{"id":407264,"date":"2025-01-27T16:41:24","date_gmt":"2025-01-27T16:41:24","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/gravity-acceleration-in-harmonic-analysis-wrong-results\/"},"modified":"2025-01-27T16:41:24","modified_gmt":"2025-01-27T16:41:24","slug":"gravity-acceleration-in-harmonic-analysis-wrong-results","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/gravity-acceleration-in-harmonic-analysis-wrong-results\/","title":{"rendered":"Gravity acceleration in harmonic analysis: wrong results."},"content":{"rendered":"<p>&lt;p&gt;I&#8217;m calculating the behaviour of a pendulum and Ansys is producing wrong results.&lt;\/p&gt;&lt;p&gt;The details are as follows:&lt;\/p&gt;<\/p>\n<ul>\n<li><strong>Analysis:<\/strong> harmonic with the mode superposition method.<\/li>\n<li><strong>Physical system:<\/strong> a pendulum composed of a long rigid rod pivoted at its upper end by a very soft angular spring, and a mass at the lower end of the rod.<\/li>\n<li><strong>Load:<\/strong> a horizontal force F, with the same value at all frequencies, acting on the mass.<\/li>\n<li><strong>Parameter calculated:<\/strong> velocity V of the mass in the direction of the force.<\/li>\n<li><strong>Gravity acceleration:<\/strong> set in the pre-stress analysis, not in the harmonic analysis.<\/li>\n<li><strong>Structural damping of the material:<\/strong> the loss angle is 1e-9, which corresponds to single crystal silicon.<\/li>\n<\/ul>\n<p>&lt;p&gt;The parameter of interest is the <strong>real part of the velocity<\/strong>, which is physically related to the damping of the pendulum and is less than that of the material because the effect of the gravitational field, a phenomena called dissipation dilution.&nbsp;&lt;\/p&gt;&lt;p&gt;It is possible to judge whether the calculation is correct or not, by fitting an analytical model to the numerically generated data and comparing the fitted parameter in the model with an expectation. The Ansys calculation produces the wrong result, whereas an independent rigid-body model produces the correct one. This situation is shown in this plot, where the admittance is the ratio V\/F:&lt;\/p&gt;&lt;p&gt;<img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2025\/01\/27-01-2025-1737991441-mceclip0.png\" width=\"523\" height=\"410\" \/>&lt;\/p&gt;&lt;p&gt;<strong>The correct value of phi_pen is 3.2e-12, whereas the value calculated with Ansys is 1e-9<\/strong>, which equals the loss angle of the material! The ratio of the two equals the value of a parameter called the dissipation dilution factor, which in this case is 312. The plot only shows low frequencies, where there are no intermal resonances of the rod in the flexible model.&nbsp;&lt;\/p&gt;&lt;p&gt;<strong>This symptom suggests that the acceleration due to gravity is not considered correctly in the harmonic analysis<\/strong>, whereas it seems to be correctly included in the structural and modal analyses, on which the harmonic analysis relies. It is important to point out that although gravity does not oscillate itself, it is the physical element that makes a pendulum oscillate.&lt;\/p&gt;&lt;p&gt;Another interesting element of the problem is that the imaginary part of the velocity is not affected, as shown in this plot:&lt;\/p&gt;&lt;p&gt;<img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2025\/01\/27-01-2025-1737995414-mceclip1.png\" width=\"509\" height=\"397\" \/>&lt;\/p&gt;&lt;p&gt;The imaginary part is several orders of magnitude larger than the real part.&lt;\/p&gt;&lt;p&gt;Given this situation, I have these questions:&lt;\/p&gt;<\/p>\n<ul>\n<li>How is the acceleration of gravity included in the harmonic analysis, only through the static structural and modal analyses or also in the equations solved in the harmonic analysis?<\/li>\n<li>I noticed it is not possible to set the acceleration of gravity as a load in the harmonic analysis, is there a particular reason for this?<\/li>\n<li>Would the situation change using a full harmonic analysis instead of modal decomposition?<\/li>\n<li>Are there possible workarounds to try?<\/li>\n<\/ul>\n<p>&lt;p&gt;Thank you very much in advance!&lt;\/p&gt;&lt;p&gt;Fabi&aacute;n&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-407264","topic","type-topic","status-publish","hentry"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27791"],"_bbp_topic_id":["407264"],"_bbp_subscription":["350467","20905"],"_bbp_author_ip":["2806:250:416:ba7f:5da4:4c8b:a2cc:854e"],"_bbp_last_reply_id":["407425"],"_bbp_last_active_id":["407425"],"_bbp_last_active_time":["2025-01-28 16:09:07"],"_bbp_reply_count":["1"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["2"],"_bbp_engagement":["350467","20905"],"_btv_view_count":["231"],"_bbp_topic_status":["unanswered"],"_bbp_notification_enabled":["480261"]},"test":"2fa7ea0b84bb478601ff6bbecbbf86dc3b0d86aa"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/407264","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\/407264\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=407264"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}