Earticle

본 연구는 연속주조 설비에서 턴디쉬 내의 용강이 유출구를 통해 유출될 때 발생되는 와류 유동에 의해 슬래그가 침지 노즐(SEN) 내로 혼입되는 것을 억제하기 위해 턴디쉬 하부의 소정의 위치에 장착하는 내화 조립체에 관한 발명의 사상을 형상 설계로 구체화한 것이다. 이 내화 조립체는 턴디쉬에서 유출되는 용강이 유입되는 유입구와 침지 노즐이 부착되는 유출구가 서로 대향되게 형성되어 있는 하우징과 이 하우징을 경사방향으로 관통하여 미끄럼 이동하는 다수의 구멍이 형성되어 있는 슬라이드 부재로 구성되어, 그 내부 구조는 전체적으로 S자형 용강 유동경로가 형성되도록 되어 있고 그 외부 형상은 턴디쉬 하부의 소정의 위치에서 그 상부 일부가 턴디쉬 용탕 내로 매입되는 상태에서 턴디쉬 내부와 외부 사이에 장착시켜 턴디쉬 내의 용강 유출 시에 어떠한 규모의 와류도 발생하지 않도록 한 것을 주요 특징으로 한다. 이에 따라, 침지 노즐로 유입되는 용강 유량의 조절은 슬라이드 부재의 미끄럼 이동 제어나 종래의 스토퍼에 의해 달성되면서 슬래그의 혼입은 적극 억제하여 용강청정도 향상과 잔류용탕(잔탕)을 최소화하여 실수율을 향상시킬 수 있는 효과를 발휘한다.

This study is an embodiment of the spirit of the invention into a geometrical shape design, which is related to a refractory assembly for preventing carried-over slag by suppressing the vortex flows of the molten steel discharging from tundish in continuous casting process. The refractory assembly is characterized in comprising a housing having a inlet and a outlet, and a slide member having a plurality of holes; the inlet and oulet being opposedly disposed each other, the inlet for the molten steel discharging from a tundish, the outlet for connecting to a submerged entry nozzle (SEN), and the slide member for sliding with being inserted through the slits on the front and rear walls of the assembly; and the overall internal configuration of the assembly for defining a certain “S" shaped flow path for molten steel, the outer shape being suitable to installed snugly between the interior and exterior of the tundish, and the upper part of the assembly being embedded into the molten steel bath within the tundish.

Numerical analysis has been carried out to investigate the characteristics of seawater flow fields and turbine output in a compact double current tidal power generation system for various level differences. There are growing concerns for the development of efficient tidal power generation which is stable and less affected by environmental circumstances as ocean energy. Especially the flow field characteristics in the compact tidal power generation system have a large influence on the system power generation performance. Flow velocity, pressure, and streamline distributions are compared including vertical type turbine out, and it can be predicted that seawater is accelerated by vortex flow in front of the turbine and there is severe turbine output variation due to the water level difference with pressure difference. These results can be applied as basic data for the effective development of compact tidal power generation system.

본 연구는 안쪽 실린더가 회전하고 바깥쪽 실린더가 고정된 반경비가 0.52인 동심환형관내 천이유동 특성을 관찰했다. 안쪽실린더가 0~600rpm으로 회전하고 물과 44% 글리세린수용액을 사용한 완전히 발달된 유동에서 압력강하와 표면마찰계수를 측정하였다. 천이유동은 표면마찰계수에 대한 레이놀즈수와 로스비수 관계를 나타내기 위해 압력강하를 측정하였다. 결과는 단지 층류영역에서 로스비수에 대해 표면마찰계수가 중요한 관계를 보인다. 천이의 발생은 레이놀즈수에 관해 압력손실과 표면마찰계수의 구배변화에 의해 조사했다. 회전에 의한 표면마찰계수의 증가율은 층류영역에서는 균일하고 반면에 천이영역에서는 갑자가 감소하며 난류영역으로 갈수록 점차적으로 감소한다. 결과적으로 임계레이놀즈수는 회전속도가 증가함에 따라 감소함을 알 수 있다.

This experimental study concerns the characteristics of a transitional flow in a concentric annulus with a radius ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure drops and skin-friction coefficients have been measured for the fully developed flow of water and that of glycerine-water solution (44%) at the inner cylinder rotational speed of 0~600rpm. The transitional flow has been examined by the measurement of pressure drops to reveal the relationship between the Reynolds and Rossby numbers considering the skin-friction coefficients. The results show that the skin-friction coefficients have a significant correlation with the Rossby numbers, only for laminar regime. The occurrence of transition has been checked by the gradient changes of pressure drops and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, is gradually declined for turbulent flow regime. Consequently, the critical(axial-flow) Reynolds number decreases as the rotational speed increases.

