Chiral Dynamics with Explicit Symmetry Breaking Interactions
Explicit Dynamics Chapter 8 Analysis Settings
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Transcript of Explicit Dynamics Chapter 8 Analysis Settings
1-1ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
February 27, 2009Inventory #002665
Chapter 8
Explicit Dynamics: Analysis Settings
ANSYS Explicit Dynamics
Explicit Dynamics: Analysis Settings
1-2ANSYS, Inc. Proprietary© 2009 ANSYS, Inc. All rights reserved.
February 27, 2009Inventory #002665
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
Explicit Dynamics: Analysis Settings
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February 27, 2009Inventory #002665
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.
Explicit Dynamics: Analysis Settings
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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)
Explicit Dynamics: Analysis Settings
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Training ManualStep Controls• Example energy conservation graph for model with symmetry plane and erosion
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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.
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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Training ManualArtificial Viscosity
Quadratic Viscosity ensures stability Linear Viscosity reduces noise
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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
Explicit Dynamics: Analysis Settings
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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