# Altair OptiStruct 2021.1 Release Notes

## Highlights

• Aeroelasticity Flutter
• Combined Hardening Material
• MPC TIE for Large Displacement Nonlinear Analysis
• 1D Fluid for Thermal analysis (CAFLUID)
• Forced Convection Heat Transfer analysis for Topology optimization

## New Features

Stiffness, Strength and Stability
Shear and Volumetric test data for viscoelasticity
Shear and Volumetric test data are supported for viscoelasticity (MATVE) and frequency domain viscoelasticity (MATFVE). For both entries, the MODEL field can be set to:
• RTEST: Provides Shear and Volumetric test data for Relaxation.
• CTEST: Provides Shear and Volumetric test data for Creep.
In each case, the SHEAR and/or BULK continuation lines can be used to define the shear and volumetric test data for viscoelasticity. The COMB continuation line is also available to specify both shear and volumetric data together.
TOTALFORCE output
TOTALFORCE is the sum of the applied force and the reaction force at any given grid points. This output is supported for linear and nonlinear static subcase in the .h3d file through the TOTALFORCE output request.
Preloaded normal mode analysis is now supported, even if the preloading subcase is nonlinear static and using initial stress input through INISTRS Subcase/Bulk Data pair.
Hyperfoam
Hyperfoam material type is supported with TYPE=FOAM in the MATHE Bulk Data Entry. Hyperfoam is supported for solid elements and axisymmetry, plane strain elements. Both direct parameter input and curve fitting through test data are supported for both implicit and explicit nonlinear analyses.
Adaptive Penalty for Nonlinear Contact analysis is supported and can be turned on by CONTPRM,TUNESTF,1 (“0” is the default and adaptive penalty is off by default). Adaptive penalty approach will adjust the contact penalty during Newton-Rapson iterations, while keeping the penetration within the user specified value. The maximum allowed penetration is chosen as MAXPNTRL*L, where L is the characteristic edge length (the average edge length on the main surface) of the contact. MAXPNTRL is defined through CONTPRM. Adaptive penalty approach can be tried in case the convergence difficulty is met with default linear penalty.
New convergence criteria
Maximum residual grid point force-based convergence criteria available with NLADAPT,ERRFINF,MAX. This is currently supported for Large Displacement nonlinear static and nonlinear transient analysis. This parameter supports input of two values: PARAM,ERRFINF,MAX,<FTOL>.
If NLADAPT,ERRFINF,MAX is set, then the maximum residual grid point force of any grids should be smaller than FTOL * Maximum grid point force. FTOL is the optional second value allowed for this parameter and the default is FTOL=0.005.
If NLADAPT,ERRFINF is defined, then nonlinear convergence criteria from NLPARM, as well as NLADAPT, ERRFINF should be satisfied.
Temperature-Dependent Hyperelasticity
Temperature-dependent hyperelastic material are available via the new MATTHE Bulk Data Entry. All material models currently supported with MATHE are also supported on the new MATTHE entry which adds temperature-dependency. Currently, only direct parameter input is supported for MATTHE and table input for curve-fitting is not supported.
MODCHG when the elements are attached to MPC or RBODY
MODCHG is now supported when the elements to be changed are attached to MPC or RBODY.
Contact interference
Contact interference fit control is available with CNTITF Bulk and Subcase Entry. When there is an overclosure of contact surface, the contact interference fit is triggered automatically to resolve the overclosure. For this case, the overclosure will be resolved gradually over the subcase.
With CNTITF, the following options are available:
• Contact interference fit can be resolved through a user-defined TABLE (time versus the multiplier to the initial penetration). This option allows the interference fit on different contact interfaces to be resolved at different times or subcases.
• Amount of allowable penetration can be specified.
• CNTITF can be specified for different contact interfaces.
Pressure-overclosure TABLE
Pressure-overclosure relationship can be specified through TABLEG/TABLES1 and defined in PCONT.
Threaded bolts can be defined as part of a CONTACT interface by setting the CLEARANCE field to reference a CLRNC Bulk Data Entry.
Half pitch angle (ALPHA), pitch (PITCH), bolt thread diameter (DMAJOR and DMEAN), the number of starts (NSTART), handedness (HANDED), clearance (CLEARANCE), and the bolt axis (XA/YA/ZA,
XB/YB/ZB) can be defined on the CLRNC Bulk Entry.
Each CLRNC entry is used to define one type of bolt geometry and this geometry can also be applied to multiple contact interfaces, if required. Multiple GSET_i continuation lines can be defined on a single CLRNC entry. This allows modeling multiple bolts with a single threaded bolt geometry. As long as these multiple GSET_i individually contain grids belonging to secondary sides of separate contact interfaces, then all these contact interfaces are applied the same threaded bolt geometry.
