• Journal of the Korean GEO-environmental Society
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• v.6 no.1
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• pp.45-51
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• 2005

The distribution of shaft resistance is measured by the static load test with the strain gauge or stress gauge, so that the long-term load distribution must be considered for the pile design. However, the measurement by strain gauge generally assumes the 'zero reading', which is the reading taken at 'zero time' with 'zero' load and the residual stress, which is the negative skin friction(or the negative shaft resistance) caused by the pile construction, is neglected. Therefore, the measured value by strain gauge is different from the true load-distribution because residual stresses were neglected. In this study, the three drilled shafts were constructed, and the strain measurements were carried out just after shaft construction. As a result of this study, it is shown that the true load-distribution of drilled shaft is quite different with known load distribution and the true load-distribution of drilled shaft changed from the negative skin friction to the positive skin according to the load increment.

• Journal of the Society of Naval Architects of Korea
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• v.56 no.4
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• pp.361-372
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• 2019

The flow pattern of air layers and skin-friction drag reduction by air injection are investigated to find the suitable multiphase flow model using unstructured finite-volume CFD solver for the Reynolds-averaged Navier-Stokes equations. In the present computations, two different multiphase flow modeling approaches, such as the Volume of Fluid (VOF) and the Eulerian Multi-Phase (EMP), are adopted to investigate their performances in resolving the two-phase flow pattern and in estimating the frictional drag reduction. First of all, the formation pattern of air layers generated by air injection through a circular opening on the bottom of a flat plate are investigated. These results are then compared with those of MMkiharju's experimental results. Subsequently, the quantitative ratios of skin-friction drag reduction including the behavior of air layers, within turbulent boundary layers in large scale and at high Reynolds number conditions, are investigated under the same conditions as the model test that has been conducted in the US Navy's William B. Morgan Large Cavitation Channel (LCC). From these results, it is found that both VOF and EMP models have similar capability and accuracy in capturing the topology of ventilated air cavities so called'air pockets and branches'. However, EMP model is more favorable in predicting quantitatively the percentage of frictional drag reduction by air injection.

• Journal of the Korean Geotechnical Society
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• v.26 no.5
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• pp.5-13
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• 2010

For the development of design chart for drilled shafts socketed into weathered zone, the 6 bi-directional pile load tests with load transfer measurements done in two in-situ sites were performed. Also, DCPTs were performed in each test point. Maximum unit skin frictions and maximum unit end bearing capacities from pile load test results were analyzed. Inter-relationships between DCPT's characteristics were also analyzed. In the soils, the inter-relationships of maximum unit skin friction and DCPT appeared so low. But in the weathered zones, inter-relationships between maximum unit skin friction / maximum unit end bearing capacity and DCPT were so high that the coefficient of correlation is over 0.70.

• Proceedings of the Korean Geotechical Society Conference
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• 1999.03a
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• pp.249-258
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• 1999

Dynamic load and static load tests are performed on steel pipe piles and concrete piles at five construction sites in highway to compare the difference of load bearing mechanisms. At each site, one steel pile is instrumented with electric strain gages and dynamic tests are performed on the pile during installation. Damages of strain gages due to the installation are checked and static test is performed upon the same pile after two or seven days as well. It shows that load transfer from side friction to base resistance behaves somewhat differently according to the results of load-settlement analysis obtained from PDA and static load test. Initial elastic stage of load settlement curves of two load tests is almost similar. But after the yielding point, dynamic resistance of pile behaves more stiffer than static resistance, thus, dynamic load test result might overestimate the real pile capacity compared with static result. Analysis of gage readings shows that unit skin friction increases exponentially with depth. The skin friction is mobilized at the 1∼2m above the pile tip and contributes to the considerable side resistance. Comparison of side and base resistances between the measured value and the calculated value by Meyerhof's bearing capacity equation using SPT N value shows that the calculated base resistance is higher than the measured. Therefore, contribution of side resistance to total capacity shouldn't be ignored or underestimated. Finally, based upon the overall test results, a construction control procedure is suggested.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.463-466
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• 2002

In order to examine the possibility of using a cavity as a passive device for reduction of skin friction and heat transfer, an intensive parametric study over a broad range of the cavity depth and length at different Reynolds numbers is performed for both laminar and turbulent boundary layers in the present study. Direct and large eddy simulation techniques are used for turbulent boundary layers at low and moderate Reynolds numbers, respectively. for both laminar and turbulent boundary layers over a cavity, a flow oscillation occurs due to the shear layer instability when the cavity depth and length are sufficiently large and it plays an important role in the determination of drag and heat-transfer increase or decrease. For a cavity sufficiently small to suppress the flow oscillation, both the total drag and heat transfer are reduced. Therefore, the applicability of a cavity as a passive device for reduction of drag and heat transfer is fully confirmed in the present study. Scaling based on the wall shear rate of the incoming boundary layer is also proposed and it is found to be valid in steady flow over a cavity.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• v.12 no.4
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• pp.201-202
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• 2008

