• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.33-38
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• 2001

Bifurcation of unstable symmetric flow patterns to stable asymmetric ones in laminar sudden-expansion flow has been numerically investigated. Computations were carried out for an expansion ratio of 3 and over a range of the flow Reynolds numbers by using numerical methods of second-order time accuracy and a fractional-step method that guarantees divergence-free flowfields at all times. The critical Reynolds number above which bifurcation of pitchfork type to asymmetric flow pattern takes place is lower in a flow with a higher expansion ratio, in agreement with the previously reported results. The bifurcation diagrams show that the bifurcation takes place at a Reynolds number, $Re_c = 86.3$, higher than the value that has been reported. The lower critical Reynolds number may be due to deficiencies in their computations which employed SIMPLE-type relaxation methods rather than the initial-value approach of the present study. Characteristics of the flow development during the transition to asymmetric stable flow have been investigated by using spectral analysis of the velocity signals obtained by the simulations.

• Korea-Australia Rheology Journal
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• 제18권3호
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• pp.161-167
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• 2006

This study presents results on the numerical simulation of Newtonian and non-Newtonian flow in a channel obstructed by an asymmetric array of obstacles for clarifying the descriptive ability of current non-Newtonian constitutive equations. Jones and Walters (1989) have performed the corresponding experiment that clearly demonstrates the characteristic difference among the flow patterns of the various liquids. In order to appropriately account for flow properties, the Navier-Stokes, the Carreau viscous and the Leonov equations are employed for Newtonian, shear thinning and extension hardening liquids, respectively. Making use of the tensor-logarithmic formulation of the Leonov model in the computational scheme, we have obtained stable solutions up to relatively high Deborah numbers. The peculiar characteristics of the non-Newtonian liquids such as shear thinning and extension hardening seem to be properly illustrated by the flow modeling. In our opinion, the results show the possibility of current constitutive modeling to appropriately describe non-Newtonian flow phenomena at least qualitatively, even though the model parameters specified for the current computation do not precisely represent material characteristics.

• 대한기계학회논문집B
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• 제30권5호
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• pp.480-488
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• 2006

Numerical simulations of flow over two spheres placed in a tandem arrangement are conducted to investigate the effect of the inter-sphere spacing on the flow characteristics. The Reynolds numbers considered are 100, 250, 300 and 425, corresponding to steady axisymmetric, steady planar-symmetric, unsteady planar-symmetric, and unsteady asymmetric flows, respectively, in the case of a single sphere. For small inter-sphere spacings, the flow past two spheres is more stable than that past a single sphere. For example, with the spacing of the sphere radius, the flow is steady axisymmetric up to Re=300. However, for relatively large spacings, the flow past two spheres becomes unstable and vortex shedding takes place even at Re=250. The drag coefficient of the rear sphere decreases significantly with decreasing inter-sphere spacing due to reduction of the stagnation pressure, thus being smaller than that of the front sphere. Also, the rear sphere shows large fluctuations of the lift force as compared to the front one in the case of unsteady flow.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2434-2439
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• 2008

This paper was studied on the cavitation instability of a Solution Pump Inducer in an absorption chiller-heater. Inlet pressure of LiBr and rotational speed at nominal mode are 2,800 Pa and 3,500 rpm respectively. Due to the marginal operation of available NPSH, the cavitation performance of the inducer is critical for the stable operation without the deterioration of head performance. In the study, cavitation performance and its mode of instability was investigated experimentally. Water was used as the working fluid and the test inducer was scaled up as 1.75 times for detail measurements and flow visualization. Inlet pressure was controlled by a vacuum pump. This research focused on types of cavitation instability and phenomena to investigate the possibility of harmful damage due to cavitation instability. Casing wall pressure and instantaneous inlet pressure was measured to observe the unsteady flow characteristics. Through the visualization and spectrum analysis of pressure, the occurrence region and intensity of asymmetric cavitation and cavitation surge are analyzed in the test inducer.

• 혜화의학회지
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• 제15권2호
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• pp.263-272
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• 2006

Purpose Brain vessles have autoregulation function, so even when perfusion pressure drops, cerebral blood flow remain stable by vasodilation. Latest research on this reserve of cerebral vessels is being done using TCD, which measures the reserve of the vessels. We did a research comparing cerebral vessel and peripheral vessel reserve between Taeumin, who are more likely to suffer CVA, and the normal. We observed blood flow of Internal carotid artery siphon and radial indicis artery of the two group with TCD. Method We picked 20 people out of patients diagnosed as cerebral infarction at Cheon-An Oriental hospital of Daejeon University. They were diagnosed as Taeumin with QSCCII questionnaire and constitutional differentiation. Using TCD, we measured highest blood flow rate, mean blood flow and asymmetric counting blood flow of Internal carotid artery siphon and radial indicis artery at rest. And then we measured again after stimulating cerebral vessels, by triggering hypercapnia by self apnea and peripheral vessels by palm heating. Result At rest, mean blood flow rate of Internal carotid artery siphon showed significant decrease compared to control group. Blood flow rate of Internal carotid artery siphon after hypercapnia showed significant decline in highest blood flow rate and mean blood flow compared to control group. Cerebral vessel reaction after the hypercapnia induction showed great change in experiment group than the control group. Peripheral vessel reaction after palm heating showed significant decline in experiment group compared to control group. Conclusion In conclusion, measuring the alteration of blood flow used in diagnosing cerebral infarction, is more sensitive when vessel stimulation is done. Non-invasive TCD is effective especially in case of Taeumin who are more likely to suffer vascular disorder than others.

