• Wind and Structures
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• v.26 no.5
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• pp.305-315
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• 2018

A numerical study based on a delayed detached eddy simulation (DDES) is conducted to investigate the aerodynamic mechanism behind the suppression of vortex-induced vibrations (VIVs) of twin box girders by central grids, which have an inhibition effect on VIVs, as evidenced by the results of section model wind tunnel tests. The mean aerodynamic force coefficients with different attack angles are compared with experimental results to validate the numerical method. Next, the flow structures around the deck and the aerodynamic forces on the deck are analyzed to enhance the understanding of the occurrence of VIVs and the suppression of VIVs by the application of central grids. The results show that shear layers are separated from the upper railings and lower overhaul track of the upstream girder and induce large-scale vortices in the gap that cause periodical lift forces of large amplitude acting on the downstream girder, resulting in VIVs of the bridge deck. However, the VIVs are apparently suppressed by the central grids because the vortices in the central gap are reduced into smaller vortices and become weaker, causing slightly fluctuating lift forces on the deck. In addition, the mean lift force on the deck is mainly caused by the upstream girder, whereas the fluctuating lift force is mainly caused by the downstream girder.

• Tunnel and Underground Space
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• v.9 no.4
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• pp.288-295
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• 1999

Concentrated stresses due to the underground tunnel excavation easily cause many problems such as yielding, popping, and failure at the immediate roof, wall and floor of tunnel. Therefore, it is very important to predict the possibility of these problems when a tunnel is excavated underground. There are two typical methods to predict these problems. The one is to predict problems from the analysis of field monitoring data and the other is to predict them from computer simulations using good site investment data. Using the second method, this study attempted to describe the time-dependent or progressive manner of immediate roof and wall due to the underground tunnel excavation. An iterative technique was used to represent progressive failure of rockmass with the Hoek and Brown theory. By developing and simulating three different shapes of twin tunnels, this research estimated the proper size of critical pillar width between tunnels, distributed stresses on the tunnel walls, and convergences of tunnel crowns. Moreover, results out of progressive failure technique based on the Hoek and Brown theory were compared with the results out of Mohr-Coulomb theory.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.4
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• pp.53-63
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• 2002

The high speed wind tunnel test for the study of the basic aerodynamic characteristics of the rocket with twin strap-on boosters was performed using ADD trisonic wind tunnel on the Mach number range of 0.4~4.0. The 6 % scale model of the early design version of Korean sounding rocket was tested. The tested configurations were core only, core/fins, core/boosters and core/boosters/fins. The effects of core length, gap between core and booster, and bank angle were investigated.

• Wind and Structures
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• v.1 no.3
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• pp.225-241
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• 1998

The high-rise supporting towers of long-span suspension and cable-stayed bridges commonly comprise a pair of slender prisms of roughly square cross-section with a center-to-centre spacing of from perhaps 2 to 6 widths and connected by one or more cross-ties. The tower columns may have a constant spacing as common for suspension bridges or the spacing may reduce towards the top of the tower. The present paper is concerned with the aerodynamics of such towers and describes an experimental investigation of the overall aerodynamic forces acting on a pair of square cylinders in two-dimensional flow. Wind tunnel pressure measurements were carried out in smooth flow and with a longitudinal intensity of turbulence 0.10. Different angles of attack were considered between $0^{\circ}$ and $90^{\circ}$, and separations between the two columns from twice to 13 times the side width of the column. The mean values of the overall forces proved to be related to the bias introduced in the flow by the interaction between the two cylinders; the overall rms forces are related to the level of coherence between the shedding-induced forces on the two cylinders and to their phase. Plots showing the variation of the force coefficients and Strouhal number as a function of the separation, together with the force coefficients spectra and lift cross-correlation functions are presented in the paper.

• Geomechanics and Engineering
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• v.7 no.1
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• pp.75-85
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• 2014

The application of the mechanized tunnelling has been extended in recent years. There are at present different approaches that are used in the design of segmental tunnel linings supported in mechanized tunnels. Even though segmental lining is utilized for mechanized tunnels, its behaviour is still quite unclear under in situ stress and there is a lack of data regarding the distribution of stresses inside segmental linings. So far no single effective calculation method exists for segmental lining design. The lack of clear solutions makes the use of segmental lining to be more expensive due to the adoption of greater safety factors. Therefore, a particular attention must be given in order to obtain data from monitored tunnels which permits to validate design methods. In this study, strain measurements, which were conducted during the construction of twin tunnels in the Bologna-Florence railway line, have been presented. The behaviour of segmental lining during the excavation and the influence of a new tunnel excavation on an existing tunnel have been shown through the measured data. The data are then compared with the results obtained with Einstein and Schwartz's method and Duddeck and Erdmann's method, which permits to highlight the fact that the two analytical methods underestimate structural forces induced in the segmental lining and then must be used with caution.

