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for different frequencies (between 2 and 6) across the frequency range, and then it computes a polynomial approximation of the values.

The harmonic sweep performs accurate approximations of the results across the frequency range in user-specified steps.

If the loading, material properties, or real constants defined with a data table (BF ,TBFIELD command, or COMBIN14) are frequency-dependent and too few data points exist in the table, the polynomial interpolation of the values will be poor. In case of abrupt variations for a given frequency f2, ANSYS, Inc. recommends splitting the frequency range [f1 f3] into two ranges (e.g., [f1 f2] ; [f2 f3] ) and performing separate analyses on each range.

The following Variational Technology topics are available for harmonic sweep:

2.1.1.Structural Elements Supporting Frequency-Dependent Properties

2.1.2.Harmonic Sweep for Structural Analysis with Frequency-Dependent Material Properties

2.1.1. Structural Elements Supporting Frequency-Dependent Properties

Frequency-dependent property structural elements can be used with the following elements:

 

Frequency-Dependent Material Properties

Element

(defined using TB,ELAS and TB,SDAMP com-

 

mands)

 

 

PLANE182

EX, SDAMP

PLANE183

EX, SDAMP

SOLID185

EX, SDAMP

SOLID186

EX, SDAMP

SOLID187

EX, SDAMP

SOLID272

EX, SDAMP

SOLID273

EX, SDAMP

SOLID285

EX, SDAMP

2.1.2. Harmonic Sweep for Structural Analysis with Frequency-Dependent Material Properties

The harmonic sweep feature of the VT Accelerator module allows you to define the material elastic properties as frequency-dependent and efficiently compute the frequency response over an entire frequency range. For more information, see the documentation for the TB,ELAS and TB,SDAMP commands. To use this formulation, use the hysteretic damping formulation with the HROPT command.

If you define the damping ratio (TB,SDAMP) as a linear function of the frequency, the damping exhibits behavior similar to that of viscous damping. See the Mechanical APDL Theory Reference for more information about the hysteretic and structural damping formulations.

Limitations

• The frequency-dependent tables (defined by the TB,ELAS and TB,SDAMP commands) define piecewise linear functions. These are used to define the stiffness and damping matrices, which in turn are fitted with a polynomial over the entire frequency range to compute their derivatives as a function of frequency;

 

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Harmonic Sweep Using VT Accelerator

therefore, if the piecewise linear approximation of the material properties is too coarse, the results will be poor.

2.1.2.1. Beam Example

Consider a cantilever beam, with a Young's modulus of 20e6 psi for static condictions and 30e6 for 500Hz.

ETYPE=186

 

LF = 10

! STARTING FREQUENCY

UF = 500

! ENDING FREQUENCY

N = 50

! NUMBER OF SUBSTEP

/PREP7

 

!*

 

ET,1,ETYPE

 

/com, * ==============================================

/com, *

 

 

/com, *

Frequency dependent material properties

/com,

*

 

 

/com,

* ==============================================

TB,ELASTIC,1,,2

! Elastic data table

TBFIELD , FREQ,1 ! First frequency value

TBDATA,1,20e6,0.3 ! E and m

TBFIELD ,FREQ,500 ! Fifth frequency value

TBDATA,1,30e6,0.3

TB,SDAMP,1, ,1

!

damping data table

TBFIELD

,

FREQ,1 ! First

frequency value

TBDATA,1,

0.02

!

Damping co.

TBFIELD

,FREQ,500

! Fifth frequency value

TBDATA,1,

0.01

 

 

MP,DENS,1,.10

BLOCK,0,10,0,2,0,2 LSEL,S,LOC,X,-.5,0.5 LESIZE,ALL,,,2 LSEL,S,LOC,X,9.5,10.5 LESIZE,ALL,,,2 LSEL,S,LOC,X,2,8 LESIZE,ALL,,,5 VMESH,ALL

FINISH /SOL

LSEL,S,LOC,X,-.5,0.5 DL,all,,all KSEL,S,LOC,X,8,12 KSEL,R,LOC,Y,-.5,.5 FK,ALL,FY,1000 ASEL,S,LOC,Y,1.8,2.2 SFA,ALL,,PRES,1000, allsel

FINI

/com, * ==============================================

/com, *

/com,

*

VT Harmonic Analysis

/com,

*

 

/com, * ==============================================

/VT

VTCLR,ALL

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VTRFIL VTFREQ,frq,LF,UF,N

VTRSLT,disp,NODE,U,ALL,0.01,ALL FINISH

/SOLU

ANTY,HARM

HROUT,OFF ! Print complex displacements as amplitude and phase angle KBC,1

HROPT,VT,,,,HYST Solve

FINISH

/show,post

/post26

nsol,10,57,u,y,d1

prvar,10

The following figure shows the difference in the Y displacement at the end of the beam, taking constant material properties for 0Hz, constant properties for 500 Hz, and variable properties:

Figure 2.1: Y-Displacement Difference at End of Beam

Because of the frequency step used to create the frequency response, the first peak is higher on the variable curve than the other two.

 

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