# Frequency response analysis

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Hello,

I have query regarding frequency response analysis.

My component is automobile lamp( eg: Head lamp of a car body)

i have meshed the component using tetra mesh. i have performed the modal analysis.

Now my query is using these frequency values obtained from the modal analysis i would like to know in detail how to perform Frequency response analysis. Please help me.

Regards,

Rajesh

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Hi Rajesh,

Frequency response analysis is used to calculate the response of a structure under a harmonic excitation.The analysis is to compute the response of the structure, which is actually transient, in a static frequency domain.  The loading is sinusoidal.(Time dependent or frequency dependent dynamic load) A simple case is a load that has amplitude at a specified frequency.  The response occurs at the same frequency, and damping would lead to a phase shift.See the attachment.(OS-1300: Direct Frequency Response Analysis of a Flat Plate)Tutorial available for Optistruct solver

Basically Normal modal analysis would give output as Frequency and mode shape.In frequency response case if excitation frequency is close to natural frequency of the component then it may cause resonance.To avoid resonance it is prequisite to perform modal analysis.

Regards

Rahul R

Jodha likes this

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Thanks a lot for the reply.

The following are conditions for the analysis

Sweep frequency -10-55-10hz

Amplitude-5g

Using these values how can i perform the frequency response analysis.

Can you let me know where can i apply the load. As I said before my component is a headlamp of a car.

Regards,

Rajesh.

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Dear rahul,

I just want perform a analysis which is not depend on the time. Simple one cycle analysis using frequency range, amplitude.

Regards,

Rajesh

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Dear rahul,

In the example said by you, uniform structural damping coefficient is calculted but how to calculate this in my component.

What value should I keep in WTM_V1??

Regards,

Rajesh

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Hi,

If you have frequency dependent dynamic load. You an use Tabled1 card to insert your loading. If loading is in the form of force use DAREA card, or if it is in the form of velocity acceleration or displacement used SPCD card.

There are multiple ways to input damping to a structure. The two types of damping which are generally used in linear-elastic materials are,

1. Viscous damping

2. Structural damping

The viscous damping force is proportional to velocity, and the structural damping force is proportional to displacement. Which type to use depends on the physics of the energy dissipation mechanism(s) and is sometimes dictated by regulatory standards.

Structural damping is specified on the MATi and PARAM,G, GFL Bulk Data entries. The GE field on the MATi entry is used to specify overall structural damping for the elements that reference this material entry. This definition is via the structural damping coefficient GE.

An alternate method for defining structural damping is through PARAM,G. This parameter multiplies the stiffness matrix to obtain the structural damping matrix. The default value for PARAM,G is 0.0. The default value causes this source of structural damping to be ignored

Converts weights to masses using this multiplier.

REAL > 0.0
Default = 1.0

Parameter

Values

Description

WTMASS

Real > 0.0

Default = 1.0

The WTMASS (Weight-To-MASS) parameter is used as a multiplier for the terms of the structural mass matrix.

Note:

PARAM, WTMASS cannot be applied to superelements (.h3d or .pch) that are read into the model. If the unit of mass is incorrect on the MAT# entries and PARAM, WTMASS is required to update the structural mass matrix; then this should be done in the creation run.

Hope it helps.

Regards

Rahul R

Rahul Ponginan and Jodha like this

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mr rahul

natural frequencies from direct frf is not same with results from normal mod analysis

because in direct frf or modal frf analysis we have to use darea or spcd so this function shift resoncance frequencies or this function district a dof on any node

this situation must be change

analysis are not agree with experimental results

in other software I have never seen this situation

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I am not clear on your above comment. Normal modal analysis comes under the category of free vibration where we don't consider damping whereas in direct/modal method of frequency response analysis we consider damping.

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oK I see you but it is strange that changing of natural frequencies in both frf analysis

in frf analysis does peaks give me natural frequencies?

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FRF analysis peaks would help you to know, at what frequency output of displacement/stress/velocity/acceleration is high.

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ok, thanks for all

I have a model for base acceleration exciting also I would like to this study re-perform

and I HOPE FRF added in attechment

I have a model in prepared in hypermesh however get SPCD error

can you help me?

aaaaaa.fem

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PFA updated fem file. Made following changes in the deck:

Uncheck shear modulus

Define Exciteid to SPCD in RLOAD2 card

Changed freq1 card f1 to 0.1

SPCD.fem

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