{"id":291514,"date":"2023-07-04T17:35:56","date_gmt":"2023-07-04T17:35:56","guid":{"rendered":"\/forum\/forums\/reply\/291514\/"},"modified":"2023-07-04T17:35:56","modified_gmt":"2023-07-04T17:35:56","slug":"291514","status":"publish","type":"reply","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/","title":{"rendered":"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period?"},"content":{"rendered":"<p>&lt;p&gt;Hello,&lt;\/p&gt;&lt;p&gt;The Natural Modes result uses the <strong>linear<\/strong> hydrostatic stiffness matrix at the cone&#8217;s equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the <strong>nonlinear<\/strong> hydrostatics over the instantaneous wetted surface of the cone at each time step.&lt;\/p&gt;&lt;p&gt;In the time domain Analysis Settings, try setting the &#8216;Use Linear Stiffness Matrix to Calculate Hydrostatic&#8217; option to &#8216;Yes&#8217;. This should give you a better agreement between the Natural Modes result and your estimated natural period in the time domain.&lt;\/p&gt;&lt;p&gt;You can also try setting the &#8216;Convolution&#8217; option to &#8216;No&#8217;, which causes the Aqwa solver to use the acceleration RAOs to calculate the radiation forces\/moments (rather than a convolution integral method). This may give you a better match with your own eigenvalue calculation.&lt;\/p&gt;&lt;p&gt;I should point out that the natural period from your original time domain analysis (with nonlinear hydrostatics and convolution method for radiation terms) should be the most accurate estimate of the real system behaviour.&lt;\/p&gt;&lt;p&gt;I hope this helps!&lt;\/p&gt;&lt;p&gt;Mike&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-291514","reply","type-reply","status-publish","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO 4.9.10 - aioseo.com -->\n\t<meta name=\"description\" content=\"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone&#039;s equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the nonlinear hydrostatics over the instantaneous wetted surface of the cone at each time step.In the time domain Analysis\" \/>\n\t<meta name=\"robots\" content=\"max-image-preview:large\" \/>\n\t<link rel=\"canonical\" href=\"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/\" \/>\n\t<meta name=\"generator\" content=\"All in One SEO (AIOSEO) 4.9.10\" \/>\n\t\t<meta property=\"og:locale\" content=\"en_US\" \/>\n\t\t<meta property=\"og:site_name\" content=\"Ansys Learning Forum | Ansys Innovation Space\" \/>\n\t\t<meta property=\"og:type\" content=\"article\" \/>\n\t\t<meta property=\"og:title\" content=\"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period? | Ansys Learning Forum\" \/>\n\t\t<meta property=\"og:description\" content=\"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone&#039;s equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the nonlinear hydrostatics over the instantaneous wetted surface of the cone at each time step.In the time domain Analysis\" \/>\n\t\t<meta property=\"og:url\" content=\"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/\" \/>\n\t\t<meta property=\"article:published_time\" content=\"2023-07-04T17:35:56+00:00\" \/>\n\t\t<meta property=\"article:modified_time\" content=\"2023-07-04T17:35:56+00:00\" \/>\n\t\t<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n\t\t<meta name=\"twitter:title\" content=\"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period? | Ansys Learning Forum\" \/>\n\t\t<meta name=\"twitter:description\" content=\"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone&#039;s equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the nonlinear hydrostatics over the instantaneous wetted surface of the cone at each time step.In the time domain Analysis\" \/>\n\t\t<script type=\"application\/ld+json\" class=\"aioseo-schema\">\n\t\t\t{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"BreadcrumbList\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/forums\\\/reply\\\/291514\\\/#breadcrumblist\",\"itemListElement\":[{\"@type\":\"ListItem\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum#listItem\",\"position\":1,\"name\":\"Home\",\"item\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\",\"nextItem\":{\"@type\":\"ListItem\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/forums\\\/reply\\\/291514\\\/#listItem\",\"name\":\"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period?\"}},{\"@type\":\"ListItem\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/forums\\\/reply\\\/291514\\\/#listItem\",\"position\":2,\"name\":\"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period?