Explicit Dynamics Chapter 8 Analysis Settings

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February 27, 2009Inventory #002665

Chapter 8

Explicit Dynamics: Analysis Settings

ANSYS Explicit Dynamics




Training ManualAnalysis Settings

• Analysis Settings are grouped in six categories

– Step Controls

– Solver Controls

– Damping Controls

– Erosion Controls

– Output Controls

– Analysis Data Management

• Specifies directory where project data will be stored

• End Time is the only required input– All other options have defaults




Training ManualStep Controls• Solve Initiation

– Resume From Cycle• Specifies the cycle (time step) from which to start the Solve.

– Default (cycle 0) is to start at the beginning (time zero).• When resuming a simulation, changes to analysis settings will be

respected where possible. – e.g. you often wish to resume a simulation with an extended End Time.

• Changes to any other features in the model (geometry suppression, connections, loads, etc….) will not be respected.

• List of cycles from which to resume will only be populated if a previous solve has been executed and restart files generated.

• Solve Termination

– Maximum Number of Cycles• Specifies the maximum number of cycles (time increments)

allowed for the simulation. – The simulation will stop once the specified value is reached. – Enter a large number (default) to ensure simulation runs to the End

Time.

– End Time (no default)• Defines the timescale that you want the simulation to run.• Enter a reasonable estimate for this parameter since it controls

the length of time the simulation will take to run.• Used in other pre-processing objects (such as tabular loads) and

also to define the scale for Chart output of results objects.




Training ManualStep Controls

• Maximum Energy Error

– Solution stops if the energy error exceeds the Maximum Energy Error (expressed as a fraction)

Reference Energy = Internal Energy + Kinetic Energy + Hourglass Energy

Current Energy = Internal Energy + Kinetic Energy + Hourglass Energy

Work Done = Work done by constraints

+ Work done by loads

+ Work done by body forces

+ Energy removed from system by element erosion

+ Work done by contact penalty forces

• Reference Energy Cycle

– Defines the cycle at which the solver calculates the reference energy

• Usually the start cycle (default)




Training ManualStep Controls• Example energy conservation graph for model with symmetry plane and erosion




Training ManualStep Controls

• Time Step Controls

– Initial Time Step

• If left on Program Controlled (recommended), the initial time step will be automatically set to ½ the smallest initial element stability time step

– Minimum Time Step

• If the time drops below this value the simulation will stop

• If left on Program Controlled (recommended), the value will be set to 1/10th the Initial Time Step

– Maximum Time Step

• Solver will use the minimum of this value or the computed stability time step

• Program Controlled is recommended

– Time Step Safety Factor

• Safety factor is applied to the computed stability time step– Default (0.9) should work for most simulations




Training ManualStep Controls• Automatic Mass Scaling

– Masses in smaller elements are scaled up to increase the time step used in the simulation

– Additional input is required for this option

• Minimum CFL Time Step– Minimum CFL time step to be obtain with mass scaling

• Should be larger that the observed CFL time step

• Maximum Element Scaling.– Limits the ratio of scaled mass / physical mass that can be applied to

each element in the model.• If this ratio is exceeded, the simulation will stop with an error message.

• Maximum Part Scaling– Limits the ratio of scaled mass / physical mass that can be applied to

an individual body. • If this ratio is exceeded, the simulation will stop with an error message.

• Update Frequency– The frequency (in cycles) that mass scaling is performed.

• A value of zero (default) means mass scaling is only done once, at the start of the simulation

• Caution! Mass scaling introduces additional mass into the system to increase the CFL time step. Introducing too much mass can lead to unphysical result.




Training ManualSolver Controls

• Solve Units– The units the solver will use for the simulation

• For accuracy, only (mm, mg, ms) are allowed

• Different units can be used to set up problems and view results.

• Beam Solution Type– Bending (default, most accurate)– Truss

• Beam Time Step Safety Factor

• Hex Integration Type– Exact (default, most accurate)– 1pt Gauss (faster)

• Shell Sublayers– Used to compute Stress Resultants and Bending

Moments– Default (3) usually provides sufficient accuracy




Training ManualSolver Controls• Shell Shear Correction Factor

• Shell BWC Warp Correction

• Shell Thickness Update– Nodal– Elemental

• Tet Pressure Integration– Average Nodal– Constant

• Shell Inertia Update– Recompute (default, most accurate)– Rotate (faster)

• Density Update– Program Controlled (default, recommended)– Incremental– Total




Training ManualSolver Controls

• Minimum Velocity– Computed nodal velocities below the

Minimum Velocity will be set zero• Eliminates small velocities cause by noise• Default is usually OK

• Maximum Velocity– Computed nodal velocities above the

Maximum Velocity will be set equal to the Maximum Velocity

• Can increase time step by eliminating high velocities that are not influencing the required solution

• Default is very large

• Radius Cutoff– Nodes with the Radius Cutoff of a

symmetry plane will be snapped onto the symmetry plane




Training ManualDamping Controls

• Three types of damping can be applied and controlled for Explicit Dynamic Analyses

– Artificial Viscosity• Introduced to prohibit instabilities developing

from shock formation / propagation

– Hourglass Damping• Introduced to prohibit “hourglass” deformation

modes developing in solid hex elements and quad shell elements

– Static Damping• Applied to allow a static equilibrium solution to

be obtained from an Explicit Dynamic analysis




Training ManualArtificial Viscosity

• Shock discontinuities are generally not stable

• Usually, the diffusion inherent in numerical solutions is sufficient to keep them stable, but solutions can be noisy

