GUI Path Command
Option General Options
Main Menu> Solution> Load Step Opts>
Time/Frequenc> Freq and Substeps NSUBST
Number of Harmonic Solutions
Main Menu> Solution> Load Step Opts>
Time/Frequenc> Time - Time Step or Freq and Substeps
KBC Stepped or Ramped Loads
Dynamics Options
Main Menu> Solution> Load Step Opts>
Time/Frequenc> Freq and Substeps HARFRQ
Forcing Frequency Range
Main Menu> Solution> Load Step Opts>
Time/Frequenc> Damping ALPHAD, BETAD,
DMPRAT Damping
Main Menu> Solution> Load Step Opts> Other>
Change Mat Props> Material Models> Structur-al> Damping
MP,DAMP, MP,DM-PR
Output Control Options
Main Menu> Solution> Load Step Opts> Output Ctrls> Solu Printout
OUTPR Printed Output
Main Menu> Solution> Load Step Opts> Output Ctrls> DB/ Results File
OUTRES Database and Results File Output
Main Menu> Solution> Load Step Opts> Output Ctrls> Integration Pt
ERESX Extrapolation of Results
4.5.3.4.1. General Options
General options include the following:
• Number of Harmonic Solutions (NSUBST)
Section 4.5: How to Do Harmonic Response Analysis
You can request any number of harmonic solutions to be calculated. The solutions (or substeps) will be evenly spaced within the specified frequency range (HARFRQ). For example, if you specify 10 solutions in the range 30 to 40 Hz, the program will calculate the response at 31, 32, 33, ..., 39, and 40 Hz. No response is calculated at the lower end of the frequency range.
• Stepped or Ramped Loads (KBC)
The loads may be stepped or ramped. By default, they are ramped, that is, the load amplitude is gradually increased with each substep. By stepping the loads (KBC,1), the same load amplitude will be maintained for all substeps in the frequency range.
Note — Surface and body loads do not ramp from their previous load step values, except for those applied to PLANE2, SOLID45, SOLID92, and SOLID95 element types. The remaining element types always ramp from zero or from the value specified via BFUNIF.
4.5.3.4.2. Dynamics Options
Dynamics options include the following:
• Forcing Frequency Range (HARFRQ)
The forcing frequency range must be defined (in cycles/time) for a harmonic analysis. Within this range, you then specify the number of solutions to be calculated.
• Damping
Damping in some form should be specified; otherwise, the response will be infinity at the resonant fre-quencies. ALPHAD and BETAD result in a frequency-dependent damping ratio, whereas DMPRAT specifies a constant damping ratio to be used at all frequencies. Damping can also be specified for individual ma-terials using MP,DAMP and MP,DMPR. See Section 5.10.3: Damping for further details.
Note — If no damping is specified in a direct harmonic analysis (full or reduced), the program uses zero damping by default.
• Alpha (Mass) Damping (ALPHAD)
• Beta (Stiffness) Damping (BETAD)
• Constant Damping Ratio (DMPRAT)
• Material Dependent Damping Multiplier (MP,DAMP)
• Constant Material Damping Coefficient (MP,DMPR)
4.5.3.4.3. Output Controls
Output control options include the following:
• Printed Output (OUTPR)
Use this option to include any results data on the output file (Jobname.OUT).
• Database and Results File Output (OUTRES)
This option controls the data on the results file (Jobname.RST).
• Extrapolation of Results (ERESX)
Use this option to review element integration point results by copying them to the nodes instead of ex-trapolating them (default).
4.5.3.5. Save a Backup Copy of the Database to a Named File
You can then retrieve your model by reentering the ANSYS program and issuing RESUME.
Command(s): SAVE
GUI: Utility Menu> File> Save as
4.5.3.6. Start Solution Calculations
Command(s): SOLVE
GUI: Main Menu> Solution> Solve> Current LS
4.5.3.7. Repeat for Additional Load Steps
Repeat the process for any additional loads and frequency ranges (that is, for additional load steps). If you plan to do time-history postprocessing (POST26), the frequency ranges should not overlap from one load step to the next. Another method for multiple load steps, which allows you to store the load steps on files and then solve them at once using a macro, is described in the ANSYS Basic Analysis Guide.
4.5.3.8. Leave SOLUTION
Command(s): FINISH
GUI: Close the Solution menu.
4.5.4. Review the Results
The results data for a harmonic analysis are the same as the data for a basic structural analysis with the following additions: If you defined damping in the structure, the response will be out-of-phase with the loads. All results are then complex in nature and are stored in terms of real and imaginary parts. Complex results will also be produced if out-of-phase loads were applied. See Section 2.3.6: Review the Results in Chapter 2, “Structural Static Analysis”.
4.5.4.1. Postprocessors
You can review these results using either POST26 or POST1. The normal procedure is to first use POST26 to identify critical forcing frequencies - frequencies at which the highest displacements (or stresses) occur at points of interest in the model - and to then use POST1 to postprocess the entire model at these critical forcing frequen-cies.
• POST1 is used to review results over the entire model at specific frequencies.
• POST26 allows you to review results at specific points in the model over the entire frequency range.
Some typical postprocessing operations for a harmonic response analysis are explained below. For a complete description of all postprocessing functions, see Chapter 4, “An Overview of Postprocessing” in the ANSYS Basic Analysis Guide.
