FastFRS (Fast Frequency Response Solver)

A solver developed by the University of Texas at Austin. It is very efficient for a certain class of large modal frequency response problems, such as NVH problems. OptiStruct has an interface to FastFRS.

OptiStruct writes the file FastFRS_gen.in as input for FastFRS, and reads results from FastFRS_gen.out. FastFRS will run in the directory specified by the environment variable AMLSDIR, or the current directory if AMLSDIR is not specified. FastFRS looks for the input and output files in the directory specified by AMLSDIR.

The following parameters can be used within OptiStruct to control the FastFRS solver.
  1. Set the environment variable FASTFRS_EXE to point to the location of the FastFRS executable.
  2. The run option -ffrs yes or the parameter PARAM,FFRS,YES can be used to activate FastFRS.
  3. Add the following optional parameters to adjust the settings for FastFRS runs:
    PARAM,FFRSLFRQ
PARAM, K4METH (see Notes 4 through 7)
PARAM,LOWRANK (This parameter is obsolete. Use PARAM, K4METH, see Notes 4 through 7.)
PARAM,K4CUTOFF
PARAM,CSTOL
PARAM,FFRSNCPU (or the run option –ffrsncpu)
PARAM,FFRSMEM (or the run option –ffrsmem or the environment variable FFRS_MEM)
Note:
  1. OptiStruct version 13.0 or above is required to run FastFRS version 2 or above.
  2. If FFRSNCPU is not set (using either the parameter or the run option), and AMLSNCPU is set, then FastFRS will use the number of CPU's specified by AMLSNCPU.
  3. For FastFRS versions 2 and above, use the run option -ffrsmem, the environment variable FFRS_MEM, or the parameter PARAM, FFRSMEM to set the amount of memory in Gigabytes used by FastFRS. By default, FastFRS will use the same amount of memory used by OptiStruct. The run option overrides the environment variable and the parameter. If both FFRS_MEM and PARAM, FFRSMEM are set, the value specified by the environment variable is used. The minimum memory value allowed is equal to 1 GB. If a value lower than 1 GB is specified, it is automatically reset to 1 GB.
  4. If the value of LOWRANK is 0, FastFRS computes a full eigensolution using the data found in the diagonal stiffness and structural damping data blocks.
    If the value of LOWRANK is -1, a low rank representation of the matrix found in structural damping is used. A special case of this option occurs when acoustic fluid is present in the model and the matrices found in the fluid mass matrix, fluid stiffness matrix, and fluid viscous damping matrix data blocks are all diagonal. FastFRS takes advantage of this special case when the value of LOWRANK is set to 1 by treating the matrix found in the structural damping data block and the fluid matrices as low rank representations. For optimization problems, the mass and stiffness matrices are full and a low rank representation cannot be used.
    • If both K4METH and LOWRANK are not specified, LOWRANK would be determined based on the material damping matrix and number of fluid modes.
    • If LOWRANK is specified, but K4METH is not specified, the value of LOWRANK would be used.
    • If K4METH is specified, LOWRANK would be set according to K4METH.
  5. FASTFR is supported for Component Dynamic Synthesis solutions.
Attention: The Fast Frequency Response Solver cannot be used (and the model is run using the standard solution method) for Modal Frequency Response Analysis, if:
  1. The number of viscous damping degrees-of-freedom is greater than the number of modes.
  2. The model includes poroelastic material, frequency-dependent material (MATFi), frequency-dependent bushing (via PBUSHT), or frequency-dependent dampers (via CAABSF/PAABSF).
  3. Mode selection via MODESELECT I/O Option Entry, PARAM, LFREQ, PARAM, HFREQ, PARAM, LFREQFL, and PARAM, HFREQFL is active.
  4. The Component Dynamic Synthesis Method (CDSMETH) is used.
  5. Rotor dynamics solution is active.