• Proceedings of the Korea Water Resources Association Conference
• /
• 2015.05a
• /
• pp.168-168
• /
• 2015

자연하천의 주요한 특징 중 하나인 하천의 사행은 직선수로에서 예측되는 유속분포를 왜곡시키며 매우 복잡한 흐름구조를 형성한다. 이는 하상 경계면에서 발생하는 전단응력 분포의 변화를 야기하는데 하상 경계면에서의 전단응력은 다양한 경험적 관계에 의존하는 유사이동의 한계 소류력 산정 및 오염물질 거동해석의 분산계수 산정에 많은 영향을 미치게 된다. 물리적인 관측을 통한 하상 경계면에서의 전단응력의 관측은 다소 제한적이며 많은 비용을 요구한다. 따라서 하상 경계면에서 발생하는 전단응력의 경우 수심의 20% 이하의 연직 유속분포를 벽법칙에 적용하여 추정하는 방법이 주로 이루어지고 있다. 벽법칙을 이용한 하상 경계면의 전단응력을 계산하는 경우 대수중복층의 유속 분포 $u/u^*=(1/{\kappa})ln(zu^*/{\nu})+B$에서 무차원상수 ${\kappa}$와 B의 적절한 추정이 요구되어 진다. 일반적으로 무차원상수 ${\kappa}$와 B는 수리학적으로 매끄러운 벽면에서 대략 ${\kappa}=0.4$, B=5.5로서 경험적으로 이용되고 있다. 본 연구에서는 직선수로 및 다양한 사행수로의 3차원 흐름장 모의를 수행하여 벽법칙의 대수 중복층을 따르는 주흐름 방향 유속의 연직분포를 비교하였다. 수치모의 소프트웨어로서 Linux 기반의 OpenFOAM이 사용되었으며 모델의 검증을 위해 Chang(1971)에 의해 수행 된 사행수로에서의 유속장 관측 결과와 비교하였고 수치모의 결과가 실험 관측치와 잘 일치하는 것으로 판단되었다. 수치모의에 적용 된 사행수로의 형상은 Hey(1976)에 의해 제안 된 사행하천의 지형학적 인자들 간에 관계를 이용하여 사행도 1.03에서 2.42까지 총 7개의 사행수로 지형을 생성하였다. 사행도의 변화에 따라 만곡부 정점에서 대수중복층 구간의 주흐름 방향 유속의 연직분포를 비교한 결과, 본 연구에서 생성 된 모든 사행수로에서 대수중복층 구간의 무차원 유속 $u^+$와 무차원 거리 $z^+$가 로그 분포를 따르는 것으로 나타났으나 경험적으로 사용되었던 무차원상수 B의 경우 사행도가 증가 할수록 대수적으로 감소하는 경향이 나타났다. 본 연구에서는 이러한 관계가 무차원 상수 B값에 미치는 영향을 반영하여 수리학적으로 매끄러운 벽면에서 적용이 가능한 수정된 대수중복층 식을 제시하고자 한다.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2009.05a
• /
• pp.445-449
• /
• 2009

With the recent increase in the demand for the net-shape forming, numerical simulations are being commonly adopted to increase the efficiency and effectiveness of design of bulk metal forming processes. Proper consideration of tribological problems at the contact interface between the tool and workpiece is crucial in such simulations. In other words, lubrication and friction play important roles in metal forming by influencing the metal flow, forming load and die wear. In order to quantitatively estimate such friction condition or lubricant characteristic, the constant shear friction model is widely used for bulk deformation analyses. For this, new friction testing method based on the forward or backward extrusion process is proposed to predict the shear friction factor in this work. In this method, the tube-shaped punch pressurizes the workpiece so that the heights at the center and outer of punch (or mandrel) become different according to the friction condition. That is, the height at the center of punch is higher than that at the outer of the punch when the friction condition at the contact interface is severe. From this founding, the proposed friction testing method can be applied to effectively evaluate the friction condition in bulk metal forming processes.

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
• /
• 1995.10a
• /
• pp.51-56
• /
• 1995

일방향 흐름에 표면파가 중복하여 작용할 때 해저면 마찰력의 증가로 토사이동량의 증대가 예상된다. 표면파는 왕복운동 특성을 갖고 있으며, 유속과 해저전단력간의 위상차와 해저잔단력과 토사이동발생 시각간의 위상차가 존재하기 때문에 토사이동량 산정에 많은 어려움이 따른다. 그러나 이렇게 복잡한 현상이 내포되어 있음에도 불구하고 일반적인 기법으로 하천 토사이동량 산정식을 수정하거나 비례상수를 재조정하여 파와 해류에 의한 토사이동량을 산정해 왔다. (중략)

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.26 no.7
• /
• pp.1340-1347
• /
• 2002

Finite element simulations are being widely used to increase the efficiency and effectiveness of design of bulk metal forming processes. In such simulations, proper consideration of friction condition is crucial in obtaining reliable results. For this purpose, tip test based on backward extrusion was proposed recently. In this lest, a cylindrical billet is positioned in a shallow groove of a counter punch for centering purpose and formation of a radial tip is induced on the extruded end of the workpiece. In this study, the effect of centering groove on tip test was investigated. The quantitative ratio of the shear friction factors between the punch and die was numerically determined depending on the shape of centering groove. Also, surface expansion and pressure distribution along the punch and die were considered in order to better understand the reason that friction condition at the punch compared to the one of die was more severe.

