{"id":359739,"date":"2024-03-31T17:37:12","date_gmt":"2024-03-31T17:37:12","guid":{"rendered":"\/forum\/forums\/topic\/negative-frequency-during-eigenvalue-analysis\/"},"modified":"2024-04-03T16:22:27","modified_gmt":"2024-04-03T16:22:27","slug":"negative-frequency-during-eigenvalue-analysis","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/negative-frequency-during-eigenvalue-analysis\/","title":{"rendered":"Negative frequencies during eigenvalue analysis"},"content":{"rendered":"<p class=\"whitespace-pre-wrap break-words\"><span style=\"font-size: 12pt\">Hello!<\/span><\/p>\n<p class=\"whitespace-pre-wrap break-words\"><span style=\"font-size: 12pt\">I generated a model which is a cylinder resting on a foundation, and I extracted the natural frequencies from it to check its accuracy. The cylinder and the foundation pad have the `*CONTACT_AUTOMATIC_SURFACE_TO_SURFACE` defined between them. In the keyword, I just selected the master and slave surfaces and changed the FS and FD parameters.<\/span><\/p>\n<p class=\"whitespace-pre-wrap break-words\"><span style=\"font-size: 12pt\">However, after checking the eigout file, I found that the first couple of rigid body modes are showing negative eigenvalues. I checked with different meshing of the foundation pad and the cylinder, and sometimes I can see the negative eigenvalues, and sometimes not.<\/span><\/p>\n<p class=\"whitespace-pre-wrap break-words\"><span style=\"font-size: 12pt\">I want to understand why this is happening and if it indicates that I am doing something wrong in my model. Also, apart from doing a convergence study, how to understand if the extracted frequencies are nearly accurate?<\/span><\/p>\n<p class=\"whitespace-pre-wrap break-words\"><span style=\"font-size: 12pt\">Thank you in advance for your time and response.<\/span><\/p>\n<p class=\"whitespace-pre-wrap break-words\"><span style=\"font-size: 12pt\">Regards,Tonmoy.<\/span><\/p>\n","protected":false},"template":"","class_list":["post-359739","topic","type-topic","status-closed","hentry","topic-tag-contact-behavior-1","topic-tag-eigenfrequency-1","topic-tag-eigenvalue","topic-tag-implicit","topic-tag-ls-dyna"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["273386","13694"],"_bbp_author_ip":["23.206.193.146"]," _bbp_last_reply_id":["0"]," _bbp_likes_count":["0"],"_btv_view_count":["1387"],"_bbp_topic_status":["answered"],"_edit_lock":["1711926974:203800"],"_bbp_topic_id":["359739"],"_bbp_forum_id":["27814"],"_bbp_engagement":["13694","273386"],"_bbp_voice_count":["2"],"_bbp_last_reply_id":["360351"],"_bbp_last_active_id":["360351"],"_bbp_last_active_time":["2024-04-03 16:59:35"]},"test":"tonmoymailinator-com"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/359739","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":2,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/359739\/revisions"}],"predecessor-version":[{"id":359746,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/359739\/revisions\/359746"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=359739"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}