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"Noh, Hee-Min"
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أغاني الأشجار : حياة شجرة
يتناول كتاب (أغاني الأشجار : حياة شجرة) لمؤلفه (مين كيونج كانج) معلومات عن النباتات ويجيب هذا الكتاب على بعض الأسئلة منها : ترى من هي الملكة المتوجة على عرش الغابات الشجرية ؟ هل هي تلك التي تمنح العالم حاجته من الأكسجين والعناصر المغذية ؟ هيا بنا نتعرف على حقائق مدهشة عن نباتات لم نعرفها من قبل وعلى أساليبها المتنوعة في التكيف مع الحياة وحيويتها التي لا حد لها.
Book
Enhancement of sound transmission loss of railway frame using ribless panels
The railway car body is made of aluminium extrusion comprising various rib shapes. The ribs are designed to enhance the stiffness of the car structure. Although the ribs are intended to support large panels of the aluminium extrusion, they can degrade the noise reduction effect owing to multiple resonant vibrations at high frequencies. Therefore, a scientific attempt was made to improve the railway frame by using ribless panels. First, a structural analysis of the original aluminium extrusion was conducted using computer simulation. The simulation results revealed that the ribs of the frames transmitted noise owing to their resonance in the high-frequency range. Therefore, a new railway frame model, named the ribless panel, was developed in this study. Vibration simulations indicated that the ribless panel was effective in reducing noise and vibrations in the high-frequency range. Furthermore, the ribless panel was reinforced with acrylic plastic to reduce the resonant vibrations of the plates. After examining the transmission loss in cross-sectional railway frames, a comparative test of the noise reduction performance was conducted in the reverberation chamber. The results indicated that the reinforced ribless panel improved the transmission loss in the overall high-frequency range. Specifically, 6.7 dB of transmitted noise was reduced using the new panel suggested in this research.
Journal Article
Aluminum panel vibration reduction using single lead zirconate titanate and resonant circuit
This study aimed to reduce the vibration of an aluminum plate using a single lead zirconate titanate (PZT) electrical circuit. First, the natural frequencies and modes of the aluminum plate were analyzed using an analytical method to determine the dynamic characteristics of the plate. The frequency domain was used to derive the vibration characteristics of the acoustically excited aluminum plate. It was noted that the resonance region where the vibration of the panel increased rapidly was at 390 Hz, which was then set as the reduction target. Thereafter, the vibration reduction performances of the circuit that connects only the resistance element to the PZT panel and the resonant circuit that uses an inductor were compared. Through actual acoustic excitation, the vibration reduction of the aluminum panel using the PZT panel based on resistance and resonance circuits was validated. The results demonstrated that the vibration in the resonant frequency region of 384 Hz was reduced by 20% through the implementation of a resonant circuit, comprising an inductor and a resistor, added to a single PZT, whose weight was 0.08% of the weight of the aluminum panel. Therefore, this study confirmed that the vibration of aluminum plates can be effectively reduced using a low-weight PZT patch.
Journal Article
Acoustic energy harvesting using piezoelectric generator for railway environmental noise
High-speed trains have a sustained high-noise level for long periods during operation. Although such high-noise levels are effective for acoustic energy harvesting, a practical design for an acoustic energy harvesting system from a high-speed train is lacking. In this study, the design of an energy harvesting system was implemented utilizing noise from a high-speed train during practical operation. We investigated the noise generated from a high-speed train and derived the characteristics of the main noise sources. The results confirmed that low-frequency noise of 50–200 Hz was generated in the passenger, cab, and between car sections. Results from this investigation were used to design a Helmholtz resonator for a target noise of 174 Hz based on a theoretical model. Moreover, numerical simulation was conducted using sound source speakers to investigate vibrations in the walls of the resonator. Finally, energy harvesting experiments were conducted using various types of piezoelectric elements such as rectangular and circular plates. Experimental results indicate that approximately 0.7 V was generated for an incident sound pressure level of 100 dB using a large rectangular plate. Such power level is sufficient to power a variety of low-power electric devices.
Journal Article
Improving transmission loss performance of aluminum extrusion in railway vehicles using lightweight material
This study developed a method for improving the transmission loss performance of aluminum extrusion by reinforcing the inner space of the extruded material with a lightweight material, such as acrylic plastic. First, the dynamic characteristics of the aluminum extrusion were analyzed, and the characteristics of noise transmission were evaluated through an acoustic mode analysis of the air layer for the inner space of the extruded material. Furthermore, to improve the transmission loss performance of the aluminum extrusion, methods were developed for increasing the vibration stiffness and altering the acoustic mode of the inner space. Additionally, a simulation analysis of the vibration mode and a response analysis based on the excitation of sound were performed to validate the proposed model; the effectiveness of the model for noise and vibration reduction was compared with that of the existing model. This analysis was conducted using models wherein the acrylic plastic was partially and totally inserted into the interior space of aluminum extrusion. The results indicated that the noise performance of the short reinforcement model with the partially inserted acrylic plastic was improved by 3 dB and that of the long reinforcement model with the totally inserted acrylic plastic was degraded by > 4 dB compared with the existing aluminum panel.
