
Re: system of differential equations mathematica help
Posted:
Jan 8, 2013 11:39 PM


On Mon, 7 Jan 2013, 01af wrote:
> Before anyone starts to type: I have read the top post on a similar matter, but the reply makes no sense to me so if anyone can give my issue a read through I would be grateful. Thanks. > > ok I am modelling airflow in the upper airway for application i obstructive sleep apnoea, but I have hit a brick wall with mathematica. I have a system of 3 differential equations with boundary conditions, and I need to solve to find 3 unknown functions numerically so that they may be plotted in various graphs. > > The equations are as follows: > > D[a[x]*u[x], x] == 0, > u[x] u'[x] == p'[x], > p[x]  1 == 2 (1  ((a[x])^(3/2)))  50 (a''[x]). > > with boundary conditions: > > u[0] == 0.1, a[0] == 1, a[10] == 1, p[10] == 1. > > so initially I tried to use NDSolve like so.. > > Code: > > NDSolve[{D[a[x]*u[x], x] == 0, u[x] u'[x] == p'[x], > p[x]  1 == 2 (1  ((a[x])^(3/2)))  50 (a''[x]), u[0] == 0.1, > a[0] == 1, a[10] == 1, p[10] == 1}, {a}, {x, 0, 10}] > > but mathematica does this: > > Code: > > Power::infy: "Infinite expression 1/0.^(3/2) encountered. " > Infinity::indet: Indeterminate expression 0. ComplexInfinity encountered. >> > Infinity::indet: Indeterminate expression 0. ComplexInfinity encountered. >> > General::stop: Further output of Infinity::indet will be suppressed during this calculation. >> > NDSolve::ndnum: Encountered nonnumerical value for a derivative at x == 0.`. >> > > which is super annoying, any pointers as to where I'm going wrong would be great. I'm not even sure if I should be using NDSolve so let me know what you think. > thanks in advance > a. > >
Hi 01af,
to be quite honest I did not see what the issue was and had to resort asking colleagues myself. What you can do is:
NDSolve[{D[a[x]*u[x], x] == 0, u[x] u'[x] == p'[x], p[x]  1 == 2 (1  ((a[x])^(3/2)))  50 (a''[x]), u[0] == 0.1, a[0] == 1, a[10] == 1, p[10] == 1}, {a}, {x, 0, 10}, Method > {"BoundaryValues" > {"Shooting", "StartingInitialConditions" > {a[0] == 1, a'[0] == 0, p[0] == 0, u[0] == .1}}}]
The issue is that the default starting conditions produce singularities  so I filed this as a suggestion for future improvement that in such a case other starting conditions are attempted.
Hope this helps,
Oliver