본 논문은 안쪽 실린더가 회전하고 바깥쪽 실린더가 고정된 반경비가 0.52인 동심환형관내 비뉴튼유체의 천이유동 특성을 관찰했다. 안쪽실린더가 0~600 rpm으로 회전하고 0.2% CMC 수용액을 완전히 발달된 유동에서 압력강하와 표면마찰계수를 측정하였다. 천이유동은 표면마찰계수에 대한 레이놀즈수와 로스비수 관계 및 유동의 불안정성 메카니즘을 나타내기 위해 압력강하를 측정하였다. 천이의 발생은 레이놀즈수에 관해 압력손실과 표면마찰계수의 구배변화에 의해 조사했다. 결과적으로 임계레이놀즈수는 회전속도가 증가함에 따라 감소함을 알 수 있다. 따라서 안쪽실린더의 회전은 Taylor 와류의 자극 때문에 천이의 시작을 촉진시킨다.

The paper concerns the characteristics of transitional flow of non-Newtonian fluids in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary while inner one is rotating. The pressure losses and skin friction coefficients have been measured for the fully developed flows of 0.2% aqueous solution of sodium carboxymethyl cellulose (CMC) at inner cylinder rotational speed of 0~600 rpm. The transitional flow has been examined by the measurement of pressure losses and visualization of flow fields to reveal the relationship between the Reynolds and Rossby numbers in relation to the skin friction coefficients and to understand the flow instability mechanism. The occurrence of transition has been checked by the gradient change of pressure losses and skin-friction coefficients with respect to the Reynolds numbers. Consequently, the critical Reynolds number decreases as the rotational speed increases. Thus the rotation of inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.

Vortex tubes are simple tubes that can separate hot and cold air from compressed air without any internal device configuration and are used in many industries. If the vortex tube with these advantages is applied to an air conditioning system, it will be free from the use of refrigerant. In this study, basic study on discharge flow rate and temperature separation characteristics was carried out by varying the number of generator nozzle hole in various shapes of the vortex tube according to the inlet pressure of the vortex tube. Experimental results show that as the number of nozzles increases, the discharge flow rate tends to decrease and the temperature separation characteristics were excellent in the number of nozzles 6~7.

The shock wave is deformed and the vortex is elongated simultaneously during the shock-vortex interaction. More precisely, the shock wave is deformed to a S-shape, consisting of a leading shock and a lagging shock by which the corresponding local vortex flows are accelerated and decelerated, respectively: the vortex flow swept by the leading shock is locally expanded and the one behind the lagging shock is locally compressed. As the leading shock escapes the vortex in the order of microseconds, the expanded flow region is quickly changed to a compression region due to the implosion effect. An induced shock is developed here and propagated against the vortex flow. This happens for a strong vortex because the tangential flow velocity of the vortex core is high enough to make the induced-shock wave speed supersonic relative to the vortex flow. For a weak shock, the vortex is basically subsonic and the induced shock wave is absent. For a vortex of intermediate strength, an induced shock wave is developed in the supersonic region but dissipated prematurely in the subsonic region. We have expounded these three shock-vortex interaction patterns that depend on the vortex flow regime using a third-order ENO method and numerical shadowgraphs.

본 연구에서는 도시지역의 하나인 주거지역에서 배출되는 강우유출수 내에 중금속이 장치형 비점 오염저감시설 중에 하나인 와류형시설에서 제거되는 특성을 파악하고자, 대상 중금속을 Zn으로 삼고 2007년 4월부터 2008년 11월까지 모니터링을 수행하였다. 유출수 모니터링은 와류형 시설의 유입부와 유출부에서 유량과 수질을 각각 시간변화에 따라 측정하였으며, 시설유입부와 유출부, 그리고 시설하부에 쌓인 침전물을 채취하여 침전물 모니터링도 수행하였다. 그 결과, 높은 강우강도에서 발생된 강우유출수는 와류형 시설내에 HRT를 감소시켜 Zn의 제거효율이 낮게 관측되었으며, 특히 HRT가 20분이내의 조건이 될 경우에는 처리효과가 없는 것으로 확인되었다. Zn는 입자성물질과 밀접한 관계를 맺으며 제거되는 특성을 보였으며, 입자성물질이 스크린에 의한 여과 및 침전작용이 일어날 때 입자성물질에 부착되어 거동하는 특성을 보였다. 그 중에 0.075mm 이하의 미세한 입자에 부착된 고농도의 Zn는 제거되지 못하였고, 와류형 시설 후단에 후처리시설로서 저류시설을 두어 충분한 HRT를 제공한 결과, 와류형 시설만을 운전하였을 때와 비교하여 높은 제거특성을 보이게 되었다.