4.5.4.2. Points to Remember
The points to remember for a harmonic analysis are the same as those for most structural analyses. See Sec-tion 2.3.6.2: Points to Remember in Chapter 2, “Structural Static Analysis”.
Section 4.5: How to Do Harmonic Response Analysis
4.5.4.3. Using POST26
POST26 works with tables of result item versus frequency, known as variables. Each variable is assigned a reference number, with variable number 1 reserved for frequency.
1. Define the variables using these options:
Command(s): NSOL, ESOL, RFORCE
GUI: Main Menu> TimeHist Postpro> Define Variables
Note — The NSOL command is for primary data (nodal displacements), the ESOL command for derived data (element solution data, such as stresses), and the RFORCE command for reaction force data. To specify the total force, static component of the total force, damping component, or the inertia component, use the FORCE command.
2. Graph the variables (versus frequency or any other variable). Then use PLCPLX to work with just the amplitude, phase angle, real part, or imaginary part.
Command(s): PLVAR, PLCPLX
GUI: Main Menu> TimeHist Postpro> Graph Variables Main Menu> TimeHist Postpro> Settings> Graph
3. Get a listing of the variable. To list just the extreme values, use the EXTREM command. Then use the PRCPLX command to work with amplitude and phase angle or real and imaginary part.
Command(s): PRVAR, EXTREM, PRCPLX
GUI: Main Menu> TimeHist Postpro> List Variables> List Extremes Main Menu> TimeHist Postpro> List Extremes
Main Menu> TimeHist Postpro> Settings> List
Many other functions, such as performing math operations among variables (in complex arithmetic), moving variables into array parameters, moving array parameters into variables, etc., are available in POST26; see Chapter 6,
“The Time-History Postprocessor (POST26)” in the ANSYS Basic Analysis Guide for details.
By reviewing the time-history results at strategic points throughout the model, you can identify the critical fre-quencies for further POST1 postprocessing.
4.5.4.4. Using POST1
1. Read in results for the desired harmonic solution. You can use the SET command for this purpose, but it will read in either the real component or the imaginary component, not both at the same time. The true magnitude of the results is given by an SRSS (square-root-of-sum-of-squares) combination of the real and imaginary components (see Figure 4.2: “Relationship Between Real/Imaginary Components and Amplitude/Phase Angle”) and can be done for specific points in the model in POST26.
2. Display the deformed shape of the structure, contours of stresses, strains, etc., or vector plots of vector items (PLVECT). To obtain tabular listings of data, use PRNSOL, PRESOL, PRRSOL, etc.
• Option: Display Deformed Shape Command(s): PLDISP
GUI: Main Menu> General Postproc> Plot Results> Deformed Shape
• Option: Contour Displays
Command(s): PLNSOL or PLESOL
GUI: Main Menu> General Postproc> Plot Results> Contour Plot> Nodal Solu or Element Solu
Use these options to contour almost any result item, such as stresses (SX, SY, SZ...), strains (EPELX, EPELY, EPELZ...), and displacements (UX, UY, UZ...).
The KUND field on PLNSOL and PLESOL gives you the option of overlaying the undeformed shape on the display.
• Option: Vector Plots
Command(s): PLVECT
GUI: Main Menu> General Postproc> Plot Results> Vector Plot> Predefined
Use PLNSOL or PLESOL to contour almost any result item, such as stresses (SX, SY, SZ...), strains (EPELX, EPELY, EPELZ...), and displacements (UX, UY, UZ...).
• Option: Tabular Listings
Command(s): PRNSOL (nodal results) PRESOL (element-by-element results) PRRSOL (reac-tion data) etc. NSORT, ESORT
GUI: Main Menu> General Postproc> List Results> Nodal Solution Main Menu> General Postproc> List Results> Element Solution Main Menu> General Postproc> List Results> Reaction Solution Use the NSORT and ESORT commands to sort the data before listing them.
Many other functions, such as mapping results on to a path, transforming results to different coordinate systems, load case combinations, etc., are available in POST1; see Chapter 3, “Solution” in the ANSYS Basic Analysis Guide for details.
See the ANSYS Commands Reference for a discussion of the ANTYPE, HROPT, HROUT, HARFRQ, DMPRAT, NSUBST, KBC, NSOL, ESOL, RFORCE, PLCPLX, PLVAR, PRCPLX, PRVAR, PLDISP, PRRSOL, and PLNSOL com-mands.
4.6. Sample Harmonic Response Analysis (GUI Method)
In this sample problem, you will determine the harmonic response of a two-mass-spring system.
4.6.1. Problem Description
Determine the response amplitude (Xi) and phase angle (Φi) for each mass (mi) of the system shown below when excited by a harmonic force (F1sin Ωt) acting on mass m1.
4.6.2. Problem Specifications
Material properties for this problem are:
m1 = m2 = 0.5 lb-sec2/in k1 = k2 = kc = 200 lb/in Loading for this problem is:
F1 = 200 lb
The spring lengths are arbitrarily selected and are used only to define the spring direction. Two master degrees of freedom are selected at the masses in the spring direction. A frequency range from zero to 7.5 Hz with a
Section 4.6: Sample Harmonic Response Analysis (GUI Method)
solution at 7.5/30 = 0.25 Hz intervals is chosen to give an adequate response curve. POST26 is used to get an amplitude versus frequency display.