New question #667812 on Yade:
https://answers.launchpad.net/yade/+question/667812

Dear all,
I have been struggling recently to apply the RotationEngine on a clump of 
spheres.
My goal is basically to let the clump roll over the floor, as a wheel would do. 
The forward movement comes from the traction between the wheel and the floor, 
so the TranslationEngine is not used.

My questions are:
- what am I doing wrong? I tried several things: apply the rotation engine on 
the individual spheres or on the clump ID, rotate around the center of mass, ...
- Is the same thing possible with a clump of facets?


Here's an example:
I'm using Yade 2018.02b-50-e0894fb~xenial
(I want to work with cohesive materials, hence the cohesive physics. I guess 
this doesn't matter)

## MWE
from yade import plot, utils, pack, geom, ymport, polyhedra_utils, timing

## MATERIALS
CohFrictMat02=CohFrictMat(label='CohMat1',id=2,
                          
density=2.6e3,young=1e6,poisson=1e0,frictionAngle=numpy.pi/2*0.8,
                          isCohesive=False)

## DEFINE BODIES
# Fixed Reference
sp=pack.regularOrtho(pack.inAlignedBox([-0.4,-0.4,-0.11],[0.4,2.0,0.1]),
                     radius=0.1,
                     gap=0.0,
                     material = CohFrictMat02,
                     fixed = True)
O.bodies.append(sp)

# Clump to be rotated
sp=pack.regularHexa(pack.inAlignedBox([-0.3,-0.3,0.2],[0.3,0.3,0.8]),
                    radius=0.1,
                    gap=0.0,
                    material = CohFrictMat02,
                    wire = True,
                    fixed = False,
                    )

RotatingClumpID_L = O.bodies.appendClumped(sp) ## APPEND CLUMPED
RotatingClumpID = RotatingClumpID_L[0]
RotatingSpheresID = RotatingClumpID_L[1]
# RotatingSpheresID = O.bodies.append(sp) ## APPEND NOT CLUMPED (for testing)

## dT
dt=O.dt=polyhedra_utils.PWaveTimeStep() # 

## MODEL
Gravity = -9.8
# Gravity = 0.0

## Engines
O.timingEnabled=True
EnlargeDetectionRadius = 1.1

RotEngine = RotationEngine(angularVelocity = -10.0,
                           rotationAxis = (1,0,0),
                           ids = [RotatingClumpID,],
                           # ids = RotatingSpheresID,
                           # ids = [RotatingClumpID,]+RotatingSpheresID,
                           # ids = [RotatingSpheresID[0],],
                           rotateAroundZero=1,
                           zeroPoint=O.bodies[RotatingClumpID].state.pos,
                           # rotateAroundZero=0,
                           label = 'RotationTootToot',)

O.engines=[
    ForceResetter(),
    InsertionSortCollider([
        Bo1_Sphere_Aabb(aabbEnlargeFactor = EnlargeDetectionRadius),
        ]),
    InteractionLoop(
        [
        Ig2_Sphere_Sphere_ScGeom6D(interactionDetectionFactor = 
EnlargeDetectionRadius),
        ],
        [Ip2_CohFrictMat_CohFrictMat_CohFrictPhys()],
        [Law2_ScGeom6D_CohFrictPhys_CohesionMoment(always_use_moment_law = 
False,
                                                   creep_viscosity = 1,
                                                   shear_creep = False,
                                                   twist_creep = False,
                                                   )],
    ),
    NewtonIntegrator(gravity=(0,0,Gravity),damping=0.8), # Gravity
    RotEngine,
    
PyRunner(iterPeriod=1,command="RotEngine.zeroPoint=O.bodies[RotatingClumpID].state.pos"),
    ]


Thanks
Jan

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