{"id":35553,"date":"2019-05-24T02:02:16","date_gmt":"2019-05-24T02:02:16","guid":{"rendered":"\/forum\/forums\/topic\/difference-between-define-by-substeps-vs-time-in-fsi\/"},"modified":"2019-05-24T02:02:16","modified_gmt":"2019-05-24T02:02:16","slug":"difference-between-define-by-substeps-vs-time-in-fsi","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/difference-between-define-by-substeps-vs-time-in-fsi\/","title":{"rendered":"Difference between &#8220;Define By&#8221; Substeps vs. Time in FSI"},"content":{"rendered":"<p>Hello,<\/p>\n<p><\/p>\n<p>&nbsp;<\/p>\n<p><\/p>\n<p>I am performing FSI analysis through <strong>System Coupling<\/strong> with <strong>Transient Structural<\/strong> and <strong>Fluent<\/strong>.&nbsp; In the process of refining the model I found that I needed to figure out how the mechanical side handles damping, due to oscillations that are present in my model but not in the physical system.&nbsp; To explore this I made a simpler model that doesn&#8217;t take as long to run but that has some of the salient features of my actual model. All these images and gifs here portray the simpler model.&nbsp; The simpler model is to help me figure out how to get rid of the oscillations in my real model.&nbsp; The model is a simple hollow cylinder filled with air.&nbsp; The cylinder material is hyperelastic and the air is ideal gas compressible.&nbsp; One end of the cylinder has a fixed support and the other has a pressure that is imposed by tabular data in 40 increments.&nbsp; So the pressure increases halfway through, then suddenly drops in order to induce oscillations.<\/p>\n<p><\/p>\n<p><img decoding=\"async\" src=\"https:\/\/us.v-cdn.net\/6032193\/uploads\/attachments\/8babacfe-69bd-457f-8c79-a9e90018d38c\/18898a53-4d33-4bd2-a081-aa57001db8c7_pressure-input-substeps-ann.jpg?width=690&amp;upscale=false\" alt=\"\"><\/p>\n<p>Along the way I started to wonder if there was some difference between ramping pressures with <strong>Substeps<\/strong> vs. <strong>Timesteps<\/strong>, as far as the system response.&nbsp; So I made two copies of the same system, the first one uses 40 substeps over 1 second, as seen in these <strong>Analysis Settings<\/strong>:<\/p>\n<p><\/p>\n<p><img decoding=\"async\" src=\"https:\/\/us.v-cdn.net\/6032193\/uploads\/attachments\/8babacfe-69bd-457f-8c79-a9e90018d38c\/e33f307f-c3c1-4336-8275-aa57001dff3f_analysis-settings-substeps.jpg?width=690&amp;upscale=false\" alt=\"\"><\/p>\n<p><\/p>\n<p>The solution using substeps is shown in the following gif.&nbsp; As can be seen, the cylinder compresses and when the pressure drops it oscillates as expected.<\/p>\n<p><\/p>\n<p><img decoding=\"async\" src=\"https:\/\/i.imgur.com\/HKmA8Fs.gif?width=690\" alt=\"\"><\/p>\n<p><\/p>\n<p>The second model uses a time step size of 0.025s from 0 to 1 second for a total of 40 time steps, with <strong>Define By<\/strong> set to <strong>Time<\/strong>, as shown below:<\/p>\n<p><\/p>\n<p><img decoding=\"async\" src=\"https:\/\/us.v-cdn.net\/6032193\/uploads\/attachments\/8babacfe-69bd-457f-8c79-a9e90018d38c\/9e07ae06-810e-47e5-bcdf-aa57001f33ca_analysis-settings-time.jpg?width=690&amp;upscale=false\" alt=\"\"><\/p>\n<p><\/p>\n<p>This is the <em>only difference<\/em> between the two systems.&nbsp; The time stepping model seems to give a similar solution until the pressure drop, at which point it fails with a &#8220;Highly Distorted Element&#8221; error.&nbsp; The animation for this model is shown below:<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/i.imgur.com\/0NoTVEn.gif?width=690\" alt=\"\"><\/p>\n<p>My questions:<\/p>\n<p><\/p>\n<p><strong>1.&nbsp; What is the substantive difference between using Substeps and Time in the &#8220;Define By&#8221; setting?&nbsp; I couldn&#8217;t find anything in the manual other than that the user can choose one or the other.&nbsp; <\/strong><\/p>\n<p><\/p>\n<p><strong>2. The solution using Substeps took 1.6 hours to complete.&nbsp; The solution that solved only halfway through using TimeSteps took 3 minutes before the error.&nbsp; Extrapolating this, the TimeSteps solution would solve in under 10 minutes.&nbsp; Why the dramatic difference?<\/strong><\/p>\n<p><\/p>\n<p>&nbsp;<\/p>\n","protected":false},"template":"","class_list":["post-35553","topic","type-topic","status-closed","hentry"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_old_topic_id":["7725"],"_bbp_old_topic_author_name_id":["Anonymous"],"_bbp_old_is_topic_anonymous_id":["false"],"_bbp_old_closed_status_id":["publish"],"_bbp_author_ip":[null],"_bbp_old_sticky_status_id":["normal"],"_bbp_likes_count":["0","0"],"_btv_view_count":["1396"],"_bbp_topic_status":["unanswered"],"_bbp_status":["publish"],"_bbp_topic_id":["35553"],"_bbp_forum_id":["27791"],"_bbp_engagement":["20093","162192"],"_bbp_voice_count":["2"],"_bbp_reply_count":["2"],"_bbp_last_reply_id":["86023"],"_bbp_last_active_id":["86023"],"_bbp_last_active_time":["2019-05-25 16:29:10"]},"test":"ansysuser"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/35553","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\/35553\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=35553"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}