• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.23 no.11
• /
• pp.1399-1409
• /
• 1999

It is known that previous models are unsatisfactory in predicting adverse pressure gradient turbulent flows. In the present paper, a revised low Reynolds number $k-{\varepsilon}$ model is proposed. In this model, a newly developed term is added lo the dissipation rate equation. In order to reflect appropriate effects for an adverse pressure gradient. The added tenn is derived by considering the distribution of mean velocity and turbulent properties in the turbulent flow with, adverse pressure gradient. The new $k-{\varepsilon}$ model was applied to calculations of flat plate flow with adverse pressure gradient, conical diffuser flow and backward facing step flow. It was found that the three numerical results showed better agreement than other models compared with DNS results and experimental ones.

• Journal of the Korean Society for Industrial and Applied Mathematics
• /
• v.11 no.1
• /
• pp.31-39
• /
• 2007

A pedagogic model for blood flow is introduced to help medicine majors understand a simplified version of Navier-Stokes equations which is known to be a good tool for interpreting the phenomena in blood flow. The pressure gradient consists of a time-independent part known as Hagen-Poiseuille's gradient and a time-dependent part known as Sexl's, and the model formula for the volume rate of blood flow is reduced to a very simple form. For demonstration, the blood rate in human aorta system is analyzed in connection with the time-dependence of pressure gradient. It is shown for Sexl's part that the flow rate lags the pressure gradient by ${\pi}/2$, which is thought to be due to the relaxation process involved.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
• /
• 2001.10a
• /
• pp.160-163
• /
• 2001

In order to develop a nodule conveying system through a flexible pipe out of the deep-seabed manganese nodule miner, an experimental study of the solid-water slurry flow in vertical pipe is performed as the first stage of total experiments. Hydraulic characteristics of the pipe slurry flow such as slip velocity, transport concentration and pressure gradient are investigated for the size of particle, load ratio, and flow rate of water. The higher the load ratio is, the larger the transport concentration and pressure gradient become. The bigger the size of particles is, the larger the pressure gradient becomes. The effectiveness of the flow rate to hydraulic performance is also investigated. This results are to be used for designing crusher and pump, and operating the conveying device.

• 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.

• 한국가시화정보학회:학술대회논문집
• /
• 2002.11a
• /
• pp.49-50
• /
• 2002

In micro-channels, the electro-viscous effect is caused by the electrical double layer on pressure-driven liquid flow. Velocity fields of flow inside micro-channels were measured using micro-PIV system for investigating the electro-viscous effect. De-ionized water and aqueous NaCl solutions with four different concentrations were used as working fluid in a PDMS micro-channel of $100{\mu}m$ width and $66{\mu}m$ height. The pressure gradient, dP/dx, was determined from the pre-determined input flow rate Q of syringe pump. The mean velocity $u_m$ used for calculating Reynolds number was obtained from the PIV velocity field data. These are used to plot the pressure gradient as a function of Reynolds numbers. The pressure gradient far lower concentration solution $(10^{-5}\;M)$ was higher than that for the higher concentration solution. The increase of flow resistance was about $30\%\;and\;37.5\%$ at Re=0.02 and 0.06, respectively.

• Journal of Biosystems Engineering
• /
• v.20 no.1
• /
• pp.58-65
• /
• 1995

A numerical study for a two dimensional multi-jet with crossflow of the spent fluid has been carried out. Three different distributions of mass-flow rate at 5 jet exits were assumed to see their effects upon the flow characteristics, especially in the jet-flow region. For each distribution, various Reynolds numbers ranging from laminar to turbulent flows were considered. Calculations drew the following items as conclusion. 1) The development of the free jets issued from downstream jets was hindered by the crossflow formed due to jets. Consequently, the free jet was developed into the channel flow without any evident symptom of impingement jet flow characteristics 2) The crossflow induced the pressure gradient along the cross section of jet exits and the value of the pressure gradient increased as going downstream. The crossflow generated also the turbulent kinetic energy as it collied with the downstream jets. 3) The skin friction coefficient along the impingement plate was affected more by the distribution of mass flow rate at jet exits rather than by the Reynolds number. The skin friction coefficient was inversely proportional to the square root of the Reynolds number, regardless of flow regime when a fully developed flow was formed in the jet flow region. 4) The distribution of the skin friction coefficient along the impingement plate was found to be controlled by adjusting the distribution of mass flow rate at jet exits.

• The KSFM Journal of Fluid Machinery
• /
• v.14 no.4
• /
• pp.57-63
• /
• 2011

Regenerative blower is suitable for hydrogen recirculation in fuel cell vehicle due to its capability of high pressure rise in single stage. Numerical models were applied to investigate inner gap leakage flow characteristics. A leakage flow in the inner gap is dominantly affected by pressure gradient. Therefore a blower with concentric channel type was suggested as one of modified models for reducing the inner gap pressure gradient. Also numerical results such as pressure rise, efficiency, leakage flow rate and torque were compared between modified and reference models. The performance of concentric channel type was improved as a result of reduced leakage flow.

• 한국전산유체공학회:학술대회논문집
• /
• 2008.03b
• /
• pp.341-344
• /
• 2008

The complicated helical flow formed in the regenerative turbomachines is usually decomposed into a peripheral component and a circulatory component. On the basis of the momentum exchange theory, the circulatory flow plays a critical role of imparting angular momentum to the peripheral flow. Therefore, the accuracy of performance prediction is dominated by the circulatory flow modeling. Until now the circulatory flow has been accounted of a standstill flow normal to the peripheral flow. However, the circulatory path from the impeller exit to the re-entrance inlet is exposed to the adverse pressure gradient, so it would be more realistic to describe that the circulatory flow is formed on the skewed plane not perpendicular to the peripheral flow. Present study suggests new circulatory flow loss model including the effect of adverse pressure gradient and modifies the effective circulatory flow rate and circulatory pivot which were previously published.

• Journal of the Korean Society for Precision Engineering
• /
• v.14 no.8
• /
• pp.146-154
• /
• 1997

Solenoid-folw control type ABS is used with a 'dump and reapply' pressure control arrangement instead of using 2/2 (normal open/close) solenoid valves in convensional systems(sol. -sol. control type), a flow control valve is used which replaces the (no) inlet valve. The flow control valve controls fluid flow providing a nearly constant reapply rate( .theta. ) after the dump plase of ABS operation. In this study, to investigate a characteristics of brake pressure by PWM control, test rig was consisted of ABS hydraulic modulator, digital controller, pneumatic power supply and brake master cylinder. For comparison with experi- mental results, system modelling and computer simulation were performed. As a result, experiment results showed fairly agreement with the simulation. Also, it is shown that the pressure gradient (tan .theta. ) is affected by pressure, frequency, duty ratio and expressed with an exponential funtion.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.17 no.6
• /
• pp.1541-1546
• /
• 1993

To extend the widely used Gibson and Launder's second order closure model to the low-Reynolds-number region near a wall, modifications have been made for velocity pressure-gradient interaction and dissipation terms in the stress equations, and also for the dissipation rate equation. From the computation of fully developed plane channel flow, it is found that the results with present model agree well with the data of direct numerical simulation in the predictions of stress components. And, the computed mean velocity profile coincides with the universal velocity law.