User:Tohline/Appendix/Mathematics/ScaleFactors

Scale Factors for Orthogonal Curvilinear Coordinate Systems

Here we lean heavily on the class notes and associated references that have been provided by P. A. Kelly in a collection titled, Mechanics Lecture Notes: An Introduction to Solid Mechanics, as they appeared online in early 2021. See especially the subsection of Part III in which the properties of Vectors and Tensors are discussed.

Getting Started

Following Kelly, we will use <math>~\hat{e}_i</math> and <math>~x_i</math> when referencing, respectively, the three (i = 1,3) basis vectors and coordinate "curves" of the Cartesian coordinate system; and we will use <math>~\hat{g}_i</math> and <math>~\Theta_i</math> when referencing, respectively, the three (i = 1,3) basis vectors and coordinate curves of some other, curvilinear coordinate system.

2D Oblique Coordinate System Example

Consider a vector, <math>~\vec{v}</math>, which in Cartesian coordinates is described by the expression,

<math>~\vec{v}</math>

<math>~=</math>

<math>~ \hat{e}_1 v_x + \hat{e}_2 v_y \, . </math>

Referencing Figure 1.16.4 of Kelly's Part III, we appreciate that in a two-dimensional (2D) oblique coordinate system where <math>~\alpha</math> is the (less than 90°) angle between the two basis vectors, the same vector will be represented by the expression,

<math>~\vec{v}</math>

<math>~=</math>

<math>~ \hat{g}_1 v^1 + \hat{g}_2 v^2 \, . </math>

The angle between <math>~\hat{g}_2</math> and <math>~\hat{e}_2</math> is, (π/2 - α), so we appreciate that,

<math>~v_y</math>	<math>~=</math>	<math>~v^2\cos\biggl(\frac{\pi}{2} - \alpha \biggr) = v^2 \sin\alpha</math>
<math>~\Rightarrow~~~v^2</math>	<math>~=</math>	<math>~\frac{v_y}{\sin\alpha} \, .</math>

Next, from a visual inspection of the figure, we appreciate that <math>~v_x</math> is longer than <math>~v^1</math> by the amount, <math>~v^2\cos\alpha</math>; that is,

<math>~v_x</math>	<math>~=</math>	<math>~v^1 + v^2\cos\alpha = v_1 + \frac{v_y}{\tan\alpha}</math>
<math>~\Rightarrow ~~~ v^1</math>	<math>~=</math>	<math>~v_x - \frac{v_y}{\tan\alpha} \, .</math>

(These are the same pair of transformation relations that appear as Eq. (1.16.3) of Kelly's Part III.)

Covarient:   The set of basis vectors, <math>~\hat{g}_1</math> and <math>~\hat{g}_2</math> (note the subscript indices), that are aligned with the coordinate directions, <math>~\Theta_1</math> and <math>~\Theta_2</math>, are generically referred to as covariant base vectors. Contravarient:   A second set of vectors, which will be termed contravariant base vectors, <math>~\hat{g}^1</math> and <math>~\hat{g}^2</math> (denoted by superscript indices), will be aligned with a new set of coordinate directions, <math>~\Theta^1</math> and <math>~\Theta^2</math>. This new set of base vectors is defined as follows (see Fig. 1.15.5 of Kelly's Part III): the base vector <math>~\hat{g}^1</math> is perpendicular to <math>~\hat{g}_1</math> — that is, <math>~\hat{g}^1 \cdot \hat{g}_2 = 0</math> — and the base vector <math>~\hat{g}^2</math> is perpendicular to <math>~\hat{g}_2</math> — that is, <math>~\hat{g}_1 \cdot \hat{g}^2 = 0</math>. Further, we ensure that, <math>~\hat{g}_1 \cdot \hat{g}^1 = 1 \, ,</math>       and,       <math>~\hat{g}_2 \cdot \hat{g}^2 = 1 \, .</math>

Continuing with our 2D oblique coordinate system example and appreciating that Kelly has chosen to align the <math>~\hat{g}_1</math> basis vector with the <math>~\hat{e}_1</math> (Cartesian) basis vector, we see that the transformation between the two sets of covariant basis vectors is given by the relations,

<math>~\hat{g}_1 = \hat{e}_1 \, ,</math>

and,

<math>~\hat{g}_2 = \hat{e}_1\cos\alpha + \hat{e}_2\sin\alpha \, .</math>

These conditions lead to the following complementary set of contravariant basis vectors:

<math>~\hat{g}^1 = \hat{e}_1 - \hat{e}_2 \biggl( \frac{1}{\tan\alpha}\biggr) \, ,</math>

and,

<math>~\hat{g}^2 = \hat{e}_2 \biggl( \frac{1}{\sin\alpha} \biggr) \, .</math>

Note that, as defined herein, the magnitude (i.e., scalar lengths) of these contravariant basis vectors is not unity; they are, instead,

<math>~|\hat{g}^1| \equiv \biggl[ \hat{g}^1 \cdot \hat{g}^1 \biggr]^{1 / 2}</math>

<math>~=</math>

<math>~\biggl\{ \biggl[\hat{e}_1 - \hat{e}_2 \biggl( \frac{1}{\tan\alpha}\biggr)\biggr] \cdot \biggl[\hat{e}_1 - \hat{e}_2 \biggl( \frac{1}{\tan\alpha}\biggr)\biggr] \biggr\}^{1 / 2} </math>

See Also

© 2014 - 2021 by Joel E. Tohline |   H_Book Home   |   YouTube   | Appendices: | Equations | Variables | References | Ramblings | Images | myphys.lsu | ADS | Recommended citation:   Tohline, Joel E. (2021), The Structure, Stability, & Dynamics of Self-Gravitating Fluids, a (MediaWiki-based) Vistrails.org publication, https://www.vistrails.org/index.php/User:Tohline/citation