• Structural Engineering and Mechanics
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• v.75 no.4
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• pp.425-434
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• 2020

Practical non-synoptic fluctuating wind often exhibits nonstationary features and should be modeled as nonstationary random processes. Generally, the coherence function of the fluctuating wind field has time-varying characteristics. Some studies have shown that there is a big difference between the fluctuating wind field of the coherent function model with and without time variability. Therefore, it is of significance to simulate nonstationary fluctuating wind field with time-varying coherent function. However, current studies on the numerical simulation of nonstationary fluctuating wind field with time-varying coherence are very limited, and the proposed approaches are usually based on the traditional spectral representation method with low simulation efficiency. Especially, for the simulation of multi-variable wind field of large span structures such as transmission tower-line, not only the simulation is inefficient but also the matrix decomposition may have singularity problem. In this paper, it is proposed to conduct the numerical simulation of nonstationary fluctuating wind field in one-spatial dimension with time-varying coherence based on the wavenumber-frequency spectrum. The simulated multivariable nonstationary wind field with time-varying coherence is transformed into one-dimensional nonstationary random waves in the simulated spatial domain, and the simulation by wavenumber frequency spectrum is derived. So, the proposed simulation method can avoid the complicated Cholesky decomposition. Then, the proper orthogonal decomposition is employed to decompose the time-space dependent evolutionary power spectral density and the Fourier transform of time-varying coherent function, simultaneously, so that the two-dimensional Fast Fourier transform can be applied to further improve the simulation efficiency. Finally, the proposed method is applied to simulate the longitudinal nonstationary fluctuating wind velocity field along the transmission line to illustrate its performances.

• Proceedings of the IEEK Conference
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• 2003.07b
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• pp.943-946
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• 2003

This paper proposes new SPICE Macro-Model of MTJ(Magnetic Tunnel Junction). This Macro-Model has five I/O terminals, reproduces MR characteristics including hysteresis and behaves correctly to time varying input signals. Furthermore, this Model can be easily modified to various MTJs with different characteristics by simply varying internal parameters.

• The Journal of the Acoustical Society of Korea
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• v.8 no.4
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• pp.13-22
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• 1989

Underwater channels can be regarded as time-varying systems in view of the acoustical characteristics due to the fact that the characteristics of the channel are affected by the environmental and geometrical conditions. Especially in shallow water case, the surface and bottom conduct as a waveguide so echo effect due tu the multipath reflections are severe. Therefore in shallow water communications, it is very important to equalize the transmitted signals distorted by the underwater channels with time-varying multipath fading. In this paper an equalizer system which employs the frequency domain adaptive filter to equalize the channels using inverse modeling technique is introduced.

• Journal of the Korean Society for Railway
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• v.12 no.4
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• pp.465-471
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• 2009

In this paper, a relationship between the noise and vibration of a high-speed train's wheelset is examined by using time-varying frequency analysis with random data analysis which together contributes to a reduction in the number of experimental running. The noise and vibration of the wheelset is mainly caused by an interaction between the wheel and railway which shows in non-stationary characteristics. For the analysis, they are measured by some microphones and accelerometers, and those signals are post-processed by time-varying frequency analysis with random data analysis. From the analysis, their methods are useful for analyzing the noise and vibration of high-speed train's wheelset.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.11a
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• pp.517-521
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• 2001

Measurement techniques for the in-duct source characteristics of fluid machines can be classified into direct method and load method, according to whether the technique employs an external acoustic source or not. It has been known that the two methods yield different results and the load method used to come up with a negative source resistance, in spite of the fact that a very accurate prediction of radiated noise can be obtained by using any result. This paper is focused to the effect of time-varying nature of fluid machines on the output result. For this purpose, a simplified fluid machine consisting of a reservoir, a valve and a pipe is considered as representing a typical linear, periodic, time-varying system and the measurement techniques are simulated by utilizing the Hill equation and its steady-state forced response. In the load method, the source impedance turns out being dependent on the valve impedance at the calculation frequency and the valve and load impedances at other frequencies as well.

• 제어로봇시스템학회:학술대회논문집
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• 2002.10a
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• pp.78.5-78
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• 2002

We propose an evolutionary reinforcement learning (RL) system with time-varying parameters that can deal with a dynamic environment. The proposed system has three characteristics: 1) It can deal easily with a dynamic environment by using time-varying parameters; 2) The division of state space is acquired evolutionarily by genetic algorithm (GA); 3) One does not have to design the rules constructing an agent in advance. So far many RL systems have been proposed. These systems adjust constant or non time-varying parameters; by those systems it is difficult to realize appropriate behavior in complex and dynamic environment. Hence, we propose the RL system whose parameters can vary temporally. T...

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.385-392
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• 2001

Time-dependent frequency and energy of free vibration of the Spagetti problem, that is the axially moving continuum with time-varying length, are investigated. Exact expressions for the natural frequency and time-varying vibration energy are derived by dealing with traveling waves. When the string length is increased, the vibration period increases, but the free vibration energy varies as a function of both translating velocity and boundary velocity of the continuum. However, when the string undergoes retraction, the vibration energy increases with time, String tension together with non-zero instantaneous velocity at the moving boundary results in energy variation.

• Journal of the Korean Society for Railway
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• v.3 no.3
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• pp.131-138
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• 2000

The design of current collection system of high speed train requires the fundamental understandings for the dynamic characteristics of catenary system and pantograph. The stiffness of catenary system of high speed train has the varying characteristics for the change of contact point with pantograph, since the supporting pole and hanger make the different boundary conditions for the up-down stiffness of a trolley wire. The variation of stiffness results in Mathiue equation, which characterizes the stability of the system. However, the two-term variation of the stiffness due to span length and hanger distance cannot be solved analytically. In this paper, the stiffness variations are calculated and the physical reasoning of linear model and one term Mathieu equation are reviewed. And the numerical analysis for the two-term variation of the stiffness is done for the several design parameters of pantograph.

• The Transactions of the Korean Institute of Power Electronics
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• v.19 no.2
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• pp.124-132
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• 2014

In this paper, we implement a non-time-varying transfer function of the duty ratio to improve the control characteristics in a control system that Input voltage and the output voltage is varied, DC / DC converters for bi-directional charging. When control is performed with using controller gain of conventional design, characteristics of the control is varied to fluctuations of the input voltage. The proposed method is the equivalently removing method for duty ratio in entire control block, by voltage controller gain is changed for inverse of the duty ratio. The proposed non-time-varying duty ratio transfer function is applied to DC / DC converter for bi-directional charging. In this paper, feasibility and superiority is verified through PSIM simulations and experiments.

• Proceedings of the KIEE Conference
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• 1999.11b
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• pp.113-115
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• 1999

A more reliable relaying algorithm for detecting a high impedance fault (HIF) requires fault currents at the relaying point containing information of various HIF characteristics as well as load conditions. As HIF characteristics, there are buildup, shoulder, nonlinearity and asymmetry. This paper presents a modeling method of a HIF in a distribution system using EMTP. In order to represent HIF characteristics, the proposed method uses two time-varying resistances. The first TVR models nonlinearity and asymmetry using voltage as the input. The second TVR models buildup and shoulder using time information as the input. The proposed method is implemented in EMTP.