• 대한기계학회논문집B
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• 제21권1호
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• pp.99-112
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• 1997

Mixing process of impinging jets of liquid oxidizer and liquid fuel is simulated by using water and sodium carbonate (Na$_{2}$CO$_{3}$) solution. The shapes of liquid sheets are visualized and flowrate distributions are measured by collecting droplets using measuring cells. Mixing charateristics are studied by using acid-base titration. Stable liquid sheets are formed and two liquid jets are well mixed for symmetric impinging jets. Similarity in flowrate distribution for various measuring heights is observed. For asymmetric impinging jets, liquid sheets become unstable as the difference in the velocities of jets increases. In some extreme cases, liquid sheets are not formed and the jets are separated. Dimensionless variables are adopted demonstrating similarly in flowrate distribution. Mixing characteristics vary significantly with experiment conditions.

• 한국항공우주학회지
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• 제43권8호
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• pp.677-683
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• 2015

하이브리드 로켓은 특정한 연소조건에서 10~30Hz 저주파수 연소불안정이 나타난다. 후연소실의 와류 흘림 현상이 저주파수 불안정 발생과 직접적인 관련이 있는 것으로 판단되며, 이를 확인하기 위하여 발광하는 연소가스의 후연소실 내부유동을 직접 촬영하여 광도 분석과 유동 가시화를 시도하였다. 저주파수 연소불안정이 발생하면 일정한 주기(~18Hz)를 갖는 광도 변화가 나타났으며, 압력교란(p')의 위상(phase)이 거의 일치하고 있어 비-음향 불안정임에도 불구하고 압력과 연소교란이 상호 간섭하고 있음을 확인하였다. POD에 의한 유동 모드 분석결과 안정한 연소가 발생하면 후연소실 유동은 공간적으로 상하 대칭 모드 형태를 이루지만 불안정 연소에서는 수직방향 경사각을 이루는 대칭축을 중심으로 변화하는 유동모드가 나타난다. 특히 3번 모드는 불안정 연소가 발생하는 경우에만 나타나는 유동 모드이다. 불안정 연소가 발생하는 경우에 수직선을 대칭으로 변하는 모드가 나타나는 것은 저주파수 연소불안정이 발생할 때 와류 흘림인 것으로 판단할 수 있다.

• 한국압력기기공학회 논문집
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• 제17권1호
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• pp.18-27
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• 2021

This study aims to develop an improved evaluation technology for assessing CANDU-6 safety. For this purpose, the multiple steam generator tube rupture (mSGTR) followed by an unmitigated station blackout (SBO) in a CANDU-6 plant was selected as a hypothetical event scenario and the analysis model to evaluate the plant responses was envisioned into the MARS-KS input model. The model includes logic models for controlling the pressure and inventory of the primary heat transport system (PHTS) decreasing due to the u-tubes' rupture, as well as the main features of PHTS with a simplified model for the horizontal fuel channels, the secondary heat transport system including the shell side of steam generators, feedwater and main steam line, and moderator system. A steady state condition was successfully achieved to confirm the stable convergence of the key parameters. Until the turbine trip, the fuel channels were adequately cooled by forced circulation of coolant and supply of main feedwater. However, due to the continuous reduction of PHTS pressure and inventory, the reactor and turbine were shut down and the thermal-hydraulic behaviors between intact and broken loops got asymmetric. Furthermore, as the conditions of low-flow coolant and high void fraction in the broken loop persisted, leading to degradation of decay heat removal, it was evaluated that the peak cladding temperature (PCT) exceeded the limit criteria for ensuring nuclear fuel integrity. This study is expected to provide the technical bases to the accident management strategy for transient conditions with multiple events.

• 경제분석
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• 제27권1호
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• pp.67-90
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• 2021

본 연구는 투자자의 거래행태를 민감하게 반영하는 비트코인(BTC) 시장 자료를 이용한 실증분석을 통해 COVID-19 팬데믹이 발생하기 전과 후에 변동성과 거래량의 비선형 관계가 바뀌었다는 흥미로운 사실을 발견하였다. 즉, COVID-19 팬데믹 이전의 안정적 시장 상태에서는 정보 유입 패러다임에 근거한 이론처럼 두 변수의 관계가 양(+)으로 나타났지만 COVID-19 팬데믹 기간에 발생한 극단적 시장 스트레스 상태에서는 두 변수의 의존관계 구조가 달라지고 심지어 음(-)의 관계가 나타났다. 이는 행태경제학적 관점에서 COVID-19 팬데믹 기간의 시장 스트레스 증가가 투자자의 거래 행태(behavior)를 변화시켜 자산시장에 구조변화를 일으켰으며, 변동성과 거래량의 비선형 의존관계(특히, 극단적 분위수(quantiles)의 의존성)에 중대한 영향을 미친 결과라고 추론해 볼 수 있다. 따라서 정보 유입 외에 시장 스트레스로 인한 행태적 편의나 군집행동(herding)과 같은 심리현상이 두 변수의 의존관계 구조를 변화시키는 주요인이 될 수 있다는 전제하에 이를 검정해보았다. 본 연구는 실증분석을 위해 Ross (2015)의 구조변화 탐지 검정을 수행하였으며, 독립적이고 동일하게 분포하는(i.i.d.) 임의변수 가정 없이 비선형 관계구조와 분포 꼬리 부분의 비대칭적 의존관계를 면밀히 파악할 수 있는 Copula 회귀분위수(CRQ) 접근법을 제안하였다.