5. Keyword ADAPTATION#
Adapting the time step consists in calculating a new time step \(\Delta {t}^{i+1}\), based on the information from the current time step \(\Delta {t}^{i}\) (and the previous time steps \(\Delta {t}^{i-1}\), \(\Delta {t}^{i-2}\),…).
So the aim is to calculate \(\Delta {t}^{i+1} = c \cdot \Delta {t}^{i}\), where \(c\) is a real coefficient.
The process is as follows:
For the \(k\) occurrence of the ADAPTATION keyword, an event and a method of calculation of \(\Delta {t}^{i+1}\) are defined. If the event is verified, then \({c}^{k}\) is calculated using the chosen calculation method.
A summary table of this type is obtained:
Then, we choose \(c=min({c}^{k})\) among the occurrences whose event is verified. In this example, the new time step will be worth \(\Delta {t}^{i+1} = 1.7 \cdot \Delta {t}^{i}\).
To use MODE_CALCUL_TPLUS =” IMPLEX “, this keyword must only be used once.
5.1. Operand EVENEMENT#
This operand allows you to specify the trigger criterion for adapting the time step.
If EVENEMENT =” SEUIL “, then the event is only verified if the threshold is crossed.
If EVENEMENT =” TOUT_INST “, then the event is checked at all times; this case is mandatory with the IMPLEX method.
If EVENEMENT =” AUCUN “, then the event is never verified.
5.2. Operands NB_INCR_SEUIL, NOM_PARA, CRIT_COMP, VALE_I#
These operands should only be filled in the event of a threshold event (EVENEMENT =” SEUIL “). The event is verified if we have a « green light » NB_INCR_SEUIL times in a row, a green light being characterized by the condition defined by NOM_PARA, CRIT_COMP and VALE_I.
For example if NB_INCR_SEUIL =2, NOM_PARA =” NB_ITER_NEWTON “, CRIT_COMP =”LE”, and VALE_I =5, then the event is true if we have done 2 times in a row less than 5 Newton iterations.
- note
There is no default value for VALE_I in the DEFI_LIST_INST operator catalog. However, if its value is not entered, we will take VALE_I to be half of Newton’s maximum number of iterations (declared in STAT_NON_LINE/CONVERGENCE/ITER_GLOB_MAXI).
5.3. Operand MODE_CALCUL_TPLUS#
This operand is used to specify how to calculate the new time step (in fact, the \(c\) coefficient).
If MODE_CALCUL_TPLUS =” FIXE “, then the coefficient \(c\) is fixed. Its value is determined using the PCENT_AUGM operand. The new step of time will then be worth \(\Delta {t}^{i+1} = (1+\frac{\text{PCENT_AUGM}}{100.})\Delta {t}^{i}\).
If MODE_CALCUL_TPLUS =” DELTA_GRANDEUR “, then the coefficient \(c\) is linked to the variation over the time step of a selected quantity. The following formula is used:
with \(N\) the set of knots or Gauss points for which \(\Delta {V}_{i}\) is not zero. The reference value \({V}_{ref}\) is chosen by the user.
If field \(\Delta V\) is identically zero, then it is considered that the criterion (the event) is not verified. By default, all component values are examined. However, you can filter the position using the keywords GROUP_MA (for fields SIEF_ELGA and VARI_ELGA) or GROUP_NO (for field DEPL). In this case, the evaluation only relates to the list of mesh groups or the list of groups of nodes.
If MODE_CALCUL_TPLUS =” ITER_NEWTON “, then the coefficient \(c\) is related to the variation over the time step in the number of iterations of Newton. The following formula is used:
where \({N}_{it}\) is the number of iterations of Newton performed and \({N}_{ref}\) is a reference value chosen by the user. We recall that the prediction is considered to be Newton’s iteration No. 0, which means that if we converge from the prediction then \(c=\sqrt{{N}_{ref}}\).
If MODE_CALCUL_TPLUS =” IMPLEX “, then the coefficient \(c\) is linked to the increments of internal variables (cumulative plastic deformation or damage) of the previous step (see [R5.03.81]). In this case, the ADAPTATION keyword should only be used once. The coefficient \(c\) will necessarily be between 0.5 and 1.2. Moreover, by definition of method IMPLEX there will be no cause for failure.
5.4. Operand PCENT_AUGM#
This operand is used to specify the percentage increase in the time step if MODE_CALCUL_TPLUS = “FIXE”. By default, we increase by 100%, that is to say we double the value of the time step. The value may be negative and must be strictly greater than -100.
5.5. Operands VALE_REF, NOM_CHAM, NOM_CMP#
These operands are used to specify the calculation of the coefficient \(c\) for increasing the time step if MODE_CALCUL_TPLUS =” DELTA_GRANDEUR “(cf. Event “DELTA_GRANDEUR”) with VALE_REF the reference value \({V}_{ref}\) and NOM_CHAM the name of the magnitude field in question.
5.6. Operand NB_ITER_NEWTON_REF#
This operand is used to fill in the reference number of iterations of Newton, noted \({N}_{ref}\) (cf. Operand MODE_CALCUL_TPLUS) in the case where MODE_CALCUL_TPLUS = “ITER_NEWTON”.
6. Operand INFO#
For the moment, this operand is not used.