• Journal of the Korean Solar Energy Society
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• v.35 no.1
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• pp.35-43
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• 2015

A numerical simulation on the wake flow of a wind turbine which is a scaled version of a multi-megawatt wind turbine has been performed. Two different inlet conditions of averaged wind speed including one below and one above the rated wind speed were used in the simulation. Steady-state pitch angles of the blade associated with the two averaged wind speeds were imposed for the simulation. The steady state analysis based on the Reynolds averaged Navier-Stokes equations with the method of frame motion were used for the simulation to find the torque of the rotor and the wake field behind the wind turbine. The simulation results were compared with the results obtained from the wind tunnel testing. From comparisons, it was found that the simulation results on the turbine power are pretty close to the experimental values. Also, the wake results were relatively close to the experimental results but there existed some discrepancy in the shape of velocity deficit. The reason for the discrepancy is considered due to the steady state solution with the frame motion method used in the simulation. However, the method is considered useful for solutions with much reduced calculation time and reasonably good accuracy compared to the transient analysis.

• KIPS Transactions on Computer and Communication Systems
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• v.8 no.4
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• pp.93-102
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• 2019

As interest in wellness grows, There is a lot of research about monitoring individual health using wearable devices. Accordingly, a variety of methods have been studied to distinguish exercise from daily activities using wearable devices. Most of these existing studies are machine learning methods. However, there are problems with over-fitting on individual person's learning, data discontinuously recognition by independent segmenting and fake activity. This paper suggests a detection method for exercise activity based on the physiological response principle of heart rate up and down during exercise. This proposed method calculates activity intensity and heart rate from triaxial and photoplethysmography sensor to determine a heart rate recovery, then detects exercise by estimating activity intensity or detecting a heart rate rising state. Experimental results show that our proposed algorithm has 98.64% of averaged accuracy, 98.05% of averaged precision and 98.62% of averaged recall.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.43 no.5
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• pp.631-638
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• 2023

This paper is to introduce methodologies of travel time reliability evaluation using daily traffic volumes. The methodologies include desirable speed concept, the relation between averaged daily speeds and daily traffic volumes, the standardized way in integration of travel time deviations obtained from continuous highway sections. The study began with traffic data collection from a freeway line with long lasting congestion. And then, provided establishment of the relation between averaged daily speeds and daily traffic volumes, and usability of Point estimate method (PEM) to integrate travel time deviations of sections. Based on the relation between averaged daily speeds and daily traffic volumes, it is identified that travel time reliability begins to decrease around LOS (Level of Service) C even before LOS D or F. It may be concluded that travel time reliability could be evaluated based on daily traffic volumes in highway sections using standardized PEM. The methodologies introduced in the paper, could be useful in practicing evaluation of travel time reliability during the works of highway operation or highway planning.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.10
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• pp.1457-1463
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• 2003

In this paper, the response surface method using three-dimensional Navier-Stokes analysis to optimize the shape of a multi-blade centrifugal fan, is described. For numerical analysis, Reynolds-averaged Navier-Stokes equations with standard k - c turbulence model are transformed into non-orthogonal curvilinear coordinate system, and are discretized with finite volume approximations. Due to the large number of blades in this centrifugal fan, the flow inside of the fan is regarded as steady flow by introducing the impeller force models for economic calculations. Linear Upwind Differencing Scheme(LUDS) is used to approximate the convection terms in the governing equations. SIMPLEC algorithm is used as a velocity-pressure correction procedure. Design variables, location of cur off, radius of cut off, expansion angle of scroll and width of impeller were selected to optimize the shapes of scroll and blades. Data points for response evaluations were selected by D-optimal design, and linear programming method was used for the optimization on the response surface. As a main result of the optimization, the efficiency was successfully improved. It was found that the optimization process provides reliable design of this kind of fans with reasonable computing time.

