Explicit Dynamics Chapter 8 Analysis Settings

Chapter 8 Explicit Dynamics: Analysis Settings

ANSYS Explicit Dynamics

ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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February 27, 2009 Inventory #002665

Explicit Dynamics: Analysis Settings

Analysis Settings

Training Manual

• 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


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Step Controls

Training Manual

• 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. ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Step Controls

Training Manual

• 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)


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Step Controls

Training Manual

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


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Step Controls

Training Manual

• 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


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Step Controls

Training Manual

• 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. ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Solver Controls

Training Manual

• 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 ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Solver Controls

Training Manual

• 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


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Solver Controls

Training Manual

• 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 ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Damping Controls

Training Manual

• 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


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Artificial Viscosity

Training Manual

• 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) 2   V    V  V     q    C Q d    C L c  for 0 V V V        V 0 for 0 V

CQ is the Quadratic Artificial Viscosity coefficient CL 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


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Artificial Viscosity

Training Manual

Quadratic Viscosity ensures stability


Linear Viscosity reduces noise

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Hourglass Damping

Training Manual

• 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.

FH  C H cV

2

3

* f KF X 

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 f X  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.

FH  C H cV

2

3

* f FB X 

• 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. ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Hourglass Damping

Training Manual

• 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


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Static Damping

Training Manual

• 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

Rd 

2 t T 1  2  t T

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. ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Erosion Controls

Training Manual

• Erosion is a numerical mechanism for the automatic removal (deletion) of elements during a simulation.

Defaults

– 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

 eff 







2 2 2 2  1  2  3   1 2  2 3  3 1 3  122  232  312 3



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– Custom result EFF_STN can be used to review effective strain. ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Erosion Controls

Training Manual

• On Material Failure – An element erodes immediately upon material failure.

Defaults

• 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.


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Output Controls

Training Manual

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

Defaults

– 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. ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Output Controls

Training Manual

• Result Tracker data – Result Tracker files contains time history data for probes

Defaults

– 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. ANSYS, Inc. Proprietary © 2009 ANSYS, Inc. All rights reserved.

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Explicit Dynamics Chapter 8 Analysis Settings

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