• Journal of Korean Society of Coastal and Ocean Engineers
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• v.9 no.2
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• pp.57-62
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• 1997

Based on the facts that significant parts of the harbor response spectrum usually reside in the vicinity of the Helmholtz mode in the eastern part of Korea. economically feasible redemption measures of seiches for malfunctioned harbors already in service is proposed by extending the wisdom of perforated breakwaters and adjusting the width of entrance channel as a control tool. It turns out that as the entrance channel is getting narrower, the harbor system is getting slender due to the increase of added hydrodynamic length so that harbor response can be effectively diminished and separated from the incident wave spectrum where considerable amount of wave energy is located at the lower frequency range.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.68-77
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• 2017

High-frequency combustion instability results from a feedback coupling between the unsteady heat release rate and the acoustic waves formed resonantly in the combustion chamber. It can be modeled as thermoacoustic problems with various degrees of the assumptions and simplifications. This paper presents numerical analysis of self-excited combustion instabilities in a variable-length lean-premixed combustor and designs of passive control devices such as baffle and acoustic resonators in a framework of 3-D FEM Helmholtz solver. Nonlinear behaviors such as steep-fronted shock waves and a finite amplitude limit cycle are also investigated with a compressible flow simulation technique.

• Journal of Astronomy and Space Sciences
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• v.32 no.1
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• pp.1-11
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• 2015

Earth's magnetopause separating the fast and often turbulent magnetosheath and the relatively stagnant magnetosphere provides various forms of free energy that generate low-frequency surface waves. The source mechanism of this energy includes current-driven kinetic physical processes such as magnetic reconnection on the dayside magnetopause and flux transfer events drifting along the magnetopause, and velocity shear-driven (Kelvin-Helmholtz instability) or density/pressure gradient-driven (Rayleigh-Taylor instability) magnetohydro-dynamics (MHD) instabilities. The solar wind external perturbations (impulsive transient pressure pulses or quasi-periodic dynamic pressure variations) act as seed fluctuations for the magnetopause waves and trigger ULF pulsations inside the magnetosphere via global modes or mode conversion at the magnetopause. The magnetopause waves thus play an important role in the solar wind-magnetosphere coupling, which is the key to space weather. This paper presents recent findings regarding the generation of surface waves (e.g., Kelvin-Helmholtz waves) at the Earth's magnetopause and analytic and observational studies accountable for the linking of the magnetopause waves and inner magnetospheric ULF pulsations, and the impacts of magnetopause waves on the dynamics of the magnetopause and on the inner magnetosphere.

• Journal of Magnetics
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• v.21 no.4
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• pp.616-621
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• 2016

This paper presents a new multifunctional active guidewire system for medical applications that uses a magnetic microrobot. The study demonstrated that the proposed microrobot system could swim and be controlled under Low-Reynolds-number (Re) environments in blood vessel models. The prototype of the robotic guidewire, which is driven within a three-axis Helmholtz coil system, consists of a guide-wire, spiral blade, drilling tip, and permanent magnet. The spiral-type microrobot showed stable active locomotion between 3 kA/m and 9.1 kA/m under driving frequency up to 70 Hz in a silicone oil (of viscosity 1000 cst). The microrobot produced a maximum moving velocity of $8.08{\times}10^{-3}m/s$ at 70 Hz and 9.1 kA/m. In particular, the robotic guidewire produced 3D locomotion with drilling in the three-axis Helmholtz coil system. We verified active locomotion, towing of guidewire, steering, and drilling of the proposed robotic guidewire system through experimental analyses.

