• 한국콘크리트학회:학술대회논문집
• /
• 한국콘크리트학회 2003년도 가을 학술발표회 논문집
• /
• pp.675-678
• /
• 2003

Experimental results for variation of concrete strength according to vibration time control for fresh concrete were given. Vibration velocity, time before vibration and vibrating time were used as experimental parameters. Compressive strength, split tensile strength, ana bond strength were investigated and then fracture surfaces of split tensile strength specimen were observed. From the experimental results, it could be concluded that there may be no decrease in concrete strengths if time before vibration will be sustained at least for more than 3 hours.

• 지구물리와물리탐사
• /
• 제4권3호
• /
• pp.84-88
• /
• 2001

탄성파 구조보정에서 초기속도모델과 실제지층속도와의 오차는 심각한 이미지 왜곡을 초래할 수 있다. 따라서, 초기속도 모델의 설정은 성공적인 구조보정을 위한 중요한 요소 중의 하나이다. 초기속도모델로서 단순지층 모델을 적용할 경우, 속도 차가 큰 지층경계면에서는 기존의 주시계산 방법으로는 정확한 주시를 계산할 수 없다. 또한 실제 지하내부가 갖는 선형적 속도변화를 적절히 표현할 수 없다. 본 연구에서 초기모델로 적용한 Smooth Background Model(이하 SBM)은 깊이에 따라 지층속도가 선형적으로 변화하는 모델로서 지하내부의 특성을 적절히 표현할 수 있고, Vidale 방법과 같은 주시계산 알고리즘을 적절히 적용할 수 있다. 본 연구에서는 중합전 구조보정을 위해 키리히호프 연산자를 사용하였으며 모델링을 통해 얻은 절대 진폭값을 가중치로 적용하므로써 초기 모델에 대한 진진폭을 고려하였다. 구조보정을 위한 초기모델은 중합속도를 이용하여 결정하였고, 이를 실제자료에 적용하여 보았다.

• 한국가스학회지
• /
• 제23권3호
• /
• pp.27-32
• /
• 2019

본 연구에서는 단일 원관에서 입구의 유속이 일정한 경우와 주기적인 변동이 있는 경우에 대하여 시간에 따라 원관 주위에서 와류 분포와 온도 분포 변화를 비교 분석하였다. 또한, 양력과 항력의 시간 변화와 PSD(power spectral density)를 분석하여 유동의 주파수 특성을 규명하였다. 일정 입구 유속의 경우는 잘 알려진 칼만 와류 분포를 보여 주었으며 원관의 상하에서 교대로 와류가 발생하는 것을 관찰할 수 있었다. 주기적인 입구 유속 변동의 경우는 원관의 상하에서 동시에 와류가 발생하는 것을 관찰할 수 있었다. 두 가지 모두 시간에 따른 온도 분포 변화는 와류 분포 변화와 거의 유사하게 거동하는 것을 확인할 수 있었다. 일정 입구 유속의 경우의 와류 유동 주파수는 31.15 Hz 이며 주기적인 입구 유속의 경우는 와류 유동 주파수는 입구 유속의 주파수와 동일하게 15.57 Hz으로 나타났다. 원관 표면 평균 누셀수는 일정 입구 유속의 경우는 99.6이며 주기적인 입구 유속 변동의 경우는 110.7로 나타나서 주기적인 입구 속도 변동의 경우가 열전달이 11.1% 증가하는 것을 알 수 있었다.

• 한국기계가공학회지
• /
• 제14권3호
• /
• pp.57-64
• /
• 2015

In this paper, a model reference adaptive control (MRAC) scheme is applied for the precise and robust motion control of a pneumatic system with load variation. The reference model for MRAC is designed systematically using linear quadratic Gaussian control with loop transfer recovery (LQG/LTR). The sigmoid function of inverse velocity is used to compensate for the nonlinear friction force between the sliding parts. The experimental results show that MRAC effectively overcame the limit of the PID controller when there was unknown disturbance, including abrupt load variation and model uncertainty in the pneumatic control system.

• 대한기계학회논문집B
• /
• 제32권12호
• /
• pp.970-976
• /
• 2008

Characteristics of methane turbulent non-premixed flame have been studied experimentally in coflow jets with initial temperature variation. The results showed that the premixed flame model and the large-scale mixing model for turbulent flame stabilization were effective for methane fuel considered initial temperature variation. Especially, the premixed flame model has been improved by considering nitrogen dilution for the liftoff height of turbulent lifted flame. In estimating blowout velocity and the liftoff height at blowout with the premixed flame model and the large-scale mixing model, the two turbulent models were excellently correlated by considering the effect of physical properties and buoyancy for the initial temperature variation.

