yade-users team mailing list archive

[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

kan a écrit :
> Thank you, Vaclav,
>  
> So the step for this is :
> during each timestep, the position of each particle/element keeps the 
> same, so we can calculate the force brom the force-displacement 
> relationship(in this case, the order does not affect the calculation 
> since all the particle/elements do not move), after we get the forces, 
> we use this forces to calculate the acceleration, and then the 
> velocity(average velocity),  and then the movement/displacement in 
> this timestep, and then update the position of each particle/element 
> to new positions by add movement/displacement to current position. 
> (until now, the calculation cycle of this timestep is completed)
> then go to next timestep by use the new positions of 
> particles/elements as the current position to do the next calculation 
> cycle.
>  
> is my understanding right?

Exactly right. What you describe here is the centered second order 
finite diffence scheme (also called leap-frog), which is used in Yade. 
It is probably the most common (and most simple) scheme in DEM codes. 
Other schemes may be possible but are not implemented.

Bruno

>  
>  
>  
> Thanks again.
>  
> Kan
>
>
>  
> On 1/22/08, *yade-users-request@xxxxxxxxxxxxxxxx 
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>     Today's Topics:
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>       1. How to get the micro-force from the macro-stress (kan)
>       2. Re: How to get the micro-force from the macro-stress
>          (V?clav ?milauer)
>
>
>     ----------------------------------------------------------------------
>
>     Message: 1
>     Date: Mon, 21 Jan 2008 13:29:54 -0600
>     From: kan <nicessgg@xxxxxxxxx <mailto:nicessgg@xxxxxxxxx>>
>     Subject: [Yade-users] How to get the micro-force from the
>     macro-stress
>     To: yade-users@xxxxxxxxxxxxxxxx <mailto:yade-users@xxxxxxxxxxxxxxxx>
>     Message-ID:
>            <
>     45213f870801211129l171d2a5cs69da2382202961f8@xxxxxxxxxxxxxx
>     <mailto:45213f870801211129l171d2a5cs69da2382202961f8@xxxxxxxxxxxxxx>>
>     Content-Type: text/plain; charset="utf-8"
>
>     Hi, buddies,
>
>     I have some questiones.
>     1. how to get the micro-force from the macro-stress?
>     As we know,for most of current studies, the concept of stress is
>     used to
>     describe the stress status. but when we look into the micro level,
>     we have
>     to convert the macro-stress to descrete force that applied at each
>     particle,
>     then how to do this convert?
>
>     2. what is the calculation process in each time step?
>     The problem is that, as see the relation ship below,
>     ___\|/____\|/___\|/________
>     01---02---03---04---05---06...
>     |   \   |  /  |      |       |      | ...
>     11---12---13---14---15---16...
>     |      |      |   /  |   \  |      |...
>     21---22---23---24---25---26...
>     ...
>     ...
>     x1---x2---x3---x4---x5---x6...
>
>     in which, the number is the ID of a particle,
>     and "--- " , "/" ,"\" and  "|" between the numbers mean there is a
>     contact
>     between the particles
>     "\|/" means force.
>     assume the 01-02-03-04...is the upper bounday layer, and the force is
>     applied as shown "\|/".
>     when the calculation begins, we need to detecte the contact and
>     update the
>     position of each particles,
>     now my problem comes:
>     what is the procedure to calculation and update the particles'
>     information,
>     you know the force is from up layer, so the first layer
>     ("01-02-03-04...")
>     should be calculated, then the second layer,and then the third
>     layer.... but
>     in DEM, the particle is independent to move, so after some time, some
>     particles in the first layer may move to the second layer and some
>     particles
>     in second layer may  move to the first layer(other other
>     layer),then how
>     should I do in order to know which particle should be calculated
>     and updated
>     first and which should be done later in the same timestep?
>     For example,
>     01 has contacting neighbors 02 and 11, in a same timestep, after
>     we have
>     calculated 01's properties( velocity, position....), we should
>     calculate 02
>     first, then 03(the neighbor of 02), after the layer of
>     01-02-03...is done,
>     then we can step to calculate the 11, which is to say, in a same
>     timestep,
>     though 02 and 11 are both contacting neighbors of 01, we should
>     calculate
>     02, and 11 later though 02 and 11 are both affected because of teh
>     movement
>     of 01. so, how to indentify which particle should be calculated
>     and update
>     first , and which should be later?
>     (because it is a dynamic process, so just use the order of  ID  is
>     not  a
>     way to solve this)
>
>     Hope I have described the problem 2 clearly,
>
>     Need your help, thanks so much.
>
>     Kan
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>     ------------------------------
>
>     Message: 2
>     Date: Tue, 22 Jan 2008 09:20:53 +0100
>     From: V?clav ?milauer < eudoxos@xxxxxxxx <mailto:eudoxos@xxxxxxxx>>
>     Subject: Re: [Yade-users] How to get the micro-force from the
>            macro-stress
>     To: yade-users@xxxxxxxxxxxxxxxx <mailto:yade-users@xxxxxxxxxxxxxxxx>
>     Message-ID: < 4795A765.9040403@xxxxxxxx
>     <mailto:4795A765.9040403@xxxxxxxx>>
>     Content-Type: text/plain; charset=UTF-8
>
>
>     > so, how to indentify which particle should be calculated and update
>     > first , and which should be later?
>     > (because it is a dynamic process, so just use the order of  ID  is
>     > not  a way to solve this)
>     The trick is that you have constant positions during the whole
>     timestep,
>     you calculate forces from all contacts (order doesn't matter, elements
>     don't move during calculation) and then, before next timestep begins,
>     there is LeapFrogPositionIntegrator, that calculates displacements
>     based
>     on velocity, inertia, applied forces, ... and moves elements to
>     the new
>     position. Obviously, your timestep must be small enough so that
>     you get
>     convergence.
>
>     HTH, Vaclav
>
>
>
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-- 
 
_______________
Chareyre Bruno
Maitre de conference

Institut National Polytechnique de Grenoble
Laboratoire 3S (Soils Solids Structures) - bureau E145
BP 53 - 38041, Grenoble cedex 9 - France
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