# Maximum length scale for area density

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 January 28, 2014, 11:29 Maximum length scale for area density #1 New Member   Join Date: Oct 2013 Posts: 21 Rep Power: 5 Sponsored Links Hello everybody, i'm simulating a multiphase flow with the free surface model. The simulation is very unstable because there is much dynamic in the system. I figured out that a little value of "Maximum length scale for area density" stabilizes my calculation. I also see an effect in the solution. Unfortunately the documentation gives no informations about the meaning of this setting. In the forum I also didn't found anything related to this theme. Could anybody explain the physical relationship please? Greetings Thomas
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 September 1, 2014, 03:41 #2 New Member   Join Date: Oct 2013 Posts: 21 Rep Power: 5 Does nobody have an idea??

 September 1, 2014, 06:44 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,724 Rep Power: 106 I am not familiar with that setting so cannot comment. But your instability is probably due to more than just that one single parameter. This FAQ lists a number of things to look at when you simulation is unstable: http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F

 September 1, 2014, 09:49 #4 New Member   Michael Join Date: Jan 2012 Location: Erlangen, Germany Posts: 2 Rep Power: 0 Hello Thomas, i hope the citation from ANSYS HELP is allowed in this forum: The parameter is explained in the CFX Theory Guide (cit. without formula): >>5.2.1.1.3. The Free Surface Model >> >>The free surface model attempts to resolve the interface between the >>fluids. If there are just two phases in the simulation, the following >>equation is used for interfacial area density: >> >> >>(5–16) >> >>This area density is clipped to be no smaller than L_max, where L_max is >>controlled by the parameter Maximum Length Scale for Area Density, >>which takes a default value of 1 m. The interfacial area density is explained in chapter 5.2.1.1 Greetings Michael

 September 2, 2014, 02:54 #5 New Member   Join Date: Oct 2013 Posts: 21 Rep Power: 5 Hi Michael, thanks for your advice. In the meantime I found the section too. So I influence the "interficial area density" with this parameter. That means: If the value is very low the interficial area density in my flow field is constant and very high, which is nonsense but it stabilises my calculation. I've no explanation for that behaviour. Maybe the interfaces are resolved too bad and that produces problems. What does the interfacial area density influence? The drag force? The heat transfer? I also figured out, that a value of the drag coefficient which is much higher than the default value of 0,44 has a similar effect. The convergence is much better and the flow field looks like if I reduce the "max. length scale for area density". Does anybody have an advice for this value? 0,44 is related to the flow around a spherical. Thomas

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