• Journal of computational fluids engineering
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• v.18 no.2
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• pp.66-71
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• 2013

In this paper, the supersonic flows around space launch vehicles have been numerically simulated by using a 3-D RANS flow solver. The focus of the study was made for investigating plume-induced flow separation(PIFS). For this purpose, a vertex-centered finite-volume method was utilized in conjunction with 2nd-order Roe's FDS to discretize the inviscid fluxes. The viscous fluxes were computed based on central differencing. The Spalart-Allmaras model was employed for the closure of turbulence. The Gauss-Seidel iteration was used for time integration. To validate the flow solver, calculation was made for the 0.04 scale model of the Saturn-5 launch vehicle at the supersonic flow condition without exhaust plume, and the predicted results were compared with the experimental data. Good agreements were obtained between the present results and the experiment for the surface pressure coefficient and the Mach number distribution inside the boundary layer. Additional calculations were made for the real scale of the Saturn-5 configuration with exhaust plume. The flow characteristics were analyzed, and the PIFS distances were validated by comparing with the flight data. The KSLV-1 is also simulated at the several altitude conditions. In case of the KSLV-1, PIFS was not observed at all conditions, and it is expected that PIFS is affected by the nozzle position.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.5 s.236
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• pp.577-589
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• 2005

The present study has numerically investigated two-dimensional laminar flow over a steadily rotating circular cylinder with a uniform planar shear, where the free-stream velocity varies linearly across the cylinder. It aims to find the combined effect of rotation and shear on the flow. Numerical simulations using the immersed boundary method are performed for the ranges of $-2.5{\le}\alpha{\le}2.5$ and $0{\le}K{\le}0.2$ at a fixed Reynolds number of Re=100, where a and K are respectively the dimensionless rotational speed and velocity gradient. Results show that the positive shear, with the upper side having the higher free-stream velocity than the lower one, favors the effect of the counter-clockwise rotation $(\alpha<0)$ but countervails that of the clockwise rotation $(\alpha>0)$. Accordingly, the absolute critical rotational speed, below which vortex shedding occurs, decreases with increasing K for $(\alpha>0)$, but increases for $\alpha>0$. The vortex shedding frequency increases with increasing \alpha (including the negative) and the variation becomes steeper with increasing K. The mean lift slightly decreases with increasing K regardless of the rotational direction. However, the mean drag and the amplitudes of the lift- and drag-fluctuations strongly depend on the direction. They all decrease with increasing K for $\alpha>0$, but increase for $\alpha<0$. Flow statistics as well as instantaneous flow folds are presented to identify the characteristics of the flow and then to understand the underlying mechanism.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.23 no.11 s.170
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• pp.2014-2021
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• 1999

The present investigation is concerned with the degradation characteristics of cast stainless steel(CF8M), exposed to the $\sigma$-phase degradation at $700^{\circ}C$. In the present paper, the degradation of CF8 M at $700^{\circ}C$ is evaluated by a non-destructive test, DL-EPR(double loop electrochemical potentiokinetic reactivation). The surface of specimens is observed by using scanning electron microscopy after DL-EPR test. Also. chromium contents of matrix, grain boundary and ferrite phase are analyzed by electron probe X-ray micro analyzer. Through the experiments, the following results are obtained 1) The degree of sensitization(DOS) of CF8M aged up to 15hr at $700^{\circ}C$ is increased with acing time while that is decreased with aging time from 15hr to 150hr. 2) The impact energy decreases with increase of $\sigma$-phase while DOS increases with $\sigma$-phase until aging time reaches to 15hr. After the aging time. 15hr, the $\sigma$-phase and the rate of impact energy with respect to aging time decreases. Therefore the degradation behavior of the CF8M can be evaluated by comparing SEM micrographs and the value of DOS.

