squashing modelling

Hello,

See https://answers.launchpad.net/yade/+question/250196

Cheers,

Robert

Hello Robert,

Thank you for your answer but it has no link with my problem because I do not deal with compressible flows !
In my situation, the vertical mechanical compression induces lateral mechanical expansion.
Thank you for your replies!

All the best

Fr.

>>Thank you for your answer but it has no link with my problem

The problem in the link I posted is, in fact, identical to your problem :-). You are describing undrained triaxial compression. While that thread discusses how you might go about re-posing that ill-posed flow problem in Yade.

Hello Robert,

Indeed, it is only advised by Bruno to use "custom tricks" (?) to deal with the Undrained/incompressible case.
Can you tell me more for my specific case of squashing ??

Thanks,

Fr.

I'll echo Bruno's recommendation to use the compressible scheme:

"3/ switching to compressible scheme does not imply to change other parameters, no. It is more the other way around: the stability of the incompressible scheme requires the smallest timestep, for instance. So switching to compressible should be 100% safe in general."

Hello,

-OK, Robert, I introduced a fluid bulk modulus: the error message does not appear anymore immediately but later one in the middle of the compression process. Apparently the time of apparition of the error message depends on the value given to the fluid bulk modulus....what are the units of this parameter in yade ?? typical values ??

- I have still my second question open concerning the force and position of the top plate (see question 2 above): it does not vary during the compression process !

Thanks for your help,

Best,

Fr.

Hi,
1- Isn't it just scheme instability due to the timestep? Did you try a smaller one?
> units of this parameter in yade
Same unit as Young (both dimensionless in my view, but that's up to you).

2. PLease show a minimal working example.

Bruno

For 2/
You write force component in some variable at some point, then you record
that variable. Of course it's constant.
Like saying that speed of wind at time t_0 is independent of time t, which
is true and expected.
Bruno

Le mar. 19 mars. 2019 17:08, Rioual <email address hidden>
a écrit :

