Modal Forces
A modal force, or MFORCE, allows you to distribute a force to one or more, or all nodes of a flexible body. The force can vary in time or position and can even be made dependent on a state variable. Examples of modal force applications are pressures on journal bearings, simulating magnetically induced fields, or the modeling of airfoil flutter. Modal forces are a special class of forces called distributed loads that can only be applied to flexible bodies.
For a detailed overview of distributed loads and a tutorial that steps you through an example of adding modal forces to your model, see Modeling Distributed Loads and Predeformed Flexible Bodies.
Adams View provides three options for defining MFORCEs on flexible bodies. All options require additional work outside of Adams View to complete the definition and simulation of modal forces.
■Reference and scale a load case defined in the flexible body's modal load matrix. This option can only be used in Adams View on flexible bodies that have been built with Modal Neutral File (MNF) that contains modal load case information. For more information on flexible bodies, their modal load matrix, and how to generate modal load case information in an MNF, see Creating Loadcase Files.
■Specify the modal force as a product of a modal load case and scale function defined in a User-written subroutine. The scale function can depend on time or the state of the system. The load case can only be a function of time.
■Directly specify the components of a modal force in a User-written subroutine. Each component can depend on time or the state of the system. This option is only available in Adams Solver (C++).
The last two options provide much more capability in defining modal forces. To take advantage of these options, however, you need to develop a MFOSUB routine that is built into the Adams Solver. For more information, see the Subroutines section of the Adams Solver online help.
More than one modal force can be defined on a flexible body. For each modal force defined on a flexible body a modal force icon appears at its local part reference frame. You can transfer modal forces from one flexible body to another.
Learn more:
Creating Modal Forces
To create a modal force:
The Create/Modify Modal Force dialog box appears.
2. In the Create Modal Force dialog box, specify the following:
Options in Create Modal Force Dialog Box
To: | Do the following: |
|---|---|
Assign a name to the MFORCE | In the Force Name text box, enter the name of the modal force to be created. Adams View automatically assigns a default name of MFORCE followed by an underscore and a number to make the name unique (for example, MFORCE_1). |
Specify the flexible body to which the MFORCE is applied | In the Flexible Body text box, enter the name of the flexible body. Tips on Entering Object Names in Text Boxes. |
Apply the reaction of the modal force resultant to a part | If desired, in the Reaction Part text box, enter the name of an existing part. If you enter a part name, Adams View automatically creates a Floating marker associated with this part when it creates the MFORCE. Adams View keeps the marker coincident with the flexible body analysis coordinate system during the simulation. Therefore, the need for the point of reaction to be a floating marker. In addition, because floating markers cannot be defined on flexible bodies, the reaction part is restricted to rigid bodies only. Note: You can use the Info command to see the floating marker that Adams View creates when you reference a reaction part. Learn about Displaying Object Information and Accessing Information Window. |
Select how you want to define the modal force. | Select the following from Define Using: ■Function - Lets you select the modal load case and scale function of the MFORCE. Note that you cannot select Function when defining an MFORCE on a flexible body that does not contain any modal load case information in its corresponding MNF. ■Subroutine - Lets you specify up to thirty user-defined constants to be passed to the user-defined subroutine, MFOSUB to directly compute the modal load case and scale function whose product is the modal force applied to the flexible body. The scale function can depend on time or the state of the system. The load case can only be a function of time. ■Force - Lets you specify up to thirty user-defined constants to be passed to the user-defined subroutine, MFOSUB to directly compute the modal force on the flexible body. Each component of the modal force can depend on time or the state of the system. (Adams Solver (C++) only. Learn about switching solvers with Solver Settings - Executable dialog box help.) To use a subroutine, you need to build a version of the Adams Solver that contains your version of the MFOSUB routine that quantifies the modal force. For more information, see the Subroutines section of the Adams Solver online help. You can also specify an alternative library and name for the user subroutine in the Routine text box. Learn about specifying your own routine with ROUTINE Argument. |
3. If you select to specify a flexible body with modal load case information, you also specify:
♦Load Case - Lets you select a modal load case label from a list. The list of modal load case labels is generated from the MNF. Learn about Creating Loadcase Files.