• 한국해양학회지
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• v.14 no.2
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• pp.71-77
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• 1979

Tidal waves and the fluctuation of current are studied by use of observed data on tidal level, flow velocity and river discharge in the estuarine region of the Nakdong River. Observed data on the tidal level at five stations are used to obtain the fluctuation of amplitude and phase of tides, and the change of the wave speed versus distance from the river mouth. Comnining these tidal data with the vertical distribution of horizontal velocity data, some characteristics of the periodic tidal flow are deduced: (1)Diminishing rates of the tidal amplitude ratio η / η$\_$0/ at high tide were 0.058η$\_$0H/ /Km at neap tides. The constant of phase change, K, was 0.035rad/km. (2)While proceeding landward, the shape of the tidal wave changes from symmetrical to asymmetrical. The traveling speed of the tidal wave crest was estimated to be 3.6∼5.2m/sec, while that of the tidal wave trough was 2.4∼ 3.5m/sec. (3)The flowing speed of the water varies periodically in accordance with the tidal period. The maximum speed of landward flow appeared approximately at two hours before the high tide, while that of seaward flow at two hours before the low tide. (4)The upstream boundary is deduced approximately to be 50km at spring tide and 44km at neap tide from the tidal velocity decreasing. the tidal influence area is estimated approximately to be 65km from the tidal amplitude damping.

• Laser Solutions
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• v.10 no.3
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• pp.25-32
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• 2007

The effect of welding speed on the weldability, microstructures, hardness, tensile property of Nd:YAG laser welding joint in 600MPa grade precipitation hardening high strength steel was investigated. A shielding gas was not used, and bead-on-plate welding was performed using various welding speeds at a power of 3.5kW. Porosity in the joints occurred at 1.8m/min, but were not observed over the welding speed of 2.1m/min. However, spatter occurred over the welding speed of 6.6m/min. The hardness was the highest at heat affected zone(HAZ) near fusion zone(FZ), and was decreased on approaching to the base metal. The maximum hardness increased with increasing welding speed. The microstructure of FZ was composed of coarse grain boundary ferrite and bainite(upper) but the HAZ near the FZ contained bainite(Lower) and fine ferrite at a low welding speed. With increasing welding speed, ferrite at the FZ and the HAZ became finely and upper binite changed to lower bainite. In a perpendicular tensile test to the weld line, all specimens were fractured at the base metal, and the tensile strength and the yield strength of joints was equal to those of raw material. Elongation was found to be lower than that of the raw material.

• Steel and Composite Structures
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• v.20 no.5
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• pp.1023-1042
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• 2016

Based on Hamilton's principle, the flexural vibration differential equations and boundary conditions of the steel-concrete composite beam (SCCB) with comprehensive consideration of the influences of the shear deformation, interface slip and longitudinal inertia of motion were derived. The analytical natural frequencies of flexural vibration were compared with available results previously observed by the experiments, the results calculated by the FE model and the other similar beam theories available in the open literatures. The comparison results showed that, the calculation results of the analytical and Timoshenko models had a good agreement with the results of the experimental test and FE model. Finally, the influences of shear deformation and interface slip on the flexural natural frequencies of the SCCB were discussed. The shear deformation effect increases with the increase of the mode orders of flexural natural vibration, and the flexural natural frequencies of the higher mode orders ignoring the influence of shear deformations effect would be overestimated. The interface slip effect decrease with the increase of the mode orders of flexural natural vibration, and the influence of the interface slip effect on flexural natural frequencies of the low mode orders is significant. The influence of the degree of shear connection on shear deformation effect is insignificant, and the low order modes of flexural natural vibration are mainly composed of the rotational displacement of cross sections.

• Geomechanics and Engineering
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• v.19 no.5
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• pp.463-472
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• 2019

The mode perturbation method (MPM) is suitable and efficient for solving the eigenvalue problem of a nonuniform soil deposit whose property varies with depth. However, results of the MPM do not always converge to the exact solution, when the variation of soil deposit property is discontinuous. This discontinuity is typical because soil is usually made up of sedimentary layers of different geologic materials. Based on the energy integral of the variational principle, a new mode perturbation method, the energy-based mode perturbation method (EMPM), is proposed to address the convergence of the perturbation solution on the natural frequencies and the corresponding mode shapes and is able to find solution whether the soil properties are continuous or not. First, the variational principle is used to transform the variable coefficient differential equation into an equivalent energy integral equation. Then, the natural mode shapes of the uniform shear beam with same height and boundary conditions are used as Ritz function. The EMPM transforms the energy integral equation into a set of nonlinear algebraic equations which significantly simplifies the eigenvalue solution of the soil layer with variable properties. Finally, the accuracy and convergence of this new method are illustrated with two case study examples. Numerical results show that the EMPM is more accurate and convergent than the MPM. As for the mode shapes of the uniform shear beam included in the EMPM, the additional 8 modes of vibration are sufficient in engineering applications.

