• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.15 no.3
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• pp.177-186
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• 2003

In the present study, the flow characteristics of developing turbulent flows are investigated at the exit region of a square cross-sectional 180" curved duct with dimensions of 40mm$\times$40mm$\times$4000mm (height $\times$ width $\times$length). Smoke particles produced from mosquito coils were used as seed particles for the LDV measurement. Experiments were carried out to measure axial velocity profiles, shear stress distributions and entrance lengths by using an LDV system and Rotating Machinery Resolver RMR with PHASE software. Experimental results clearly show that the time-averaged Reynolds number does not affect oscillatory flow characteristics because the turbulent components tend to balance the oscillatory components in the fully developed flow region. Also, the velocity profiles are in good agreement with 1/7power law such as the results of steady turbulent flows. The turbulent intensity linearly increases along the walls and is slightly higher, especially in the period of deceleration. On the other hand, the LDV measurements show that shear stress values in slightly higher in the period of deceleration due to the flow characteristics in the exit region. The entrance length where flows become stable appears at the point that is 40 times the length of hydraulic diameter.eter.

• Journal of Advanced Navigation Technology
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• v.18 no.5
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• pp.483-489
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• 2014

Proposed electronic ballast circuit for harmonic reduction includes adding lossless snubber circuit to DC-DC converter. To get inverter for changing DC-AC, a lamp has been changed to equivalence resistance in the standard lamp voltage and current data, also inverter circuit has been interpreted by being changed to R, L, C equivalence circuit. Using converted equivalent circuit, the most suitable circuit constant which can satisfy the characteristics of fluorescent lamps has been decided on and finally designed. It could not only eliminate distortion waveform from pulsation frequency in inverter direct side current but also reduce considerably pulsation rate and switching loss by making input current of inverter discontinuous. The validity for the results of this study has been verified through the experiment to measure harmonic occurrence after applying a newly-manufactured product of electronic ballast to 40 W line tube style fluorescent lamps.

• Proceedings of the KSME Conference
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• 2000.11b
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• pp.397-403
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• 2000

In the present study, flow characteristics of turbulent pulsating flow in a square-sectional $180^{\circ}$ curved duct were experimentally investigated. Experimental studies for air flows were conducted to measure axial velocity and wall shear stress distributions and entrance length in a square-sectional $180^{\circ}$ curved duct by using the LDV with the data acquisition and the processing system. The experiment was conducted in seven sections from the inlet (${\phi}=0^{\circ}$) to the outlet (${\phi}=180^{\circ}$) at $30^{\circ}$ intervals of the duct. The results obtained from the experimentation were summarized as follows ; (1) When the ratio of velocity amplitude ($A_1$) was less than one, there was hardly any velocity change in the section except near the wall and any change in axial velocity distributions along the phase. When the ratio of velocity amplitude ($A_1$) was 0.6, the change rate of velocity was slow. (2) Wall shear stress distributions of turbulent pulsating flow were similar to those of turbulent steady flow. The value of the wall shear stress became minimum in the inner wall aid gradually increased toward the outer wall where it became maximum. (3) The entrance length of turbulent pulsating flow reached near the region of bend angle of $90^{\circ}$, like that of turbulent steady flow. The entrance length was changed by the dimensionless angular frequency (${\omega}^+$).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.8
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• pp.857-864
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• 2012

A linear stability analysis of a diffusion flame with radiation heat loss is performed to identify linearly unstable conditions for the Damk$\ddot{o}$hler number and radiation intensity. We adopt a counterflow diffusion flame with unity Lewis number as a model. Near the kinetic limit extinction regime, the growth rates of disturbances always have real eigenvalues, and a neutral stability condition perfectly falls into the quasi-steady extinction. However, near the radiative limit extinction regime, the eigenvalues are complex, which implies pulsating instability. A stable limit cycle occurs when the temperatures of the pulsating flame exceed the maximum temperature of the steady-state flame with real positive eigenvalues. If the instantaneous temperature of the pulsating flame is below the maximum temperature, the flame cannot recover and goes to extinction. The neutral stability curve of the radiation-induced instability is plotted over a broad range of radiation intensities.

