Surface tension is the force that holds the interface together. If an
interface is cut, the magnitude of the surface tension force per unit length is
called the surface tension coefficient, 
. Surface
tension has a number of important physical effects, including:
If the interface is curved, it induces a force normal to the interface. For a droplet at rest, this induces a pressure rise within the droplet of
,
where 
is the droplet curvature.
The effect of this normal force is to smooth regions of high curvature;
it tends to reduce the surface area of droplets.When the surface tension coefficient is not constant, the surface tension force has a tangential component that tends to move fluid along the interface toward regions of high surface tension coefficient. This is often called Marangoni convection.
When the free surface interface touches a wall, the wall may attract the liquid (wetting wall) or repel the liquid (non-wetting wall). In a static situation, the wall contact angle may be measured. This phenomenon, known as wall adhesion, gives rise to effects such as the meniscus and capillary rise in tubes.
For free surface flows, you have the option of including surface tension effects. When it is enabled, a surface tension coefficient must be specified. If the surface tension coefficient is variable, the Marangoni force is automatically included. If the surface tension model is activated, using double precision is often required to avoid roundoff errors in the curvature calculation.
Additional information on the surface tension implementation in Ansys CFX is available in Surface Tension in the CFX-Solver Theory Guide.
When evaluating the interface normal vector for the curvature and surface
tension force, it is usually beneficial to take the gradient of a smoothed
volume fraction field.
The volume
fraction smoothing method may be optionally set in CFX-Pre. The options are
None, Laplacian, and
Volume-Weighted.
Volume-Weighted is the default.
Laplacian is slightly more accurate, but also
slightly more expensive, than Volume-Weighted.
Occasionally, when the surface tension force is driven only by wall adhesion
and there is no interior curvature, it may be beneficial to choose
None.
When setting initial conditions for surface tension calculations, it may also be beneficial to assign a smeared volume fraction rather than a discontinuous one. The hyperbolic tangent function (available in CEL) is useful for this purpose. For example, the following expressions defined a smeared droplet of radius 0.3 m centered at (x,y) = (0.5 [m], 0.5 [m]):
rdrop = 0.3 [m]
dist = rdrop -sqrt((x - 0.5[m])^2 + (y -
0.5[m]^2)
delta = 0.01 [m]
drop = 0.5*tanh(dist/delta)+0.5
These expressions represent:
rdrop - droplet radius
dist - signed distance to the droplet interface
delta - smearing distance
drop - the smeared volume fraction field: 1 inside the
droplet, 0 outside, and smoothly varying between 0 and 1 over a range of about
3*delta on each side of the interface.
To account for wall adhesion, you should specify a contact angle when setting wall boundary conditions. This is the angle the interface makes with the wall through the Primary Fluid that is prescribed on the Fluid Pairs tab of the Domain details view when the Surface Tension Coefficient is set. If the Primary Fluid is a liquid and the secondary fluid is a gas then, for non-wetting walls, the contact angle is greater than 90 degrees, while for wetting walls the contact angle is less than 90 degrees.