Hertz mindlin, rolling back sphere

Hi Anna,

try to decrease a friction angle.
It should "help". But the physics of the process and its
implementation in Yade should be checked.

Anton

Hi,
I tried to change the friction angle but the result is the same,the only
difference is the point where the sphere is rolling forward and back.

Cheers,
Anna

You are using rotational stiffness, so, to me, there is nothing wrong.
Instead, it prooves robustness!
Try with krot=0, it will roll as you expect.
Nice script anyway.

B

Hi Anna,

Are you aware that you input in the model some rolling resistance (if you try with the bool includeMoment=False you will see what I mean)? I think that explains the behaviour. Can you please try to explain here the purpose of your test? It may help us to give you a better answer.

Thank you,
Chiara

On 29 Jun 2012, at 07:01, anna effeindzourou wrote:

> New question #201769 on Yade:
> https://answers.launchpad.net/yade/+question/201769
>
> Hi all,
> I am using the Hertz Mindlin contact law with a simple example a sphere rolling on a inclined facet and then on a horizontal facet. I've observed that the sphere is rolling up and down.
> Here is a small script to help you understand the phenomenon.
>
>
> #!/usr/bin/python
> # -*- coding: utf-8 -*-
> from yade import utils
>
> h=0.2
>
> O.materials.append(FrictMat(young=1.7e8,poisson=0.20588,frictionAngle=radians(17),density=2650))
> s1=O.bodies.append( utils.sphere([0.0000001,0.5,h+0.1],.1,color=[0,1,0],fixed=False,wire=True))
>
>
> v1=(0,0,h)
> v6=(0,1,h)
> v2=(.5,0,0)
> v3=(2,0,0)
>
> v4=(2,1,0)
> v5=(0.5,1,0)
>
> f1=O.bodies.append( utils.facet([v1,v2,v5], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))
> f2=O.bodies.append( utils.facet([v1,v5,v6], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))
> f3=O.bodies.append( utils.facet([v2,v3,v4], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))
> f4=O.bodies.append( utils.facet([v2,v4,v5], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))
>
> ## time step
> O.dt=1e-5
>
> O.engines=[
> ForceResetter(),
> InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb()]),
> InteractionLoop(
> [Ig2_Facet_Sphere_ScGeom(),Ig2_Sphere_Sphere_ScGeom()],
> [
> Ip2_FrictMat_FrictMat_MindlinPhys(en=0.1, krot=1,eta=1)
> ],
> [Law2_ScGeom_MindlinPhys_Mindlin(includeMoment=True),
> ]),
> NewtonIntegrator(damping=0,gravity=(0,0,-9.81)),
> ]
>
> Any ideas?
>
> cheers,
> Anna
>
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Hi,
The purpose of this test is to investigate the influence of the rolling
resistance,
that's why the boolean includeMoment is true. But I just didn't expect that
the sphere could be able to roll up the hill.

Thank you,
Anna

Hi anna,

I tested your script and it looks strange to me too. I would not expect, that sphere comes back, when rolling on the horizontal facets. It should roll along the plane and should get slower while rolling. But it looks like the sphere is draged to the break of slope. I am also wondering about this behavior, ...

regards,

christian

Ok, so it behaves like a pendulum, where anchor point is the middle point of two facets.
I modified annas script a little bit. Now one can see the pendulum effect.

#!/usr/bin/python
# -*- coding: utf-8 -*-
from yade import utils

h=0.2
y_ext = 100
x_ext = 100

O.materials.append(FrictMat(young=1.7e8,poisson=0.20588,frictionAngle=radians(17),density=2650))
s1=O.bodies.append( utils.sphere([0.0000001,y_ext/2,h+0.5],.5,color=[0,1,0],fixed=False,wire=True))

v1=(0,0,h)
v6=(0,y_ext,h)
v2=(.5,0,0)
v3=(x_ext,0,0)

v4=(x_ext,y_ext,0)
v5=(0.5,y_ext,0)

f1=O.bodies.append( utils.facet([v1,v2,v5], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))
f2=O.bodies.append( utils.facet([v1,v5,v6], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))
f3=O.bodies.append( utils.facet([v2,v3,v4], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))
f4=O.bodies.append( utils.facet([v2,v4,v5], dynamic=None, fixed=True, wire=True, color=None, highlight=False, noBound=False, mask=1))

## time step
O.dt=1e-4

O.engines=[
        ForceResetter(),
        InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb()]),
        InteractionLoop(
        [Ig2_Facet_Sphere_ScGeom(),Ig2_Sphere_Sphere_ScGeom()],
        [
        Ip2_FrictMat_FrictMat_MindlinPhys(en=0.1, krot=0.24, eta=1)
        ],
        [Law2_ScGeom_MindlinPhys_Mindlin(includeMoment=True),
        ]),
        NewtonIntegrator(damping=0,gravity=(0,0,-9.81)),
]

Thank you guys

Thanks Chareyre, that solved my question.