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doc:user:remeshing:remeshing [2016/06/13 16:53]
joris [Post-remeshing operations]
doc:user:remeshing:remeshing [2018/05/04 15:47]
boman ↷ Links adapted because of a move operation
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 Which is why all of this has been kindly automatized, so that you just have a few settings to define, click on execute, and just sit back, relax, and enjoy the beauty of FENRIR ! Which is why all of this has been kindly automatized, so that you just have a few settings to define, click on execute, and just sit back, relax, and enjoy the beauty of FENRIR !
-===== How does FENRIR work ? =====+===== How does remeshing work ? =====
  
 First, you need to have an operational 2D test case, and the will to introduce remeshing into it.  First, you need to have an operational 2D test case, and the will to introduce remeshing into it. 
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 Second, [[doc/user/meshtransfer/datatransferbetweenmeshes|data must be transferred]] from the old to the new mesh. As you very well know, the Gauss points contains information about strains and stresses, and the nodes about displacements, velocities and temperatures. However, just after remeshing, the newly defined nodes and Gauss points do not know anything, since they were just created ! If we do not want to loose what was computed during the previous time integration, then data (stresses, temperatures, velocities...) must be transferred to the new mesh. Second, [[doc/user/meshtransfer/datatransferbetweenmeshes|data must be transferred]] from the old to the new mesh. As you very well know, the Gauss points contains information about strains and stresses, and the nodes about displacements, velocities and temperatures. However, just after remeshing, the newly defined nodes and Gauss points do not know anything, since they were just created ! If we do not want to loose what was computed during the previous time integration, then data (stresses, temperatures, velocities...) must be transferred to the new mesh.
  
-Once this is done (it used to take a bloody hell of a f***ing time, but now the time required is more reasonable, see [[commit:01_19]] for details), we have a good new mesh, containing data from the previous computation. From this, the simulation can be restarted, which means that the time integration is carried out further, until the remeshing criterion is met again and another remeshing operation takes place.+Once this is done (it used to take a bloody hell of a f***ing time, but now the time required is more reasonable, see [[commit:2016:01_19]] for details), we have a good new mesh, containing data from the previous computation. From this, the simulation can be restarted, which means that the time integration is carried out further, until the remeshing criterion is met again and another remeshing operation takes place.
  
 And that's it for the basics ! But as they say, a picture is worth a thousand words, and I guess that a video is worth a thousand pictures, so just enjoy the following video, about the modelling of forging, to see how it works : And that's it for the basics ! But as they say, a picture is worth a thousand words, and I guess that a video is worth a thousand pictures, so just enjoy the following video, about the modelling of forging, to see how it works :
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         autoRemesh.setBalancing(True)         autoRemesh.setBalancing(True)
-            +     
 +        autoRemesh.makeAnim = False 
 +    
         autoRemesh.execute()         autoRemesh.execute()
                  
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   * ''setInteractionToPostStep'' gives to FENRIR the user number of the interaction on which post-remeshing operations should be executed (typically, any contact interaction), here number 1.   * ''setInteractionToPostStep'' gives to FENRIR the user number of the interaction on which post-remeshing operations should be executed (typically, any contact interaction), here number 1.
      
-  * Finally, the line ''autoRemesh.setBalancing(True)'' is the magical ingredient that makes everything taste better... I will talk more about this in a [[#Balancing|following section]].+  * The line ''autoRemesh.setBalancing(True)'' is the magical ingredient that makes everything taste better... I will talk more about this in a [[#Balancing|following section]]
 + 
 +  * Finally, the line ''autoRemesh.makeAnim = False'' indicates that we have no wish to make a save a picture every time that a fac is saved (to make an anim later on). If set to ''True'', then after each time integration, all facs will be loaded and a screenshot will be saved as a .bmp in the folder Anim, located in the workspace, just as is traditionally done when clicking on the button ''makeAnimation'' in Metafor GUI.
  
 Now that you have altered your test case to include remeshing, and that you have defined this little script, all that remains is to launch Metafor and execute your script (execute, not load/meta). Everything should work out perfectly. Now that you have altered your test case to include remeshing, and that you have defined this little script, all that remains is to launch Metafor and execute your script (execute, not load/meta). Everything should work out perfectly.
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 ==== Changing the critical remeshing value during the computation ==== ==== Changing the critical remeshing value during the computation ====
  
-It may be possible that we want to change the critical value after a few remeshing. If you recall, this value is defined in the [[#getParameters()|''getParameters()'' function of the script]].+It may be possible that we want to change the critical value after a few remeshing (see for example ''apps.remeshing2.fullAuto.cont2bRemeshing''). If you recall, this value is defined in the [[#getParameters()|''getParameters()'' function of the script]].
  
