Mode before and after augmentation of residual vectors 1

[kammieorama]

thread825-341144

can someone please answer this.

even i have the same doubt

what is the difference between mode shapes before and after augmentation of residual vectors?

and most importantly what do we mean by residual vectors..?

thank you in advance..!!

best regards
Kammieorama

 

[BlasMolero]

Dear Kammieorama,
Residual Vectors consideration now is a must when performing a modal eigenvalue analysis in Dynamic Force Response analysis, previosly the user need to activate it using PARAM,RESVEC,YES but now since NX NASTRAN REV9.0 residual vectors are ised by default.

The modal method is a common approach for dynamic forced response solutions because of the computational efficiencies gained by the normal mode reduction. In practice, a small subset of the modes are computed and used for the computation while the effects of truncated higher frequency modes are ignored. The typical rule of thumb for modal sufficiency is to compute modes up to a frequency twice as high as the excitation frequency range. This is suitable for most responses, but some responses, in particular force and stress, can have errors because of the missing truncated modes. Since the missing modes have high frequencies, their omission leads to errors in the static response contributions. Residual vectors are an effective means for reducing the response errors by adding the missing static flexibility from the truncated modes.

New RESVEC Defaults in NX NASTRAN V9.0 (2013)
• The defaults now apply even when the RESVEC case control command is undefined. Previously the RESVEC command was required to request residual vectors.
• For SOLs 103, 106 (PARAM,NMLOOP,0), 110, 115, 153 (PARAM,NMLOOP,0), and 187, the new default is RESVEC=COMPONENT.
• For SOLs 111, 112, 118, 146, and 200 (ANALYSIS=MODES, MCEIG, MTRAN, or MFREQ), the default is RESVEC=YES.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48004 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[kammieorama]

Dear Blas,

Thank you for answering my question.

SO if my understanding is correct then considering residual vectors means cosidering static flexiblity.

Can you please explain what is meant by truncated modes and static flexiblity ?

Also if I use NxNastran versions before 9.0 (ie Version 7.1) and we do not consider RESVEC=YES so the mode values that we get are without consideration of static flexiblity?

And when we talk of high frequencies is there any estimated frequency range above which we can expect marginal differeneces?


thanks in advance

best regards
Kammieorama