• Title/Summary/Keyword: Q-curvature

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LOXODROMES AND TRANSFORMATIONS IN PSEUDO-HERMITIAN GEOMETRY

  • Lee, Ji-Eun
    • Communications of the Korean Mathematical Society
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    • v.36 no.4
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    • pp.817-827
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    • 2021
  • In this paper, we prove that a diffeomorphism f on a normal almost contact 3-manifold M is a CRL-transformation if and only if M is an α-Sasakian manifold. Moreover, we show that a CR-loxodrome in an α-Sasakian 3-manifold is a pseudo-Hermitian magnetic curve with a strength $q={\tilde{r}}{\eta}({\gamma}^{\prime})=(r+{\alpha}-t){\eta}({\gamma}^{\prime})$ for constant 𝜂(𝛄'). A non-geodesic CR-loxodrome is a non-Legendre slant helix. Next, we prove that let M be an α-Sasakian 3-manifold such that (∇YS)X = 0 for vector fields Y to be orthogonal to ξ, then the Ricci tensor 𝜌 satisfies 𝜌 = 2α2g. Moreover, using the CRL-transformation $\tilde{\nabla}^t$ we fine the pseudo-Hermitian curvature $\tilde{R}$, the pseudo-Ricci tensor $\tilde{\rho}$ and the torsion tensor field $\tilde{T}^t(\tilde{S}X,Y)$.

Geometric Processing for Freeform Surfaces Based on High-Precision Torus Patch Approximation (토러스 패치 기반의 정밀 근사를 이용한 자유곡면의 기하학적 처리)

  • Park, Youngjin;Hong, Q Youn;Kim, Myung-Soo
    • Journal of the Korea Computer Graphics Society
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    • v.25 no.3
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    • pp.93-103
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    • 2019
  • We introduce a geometric processing method for freeform surfaces based on high-precision torus patch approximation, a new spatial data structure for efficient geometric operations on freeform surfaces. A torus patch fits the freeform surface with flexibility: it can handle not only positive and negative curvature but also a zero curvature. It is possible to precisely approximate the surface regardless of the convexity/concavity of the surface. Unlike the traditional method, a torus patch easily bounds the surface normal, and the offset of the torus becomes a torus again, thus helps the acceleration of various geometric operations. We have shown that the torus patch's approximation accuracy of the freeform surface is high by measuring the upper bound of the two-sided Hausdorff distance between the freeform surface and set of torus patches. Using the method, it can be easily processed to detect an intersection curve between two freeform surfaces and find the offset surface of the freeform surface.

On Generalized 𝜙-recurrent Kenmotsu Manifolds with respect to Quarter-symmetric Metric Connection

  • Hui, Shyamal Kumar;Lemence, Richard Santiago
    • Kyungpook Mathematical Journal
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    • v.58 no.2
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    • pp.347-359
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    • 2018
  • A Kenmotsu manifold $M^n({\phi},\;{\xi},\;{\eta},\;g)$, (n = 2m + 1 > 3) is called a generalized ${\phi}-recurrent$ if its curvature tensor R satisfies $${\phi}^2(({\nabla}_wR)(X,Y)Z)=A(W)R(X,Y)Z+B(W)G(X,Y)Z$$ for all $X,\;Y,\;Z,\;W{\in}{\chi}(M)$, where ${\nabla}$ denotes the operator of covariant differentiation with respect to the metric g, i.e. ${\nabla}$ is the Riemannian connection, A, B are non-vanishing 1-forms and G is given by G(X, Y)Z = g(Y, Z)X - g(X, Z)Y. In particular, if A = 0 = B then the manifold is called a ${\phi}-symmetric$. Now, a Kenmotsu manifold $M^n({\phi},\;{\xi},\;{\eta},\;g)$, (n = 2m + 1 > 3) is said to be generalized ${\phi}-Ricci$ recurrent if it satisfies $${\phi}^2(({\nabla}_wQ)(Y))=A(X)QY+B(X)Y$$ for any vector field $X,\;Y{\in}{\chi}(M)$, where Q is the Ricci operator, i.e., g(QX, Y) = S(X, Y) for all X, Y. In this paper, we study generalized ${\phi}-recurrent$ and generalized ${\phi}-Ricci$ recurrent Kenmotsu manifolds with respect to quarter-symmetric metric connection and obtain a necessary and sufficient condition of a generalized ${\phi}-recurrent$ Kenmotsu manifold with respect to quarter symmetric metric connection to be generalized Ricci recurrent Kenmotsu manifold with respect to quarter symmetric metric connection.

