{"id":455787,"date":"2026-04-05T08:57:40","date_gmt":"2026-04-05T08:57:40","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/issue-with-continuity-residuals-when-using-time-varying-inlet-flow-udf-in-dynami\/"},"modified":"2026-04-05T08:57:40","modified_gmt":"2026-04-05T08:57:40","slug":"issue-with-continuity-residuals-when-using-time-varying-inlet-flow-udf-in-dynami","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/issue-with-continuity-residuals-when-using-time-varying-inlet-flow-udf-in-dynami\/","title":{"rendered":"Issue with Continuity Residuals When Using Time-Varying Inlet Flow UDF in Dynami"},"content":{"rendered":"<p>&lt;p&gt;<img decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2026\/04\/05-04-2026-1775379046-mceclip0.png\" \/><img decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2026\/04\/05-04-2026-1775379058-mceclip1.png\" \/>&lt;\/p&gt;&lt;p data-start=&#8221;85&#8243; data-end=&#8221;204&#8243;&gt;From the current setup, it can be observed that there are a chamber, a valve, an air port inlet, and a fuel port inlet.&lt;\/p&gt;&lt;p data-start=&#8221;206&#8243; data-end=&#8221;527&#8243;&gt;The objective of this study is to quantitatively evaluate the mixing quality as air and fuel enter the chamber. To achieve this, I attempted to implement the valve using a UDF with a dynamic mesh. Similarly, I applied a dynamic mesh at the bottom surface, like a piston, to create an expansion effect in the lower region.&lt;\/p&gt;&lt;p data-start=&#8221;529&#8243; data-end=&#8221;809&#8243;&gt;For the inlet conditions, I believed that assigning constant mass flow rates for both air and fuel would not be appropriate. Therefore, I implemented UDFs in which the mass flow rates follow a cosine function&mdash;starting low, reaching a peak in the middle, and then decreasing again.&lt;\/p&gt;&lt;p data-start=&#8221;811&#8243; data-end=&#8221;1097&#8243;&gt;However, when running the simulation with this setup, I observed that the continuity residual increases at every new time step. Although it eventually decreases back to a certain level, it rises again in the next step. This repetitive increase led me to believe that there is a problem.&lt;\/p&gt;&lt;p data-start=&#8221;1099&#8243; data-end=&#8221;1316&#8243;&gt;When I changed the air flow rate to a constant value, the continuity residual stabilized around 1. Based on this, I suspect that the way I implemented the time-varying flow rate using a UDF might be causing the issue.&lt;\/p&gt;&lt;p data-start=&#8221;1318&#8243; data-end=&#8221;1485&#8243;&gt;I would like to ask for your opinion on whether the UDF-based flow rate implementation could indeed be the source of the problem, and how this issue might be resolved.&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-455787","topic","type-topic","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=\"From the current setup, it can be observed that there are a chamber, a valve, an air port inlet, and a fuel port inlet.The objective of this study is to quantitatively evaluate the mixing quality as air and fuel enter the chamber. 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