• Proceedings of the Korean Magnestics Society Conference
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• 2012.11a
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• pp.104-105
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• 2012

The temperature dependence of the effective magnetic anisotropy constant K(T) of ferrite nanoparticles is obtained based on the measurements of SQUID magnetometry. For this end, a very simple but intuitive and direct method for determining the temperature dependence of anisotropy constant K(T) in nanoparticles is introduced in this study. The anisotropy constant at a given temperature is determined by associating the particle size distribution f(r) with the anisotropy energy barrier distribution $f_A(T)$. In order to estimate the particle size distribution f(r), the first quadrant part of the hysteresis loop is fitted to the classical Langevin function weight-averaged with the log?normal distribution, slightly modified from the original Chantrell's distribution function. In order to get an anisotropy energy barrier distribution $f_A(T)$, the temperature dependence of magnetization decay $M_{TD}$ of the sample is measured. For this measurement, the sample is cooled from room temperature to 5 K in a magnetic field of 100 G. Then the applied field is turned off and the remanent magnetization is measured on stepwise increasing the temperature. And the energy barrier distribution $f_A(T)$ is obtained by differentiating the magnetization decay curve at any temperature. It decreases with increasing temperature and finally vanishes when all the particles in the sample are unblocked. As a next step, a relation between r and $T_B$ is determined from the particle size distribution f(r) and the anisotropy energy barrier distribution $f_A(T)$. Under the simple assumption that the superparamagnetic fraction of cumulative area in particle size distribution at a temperature is equal to the fraction of anisotropy energy barrier overcome at that temperature in the anisotropy energy barrier distribution, we can get a relation between r and $T_B$, from which the temperature dependence of the magnetic anisotropy constant was determined, as is represented in the inset of Fig. 1. Substituting the values of r and $T_B$ into the $N{\acute{e}}el$-Arrhenius equation with the attempt time fixed to $10^{-9}s$ and measuring time being 100 s which is suitable for conventional magnetic measurement, the anisotropy constant K(T) is estimated as a function of temperature (Fig. 1). As an example, the resultant effective magnetic anisotropy constant K(T) of manganese ferrite decreases with increasing temperature from $8.5{\times}10^4J/m^3$ at 5 K to $0.35{\times}10^4J/m^3$ at 125 K. The reported value for K in the literatures is $0.25{\times}10^4J/m^3$. The anisotropy constant at low temperature region is far more than one order of magnitude larger than that at 125 K, indicative of the effects of inter?particle interaction, which is more pronounced for smaller particles.

• Journal of Mechanical Science and Technology
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• v.16 no.7
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• pp.959-965
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• 2002

Effective hydraulic conductivity of a statistically anisotropic heterogeneous medium is obtained for steady two-dimensional flows employing stochastic analysis. Flow equations are solved up to second order and the effective conductivity is obtained in a semi-analytic form depending only on the spatial correlation function and the anisotropy ratio of the hydraulic conductivity field, hence becoming a true intrinsic property independent of the flow field. Results are obtained using a statistically anisotropic Gaussian correlation function where the anisotropy is defined as the ratio of integral scales normal and parallel to the mean flow direction. Second order results indicate that the effective conductivity of an anisotropic medium is greater than that of an isotropic one when the anisotropy ratio is less than one and vice versa. It is also found that the effective conductivity has upper and lower bounds of the arithmetic and the harmonic mean conductivities.

• Transactions of Materials Processing
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• v.22 no.6
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• pp.310-316
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• 2013

Recently, numerical predictions of surface deflection based on curvature analysis have been developed. In the current study, a measure of surface deflection is proposed as the maximum variation of curvature difference between the panel and the tool in order to account for surfaces that have high curvature. The current study focused on the assessment of accuracy for the surface deflection prediction with the consideration of planar anisotropy. As an example, a shallow rectangular drawn part with rectangular embossing was considered. In terms of the proposed surface deflection measure, the maximum variation of curvature difference, the prediction with a planar anisotropic model shows better correspondence with experiment than the one using a normal anisotropic model.

• Proceedings of the KIEE Conference
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• 2007.07a
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• pp.858-859
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• 2007

This paper deals with the characteristics analysis of Synchronous Reluctance (SynRM) with anisotropy rotor using a coupled FEM & Preisach model. The focus of this paper is the design relative to the output power on the basis of rotor materials of a SynRM. The coupled Finite Elements Analysis (FEA) & Preisach model have been used to evaluate nonlinear solutions. Comparisons are given with characteristics of normal synchronous reluctance motor and those of anisotropy rotor SynRM (ANISO-SynRM), respectively.

• Proceedings of the KSMRM Conference
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• 2003.10a
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• pp.20-20
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• 2003

Purpose： To investigate the difference of fractional anisotropy (FA) values between 6- and 25-direction diffusion tensor images (DTI) in normal adult brain. Materials and Methods： DTI was peformed in 28 normal subjects (15 subjects with 6-direction, 13 subjects with 25-direction) in a 1.5 T MR system. DTI was done with SE-EPI sequence with TR/TE/NEX 10000/84/1, 5mm slice thickness and b=1000 s/mm2. FA values were measured from 8 different anatomical locations which included both cerebral peduncles, both posterior limbs of the internal capsules, both corona radiata, genu and splenium of the corpus callosum. Statistical difference of FA was tested between 6-and 25-direction DTI.

