Maximizing the First Eigenfrequencies

You can increase several lowest eigenfrequencies.

Note: It is important to consider more than the first natural eigenfrequency when increasing the natural frequencies using optimization. At least the next two first natural frequencies should be considered in the optimization.

This task shows you how to:

Before you begin:
  • All natural eigenfrequencies requested in the FE model are applied in the optimization if ALL option is applied in the LC_SET parameter.
  • During the optimization, the various natural frequencies are automatically weighted by their distance from the lowest natural frequency, i.e., when the other natural frequencies approach the first natural frequency during the optimization, the more they will be weighted. Generally, the first natural frequency is always maximized.
  • The design response is defined using the Kreisselmaier-Steinhauser formulation. For more information, see Overview of Eigenfrequency.
  • Any number of natural frequencies in the design response can be specified using the DRESP command.
Related Topics
Overview of Eigenfrequency
  • In this example, all calculated natural frequencies are considered for the objective function using the Kreisselmaier-Steinhauser formulation and are defined as follows:

    DRESP
 ID_NAME = all_lowest_eigenfrequencies
 DEF_TYPE = SYSTEM
 TYPE = DYN_FREQ_KREISSEL
 LC_SET = MODAL, ALL, ALL
END_
OBJ_FUNC
 ID_NAME = maximize_eigenfrequencies
 DRESP = all_lowest_eigenfrequencies
 TARGET = MAX
END_

    If one has requested 10 eigenfrequencies in the finite element input model but only the first 5 eigenfrequencies are to be used in the optimization definition, then the design response for 5 eigenfrequencies is defined as follows:

    DRESP
 ID_NAME = all_lowest_eigenfrequencies_1_5
 DEF_TYPE = SYSTEM
 TYPE = DYN_FREQ_KREISSEL
 LC_SET = MODAL, ALL, 1-5
END_
OBJ_FUNC
 ID_NAME = maximize_eigenfrequencies_1_5
 DRESP = all_lowest_eigenfrequencies_1_5
 TARGET = MAX
END_

Define an Eigenfrequency Objective Function in Tosca ANSA® environment

  1. To add a new objective term for the eigenfrequency optimization, select New > EIGENFREQUENCY command on OBJ_FUNC item.

    The OBJ_FUNC_TERM dialog appears:

  2. In this dialog, choose TYPE = DYN_FREQ_KREISSEL.

  3. Select the appropriate load cases in LC_SET field either directly or by pressing "?" key in LC_SET field and using the window that appears.

    Note: The 3rd parameter is the list of required eigenfrequencies, starting with 1 for the lowest one.

Define an Eigenfrequency Design Response in Tosca Structure.gui

  1. Create a new Design Response.

  2. Set DefType = System, Category = Base and Type = DYN_FREQ_KREISSEL.

  3. Choose the load cases by pressing Add LC button and setting Analysis Type to MODAL, entering the load case ID and the numbers of eigenmodes (e.g., 1-5) in Eigenmode/Subcase field.