Combined Hardening Material
Combining hardening material can be used for analysis with cyclic loading, to capture shakedown, ratcheting effect, and so on. It consists of two nonlinear hardening rules, the nonlinear kinematic (NLKIN) and nonlinear isotropic (NLISO) hardening methods. Generally, the isotropic part is closely related to the von Mises criteria, and the kinematic part is described by the evolution law of back stress.
Combined hardening can be activated by setting HR=6 on the MATS1 Bulk Data. Nonlinear kinematic hardening material data can be specified with the NLKIN continuation line and nonlinear isotropic hardening material can be specified with the NLISO continuation line in the MATS1 Bulk Entry.
For nonlinear kinematic hardening, the input material data can be either:
• Parameter Input (TYPKIN=PARAM): Input of kinematic hardening parameters directly.
• Test Data (TYPKIN=HALFCYCL): Specifies stress – plastic strain curve. Total stress is provided from experiment versus the equivalent plastic strain.
For nonlinear isotropic hardening, the input material data can be either:
• Parameter Input (TYPISO=PARAM): Input of isotropic hardening parameters directly.
• Test Data (TYPISO=TABLE): Specifies stress – plastic strain curve. Isotropic part of yield stress stress is provided from experiment versus the equivalent plastic strain.
Heat Transfer Analysis
1D FLUID (CAFLUID)
CAFLUID Bulk Data Entry is an 1D element with the ability to conduct heat and transmit fluid between its two primary nodes (G1 and G2). Heat flow occurs both due to the conduction within the fluid and the mass transport of fluid.
Property (PAFLUID) of CAFLUID defines the associated material property (MID), hydraulic diameter (D) that defines the flow cross sectional area, the mass flow rate (W) and the type of shape function (SF - linear or exponential).
Convection may be accounted for either with ambient nodes (G3, G4) on the CAFLUID entry or by referencing CAFLUID primary grid(s) on corresponding TAi fields on the CONV Bulk Data Entry associated with CHBDYE surface elements.
The MAT4 Bulk Data can be referenced on the PAFLUID entry to provide fluid material properties.
Aeroelasticity
Flutter Analysis
Aeroelastic flutter is a dynamic instability of a structure associated with the interaction of aerodynamic, elastic, and inertial loads. Flutter analysis of aeroelastic systems involves determining the velocity (and hence Mach Number) of the system and the frequency of oscillation at which the system attains the state of flutter. In this phenomenon, the aerodynamic loads on a flexible body couple with its natural modes of vibration to produce oscillatory motions with increasing amplitude. This may lead to catastrophic structural failure. Therefore, structures exposed to aerodynamic loads must be carefully designed to avoid flutter.
In finite element analysis, the prediction of flutter involves a series of complex eigenvalue solutions. OptiStruct uses the modal approach where the structural-vibration modes in a selected frequency range are used as the degrees of freedom.
There are four different methods for flutter analysis supported in OptiStruct, K, KE, PK, and PKNL methods.
The following entries are relevant for flutter analysis in OptiStruct:
Bulk Data Entry
Description
AERO
Defines flight conditions.
MKAERO1/MKAERO2
Specifies the Mach number and reduced frequency pairs for the explicit computation of the aerodynamic matrix.
FLFACT
Specifies the values of flutter parameters (density ratios, velocities, and reduced frequencies) for flutter analysis.
FLUTTER
Selects the method (K/KE/PK/PKNL) and parameters for flutter analysis. This entry also references the definitions of FLFACT.
EIGC
Selects the complex eigenvalue method for the K method.
EIGRL/EIGRA
Selects how the structural modes are computed and the number of them. The number of structural modes can be changed with LMODES/LFREQ/HFREQ. NVALUE in FLUTTER entry can be used to limit the number of eigenvalues printed in the .flt file.
PARAM, VREF
Used to scale the output velocity: Vout = V/Vref.
Subcase Entry
Description
FMETHOD
References the FLUTTER entry.
CMETHOD
References the EIGC entry, for complex eigenvalue extraction (K method only).
Composite
Strength Ratio output for PCOMPLS
Strength ratio output for PCOMPLS with PARAM,SRCOMPS is now available.
Max Stress Criteria with FT=STRS for PCOMP(G) and PCOMPP
Max stress criteria with FT=STRS is now available for PCOMP(G) and PCOMPP.
Optimization
Convection Topology Optimization with Darcy Flow
Forced Convection Heat Transfer is available via Darcy Flow analysis. Currently, this is supported for Linear steady-state heat transfer analysis only and both optimization and analysis only runs are supported.
