Toroidal-Coordinate Integration Limits

In support of our accompanying discussion of the gravitational potential of a uniform-density circular torus, here we explain in detail what limits of integration must be specified in order to accurately determine the volume — and, hence also the total mass — of such a torus using toroidal coordinates.

Preamble

Referencing the illustration displayed in the left-hand panel of the following figure, our goal is to determine the gravitational potential at any cylindrical-coordinate location <math>~(R_0, Z_0)</math> due to a uniform-density circular torus whose major radius is <math>~\varpi_t</math> and whose cross-sectional radius is <math>~r_t</math>. Here we explain how a toroidal coordinate system — as defined, for example, by MF53 (see the schematic illustration in the right-hand panel of the following figure) — can be used to reduce the geometric complexity of this problem. In particular, we show how the three-dimensional integral over the mass distribution can be reduced to the sum of a small number (1 - 4) of one-dimensional integrals over the <math>~\xi_1</math> "radial" coordinate in toroidal coordinates.

Figure 1: Meridional slice through …
(Pink) Circular Torus	Toroidal Coordinate System (schematic) (see also Wikipedia's Apollonian Circles)

Quantitative Illustration of Employed Toroidal Coordinate System

Schematic Zones

Zone I

for any <math>~a</math>

Zone II

and

Zone III

and

<math>~\varpi_t - \sqrt{r_t^2 - Z_0^2} < a < \varpi_t + \sqrt{r_t^2 - Z_0^2}</math>

<math>~\frac{V_i}{V_\mathrm{torus}}</math>

<math>~\frac{a^3}{2\pi \varpi_t r_t^2} \int\limits_{\xi_1 = \lambda_i}^{\xi_1 = \Lambda_i} d\xi_1 \biggl\{ \frac{(1-\xi_2^2)^{1/2} [ 4\xi_1^2 - 3\xi_1 \xi_2 - 1]}{(\xi_1^2-1)^2 (\xi_1 - \xi_2)^2} + \biggl[ \frac{(2\xi_1^2 + 1)}{(\xi_1^2-1)^{5/2}}\biggr] \cos^{-1}\biggl[ \frac{(\xi_1\xi_2 - 1 )}{(\xi_1- \xi_2)} \biggr] \biggr\}_{\xi_2 = \gamma_i}^{\xi_2 = \Gamma_i} \, . </math>

<math>~\Phi_i(a,Z_0)</math>	<math>~=</math>	<math>~\frac{2^{5/2} G \rho_0 a^{2}}{3} \int\limits_{\xi_1 = \lambda_i}^{\xi_1 = \Lambda_i} \frac{(\xi_1+1)^{1/2}K(\mu) d\xi_1}{(\xi_1^2 - 1)^2 [ (\xi_1^2 - 1)^{1/2}+\xi_1 ]^{1/2} } \biggr[ \frac{\sin \theta(5\xi_1^2 - 4\xi_1 \cos \theta - 1)}{(\xi_1+1)^{1/2} (\xi_1 - \cos \theta)^{3/2}} </math>
		<math>~ - 4\xi_1 E\biggl( \frac{\pi-\theta}{2} \, , \sqrt{\frac{2}{\xi_1 + 1}} \biggr) + (\xi_1-1) F\biggl( \frac{\pi-\theta}{2} \, , \sqrt{\frac{2}{\xi_1 + 1}} \biggr) \biggr]_{\theta = \cos^{-1}(\gamma_i)}^{\theta = \cos^{-1}(\Gamma_i)} \, . </math>

User:Tohline/2DStructure/ToroidalCoordinateIntegrationLimits

Contents

Toroidal-Coordinate Integration Limits

Preamble

See Also

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