Re: [Yade-users] [Question #706553]: About simulating constant volume condition in a triaxial test
Question #706553 on Yade changed: https://answers.launchpad.net/yade/+question/706553 Bruno Chareyre posted a new comment: Hi, I think strain damping could give a null volumetric rate in this case. It's consistent with the fact that even with 0.99 ev is still (somehow) small. The algorithm implies an exponential convergence to the goal rates, approximately like this: rate += (goal-rate)*(1-damping) (a discrete form of d(rate)/dt = - a*(goal-rate))[1]. If I'm not wrong, it leads to proportional increases of the rates in the different directions, and so the trace of the strain rate tensor should be always zero. In this reasoning I'm assuming that all strains start from zero, though. If you start the constant volume stage from a state where the velocities are not zero (which is the case if you impose stresses for instance), then there can be small offsets of the different components during the convergence. If that is the cause of the small volume changes, there could be multiple solutions, e.g. - assign goals=0 and run a few iterations, then shear - assign velocities=0 directly (wall.state.vel=(0,0,0)), then shear Another option is to simply set set strainDamping=0, and implement your own ramp in the script, possibly reproducing [1] or with a different equation, changing the goals incrementally (this is to reduce elastic waves mainly). Bruno -- You received this question notification because your team yade-users is an answer contact for Yade. ___ Mailing list: https://launchpad.net/~yade-users Post to : yade-users@lists.launchpad.net Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
Re: [Yade-users] [Question #706553]: About simulating constant volume condition in a triaxial test
Question #706553 on Yade changed: https://answers.launchpad.net/yade/+question/706553 Status: Answered => Solved Leonard confirmed that the question is solved: Thanks Karol Brzezinski, that solved my question. -- You received this question notification because your team yade-users is an answer contact for Yade. ___ Mailing list: https://launchpad.net/~yade-users Post to : yade-users@lists.launchpad.net Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
Re: [Yade-users] [Question #706553]: About simulating constant volume condition in a triaxial test
Question #706553 on Yade changed: https://answers.launchpad.net/yade/+question/706553 Leonard posted a new comment: Hi Karol, Thanks very much for your reply. Yes, the strainDamping is a good clue. I add one line of code "triax.strainDamping=0.01" in the MWE and run the simulation. I compare the results with the original MWE results whose triax.strainDamping=0.99 by default. I found: triax.strainDamping=0.99 gives ev=4.3e-4 at e22=0.4 triax.strainDamping=0.01 gives ev=2.2e-6 at e22=0.4, which is much close to zero. This suggests that using a small strainDamping can make it close to a perfect constant volume condition, although the stress-strain results also change. Thanks Leonard -- You received this question notification because your team yade-users is an answer contact for Yade. ___ Mailing list: https://launchpad.net/~yade-users Post to : yade-users@lists.launchpad.net Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
Re: [Yade-users] [Question #706553]: About simulating constant volume condition in a triaxial test
Question #706553 on Yade changed: https://answers.launchpad.net/yade/+question/706553 Karol Brzezinski posted a new comment: After second thought I think that I misunderstood the problem. Because you are actually checking if the obtained strain is equall to the value expected from the prescribed rate. Maybe strainDamping is [1] is the source of the error... I cannot check right now, but it looks like a good clue, since affects the change in strain rate. Cheers, Karol [1] https://yade- dem.org/doc/yade.wrapper.html?highlight=triaxialstresscontroller#yade.wrapper.TriaxialStressController.strainDamping -- You received this question notification because your team yade-users is an answer contact for Yade. ___ Mailing list: https://launchpad.net/~yade-users Post to : yade-users@lists.launchpad.net Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
Re: [Yade-users] [Question #706553]: About simulating constant volume condition in a triaxial test
Question #706553 on Yade changed: https://answers.launchpad.net/yade/+question/706553 Karol Brzezinski proposed the following answer: After some 'gymnastics', I would propose: triax.goal2=-(2*rate+rate**2)/(1+2*rate+rate**2) triax.goal1=rate triax.goal3=rate Cheers, Karol -- You received this question notification because your team yade-users is an answer contact for Yade. ___ Mailing list: https://launchpad.net/~yade-users Post to : yade-users@lists.launchpad.net Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
Re: [Yade-users] [Question #706553]: About simulating constant volume condition in a triaxial test
Question #706553 on Yade changed: https://answers.launchpad.net/yade/+question/706553 Status: Open => Answered Karol Brzezinski proposed the following answer: Hi Leonard, triax.goal2=rate triax.goal1=-0.5*rate triax.goal3=-0.5*rate Comes from the fact that volumetric strain is equal to e11+e22+e33 for small deformations. One assumes e22=e33 and wants e11+e22+e33 = 0. So 2*e22 = -e11, hence e22=e33=0.5*e11. The error comes from the inaccuracy of the formula for large strains (apparently, your strains are large if the error is not acceptable). You can propose your own triax.goals by taking into account that the actual volumetric strain is (1+e11)*(1+e22)*(1+e33). Cheers, Karol -- You received this question notification because your team yade-users is an answer contact for Yade. ___ Mailing list: https://launchpad.net/~yade-users Post to : yade-users@lists.launchpad.net Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
