• The Journal of the Acoustical Society of Korea
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• 제40권1호
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• pp.55-63
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• 2021

In this study, a numerical methodology is proposed for evaluating valve flow noise in a pipe conveying high pressure gas, and the effects of perforated plates on reduction of such valve flow noise are quantitatively analyzed. First, high-accurate unsteady compressible Large Eddy Simulation techniques are utilized to predict flow and flow noise by a valve in a high-pressure pipe. The validity of the numerical result is confirmed by comparing the predicted wall pressure spectrum with the measured one. Next, the acoustic power of downstream-propagating acoustic waves due to the valve flow is analyzed using an acoustic power formula for acoustic waves propagating on mean flow in a pipe. Based on the analysis results, perforated plates are designed and installed downstream of the valve to suppress the valve flow noise and the acoustic power of downstream-going acoustic waves is predicted by using the same numerical procedure. The reduction by 9.5 dB is confirmed by comparing the predicted result with that of the existing system. Based on these results, the current numerical methodology is expected to be used to reduce valve flow noise in an existing system as well as in a design stage.

• The Journal of the Acoustical Society of Korea
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• 제40권6호
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• pp.555-565
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• 2021

The goal of this study is to improve flow and noise performances of existing backward-curved blade centrifugal fan system used for circulating cold air in a refrigerator freezer by optimally designing airfoil shape. The unique characteristics of the system is to drive cold airflow with two volute tongues in combination with duct system in a back side of a refrigerator without scroll housing generally used in a typical centrifugal fan system. First, flow and noise performances of existing fan system were evaluated experimentally. A P-Q curve was obtained using a fan performance tester in the flow experiment, and noise spectrum was measured in an anechoic chamber in the noise experiment. Then, flow characteristics were numerically analyzed by solving the three-dimensional unsteady Navier-Stokes equations and noise analysis was performed by solving the Ffowcs Williams and Hawkins equation with input from the flow simulation results. The validity of numerical results was confirmed by comparing them with the measured ones. Based on the verified numerical method, blade inlet and outlet angles were optimized for maximum flow rate using the two-factor central composite design of the response surface method. Finally, the flow and noise performances of a prototype manufactured with the optimum design were experimentally evaluated, which showed the improvement in flow and noise performance.

• The Journal of the Acoustical Society of Korea
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• 제38권2호
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• pp.168-176
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• 2019

When an array receives a signal with a frequency higher than the design frequency, there is an ambiguity in beamforming due to spatial aliasing. In order to overcome this problem, Abadi proposed frequency-difference beamforming. However, there is a constraint that the minimum frequency bandwidth is required according to the value of the difference frequency. In this paper, we propose a method to find the direction of the target signal with spatial aliasing based on the frequency-wavenumber spectrum combined with Radon transform. The proposed method can estimate the direction of the target without ambiguities when the signal has nonnegligible bandwidth. We tested the algorithm by simulating a broadband signal and verified the results with the frequency-difference beamforming method using SAVEX15 (Shallow Water Acoustic Variability EXperiment 2015)'s shrimp noise data.

• Journal of KSNVE
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• 제11권3호
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• pp.428-436
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• 2001

The rotating inner cylinder executes a periodic translational motion in concentric annulus while the outer one is stationary. In the study of flow-induced vibrations and relaxed instabilities, it is of interest to evaluate the fluid-dynamic forces acting on the rotating inner cylinder. In the present work, the governing equations for the confined flow are expressed as Navier-Stokes equations, including the steady and unsteady terms. The fluid parameters for steady flow generated by the rotating cylinder are determined analytically while the unsteady ones by the oscillatory motion are evaluates by a numerical method based on the spectral collocation method. In order to validate the numerical approach, the numerical results are compared wish the analytical ones given by existing theories, for simple cases where the both approaches are applicable. Good agreement was found between the results. It is found the effects of the Reynolds number, defined by rotating velocity, on the fluid-dynamic forces are important for the case of relatively low oscillatory Reynolds number, defined by oscillatory frequency : j.e., in case of $Re_\omega\gg Re_S$.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 2007년도 추계학술대회논문집
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• pp.1014-1017
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• 2007