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0～600 rpm. Also, the visualization of vortex flows has been performed to observe the unstable waves. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number Ro with respect to the skin friction coefficients. In somehow, they show the existence of flow instability mechanism. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and. then, it gradually approach to zero for the turbulent flow regime. Consequently, the critical (bulk flow) Reynolds number Re$\_$c/ decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.

The accurate prediction of the flow and the pressure distribution near the tip of the blade is crucial in determining the tip vortex cavitation inception which usually occurs on the blade tip or inside the core of the tip vortex just downstream of the blade tip. An improved boundary element method is applied to the prediction of the flow around propeller blades, with emphasis at the tip region. In the method, the Blow adapted grid and a higher order panel method, which combines a hyperboloidal panel geometry with a hi-quadratic dipole distribution, are used in order to accurately model the trailing wake geometry and the highly rolled-up regions in the wake. The method is applied to several propeller geometries and the results have been found to agree well to the existing experimental data. Inviscid flow methods are able to predict the pressures at the tip as well as the shape of the trailing wake. On the other hand, they are unable to determine the flow inside the viscous core of the tip vortex, where cavitation inception often occurs. Thus, a method is presented that treats the flow inside the viscous core. The inner flow is treated with a 2-D Clavier-stokes solution without making any assumptions for axisymmetric flow and conicity of the flow along the tip trajectory. The method can thus allow the treatment of general propeller blade configurations. The velocity and pressure distributions inside the core are shown and compared to those from other numerical methods.

Numerical simulation was applied to investigate the Taylor vortex flow inside the concentric cylinders with a constant radial temperature gradient. The reliability of numerical simulation method was verified by the experimental results of PIV. The radial velocity and temperature distribution in plain and 12-slit model at different axial locations were compared, and the heat flux distributions along the inner cylinder wall at different work conditions were obtained. In the plain model, the average surface heat flux of inner cylinder increased with the inner cylinder rotation speed. In slit model, the slit wall significantly changed the distribution of flow field and temperature in the annulus gap, and the radial flow was strengthen obviously, which promoted the heat transfer process at the same working condition.

The operational characteristics of superconducting amplifier using vortex flux flow were analyzed from an equivalent circuit in which its current-voltage characteristics for the vortex motion in YBCO microbridge were reflected. For the analysis of operation as an amplifier, dc bias operational point for the superconducting amplifier is determined and then ac operational characteristics for the designed superconducting amplifier were investigated. The variation of transresistance, which describes the operational characteristics of superconducting amplifier, was estimated with respect to conditions of dc bias. The current and the voltage gains, which can be derived from the circuit for small signal analysis, were calculated at each operational point and compared with the results obtained from the numerical analysis for the small signal circuit. From our paper, the characteristics of amplification for superconducting flux flow transistor (SFFT) could be confirmed. The development of the superconducting amplifier applicable to various devices is expected.

This is the continuation of a numerical study on vortex shedding from a blunt trailing-edge of a hydrofoil. In our previous work (Lee et al., 2015), numerical schemes for efficient computations were successfully implemented; i.e. multiple domains, the approximation of domain boundary conditions using cubic spline functions, and particle-based domain decomposition for better load balancing. In this study, numerical results through a hybrid particle-mesh method which adopts the Vortex-In-Cell (VIC) method and the Brinkman penalization model are further rigorously validated through comparison to experimental data at the Reynolds number of $2{\times}10^6$. The effects of changes in numerical parameters are also explored herein. We find that the present numerical method enables us to reasonably simulate vortex shedding phenomenon, as well as turbulent wakes of a hydrofoil.

The effect of molecular parameters on the steady vortex behaviors in the inertial viscoelastic flow past a cylinder has been investigated. FENE-CR model was considered as a constitutive equation. A recently developed iterative solution method (Kim et al., (in press)) was found to be successfully applicable to the computation of inertial viscoelastic flows. The high-resolution computations were carried out to understand the detailed flow behaviors based on the efficient iterative solution method armed with ILU(0) type pre-conditioner and BiCGSTAB method. The discrete elastic viscous split stress-G/streamline upwind Petrov Galerkin (DEVSS-G/SUPG) formulation was adopted as a stabilization method. The vortex size decreased as elasticity increases. However, the vortex enhancement was also observed in the case of large extensibility, which means that the vortex behavior is strongly dependent upon the material parameters. The longitudinal gradient of normal stress was found to retard the formation of vortex, whereas the extensional viscosity played a role in the vortex enhancement. The present results are expected to be helpful for understanding the inertial vortex dynamics of viscoelastic fluids in the flow past a confined cylinder.