Journal Article
Improvement of transmission loss of bellows through thickness improvement and structural modification
The bellows of the vehicle are vulnerable to noise because of the low transmission loss among the components. Therefore, in this study, we modified the thickness and the structure of the bellows to improve transmission loss. Based on the impedance tube test, the transmission loss of the silicon rubber specimen – the main material of the bellows – was analyzed; the results confirmed low transmission loss in the low-frequency region. An analysis of the natural vibration model of the simple model indicated that in the low-frequency region of the bellows, a number of vibrations occurred because of the vibration of the outer and inner components. Accordingly, to improve transmission loss, the improvement introduced by varying the thickness was analyzed, and the results confirmed that the noise performance improved by more than 3 dB for a thickness of 3.5 mm in the double-layer structure. In addition, the transmission loss improved in the low-frequency region after acoustic-structure coupling analysis was performed based on a simple model. To compare the actual performance between the existing and improved bellows, a noise comparison test was performed. The test results confirm that the existing noise reduction index improved by 3 dB from 30 to 33 dB when the thickness was increased to 3.5 mm; in the frequency domain, the highest noise performance was improved with an improvement of 5.6 dB at 160 Hz.
Journal Article
Wind tunnel test analysis to determine pantograph noise contribution on a high-speed train
In this study, we investigated the characteristics and the influence of the aero-acoustic noise generated from a pantograph using various experimental approaches in a wind tunnel. First, the noise generated at various flow velocities was measured and analyzed using a full-scale pantograph model. Then, the noise generated from the main position of the pantograph was derived using a microphone array attached to one side of a wind tunnel. The noise contributions of the main components of the pantograph were derived from the noise measurements obtained from a step-by-step disassembly of the full-scale model. In addition, the noise reduction achieved by panhead collectors, which are some of the most important noise sources on a pantograph, was examined by studying the results obtained when varying their geometry. In order to analyze the noise-reduction effect achieved by varying the height of the collector, different types of collectors were fabricated and wind tunnel tests were conducted. Through this study, we have investigated the aero-acoustic noise contribution of the major components of a pantograph, and we have developed effective noise-reduction measures for the panhead collector.
Journal Article
Numerical analysis of aerodynamic noise from pantograph in high-speed trains using lattice Boltzmann method
A pantograph in contact with a catenary for power supply is one of the major aerodynamic noise sources in high-speed trains. To reduce pantograph noise, it is essential to understand the noise generation mechanism of the pantograph. However, it is difficult to determine this mechanism through measurement. Therefore, in this study, the aerodynamic and acoustic performances of a pantograph in a high-speed train were investigated through numerical analysis using the lattice Boltzmann method. First, a real-scaled pantograph was modeled through computer-aided design. Then, the surface and volume meshes of the pantograph model were generated for simulation analysis. Numerical simulation was conducted at a speed of 300 km/h based on the lattice Boltzmann method. Based on the time derivative analysis of flow pressures, it was concluded that the panhead, joint, and base were the dominant noise sources in the pantograph. In particular, various vortexes were generated from the metalized carbon strip of the panhead. The peaks of the sound pressure level propagated from the panhead were 242, 430, and 640 Hz. The noise generation mechanism was analyzed through numerical simulation using noise characteristics.
Journal Article
Noise reduction in high-speed train gangways using fairings and side barriers
Interior noise in high-speed trains creates passenger discomfort and fatigue. In particular, the noise generated around the gangway between carriages tends to be easily transmitted to the passenger spaces due to the large noise component in the low-frequency region. In addition, the noise from the between-cars space around the gangway exterior tends to increase significantly when the high-speed train is travelling inside a tunnel. Therefore, this study analyses the cause of the noise generated in the gangways and identifies an effective method for reducing it. First, the mechanism for noise generation around the gangways was determined by computational flow noise analysis using a simple two-dimensional cavity model. From this simulation, it was confirmed that noise is generated when air flow enters the cavity. To investigate the influence of noise reduction in the between-cars section, blocking the airflow indirectly by installing fairings or directly by using side barriers was proposed. These noise reduction methods were examined by flow noise analysis, and then applied to an actual high-speed train. The interior noise was measured, and the results show that the use of side barriers was effective in reducing interior noise by 10 dB or more. In particular, a high noise reduction effect was exhibited in the low-frequency region. Therefore, this study verifies that directly blocking the inflow into the between-cars section is an effective noise reduction method.
Journal Article
Improvement of noise reduction performance in bellows using multilayer perforated panels
When the speed of a railway vehicle increases, the level of noise inside the vehicle inevitably increases as well, which is a major cause of discomfort to passengers. The most effective method is to improve the overall noise reduction performance of a vehicle. In particular, the gangway of the railway vehicle is made of silicone rubber; therefore, its noise reduction performance is inferior to that of other components of the vehicle. Thus, it is essential to improve the interior noise performance of railway vehicles. This study aims to reduce the noise in the low-frequency region of a railway vehicle gangway. It examines the applicability of the multi-layered resonance type panel, which has not been previously applied to the bellows in railway vehicles. In particular, the transmission loss was improved by changing the structure without filling the bellows with sound-absorbing material. First, a theoretical review of the noise reduction performance of a perforated multilayer structure was performed. Based on this, the major design parameters of the perforated multilayer structure that are effective in reducing noise in the low-frequency region of the bellows were derived. Through this, it was confirmed that in the multilayered structure, the hole diameter of 1 mm was effective in increasing the transmission loss in the low-frequency region, and the transmission loss was improved at 1% of the porosity. In addition, through a simple two-dimensional analysis model, it was confirmed that the transmission loss of the porous panel was improved at low frequencies of 100 to 400 Hz. Based on this result, a gangway with perforated multilayer structures was developed and tested. Through this verification test, it was confirmed that the noise performance of 9.2 dB was an improvement in the low frequency range of 100 Hz.
Journal Article