{"id":157668,"date":"2020-09-17T19:41:45","date_gmt":"2020-09-17T19:41:45","guid":{"rendered":"\/forum\/forums\/topic\/why-dont-i-get-zero-frequency-modes-in-a-modal-analysis-for-the-dofs-that-are-not-constrained-2\/"},"modified":"2020-09-21T12:07:41","modified_gmt":"2020-09-21T12:07:41","slug":"why-dont-i-get-zero-frequency-modes-in-a-modal-analysis-for-the-dofs-that-are-not-constrained-2","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/why-dont-i-get-zero-frequency-modes-in-a-modal-analysis-for-the-dofs-that-are-not-constrained-2\/","title":{"rendered":"Why dont I get zero frequency modes in a Modal analysis for the DOFs that are not constrained?"},"content":{"rendered":"<div class=\"Item-Body\">\n<div class=\"Message userContent\">\n<p>This could be due to the bonded contacts with large gaps present in the Modal analysis. Gap\/Penetration at bonded contact adds extra stiffness to the model in Modal Analysis when the contact formulation is set to program controlled (penalty method).&nbsp;<\/p>\n<p><\/p>\n<\/p>\n<p><strong>Background:<\/strong> With large gaps, Bonded contacts using penalty methods can have accuracy issues. There is a warning issued for the user to either clean-up the model using cnch.adjust or morph or switch to MPC.&nbsp;<\/p>\n<p>Warning message from the output is copied below.&nbsp;<\/p>\n<p>**WARNING*: The initial penetration\/gap is relatively large.&nbsp;Bonded\/no&nbsp;separation option may cause an accuracy issue.&nbsp;Switch to MPC&nbsp;algorithm or you may use the CNCHECK,ADJUST\/MORPH command to move the&nbsp;contact nodes towards the target surface.&nbsp;&nbsp;&nbsp;<\/p>\n<p><strong>Resolution:<\/strong> Change the bonded contact formulation with these warnings to MPC formulation. This should give zero frequency modes for the unconstrained DOFs. As a check, compare higher frequency modes using both MPC and penalty contact formulations. There should be minor changes in the higher frequencies for the same mode shapes.<\/p>\n","protected":false},"template":"","class_list":["post-157668","topic","type-topic","status-closed","hentry","topic-tag-mechanical","topic-tag-modal-analysis"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_author_ip":[""],"_bbp_old_reply_author_name_id":["Anonymous"],"_bbp_old_is_reply_anonymous_id":["false"],"_btv_view_count":["517"],"_bbp_likes_count":["0"],"_bbp_subscription":["245900"],"_bbp_topic_status":["unanswered"],"_bbp_status":["publish"],"_bbp_topic_id":["157668"],"_bbp_forum_id":["27791"],"_bbp_engagement":["33","200","20905"],"_bbp_voice_count":["3"],"_bbp_reply_count":["2"],"_bbp_last_reply_id":["176063"],"_bbp_last_active_id":["176063"],"_bbp_last_active_time":["2020-09-24 16:30:48"]},"test":"cheta"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/157668","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":0,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/157668\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=157668"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}