• Transactions of the Korean Society of Mechanical Engineers
• /
• v.19 no.9
• /
• pp.2308-2315
• /
• 1995

We studied the effects on the ignitability of mixture, the combustion duration, and the maximum combustion pressure, of various initial combustion factors such as temperature, pressure, and each equivalence ratio in order to identify the combustion characteristics of lean mixture and improve ignitability through the proper control of the ignition energy. It is concluded that there is an optimum turbulent intensity that enables the combustion to have the best ignitability and the shortest duration under each equivalent ratio, and the combustion duration is only dependent upon the distribution and magnitude of discharge energy within the limit of inflammability.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.23 no.7
• /
• pp.937-944
• /
• 1999

The near-wall flow structures of turbulent boundary layer over riblets having semi-circular grooves were investigated experimentally for the drag decreasing ($s^+=25.2$) and drag increasing ($s^+=40.6$) cases. The field of view used for tho velocity field measurement was $6.75{\times}6.75mm^2$ in physical dimension, containing two grooves. One thousand instantaneous velocity fields over the riblets were extracted for each case of drag increase and decrease. For comparison, five hundreds instantaneous velocity fields over a smooth flat plate were also obtained under the same flow conditions. To see the global flow structure qualitatively, the flow visualization was also performed using the synchronized smoke-wire technique. For the drag decreasing case ($s^+=25.2$), most of the streamwise vortices stay above the riblets, interacting with the riblet tips. The high-speed in-rush flow toward the riblet surface rarely influences the flow inside tho riblet valleys submerged in the viscous sublayer. The riblet tips seem to impede the spanwise movement of the longitudinal vortices and induce secondary vortices. The turbulent kinetic energy in the riblet valley is sufficiently small to compensate the increased wetted area of the riblets. In addition, in the logarithmic region, the turbulent kinetic energy are small or almost equal to that of a smooth flat plato. For the drag increasing case ($s^+=40.6$), however, the streamwise vortices move into the riblet valley freely, interacting directly with the riblet inner surface. The penetration of the high-speed in-rush flow on the riblets increases tho skin-friction. The turbulent kinetic energy is increased in the riblet valleys and even in the outer region compared to that over a flat plate.

• Journal of Mechanical Science and Technology
• /
• v.19 no.2
• /
• pp.704-715
• /
• 2005

In this study, to investigate in-cylinder tumble or swirl intake flow of a gasoline engine, the flow characteristics were examined with opening control valve (OCV) and several swirl control valves (SCV) which intensify intake flow through steady flow experiment, and also turbulent characteristics of in-cylinder flow field were investigated by 2-frame cross-correlation particle image velocimetry (PIV) method. In the investigation of intake turbulent characteristics using PIV method, the different flow characteristics were showed according to OCV or SCV figures. The OCV or SCV installed engine had higher vorticity and turbulent kinetic energy than a baseline engine, especially around the wall and lower part of the cylinder. Above all, SCV B type was superior to the others. About energy dissipation and reynolds shear stress distribution, a baseline engine had larger loss than OCV or SCV installed one because flow impinged on the cylinder wall. It should be concluded, from what has been said above, as swirl component was added to existing tumble flow adequately, it was confirmed that turbulent intensity was enlarged, flow energy was conserved effectively through the experiment. In other words, there is a suggestion that flow characteristics as these affected to in-cylinder combustion positively.

• Nuclear Engineering and Technology
• /
• v.46 no.2
• /
• pp.195-206
• /
• 2014

Heat transfer characteristics in a narrow rectangular channel are experimentally investigated for upward and downward flows. The experimental data obtained are compared with existing data and predictions by many correlations. Based on the observations, there are differences from others: (1) there are no different heat transfer characteristics between upward and downward flows, (2) most of the existing correlations under-estimate heat transfer characteristics, and (3) existing correlations do not predict the high heat transfer in the entrance region for a wide range of Re. In addition, there are a few heat transfer correlations applicable to narrow rectangular channels. Therefore, a new set of correlations is proposed with and without consideration of the entrance region. Without consideration of the entrance region, heat transfer characteristics are expressed as a function of Re and Pr for turbulent flows, and as a function of Gz for laminar flows. The correlation proposed for turbulent and laminar flows has errors of ${\pm}18.25$ and ${\pm}13.62%$, respectively. With consideration of the entrance region, the heat transfer characteristics are expressed as a function of Re, Pr, and $z^*$ for both laminar and turbulent flows. The correlation for turbulent and laminar flows has errors of ${\pm}19.5$ and ${\pm}22.0%$, respectively.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2012.05a
• /
• pp.378-385
• /
• 2012

Vent mixer can provide main flow directly into a recirculation region downstream of the mixer to enhance fuel-air mixing efficiency. Based on experimental results of three-dimensional velocity, vorticity and turbulent kinetic energy obtained by a stereoscopic PIV method, the performance of the vent mixer was compared with that of the step mixer which was used as a basic model. Thick shear layers of the vent mixer induced the increase of the penetration height. The turbulent kinetic energy mainly distributed along a boundary layer between the main flow and the jet plume. This turbulent field activates mass transfer in a mixing region, leading to the mixing enhancement.

