Maximizing the Eigenfrequency of a Certain Eigenmode

Often it is needed to increase a specific eigenfrequency in the eigenfrequency spectrum belonging to a certain eigenmode. In this case, a specific eigenfrequency is defined using TYPE = DYN_FREQ.

This task shows you how to:

Related Topics
Overview of Eigenfrequency

Context:

When optimizing a specific eigenfrequency, the order of the eigenfrequencies might change during the optimization iterations. Consequently, the eigenfrequencies might have to be tracked during the optimization iterations. This is done by mode tracking. For more information, see Settings for the General Sensitivity-Based Optimization.

By default the modes are not tracked during the optimization. Mode tracking is activated in OPT_PARAM command:

DRESP
 ID_NAME = single_lowest_eigenfrequency_1
 DEF_TYPE = SYSTEM
 TYPE = DYN_FREQ
 LC_SET = MODAL, ALL, 1
END_

OBJ_FUNC
 ID_NAME = maximize_single_eigenfrequency
 DRESP = single_lowest__1
 TARGET = MAX
END_

OPT_PARAM
 ID_NAME = opt_params
 OPTIMIZE = maximize_single_eigenfrequency
 MODETRACKING = ON
 MODENUMBERS = 8
END_

For this example, at least 8 eigenfrequencies should be requested in the finite element input model defined by the user.

Define the OPT_PARAM Command in Tosca Extension for ANSYS® Workbench

  1. In the Mechanical application, select Static Structural > Analysis Settings.

  2. In the appeared dialog, define additional optimization parameters, e.g. Modetracking and Number of Modes.

Define an Objective Function for a Minimization of a Specific Eigenfrequency in Tosca ANSA® environment

  1. To add a new objective term for the optimization, apply OBJ_FUNC > New.

    The OBJFUNC_TERM dialog appears:

  2. Choose TYPE = DYN_FREQ_KREISSEL.

  3. Select the appropriate load case 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 eigenfrequency number, so that 1 corresponds to the smallest eigenfrequency.

Define a Design Response for a Specific Eigenfrequency in Tosca Structure.gui

  1. To create a new Design Response, select Command > DRESP.

    The DRESP dialog appears.

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

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

Define a Design Response for a Specific Eigenfrequency in Tosca Extension for ANSYS® Workbench

Before you begin: Eigenfrequency Design Responses are based on a Modal system that identifies the eigenfrequencies of a model first.

  1. To add a Design Response, select Optimization > Design Response.

  2. In this example, select Eigenfrequency as Category, select DYN_FREQ as Response Type.

  3. As Load Cases select Manual.

  4. To add the needed tabular data, create a new tab.

  5. Choose Modal as Analysis, 1 as Steps and 1 as Substep/Mode as shown in the following figure:

    In this case only the very first eigenfrequency is considered.

  6. Then call the predefined Design Response within an Objective Function: Select Optimization > Objective Function.

  7. As Target select Maximize sum and call the Design Response that contains the eigenfrequency.