MATLAB: Is there a way to add constraints (other than nonnegativity) on ode15s

odeode15s

I am using ode15s to solve a DAE system describing oil-water flow in a pipe. Some of the variables in the solution are physically unrealistic, and I was wondering whether there is a way to add specific constraints?

Thank you.

Best Answer

Only in the sense that you could add an event function to detect the value going out of range, and terminating when it does. If you wanted it to somehow "bounce off" of the boundary then you would have to signal the termination and then call ode15s again with revised conditions and tspan. See for example ballode()

Related Solutions

MATLAB: How to speed up ODE15s

Some things to try...

For stiff problems you should always specify the Jacobian or its sparsity pattern via the options structure. Sometimes this can greatly improve performance since the solver estimates it numerically when you don't specify it. See odeset for more info.
Try some of the other stiff solvers since they use a slightly cruder tolerance to calculate steps ( ode23s, ode23t, ode23tb). Since your ODE function contains a loop the fastest solver is likely to be the one with the fewest function evaluations. You can check that by setting 'Stats','on' with odeset and comparing the stats of each solver.
If your equations include some conservation equations then you might have a system of Differential Algebraic Equations. If that's the case then you can continue using ode15s but might need to set some options as outlined on the linked page.

MATLAB: How to solve DAE system with non-negativity constraint

The usual strategies are:

1. Choose smaller error tolerances for RelTol and/or AbsTol

2. Make a copy of the solution variables in the subroutine where you supply the time derivatives. Then check the copy for negative values and reset them to small positive values within the forthcoming calculations, e.g.

conc_A = y(1);

if conc_A < 0

   conc_A=1e-4;

end

Best wishes

Torsten.

Best Answer

Related Solutions

MATLAB: How to speed up ODE15s

MATLAB: How to solve DAE system with non-negativity constraint

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