/DT/AMS

Engine Keyword Time step control for Advanced Mass Scaling. Advanced Mass Scaling is an elementary time step method that increases the time step to a higher value than the usual elementary or nodal time step.

Format

/DT/AMS/Iflag

Δ T sca Δ T min

If Iflag=1 or 2, insert the next line

Tol_AMS

If Iflag=2, insert the next line

Niter Nprint

Definitions

Data Description SI Unit Example
Iflag Number of additional (optional) lines.
= 0 or blank
Only one card for Δ T sca and Δ T min is read.
= 1 or 2
Tol_AMS is read from input.
= 2
Tol_AMS, Niter, and Nprint are read from input.
 
Δ T sca Scale factor on critical time step.

It is recommended to set Δ T sca to 0.67.

Default = 0.9

 
Δ T min Minimum time step. 2  
Tol_AMS Tolerance for AMS convergence.

Default = 10-3

 
Niter Maximum number of iterations in conjugate gradient.

Default = 1000

 
Nprint Frequency (number of cycles) for writing additional output about the number of iterations before convergence in the conjugate gradient.

Default = 0

 

Comments

  1. To apply Advanced Mass Scaling, the Engine keyword /DT/AMS and Starter keyword /AMS must be defined.
  2. The AMS time step method is applied on an element when the time step of an element becomes less than:

    Δ T min Δ T s c a MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbwvMCKf MBHbqefqvATv2CG4uz3bIuV1wyUbqedmvETj2BSbqefm0B1jxALjhi ov2DaebbnrfifHhDYfgasaacH8akY=xipgYlh9vqqj=hEeei0xXdbb a9frFf0=yqFf0dbba91qpepeI8k8fiI+fsY=rqaqpepae9pg0Firpe pesP0xe9Fve9Fve9qapdbaGaaiGadiWaamaaceGaaqaacaqbaaGcba WaaSaaaeaacqqHuoarcaWGubWaaSbaaSqaaiGac2gacaGGPbGaaiOB aaqabaaakeaacqqHuoarcaWGubWaaSbaaSqaaiaadohacaWGJbGaam yyaaqabaaaaaaa@445E@

  3. If Iflag=0, the default values for Tol_AMS, Niter, and Nprint are used.
  4. At each cycle, AMS solves iteratively the nodal accelerations [M] γ = F using a conjugate gradient algorithm (Mass matrix is not diagonal anymore when using AMS).
  5. Convergence of the conjugate gradient is assumed when:(1)
    F [M] γ Tol _ AMS F
  6. If Iflag = 2, the maximum number of iterations in conjugate gradient Niter, and the frequency for additional output Nprint, must be entered. Otherwise, the maximum number of iterations in conjugate gradient is set to its default value (1000), and no additional output is provided.
  7. If more than Niter iterations have been performed before convergence of the conjugate gradient, the following error message is output and the computation stops:
    "** ERROR: AMS IS LIKELY DIVERGING"
  8. If Nprint is specified, then at each Nprint cycle an additional output is provided including: the number of iterations before convergence of the conjugate gradient at this cycle, the final residual norm, and the force vector norm.
  9. It is possible to apply Advanced Mass Scaling to a group of parts. The part group must then be specified in Radioss Starter Input deck (/AMS). For optimized computational performance, it is recommended to apply a classical mass scaling, /DT/NODA/CST to the parts not belonging to /AMS part group.
    Example:
    /DT/AMS/1
    
                                            
                                                
                                                    Δ
                                                    
                                                      T
                                                      
                                                      s
                                                      c
                                                      a
                                                      
                                                    
                                                    
                                                    Δ
                                                    
                                                      T
                                                      
                                                      min
                                                      
                                                    
                                                
                                                MathType@MTEF@5@5@+=
                                                    feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbwvMCKf
                                                    MBHbqefqvATv2CG4uz3bIuV1wyUbqedmvETj2BSbqefm0B1jxALjhi
                                                    ov2DaebbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY=Hhbbf9v8
                                                    qqaqFr0xc9pk0xbba9q8WqFfea0=yr0RYxir=Jbba9q8aq0=yq=He9
                                                    q8qqQ8frFve9Fve9Ff0dmeaacaGacmGadaWaaiqacaabaiaafaaake
                                                    aacaqGuoGaamivamaaBaaaleaacaWGZbGaam4yaiaadggaaeqaaOGa
                                                    aGjbVlaabs5acaWGubWaaSbaaSqaaiGac2gacaGGPbGaaiOBaaqaba
                                                    aaaa@4444@ 
                                            
                                        
    
    Tol_AMS
    /DT/NODA/CST
    
                                            
                                                
                                                    Δ
                                                    
                                                      T
                                                      
                                                      s
                                                      c
                                                      a
                                                      
                                                    
                                                    
                                                    Δ
                                                    
                                                      T
                                                      
                                                      min
                                                      
                                                    
                                                
                                                MathType@MTEF@5@5@+=
                                                    feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbwvMCKf
                                                    MBHbqefqvATv2CG4uz3bIuV1wyUbqedmvETj2BSbqefm0B1jxALjhi
                                                    ov2DaebbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY=Hhbbf9v8
                                                    qqaqFr0xc9pk0xbba9q8WqFfea0=yr0RYxir=Jbba9q8aq0=yq=He9
                                                    q8qqQ8frFve9Fve9Ff0dmeaacaGacmGadaWaaiqacaabaiaafaaake
                                                    aacaqGuoGaamivamaaBaaaleaacaWGZbGaam4yaiaadggaaeqaaOGa
                                                    aGjbVlaabs5acaWGubWaaSbaaSqaaiGac2gacaGGPbGaaiOBaaqaba
                                                    aaaa@4444@ 
                                            
                                        

    If /DT/AMS and /DT/NODA/CST are used together, /DT/NODA/CST is only applied to parts not defined with AMS using the part group in /AMS. It is also recommended to use the same Δ T min , because the simulation will always use the minimum time step. If no part group is specified on /AMS, then AMS applied to the whole model even if /DT/NODA/CST.

  10. Advanced Mass Scaling (AMS) does not modify the global mass, so that the global momentum of the related nodes is conserved. It is, therefore, more accurate than /DT/NODA/CST.
  11. /DT/Eltyp/Keyword3 is compatible with /DT/AMS (except /DT/INTER/CST, that only applies to the nodes where /DT/AMS does not apply.
  12. When using /AMS with nonlinear stiffness contact interfaces TYPE7, TYPE11 or TYPE19, it may be necessary to use /DT/INTER/DEL in the Radioss Engine input deck; otherwise, AMS may converge slowly, or may even diverge.
  13. For information about limitations related to /DT/AMS, refer to Capabilities and Limitations in Advanced Mass Scaling (AMS) in the User Guide.