Because of the importance of suction or injection in the fields of aerodynamics, space science and many other industrial applications, our present study is motivated. The effect of natural convection on MHD flow past a vertical plate with suction or injection is studied. We have tried to solve the dimensionless governing equations by using finite difference scheme. To ensure the validity of our numerical solutions, we have compared our numerical solutions for temperature and velocity for the case of suction and injection for unit Prandtl number with the available exact solutions in the literature. The corresponding codes were written in Mathematica 5.0 for calculating numerical solutions for temperature and velocity and the comparison between the exact and numerical solutions. For the purpose of discussing the results some numerical calculations are carried out for non-dimensional temperature T, velocity u, skin friction ${\tau}$ and the Nusselt number $N_u$, by making use of it, the rate of heat transfer is studied.

• 한국가시화정보학회:학술대회논문집
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• 2005.12a
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• pp.17-22
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• 2005

An experimental study was carried out to investigate the effect of local ultrasonic forcing on a turbulent boundary layer. Stereoscopic particle image velocimetry (SPIV) was used to probe the characteristics of the flow. A ultrasonic forcing system was made by adhering six ultrasonic transducers to the local flat plate. Cavitation which generates uncountable minute air-bubbles having fast wall normal velocity occurs when ultrasonic was projected into water. The SPIV results showed that the wall normal mean velocity is increased in a boundary layer dramatically and the streamwise mean velocity is reduced. The skin friction coefficient ($C_{f}$) decreases $60\%$ and gradually recovers at the downstream. The ultrasonic forcing reduces wall-region streamwise turbulent intensity, however, streamwise turbulent intensity is increased away from the wall. Wall-normal turbulent intensity is almost the same near the wall but it increases away from the wall, In tile vicinity of the wall, Reynold shear stress, sweep strength and production of turbulent kinetic energy were decreased. This suggests that the streamwise vortical structures are lifted by ultrasonic forcing and then skin friction is reduced.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.11
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• pp.1481-1489
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• 1999

The sliding-belt concept introduced by Bechert et al. (AIAA J., Vol. 34, pp. 1072~1074) is numerically applied to a turbulent boundary layer flow for the skin-friction reduction. The sliding belt is moved by the shear force exerted on the exposed surface of the belt without other dynamic energy input. The boundary condition at the sliding belt is developed from the force balance. Direct numerical simulations are performed for a few cases of belt configuration. In the ideal case where the mechanical losses associated with the belt can be ignored, the belt velocity increases until the integration of the shear stress over the belt surface becomes zero, resulting in zero skin friction on the belt. From practical consideration of losses occurred In the belt device, a few different belt velocities are given to the sliding belt. It is found that the amount of drag reduction is proportional to the belt velocity.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• v.19 no.1
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• pp.57-68
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• 2015

When a straight channel formed by two parallel porous plates, through which two immiscible liquids occupying different heights are flowing a secondary motion is set up. The motion is caused by moving the upper plate with a uniform velocity about an axis perpendicular to the plates. The solutions are exact solutions. Here we discuss the effect of suction parameter and the position of interface on the flow phenomena in case of Couette flow. The velocity distributions for the primary and secondary flows have been discussed and presented graphically. The skin-friction amplitude at the upper and lower plates has been discussed for various physical parameters.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.9
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• pp.2431-2442
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• 1994

In this paper the efficient space marching Viscous Shock Layer and Parabolized Navier-Stokes method have been applied to study the complex 3-D hypersonic equilibrium chemically reacting flowfilelds over sphere-cone($10^{\circ}$) vehicle at low angles of attack($0^{\circ}~5^{\circ}), Mach 20, and an altitude of 35km. The current bluntbody/afterbody space marching numerical method predicts the complex flowfields accurately and efficiently even on a small computer. The shock thickness from equilibrium air model is thinner than that from the perfect gas model. The windside wall heat-transfer rate, pressure and skin friction force were increased significantly when compared with those of leeside. The CA, CN, CM were increased almost linearly with the angle of attack in this region. The wall pressure, heat transfer, skin friction and axial force coeffient from equilibrium model were much higher than those from perfect gas model. The center of pressure moved forward with the increase of angle of attack.