Hi Grace, and welcome, My reply is interleaved with your comments.
On Tue, Jan 14, 2014 at 06:40:03PM +0000, Grace Roberts wrote: > Hi, > I'm a python beginner, currently using scipy's 'odeint' to compute to > a set of ODEs (obtained by splitting Newton's law of gravity > ma=-Gm1m2r/r^3 in two ordinary differentials). The idea is that I'm > modelling the orbit of a satellite as it approaches Mars. Unfortunately, I don't think there is anyone here who is very familiar with scipy. Including me. I'll try my best to give some helpful comments, but take them with a grain of salt. > I have two > problems:-For certain initial conditions the programme displays > impossible orbits, showing the satellite making immediate sharp turns > or jumping up and down in velocity. I'm not an expert on numeric code, but my first instinct on reading this is to think that you're suffering numerical instability in your ODEs. It's been too many years since I've done ODEs to be much help to you here, but can you double check that the ODE you are using is mathematically correct and any constants are expressed to sufficient accuracy? > The exact same code can also end > up displaying different graphs when run multiple times. For example > when initial conditions are set at: > xx0=[1000.,10000.,1000.,10000.]. If what you say is accurate, that strongly suggests that the system you are modelling is chaotic, and tiny changes in initial conditions lead to major changes in the output. This problem may be inherent in the physical system. > -Often when run an error message > appears saying: "Excess work done on this call (perhaps wrong Dfun > type). Run with full_output = 1 to get quantitative information." You > can see that as part of the odeint function I tried to stop the excess > work by increasing the maxstep and I also tried to run with > full_output =1. It's quite possible I haven't done this right. If I > have, what else can I do to stop this message from appearing? I've > attached a quick print-screen of the code. Any help is much > appreciated. Unfortunately print-screen is not so helpful here. What we really need is your code copy and pasted as text, if possible directly in the body of your email. If it is more than a hundred lines or so, it will probably be better to attach your .py file to the email. I will try to find a program that will let me view your .docx file (like many of us here, I'm a Linux user and don't have access to Microsoft Office) so I can at least see the print-screen, but being able to access it as text rather than just a screenshot would make it much easier to help you. Regards, -- Steven _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