• Journal of Ocean Engineering and Technology
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• v.26 no.1
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• pp.9-16
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• 2012

This paper presents the numerical results of a simulation of the free surface flow around a blunt bow ship model and focuses on the validation of the proposed method with a brief investigation of the relation between the resistance and free surface behavior. A finite volume method based on the Reynolds Averaged Navier-Stokes (RANS) approach is used to solve the governing flow equations, where the free surface, including wave breaking,is captured by using a two-phase Level-Set (LS) method. For turbulence closure, a two equation k-${\varepsilon}$ model with the standard wall function technique is used. Finally, the numerical results are compared with the available experimental data, showing good agreement.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.40 no.6
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• pp.384-392
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• 2003

The algorithm for calculating the base-collector breakdown voltage of NPN BJT for integrated circuits is proposed. The method of three-dimensional mesh generation to minimize the time required for device simulation is presented and the method for calculating the breakdown voltage using solutions of the Poisson´s equation is presented. To verify the proposed method, the breakdown voltage between base and collector of NPN BJT using 20V process and 30V process is compared with the measured data. The breakdown voltage from the proposed method of NPN BJT using 20V process shows an averaged relative error of 8.0% compared with the measured data and the breakdown voltage of NPN BJT using 30V process shows an averaged relative error of 4.3% compared with the measured data.

• Journal of the Korean Society of Safety
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• v.14 no.4
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• pp.37-42
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• 1999

Modifications were made in the vortex particle method by reducing the number of numerical parameters and adapting more accurate integration schemes. The method was applied to 0.15, 0.2 and 0.25 rectangles where the original method yielded poor results. Structure of vortex formation and its shedding in the wake was clearly shown, and vortex shedding was more regular than that without the modifications, while the time-averaged drag coefficients were nearly the same. It was confirmed the modified method could be used in the viscous vortex particle method.

• Journal of IKEEE
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• v.15 no.1
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• pp.64-69
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• 2011

In this paper, a semi-analytical approach is proposed for decode-and-forward(DF) relay systems over rayleigh fading channels. At first, we derive the general form of the averaged bit error rate(BER) based on error-events at relay nodes in which a selection scheme is not used. It is confirmed that an erroneous detection and transmission at relay nodes can cause the degradation of the received signal-to-noise ratio (SNR) and the averaged BER performance. Furthermore, the proposed method can be extended to selective-DF(SDF) relay schemes so that it is verified to be another general solution for DF relay systems. Also, proposed semi-analytical expressions have been verified by comparing with simulations.

• Journal of Aerospace System Engineering
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• v.2 no.4
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• pp.7-12
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• 2008

Averaged heat transfer coefficients were measured in a turbine blade internal cooling passage model with three blockage walls. Each blockage wall was equipped with 9 staggered holes or slots in order to create different shaper of repeated jet impingement. The effect of jet shape on the averaged heat transfer coefficient was studied by the copper-thermocouple method and three Reynolds number of 10,000, 20,000, and 30,000 were tested. Results showed that the repeated stagger jets could increase the averaged heat transfer coefficient by at least 9 times compared to the smooth channel cases. Due to the large pressure drop induced by the repeated jet impingement, the thermal performance was less than 1 for all cases and decreased as the Reynolds number increased. Among the tested cases, the widest slot showed the best thermal performance. The measurement results showed that the thermal performance of the heat transfer augmentation by repeated stagger jets could be improved by altering the jet shape, and other shape of impingement jet will be studied in near future.

• 한국전산유체공학회:학술대회논문집
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• 2008.03a
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• pp.104-110
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• 2008

Detached Eddy Simulation (DES) is applied to an axisymmetric base flow at supersonic mainstream. DES is a hybrid approach to modeling turbulence that combines the best features of the Reynolds-averaged Navier-Stokes RANS) and large-eddy simulation (LES) approaches. In the Reynolds-averaged mode, the model is currently based on either the Spalart-Allmaras (S-A) turbulence model. In the large eddy simulation mode, it is based on the Smagorinski subgrid scale model. Accurate predictions of the base flowfield and base pressure are successfully achieved by using the DES methodology with less computational cost than that of pure LES and monotone integrated large-eddy simulation (MILES) approaches. The DES accurately resolves the physics of unsteady turbulent motions, such as shear layer rollup, large-eddy motions in the downstream region, small-eddy motions inside the recirculating region. Comparison of the results shows that it is necessary to resolve approaching boundary layers and free shear-layer velocity profiles from the base edge correctly for the accurate prediction of base flows. The consideration of an empirical constant CDES for a compressible flow analysis may suggest that the optimal value of empirical constant CDES may be larger in the flows with strong compressibility than in incompressible flows.