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April 21, 2021

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Ansys Random Vibration (PSD) Analysis

Random Vibration (PSD) Analysis with Ansys Mechanical

The purpose of random vibration analysis is to determine the response of structures to vibration loads that are random in nature. An example would be the response of a sensitive electronic component mounted in a car subjected to the vibration from the engine, pavement roughness, and acoustic pressure.

The frequency content of the time history (spectrum) is captured along with the statistics and used as the load in the random vibration analysis. This spectrum, for historical reasons, is called Power Spectral Density or PSD. In a random vibration analysis, since the input excitations are statistical in nature, so are the output responses such as displacements, stresses, and so on.

ElectroMechanical Transducer Permittivity in ANSYS Mechanical

EM Transducer is a boundary condition commonly used in the simulations of MEMS devices.  Workbench users can download the Piezo & MEMS ACT module from the ANSYS Customer portal to gain access to EM transduce capabilities.  EM Transducer automatically generates TRANS126 elements in Workbench based on the EMTGEN APDL command.

The EM Transducer object uses permittivity of Air as a default and does not allow users to define their own permittivity.  If a user wants to define a custom permittivity, they can create an APDL command object that updates the 7th real constant of the generated TRANS126 elements.  See the sample code below:



epsr=ARG1                                          ! USED TO DEFINE THE RELATIVE PERMITTIVITY
                                                               ! IN THE DETAILS OF THE COMMAND FILE
fini                                                         ! LEAVE THE SOLUTION PROCESSOR
*get,nelems,elem,,count                   ! nelems = NUMBER OF THESE ELEMENTS
elm=0                                                   ! INITIALIZE ELEMENT NUMBER
*do,i,1,nelems elm=elnext(elm)      ! elm = NEXT HIGHEST ELEMENT NUMBER
*get,rnum,elem,elm,attr,real           ! rnum = REAL CONSTANT ID # ASSIGNED TO ELEMENT elm
*get,r7,rcon,rnum,const,7                 ! r7 = VALUE OF 7th REAL CONSTANT IN REAL CONSTANT SET #rnum
r7_new=epsr*r7                                  ! MODIFIED VALUE TO SUBSTITUTE FOR ORIGINAL VALUE
rmod,rnum,7,r7_new                         ! MODIFY THE 7th REAL OF REAL CONSTANT SET ID #rnum


fini                                                          ! LEAVE PREP7
/solu                                                       ! REENTER THE SOLUTION PROCESSOR

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How to Parameterize Everything in ANSYS

kaan-80x80One of the best ways to get value out of your simulation is to do parametric analysis. With very little marginal work a completed model can be parameterized to simulate scenarios limited only by your computational time and resources (have you considered a Parametric Pack for parallel design solves?). With ANSYS R18’s promise of digital exploration, DesignXplorer is now included with all CFD, Mechanical and Multiphysics bundles. Sophisticated parametric exploration, optimization and robustness is now at your fingertips. While ANSYS Workbench and DesignXplorer manage your parameters in a consistent interface, setting up a parameter is different in each of the software tools and not always obvious. This document is intended to be a quick reference of how to do so, letting you get over this initial hurdle to take full advantage of the promise of Digital Exploration.

Table of Contents (for quick jumping around):


SpaceClaim Direct Modeler

SpaceClaim let’s you create a parameter from almost any operation:

Param - SpaceClaim Simple

Your parameters show up in the Groups tab.

For more advanced driving dimensions you need to have a dimension on an annotation plane:

Param - SpaceClaim Advanced



DesignModeler has the familiar checkbox to “promote” parameters out to Workbench:

Param - DesignModelerYou can create relations between promoted parameters and other dimensions in the model with the Parameters Pane.

CAD Systems

With the appropriate associative interface licensed and configured, you can make Workbench aware of CAD parameters.

Param - CADBe sure to either use the DS or ANS prefix or else ANSYS will ignore your parameter. You can modify this with the Parameter Key property shown above.


In Mechanical, anywhere you see the checkbox can be promoted to a parameter for Workbench to use:

Param - MechanicalMechanical Command Snippets

APDL command snippets used in Mechanical can also be parameterized, both as input parameters:

Param - Mechanical APDL In

and output parameters (any variable with the Output Search Prefix will be retrieved):

Param - Mechanical APDL OutBe aware that APDL command snippets are not units aware!

Mechanical APDL (ANSYS Classic)

You can also easily use script files from the older user interface in Workbench, easily. With the Mechanical APDL component, specify the input file (along with supporting files as a reference file) and it will be parsed for all of it’s variables. All that’s left is to specify what’s an output and what’s an input:

Param - MAPDL


In Fluent, most places that you can enter a value will have a dropdown that allows you to specify a parameter instead:

Param - Fluent


In CFX-Pre you can specify expressions as parameters and use them as inputs in other parts of the model:

Param - CFX


In CFD-Post you base parameters off of expressions as well, making sure to use the nice right click menu to help with building expressions:

Param - CFD-Post

Maxwell & HFSS

The user interface is similar enough in these tools so that the same instructions apply. When accessing these from Workbench, a DesignXplorer node is created under the Optimetrics portion of the tree. Optimetrics is an EMAG specific optimization tool that is complimentary to DesignXplorer. In most places in Maxwell and HFSS, enter an identifier instead of a number to automatically create a parameter. Promote it out to Workbench in the DesignXplorer node. In the DesignXplorer node, output variables can be created in the Calculate tab.

Param - Maxwell HFSS


Similar to the above, in the user interface where you would normally enter a number, instead enter an identifier preceded by $. Then in the Define Trials dialog, expose the parameter to Workbench.

Param - Icepak

And that’s it! Remember that the power of an integrating platform like Workbench allows you to have several of these software tools be connected in the same analysis flow. From CAD Parameter interaction to coupled field analysis, it’s all possible. Now go forth and digitally explore your design space!

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