• International Journal of Fluid Machinery and Systems
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• v.4 no.3
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• pp.349-359
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• 2011

An unsteady numerical analysis has been carried out to study the strong impeller volute interaction of a centrifugal pump with six backward swept blades shrouded impeller. The numerical analysis is done by solving the three-dimensional Reynolds Averaged Navier-Stokes codes with standard k-${\varepsilon}$ two-equations turbulence model and wall regions are modeled with a scalable log-law wall function. The flow within the impeller passage is very smooth and following the curvature of the blade in stream-wise direction. However, the analysis shows that there is a recirculation zone near the leading edge even at design point. When the flow is discharged into volute casing circumferentially from the impeller outlet, the high velocity flow is severely distorted and formed a spiraling vortex flow within the volute casing. A spatial and temporal wake flow core development is captured dynamically and shows how the wake core diffuses. Near volute tongue region, the impeller/volute tongue strong interaction is observed based on the periodically fluctuating pressure at outlet. The results of existing analysis also proved that the pressure fluctuation periodically is due to the position of impeller blade relative to tongue.

• Journal of computational fluids engineering
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• v.21 no.2
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• pp.47-53
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• 2016

This study investigated natural convection flow behavior in airflow path designed in concrete dry storage cask to remove the decay heat from spent nuclear fuels. Using FLUENT 16.1 code, thermal analysis for natural convection was carried out for three dimensional, 1/4 symmetry model under the normal condition that inlet ducts are 100% open. The maximum temperatures on other components except the fuel regions were satisfied with allowable values suggested in nuclear regulation-1536. From velocity and temperature distributions along the flow direction, the flow behavior in horizontal duct of air inlet and outlet duct, annular flow-path and bent pipe was delineated in detail. Theses results will be used as the theoretical background for the composing of airflow path for the designing of passive heat removal system by understanding the flow phenomena in airflow path.

• Journal of the Korean Society of Visualization
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• v.3 no.1
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• pp.58-62
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• 2005

Flow characteristics at secondary recirculation zone in a liquid fuel ramjet combustor were investigated using CFD and Stereoscopic PIV method. The combustors have two rectangular inlets that form 90 degree each other. Three guide vanes were installed in each rectangular inlet to improve the flow stability. The tested angle of the air intakes was 60 degree. The experiments were performed in the water tunnel test with the same Reynolds number in the case of Mach 0.3 at inlet. The computational and experimental results showed that the secondary recirculation flow occurred at the front junction of inlet main stream and combustor chamber. The size of secondary recirculation regions are increased with approaching closer to the center of the combustor. Since the performance of combustor is closely dependent not only on the main recirculation in the dome region but also on the secondary recirculation flow in a junction region, the optimal angle of the air intakes should be considered the recirculation size as frame holder.

• Wind and Structures
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• v.18 no.1
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• pp.69-82
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• 2014

Flow around a small white fir tree was investigated with varying the length of the bottom trunk (hereafter referred to as bottom gap). The velocity fields around the tree, which was placed in a closed-type wind tunnel test section, were quantitatively measured using particle image velocimetry (PIV) technique. Three different flow regions are observed behind the tree due to the bottom gap effect. Each flow region exhibits a different flow structure as a function of the bottom gap ratio. Depending on the gap ratio, the aerodynamic porosity of the tree changes and the different turbulence structure is induced. As the gap ratio increases, the maximum turbulence intensity is increased as well. However, the location of the local maximum turbulence intensity is nearly invariant. These changes in the flow and turbulence structures around a tree due to the bottom gap variation significantly affect the shelter effect of the tree. The wind-speed reduction is increased and the height of the maximum wind-speed reduction is decreased, as the gap ratio decreases.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.137-140
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• 2003

The flow characteristics of secondary recirculation region in a liquid fuel ramjet combustor are measured using PIV method. The model combustor has two rectangular inlets that form 90 degree angle each other. The tested angles of the air intakes were 30, 45 and 60. Three guide vanes are installed in each rectangular inlet to improve the flow stability. The experiments are performed in the water tunnel test with the same Reynolds number as the case of Mach 0.3 at the inlet. PIV software is developed to measure the characteristics of the flow field in the combustor. The accuracy of the developed PIV program is verified with rotating disk experiment and standard data. The experimental results show that the secondary recirculation flow occurred at the front junction of inlet main stream and combustorchamber. The size of secondary recirculation regions are increased with increasing air inlet angles. Since the performanceof combustor is very dependant on not only the main recirculation in the dome region but also the secondary recirculation flow in a junction region, the optimal angle of the air intakes should consider the both recirculation size as a frame holder.