Experimental study on creep behavior of fly ash concrete filled steel tube circular arches

  • Yan, Wu T.;Han, Bing;Zhang, Jin Q.;Xie, Hui B.;Zhu, Li;Xue, Zhong J.
    • Steel and Composite Structures
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    • v.27 no.2
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    • pp.185-192
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    • 2018
  • Fly ash can significantly improve concrete workability and performance, and recycling fly ash in concrete can contribute to a cleaner environment. Since fly ash influences pozzolanic reactions in concrete, mechanical behaviors of concrete containing fly ash differ from traditional concrete. Creep behaviors of fly ash concrete filled steel tube arch were experimentally investigated for 10% and 30% fly ash replacement. The axes of two arches are designed as circular arc with 2.1 m computed span, 0.24 m arch rise, and their cross-sections are all in circular section. Time dependent deflection and strain of loading and mid-span steel tube were measured, and long term deflection of the model arch with 10% fly ash replacement was significantly larger than with 30% replacement. Considering the steel tube strain, compressive zone height, cross section curvature, and internal force borne by the steel tube, the compressive zone height and structural internal forces increased gradually over time due to concrete creep. Increased fly ash content resulted in more significant neutral axis shift. Mechanisms for internal force effects on neutral axis height were analyzed and verified experimentally.

CONTACT THREE CR-SUBMANIFOLDS OF A (4m + 3)-DIMENSIONAL UNIT SPHERE

  • Kim, Hyang-Sook;Kim, Young-Mi;Kwon, Jung-Hwan;Pak, Jin-Suk
    • Journal of the Korean Mathematical Society
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    • v.44 no.2
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    • pp.373-391
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    • 2007
  • We study an (n+3)($n\;{\geq}\;7-dimensional$ real submanifold of a (4m+3)-unit sphere $S^{4m+3}$ with Sasakian 3-structure induced from the canonical quaternionic $K\"{a}hler$ structure of quaternionic (m+1)-number space $Q^{m+1}$, and especially determine contact three CR-submanifolds with (p-1) contact three CR-dimension under the equality conditions given in (4.1), where p = 4m - n denotes the codimension of the submanifold. Also we provide necessary conditions concerning sectional curvature in order that a compact contact three CR-submanifold of (p-1) contact three CR-dimension in $S^{4m+3}$ is the model space $S^{4n_1+3}(r_1){\times}S^{4n_2+3}(r_2)$ for some portion $(n_1,\;n_2)$ of (n-3)/4 and some $r_1,\;r_2$ with $r^{2}_{1}+r^{2}_{2}=1$.

Resonance Characteristics and Radiation Characteristics of a Spherical Patch on a Dielectric Sphere (구형 유전체 위에 있는 구형 패치의 공진 특성과 방사 특성)

  • Jeong, Yi-Ru;Hong, Ic-Pyo;Lee, Myung-Gun;Chun, Hueng-Jae;Yook, Jong-Gwan
    • The Journal of Korean Institute of Electromagnetic Engineering and Science
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    • v.23 no.4
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    • pp.515-523
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    • 2012
  • In this paper, the resonance characteristics and radiation characteristics of a spherical patch on a dielectric sphere are analyzed. Resonance characteristics can be obtained from the resonant frequency and the quality factor. Radiation characteristics can be also analyzed from the E-field in the far region. In order to calculate these parameters, spectral domain analysis method is applied. Algebraic equation can be obtained in spectral domain through Vector Legendre transform pair and Galerkin's method. So, efficient calculation is possible numerically. It is investigated that radius, curvature of a spherical patch, and dielectric constant of a dielectric sphere have an effect on characteristics of a spherical patch.

