04-UsingExpressions.adoc

Defining and using expressions

The next step is to construct a function space over the mesh.

Step by step explanations

• We start by loading a Mesh in 2D.

link:{examplesdir}/04-myexpression.cpp[role=include]

• then we define some expression through the command line of config file: g is a scalar field and f is a vector field, here is an example how to enter them :

./feelpp_tut_myexpression --a=3 --functions.g="a*x*y:x:y:a" --functions.f="{sin(pi*x),cos(pi*y)}:x:y"

You can print back the expression to the screen to check that everything is ok. You want to use as expression a*x+b*y, you have to define a and b as option (either in your code, either in the library).

• then we compute the gradient of g and f.

link:{examplesdir}/04-myexpression.cpp[role=include]

Note	template argument are given to grad to specify the shape of the gradient: in the case of \$\nabla g\$, it is \$1\times2\$ and \$2\times 2\$ for \$\nabla f\$ since we are in 2D.

• then we compute the laplacian of g and f.

link:{examplesdir}/04-myexpression.cpp[role=include]

• then we compute the divergence of f.

link:{examplesdir}/04-myexpression.cpp[role=include]

• and the curl of f

link:{examplesdir}/04-myexpression.cpp[role=include]

• Finally we evaluate these expressions at one point given by the option x and y.

Implementation

An implementation reads as follows:

link:{examplesdir}/04-myexpression.cpp[role=include]

and the associated config file

link:{examplesdir}/04-myexpression.cfg[role=include]

Execution

$ ./feelpp_tut_myexpression

or

$ ./feelpp_tut_myexpression --a=3 --functions.g="<your_function>" --functions.f="<your_function>"

We start with the following function g=1 and f=(1,1).

 $./feelpp_tut_myexpression --functions.g=1:x:y --functions.f="{1,1}:x:y"

and get something like this

g=1
f={1,1}
i=(x-aVal)*y
grad(g)=[[0,0]]
grad(f)=[[0,0],[0,0]]
laplacian(g)=[[0]]
laplacian(f)=[[0],[0]]
div(f)=[[0]]
curl(f)=[[0]]
Evaluation  at  (0,0):
           g(x,y)=1
           f(x,y)=1
1
           i(x,y)=-0
Gradient:
     grad(g)(x,y)=0 0
     grad(f)(x,y)=0 0
0 0
Divergence:
      div(f)(x,y)=0
Curl:
     curl(f)(x,y)=0
Laplacian:
laplacian(g)(x,y)=0
laplacian(f)(x,y)=0
0

The symbolic calculus system worked as expected.