• Title/Summary/Keyword: gradient flow

Search Result 1,171, Processing Time 0.034 seconds

A Coupled Moisture and Bent Flow Analysis Model in Unsaturated Soil (불포화토에서의 복합적 습기와 열흐름의 분석모델)

  • Kim, Suk-Nam;Kim, Suk-Nam
    • Journal of the Korean Geotechnical Society
    • /
    • v.18 no.5
    • /
    • pp.67-72
    • /
    • 2002
  • Water content of soils within pavement varies seasonally depending on climatic factors such as rainfall, temperature and so on, since a hydraulic gradient due to rainfall causes moisture flow, and a thermal gradient due to temperature change induces not only heat flow but also moisture flow directly and indirectly. Soils within pavement are usually in an unsaturated state, and heat flow and moisture flow have been recognized as coupled processes with complex interactions between them. This paper presents a one-dimensional analysis model by the finite element method for the coupled heat flow and moisture flow in unsaturated soils. The model can be used to predict not only the change of temperature and water content, but also frist heave with time. It will be a meaningful work for the design and maintenance of pavement to predict the change of the temperature and water content and frist heave. The model is tested through comparisons with the results by other models.

The Generation and Function of the three $\breve{U}$m and three Yang meridian (삼음삼양경맥(三陰三陽經脈)의 발생(發生)과 기능(機能))

  • Park, Chan-Guk
    • Journal of Korean Medical classics
    • /
    • v.12 no.2
    • /
    • pp.166-174
    • /
    • 1999
  • Meridians are often defined as passway of Ki and Blood or something that can control funtions of the body. It is true but I think meridians have something more than that. Meridians are not just passway of Ki and Blood, rather they receive Ki from outside and transform it into Essence-Ki(精氣). If we draw a line in a body, we have Chang-Pu inside and meridians outside. Chang-Pu whim is inside our body hold Essence-Ki and manipulate it. These Chang-Pu also have variation of Ki accumulation-Tai-Yang, Soyang, T'ae$\breve{u}$m, Soum. Nevertheless, their gradients are not great so Ki flow among them are not great either. If there are much Ki flow in our body there will be much Ki consumption resulting in exhaustion of Essence-Ki, which is very hard to acquire. Therefore Chang-Pu keeps less gradient by not moving Ki a lot to preserve Essence-Ki. Chang-Pu, inside, are suitable for storing Ki while meridians, outside, are for producing Ki. Meridinas have great difference in Ki accumulation so there are great flow of Ki. This nature is suitable for producing Ki. For example, roots and limbs of a tree don't have much gradient in Ki. They are concentrated and their shape are not very distinct. On the other hand, leaves are wide and it's easy to tell front from back. It means their Ki gradient is great and their Ki flow is also great. Therefore they suitable for producing Ki. Just like this, meridians in our body are suitable for producing Ki. Areas that meridians cover are much wider than that of Chang-Pu. Four limbs and surface of our body are very distinctive. Ulnar side is high in Ki accumulation but is small in volume so it's better to store Ki there. Radial side is low in Ki accumulation but big in volume so it's better to receive and consume Ki there. Meridians are deeply involved in producing and storing Ki.

  • PDF

Nutrient Variations in the Jindong Bay during Summer by Ecosystem Modeling (해양생태계모델에 의한 하계 진동만의 영양염변동)

  • 김동선;홍철훈
    • The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
    • /
    • v.8 no.2
    • /
    • pp.164-176
    • /
    • 2003
  • During summer, the DIN (dissolved inorganic nitrogen) and DIP (dissolved inorganic phosphate) observed in the Jindong Bay in the southern sea of Korea show much higher values in the inner area of the bay. In general, they have high values in the upper (0-1 m) and lower layers (8 m-bottom), but are relatively lower in the middle layer (1-8 m). These features in their distribution are examined using an ecosystem model with considering the wind, tidal current, horizontal gradient of water density and residual flow. The experiments were focused on how to influence nutrients associated with these conditions. In the experiment with tide-induced residual flow, the values of nutrients appeared lower than the observation, and were well corresponded to it when the effects of wind, tide-induced residual current and horizontal gradient of water density were additionally imposed. A statistical analysis identifies these results. This paper suggests that variation of nutrient in the Jindong Bay during summer should be seriously a(footed wind-driven current by the wind and density-driven current is induced by the horizontal gradient of water density as well as tidal current.

