Direction Cosines

Basic Definitions and Relations

Here we follow the notation of MF53.

<math> \gamma_{ni} = \frac{1}{h_n} \frac{\partial x_i}{\partial \xi_n} = h_n \frac{\partial\xi_n}{\partial x_i} . </math>

This means that the following inverse relationship applies in general:

<math> \frac{\partial x_i}{\partial \xi_n} = h_n^2 \frac{\partial\xi_n}{\partial x_i} . </math>

Let's define a delta function, <math>\delta_{mn}</math> such that <math>\delta_{mn} = 1</math> if <math>m = n</math> but <math>\delta_{mn}=0</math> if <math>m \ne n</math>. The coordinate system <math>(\xi_1, \xi_2, \xi_3)</math> is orthogonal if all the direction cosines obey the following relation:

<math>\sum_s \gamma_{ms}\gamma_{ns} = \delta_{mn} .</math>

Usage

Scale Factors

The above relations can be used to define the scale factors <math>(h_1, h_2, h_3)</math>. For example,

<math> \sum_s \gamma_{1s}\gamma_{1s} = \sum_s \biggl( h_1 \frac{\partial\xi_1}{\partial x_s} \biggr)^2 = 1 </math>

<math> \Rightarrow ~~~~~ h_1^2 = \biggl[ \biggl(\frac{\partial\xi_1}{\partial x} \biggr)^2 + \biggl(\frac{\partial\xi_1}{\partial y} \biggr)^2 + \biggl(\frac{\partial\xi_1}{\partial z} \biggr)^2 \biggr]^{-1} ; </math>

or,

<math> \sum_s \gamma_{1s}\gamma_{1s} = \sum_s \biggl( \frac{1}{h_1} \frac{\partial x_s}{\partial\xi_1} \biggr)^2 = 1 </math>

<math> \Rightarrow ~~~~~ h_1^2 = \biggl[ \biggl(\frac{\partial x}{\partial\xi_1} \biggr)^2 + \biggl(\frac{\partial y}{\partial\xi_1} \biggr)^2 + \biggl(\frac{\partial z}{\partial\xi_1} \biggr)^2 \biggr] . </math>

Orthogonality

How can we check to make sure that the coordinate <math>\xi_1</math> is everywhere orthogonal to the coordinate <math>\xi_2</math>? Here we'll illustrate how orthogonality can be checked for any axisymmetric coordinate system; that is, we'll examine behavior only in the <math>(\varpi,z)</math> plane. First, note that,

<math> \frac{\partial\varpi}{\partial x} = \frac{\partial}{\partial x} (x^2 + y^2)^{1/2} = \frac{x}{\varpi} , </math>

and,

<math> \frac{\partial\varpi}{\partial y} = \frac{\partial}{\partial x} (x^2 + y^2)^{1/2} = \frac{y}{\varpi} , </math>

Hence,

<math> \frac{\partial\xi_i}{\partial x} = \frac{\partial\xi_i}{\partial \varpi}\frac{\partial\varpi}{\partial x} = \biggl(\frac{x}{\varpi}\biggr) \frac{\partial\xi_i}{\partial \varpi} , </math>

and,

<math> \frac{\partial\xi_i}{\partial y} = \frac{\partial\xi_i}{\partial \varpi}\frac{\partial\varpi}{\partial y} = \biggl(\frac{y}{\varpi}\biggr) \frac{\partial\xi_i}{\partial \varpi} . </math>

Therefore also,

<math> \biggl( \frac{\partial\xi_i}{\partial x} \biggr)^2 + \biggl( \frac{\partial\xi_i}{\partial y } \biggr)^2 = \biggl( \frac{\partial\xi_i}{\partial\varpi} \biggr)^2 </math>

<math> \Rightarrow ~~~~~ h_i^2 = \biggl[ \biggl(\frac{\partial\xi_i}{\partial \varpi} \biggr)^2 + \biggl(\frac{\partial\xi_i}{\partial z} \biggr)^2 \biggr]^{-1} . </math>

The relationship between the direction cosines when <math>m \ne n</math> gives a key orthogonality condition. Take, for example, <math>m=1</math> and <math>n=2</math>:

<math>\sum_s \gamma_{1s}\gamma_{2s} = 0 .</math>

This means that if <math>\xi_1</math> is orthogonal to <math>\xi_2</math>,

<math> h_1 \frac{\partial\xi_1}{\partial x} \cdot h_2 \frac{\partial\xi_2}{\partial x} + h_1 \frac{\partial\xi_1}{\partial y} \cdot h_2 \frac{\partial\xi_2}{\partial y} + h_1 \frac{\partial\xi_1}{\partial z} \cdot h_2 \frac{\partial\xi_2}{\partial z}= 0 </math>

<math> \Rightarrow ~~~~~ h_1 h_2\biggl[ \biggl( \frac{x^2}{\varpi^2} \biggr) \frac{\partial\xi_1}{\partial \varpi} \cdot \frac{\partial\xi_2}{\partial \varpi} + \biggl( \frac{y^2}{\varpi^2} \biggr) \frac{\partial\xi_1}{\partial \varpi} \cdot \frac{\partial\xi_2}{\partial \varpi} + \frac{\partial\xi_1}{\partial z} \cdot \frac{\partial\xi_2}{\partial z} \biggr] = 0 .

</math>

Hence,

An Example Orthogonality Condition
<math> \frac{\partial\xi_1}{\partial \varpi} \cdot \frac{\partial\xi_2}{\partial \varpi} = - \frac{\partial\xi_1}{\partial z} \cdot \frac{\partial\xi_2}{\partial z} . </math>

Unit Vectors

Direction cosines can be used to switch between the basis vectors of different orthogonal coordinate systems. For example,

<math> \hat\imath </math>	<math> = </math>	<math> \hat{e}_1 \gamma_{11} + \hat{e}_2 \gamma_{21} + \hat{e}_3 \gamma_{31} ; </math>
<math> \hat\jmath </math>	<math> = </math>	<math> \hat{e}_1 \gamma_{12} + \hat{e}_2 \gamma_{22} + \hat{e}_3 \gamma_{32} ; </math>
<math> \hat{k} </math>	<math> = </math>	<math> \hat{e}_1 \gamma_{13} + \hat{e}_2 \gamma_{23} + \hat{e}_3 \gamma_{33} . </math>

Position Vector

And, employing these relations tells us that in general the position vector is,

<math> \vec{x} </math>	<math> = </math>	<math> \hat\imath x + \hat\jmath y + \hat{k}z </math>
	<math> = </math>	<math> (\hat{e}_1 \gamma_{11} + \hat{e}_2 \gamma_{21} + \hat{e}_3 \gamma_{31}) x + (\hat{e}_1 \gamma_{12} + \hat{e}_2 \gamma_{22} + \hat{e}_3 \gamma_{32})y + (\hat{e}_1 \gamma_{13} + \hat{e}_2 \gamma_{23} + \hat{e}_3 \gamma_{33})z </math>
	<math> = </math>	<math> \hat{e}_1(x\gamma_{11} + y\gamma_{12} + z\gamma_{13} ) + \hat{e}_2(x\gamma_{21} + y\gamma_{22} + z\gamma_{23} ) + \hat{e}_3 (x\gamma_{31} + y\gamma_{32} + z \gamma_{33}) . </math>

User:Tohline/Appendix/Ramblings/DirectionCosines

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