Section 1.16 Solid Mechanics Part III Kelly 1351.16 Curvilinear Coordinates Up until now, a rectangular Cartesian coordinate system has been used, and a set of orthogonal unit base vectors iehas been employed as the basis for representation of vectors and tensors. This basis is independent of positionand provides a simple formulation. Two exceptions were in §1.6.10 and §1.14.4, where cylindrical and spherical coordinate systems were used. These differ from the Cartesian system in that the cylindrical and spherical base vectors do depend on position. However, although the directions of these base vectors may change with position, they are always orthogonal to each other. In this section, arbitrary bases, with base vectors not necessarily orthogonal nor of unit length, are considered. It will be seen how these systems reduce to the special cases of orthogonal (e.g. cylindrical and spherical systems) and Cartesian systems. 1.16.1 Curvilinear Coordinates A Cartesian coordinate system is defined by the fixed base vectors 321,,eeeand the coordinates ),,(321xxx, and any point pin space is then determined by the position vector iixex=(see Fig. 1.16.11). This can be expressed in terms of curvilinear coordinates),,(321ΘΘΘby the transformation (and inverse transformation) ()),,(,,321321ΘΘΘ=Θ=Θiiiixxxxx(1.16.1) In order to be able to solve for the iΘgiven the ix, and to solve for the ixgiven the iΘ, it is necessary and sufficient that the following determinants are non-zero – see Appendix 1.A.2 (the first here is termed the JacobianJof the transformation): JxxxxJjijijiji1det,det=∂Θ∂=⎥⎦⎤⎢⎣⎡∂Θ∂Θ∂∂=⎥⎦⎤⎢⎣⎡Θ∂∂≡, (1.16.2) the last equality following from (1.15.2, 1.10.18d). If 1Θis varied while holding 2Θand 3Θconstant, a space curve is generated called a 1Θcoordinate curve. Similarly, 2Θand 3Θcoordinate curves may be generated. Three coordinate surfacesintersect in pairs along the coordinate curves. On each surface, one of the curvilinear coordinates is constant. Note: •This Jacobian is the same as that used in changing the variable of integration in a volume integral, §1.7; from Cartesian coordinates to curvilinear coordinates, one has ∫∫ΘΘΘ→VVddJddxdxdx3213211superscripts are used here and in much of what follows for notational consistency (see later)
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