Numerical Investigation on Seepage Stability in Offshore Bucket Cut-off Walls (수치해석을 이용한 대형원형강재 가물막이의 침투 안정성 분석)

  • Ssenyondo, Vicent;Tran, Van An;Kim, Sung-Ryul
    • Journal of the Korean Geotechnical Society
    • /
    • v.33 no.11
    • /
    • pp.73-82
    • /
    • 2017
  • Recently, offshore bucket cut-off walls were developed to solve several problems in conventional offshore cut-off walls. In this study, a numerical analysis was carried out to investigate the seepage stability of offshore bucket cut-off walls. The ground was assumed as uniform homogeneous sand and steady state flow conditions were applied. The flow condition was compared among 2-dimensional flow (2-D), 2-dimensional concentrated flow (2-DC), and axisymmetric flow. The analysis results showed that the seepage velocities in axisymmetric flow were about 1.5 and 2 times larger than those of 2-DC and 2-D flow conditions, respectively. Thereafter, the axisymmetric flow condition was applied because the seepage flow was concentrated toward the center of the circular-shaped wall. A parametric study was performed varying bucket radius, penetration depth, total head difference between in and outside of the wall. The exit gradient, which used for the calculation of piping stability, decreased with increase of the penetration depth and bucket radius. Design charts were proposed to estimate the factor of safety and the exit gradient at various analysis conditions. Finally, the design equation was proposed to calculate the exit gradient for the preliminary design of the bucket cut-off wall.

The Effect of Serrated Fins on the Flow Around a Circular Cylinder

  • Boo, Jung-Sook;Ryu, Byong-Nam;Kim, Kyung-Chun
    • Journal of Mechanical Science and Technology
    • /
    • v.17 no.6
    • /
    • pp.925-934
    • /
    • 2003
  • An experimental study is performed to investigate the characteristics of near wake flow behind a circular cylinder with serrated fins using a constant temperature anemometer and flow visualization. Various vortex shedding modes are observed. Fin height and pitch are closely related to the vortex shedding frequency after a certain transient Reynolds number. The through velocity across the fins decreases with increasing fin height and decreasing fin pitch. Vortex shedding is affected strongly by the velocity distribution just on top of the finned tube. The weaker gradient of velocity distribution is shown as increasing the freestream velocity and the fin height, while decreasing the fin pitch. The weaker velocity gradient delays the entrainment flow and weakens its strength. As a result of this phenomenon, vortex shedding is decreased. The effective diameter is defined as a virtual circular cylinder diameter taking into account the volume of fins, while the hydraulic diameter is proposed to cover the effect of friction by the fin surfaces. The Strouhal number based upon the effective diameters seems to correlate well with that of a circular cylinder without fins. After a certain transient Reynolds number, the trend of the Strouhal number can be estimated by checking the ratio of effective diameter to inner diameter. The normalized velocity and turbulent intensity distributions with the hydraulic diameter exhibit the best correlation with the circular cylinder's data.

Characteristics of Near Wake Behind a Circular Cylinder with Serrated Fins (II) - Comparison of Time Mean Flow Fields- - (톱니형 휜이 부착된 원주의 근접후류특성 연구 (II) - 시간평균 유동장 비교 -)

  • Ryu, Byeong-Nam;Kim, Gyeong-Cheon;Bu, Jeong-Suk
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.26 no.8
    • /
    • pp.1191-1200
    • /
    • 2002
  • The measurements of velocity vectors are made in the near wake(X/d=5.0) of a circular cylinder with serrated fins. Velocity of fluid which flow through fins decreases as increasing fin height and freestream velocity and decreasing fin pitch. Therefore the velocity distribution at X/d=0.0 has lower gradient with increasing freestream velocity and fin height and decreasing fin pitch. The discontinuity of the streamwise velocity gradient is observed near the fin edge and causes significant changes in V-component velocity distribution in the near wake. This change attributes to the differences in Strouhal number and entraintment flow behavior. Increased turbulent intensity around a circular cylinder due to the serrated fins and entrainment flow are important factors for the recovery of velocity defect. The widths of velocity and turbulent intensity distribution of fin tubes are wider than those of a circular cylinder. The normalized velocity and turbulent intensity distributions with a hydraulic diameter which is proposed in this paper are in closer agreement with those of a circular cylinder.

A Study on the Characteristics of Cylinder Wake Placed in Thermally Stratified Flow(II)(Par II. Turbulent Characteristics of Stratified Wake) (열성층유동장에 놓인 원주후류의 특성에 대한 연구(2)(Part 2. 성층후류의 난류유동특성))

  • 김경천;정양범;강동구
    • Transactions of the Korean Society of Mechanical Engineers
    • /
    • v.18 no.5
    • /
    • pp.1322-1329
    • /
    • 1994
  • The effect of thermal stratification on the stratified flow past a circular cylinder was examined in a wind tunnel. Turbulent intensities, the rms values of temperature and turbulent convective heat flux as well as the velocity and temperature profiles in the cylinder wake with a strong thermal gradient of $200^{\circ}C/m$ were measured by using a hot-wire and cold-wire combination probe. It is found that the temperature field affects as an active contaminant, so that the vertical growth of vortical structure is suppressed and the strouhal number decreases with increasing the extent of stratification. And also, the wake structure can not sustain their symmetricity about the wake centerline and vertical turbulent motion dissipates faster than that of the neutral case when such a strong thermal gradient is superimposed. It is evident that the turbulent mixing in the upper half section is stronger than that of the lower of the wake in a stably stratified flow because the turbulent intensities and convective heat flux in the upper half section are larger than those of the lower half of the wake.