\",\"previousItem\":{\"@type\":\"ListItem\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum#listItem\",\"name\":\"Home\"}}]},{\"@type\":\"Organization\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/#organization\",\"name\":\"Ansys Learning Forum\",\"description\":\"Ansys Innovation Space\",\"url\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/\"},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/forums\\\/reply\\\/291514\\\/#webpage\",\"url\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/forums\\\/reply\\\/291514\\\/\",\"name\":\"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period? | Ansys Learning Forum\",\"description\":\"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone's equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the nonlinear hydrostatics over the instantaneous wetted surface of the cone at each time step.In the time domain Analysis\",\"inLanguage\":\"en-US\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/#website\"},\"breadcrumb\":{\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/forums\\\/reply\\\/291514\\\/#breadcrumblist\"},\"datePublished\":\"2023-07-04T17:35:56+00:00\",\"dateModified\":\"2023-07-04T17:35:56+00:00\"},{\"@type\":\"WebSite\",\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/#website\",\"url\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/\",\"name\":\"Ansys Learning Forum\",\"description\":\"Ansys Innovation Space\",\"inLanguage\":\"en-US\",\"publisher\":{\"@id\":\"https:\\\/\\\/innovationspace.ansys.com\\\/forum\\\/#organization\"}}]}\n\t\t<\/script>\n\t\t<!-- All in One SEO -->\n\n","aioseo_head_json":{"title":"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period? | Ansys Learning Forum","description":"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone's equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the nonlinear hydrostatics over the instantaneous wetted surface of the cone at each time step.In the time domain Analysis","canonical_url":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/","robots":"max-image-preview:large","keywords":"","webmasterTools":{"miscellaneous":""},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"BreadcrumbList","@id":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/#breadcrumblist","itemListElement":[{"@type":"ListItem","@id":"https:\/\/innovationspace.ansys.com\/forum#listItem","position":1,"name":"Home","item":"https:\/\/innovationspace.ansys.com\/forum","nextItem":{"@type":"ListItem","@id":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/#listItem","name":"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period?"}},{"@type":"ListItem","@id":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/#listItem","position":2,"name":"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period?","previousItem":{"@type":"ListItem","@id":"https:\/\/innovationspace.ansys.com\/forum#listItem","name":"Home"}}]},{"@type":"Organization","@id":"https:\/\/innovationspace.ansys.com\/forum\/#organization","name":"Ansys Learning Forum","description":"Ansys Innovation Space","url":"https:\/\/innovationspace.ansys.com\/forum\/"},{"@type":"WebPage","@id":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/#webpage","url":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/","name":"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period? | Ansys Learning Forum","description":"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone's equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the nonlinear hydrostatics over the instantaneous wetted surface of the cone at each time step.In the time domain Analysis","inLanguage":"en-US","isPartOf":{"@id":"https:\/\/innovationspace.ansys.com\/forum\/#website"},"breadcrumb":{"@id":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/#breadcrumblist"},"datePublished":"2023-07-04T17:35:56+00:00","dateModified":"2023-07-04T17:35:56+00:00"},{"@type":"WebSite","@id":"https:\/\/innovationspace.ansys.com\/forum\/#website","url":"https:\/\/innovationspace.ansys.com\/forum\/","name":"Ansys Learning Forum","description":"Ansys Innovation Space","inLanguage":"en-US","publisher":{"@id":"https:\/\/innovationspace.ansys.com\/forum\/#organization"}}]},"og:locale":"en_US","og:site_name":"Ansys Learning Forum | Ansys Innovation Space","og:type":"article","og:title":"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period? | Ansys Learning Forum","og:description":"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone's equilibrium position to find the natural periods of the system. On the other hand, in your time domain analysis the Aqwa solver calculates the nonlinear hydrostatics over the instantaneous wetted surface of the cone at each time step.In the time domain Analysis","og:url":"https:\/\/innovationspace.ansys.com\/forum\/forums\/reply\/291514\/","article:published_time":"2023-07-04T17:35:56+00:00","article:modified_time":"2023-07-04T17:35:56+00:00","twitter:card":"summary_large_image","twitter:title":"Reply To: AQWA Eigenvalue vs Free Decay Test Resulting in Different Heave Natural Period? | Ansys Learning Forum","twitter:description":"Hello,The Natural Modes result uses the linear hydrostatic stiffness matrix at the cone's equilibrium position to find the natural periods of the system. 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