• Journal of the Korean Society of Propulsion Engineers
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• v.23 no.6
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• pp.1-10
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• 2019

The wave motion inside the nozzle is known as one of the major damping elements of the rocket's combustion instability by it's aeroacoustic effects that result from the flow passage through the nozzle throat. These effects can be quantitatively evaluated by the nozzle admittance. In this study, one-dimensional linearized Euler equation was adopted to calculate the nozzle admittance, and trend analysis was performed depending on the nozzle's main design variables. As a result, when nozzle converging part shortens, it is verified that the frequency dependency of the nozzle admittance is decreased due to the widened frequency range with lowered longitudinal nozzle admittance. Also, admittance estimation using the short nozzle theory is not appropriate when the first tangential mode of the pressure perturbation arises.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.7
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• pp.653-657
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• 2015

A numerical investigation to simulate the selective growth of silicon-germanium quantum-dots via local surface diffusivity enhancement is presented. A nonlinear equation for the waviness evolution of film surface is derived to consider the effects of spatially-varying diffusivity, influenced by a surface temperature profile. Results show that the morphology of the initially planar film shapes into an undulated surface upon perturbation, and a steady-state solution describes a fully grown quantum-dot. The present study points toward a fabrication technique that can obtain selectivity for self-assembly.

• Nuclear Engineering and Technology
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• v.12 no.4
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• pp.267-273
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• 1980

The quasistatic approximation is incorporated in HEXKIN, a 2-group, 2-dimensional reactor kinetics code specially developed for a hexagonal lattice-type reactor. The code allows maximum 15 delayed neutron groups, 279 lattice points, and 500 different driving functions to be able to initiate perturbation at each lattice point. Reactivity feedback due to power-dependent fuel temperature change is also involved. To check the accuracy of the code, a result of numerical experiment is compared with the measurement at the Savannah River Laboratory. The experiment was specifically designed to emphasize delayed neutron holdback. The calculated flux tilts agree with the measured flux tilts within the small uncertainty of the measurements.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.1
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• pp.41-51
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• 2004

A direct numerical simulation is performed to analyze the effects of a localized time-periodic blowing on a turbulent boundary layer flow at R $e_{+}$=300. Main emphasis is placed on the blowing frequency effect on near-wall turbulent flow structures at downstream. Wall-normal velocity on a spanwise slot is varied periodically at different frequencies (0.004$\leq$ $f^{＋}$$\leq$0.080). The amplitude of periodic blowing is $A^{＋}$=0.5 in wall nit, which corresponds to the value of $v_{rms}$ at $y^{＋}$=15 without blowing. The frequency responses are scrutinized by examining the phase or time-averaged turbulent statistics. The optimal frequency ( $f^{＋}$=0.03) is observed, where maximum increase in Reynolds shear stress, streamwise vorticity fluctuations and energy redistribution occurs. The phase-averaged stretching and tilting term are investigated to analyze the increase of streamwise vorticity fluctuations which are closely related to turbulent coherent structures. It is found that the difference between PB and SB at a high blowing frequencies is negligible.e.e.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.6
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• pp.1445-1451
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• 1994

To calculate the wave pressure acting on coastal structures under the design wave condition, it is often necessary to numerically reproduce the big standing wave profiles close to wave breaking condition. For this, the governing equation and all nonlinear terms occurring in boundary conditions should be effectively considered in the numerical wave profile. In particular, the velocity square term in the free surface boundary condition is very important. A boundary element method is applied here to calculate the standing wave profile with the velocity square term fully treated by Newton iterative method. In order to check the validity of the method, the numerical wave profiles are compared to ones calculated by the perturbation method, the Fourier approximation method and the hydraulic experiment.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.2
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• pp.83-93
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• 2020

Dynamic pressure transducer needs to be flush-mounted on hardware due to frequency response characteristics of pressure transmission system. However, it is sometimes necessary to be mounted in recessed configuration due to insufficient space for sensor installation and for protection of sensor from thermal damage. Dynamic response characteristics should be considered due to distortion of original dynamic pressure signal in the pressure transmission system. In this study, small perturbation model and 2nd order reduced model were compared with experiments and a guideline for selecting a frequency response model was suggested.

• The Journal of the Acoustical Society of Korea
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• v.18 no.8
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• pp.54-60
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• 1999

The paper describes a theoretical study on the speed of the torsional elastic waves propagating in a circular cylinder whose outer radius varies periodically as a harmonic function of the axial coordinate. The approximate solution for the phase speed has been obtained using the perturbation technique for sinusoidal modulation of small amplitude. It is shown that the wave speed in the cylinder with a corrugated outer surface is less than that in a smooth cylinder by the square of the amplitude of the surface perturbation. This theoretical prediction agrees reasonably with an experimental observation reported earlier. It is also shown that the wave speed reduction due to the surface corrugation becomes larger for a thinner cylinder and for a bigger density of corrugation.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.15 no.3
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• pp.301-310
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• 2004

In this paper, a compact dual-mode filter structure without coupling gaps is proposed. The novel design is achieved by embedding a pair of equal crossed slots and spur-lines. Without coupling gaps between feed lines and patch resonator, the new filter can provide low insertion loss. It is found that this design has wide coupling range for dual-mode operation. It means that these characteristics of the proposed filter can reduce uncertainty in fabrication. By using two PIN diodes mounted inside a pair of spur-lines, the proposed structure works as a switchable filter. Also, it has a size reduction of about 34.7 %, compared with conventional dual-mode filters.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.39 no.10
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• pp.1066-1074
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• 1990

In this paper, we attempt to control induction motors with high power efficiency as well as high dynamic performance by utilizing the recently developed theories : singular perturbation technique and noninteracting feedback control. Our controller consists of three subcontrollers` a saturation current controller, a decoupling controller, and a well-known flux simulator. The decoupling controller decouples rotor speed (or motor torque) and rotor flux linearly. Our controller does not need the rotor resistance that varies widely with the machine temperature. To illuminate the practical significance of our results, we present simulation and experimental results as well as mathematical performance analysis.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.04a
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• pp.13-16
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• 2007

Nonlinear acoustic damping of a half-wave acoustic resonator in a combustion chamber is investigated numerically. First, in a baseline chamber without any resonators, acoustic behavior is investigated over the wide range of acoustic amplitude from 80 dB to 150 dB. Decay rate increases nonlinearly with acoustic amplitude and nonlinearity becomes appreciable at acoustic amplitude above 125 dB. Next, damping effect of a half-wave resonator is investigated. Nonlinear acoustic excitation does not affect optimum tuning condition of the resonator, which is derived from linear acoustics. A half-wave resonator is effective even for acoustic damping of high-amplitude pressure oscillation, but its function of acoustic damper is relatively weakened compared with the case of linear acoustic excitation.