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Re: Relative velocity (Shear force)

 

On 18 March 2010 16:46, Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx> wrote:

>
>
>> v_rel = deltaV_2 - deltaV_1
>>
>>
>> Question: is that what we do in this line:
>>
>> Vector3r relativeVelocity = (de2->vel + de2->angVel.Cross(_c2x_)) -
>> (de1->vel + de1->angVel.Cross(_c1x_));
>> ?
>>
>> I know what that line means, but it is to say the same thing as I
>> expressed above?
>>
> Yes, exactly the same thing, provided deltaV_2 is the disp. of the material
> point at contact (not just the disp of the body's center - that explains the
> rotational term with cross product).
>
> I discover this long discussion on sign/sense convention now after few days
> of network problem...
> Just one remark : the initial convention (ScGeom) has never been
> explicitely linked to the sign of compression/traction in my brain (as
> opposed to what was suggested by remarks on geomechanical sign conventions).
>
> "My" explanation of the convention was : if the normal is defined as b1->b2
> (i.e. "from b1 to b2"), it is more intuitive to define the force as well as
> "applied by b1 on b2".
> The fact that it will give a positive product (normal*force) for
> compressive interactions is a consequence, not an objective of the
> convention.
>
> Exactly what I said in one of my previous mails, it is no more a convention
once we define the normal vector. That is why I do not understand what is
going on in the Dem3Dof, since the normal vector is the same as in ScGeom
but it seems that the so far called convention (that is not), is different
in that case.



> I hope it creates even more trouble... ;)
>
> Bruno
>
>
>
>
>> **************************
>>
>> One more observation about the relative velocity. I am quite confident to
>> repeat that we cannot predict the sense of this vector. This is a result of
>> the motion, and the shear increment that we get can give a positive
>> contribution or a deduction to the total shear force according to that. Now
>> (perhaps) it does not make too much different to say
>>
>> rel_vel = body1_vel - body2_vel
>> OR
>> rel_vel = body2_vel - body1_vel
>>
>> Therefore it should be fine both saying
>>
>> shearForce -= rel_vel * dt * ks
>> OR
>> shearForce += rel_vel * dt * ks
>>
>> since is the sense of the force increment that at each time step will give
>> the right contribution. Please, correct me if I am wrong (I have a lot of
>> doubts on this side).
>> Thanks, Chiara
>> ------------------------------------------------------------------------
>>
>>
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>>
>
>
> --
> _______________
> Bruno Chareyre
> Associate Professor
> Grenoble INP
> Lab. 3SR
> BP 53 - 38041, Grenoble cedex 9 - France
> Tél : 33 4 56 52 86 21
> Fax : 33 4 76 82 70 43
> ________________
>
>
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