• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.743-750
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• 2008

Asymmetric tip clearance in an axial compressor induces pressure and velocity redistributions along the circumferential direction in an axial compressor. This paper presents the mechanism of the flow redistribution due to the asymmetric tip clearance with a simple numerical modeling. The flow field of a rotor of an axial compressor is predicted when an asymmetric tip clearance occurs along the circumferential direction. The modeling results are supported by CFD results not only to validate the present modeling but also to investigate more detailed flow fields. Asymmetric tip clearance makes local flow area and resultant axial velocity vary along the circumferential direction. This flow redistribution 'seed' results in a different flow patterns according to the flow coefficient. Flow field redistribution patterns are largely dependent on the local tip clearance performance at low flow coefficients. However, the contribution of the main flow region becomes dominant while the tip clearance effect becomes weak as the flow coefficient increases. The flow field redistribution pattern becomes noticeably strong if a blockage effect is involved when the flow coefficient increases. The relative flow angle at the small clearance region decreases which result in a negative incidence angle at the high flow coefficient. It causes a recirculation region at the blade pressure surface which results in the flow blockage. It promotes the strength of the flow field redistribution at the rotor outlet. These flow pattern changes have an effect on the blade loading perturbations. The integration of blade loading perturbation from control volume analysis of the circumferential momentum leads to well-known Alford's force. Alford's force is always negative when the flow blockage effects are excluded. However when the flow blockage effect is incorporated into the modeling, main flow effects on the flow redistribution is also reflected on the Alford's force at the high flow coefficient. Alford's force steeply increases as the flow coefficient increases, because of the tip leakage suppression and strong flow redistribution. The predicted results are well agreed to CFD results by Kang and Kang(2006).

• Geomechanics and Engineering
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• v.24 no.6
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• pp.519-529
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• 2021

In coal mining activities, the abutment stress of the coal has to undergo cyclic loading and unloading, affecting the strength and seepage characteristics of coal; additionally, it can cause dynamic disasters, posing a major challenge for the safety of coal mine production. To improve the understanding of the dynamic disaster mechanism of gas outburst and rock burst coupling, triaxial devices are applied to axial pressure cyclic loading-unloading tests under different axial stress peaks and different pore pressures. The existing empirical formula is use to perform a non-linear regression fitting on the relationship between stress and permeability, and the damage rate of permeability is introduced to analyze the change in permeability. The results show that the permeability curve obtained had "memory", and the peak stress was lower than the conventional loading path. The permeability curve and the volume strain curve show a clear symmetrical relationship, being the former in the form of a negative power function. Owing to the influence of irreversible deformation, the permeability difference and the damage of permeability mainly occur in the initial stage of loading-unloading, and both decrease as the number of cycles of loading-unloading increase. At the end of the first cycle and the second cycle, the permeability decreased in the range of 5.777 - 8.421 % and 4.311-8.713 %, respectively. The permeability decreases with an increase in the axial stress peak, and the damage rate shows the opposite trend. Under the same conditions, the permeability of methane is always lower than that of helium, and it shows a V-shape change trend with increasing methane pressures, and the permeability of the specimen was 3 MPa > 1 MPa > 2 MPa.

• Steel and Composite Structures
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• v.34 no.3
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• pp.453-465
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• 2020

Influences of different variables that affect the effective flexural rigidity of reinforced concrete (RC) members are not considered in the most seismic codes. Furthermore, in the last decades, the application of steel fibers in concrete matrix designs has been increased, requiring development of an accurate analytical procedure to calculate the effective flexural rigidity of steel fiber reinforced concrete (SFRC) members. In this paper, first, a nonlinear analytical procedure is proposed to calculate the SFRC members' effective flexural rigidity. The proposed model's accuracy is confirmed by comparing the results obtained from nonlinear analysis with those recorded from the experimental testing. Then a parametric study is conducted to investigate the effects of different parameters such as varying axial load and steel fiber are then investigated through moment-curvature analysis of various SFRC (normal-strength concrete) sections. The obtained results show that increasing the steel fiber volume percentage increases the effective flexural rigidity. Also it's been indicated that the varying axial load affects the effective flexural rigidity. Lastly, proper equations are developed to estimate the effective flexural rigidity of SFRC members.

• Journal of Korean Neurosurgical Society
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• v.56 no.5
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• pp.400-404
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• 2014

Objective : Clip artifacts limit the visualization of intracranial structures in CT scans from patients after aneurysmal clipping with cobalt alloy clips. This study is to analyze the parameters influencing the degree of clip artifacts. Methods : Postoperative CT scans of 60 patients with straight cobalt alloy-clipped aneurysms were analyzed for the maximal diameter of white artifacts and the angle and number of streak artifacts in axial images, and the maximal diameter of artifacts in three-dimensional (3-D) volume-rendered images. The correlation coefficient (CC) was determined between each clip artifact type and the clip blade length and clip orientation to the CT scan (angle a, lateral clip inclination in axial images; angle b, clip gradient to scan plane in lateral scout images). Results : Angle b correlated negatively with white artifacts (r=-0.589, p<0.001) and positively with the angle (r=0.636, p<0.001) and number (r=0.505, p<0.001) of streak artifacts. Artifacts in 3-D images correlated with clip blade length (r=0.454, p=0.004). Multiple linear regression analysis revealed that angle b was the major parameter influencing white artifacts and the angle and number of streak artifacts in axial images (p<0.001), whereas clip blade length was a major factor in 3-D images (p=0.034). Conclusion : Use of a clip orientation perpendicular to the scan gantry angle decreased the amount of white artifacts and allowed better visualization of the clip site.