• Journal of computational fluids engineering
• /
• v.10 no.1
• /
• pp.56-62
• /
• 2005

Large eddy simulation of vortexing flow of molten steel in the continuous casting mold with and without DC magnetic field was conducted. The influence of the position of magnetic field to the residence time and depth of the vortex was analyzed. The mechanism of the influence of magnetic field to the vortexing flow was found. The computational results show that the vortexing flow is the result of shearing of the two un-symmetric surface flows from the mold narrow faces when they meet adjacent to the SEN; the un-symmetric flow for turbulent vortex is caused by turbulent energy of the fluid and that for biased vortex is caused by biased flow and the turbulent energy of fluid; with the moving of the magnetic field from the centerline of the outlet of the SEN to the free surface, the surface velocity is decreased gradually and the depth of the turbulent vortex and the biased vortex is decreased, the residence time is increased with the magnetic field moves from DL=120mm to DL=60mm and then decreased; the turbulent vortex and the biased vortex can be eliminated when the magnetic field is located at the free surface.

• Journal of Power System Engineering
• /
• v.15 no.2
• /
• pp.31-36
• /
• 2011

The flow characteristics on the periodically arranged semi-circular ribs in a rectangular channel for turbulent flow have been investigated numerically. The aspect ratio of the rectangular channel was AR=5, the rib height to hydraulic diameter ratio was 0.07 and rib height to channel height ratio was e/H=0.117. The v2-f turbulence model and SST k-${\omega}$ turbulence model were used to find the flow characteristics of near the wall which are suited for realistic phenomena. The numerical analysis results show turbulent flow characteristics and pressure drop at the near the wall as observed experimentally. The results predict that turbulent kinetic energy(k) is closely relative to the diffusion of recirculation flow, and v2-f turbulence model simulation results have a good agreement with experimental.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.14 no.3
• /
• pp.716-724
• /
• 1990

Oblique impinging plane jets were investigated experimentally and numerically at Reynolds number 21000. The inclination angle was varied from 90.deg.(normal to the impinging plate) to 60.deg.. The distance H between the nozzle exit and the stagnation point on the impinging plate was fixed at H/D=8. The working fluid was air. The mean velocity components and turbulent quantities were measured by a hot-wire anemometer. And the static pressure distributions on the impinging plate were measured by a Pitot tube. In numerical computation, the governing partial differential equations of elliptic type were solved with conventional k-.epsilon. turbulence model. The measurements show that, after impingement, the jet half width alone the wall increases in both directions, and that similarity for each turbulent quantity such as Reynolds shear stress or turbulent kinetic energy is revealed in the wall jet region. The computed results show some deviation from experimental data in the impingement region, where streamline curvature is significant. However, the computed results agree qualitatively well with measurements.

• Nuclear Engineering and Technology
• /
• v.49 no.6
• /
• pp.1318-1325
• /
• 2017

Direct Numerical Simulation (DNS) serves as an irreplaceable tool to probe the complexities of multiphase flow and identify turbulent mechanisms that elude conventional experimental measurement techniques. The insights unlocked via its careful analysis can be used to guide the formulation and development of turbulence models used in multiphase computational fluid dynamics simulations of nuclear reactor applications. Here, we perform statistical analyses of DNS bubbly flow data generated by Bolotnov ($Re_{\tau}=400$) and LueTryggvason ($Re_{\tau}=150$), examining single-point statistics of mean and turbulent liquid properties, turbulent kinetic energy budgets, and two-point correlations in space and time. Deformability of the bubble interface is shown to have a dramatic impact on the liquid turbulent stresses and energy budgets. A reduction in temporal and spatial correlations for the streamwise turbulent stress (uu) is also observed at wall-normal distances of $y^+=15$, $y/{\delta}=0.5$, and $y/{\delta}=1.0$. These observations motivate the need for adaptation of length and time scales for bubble-induced turbulence models and serve as guidelines for future analyses of DNS bubbly flow data.

• Journal of the Korean Society of Visualization
• /
• v.19 no.2
• /
• pp.26-31
• /
• 2021

The spanwise growth of turbulence structures in turbulent pipe flow was investigated using the direct numerical simulation data of Re_𝜏 = 544, 934 and 3008. Two-point correlations and pre-multiplied energy spectra of streamwise velocity fluctuations were examined along the spanwise direction. The arclength direction is defined as r𝛳, which is useful for an analogy with the spanwise direction for channels or boundary layers; here, r and 𝛳 are the radial distance from the core and the azimuthal angles, respectively. Both analyses showed that the arclength scales increased with increasing the wall-normal distance. It showed that the coherent structures were confined in the core region due to the crowding effect of a circular pipe geometry. The pipe flow simulation could describe a realistic geometrical flow along the azimuthal direction, unlike the simulations of turbulent channel or boundary layer flow using periodic boundary conditions along the spanwise direction. The present results provided the spanwise organization of energy-containing motions over a broad range of scales in turbulent pipe flow.