• Wind and Structures
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• v.18 no.5
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• pp.511-528
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• 2014

The characteristics of wake galloping phenomenon for two parallel/unparallel circular cylinders were investigated via wind tunnel tests. The two cylinders were initially deployed in parallel and wake galloping phenomena were observed by varying the center-to-center distance. The effect of an unparallel arrangement of two cylinders was next investigated by fixing the spacing ratio of one side of the cylinders at 5.0D and the other side at 3.0D, in which D represents the diameter of the cylinder. For the unparallel disposition, the 5.0D side showed a small, limited vibration while the 3.0D side produced much larger amplitude of vibration, resulting in a rolling motion. However, the overall amplitude appeared to decrease in unparallel disposition when compared with the amplitude of the 3.0D - 3.0D parallel case. This represents the mitigation effect of wake galloping due to the unparallel disposition between two cylinders. Flow visualization tests with particle image velocimetry were conducted to identify flow fields between two cylinders. The test results demonstrate the existence of a complex interaction of the downstream cylinder with the shear layer generated by the upstream cylinder. When the spacing ratio was large enough, the shear layer was not observed and the downstream cylinder showed only limited random vibration.

• Structural Engineering and Mechanics
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• v.83 no.4
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• pp.495-513
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• 2022

This paper aims to study the fracture mechanism of rocks under the 'u'shape cutters considering the effects of crack (pre-existing crack) distances, crack spacing and crack inclination angles. The effects of loading rates on the rock fragmentation underneath these cutters have been also studied. For this purpose, nine experimental samples with dimensions of 5 cm×10 cm×10 cm consisting of the non-persistent cracks were prepared. The first three specimens' sets had one non-persistent crack (pre-existing crack) with a length of 2 cm and angularity of 0°, 45°, and 90°. The spacing between the crack and the "u" shape cutter was 2 cm. The second three specimens" set had one non-persistent crack with a length of 2 cm and angularity of 0°, 45°, and 90° but the spacing between pre-existing crack and the "u" shape cutter was 4 cm. The third three specimens'set has two non-persistent cracks with lengths of 2 cm and angularity of 0°, 45° and 90°. The spacing between the upper crack and the "u" shape cutter was 2 cm and the spacing between the lower crack and the upper crack was 2 cm. The samples were tested under a loading rate of 0.005 mm/s. concurrent with the experimental investigation. The numerical simulations were performed on the modeled samples with non-persistent cracks using PFC2D. These models were tested under three different loading rates of 0.005 mm/s, 0.01 mm/sec and 0.02 mm/sec. These results show that the crack number, crack spacing, crack angularity, and loading rate has important effects on the crack growth mechanism in the rocks underneath the "u" shape cutters. In addition, the failure modes and the fracture patterns in the experimental tests and numerical simulations are similar to one another showing the validity and accuracy of the current study.

• The Journal of The Korea Institute of Intelligent Transport Systems
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• v.23 no.1
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• pp.82-96
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• 2024

While analyzing traffic flow, speed, traffic volume, and density are important macroscopic indicators, and acceleration and spacing are the important microscopic indicators. The speed and traffic volume can be collected with the currently installed traffic information collection devices. However, acceleration and spacing data are necessary for safety and autonomous driving but cannot be collected using the current traffic information collection devices. 'You Look Only Once'(YOLO), an object recognition technique, has excellent accuracy and real-time performance and is used in various fields, including the transportation field. In this study, to measure acceleration and spacing using YOLO, we developed a model that measures acceleration and spacing through changes in vehicle speed at each interval and the differences in the travel time between vehicles by setting the measurement intervals closely. It was confirmed that the range of acceleration and spacing is different depending on the traffic characteristics of each point, and a comparative analysis was performed according to the reference distance and screen angle to secure the measurement rate. The measurement interval was 20m, and the closer the angle was to a right angle, the higher the measurement rate. These results will contribute to the analysis of safety by intersection and the domestic vehicle behavior model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.8
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• pp.1115-1122
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• 2002

The present experiment investigates the effects of pore density f of aluminum foam heat sinks, the jet-to-jet spacing X and the nozzle plate-to-target surface spacing H of 3$\times$3 square impinging arrays on the averaged Nusselt number. The performance of the aluminum foam heat sinks and the rectangular plate heat sink is evaluated in terms of the enhancement factor. /equation omitted/. The multiple impinging jet with X/d=4.0 displays higher Nusselt numbers than single impinging jet for 12.0$\leq$H/d$\leq$20.0. With the variation of the jet-to-jet spacing, the aluminum foam heat sink of 10 PPI show higher Nusselt numbers than the 20 and 40 PPI aluminum foam heat sinks. Further, the 10 PPI aluminum foam heat sink demonstrates 26% higher enhancement factor than the rectangular plate heat sink in the range of 7000$\leq$Re$\leq$11000.

