## General Mechanical

#### Modal analysis – Fluid structure interaction – Skipping frequencies – ANSYS APDL

• pierluigi.olmati
Subscriber

Hi All,

I d like asking suggestion about the following matter. I am performing a unsymmetric modal analysis of a tank filled with fluid. Modal shapes of the fluid are convective (e.g. 0.1Hz~1Hz) and impulsive (e.g. 2Hz~20Hz). The problem is that the convective modes are a lot (1000 modes?) therefore I cannot calculate convective and impulsive modal shapes of the fluid (and structure) in one simulation because I would end up having thousands of modal shapes.

--> There is any way to skip and/or select a range of frequencies in the unsymmetric modal analysis?
Currently I am using the following commands:

ANTYPE, MODAL
MODOPT, UNSYM, 20, 0.01, 1,,, BOTH (for the convective modal shapes)
MODOPT, UNSYM, 200, 2, 20,,, BOTH (for the impulsive modal shapes)

--> There is a way to calculates modal shapes in two frequencies range in the same simulations? (e.g. 0.1Hz~1Hz and e.g. 2Hz~20Hz)

Thank you!

• Ashish Khemka
Ansys Employee

Hi,

You can select one frequency range in one analysis.

Regards,

Ashish Khemka

• pierluigi.olmati
Subscriber

Hi Dr. Khemka,

Thank you really much for your answer. Maybe you are able to help me with this related topic as well.

I use *[GET, EF_X, MODE, counter, EFFM, , DIREC, X] to get EFFM for each modal shape. However, some of them give me EFFM=0. Is it normal for an [MODOPT, UNSYM, 20, 0.01, 1,,, BOTH] analysis? Never happen to me for any traditional (symmetric) modal analysis. Also, if I do not use [BOTH] I get a sum of EFFM that is much larger than the total mass of the model.

Thank you, Pierluigi