# partial slip b.c., convergence issues

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 August 1, 2016, 13:47 partial slip b.c., convergence issues #1 Senior Member   Saideep Join Date: Apr 2015 Location: INDIA Posts: 203 Rep Power: 12 Hi guys, After struggling for quite long I have two questions. 1. I have issues with respect to convergence of capillary rise test case. I am dealing with dimensions in order of micro meters. (10e-6 is the height and 120e-6 is the length of the channel). Digging a bit into mesh independence study, I came across two possible culprits. a. Wall boundary condition (slip vs no-slip) I tried to use a partialSlip boundary condition on wall surface (thinking this as Navier slip b.c, not sure) where the "valueFraction" parameter is defined as 1/(slip length+1). My slip length variable is quite less (usually from papers, delta/2, where delta is the smallest cell size). I have a value of 0.999999. Close to a no-slip condition and I dont see any difference(even in phase volume fractions over time) between the partial Slip and no-slip b.c for this specific case. This is the 1st time I am using partialSlip and there are few threads discussing this b.c. Can any expert shed some light how can I use a valid value for the Navier slip b.c for microchannels. b. Using a dynamic c.a. instead of a static c.a. From my understanding there are 3 contact angles. Static, dynamic(advancing, receding). Within dynamic c.a. both the advancing and receding c.a. are quite different and independent. However, OF tends to solve an equation and return only one theta value. How is that possible? Would appreciate any help over these issues!! Thanks, Saideep

 August 1, 2016, 21:34 #2 Senior Member   Arjun Join Date: Mar 2009 Location: Nurenberg, Germany Posts: 1,279 Rep Power: 34 Navier slip could be very unstable so I would suspect this first.

 August 2, 2016, 06:59 #3 Senior Member   Saideep Join Date: Apr 2015 Location: INDIA Posts: 203 Rep Power: 12 Hi arjun; This is the 1st time I am using the Navier slip b.c. Why would it make things(like the contact line) unstable? Sorry if its a basic one. Saideep

August 2, 2016, 09:18
#4
Senior Member

Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 1,279
Rep Power: 34
Quote:
 Originally Posted by Saideep Hi arjun; This is the 1st time I am using the Navier slip b.c. Why would it make things(like the contact line) unstable? Sorry if its a basic one. Saideep

It is very hard to explain, but your question is not basic one. I know by experience. In fact I have a patent about making it stable

Just from my experience that navier slip could be very unstable.

 Tags dynamic contact angle, mesh independence, partial slip