• Advances in aircraft and spacecraft science
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• v.10 no.3
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• pp.203-222
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• 2023

The detonation combustion is a supersonic combustion process follows on shock wave oscillations in detonation tube. In this paper numerical studies are carried out combined effect of blockage ratio and spacing of obstacle on detonation wave propagation of hydrogen-air mixture in pulse detonation combustor. The deflagration to detonation transition of stoichiometric (ϕ=1)fuel-air mixture in channel has been analyzed for effect of blockage ratio (BR)=0.39, 0.51, 0.59, 0.71 with spacing of 2D and 3D. The reactive Navier-Stokes equation is used to solve the detonation wave propagation mechanism in Ansys Fluent platform. The result shows that fully developed detonation wave initiation regime is observed near smaller vortex generator ratio of BR=0.39 inside the combustor. The turbulent rate of reaction has also a great significance role for shock wave structure. However, vortices of rapid detonation wave are appears near thin boundary layer of each obstacle. Finally, detonation combustor demonstrates the superiority of pressure gain combustor with turbulent rate of reaction of 0.6 kg mol/m3 -s inside the detonation tube with obstacle spacing of 12 cm, this blockage enhanced the turbulence intensity and propulsive thrust. The successful detonation wave propagation speed is achieved in shortest possible time of 0.031s with a significance magnitude of 2349 m/s, which is higher than Chapman-Jouguet (C-J) velocity of 1848 m/s. Furthermore, stronger propulsive thrust force of 36.82 N is generated in pulse time of 0.031s.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.3
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• pp.561-565
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• 1988

In the present study, a new method of generating a nearly orthogonal boundary-fitted coordinate systems with automatic grid spacing control is introduced. Applications of the method to a two dimensional simply-connected region is then demonstrated. The nearly orthogonal boundary-fitted method has the following features, (a) Strong grid control in the .eta.-direction can be made, (b) The generated boundary-fitted coordinates are nearly orthoronal, (c) Both the .xi.-and .eta.-direction control function are mathematically derived. Especially the .eta.-direction control function is derived under the assumption that the .eta.-direction grid spacing is by far smaller than the .xi.-direction grid spacing when the .eta.-direction grid line is strongly clustered. (d) The grid control functions are dynamically adjusted by the metric scale factors imposed on the boundary. The control function is fully automatic and eliminates the need of user manipulation of the control function.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.1 s.256
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• pp.8-15
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• 2007

The jet impingement cooling characteristics are investigated experimentally. The study is motivated by the potential application of local hot spot cooling by means of the vortex tube. The purposes of this research are to examine the effect of the nozzle-block spacing and flow rate. The results of jet through vortex tube is compared with ones of circular Jet. Flow visualization by the smoke-wire technique is also performed to investigate the flow structure. As the nozzle-block spacing is increased and flow rate decreased, the cooling effect of the Jet through the vortex tube decreases mere remarkably than that of the circular jet. So the cooling effect for the jet through the vortex tube is higher than that for the circular jet at $H/D{\leq}3$, $Q{\geq}10m^3/h$.

• Journal of electromagnetic engineering and science
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• v.13 no.4
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• pp.240-244
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• 2013

In this paper, we present an improvement of a closed-form formula for mutual impedance computation. Depending on the center-to-center spacing between two rectangular microstrip patch antennas, the mutual impedance formula is separated into two parts. The formula based on synthetic asymptote and variable separation is utilized for spacings of more than 0.5 ${\lambda}_0$. When the spacing is less than 0.5 ${\lambda}_0$, an approximate formula is proposed to improve the computation for closely spaced elements. Simulation results are compared to computational results of mutual impedances and mutual coupling coefficients as functions of normalized center-to-center spacing in both E- and H-plane coupling configurations. A good agreement between simulation and computation is achieved.

• Tunnel and Underground Space
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• v.23 no.1
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• pp.42-53
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• 2013

The percussion rate and spacing of the button of drill bit are very important in maximizing the drilling efficiency. Therefore, a series of percussion tests using Hopkinson bar system was carried out to assess the fragmentation performance against the beat rate and spacing of a drill bit. First, single percussion test complemented with numerical simulation was performed to analyze rock fragmentation phenomenon and to describe the fragmentation process. Next, multiple percussion test that repetitively strike the rock sample moving at predetermined rate was carried out to predict drilling efficiency against the button spacing. After the tests, the fragmented volume of the rock was measured by laser scanner and the drilling performance was analyzed using the calculated percussive energy and measured negative volume. Based on the results, the single impact performance of drill bit with 102 mm diameter was predicted.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.8
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• pp.1566-1574
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• 1992

Local heat transfer characteristics for a round air jet impinging normally on a heated flat plate were experimentally investigated. The problem parameters investigated were jet Reynolds number, Re=4000,10000, and 20000, and nozzle-to-plate spacing(L/D) of 2,6, and 10. The temperature variations on the flat uniform heat flux surface were mapped using a thermo-sensitive liquid crytal sheet. The isochromatic images corresponding to the characteristic temperature of liquid crystal were analyzed with the help of a digital image processing system. The local Nusselt number, Nu decreased rapidly in the impingement region and exhibited a similar profiles in the wall jet region independent of the nozzle-to-plate spacing L/D. In the case of large Reynolds number, heat transfer rate (Nu) was proportional to 0.5 power of the Reynolds number. For L/D=2, a secondary peak in the heat transfer rate was seen in the region of X/D=1.5~3 due to the transition from laminar to turbulent boundary layer.