• Journal of the Korean Society of Radiology
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• v.4 no.3
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• pp.13-18
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• 2010

Purpose : Diffusion tensor imaging(DTI) allows the visualization of fiber tract damage in patients with cerebral infarction. The purpose of this study is to evaluate the correlation between degree of NIH stoke scale and fractional anisotropy (FA) in patient with cerebral infarction. Material and Methods : 16 patients aged 36~77 years(male : 11, female : 5, mean age : 61y), diagnosed cerebral infarction by diffusion weighted imaging(DWI), underwent 24 directional diffusion tensor imaging(DTI). Patients had the DTI taken within 3days of stroke onset. Comparison of DWI, FA value on DTI were measured infarcted area and counter part of specific region of interest (ROI). And evaluation of differences between clinically improved patient group (n=9) and unimproved patient group (n=7) until 2 week follow up after development of cerebral infarction. Clinical status was scaled by NIH stroke scale. Results : Quantitative measurements of FA confirmed statistically the significant diffusion changes in the infarct compared with the matched-counter part region. In DWI, the infarcted area shows high signal intensity, however FA value on DTI was lower than normal brain parenchyma. The FA value of clinically improved patient by NIH stroke scale was 0.49, and the value of contralateral normal brain parenchyma was 0.41. On the contrary, FA value of infarcted area shows about 15% lower than normal brain parenchyma. But, the FA value of unimproved patient by NIH stroke scale represents a half those of contralateral normal brain parenchyma (0.28 on infarcted area vs. 0.56 on normal brain parenchyma). So, the FA value of unimproved patient group was considerably less than those of improved. Conclusion : It is concluded that the unimproved patient group after cerebral infarction showed much less FA value than that of normal brain parenchyma. The FA value of DTI may be one of the useful parameter to predict outcome of cerebral infarction patients.

• Journal of the Korean Geophysical Society
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• v.3 no.1
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• pp.49-56
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• 2000

As a part of geophysical studies on segmentation of the Ulsan fault, walkaway refraction seismic data were measured at 17 stations near National Road 7 between Kyungju and Ulsan. Seismic anisotropy was analyzed in the offset range of 1-48 m. The average refraction velocity of 1787 m/s indicates the refractor is the upper boundary of weathered basement. P-wave anisotropy is computed to be 0.056 in average, which may serve as a weak evidence that the strike of major geologic structure coincide with the inferred fault direction. In the south of the province boundary between Kyungsangnam-do and Kyungsangbuk-do, the velocity anisotropy is normal in that P-wave velocity in the strike direction is faster than the one measured in the dip direction. On the contrary, it appears that the fault strikes in many directions or that fractures may be developed better in the dip direction in the northern par. Such a difference in anisotropic pattern is believed to be a seismic evidence indicating that a segmentation boundary of the Ulsan fault locates near the province boundary.

• Geotechnical Engineering
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• v.2 no.3
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• pp.27-36
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• 1986

To investigate the seepage effect on the compacted backfill of retaining walls, an expriment and an analytical study were carried out First, the expriment was performed with a two-way permeameter newly designed for the do- termination on the degree of permeability anisotropy of compacted soils. As a result, e-log(kz/ky) plot showed a linear relationship, where kz and ky were permeability coefficients for the normal and the parallel directions to the compaction. The degree of permeability anisotropy, kz/ky was 2 to 4 at Dr>90% for sands, regardless of the methods of compaction. The kz/ky of the fine sand was greater than that of the coarse sand. Second, the exprimental results were applied to the extention of Gray's theory for the investigation of the active thrust affected by the seepage of permeability anisotropy. The active thrust was decreased with the increase in the degree of permeability anisotropy, and it It.as a little effect on wall friction.

• Journal of Mechanical Science and Technology
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• v.14 no.1
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• pp.93-102
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• 2000

The objective of the present study is to investigate the pressure-strain correlation terms of the Reynolds stress models for the three dimensional turbulent boundary layer in a $30^{\circ}$ bend tunnel. The numerical results obtained by models of Launder, Reece and Rodi (LRR) , Fu and Speziale, Sarkar and Gatski (SSG) for the pressure-strain correlation terms are compared against experimental data and the calculated results from the standard k-${\varepsilon}$ model. The governing equations are discretized by the finite volume method and SIMPLE algorithm is used to calculate the pressure field. The results show that the models of LRR and SSG predict the anisotropy of turbulent structure better than the standard k-${\varepsilon}$ model. Also, the results obtained from the LRR and SSG models are in better agreement with the experimental data than those of the Fu and standard k-${\varepsilon}$ models with regard to turbulent normal stresses. Nevertheless, LRR and SSG models do not effectively predict pressure-strain redistribution terms in the inner layer because the pressure-strain terms are based on the locally homogeneous approximation. Therefore, to give better predictions of the pressure-strain terms, non-local effects should be considered.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.1
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• pp.169-178
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• 1991

A plane-strain finite element analysis of sheet metal forming is carried out by using the rigid-plastic FEM based on the membrane theory. The sheet material is assumed to possess normal anisotropy and to obey Hill's new yield criterion and its associated flow rule. A formulation of initial guess generation for the displacement field is derived by using the nonlinear elastic FEM. A method of contact treatment is proposed in which the skew boundary condition for arbitrarily shaped tools is successively used during iteration. In order to verify the validity of the developed method, plane-strain drawing with tools in analytic expression and with arbitrarily shaped tools is analyzed and compared with the published results. The comparison shows that the present method can be effectively used in the analysis of plane-strain sheet metal forming and thus provides the basis of approximate sectional analysis of panel-like sheet forming.