capillarity question.

Katt · January 21, 2018, 11:51

Greetings;

I am trying to understand "capillarity".

1. Does the surface tension force act only on the walls of a model or acts all along the interace between fluids?

2. I tried a tutorial case in OpenFoams multiphase solver (using volume of fluid) and whenever I output the capillary force: the values on the wall are 0. Whereas they have values in the internal cell faces where there is an interface.

Further, when we say capillary forces as: (perimeter of interface)*ST*cos(contact angle) is that a general value? Which we should get numerically after summation of the capillary forces acting all along the interface?

K

AliE · January 21, 2018, 14:14

Hello,

Capillary force acts along the interface and it is proportional to interface curvature, so its nomal that you find it into internal cells. The vof solver caluculates the curvature as the divergence of volume fraction gradient normalized by the gradient'module, so if you have a volume fraction gradient then you have a force in the owner cell.

AliE · January 21, 2018, 14:18

Moreover, if you want a general formula, the capillary force is caluclated ans the integral along the interface of ST*curvature*normal. For simple cases it reduces to the one that you have written.

Saideep · January 22, 2018, 12:55

Hi,

It could be more like you misunderstood textbook pictures which show the capillary force acting on the wall. You can realize the intention of the figures, where the authors generally want to say that the contact angle plays a crucial role in determine the magnitude of the force as well.

January 21, 2018, 11:51	capillarity question.	#1
Katt Member Katt Join Date: May 2017 Posts: 30 Rep Power: 8	Greetings; I am trying to understand "capillarity". 1. Does the surface tension force act only on the walls of a model or acts all along the interace between fluids? 2. I tried a tutorial case in OpenFoams multiphase solver (using volume of fluid) and whenever I output the capillary force: the values on the wall are 0. Whereas they have values in the internal cell faces where there is an interface. Further, when we say capillary forces as: (perimeter of interface)STcos(contact angle) is that a general value? Which we should get numerically after summation of the capillary forces acting all along the interface? K

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January 21, 2018, 14:14		#2
AliE Senior Member Join Date: Dec 2017 Posts: 153 Rep Power: 8	Hello, Capillary force acts along the interface and it is proportional to interface curvature, so its nomal that you find it into internal cells. The vof solver caluculates the curvature as the divergence of volume fraction gradient normalized by the gradient'module, so if you have a volume fraction gradient then you have a force in the owner cell.

January 21, 2018, 14:18		#3
AliE Senior Member Join Date: Dec 2017 Posts: 153 Rep Power: 8	Moreover, if you want a general formula, the capillary force is caluclated ans the integral along the interface of STcurvaturenormal. For simple cases it reduces to the one that you have written.

January 22, 2018, 12:55		#4
Saideep Senior Member Saideep Join Date: Apr 2015 Location: INDIA Posts: 203 Rep Power: 12	Hi, It could be more like you misunderstood textbook pictures which show the capillary force acting on the wall. You can realize the intention of the figures, where the authors generally want to say that the contact angle plays a crucial role in determine the magnitude of the force as well.