Forced convection applications include cooling solutions for Electric motors, Machine tools (casting, forming), Heat Exchangers, HVAC systems, Cooling for electronic devices including PCBs. Additionally, Topology Optimization is available for steady-state heat transfer with Darcy flow analysis.
Topology optimization considers the effect of forced convection for cooling in conjunction with structural steady-state heat transfer analysis. Topology optimization can help optimize cooling channel structure and placement for a wide range of applications.
Boundary conditions are required for both thermal structural and fluid flow analysis. The typical structural thermal boundary conditions are available via the SPC Subcase/Bulk Data.
For flow analysis, there are two options to define the boundary conditions:
Nodal Pressure
Flow analysis is solved in the same subcase as thermal analysis. The SPCP Subcase Entry and SPCP Bulk Data are available to define flow pressure boundary conditions. Both inlet and outlet flow pressures can be defined using the SPCP entry.
Inlet Velocity
Inlet velocity via the INLETVEL Subcase Entry and INLETVEL Bulk Data are alternately available, instead of inlet pressure definition via SPCP entries. The outlet pressure still has to be defined using the SPCP entry.
Material Properties for both structure and fluid for forced convection heat transfer analysis can be defined via the MAT4 Bulk Data Entry. The structural material properties are typically defined using the first line which specify the fields, K (structural conductivity), CP (structural specific heat), RHO (structural density), and H (convection heat transfer coefficient). H is only used for free convection to ambient in the presence of CONV Bulk Data. For the fluid heat transfer properties, the DARCY continuation line can be used to define KAPPA (fluid permeability), MU (fluid dynamic viscosity), K (fluid conductivity), CP (fluid specific heat), and RHO (fluid density).
Darcy Flow analysis and Convection Topology Optimization is supported for shell and solid elements. The DTPL Bulk Data Entry can be used to turn on Topology Optimization.
VERTEXM Free-shape
The following enhancements have been added for VERTEXM free-shape optimization.
• Pattern grouping (1, 2, and 3 plane symmetry are supported)
• GRIDCON with FIXED, VECTOR and PLANAR
MMO for Global Fatigue response
Multi-Model Optimization (MMO) now supports fatigue optimization with global fatigue constraints defined in the DTPL and DSIZE Bulk Data Entries.
General
Modal damping based on mode ID
Modal damping input for each mode ID (instead of frequency with TABDMP1) is available through the newly added TABDMP2 entry.
Bolt Section output for 1D bolt
Section coordinate system and Section resultant force summary are available .out file and .secres file for 1D pretension bolt section. The same results for solid bolt section has already been available in previous releases.
Option to suppress mode output or adjust the printing frequency in .out file
OUTPUT,MODES is now available, so that the normal mode results printing in the .out file can be suppressed (OUTPUT,MODES,NO) or the printing frequency can be specified (OUTPUT,MODES,n).
AVL Excite support
• Structural damping for .exb file
• SET support for .exb file
• PARAM,EXCOUT is now obsolete and disabled
• PARAM,EXCOP2 is set to NO by default
Force output in OPTI format .force file for Normal Modes Analysis
Element force output for normal modes analysis in the .force file is available with OPTI output request.
HDF5
Available enhancements for HDF5 output (.h5 file) are:
• Element force and stress for CBUSH elements
• PSD/RMS and cumulative RMS (Displacement, Velocity, Acceleration, and SPCF)
• Frequency response analysis results (Displacement, Velocity, Acceleration, and SPCF)
• CORD1R and CORD2R support
MEFFMASS
Modal effective mass output with MEFFMASS I/O Option Entry is now available. Additional options available in MEFFMASS compared to PARAM,EFFMASS is that MEFFMASS allows to specify a grid point as reference for the calculation of the rigid body mass matrix. The default is the origin of the basic coordinate system. Also, MEFFMASS has an option to output the results in the units of weight.
PSD/RMS results for beam/bar
Normal, shear and von Mises stress output on each evaluation point of beam/bar is supported for random response analysis in h3d.
CBEAM axial stress in OPTI format output (.strs file)
Axial stress/strain of beam is added at the end of line in .strs file for beam elements.
PART Superelements
• The Bulk Data section in which CID is defined in GRAV/RFORCE entries can now be specified using the MB field. This feature is useful when loading needs to be defined in a fixed coordinate system, regardless of the orientation of the superelement, defined by a partitioned Bulk Data section.
• Original user ID is retained in H3D and punch output files for each part/superelement.
• In the H3D file:
• Each part/superelement has its own component/grid/element pool.
• Component labels are highlighted with its superelement SEID.

