When enforced motion is employed in modal dynamic response analysis, using modal
augmentation vectors is highly recommended in order to improve solution accuracy. Without these vectors, your obtained results may be unacceptable.
Qualitatively, the enforced motion shapes, or “modes,” may not necessarily be represented by the component modes of the structure, leading to a truncation of the enforced motion shapes during projection to the modal space. To obtain modal residual vectors from applied loads using modal augmentation vectors, specify PARAM,RESVEC,YES in the Bulk Data section. NX Nastran automatically derives the necessary static loads from the enforced motion data; you don’t need to specify any additional loads.
In summary, the procedure for applying enforced motion in a dynamic analysis:
1. Specify the appropriate type of enforced motion to be applied via the TYPE field in RLOAD1/RLOAD2 (field 8) or TLOAD1/TLOAD2 (field 5) Bulk Data entry, as appropriate.
2. Define the desired enforced motion using the SPCD Bulk Data entry. The set IDs of these SPCD data must match the IDs appearing in the EXCITEID fields of the corresponding dynamic load data in step 1.
3. Ensure that the components referenced in the SPCD Bulk Data entry above are also specified in the SPC1 Bulk Data entry and this SPC1 is also selected in the Case Control Section.
4. Ensure PARAM,RESVEC is set to YES in the Bulk Data Section if a modal dynamic solution approach is used.
See also
• “RESVEC” in the NX Nastran Quick Reference Guide
8.10 Frequency-dependent Loads
You use the DLOAD entry to define linear combinations of frequency dependent loads that are defined on RLOAD1 or RLOAD2 entries. The RLOAD1 entry defines a frequency dependent load of the form
Equation 8-11.
where A is defined on a DAREA entry, C (f) and D (f) are defined on TABLEDi entries, θ is defined on a DPHASE entry,τ is defined on a DELAY entry, and f is defined on a FREQ, FREQ1 or FREQ2.entry. The RLOAD2 entry defines a frequency-dependent load of the form
Equation 8-12.
where A is defined on a DAREA entry, B (f) andφ (f) are defined on TABLEDi entries, θ is defined on a DPHASE entry, andτ is defined on a DELAY entry. The coefficients on the DAREA, DELAY, and DPHASE entries may be different for each loaded degree-of-freedom. The loads are applied to the specified components in the global coordinate system.
Static loading data may be used to supply DAREA values. Using a LOADSET Case Control request with LSEQ Bulk Data entries, you can build DAREA load vectors from any valid static load set. This option is available in the frequency response solutions and in superelement analysis.
The RANDPS entry defines load set power spectral density factors for use in random analysis of the form
Equation 8-13.
where G (f) is defined on a TABRNDi entry. The subscripts j and k define the subcase numbers of the load definitions. If the applied loads are independent, only the diagonal terms (j = k) need be defined. You use the RANDT1 entry to specify the time lag constants for use in the computation of the auto correlation functions.
See also
• “Frequency Response Analysis” in the NX Nastran Basic Dynamic Analysis User’s Guide (for a discussion of frequency response calculations)
• Section 12.2 of The NASTRAN Theoretical Manual (for a discussion of random response calculations)
8.11 Time-dependent Loads
You can use the DLOAD entry to define linear combinations of time-dependent loads that are defined on TLOAD1 and TLOAD2 entries. The TLOAD1 entry defines a time-dependent load of the form
Equation 8-14.
where A is defined on a DAREA entry,τ is defined on a DELAY entry, and F (t − τ) is defined on a TABLEDi entry. The TLOAD2 entry defines a time-dependent load of the form
Equation 8-15.
where T1and T2 are time constants, , A and τ are the same as on the TLOAD1 entry, B is the growth coefficient, C is the exponential coefficient, F is the frequency, and P is the phase angle.
The coefficients on the DAREA and DELAY entries may be different for each loaded
degree-of-freedom. The loads are applied to the specified components in the global coordinate system.
Two methods are available to automatically create DAREA values from static load data.
• Simple static loads that supply load values directly to grid points (such as FORCE,
MOMENT, SLOAD, and PLOAD Bulk Data entries) will add to the DAREA factors when the load identification matches the requested set.
• More complicated loads may be generated using the LOADSET Case Control command. The requested LSEQ Bulk Data entry matches DAREA sets to static loading sets. Any type of static load (e.g., LOAD, GRAV, RFORCE, and SPCD Bulk Data entries) may be generated for either superelement or residual only (non-superelement) models.
Nonlinear effects are treated as an additional applied load vector, for which the components are functions of the degrees-of-freedom. This additional load vector is added to the right side of the equations of motion and treated along with the applied load vector during numerical integration.
It is required that the points to which the nonlinear loads are applied and the degrees-of-freedom on which they depend be members of the solution set; i.e., that they cannot be degrees-of-freedom eliminated by constraints. It is further required that, if a modal formulation is used, the points referenced by the nonlinear loads be members of the set of extra scalar points introduced for dynamic analysis.