AN EXTENSION OF SCHNEIDER'S CHARACTERIZATION THEOREM FOR ELLIPSOIDS

  • Dong-Soo Kim;Young Ho Kim
    • Bulletin of the Korean Mathematical Society
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    • v.60 no.4
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    • pp.905-913
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    • 2023
  • Suppose that M is a strictly convex hypersurface in the (n + 1)-dimensional Euclidean space 𝔼n+1 with the origin o in its convex side and with the outward unit normal N. For a fixed point p ∈ M and a positive constant t, we put 𝚽t the hyperplane parallel to the tangent hyperplane 𝚽 at p and passing through the point q = p - tN(p). We consider the region cut from M by the parallel hyperplane 𝚽t, and denote by Ip(t) the (n + 1)-dimensional volume of the convex hull of the region and the origin o. Then Schneider's characterization theorem for ellipsoids states that among centrally symmetric, strictly convex and closed surfaces in the 3-dimensional Euclidean space 𝔼3, the ellipsoids are the only ones satisfying Ip(t) = 𝜙(p)t, where 𝜙 is a function defined on M. Recently, the characterization theorem was extended to centrally symmetric, strictly convex and closed hypersurfaces in 𝔼n+1 satisfying for a constant 𝛽, Ip(t) = 𝜙(p)t𝛽. In this paper, we study the volume Ip(t) of a strictly convex and complete hypersurface in 𝔼n+1 with the origin o in its convex side. As a result, first of all we extend the characterization theorem to strictly convex and closed (not necessarily centrally symmetric) hypersurfaces in 𝔼n+1 satisfying Ip(t) = 𝜙(p)t𝛽. After that we generalize the characterization theorem to strictly convex and complete (not necessarily closed) hypersurfaces in 𝔼n+1 satisfying Ip(t) = 𝜙(p)t𝛽.

Optimal design of impeller in fan motor unit of cordless vacuum cleaner for improving flow performance and reducing aerodynamic noise (무선진공청소기 팬 모터 단품의 유량성능 향상과 공력소음 저감을 위한 임펠라 최적설계)

  • Kim, KunWoo;Ryu, Seo-Yoon;Cheong, Cheolung;Seo, Seongjin;Jang, Cheolmin
    • The Journal of the Acoustical Society of Korea
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    • v.39 no.5
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    • pp.379-389
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    • 2020
  • In this study, the flow and noise performances of high-speed fan motor unit for cordless vacuum cleaner is improved by optimizing the impeller which drives the suction air through flow passage of the cordless vacuum cleaner. Firstly, the unsteady incompressible Reynolds averaged Navier-Stokes (RANS) equations are solved to investigate the flow through the fan motor unit using the computational fluid dynamics techniques. Based on flow field results, the Ffowcs-Williams and Hawkings (FW-H) integral equation is used to predict flow noise radiated from the impeller. Predicted results are compared to the measured ones, which confirms the validity of the numerical method used. It is found that the strong vortex is formed around the mid-chord region of the main blades where the blade curvature change rapidly. Given that vortex acts as a loss for flow and a noise source for noise, impeller blade is redesigned to suppress the identified vortex. The response surface method using two factors is employed to determine the optimum inlet and outlet sweep angles for maximum flow rate and minimum noise. Further analysis of finally selected design confirms the improved flow and noise performance.

An Experimental Study on the Required Performances of Roof Concrete Placed in the In-ground LNG Storage Tank (지하식 LNG 저장탱크의 지붕 콘크리트의 요구성능에 관한 실험적 연구)

  • Kwon, Yeong-Ho
    • Journal of the Korea Concrete Institute
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    • v.25 no.3
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    • pp.339-345
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    • 2013
  • This study is to derive from the required performances and the optimum mix proportion of the roof concrete placed in the in-ground LNG storage tank with a capacity of 200000 $m^3$, and propose the actual data for site concrete work. The concrete placing work without sliding and segregation in the fresh concrete condition is very important because the slope of domed roof is varied in the large range by its curvature. Also the control of hydration heat and the strength development at test ages are classified with massive section about 1.4 m thick and considered to the pre-stressing work and removal of air support after concrete placing work. Considering above condition, slump range is selected $100{\pm}25$ mm under the slope $20^{\circ}$ and $150{\pm}25$ mm over the slope $20^{\circ}$ s until 60 minutes of elapsed time. Also, the roof concrete is satisfied with compressive strength range including design strength at 91 days (30 MPa), pre-stressing work at 7 days (10 MPa), air support removal work at 21 days (14 MPa). Replacement ratio of limestone powder is determined by confined water ratio test and main design factors include water-cement ratio (W/C), sand-aggregate ratio and dosage of admixture. As test results, the optimum mix proportion of the roof concrete used low heat cement is as followings. 1) Replacement ratio of limestone powder 25% by confined water ratio test 2) Water-cement ratio 57.8% 3) Sand-aggregate ratio 42.0%. Also, test results for the adiabatic temperature rising test is satisfied with its criteria and shown the lower value compared to preceding storage tank (TK-13, 14). These required performances and the optimum mix proportion is to apply the actual construction work.