Initial and Boundary Conditions with sampled/measured Data

Initial and boundary conditions are sometimes applied with measured data that is itself pre-discretized. In order to use such data it is recommended to leverage Interpolations.jl, or DataInterpolations.jl, for better dealing with possibly noisy data (currently limited to 1D). To create a callable effectively continuous function, for example (from the Interpolations.jl docs):


using Interpolations

A_x = 1.:2.:40.
A = [log10(x) for x in A_x]
itp = interpolate(A, BSpline(Cubic(Line(OnGrid()))))
sitp1 = scale(itp, A_x)
sitp1(3.) # exactly log10(3.)
sitp1(3.5) # approximately log10(3.5)


using Interpolations 

A_x1 = 1:.1:10
A_x2 = 1:.5:20
f(x1, x2) = log10(x1+x2)
A = [f(x1,x2) for x1 in A_x1, x2 in A_x2]
itp = interpolate(A, BSpline(Cubic(Line(OnGrid()))))
sitp2 = scale(itp, A_x1, A_x2)
sitp2(5., 10.) # exactly log10(5 + 10)
sitp2(5.6, 7.1) # approximately log10(5.6 + 7.1)

Then, register the functions with ModelingToolkit:

sitp1_f(x) = sitp1(x)
sitp2_f(x, y) = sitp2(x, y)
@register_symbolic sitp1_f(y)
@register_symbolic sitp2_f(x, y)

Then as a BC or IC:

bcs = [u(0, x, y) ~ sitp2_f(x, y),
       u(t, 0, y) ~ sitp1_f(y),

Note that the measured data need not be measured on the same grid as will be generated for the discretization in MethodOfLines.jl, as long as it is defined upon the whole simulation domain it will be automatically re-sampled.

If you are using an edge_align grid, your interpolation will need to be defined ±dx/2 above and below the edges of the simulation domain where dx is the step size in the direction of that edge. Extrapolation may prove useful here.