• Structural Engineering and Mechanics
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• v.69 no.5
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• pp.487-497
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• 2019

Rapid advances in the engineering applications can bring further areas to provide the opportunity to manipulate anisotropic structures for direct productivity in design of micro/nano-structures. For the first time, magnetic affected wave characteristics of nanosize plates made of anisotropic material is investigated via the three-dimensional bi-Helmholtz nonlocal strain gradient theory. Three small scale parameters are used to predict the size-dependent behavior of the nanoplates more accurately. After owing governing equations of wave motion, an analytical approach based harmonic series is utilized to fine the wave frequency as well as phase velocity. It is observed that the small scale parameters, magnetic field and wave number have considerable influence on the wave characteristics of anisotropic nanoplates. Due to the lack of any study on the mechanics of three-dimensional bi-Helmholtz gradient plates made of anisotropic materials, it is hoped that the present exact model may be used as a benchmark for future works of such nanostructures.

• Journal of ILASS-Korea
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• v.29 no.1
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• pp.23-31
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• 2024

This study conducted a 3D thermo-acoustic analysis based on the helmholtz solver to analyze the major resonance modes causing combustion instability in a single-can combustor. The experimental investigations were carried out on a test rig designed by the Korea Institute of Machinery & Materials (KIMM) under various conditions of hydrogen co-firing and fuel staging. Through these experiments, two primary unstable frequencies were identified. To determine the resonance modes of these frequencies, a 3D thermo-acoustic analysis was conducted using temperature information from the test rig. The results confirmed that the unstable frequencies observed in the experiments were all longitudinal modes. Additionally, the mode shapes identified in the analysis facilitated a simplification of the exit geometry for the low-order network model, confirming that this did not significantly affect the fundamental resonance modes.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.249-252
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• 2003

Acoustic behavior in combustion chamber with acoustic cavity is numerically investigated by adopting linear acoustic analysis. Helmholtz-type resonator is employed as a cavity model to suppress acoustic instability. The tuning frequency of acoustic cavity is adjusted by varying the sound speed in acoustic cavity. Acoustic pressure responses of chamber to acoustic oscillating excitation are shown md acoustic damping effect of acoustic cavity is quantified by damping factor. As the tuning frequency approaches the target frequency of the resonant mode, mode split from the original resonant mode to lower and upper modes appears and thereby damping effect is degraded. Considering mode split and damping effect as a function of tuning frequency, it is desirable to make acoustic cavity tuned to maximum frequency of those of the possible splitted upper modes.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2012.05a
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• pp.86-90
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• 2012

In this paper, the combustion instabilities which may occur in the hybrid rocket were studied. The rocket combustor where the vortexes can be generated was designed, and the experiments were performed. The investigations about characteristics on the presence of the diaphragm, the length of the fuel, the diameter of the fuel port, the diameter of the diaphragm, the diameter of the nozzle throat, and the variation of the Ox massflow rate were conducted. The main resonant frequency of the combustion pressure is regarded by the Vortex shedding mode, and it is considered that the other resonant frequency of the pressure fluctuation is hybrid low frequency, or helmholtz mode.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.964-968
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• 2007

A helmholtz resonator has been widely used for the purpose of suppressing low frequency noises propagated from various heat and fluid machineries. However, the resonator has demerits that the absorption bandwidth at resonance frequency is very small and a large cavity is necessary. In order to overcome these problems, in this paper, a resonator with perforated panels at the neck and/or in the cavity is proposed. The absorption performances of resonators are measured by two-microphone method and are estimated by transfer matrix method. The experimentally measured values of normal absorption coefficients are agreed well with the corresponding values from the transfer matrix method. By introducing perforated panels at the neck of a resonator, it is shown that the absorption performances and bandwidth have a significant improvement.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.1702-1707
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• 2003

Absorptive material arrangement method for effective interior noise control is proposed. Sound field with arbitrary boundary condition is formulated by Kirchhoff-Helmholtz integral equation. A simple example such as a rectangular cavity will present physical meaning between changing boundary condition and control of sound field. The effect of changing boundary condition is expressed in modal admittance. From this formulation, an admittance map is presented. The admittance map is the figure to represent position where absorptive material is attached. The admittance map can be assigned to each resonant frequency. There, however, may be common area of those maps. Then, frequency robust arrangement of absorptive material in noise control will be presented.