MATLAB: Do I have error during usage of state.time for non-constant boundary condition

Question

Dear all,

I am trying to solve a parabolic PDE with imported 3D geometry. I create the model and everything correctly but I decided to implement on one of the faces a boundary condition that depends on the time. Basically, what it needs to do is for each integer time-step say state.time=k we need to access azimuth(state.time). For this purpose I synchronized the time steps as follows:

nframes = 5; tlist = linspace(1,nframes,nframes);

Previously in the code I have specified the function

myufun14= @(region,state)sin(azimuth(state.time))

where I recall that azimuth is a vector. It gave and error :

"Subscript indices must either be real positive integers or logicals"

I tried just to test whether state.time might not return something different from the time steps included in tlist=linspace(1,nframes,nframes). In doing so I just attempted to apply the ceiling function:

myufun14= @(region,state)sin(azimuth(ceil(state.time+1)))

I got the same error:"Subscript indices must either be real positive integers or logicals"

Please could you help me!

Best

Answer 1

You need to write your boundary conditions in the appropriate syntax. I suppose that you are setting a Dirichlet condition on your specified face.

As the documentation states, your boundary function must return an N1-by-M matrix of points, where N1 is the number of equations (probably 1 for your problem), and M is the number of evaluation points, meaning the length of region.x. So if sin(azimuth(state.time)) is a scalar, then your function should be

 myufun14 = @(region,state)sin(azimuth(state.time))*ones(1,length(region.x))

It is also possible that the solver would need times other than those you specify. So you might need to change azimuth(state.time) to azimuth(floor(state.time)).

Alan Weiss

MATLAB mathematical toolbox documentation