There are four different types of nonlinear transient forcing functions:
• Arbitrary function generators
• Multiplies
• Positive power functions
• Negative power functions
The NOLIN1 entry defines a nonlinear load of the form
Equation 8-16.
where Piis the load applied to ui, Siis a scale factor, T(uj) is a tabulated function defined with a TABLEDi entry, and ujis any permissible displacement component or velocity component.
The NOLIN2 entry defines a nonlinear load of the form
Equation 8-17.
where ujand ukare any permissible pair of displacement components and/or velocity components.
They may be the same.
The NOLIN3 entry defines a nonlinear load of the form
Equation 8-18.
where A is an exponent and ujis any permissible displacement component or velocity component.
The NOLIN4 entry defines a nonlinear load of the form
Equation 8-19.
where A is an exponent and ujis any permissible displacement component or velocity component.
See also
• “Transient Response Analysis” in the NX Nastran Basic Dynamic Analysis User’s Guide (for a discussion of transient response calculations)
• Section 11.2 of The NASTRAN Theoretical Manual
• For a discussion of nonlinear forcing functions, see Section 11.2 of The NASTRAN Theoretical Manual.
8.12 Using Static Loading Data in Dynamic Analysis
To include static loading data in a dynamic analysis, all static loads and thermal loads that have the same ID as the DAREA ID on the dynamic load entry are automatically selected, as shown in the example below.
. CEND .
$ THE FOLLOWING REQUEST SELECTS RLOAD1 ENTRY 150 DLOAD = 150
.
BEGIN BULK
$ THE FOLLOWING PLOAD4 IS AUTOMATICALLY SELECTED
$ BECAUSE ITS ID OF 100 MATCHES THE EXCITEID ID OF THE
$ SELECTED RLOAD1 ENTRY PLOAD4,100,...
RLOAD1,150,100,...
$ THE ABOVE USAGE IS EQUIVALENT TO THE USER
$ SELECTING AN LSEQ BULK DATA OF THE
FORM:-$ LSEQ,SET_ID,100,100,100 .
The DAREA Bulk Data entry is used to specify point loads in dynamic analysis. In the case of grid points, these loads are implicitly assumed to be in the displacement (or local) coordinate systems of those points. The software automatically converts all DAREA Bulk Data entries for grid and scalar points into the appropriate equivalent FORCE/MOMENT/SLOAD Bulk Data entries.
Two additional, important advantages of the above enhancement follow.
• When you perform dynamic analysis using the modal approach, you may want to employ residual vectors to improve the quality of the solution. In addition to specifying PARAM,RESVEC,YES in the Bulk Data Section, you must also specify static loads at those points that are dynamically excited. However, with the above automatic conversion feature, it isn’t necessary to explicitly specify static loads for the purpose of residual vector calculations. Such loads are automatically generated by NX Nastran.
• With the automatic conversion feature, you can use the DAREA entry to be used in static analysis as well. This is particularly advantageous when you want to apply loads at grid points in the displacement (or local) coordinate systems of those grid points.
When the software performs the automatic conversion, it issues you a message at the end of the Preface module that contains an image of each converted DAREA Bulk Data entry as well as the corresponding FORCE/MOMENT/SLOAD Bulk Data entry into which it has been converted.
See also
• “DAREA” in the NX Nastran Quick Reference Guide
8.13 Defining Loads for Cyclic Symmetry Analysis
In NX Nastran, there are several different bulk data entries that you can use to define loading for cyclic symmetry analyses.
• You can use the LOADCYH entry to define the harmonic coefficients of a load in cyclic symmetry analysis.
• You can use the LOADCYN entry to define a physical load in cyclic symmetry analysis.
• You can use the LOADCYT entry to specify a tabular load input for axisymmetric cyclic symmetry problems.
See also
• “LOADCYH” in the NX Nastran Quick Reference Guide
• “LOADCYN” in the NX Nastran Quick Reference Guide
• “LOADCYT” in the NX Nastran Quick Reference Guide
• “Understanding Cyclic Symmetry” in the NX Nastran User’s Guide
8.14 Combining Loads
You can use the LOAD case control command to combine the loads you specified on other entries to form new loading conditions. You can use LOAD to define a static loading condition that’s a linear combination (superposition) of load sets consisting of loads applied directly to grid points, pressure loads, gravity loads, and centrifugal forces.
Once you’ve used the loading bulk data entries to define the load, you can use case control commands to specify which of the loading entries you want to use in a given analysis. The case control command you use depends on the type of load or loads you’re applying:
LOAD=n DEFORM=n TEMP(LOAD)=n SPC=n
where n is a unique, user-defined set identification number (SID) in field 2 of the load Bulk Data entry.
For a given analysis, the total load the software applies is the sum of external (LOAD), thermal (TEMP(LOAD)), element deformation (DEFORM), and displacement (SPC) loads.
See also
• “LOAD” in the NX Nastran Quick Reference Guide
• “TEMPERATURE” in the NX Nastran Quick Reference Guide
• “DEFORM” in the NX Nastran Quick Reference Guide
• “SPC” in the NX Nastran Quick Reference Guide