FluxIntegral3D.m

%% Integration of a chebfun3v over a 2D surface
% Olivier Sète, June 2016

%%
% (Chebfun Example approx3/FluxIntegral3D.m)

%%
% In this example we illustrate the computation of the flux of a vector field
% through a 2D surface in 3D space with the |integral2| command.

%% 
% Assume we have a surface given by a parametrization $S = S(u,v)$ defined on
% $D = [a,b] \times [c,d]$, represented by a 3-component chebfun2v (i.e.,
% each point in the 2D domain $D$ is mapped to a point in 3D).

%%
% Let $F(x,y,z) = [F_1(x,y,z); F_2(x,y,z); F_3(x,y,z)]$ be a vector field defined
% over a box containing the surface (i.e., $S(D)$).  We can then compute its flux through the
% surface by the flux integral
% $$\int_S F \cdot \vec{dS} = \int_D F(S(u,v)) \cdot \left( \frac{\partial
% S}{\partial u}(u,v) \times \frac{\partial S}{\partial v}(u,v) \right) \,
% dudv. $$
% When $F$ is represented as a chebfun3v object, this integral can be
% computed with |integral2|.

%%
% Let us consider the vector field $F$:

format long
dom = [-5, 5, -5, 5, -1, 1];
F = chebfun3v(@(x,y,z) x+y, @(x,y,z) x.*z + y, @(x,y,z) z, dom);

%%
% As our first example, let us consider the rippled disk parametrized by

S = chebfun2v(@(r, t) r .* cos(t), @(r, t) r.*sin(t), @(r, t) cos(5*r), ...
    [0, 5, 0, 2*pi]);
surf(S), axis equal

%%
% To compute the flux of $F$ through $S$ we simply type
integral2(F, S)


%%
% As a second example, we take $S$ to be the lower half of the unit sphere,
% parametrized by

S = chebfun2v(@(phi, theta) sin(theta) .* cos(phi), ...
    @(phi, theta) sin(theta) .* sin(phi), @(phi, theta) cos(theta), ...
    [0, 2*pi, pi/2, pi]);
surf(S), axis equal

%%
% The flux of $F$ through this "bowl" is
integral2(F, S)

%%
% This matches nicely the exact value $-2\pi$
-2*pi

%%
% We can also integrate a scalar function $f = f(x,y,z)$ over a surface with the
% following surface integral:
% $$\int_S f \, dS = \int_D f(S(u,v)) \left\Vert \frac{\partial S}{\partial 
% u}(u,v) \times \frac{\partial S}{\partial v}(u,v) \right\Vert \, dudv. $$
% Note the difference to the flux integral: here we take the norm of the cross
% product.
% When $f$ is represented by a chebfun3 object, this integral can be computed
% with |integral2|.

%%
% Of course we can also integrate over curves in Chebfun3.  This is done with
% |integral|.  See 
% http://www.chebfun.org/examples/approx3/LineIntegral3D.html
% for integration of a scalar function over a curve and
% http://www.chebfun.org/examples/approx3/GaussGreenStokes.html
% for integrals of vector fields along a curve.