> Question #679320 on Yade changed:
> https://answers.launchpad.net/yade/+question/679320
>
> Status: Answered => Open
>
> Rioual is still having a problem:
> Hello Bruno
>
> Thank's for your input.
>
> 1- I tried the simulation with different smaller time scales 1e-4, 1e-5,
> 1e-6 and the error messages appear at fairly the same simulation time
> apparently:
> "
> CHOLMOD warning: matrix not positive definite
> something went wrong in Cholesky factorization, use LDLt as fallback this
> time
> 65 : Vh==NULL!! id=65 Point=-0.803797 -0.683743 1.61915 rad=0.0666365
> 147 : Vh==NULL!! id=147 Point=-0.927542 -0.647778 1.92058 rad=0.0638237
> ...etc"
>
> 2- Before the error messages, the compression is processed as can be
> observed on the graphical output (motion of the boundaries) but nothing
> changes when I print the position of the upper plate and the force on
> this plate. See a minimal working example code of the compression
> process below that shows both problems 1 and 2:
>
>
> ****************************************************************************
>
> ## ______________ First section, similar to
> triax-tutorial/script-session1.py _________________
> from yade import pack
>
> num_spheres=1000# number of spheres
> young=1e6
> C=1e6
> compFricDegree = 3 # initial contact friction during the confining phase
> finalFricDegree = 11 # contact friction during the deviatoric loading (for
> ice!!)
> mn,mx=Vector3(0,0,0),Vector3(1,1,1) # corners of the initial packing
>
>
> O.materials.append(FrictMat(young=young,poisson=0.5,frictionAngle=radians(compFricDegree),density=2600,label='spheres'))
>
> O.materials.append(FrictMat(young=young,poisson=0.5,frictionAngle=0,density=0,label='walls'))
> walls=aabbWalls([mn,mx],thickness=0,material='walls')
> wallIds=O.bodies.append(walls)
>
> sp=pack.SpherePack()
> sp.makeCloud(mn,mx,-1,0.3333,num_spheres,False, 0.95,seed=1) #"seed" make
> the "random" generation always the same
> sp.toSimulation(material='spheres')
>
> triax=TriaxialStressController(
> maxMultiplier=1.+2e4/young, # spheres growing factor (fast growth)
> finalMaxMultiplier=1.+2e3/young, # spheres growing factor (slow
> growth)
> thickness = 0,
> stressMask = 7,
> max_vel = 0.005,
> internalCompaction=True, # If true the confining pressure is
> generated by growing particles
> )
>
> newton=NewtonIntegrator(damping=0.2)
>
> O.engines=[
> ForceResetter(),
> InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Box_Aabb()]),
> InteractionLoop(
> [Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
> [Ip2_FrictMat_FrictMat_FrictPhys()],
> [Law2_ScGeom_FrictPhys_CundallStrack()],label="iloop"
> ),
> FlowEngine(dead=1,label="flow"),#introduced as a dead engine for
> the moment, see 2nd section
>
> GlobalStiffnessTimeStepper(active=1,timeStepUpdateInterval=100,timestepSafetyCoefficient=0.8),
> triax,
> newton
> ]
>
> triax.goal1=triax.goal2=triax.goal3=-10000
>
> while 1:
> print '***'
> O.run(1000, True)
> unb=unbalancedForce()
> if unb<0.001 and abs(-10000-triax.meanStress)/10000<0.001:
> break
>
> setContactFriction(radians(finalFricDegree))
>
> ##Second section: Activate flow engine and set boundary conditions for
> the compression (squashing) test, impermeable on each face:
>
> flow.dead=0
> flow.defTolerance=0.3
> flow.meshUpdateInterval=200
> flow.useSolver=3
> flow.permeabilityFactor=1
> flow.viscosity=10
> flow.boundaryUseMaxMin=[0,0,0,0,0,0]
> O.dt=1e-4
> O.dynDt=False
>
> flow.bndCondIsPressure=[False,False,False,False,False,False]
> #flow.bndCondValue=[0,0,0,0,0,0]
>
> newton.damping=0
>
> #aditional compressibility of the fluid
> flow.fluidBulkModulus=1e6
>
> triax.stressMask=5
> triax.internalCompaction=False
> triax.goal2=-0.1
> triax.goal1= 0
> triax.goal3= 0
> triax.wall_left_activated= True
> triax.wall_right_activated= True
> triax.wall_front_activated= True
> triax.wall_back_activated= True
> triax.wall_top_activated= True
>
> global plate
> plate = O.bodies[triax.wall_top_id]
>
> Ftot0 = O.forces.f(plate.id)[0]
> Ftot1 = O.forces.f(plate.id)[1]
> Ftot2 = O.forces.f(plate.id)[2]
>
> xtot = plate.state.pos[0]
> ytot = plate.state.pos[1]
> ztot = plate.state.pos[2]
>
>
> ## a function printing variables
> def history():
> print 't=', O.time, 'Force on the plate (Ftot)=', Ftot0, Ftot1,
> Ftot2
> print 't=', O.time, 'Position of the plate (xtot,ytot,ztot)=',
> xtot, ytot, ztot
>
>
> O.engines=O.engines+[PyRunner(iterPeriod=200,command='history()',label='recorder')]
>
> from yade import timing
> print "starting squashing simulation"
> O.run(200,1)
> timing.stats()
>
>
> ************************************************************************************************
>
>
> Best wishes,
>
> Fr.
>
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Hello Bruno

For 2/ mistake of a beginner indeed because of my previous bad understanding of a loop in yade with o.run, now it is clear, ok, thanks !!

For 1/ the crash still remains....

All the best
Fr.

...The force on the upper plate doesn't stop increasing during the run: the packing might be too packed to generate a lateral expansion of the material, it may bring problems (?)

Fr.

The strain rate is too high...
Thank you all, my problem is solved!