 However, since the same two files are used to generate all the remeshing, it is not possible to know how far in the computation we are. But to change the critical value, we need to know whether the current integration is the first or the tenth. However, since the same two files are used to generate all the remeshing, it is not possible to know how far in the computation we are. But to change the critical value, we need to know whether the current integration is the first or the tenth.
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 ==== Changing mesh density during the computation ==== ==== Changing mesh density during the computation ====
  
-Actually, the key ''p['integrationNumber']'' can be used to modify any and all parameters during the computation. For example, if we want to double the mesh density after two remeshing, we simply add in the ''getParameters()'' function of the script (after the update !) : +Actually, the key ''p['integrationNumber']'' can be used to modify any and all parameters during the computation (see for example ''apps.remeshing2.fullAuto.cont2bRemeshing''). For example, if we want to double the mesh density after two remeshing, we simply add in the ''getParameters()'' function of the script (after the update !) : 
  
     if p['integrationNumber'] < 3:     if p['integrationNumber'] < 3:
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 Two different remeshing criteria have been implemented yet, one based on a ''[[doc/user/results/courbes_res#valueextractor|ValueExtractor]]'', the other on time. Two different remeshing criteria have been implemented yet, one based on a ''[[doc/user/results/courbes_res#valueextractor|ValueExtractor]]'', the other on time.
  
-Previously, the one relying on a valueExtractor was used in a very simple way : on one side of the part, with the field ''IF_MESH_QUALITY''. But is can be used with any ''[[doc/user/results/courbes_res#valueextractor|ValueExtractor]]'' you want, and on any geometrical entity. For example, let us assume that we want a criterion based on the aspect ratio, which is to be evaluated on two different sides. Then we simply define a skin with these two sides and we have the following :  +Previously, the one relying on a ''valueExtractor'' was used in a very simple way : on one side of the part, with the field ''IF_MESH_QUALITY''. But is can be used with any ''[[doc/user/results/courbes_res#valueextractor|ValueExtractor]]'' you want, and on any geometrical entity. For example, let us assume that we want a criterion based on the aspect ratio, which is to be evaluated on two different sides. Then we simply define a skin with these two sides and we have the following :  
  
     #Stop Criterion     #Stop Criterion
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 But then, there is also a simple criterion based on time, if you want to remesh every few seconds or so. But then, there is also a simple criterion based on time, if you want to remesh every few seconds or so.
-In this case, you must first define a vector containing all the times at which you want to remesh (''p[stopTime]''), then use the key ''p[integrationNumber]'' to set the right remeshing time depending on the integration. See below for the example : +In this case, you must first define a vector containing all the times at which you want to remesh (''p[stopTime]''), then use the key ''p[integrationNumber]'' to set the right remeshing time depending on the integration. See below, or see ''apps.remeshing2.fullAuto.cont2cRemeshing'', for the example : 
  
     def getParameters(_p={}):     def getParameters(_p={}):
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   CopyRemesher(side, p['remeshing'].oldDomain, domain, density).execute()   CopyRemesher(side, p['remeshing'].oldDomain, domain, density).execute()
  
-Therefore, if you have a complex domain, just divide it into the appropriate sides and copy/remesh as you see fit.  +Therefore, if you have a complex domain, just divide it into the appropriate sides and copy/remesh as you see fit. However, when you want to remesh one side, copy another side, and that the two sides have a common edge, then you must first use CopyRemesher, then Gen4remesher, in order ensure that the nodes of the edges are also copied and not regenerated !
- +
-<note> When you want to remesh one side, copy another side, and that the two sides have a common edge, then you must first use CopyRemesher, then Gen4remesher, in order ensure that the nodes of the edges are also copied and not regenerated ! </note>+
  
 ==== Transfinite Meshers ==== ==== Transfinite Meshers ====
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 By default, the transfer of data is done using Philippe's complex "FVTM" routine with all its default parameters (see [[doc:user:meshtransfer:datatransferbetweenmeshes]], in French). By default, the transfer of data is done using Philippe's complex "FVTM" routine with all its default parameters (see [[doc:user:meshtransfer:datatransferbetweenmeshes]], in French).
  
-For the advanced user, changing these parameters is possible (in the ''main()'' function of the [[#File 2 : script|script]])+For the __advanced__ user, changing these parameters is possible (in the ''main()'' function of the [[#File 2 : script|script]])
  
 ==== Changing the transfer method ==== ==== Changing the transfer method ====
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   cell.addOption(INTPT_NB, 5)   cell.addOption(INTPT_NB, 5)
    
-To understand all these options, please look at [[doc:user:meshtransfer:datatransferbetweenmeshes|Philippe's doc]]+To understand all these options, please look at [[doc:user:meshtransfer:datatransferbetweenmeshes|Philippe's doc]].
  