• Journal of Construction Engineering and Project Management
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• v.7 no.1
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• pp.1-12
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• 2017

Construction delay is one of the basic constrains to achieve the project objectives in developing countries. This study aims to find the causes and effects of construction delays in developing countries. A thorough literature review has been done following the content analysis method. The relevant literature of 28 developing countries was collected from the scholarly journals published in the period of 2006 to 2016. The different developing countries are grouped into three geographic regions, i.e. South and Southeast Asia, Middle East, and Africa. In these regions, total 53 potential causes of delay under 8 major groups are identified. Frequency and ranking of these factors have been done. The factors, delay in progress payment by owner, contractors' cash flow problem, improper planning and scheduling, poor site management, and change order by owner during construction, are acknowledged as critical causes of delay in developing countries. This study will assist both academic and professional experts providing more insight about the construction delays and project management in developing countries.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.6
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• pp.823-832
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• 2002

A modified $textsc{k}$-$\varepsilon$model is proposed for calculation of transitional boundary-layer flows with changing pressure gradient. In order to develop the model for this problem, the flow is divided into three regions; pre-transition region, transition region and fully turbulent region. The effect of pressure gradient is taken into account in stream-wise intermittency factor, which bridges the eddy-viscosity models in the pre-transition region and the fully turbulent region. From intermittency data in various flows, Narashima＇s intermittency function, F(${\gamma}$), has been found to be proportional to $\chi$$^{n}$ according to the extent of pressure gradient. Three empirical correlations of intermittency factor being analyzed, the best one was chosen to calculate three benchmark cases of bypass transition flows with different free-stream turbulence intensity under arbitrary pressure gradient. It was found that the variations of skin friction and shape factor as well as the profiles of mean velocity in the transition region were very satisfactorily predicted.

• Wind and Structures
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• v.37 no.1
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• pp.57-78
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• 2023

In this study, a generalized three-degree-of-freedom (3-DoF) analytical model is formulated to predict linear aerodynamic instabilities of a prism under quasi-steady (QS) conditions. The prism is assumed to possess a generic cross-section exposed to turbulent wind flow. The 3-DoFs encompass two orthogonal horizontal directions and rotation about the prism body axis. Inertial coupling is considered to account for the non-coincidence of the mass center and the rotation center. The aerodynamic force coefficients-drag, lift, and moment-depend on the Reynolds number based on relative flow velocity, angle of attack, and the angle between the wind and the cable. Aerodynamic forces are linearized with respect to the static equilibrium configuration and mean wind velocity. Routh-Hurwitz and Liénard and Chipart criteria are used in the eigenvalue problem, yielding an analytical solution for instabilities in galloping and static divergence types. Additionally, the minimum structural damping and stiffness required to prevent these instabilities are numerically determined. The proposed 3-DoF instability model is subsequently applied to a conductor with ice accretion and a full-scale dry inclined cable. In comparison to existing models, the developed model demonstrates superior prediction accuracy for unstable regions compared with results in wind tunnel tests.

• Wind and Structures
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• v.35 no.5
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• pp.309-322
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• 2022

Due to the complex terrain around high-speed railways, the windbreaks were established along different landforms, resulting in irregular windbreak transition regions between different subgrade infrastructures (flat ground, cutting, embankment, etc). In this paper, the effect of a windbreak transition on the wind flow around railways subjected to crosswinds was studied. Wind tunnel testing was conducted to study the wind speed change around a windbreak transition on flat ground with a uniform wind speed inflow, and the collected data were used to validate a numerical simulation based on a detached eddy simulation method. The validated numerical method was then used to investigate the effect of the windbreak transition from the flat ground to cutting (the "cutting" is a railway subgrade type formed by digging down from the original ground) for three different wind incidence angles of 90°, 75°, and 105°. The deterioration mechanism of the flow fields and the reasons behind the occurrence of the peak wind velocities were explained in detail. The results showed that for the windbreak transition on flat ground, the impact was small. For the transition from the flat ground to the cutting, the influence was relatively large. The significant increase in the wind speeds was due to the right-angle structure of the windbreak transition, which resulted in sudden changes of the wind velocity as well as the direction. In addition, the height mismatch in the transition region worsened the protective effect of a typical windbreak.

• Coupled systems mechanics
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• v.2 no.2
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• pp.191-214
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• 2013

The unsteady flow past a circular cylinder which starts rotating or rotary oscillating impulsively from rest in a viscous fluid is investigated for Reynolds numbers Re=200 and 1000, rectilinear speed ratios ${\alpha}$ between 0.5 and 5.0, and forced oscillating frequencies $f_s$ between 0.1 and 2.0. Numerical solutions of the Navier-Stokes equations are obtained by using a finite volume method on an unstructured colocated grid. The objective of the study is to examine the effect of the rotating and rotary oscillating circular cylinder on the flow patterns and dynamics loads. The numerical results reveal that the $K\acute{a}rm\acute{a}n$ vortex street vanishes entirely behind the rotating cylinder when the ratio ${\alpha}$ exceeds the critical value, and the vortex shedding behind the rotary oscillating cylinder undergoes mainly three modes named 'synchronization', 'competition' and 'natural shedding' with the increase of $f_s$. Based on the amplitude spectra analysis of the lift coefficients, the regions of the classification of flow structure modes are presented, which provide important references for the flow control in the ocean engineering.