• Journal of Biosystems Engineering
• /
• 제34권5호
• /
• pp.351-357
• /
• 2009

This study was conducted to verify the simulation model through the drying test, and investigate effect of factors, such as temperature of drying air, airflow rate, and velocity of the airflow, on the drying. The low temperature drying simulation model was developed based on the circulation dry simulation model presented by Keum et al. (1987), and by modifying low temperature thin layer drying model, equilibrium moisture content model, latent heat of vaporization model, and crack ratio prediction model. The heat pump and experimental dryer with a capacity of 150kg were used for the test. The RMSE between the predicted and measured value was 0.27% (drying temperature), 0.15% (crack ratio), and 2.08% (relative humidity), so the relevance of the model was verified. In addition, the effect of drying temperature, airflow rate, and velocity of the airflow on the drying was examined. The experimental results showed that the crack ratio at drying temperature of $25{\sim}40^{\circ}C$ was allowable. Moreover, at below $30^{\circ}C$, variation of the crack ratio was slight, but drying time was delayed. Given these results, the drying temperature of over $30^{\circ}C$ was effective. As the airflow rate increased, required energy dramatically increased. Whereas drying rate slowly increased, so loss of drying efficiency was caused. Considering these results, the dryer needed to be designed and adjusted to lower than $30\;m^3/min{\cdot}ton$. As velocity of the airflow increased, required drying energy increased when the velocity of the airflow was over $5\;m^3$/hr, while crack ratio and drying rate showed little variation.

• International Journal of Aeronautical and Space Sciences
• /
• 제13권3호
• /
• pp.386-397
• /
• 2012

The stability and structure of bluff-body stabilized hydrogen flames were investigated numerically and experimentally. The velocity of coflowing air was varied from subsonic velocity to a supersonic velocity of Mach 1.8. OH PLIF images and Schlieren images were used for analysis. Flame regimes were used to classify the characteristic flame modes according to the variation of the fuel-air velocity ratio, into jet-like flame, central-jet-dominated flame, and recirculation zone flame. Stability curves were drawn to find the blowout regimes and to show the improvement in flame stability with increasing lip thickness of the fuel tube, which acts as a bluff-body. These curves collapse to a single line when the blowout curves are normalized by the size of the bluff-body. The variation of flame length with the increase in air flow rate was also investigated. In the subsonic coflow condition, the flame length decreased significantly, but in the supersonic coflow condition, the flame length increased slowly and finally reached a near-constant value. This phenomenon is attributed to the air-entrainment of subsonic flow and the compressibility effect of supersonic flow. The closed-tip recirculation zone flames in supersonic coflow had a reacting core in the partially premixed zone, where the fuel jet lost its momentum due to the high-pressure zone and followed the recirculation zone; this behavior resulted in the long characteristic time for the fuel-air mixing.

• 대한조선학회논문집
• /
• 제51권4호
• /
• pp.311-320
• /
• 2014

Test uncertainty of a towed underwater Stereoscopic Particle Image Velocimetry (SPIV) system was assessed in a towing tank. To estimate the systematic error and random error of mean velocity and turbulence properties measurement, velocity field of uniform flow was measured. Total uncertainty of the axial component of mean velocity was 1.45% of the uniform flow speed and total uncertainty of turbulence properties was 3.03%. Besides, variation of particle displacement was applied to identify the change of error distribution. In results for variation of particle displacement, the error rapidly increases with particle movement under one pixel. In addition, a nominal wake of a model ship was measured and compared with existing experimental data by five-hole Pitot tubes, Pitot-static tube, and hot wire anemometer. For mean velocity, small local vortex was identified with high spatial resolution of SPIV, but has serious disagreement in local maxima of turbulence properties due to limited sampling rate.

• 한국지반공학회:학술대회논문집
• /
• 한국지반공학회 2010년도 춘계 학술발표회
• /
• pp.176-181
• /
• 2010

The evaluation of shear modulus (or shear wave velocity) profile of the site is very important in various fields of geotechnical engineering. In the field, there exist spatial variations of shear modulus that case uncertainty in the geotechnical analysis or design. So it is necessary to evaluate the spatial variation of shear wave velocities of the soil site. In this study, the HWAW method is applied to the determination of a 3-D Vs map of soil site. The HWAW method, which is based on harmonic wavelet transforms, has been developed to determine phase and group velocities of waves. The HWAW method uses only the signal portion of the maximum local signal/noise ratio to evaluate the phase velocity in order to minimize the effect of the noise. The field testing of this method is relatively simple and fast because only one experimental setup, which consists of one pair of receivers on the surface, is needed using a short receiver spacing setup (1~3m). These characteristics make it possible to determine detailed local Vs profile in the site with lateral Vs variation and to evaluate 3-D Vs map by performing a series of tests on the grid. To estimate the applicability of the proposed method, field tests were performed. Through field applications validity and applicability of the proposed method were verified.

• 한국환경과학회지
• /
• 제17권7호
• /
• pp.783-793
• /
• 2008

The purpose of this study is to analyze the sensitivity of the RMA2 model parameters reflecting the flow characteristics of stream junction and thus understand the hydraulic characteristics of the channel confluence flow. This study dealt with the input parameters of the RMA-2 model, a two-dimensional numerical analysis model widely used for researches both at home and abroad. The parameters of the RMA-2 model are roughness coefficient, turbulent diffusion coefficient, Coriolis forces latitude, Density, and mesh size. This study those parameters estimated from actual heavy rainfall, and varied the parameter size by (-)30%${\sim}$+30% to review the characteristics of the flow characteristics of the channel section. Weobserved that when the ratio of the channel width was relatively small, the smaller the approaching angle was, the farther from the junctions became the generating place of the maximum flow velocity, however, when the ratio of the channel width was relatively large, the larger the approaching angle was, the farther the generating place of the maximum flow velocity from the junctions became. In particular, the distance between junctions and the place where the maximum flow velocity generated showed an absolute correlationover 90% of the relative channel width, but an inverse relationwas found when the distance to the place where the flow velocity generated was shortened as relative the channel width between the main channel and tributary increased.