×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Need help on defining load cases for a Forced Freq Response Analysis

Need help on defining load cases for a Forced Freq Response Analysis

Need help on defining load cases for a Forced Freq Response Analysis

(OP)
Hi folks,
We are providing design & analysis consulting services to a firm which is performing a conversion of cargo chopper in to VIP one by installing interiors.

One of the concerns was to check resonance freq of the monuments against the external freq of the main rotors. We did this by doing a SOL 103 normal modes analysis on Nastran & comparing the resonant freq with rotors one.

However in future, we want to be more accurate and possibly perform a force freq response analysis.

I am investigating on how to define load cases in Nastran for the above scenario. Lets say that the main rotor freq is around 25Hz. How do I apply this to the FE model?

Most of the examples that I have come across defines a unit load along with a freq domain loading function and solves for displacements, accelerations, stresses etc. Usually the unit load is applied to a node of interest in a particular direction.

In the above case, I am lost on which node & direction to apply the unit load. Lets just take an example of a galley like the one pictured below



I know the above structure is complicated, but we can substitute the above for anything more simpler.

RE: Need help on defining load cases for a Forced Freq Response Analysis

Hello VN1981,

is your question still current?
If I understand you correctly, you don't want to use unit load because you don't know the excitation force, and you are not sure where this force operates.
The rotor is a source of excitation, is it so?

Note that instead of excitation force you can use kinematic excitation - to excite by amplitude of displacement, velocity or acceleration. F.e. in case of combustion engine you can calculate these data from multi-body and implement them to FEM as excitation (loading). If there are more excited regions (f.e. bearings, supports etc.), beside the amplitude don't forget on phase shift if it is relevant.
Instead of entire frequency range you can of course consider only on specific frequency, or set of specific frequencies (if you assume several relevant engine orders) - in your case that 25Hz.

Please check card FREQ in Quick reference guideline, solver SOL 111.

BTW - be honest your structure will not be easy to model properly as a linear dynamic model, I cannot imagine obtaining of good FE results without validation/correlation from testing...how you will define f.e. damping?


Hope this helps,

Jan


With best regards,
Dr. Jan Vojna
Lead Engineer Development

Siemens, s.r.o.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close