SPIRAL
A Sample 2D Flow Problem

SPIRAL is a square region that is 1 unit wide and 1 unit high. Zero Dirichlet conditions have been applied on all sides. Normally, this would result in zero flow. However, the right hand sides of the two momentum equations have been devised to induce a sort of spiral flow (actually, the flow is more like a series of concentric loops).

To run the problem directly, you only need the user-supplied routines and the node data in nodes6.txt, and the element data in triangles6.txt.

You can run the program with the MATLAB command

        free_fem_stokes ( 'nodes6.txt', 'triangles6.txt' )
      

The user-supplied files needed to run the problem include:

• boundary_type.m, adjusts the program's default assignment of boundary condition types.
• constants.m, sets flow constants, in this case just the kinematic viscosity NU.
• dirichlet_condition.m, evaluates the right hand sides of the Dirichlet boundary conditions.
• neumann_condition.m, evaluates the right hand sides of the Neumann boundary conditions.
• rhs.m, evaluates the right hand sides (source terms) of the equations.

The printed output from a run is:

• spiral.out, output from a run of the program;

The geometry is defined by sets of nodes and triangles. The velocities use the full set of nodes, and quadratic (6 node) triangles.

• nodes6.txt, a text file containing the velocity nodes;
• nodes6.png, a PNG image of the velocity nodes;
• triangles6.txt, the quadratic triangulation;
• triangles6.png, a PNG image of the order 6 triangulation;

The pressures are associated with a subset of the nodes called "pressure nodes", and linear (3 node) triangles. Note that, in the order 3 triangulation, the nodes are renumbered, and do NOT inherit the labels used in the order 6 triangulation.

• nodes3.txt, a text file containing the pressure nodes;
• nodes3.png, a PNG image of the pressure nodes;
• triangles3.txt, the linear triangulation;
• triangles3.png, a PNG image of the linear triangulation;

The pressures are a scalar quantity associated with the pressure nodes, the velocities are a vector quantity associated with the vector nodes.

• pressure3.txt, a text file containing the pressure P at each linear node;
• pressure3.png, a PNG image of a contour plot of the pressure, produced by TRIANGULATION_ORDER3_CONTOUR.
• velocity6.txt, a text file containing the velocity (U,V) at each velocity node;
• velocity6_dir.png, a PNG image of the velocity direction field, created by VECTOR_PLOT.
• velocity6_vec.png, a PNG image of the velocity field, created by VECTOR_PLOT.
• velocity6_arrows.png, a PNG image of the velocity field, created by VELOCITY_ARROWS.