Newton Integrator updating of position and velocity

Hi,

add initRun=True to PyRunner:

PyRunner(initRun=True,command='addData()',iterPeriod=1),

Running script test.py
0.0 -0.981 0.9019
0.1 -1.962 0.7057
0.2 -2.943 0.4114
0.3 -3.924 0.019
0.4 -4.905 -0.4715
0.5 -5.886 -1.0601
0.6 -6.867 -1.7468
0.7 -7.848 -2.5316
0.8 -8.829 -3.4145
0.9 -9.81 -4.3955

Anton

2012/6/21 Mukesh Tiwari <email address hidden>:
> New question #201064 on Yade:
> https://answers.launchpad.net/yade/+question/201064
>
> Hi
> I am trying to look at how Yade updates postion and velocity through a simple example of a body falling freely under gravity. The code is:
>
> O.bodies.append([utils.sphere((0,0,1),.5)])
>
> b = O.bodies[0]
> #b.state.vel=(0,0,0)
> print O.time,b.state.vel[2],b.state.pos[2]
>
>
> O.engines=[
>   ForceResetter(),
>   NewtonIntegrator(damping=0,gravity=(0,0,-9.81)),
>   PyRunner(command='addData()',iterPeriod=1),
> ]
>
> O.dt = 0.1
>
>
> def addData():
>    print O.time,b.state.vel[2],b.state.pos[2]
>
> O.run(10,True)
>
> Now the output which I obtain is :
>
> 0.0    0.0     1.0
> 0.1  -1.962  0.7057
> 0.2  -2.943  0.4114
> 0.3  -3.924  0.019
> 0.4  -4.905  -0.4715
> 0.5  -5.886  -1.0601
> 0.6  -6.867  -1.7468
> 0.7  -7.848  -2.5316
> 0.8  -8.829  -3.4145
> 0.9  -9.81   -4.3955
>
> The velocity at dt =0.1  should be -0.981 but it gives the expected value at 2*dt. Similarly the position at dt=0.1 is what one expects at 6*dt. I am not sure if there is something wrong in what I do or if there is some other issue.
>
> Thanks
> Mukesh
>
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Thanks Anton,

The velocity agrees completley but the position still has an issue. I think it should be simply g t^2/2 which I don't get.

Mukesh

Hello Mukesh,

Nice "Yade exercise" ;-)

Your results are consistent with the way Yade computes things (see http://bazaar.launchpad.net/~yade-dev/yade/trunk/view/head:/pkg/dem/NewtonIntegrator.cpp, or documentation surely)

E.g, you have indeed :
position(t=0.4) = position(t=0.3) + speed(t=0.4) * 0.1, and so on for other times
(velocities are computed first, before modifiying positions)

In my opinion, the discrete (in time !) feature of this integration scheme is the only reason why you do not get the exact solution. Try with smaller dt, and you should certainly approach the result you expect.

Thanks I will take a look at it...

Thanks Chareyre, that solved my question.