• Artificial Viscosity is used to– Ensure stability by smearing shock discontinuities into rapidly varying, but

continuous, transition regions – Reduce noise

• Two terms are used to apply Artificial Viscosity– Quadratic (stabilizes the solution)– Linear (reduces noise)

CQ is the Quadratic Artificial Viscosity coefficientCL is the Linear Artificial Viscosity coefficient

• Both terms impose further restrictions on the time step– Not usually very significant

• Default Values are recommended– Use carefully to avoid over-diffusion of real solution

00

0

2

V

Vfor

V

Vfor

V

VcC

V

VdCq LQ




Training ManualArtificial Viscosity

Quadratic Viscosity ensures stability Linear Viscosity reduces noise




Training ManualHourglass Damping• Two formulations are available for the corrective forces used to

resist hourglass deformation modes of hexahedral elements.

– AUTODYN Standard (default, most efficient)

• Generates hourglass forces proportional to nodal velocity differences.• Often referred to as a viscous formulation.

Where FH is a vector of the hourglass forces at each node of the element, CH is the Viscous Coefficient, ρ is the material density, c is the material sound speed, V is the material volume and is a vector function of element nodal velocities aligned with the hourglass shape vector

– Flanagan Belytschko

• Invariant under rigid body rotation (i.e. hourglass forces sum to zero)• Recommended for simulations in which large rotations of hexahedral

elements are expected.

• Vector function of element nodal velocities is orthogonal to both linear velocity field and rigid body field.

– Viscous Coefficient usually varies between 0.05 and 0.15. The default value is 0.1.

XfcVCF KFHH*3

2

Xf

XfcVCF FBHH*3

2




Training ManualHourglass Damping

• The sum of the hourglass forces applied to an element is normally zero. – Momentum of the system is unaffected by hourglass forces.

• Energy associated with hourglass forces is – stored locally in the specific internal energy of the element – recorded globally over the entire model




Training ManualStatic Damping• Explicit Dynamics is primarily designed for solving transient

dynamic events.

• Using the static damping option, a static equilibrium solution can also be obtained.

– Introduces a damping force proportional to the nodal velocities, aimed to critically damp the lowest mode of oscillation of the static system.

– Solution is computed dynamically until it converges to an equilibrium state.

– Need to judge when the equilibrium state is achieved.

• Value of Static Damping (Rd) for critical damping of the lowest mode of vibration is

where T is the period of the lowest mode of vibration of the system (or close approximation).

– Expect solution to converge to static equilibrium in roughly 3T if critical damping is applied.

– If T is not known accurately, over-estimates it, rather than underestimate it.

– Approximate values of Δt and T can be obtained by first performing a dynamic analysis without static damping.

Rt T

t Td 2

1 2




Training ManualErosion Controls• Erosion is a numerical mechanism for the automatic removal

(deletion) of elements during a simulation.

– Removes very distorted elements before they become inverted (degenerate).

– Ensures time step remains reasonably large.– Ensures solutions can continue to the End Time. – Can be used to allow simulation of material fracture, cutting and

penetration.

• There are three options available to initiate erosion of elements.

• On Geometric Strain

– An element erodes when its Effective (geometric) strain exceeds the Geometric Strain Limit.

• Typical values range from 0.5 to 2.0. The default value of 1.5 can be used in most cases.

– Effective strain is calculated from the principal strain components as

– Custom result EFF_STN can be used to review effective strain.

Defaults

21231

223

212133221

23

22

21 3

3

2 eff




Training ManualErosion Controls

• On Material Failure

– An element erodes immediately upon material failure.• Elements using damage models will erode if the damage value reaches

1.0.

• On Minimum Element Time Step

– An element erodes when its local element time step, multiplied by the time step safety factor falls below the Minimum Element Time Step.

– Custom result TIMESTEP can be used to review local element time steps.

• Erosion options can be used in any combination. – Elements will erode if any of the criteria are met.

• Retain Inertia of Eroded Material

– If this option is selected, and all elements connected to a node in the mesh erode, the inertia of the resulting free node is retained. i.e. the free node continues to transfer momentum in subsequent impacts.

– If not selected, all free nodes are removed from the simulation.

Defaults




Training ManualOutput Controls• Results

– Results files contain data used for the main post-processing operations in Explicit Dynamics (Contour Results, Probe Results, etc…).

– Save Results on• Equally Spaced Time Points (specify Number of points)• Cycles (specify Cycles frequency)• Time (specify Time frequency)

– By default, 20 results files are generated for a Solve which terminates at the specified End Time.

• Restart files

– Restart files contain all information required by the solver to run (or restart) the simulation.

– Save Restart Files on• Equally Spaced Time Points (specify Number of points)• Cycles (specify Cycles frequency)• Time (specify Time frequency)

– By default, 5 restart files are generated for a Solve which terminates at the specified End Time.

Defaults




Training ManualOutput Controls• Result Tracker data

– Result Tracker files contains time history data for probes

– Save Result Tracker Data on• Cycles (specify Cycles frequency)• Time (specify Time frequency)

– By default result tracker data is recorded every cycle. • Frequency may need to be reduced for long running

simulations.

• Solution Output

– Solution Output contains general data for the overall solution (momentum and energy summaries, energy conservation, e.t.c.)

– Save Solution Output Data on• Cycles (specify Cycles frequency)• Time (specify Time frequency)

– By default, solution output data is recorded every 100 cycles.

• Frequency may need to be reduced for long running simulations.

Defaults

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Explicit Dynamics Chapter 8 Analysis Settings

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