• Journal of the Korean Society of Environmental Restoration Technology
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• v.18 no.1
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• pp.135-151
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• 2015

DMZ is considered as an ecologically sensitive landscape and one of the highest biodiversity regions in the Republic of Korea. There have been, albeit the significant value, increased interests in developing this region for a variety of purposes including tourism and commemorative events. As this region has been already facing a range of problems derived from previous development, natural disaster and invasive species, the necessity for active management of ecological health within this region has been increased, which weighs the importance of executing ecological restoration. The objective of this study was to develop evaluation indices as an effective management means of properly evaluating ecological restoration and sustainably maintaining the restored conditions on a long-term scale. Through literature review existing evaluation indices related to restoration were collected, and then the most suitable indices were selected based upon two interviews and one questionnaire survey targeting experts in the relevant field to ecological restoration. They were categorized by two major division and their subclasses (Ecological base - vegetation structure & composition, habitat characteristics, soil environment; landscape ecology - connectivity, landscape patch, boundary & surrounding) and 40 indices. These indices were considered helpful to comprehensively evaluate ecological restoration on degraded environments within ecologically sensitive areas, and sustainably manage target areas by employing a long-term monitoring approach. As this result played a meaningful role in providing the fundamentals of evaluating ecological restoration, it should develop a suitable evaluation system through further research.

• Journal of the Korean Society for Precision Engineering
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• v.7 no.1
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• pp.37-52
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• 1990

In this study, the characteristics of cooling and rolling during strip casting process is obtained in comparison with the experimental and analytical results. The prupose of this study is to effectively analyze the thermal and mechanical deformation of roller applying the results of the heat transfer and the pressure distribution to boundary conditions. And then the relation between strip thickness and roll deformation is shown. The second purpose is to obtain the proper condition of the continuous casting for stainless steel. The summary and conclusions can be made on the basis of the results obtained by the theories and experiments. a) The strip casting condition for the fine surface quality of tin-alloy as-cast material was obtained in accordance with the velocity of roll rotation and initial roll gap. b) The experimental condition that the dimension of the cast strip thickness coincide with that of the initial roll gap was according to the experimental result of continuous casting by twin-roll type. c) The thermoelastic finite element model to calculate the roll deformation is represented. Thermoelastic model prediction for the roll deformation are in good agreement with the experimental results considering the thermal expansion of the roll. d) The higher cooling rates were obtained by a twin-roller quenching technique. Also quenched microstructure of the rapidly solidified shell was verified. e) The magnitude of roll deformation due to the thermal expansion and roll separating force is quantit- atively represented in the analysis of continuous casting for stainless steel.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.70-70
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• 2010

In the present study, to estimate the mechanical properties of 800 MPa grade weld metal, welding was carried out using 800 and 600 MPa grade flux cored arc welding (FCAW) consumable and characteristics of the weld metals were investigated. The chemical composition of weld metals was investigated by an optical emission spectroscopy (OES) method. The microstructure of weld metals was analyzed by optical microscopy (OM) and secondary electron microscopy (SEM). The compositions and sizes of inclusions which are the dominant factors for the nuclei of acicular ferrite were analyzed by an transmission electron microscopy (TEM). In addition, mechanical properties of the weld metals were evaluated through tensile tests and charpy impact tests. Mostly the acicular ferrite phase which has high strength and toughness was observed. The 600 MPa grade weld metal was consisted of 75% acicular ferrite and 25% ferrite which was formed at high temperature (grain boundary ferrite, widmanstatten ferrite, polygonal ferrite). However, the 800 MPa grade weld metal was composed of about 73% acicular ferrite and 27% low temperature phase (bainite, martensite). Toughness was considerably decreased due to the increase of tensile strength (from 600 MPa to 800 MPa). The sizes of inclusions which were observed in both weld metal were $0.4{\sim}0.8\;{\mu}m$, it is effective size to form acicular ferrite.