Sound focusing technologies has been studied for various purposes from early 1990s. As a result, these technologies make us possible to apply in many uses. For example, we can treat tumors using focused ultrasonic waves without surgical knife and communicate in the ocean using time reversal array. Also applications for personal audio system become issues. Recently, as technologies are developing, in some applications, needs for regional focusing become increasing because previously suggested focusing methods, such as phase conjugation, time reversal and inverse filtering, were all about a point focusing. Therefore, studies on regional focusing method are essentially needed. Regional focusing method was firstly mentioned by Choi and Kim in 2002: acoustic contrast control. However, in regional focusing, physical interpretations between control variables and results are still not easy because of its complexity. In this regard, we tried to understand the relations between control variables and results in wavenumber domain and suggested a solution method for regional focusing: wavenumber spectrum matching. We also showed how to make spatially focused sound shape using the suggested method from the simplest case: line focusing.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 2007년도 추계학술대회논문집
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• pp.589-594
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• 2007

In this paper, the optimal command input is considered in order to minimize the residual vibrations of a flexible cantilever beam when the beam simply changes its position by translation or rotation. Although a cantilever beam has many modes of vibration, it is shown that the consideration of the first mode is sufficient in this case. Thus, the problem becomes a singledegree-of-freedom system subjected to a ground excitation. Two simple methods are proposed to find the optimal command input based on the Shock Response Spectrum (SRS). The first method is the simplest and can be applied to lightly damped cases, and the second method is applicable to more general problems. The second method gives almost the same results as the input shaping method. However the proposed method gives a easier and clearer control strategy.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 2008년도 추계학술대회논문집
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• pp.687-692
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• 2008

In a nuclear power plant, loose part monitoring and its diagnostic technique is one of the major issues for ensuring the structural integrity of the reactor system. Typically, accelerometers are mounted on the surface of a reactor vessel to localize impact location caused by the impact of metallic substances on the reactor system. However, in some cases, the number of the accelerometers is not enough to estimate the impact location precisely. In such a case, one of alternative plan is to utilize another type sensors that can measure the vibration of the reactor structure even though the measuring frequency ranges are different from each others. The AE sensors installed on the reactor structure can be utilized as additional sensors for loose part monitoring. In this paper, we proposed a new method to estimate impact location by using both accelerometer signal and AE signal, simultaneously. The feasibility of the proposed method is verified by an experiment. The experimental results demonstrate that we can enhance the reliability and precision of the loose part monitoring.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• 제18권3호
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• pp.323-334
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• 2008

According to the delivery condition, the breakage of a product occurs when it is delivered to the customers. Therefore product's makers evaluate the durability under the delivery process by accelerated life testing. In order to conduct this accelerated life testing accurately, it is very important to identify the acceleration-factor exactly between on-road and accelerated life test condition. In this paper, the acceleration-factor is identified by applying linear damage summation law, rain-flow cycle counting and Dirlik theory under the conditions of the random vibration. And approximated FEM model of the connecting-pipe to the compressor is developed for fatigue analysis. This model is finally verified by comparing the experiment results to the numerical analysis results.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 2002년도 추계학술대회논문집
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• pp.623-629
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• 2002

The spectrum of impulse response signal from an impulse hammer testing is widely used to obtain frequency response function(FRF) of the structure. However the FRFs obtained from impact hammer testing have not only leakage errors but also finite record length errors when the record length for the signal processing is not sufficiently long. The errors cannot be removed with the conventional signal analyzer which treats the signals as if they are always steady and periodic. Since the response signals generated by the impact hammer are transient and have damping, they are undoubtedly non-periodic. It is inevitable that the signals be acquired for limited recording time, which causes the finite record length error and the leakage error. In this paper, the errors in the frequency response function of multi degree of freedom system are formulated theoretically. And the method to remove these errors is also suggested. This method is based on the optimization technique. A numerical example of 3-dof model shows the validity of the proposed method.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 2002년도 추계학술대회논문집
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• pp.881-885
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• 2002

This paper presents the performance and the algorithm of a 1/3-octave band spectrum control system. The system is developed to provide various spectrums in a high intensity acoustic chamber. The required spectrum, which usually comes from launch vehicle company, starts from 25Hz band and ends 10kHz band. Automatic spectrum control system is preferred since the system requires short settling time to guarantee the safety of test objects and to reduce the amount of operating gas. The developed system adapted a PCI data-acquisition/signal-generation board installed in a personal computer to implement whole control logic. The control software used three cascade digital Butterworth filters using software. The filers are designed following ANSI S1.11 standard to implement 1/3 octave band filter bank. The graphical user interface of the system guides the user to follow standard operation procedure. The averaged control spectrum showed less than 0.05 dB in every running 1/3-octave band.