The Effect of a Bypass Flow Penetrating through a Gas Diffusion Layer on Performance of a PEM Fuel Cell (가스확산층을 통과하는 반응가스 우회유동이 고분자 연로전지의 성능에 미치는 영향)

  • Cho, Choong-Won;Ahn, Eun-Jin;Lee, Seung-Bo;Lee, Won-Yong
    • 한국신재생에너지학회:학술대회논문집
    • /
    • 2007.11a
    • /
    • pp.147-151
    • /
    • 2007
  • A serpentine channel geometry often used in a polymer electrolyte membrane fuel cell has a strong pressure gradient between adjacent channels in specific regions. The pressure gradient helps some amount of reactant gas penetrate through a gas diffusion layer(GDL). As a result, the overall serpentine flow structure is slightly different from intention of a designer. The purpose of this paper is to examine the effect of serpentine flow structure on current density distribution. By using a commercial code, STAR-CD, a numerical simulation is performed to analyze the fuel cell with relatively high aspect ratio active area. To increase the accuracy of the numerical simulation, GDL permeabilities are measured with various compression conditions. Three-dimensional flow field and current density distribution are calculated. For the verification of the numerical simulation results, water condensation process in the cathode channel is observed through a transparent bipolar plate. The result of this study shows that the region of relatively low current density corresponds to that of dropwise condensation in cathode channels.

  • PDF

Large Eddy Simulation of a High Reynolds Number Swirling Flow in a Conical Diffuser

  • Duprat, Cedric;Metais, Olivier;Laverne, Thomas
    • International Journal of Fluid Machinery and Systems
    • /
    • v.2 no.4
    • /
    • pp.346-352
    • /
    • 2009
  • The objective of the present work is to improve numerical predictions of unsteady turbulent swirling flows in the draft tubes of hydraulic power plants. We present Large Eddy Simulation (LES) results on a simplified draft tube consisting of a straight conical diffuser. The basis of LES is to solve the large scales of motion, which contain most of the energy, while the small scales are modeled. LES strategy is here preferred to the average equations strategies (RANS models) because it resolves directly the most energetic part of the turbulent flow. LES is now recognized as a powerful tool to simulate real applications in several engineering fields which are more and more frequently found. However, the cost of large-eddy simulations of wall bounded flows is still expensive. Bypass methods are investigated to perform high-Reynolds-number LES at a reasonable cost. In this study, computations at a Reynolds number about 2 $10^5$ are presented. This study presents the result of a new near-wall model for turbulent boundary layer taking into account the streamwise pressure gradient (adverse or favorable). Validations are made based on simple channel flow, without any pressure gradient and on the data base ERCOFTAC. The experiments carried out by Clausen et al. [1] reproduce the essential features of the complex flow and are used to develop and test closure models for such flows.

Composition and interface quality control of AlGaN/GaN heterostructure and their 2DEG transport properties

  • Kee, Bong;Kim, H.J.;Na, H.S.;Kwon, S.Y.;Lim, S.K.;Yoon, Eui-Joon
    • Journal of Korean Vacuum Science & Technology
    • /
    • v.4 no.3
    • /
    • pp.81-85
    • /
    • 2000
  • The effects of $NH_3$ flow rate and reactor pressure on Al composition and the interface of AlGaN/GaN heterostructure were studied. Equilibrium partial pressure of Ga and Al over AiGaN alloy was calculated as a function of growth pressure, $NH_3$flow rate and temperature. It was found equilbrium vapor pressure of Al is significantly lower than that of Ga, thus, the alloy composition mainly controlled by Ga partial pressure. We believe that more decomposition of Ga occur at lower $NH_3$ flow rate and higher growth pressure leads to preferred Al incorporation into AlGaN. The alloy composition gradient became larger at AlGaN/GaN heterointerface at higher reactor pressures, higher Al composition and low $NH_3$ flow rate. This composition gradient lowered sheet carrier concentration and electron mobility as well. We obtained an AlGaN/GaN heterostructure with sheet carrier density of ${\sim}2{\times}10^{13}cm^{-2}$ and mobility of 1250 and 5000 $cm^2$/Vs at 300 K and 100 K, respectively.

  • PDF