For example, if original component name is PSHELL1 in superelement SEID=1, then in HyperView, the component label is displayed as (SE1) PSHELL1.

• In the punch file, SEID is printed in title/label section.
String Label-based Input file definition
Entities can be identified by string labels in their corresponding ID field, in addition to the existing support of integer IDs. While integer-based IDs offer more flexibility when the input file is edited manually, string-based labels offer easier identification of entries in the input file, especially when many entries are defined. There are currently two types in which string-based labels can be used.
Type 1
String labels can be used to identify entries via their corresponding ID field. For example, a string label in the MID field of MAT1 entry can uniquely identify this material entry.
Type 2
Entries defined with string labels as IDs can then be referenced by other entries using their unique string labels. For example, the string label identifier of a MAT1 entry can be specified on the MID field of a PSOLID entry,
Support has been extended to more entities in this release, summarized as follows (only newly supported entities have been listed). *Support was available for certain entries in 2020 release.
Entity Type 1 Support Type 2 Support
Surfaces String labels can be defined in the SRFID field of SURF entries. String labels can be defined in the surface field of the following entries:
• CONTACT and TIE (SSID and MSID fields)
Sets String labels can be defined in the ID field of element and grid-based SET entries. String labels can be defined in the SET ID field of following entries:
• NSM1/NSML1 (element set in ID field)
• RBODY (grid and element set in ID field)
• TIE (GSETID)
Properties String labels can be defined in the PID field of PCONT. String labels can be defined in the PID field of CONTACT.
Tables String labels can be defined in the TID field of TABLES1 entries. String labels can be defined TID of MATS1 entry if it references a TABLES1 entry.
Coordinate systems String labels can be defined in the CID field of all the coordinate system definitions. String labels can be defined in the CID field of RFORCE.
High Performance Computing
Memory option for MPI runs
Memory option such as -minlen, -maxlen, -len, -fixlen are now per-host instead of per-MPI process which was the case until the previous release. -hostmem=no will revert to the per-MPI process memory allocation mechanism.
Example: mpirun -np 4 -fixlen=100
• if all 4 MPI processes are allocated on 1 host, then each process will use -fixlen=100/4=25
• if 2 hosts are used with 2 MPI processes each, then each process will use -fixlen=100/2=50
• if host 1 has 1 MPI process and host 2 has 3 MPI processes, then the process on host 1 will use -fixlen=100 while each process on host 2 will use -fixlen=100/3=33

## Resolved Issues

• Models with frequency-dependent materials previously showed sensitivity in results for repeated runs. That is, the same model running multiple times previously produced different results.
• ROMAX output through PARAM,ROMAX,YES no longer ends with a programming error.
• An MMO job no longer hangs after detecting an element distortion error.
• A plane strain N2S/S2S CONSLI model no longer fails with a programming error.
• H3D file from nonlinear analysis was not written out after the loss of license. Now the .h3d file will be written out, even if the loss of license occurs during the analysis.
• A Modal FRF model with EIGVSAVE/EIGVRETIREVE resulted in ERROR # 3478 in OptiStruct v2021 and v2020.1 while the same model ran in older versions.
• A nonlinear contact model with optimization encountered a programming error in igapst datablock.
• PARAM,AMSE4EFM no longer produces wrong results if there is viscous “B” option on PBUSH, with no value specified in that line (blank line).
• A programming error could occur if MFLUID Bulk Data is defined, but not referenced.
• With DOPTPRM,TOPDISC,YES, optimization restart run showed different density results at initial iteration than the last iteration in the original run.
• The thickness of RBODY influenced nonlinear analysis results, even if the thickness padding is “NONE” for contact.
• When there are multiple VABS cross-sections in a single deck, the VABS-OS run errored out with the ERROR # 5863. This has been fixed in the latest VABS code that is available on the APA download site.