• Journal of Korean Tunnelling and Underground Space Association
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• v.7 no.1
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• pp.63-72
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• 2005

The influence of internal water pressure under the Probable Maximum Flood (PMF) on tunnel spillway is analysed using 3D FDM analysis. HEC-RAS program including 1-D hydrologic numerical model was also implemented to estimate the maximum pressure on the lining under the PMF, thereafter determined rational internal water pressure. Tunnel spillway was designed as twin tunnel and excavation and supporting stage are fully considered. Analysis was classified into the 3 cases; pressure is applied only to the left tunnel, only to right tunnel, and to both tunnels. The maximum tensile stress and axial force in supporting materials induced by water flow were compared with the critical values to assess the stablilty of the tunnel and the locations of stress concentration parts were also examined.

• Journal of Korean Tunnelling and Underground Space Association
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• v.24 no.6
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• pp.699-714
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• 2022

The objective of this study is to suggest a safe and economical pillar reinforcement method when very near-twin tunnels with a minimum interval of 1 m passes through a soft zone such as weathered soil or weathered rock. A standard cross-sectional view of a two-lane road tunnel was applied to suggest a pillar reinforcement method for the very near-twin tunnels. The thickness of the pillar was 1 m. The ground condition around the tunnel was weathered soil or weathered rock. There were four reinforcement methods for pillar stability evaluation. These were rock bolt reinforcement, pre-stressed steel strand reinforcement, horizontal steel pipe grouting reinforcement, horizontal steel pipe grouting + prestressed steel strand reinforcement. When the ground condition was weathered soil, only the pillar reinforced the horizontal steel pipe grouting + prestressed steel strand did not failed. When the ground condition was weathered rock, there were no failure of the pillar reinforced the horizontal steel pipe grouting or the horizontal steel pipe grouting + prestressed steel strand. It is considered that the horizontal steel pipe grouting reinforcement played a role in increasing the stability of the upper part of the pillar by supporting the upper load applied to the upper part of the pillar.

• Wind and Structures
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• v.12 no.5
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• pp.413-424
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• 2009

A series of wind tunnel free-decay sectional model dynamic tests were conducted to examine the effects of torsional-to-vertical natural frequency ratio of 2DOF bridge dynamic systems on the aerodynamic and dynamic properties of bridge decks. The natural frequency ratios tested were around 2.2:1 and 1.2:1 respectively, with the fundamental vertical natural frequency of the system held constant for all the tests. Three 2.9 m long twin-deck bridge sectional models, with a zero, 16% (intermediate gap) and 35% (large gap) gap-to-width ratio, respectively, were tested to determine whether the effects of frequency ratio are dependent on bridge deck cross-section shapes. The results of wind tunnel tests suggest that for the model with a zero gap-width, a model to approximate a thin flat plate, the flutter derivatives, and consequently the aerodynamic forces, are relatively independent of the torsional-to-vertical frequency ratio for a relatively large range of reduced wind velocities, while for the models with an intermediate gap-width (around 16%) and a large gap-width (around 35%), some of the flutter derivatives, and therefore the aerodynamic forces, are evidently dependent on the frequency ratio for most of the tested reduced velocities. A comparison of the modal damping ratios also suggests that the torsional damping ratio is much more sensitive to the frequency ratio, especially for the two models with nonzero gap (16% and 35% gap-width). The test results clearly show that the effects of the frequency ratio on the flutter derivatives and the aerodynamic forces were dependent on the aerodynamic cross-section shape of the bridge deck.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.31 no.6
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• pp.309-314
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• 2018

The wind load and structural characteristics of a twin building are more complex than those of conventional high-rise buildings. The pressure load due to wind on a twin building was therefore measured via wind tunnel experiments to analyze such characteristics. The wind pressure pattern was then deduced from measured data using proper orthogonal decomposition. Channeling and vortex shedding were observed in the first and second modes, respectively. The along-wind loads on the two buildings featured a positive correlation and the cross-wind loads featured no correlation. Such a correlation affected the wind-induced displacement. The structural member connecting the two buildings had an insignificant effect on the positive correlation, but it notably reduced the wind-induced displacement with a negative correlation.

• 전기의세계
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• v.22 no.2
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• pp.40-44
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• 1973

A method for obtaining state equation for nonlinear time-varying RLC networks is presented. The nonlinear time-varying RLC elements are decomposed by using Murata method to formulate nonlinear state equation. A nonlinear time-varying RLC network containing twin tunnel diode is solved as an example. In consequence of solving the examjple, simple methods are presented for revising the original network model so that the formulation of state equation is simplified.