Selective Subcycling

Selective subcycling:

  • allows different time increments to be used for different groups of elements;

  • reduces run time for an analysis when a small region of elements in the model controls the stable time increment; and

  • is invoked by defining the subcycling zones.

This page discusses:

See Also
Explicit Dynamic Analysis
Fully Coupled Thermal-Stress Analysis
In Other Guides
*SUBCYCLING

Products Abaqus/Explicit

Introduction

The selective subcycling method in Abaqus/Explicit is based on domain decomposition. In this method subcycling zones are defined that remain unchanged during the analysis. The domain-level parallelization method (Parallel Execution in Abaqus/Explicit) is invoked automatically when subcycling zones are defined. Each subcycling zone, as well as the nonsubcycling zone, is independently decomposed into the user-specified number of parallel domains. The “nonsubcycling” domains are defined as the parallel domains that are derived from the nonsubcycling zone and are integrated with the largest stable time increment. The remaining parallel domains derived from the subcycling zones are integrated using smaller time increments, or “subcycles.”

The subcycle time increment sizes are chosen as integer divisors of the time increment used in the nonsubcycling parallel domains. Therefore, all parallel domains exactly reach the same time points as the nonsubcycling parallel domains. During subcycling, nodes that lie on the interface with the nonsubcycling zone require special treatment. The velocity at the interface nodes is taken from the nonsubcycling zone and is constant during subcycles. This produces an interface node displacement field that varies linearly during the subcycles.

Defining Subcycling Zones

Subcycling zones are defined by element sets. You can include all element types in these sets except EC3D8R, EC3D8RT, C3D10, C3D10T, PD3D, and PC3D elements. However, all parallel domains must have at least one deformable element to provide the stable time increment. Abaqus/Explicit issues an error message if there is no deformable element in a parallel domain. You can define an arbitrary number of subcycling zones. However, some modeling features cannot be split between subcycling zones. Abaqus/Explicit automatically merges subcycling zones that contain features that cannot be split. Subcycling zones are merged together when:

  • the zones overlap;

  • the zones share the same nodes;

  • a node is in one subcycling zone, but its adjacent nodes are in a different subcycling zone;

  • subcycling zones are involved in the same constraint equation, connector, or rigid body; or

  • general contact is specified in the analysis.

When subcycling zones are merged, the smallest stable time increment among the merged zones is used. The constraint, connector, or rigid body is always assigned to the subcycling zone if any one of its nodes is involved in that subcycling zone. Since the domain-level parallelization method is used, all restrictions on parallel domain decomposition apply to subcycling zones. These restrictions prevent certain features from being split across nonsubcycling parallel domains, as well as parallel domains that contain the subcycling zones (See Parallel Execution in Abaqus/Explicit). Beam elements with exact rotary inertia cannot share nodes between subcycling and nonsubcycling zones. Analytical rigid surfaces cannot be included in the general contact domain when a subcycling zone is defined.

Efficient selective subcycling requires proper choice of subcycling zones. For each subcycling zone, the time increment size should be small compared to the nonsubcycling zone, producing a large number of subcycles. The number of subcycles is the ratio of the stable time increment size in the nonsubcycling zone to the stable time increment sizes in the subcycling zones. In addition to a large number of subcycles, the number of elements in a subcycling zone should generally be small compared to the total number of elements in the model for optimal performance benefit. If a majority of elements in the model are in subcycling zones, there will not be much performance benefit.

Input File Usage

Use the following option to define a subcycling zone:

SUBCYCLING, ELSET=element_set_name

Accuracy of Results

The subcycling algorithm used in Abaqus/Explicit provides sufficient accuracy for most complex dynamic models. However, because of the relatively large time increment size used in the nonsubcycling zone and the interpolation used on zone interface nodes, subcycling solutions can introduce a truncation error, which may slightly alter results compared with traditional solutions. This error should not affect the overall dynamic behavior of the model. Special attention should be given to the interface between the subcycling zone and nonsubcycling zone when general contact (see About General Contact in Abaqus/Explicit) is involved. It is not necessary to define a pair of surfaces that have the potential for contacting each other within the same zone. However, to minimize truncation errors, it is highly recommended that a single surface that has the potential for contacting others not be split across the zones.

Output and Mass Scaling

Output (see About Output) and mass scaling (see Mass Scaling) are always performed at the same time points reached by all parallel domains.