• Journal of computational fluids engineering
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• v.8 no.4
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• pp.16-25
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• 2003

The design method for transonic turbine blades has been developed based on Wavier-Stokes equations. The present computing process is done on the four separate steps, i.e., determination of the blade profile, generation of the computational grids, cascade flow simulation and analysis of the computed results in the sense of the aerodynamic performance. The blade shapes are designed using the cubic polynomials under the control of the design parameters. Numerical methods for the flow equations are based on Van-Leer's FVS with an upwind TVD scheme on the finite volume. In the present study, numerical simulation has been done to investigate the effects of the design parameters on the aerodynamic peformance of the axial-flow turbine blades. Applications are made to the VKI transonic rotor blades. Computed results are analyzed with respect to four parameters and compared with the experimental data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.11
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• pp.1371-1378
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• 1999

Flow through turbomachinery has a very complex structure and Is Intrinsically unsteady. In addition, trend to highly loaded turbomachinery makes the flow extremely complex due to the interaction between rotor and stator. In this study, flows through UTRC LSRR turbine are numerically analyzed using 2 dimensional Navier-Stokes equations. The convective terms of the governing equations are discretized using the Van-Leer's FVS(Flux vector splitting) with an upwind TVD scheme. The conventional central differencing is used to discretize the diffusion terms on the finite volume. The accurate unsteady motion is achieved by using a 2nd order accurate, 3-point Euler implicit scheme. The quasi-conservative zonal scheme is used for calculating the flow variables on the zonal interface between the rotor and stator. The axial gap between stator and rotor has been configured in two variations, 15% and 65% of average chord length. The analysis program is validated using experimental results and the effect of axial gap is examined. The numerical analysis results are presented by time averaged pressure coefficient and pressure magnitude coefficient and compared with experimental results.

• International Journal of Fluid Machinery and Systems
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• v.7 no.4
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• pp.160-173
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• 2014

Surge phenomena in multi-stage axial flow compressors were studied with attention to the frequency behaviors. A new parameter "volume-modified reduced surge frequency" was introduced, which took into consideration the essential surge process, i.e., emptying and filling of the working gas in the delivery plenum. The behaviors of the relative surge frequencies at the stall stagnation boundaries, compared with the corresponding duct resonance frequencies, have demonstrated the existence of two types of surges; i.e., a near-resonant surge and a subharmonic surge. The former, which has fundamentally a near-resonance frequency, occurs predominantly at the stall stagnation boundary for the short -and-fat plenum delivery flow-path and the long-and-narrow delivery duct flow-path, and possibly in the intermediate conditions. The latter, which has a subharmonic frequency of the fundamental near-resonant one and occurs mainly in the intermediate zone, is considered to be caused by the reduced frequency restricted to a limited range. In relation with those dimensionless frequencies at the stall stagnation boundary, the surge frequency behaviors in more general situations away from the boundaries could be estimated, though very roughly.

• Journal of Mechanical Science and Technology
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• v.16 no.3
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• pp.386-394
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• 2002

The present work is aimed at investigating an unusual variation in flow and performance characteristics of a small propeller fan at low flow rates. A performance test of the fan showed dual performance characteristics, i.e., radial type characteristics at low flow rates and axial type at high flow rates. Dual performance characteristics of the fan are numerically investigated using viscous flow calculations. The Finite Volume Method is used to solve the continuity and Navier-Stokes equations in the flow domain around a fan. The performance parameters and the circumferentially averaged velocity components obtained from the calculations are compared with the experimental results. Numerical values of the performance parameters show good agreement with the measured values. The calculation simulates the steep variations of performance parameters at low flow rates and shows the difference in the flow structure between high and low flow rates. At a low flow coefficient of $\Phi$=0.2, the flow enters the fan in an axial direction and is discharged radially outward at its tip, which is much like the flow characteristics of a centrifugal fan. The centrifugal effect at low flow rates makes a significant difference in performance characteristics of the fan. As the inlet flow rate increases, flow around the fan changes into the mixed type at $\Phi$=0.24 and the axial discharge at $\Phi$=0.4.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.21 no.5
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• pp.805-813
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• 2012

A fan is the most common air flow machinery and is being used in various different industries such as for heavy machinery, home appliances and automobile. An axial fan has blades that force air to move parallel to the shaft about which the blades rotate. This type of fan is used in a wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels. An axial fan generating large air volume used to cool equipments, but is less efficient. A sirocco fan is a efficient device for moving air by centrifugal force and can generate high pressure. Fans that affect the performance and noise of a product are important components. It is also a time and budget consuming equipment to develop a fan through physical experiments. In order to overcome this problem, we have designed and developed a fan simulator for axial and sirocco fan's fluid analysis using supercomputer. Performance and noise prediction based on datamining without numerical analyses is also developed for the conceptual design of a fan.

• Progress in Superconductivity and Cryogenics
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• v.18 no.1
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• pp.50-54
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• 2016

In this paper, a hybrid method is proposed to design an air-core superconducting solenoid system for 6 T axial uniform magnetic field using Niobium Titanium (NbTi) superconducting wire. In order to minimize the volume of conductor, the hybrid optimization method including a linear programming and a nonlinear programming was adopted. The feasible space of solenoid is divided by several grids and the magnetic field at target point is approximated by the sum of magnetic field generated by an ideal current loop at the center of each grid. Using the linear programming, a global optimal current distribution in the feasible space can be indicated by non-zero current grids. Furthermore the clusters of the non-zero current grids also give the information of probable solenoids in the feasible space, such as the number, the shape, and so on. Applying these probable solenoids as the initial model, the final practical configuration of solenoids with integer layers can be obtained by the nonlinear programming. The design result illustrates the efficiency and the flexibility of the hybrid method. And this method can also be used for the magnet design which is required the high homogeneity within several ppm (parts per million).