• The Journal of the Korea institute of electronic communication sciences
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• v.8 no.11
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• pp.1675-1680
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• 2013

Recently, the permanent magnet linear synchronous motor as low noise, high speed and high thrust force transportation system has been proposed but this motor causes an increase of material cost because of its characteristic arranging the armature on the full length of transportation lines when this system is applied to the long distance transportation system. Therefore, we suggested discontinuous arrangement method of the armature to solve this problem. However, Detent force which causes thrust force ripple generating noise, vibration and decline of performance is generated when a mover pass between the armatures. Thus, in this paper, we examined characteristic of detent force to reduce the end edge effect according to the end edge teeth's height and auxiliary teeth and suggested the shape that can the most reduce the detent force.

• The Journal of Korean Physical Therapy
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• v.14 no.2
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• pp.145-152
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• 2002

This study was carried out of to determine effects of high voltage pulsed galvanic stimulation on the soleus. target muscle of the sciatic nerve, of white rat two condition. The coditions included normal, and denervated muscle. The histochemical, ultrastructural observations were made. The following results were obtains. 1. The histochemical observations found the inflammatory cells between muscle bundle and muscle fiber since 1-week control group. In addition, nucleus located in the muscle fiber was frequently observed. 2. The experimental group showed a similar phenomenom to the normal muscles in terms of glycogen granules in the 1-week group, where as fiber were not distinguishable in4-weeks group which indicated that the degenerative changes had occured. 3. The NADH-TR reaction showed that the red muscle slightly increased in the 2-weeks group, and the distinguished was impossible the red fiber 4. The ultrastructures of the muscles in both groups were severely bend, and a number of vacuoles were observed due to the destruction of mitochondria..

• The Journal of the Acoustical Society of Korea
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• v.43 no.1
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• pp.122-130
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• 2024

Vibration of compressor in the air-conditioner outdoor unit have known to be main noise source radiated from outdoor unit. However, as the operating speed of compressor increase, the relative contribution of flow induced vibration and noise is also increase. In this paper, the numerical method was established to predict fluid-borne noise from compressor discharging pipe of air-conditioner outdoor unit. In this step, numerical result was compared to experimental one to verify numerical method. Additionally, the effects of pressure pulsation of compressor and compressor vibration into radiated noise were investigated in frequency domain. It was confirmed that the compressor vibration contributed to the low frequency band, while the pressure pulsation dose to the high frequency band.

• Journal of the Korean Society of Visualization
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• v.4 no.1
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• pp.47-55
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• 2006

The left ventricular filling flow is now considered as an indicator which can be used for early diagnosing of cardiovascular diseases. Because the understanding of left ventricular flow physics is critical for this purpose, the downstream flow characteristics of the artificial heart valve are investigated using particle image velocimetry (PIV) method. In this study, we investigated the wake characteristics of flows passing through three different artificial valves (St.Jude medical bileaflet mechanical valve. Bjork-Shiley monostrut mechanical valve and St.Jude medical Biocor bio valve). The downstream flow field has remarkably altered according to the different valves. SJM MHV has the flow field similar to the pulsating circular jet and BS MHV has oblique pulsating jet. SJM BHV shows the similar flow field of clinical data of normal heart.

• Journal of the Korean Institute of Telematics and Electronics
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• v.26 no.11
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• pp.1819-1828
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• 1989

In this paper, we determined the modulation index for minimization of speed ripple and position error using maximum speed ripple (SRF) and maximum position error(PEF) in current source TPWM inverter. Through computer simulation, we compared with total current harmonic distortion, SRF and PEF for square wave modulation method and TPWM method. As a result, it turns out that square wave modulation method is superior to TPWM method of 3 pulses per half cycle in speed ripple and position error contents. And TPWM is better than square wave method when pulse number is more than 5. Also, in these pulse numbers, moduladtion index of minimum speed ripple and munimum position error is 0.91.

• Progress in Superconductivity and Cryogenics
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• v.3 no.2
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• pp.69-76
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• 2001

Unsteady components of the second-order axial velocity and temperature within a tapered pulse tube were obtained by using a novel hybrid method of solution which combines an analytical solution with a numerical solution. The effects of operating frequency, taper angle and cold eng temperature on the unsteady components of the second-order axial velocity and temperature were shown. The unsteady component of the second-order mass flux had the amplitude of the same order as the steady component when the velocities at the ends of the pulse tube have only first-order components.