{"id":30135,"date":"2018-08-22T15:46:24","date_gmt":"2018-08-22T15:46:24","guid":{"rendered":"\/forum\/forums\/topic\/time-step-size-and-courant-number\/"},"modified":"2018-08-22T15:46:24","modified_gmt":"2018-08-22T15:46:24","slug":"time-step-size-and-courant-number","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/time-step-size-and-courant-number\/","title":{"rendered":"Time Step Size and Courant Number"},"content":{"rendered":"<p>Hi<\/p>\n<p><\/p>\n<p>I am new to Fluent and am analyzing external flow of a CD nozzle. I did analytical calculations and came up with the BCs.<\/p>\n<p><\/p>\n<p>I was able to solve the first iteration with CFL as 5 and time step size (dt) as 1e-4 s. It wasn&#8217;t the perfect solution as I monitored the net mass flow rate and it was close to zero but not exactly. Mass flow rate value stabilized over time.<\/p>\n<p><\/p>\n<p>To experiment, I kept the same BCs but reduced the throat size of the nozzle, effectively reducing the mass flow rate. But, now I&#8217;m having a hard time finding the right combination of CFL and dt. Is there a good way of coming up with these values than trial and error? Each run takes over 5 hrs just to show that the solution isn&#8217;t correct.<\/p>\n<p><\/p>\n<p>I successfully ran both the analyses for internal flow under steady state conditions as a verification of BCs.<\/p>\n<p><\/p>\n<p>Additional details about my analysis &#8211;<\/p>\n<p><\/p>\n<ul style=\"box-sizing: border-box; margin-top: 0px; margin-bottom: 10px; font-family: 'Open Sans', Arial, Helvetica, sans-serif;\"><\/p>\n<li style=\"box-sizing: border-box;\">Model details &#8211; 2D geometry, inviscid model, Energy ON. Air as ideal gas.<\/li>\n<p><\/p>\n<li style=\"box-sizing: border-box;\">Minimum grid size &#8211; 5mm (about 350K elements, performed mesh dependence study on first problem and solution was independent of mesh size)<\/li>\n<p><\/p>\n<li style=\"box-sizing: border-box;\">Courant number condition &#8211; Currently running 5. Tried 1 as well but no luck. Both yielding high mass imbalance.<\/li>\n<p><\/p>\n<li style=\"box-sizing: border-box;\">Fixed time step or variable time step &#8211; Tried both. Fixed dt of 1e-3 and 1e-4 s. Currently running Adaptive time stepping and still seeing huge mass imbalance after 350 iterations with 50 iterations per step.<\/li>\n<p><\/p>\n<li style=\"box-sizing: border-box;\">Residual values &#8211; I have set residuals to 1e-6. Solution isn&#8217;t converging at the end of 50 iterations of each step.<\/li>\n<p><\/p>\n<li style=\"box-sizing: border-box;\">Monitoring net mass flow rate between inlet and outlet as well as inlet mass flow rate. With fixed time steps values mentioned above, I obtained stable values for both but with mass imbalance of 2000 kg\/s and inlet mass flow not matching my calculations. Adaptive time step did not yield stable flux values after 350 iterations. Running for 650 more iterations.<\/li>\n<p>\n<\/ul>\n<p><\/p>\n<p>I even tried checking the box for &#8211; &#8220;Adjust Solver Defaults Based on Setup&#8221; which raised the CFL to 50. With a dt of 1e-4 s that led to Divergence. So, I have unchecked it again.<\/p>\n<p><\/p>\n<p>&nbsp;<\/p>\n","protected":false},"template":"","class_list":["post-30135","topic","type-topic","status-closed","hentry"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_old_topic_id":["2304"],"_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","0","0","0"],"_btv_view_count":["19709"],"_bbp_topic_status":["unanswered"],"_bbp_status":["publish"],"_bbp_topic_id":["30135"],"_bbp_forum_id":["27792"],"_bbp_engagement":["25","199","22555","158448"],"_bbp_voice_count":["4"],"_bbp_reply_count":["7"],"_bbp_last_reply_id":["63552"],"_bbp_last_active_id":["63552"],"_bbp_last_active_time":["2018-09-04 21:53:06"]},"test":"pks7"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/30135","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\/30135\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=30135"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}