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ex_convdiff1.m File Reference

Description

EX_CONVDIFF1 2D Convection and diffusion equation example on a rectangle.

[ FEA, OUT ] = EX_CONVDIFF1( VARARGIN ) Convection and diffusion equation on a rectangle with exact solution u_0+c1*eta+c2*(2*cd*xi+eta^2). Accepts the following property/value pairs.

Input       Value/{Default}        Description
-----------------------------------------------------------------------------------
igrid       scalar 1/{0}           Cell type (0=quadrilaterals, 1=triangles)
hmax        scalar {1/40}          Max grid cell size
a           scalar {1}             Convection velocity in x-direction
b           scalar {2}             Convection velocity in y-direction
c1          scalar {1}             Solution constant
c2          scalar {0.8}           Solution constant
cd          scalar {0.5}           Diffusion coefficient
sfun        string {sflag1}        Shape function
iphys       scalar 0/{1}           Use physics mode to define problem    (=1)
                                   or directly define fea.eqn/bdr fields (=0)
iplot       scalar 0/{1}           Plot solution (=1)
                                                                                  .
Output      Value/(Size)           Description
-----------------------------------------------------------------------------------
fea         struct                 Problem definition struct
out         struct                 Output stuct

Code listing

 cOptDef = { ...
   'igrid',    0; ...
   'hmax',     1/40; ...
   'a',        1; ...
   'b',        2; ...
   'c1',       1; ...
   'c2',       0.8; ...
   'cd',       0.5; ...
   'sfun',     'sflag1'; ...
   'iphys',    1; ...
   'iplot',    1; ...
   'fid',      1 };
 [got,opt] = parseopt(cOptDef,varargin{:});
 fid       = opt.fid;
 xi        = [num2str(opt.a),'*x+',num2str(opt.b),'*y'];
 eta       = [num2str(opt.b),'*x-',num2str(opt.a),'*y'];
 refsol    = [num2str(opt.c1),'*(',eta,')+',num2str(opt.c2),'*(2*',num2str(opt.cd),'*(',xi,')+(',eta,')^2)'];


% Geometry definition.
 gobj = gobj_rectangle();
 fea.geom.objects = { gobj };


% Grid generation.
 switch opt.igrid
   case -1
     fea.grid = rectgrid(round(1/opt.hmax));
     fea.grid = quad2tri(fea.grid);
   case 0
     fea.grid = rectgrid(round(1/opt.hmax));
   case 1
     fea.grid = gridgen(fea,'hmax',opt.hmax,'fid',fid);
 end
 n_bdr = max(fea.grid.b(3,:));           % Number of boundaries.


% Problem definition.
 fea.sdim  = { 'x' 'y' };                % Coordinate names.
 if ( opt.iphys==1 )

   fea = addphys(fea,@convectiondiffusion);   % Add convection and diffusion physics mode.
   fea.phys.cd.sfun = { opt.sfun };           % Set shape function.
   fea.phys.cd.eqn.coef{2,4}   = { opt.cd };  % Set diffusion coefficient.
   fea.phys.cd.eqn.coef{3,4}   = { opt.a  };  % Convection velocity in x-direction.
   fea.phys.cd.eqn.coef{4,4}   = { opt.b  };  % Convection velocity in y-direction.
   fea.phys.cd.bdr.sel         = [1 1 1 1];
   fea.phys.cd.bdr.coef{1,end} = repmat({refsol},1,n_bdr);   % Set Dirichlet boundary coefficient to reference solution.
   fea = parsephys(fea);                 % Check and parse physics modes.

 else

   fea.dvar  = { 'c' };                  % Dependent variable name.
   fea.sfun  = { opt.sfun  };            % Shape function.

% Define equation system.
   fea.eqn.a.form = { [2 3 2 3;2 3 1 1] };   % First row indicates test function space   (2=x-derivative + 3=y-derivative),
% second row indicates trial function space (2=x-derivative + 3=y-derivative).
   fea.eqn.a.coef = { [opt.cd opt.cd opt.a opt.b] };   % Coefficients used in assembling stiffness matrix.

   fea.eqn.f.form = { 1 };               % Test function space to evaluate in right hand side (1=function values).
   fea.eqn.f.coef = { 0 };               % Coefficient used in right hand side.

% Define boundary conditions.
   fea.bdr.d     = cell(1,n_bdr);
  [fea.bdr.d{:}] = deal(refsol);         % Assign reference solution to all boundaries (Dirichlet).

   fea.bdr.n     = cell(1,n_bdr);        % No Neumann boundaries ('fea.bdr.n' empty).

 end


% Parse and solve problem.
 fea       = parseprob(fea);             % Check and parse problem struct.
 fea.sol.u = solvestat(fea,'fid',fid);   % Call to stationary solver.


% Postprocessing.
 s_err = ['abs(',refsol,'-c)'];
 if ( opt.iplot>0 )
   figure
   subplot(2,1,1)
   postplot(fea,'surfexpr','c','isoexpr','c')
   title('Solution c')
   subplot(2,1,2)
   postplot(fea,'surfexpr',s_err)
   title('Error')
 end


% Error checking.
 if ( size(fea.grid.c,1)==4 )
   xi = [0;0];
 else
   xi = [1/3;1/3;1/3];
 end
 err = evalexpr0(s_err,xi,1,1:size(fea.grid.c,2),[],fea);
 ref = evalexpr0('c',xi,1,1:size(fea.grid.c,2),[],fea);
 err = sqrt(sum(err.^2)/sum(ref.^2));


 if( ~isempty(fid) )
   fprintf(fid,'\nL2 Error: %f\n',err)
   fprintf(fid,'\n\n')
 end


 out.err  = err;
 out.pass = out.err<0.1;
 if ( nargout==0 )
   clear fea out
 end