• Journal of the Korean Geotechnical Society
• /
• v.19 no.3
• /
• pp.75-82
• /
• 2003

This paper presents the results of a study that was performed to evaluate fronds of shear strength parameters in stabilization of unbound soil-aggregate systems based on the theory of unsaturated soil mechanics. Two important shear strength parameters, effective cohesion and effective angle of internal friction were estimated by the proposed approach using the results from suction measurements and unconfined compressive strength test. In addition, the effect of different addition rates of stabilizing agent was compared. Due to the stabilization process, an increase in suction potential on engineering properties of geomaterials was observed by using dielectric constant measurements. In conclusion, the results from this study show that the proposed approach can be simply used for predicting shear strength parameters of the stabilized geomaterials.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 1999.03b
• /
• pp.119-124
• /
• 1999

Quantitative evaluation of the tribological conditions at the tool-workpiece interface in metal forming is usually accomplished by the ring-compressinon. test This paper describes an experimental investigation into shear friction factor under cold and hot forming conditions according to materials and lubricants using the ring compression test. Six different materials and five different lubricants were applied in the experiments. calibration curves with the friction coefficient were obtained using FEM analysis and verified by the verified by the experimental results. The influence of materials and lubricants level on friction are discussed. In the ring compression test the shear friction factor has a different from steels and aluminum water base graphite lubricants on cold working conditions and hot working conditions.

• Journal of the Korea Concrete Institute
• /
• v.18 no.5 s.95
• /
• pp.611-620
• /
• 2006

In the shear failure mechanism of a beam, beam and arch actions always exist simultaneously. According to the shear span to depth ratio, the proportion between these two actions is varied and the contribution of these actions to shear capacity is changed. Moreover, the current codes provide recommendations based on experimental results of normal strength concrete, so the application range of concrete strength must be extended. Based on this mechanism and new requirement, a simplified analytical equation for shear capacity prediction of reinforced high strength concrete beams without stirrups is proposed. To reflect the change in the contribution between these actions, stress variation in the longitudinal reinforcement along the span is considered by use of the Jenq and Shah Model. Dowel action with horizontal splitting failure and shear friction between cracks are also taken into account. ize effect is included to derive a more precise equation. Regression analysis is performed to determine each variable and simplify the equation. And, the formula derived from theoretical approaches is evaluated by comparison with numerous experimental data, which are in broad range of concrete strength(especially in high strength concrete), shear span to depth ratio, geometrical size and longitudinal steel ratio. It is shown that the proposed equation is more accurate and simpler than other empirical equations, so a wide range of a/d can be considered in one equation.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.37 no.10
• /
• pp.955-966
• /
• 2009

DES method is applied to an axisymmetric base flow at supersonic mainstream. The model is based on the Spalart-Allmaras (S-A) turbulence model in the RANS mode, and is based on the subgrid scale model in the Large-eddy simulation (LES) mode. Accurate predictions of the base flowfield and base pressure are successfully achieved by using the DES methodology which is less expensive than LES. Flow properties at the edge of base, such as boundary layer thickness, momentum thickness and skin fraction are compared with Dutton et al [experimental data to proper prediction of base flowfiled. From the present results, The DES accurately resolves the physics of unsteady turbulent motions, such as shear layer rollup, large-eddy motions in the downstream region and small eddy motions inside the recirculating region. Moreover, The present results of using an empirical constant $C_{DES}$ of 1.2 shows good agreement with experimental data than conventional empirical constant $C_{DES}$ of 0.65.

• Tunnel and Underground Space
• /
• v.32 no.1
• /
• pp.20-29
• /
• 2022

The existence of underground cavity should be considered in the assessment of georisk such as ground subsidence. Even if the shear strength of the ground around the cavity is known, it is difficult to accurately analyze the safety around the cavity due to the uncertainties related to geometric conditions such as the cavity size. In this paper, stability chart representing dimensionless stability constants was proposed based on the ground strength and geometric conditions. Numerical analysis had been carried out accounting for the stability constants such as the ground strength, the adhesion and friction angles, and the size and depth of the underground cavity. The proposed charts can help estimating the stability of ground with underground circular cavity.

• Journal of Bio-Environment Control
• /
• v.22 no.3
• /
• pp.284-290
• /
• 2013

This study analyzed the geometric, compressive, cutting and friction properties of oriental melons in order to design a gripper capable of soft handling and a cutter for cutting oriental melon vine among the end effector of oriental melon as a preliminary step for developing the end effector of the robot capable of harvesting oriental melons in protected cultivation. As a result, the average length, diameter at the midpoint, weight, volume and roundness of the oriental melons were 108 mm, 70 mm, 188 g, 333 mL and 3.8 mm. Nonlinear regression analysis was performed on the equation $W=L^a{\times}D_2^b$ with variation of the length (L) and diameter (D2) of the weight (W) of the oriental melons. As a result, it was shown that there was a correlation between a of 2.0279 and b of -0.9998 as a constant value. The average diameter of the oriental melon vine was 3.8 mm, and most vines were distributed within a radius of 5 mm from the center. The average yield value, compressive strength and hardness of the oriental melons were $36.5N/cm^2$, $185.7N/cm^2$ and $636.7N/cm^2$, respectively. The average cutting force and shear strength of the oriental melon vines were $2.87{\times}10^{-2}\;N$ and $5.60N/cm^2$, respectively. The maximum friction coefficient of the oriental melons was rubber of 0.609, followed by aluminium of 0.393, stainless steel of 0.177 and teflon of 0.079. It was considered possible to apply it to the size of the gripper and cutter, turning radius, dynamics of drive motor and selection of materials and their quality in light of the position error and safety factor according to the movement when designing end effector based on the analyzed data.