Dae-Geun Kim

High-strain, high-strain-rate behavior of tantalum

Tantalum plate produced by a forging-rolling sequence was subjected to high plastic shear strains(γ = 1 → 5.5) at high strain rates (∼4 × 104 s-1) in two experimental configurations: (a) a special hat-shaped geometry and (b) thin disks deformed in a split Hopkinson bar. In parallel experiments, the constitutive behavior of the same material was established through quasi-static and dynamic compression tests at ambient and elevated temperatures. The microstructure generated at high strain rates and retained by rapid cooling from a narrow (200-μm) deformation band progresses from dislocated, to elongated cells, to banded structures, and finally, to subgrains as the shear strain increases from 0 to 5.5. The temperature rise predictions from the constitutive description of the material indicate that the temperature reaches values of 800 K, and it is proposed that thermal energy is sufficient to produce a significant reorganization of the deformation substructure, leading to a recovered structure.

Analysis of the Flow over Broad Crested Side Weir by Using Three-Dimensional Numerical Simulation

In this study, we analyzed the flow characteristics in the wide-crested side weir of trapezoidal section by using a three dimensional numerical stimulation. From this study, as the Froude number increases in the main channel, the overflow discharge ratio and the discharge coefficient of lateral overflow tend to decrease. And it was also found that the increase of the lateral overflow reduces the channel discharge area in the downstream, and the size of recirculating zone is increasing in the opposite side of side weir. The stream-wise water surface on the side where the side weir is installed falls down rapidly in the weir starting point, gradually ascending, and rapidly rising at the end point. The reason why the water surface rapidly rises at the weir end point is because the weir end point hinders the flow.

Numerical Analysis of Dam-break Waves in an L-shaped Channel with a Movable Bed

We conducted a three-dimensional numerical simulation by using the FLOW-3D, with RANS as the governing equation, in an effort to track the dam-break wave.immediately after a dam break.in areas surrounding where the dam break took place as well as the bed change caused by the dam-break wave. In particular, we computed the bed change in the movable bed and compared the variation in flood wave induced by the bed change with our analysis results in the fixed bed. The analysis results can be summarized as follows: First, the analysis results on the flood wave in the L-shaped channel and on the flood wave and bed change in the movable-bed channel successfully reproduce the findings of the hydraulic experiment. Second, the concentration of suspended sediment is the highest in the front of the flood wave, and the greatest bed change is observed in the direct downstream of the dam where the water flow changes tremendously. Generated in the upstream of the channel, suspended sediment results in erosion and sedimentation alternately in the downstream region. With the arrival of the flood wave, erosion initially prove predominant in the inner side of the L-shaped bend, but over time, it tends to move gradually toward the outer side of the bend. Third, the flood wave in the L-shaped channel with a movable bed propagates at a slower pace than that in the fixed bed due to the erosion and sedimentation of the bed, leading to a remarkable increase in flood water level.

Estimation of Areal Reduction Factors for the Youngsan River Basin

By analyzing the concurrent rainfall data from rainfall gauges positioned in the Youngsan River basin, the areal reduction factors related to the rainfall characteristics of the Youngsan River basin are estimated. The estimated values are compared with the values of the Han River basin, and show that the rate of decrease of the areal reduction factors of the Youngsan River basin are smaller than those of the Han River basin as the basin area is increasing. That is especially true for short-term duration storm events. These findings reveal that the spatial variations in the Youngsan River basin`s storms are smaller than the spatial variations of the storms In the Han River basin, due to the size of the two basin areas in addition to the topological characteristics that affect the rainfall distributions.

A Numerical Simulations on the Flow over Ogee Spillway with Tainter Gate

In this study, overflow behaviors through a partially open tainter gate mounted on a standard ogee spillway were investigated by using the FLOW-3D. The results indicated that the discharge coefficient is in the range of 0.685 to 0.723. A relation of gate-controlled discharge to free discharge was proposed and a reasonable correlation between the free and controlled discharge was obtained. Pressures on the spillway crest and the gate were also investigated. As the gate opening rate decreases with a fixed gate opening height and the gate opening height increases at a fixed gate opening rate, negative pressures on the spillway crest and the dimensionless maximum pressures on the gate increase.