♦Scale Function - Lets you specify an expression for the scale factor to be applied to the modal load case.
4. Select OK.
Modifying Modal Forces
You can modify an existing MFORCE in the following ways:
■The flexible body to which the modal forces is applied.
■The part to which the reaction resultant of the modal force is applied.
■The definition of the modal force.
To modify a MFORCE:
2. Follow the instructions in the dialog box help.
3. Select OK.
Copying and Deleting a Modal Force
You can copy and delete MFORCEs just like you copy and delete other objects in Adams View. See Copying Objects and Deleting Objects.
Note: | When you copy a MFORCE that has a reaction part specified or as a result, a Floating marker referenced, Adams View also creates a new floating marker. In addition, when you delete a MFORCE that has a reaction part specified, Adams View does not delete its referenced floating marker. |
Viewing Modal Preloads of Flexible Bodies
A special form of a modal load in a flexible body is a modal preload. Since modal preloads are an integral property of the flexible body, you do not have the ability to modify these loads in Adams View. You can, however, inspect the values of these preloads for each mode. In Adams View, there are two ways to review the modal preloads of a flexible body.
For a detailed overview of modal preloads and a tutorial that steps you through an example of modeling preloads, see Modeling Distributed Loads and Predeformed Flexible Bodies.
To review the modal preloads using the Flexible Body Modify dialog box:
2. From the Flexible Body Modify dialog box, select Modal ICs.
The Modify Modal ICs... dialog box appears. Preloads for the flexible body appear in the last column.
3. Review the preloads, and then select Close.
To obtain a listing of the preloads using the Info command:
1. Display information on the flexible body as explained in Displaying Object Information and Accessing Information Window.
The modal preload values appear in the last column of the modal frequency table.
Viewing Modal Forces
You can review modal forces on flexible bodies in Adams PostProcessor as:
■Curves
No matter what form, the modal force results are presented with respect to the flexible body’s local part reference frame. This is unlike most other Adams force elements that are plotted with respect to the ground coordinate system, by default. For a detailed overview of modal forces and a tutorial that steps you through an example of creating a modal force, see Modeling Distributed Loads and Predeformed Flexible Bodies.
Note: | ■To create a contour or vector plot of a modal force, the MNF of the associated flexible body must contain nodal masses. You can use the MNF browser to check if the MNF contains nodal masses, see Browsing an MNF or an MD DB. ■Because modal forces can depend on the state of the system, you must run a simulation before viewing the results of a modal force. |
To review a modal force component as a curve:
The dashboard changes to show the results available for plotting.
3. From the Result Set list, select the modal force object whose characteristics you want to plot.
4. From the Component list, select the component of the modal force. FX, FY, FZ, TX, TY, and TZ are the resultant force and torque components with respect to the flexible body’s local part reference frame. FQi is the ith modal component of the modal force.
5. Select Add Curves to add the data curve to the current plot.
To review a modal force as a contour plot:
1. Set the Adams PostProcessor mode to animation.
3. From the Treeview in Adams PostProcessor, select the flexible body on which you want to display the modal force plot.
5. In the dashboard, select the Contour Plots tab.
6. Set Contour Plot Type to the component of the modal force you want to review. Remember that the modal force components are with respect to the flexible body’s local part reference frame.
Next, Adams PostProcessor computes the minimum and maximum values of the modal force. This can take a few minutes because it requires interrogating the modal force values at every node in every mode at every animation frame.
7. Select the Play button to animate the modal force contour plot.
To review a modal force as a vector plot:
2. Select the Vector Plots tab in the dashboard.
3. Set Vector Plot Type to either Force or Torque.
4. Select the Play button to animate the modal force contour plot.