 ====  Changing the fields to transfer ==== ====  Changing the fields to transfer ====
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 $\boldsymbol{F}^{int}=\boldsymbol{F}^{ext}$ $\boldsymbol{F}^{int}=\boldsymbol{F}^{ext}$
  
-After remeshing and data transfer, this equation is no longer true, so we have (where the $\boldsymbol{F}_t$ means force after transfer)+After remeshing and data transfer, this equation is no longer true, so we have (where the index $t$ in $\boldsymbol{F}_t$ means that we consider the value after transfer)
  
 $\boldsymbol{F}^{unbal} = \boldsymbol{F}^{int}_t - \boldsymbol{F}^{ext}_t$ $\boldsymbol{F}^{unbal} = \boldsymbol{F}^{int}_t - \boldsymbol{F}^{ext}_t$
  
-The post-remeshing algorithm is simply a variation of the quasi-static integration scheme where this imbalance is to be solved. First, we simply try to solve the equation as it is. If it works, then the imbalance is corrected and the real integration can start again. If it does not work, then an increasing fraction of the unbalanced forces are applied as external forces, in order to solve the imbalance little by little. +The post-remeshing algorithm is simply a variation of the quasi-static integration scheme where this imbalance is to be solved. First, we simply try to solve the equation as it is. If it works, then the imbalance is corrected and the real integration can start again. If it does not work, then an increasing fraction of the unbalanced forces are applied as external forces, in order to solve the imbalance little by little. Once verified for a given fraction, the the unbalanced forces are recomputed and the algorithm started again, until equilibrium is restored
  
 Mathematically, we are therefore trying to solve for $\boldsymbol{F}^{int}$ the equation :  Mathematically, we are therefore trying to solve for $\boldsymbol{F}^{int}$ the equation : 
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 $$ = (1-\alpha_F) \boldsymbol{F}^{\text{ext}}(t^{n+1}) + \alpha_F \boldsymbol{F}^{\text{ext}}(t^n)$$ $$ = (1-\alpha_F) \boldsymbol{F}^{\text{ext}}(t^{n+1}) + \alpha_F \boldsymbol{F}^{\text{ext}}(t^n)$$
  
-The idea is similar : after remeshing, there is some imbalance and the equation is no longer verified, so we want to apply a balancing algorithm to restore equilibrium before restarting the computation. Since the time is stopped, inertial forces are kept as they are and only internal forces are modified to satisfy equilibrium. In the same way as for a quasi-static simulation, this is done by applying as external forces an increasing fraction of the unbalanced forces. +The idea is similar : after remeshing, there is some imbalance and the equation is no longer verified, so we want to apply a balancing algorithm to restore equilibrium before restarting the computation. Since the time is stopped, inertial forces are kept as they are and only internal forces are modified to satisfy equilibrium. In the same way as for a quasi-static simulation, this is done by applying as external forces an increasing fraction of the unbalanced forces. The difference with respect to the quasi-static algorithm is the expression of the unbalanced forces, which will here also contains the inertial forces and the forces computed at the previous time step.
  
 Since we conserve the inertial forces and correct the internal ones, this post-remeshing algorithm works quite well for quasi-static and low speed dynamic computations. For high speed dynamic computations, it is best not to use it and simply let the simulation continue. Since we conserve the inertial forces and correct the internal ones, this post-remeshing algorithm works quite well for quasi-static and low speed dynamic computations. For high speed dynamic computations, it is best not to use it and simply let the simulation continue.
 +
 +==== Options ====
 +
 +The complete ''setBalancing'' command is actually
 +
 +  setBalancing(executeBalancing, velocitiesExtrapolation=True, maxItes=7, balancingTolerance=0.001)
 +  
 +The first argument has already been described and indicates whether balancing should be done or not. 
 +
 +For the advanced user, a few options can be played with by changing the three of arguments. 
 +
 +First of all, it is possible to deactivate the ''stepPredictor()''. If you do not know what it is, do not touch it. If you do, well sometimes, when transfer errors are significant, the balancing can go better if executed without this predictor, so without extrapolating the next step based on velocities and accelerations. This is fine only in quasi-static, because in dynamic computations velocities and accelerations also intervene in the computation of forces. To do so, simply set the second argument to ''False''. If have not investigated this greatly, but I believe that this could be a sign that the mesh of the computation was not fine enough, so activating this option could hide the real problem. I believe it should be reserved for battery tests, because we want to have tests with rather coarse meshes there. 
 +
 +Second, the number of iterations for each step of the balancing can be changed. By default, seven iterations are tried before increasing the fraction of the unbalanced forces and starting again.
 +
 +Finally, the tolerance can also be changed. The parameter $\alpha$, used to ponder the unbalanced forces, starts at 1 and is progressively divided by 2 when a step fails. Once $\alpha$ becomes smaller than the tolerance, the algo stops itself, returns an error, and the temporal integrationdoes not restart. The default value of 0.001 can be changed if needed. 
 +
 + --- //[[pjoris@ulg.ac.be|Pierre Joris]] 2016/06/13 //
doc/user/remeshing/remeshing.txt · Last modified: 2018/05/04 15:47 by boman