem.