By default, the film mass and momentum equations are solved sequentially in the Eulerian Wall Film model. In situations where the film height and velocities are closely coupled, a segregated solution approach will not be able to resolve the specific features of the film, such as wavy surface of a free-falling film.
In the coupled solution approach, the film mass and momentum equations are solved in parallel, where film height and velocity solutions of the previous time step are used in the solution of both equations in the next time step.
To improve the solution stability, smoothing is applied to variables used in film curvature and surface tension force calculations in the coupled solution approach. Variable smoothing is achieved by blending variable values from neighbouring faces in an iterative operation:
 | (17–53) |
where 
and 
refer to original and smoothed values of the film variable 
on a given face, respectively; 
refers to a value of 
on the 
th neighbouring face (direct or indirect), as
illustrated in Figure 17.6: Face Value Smoothing; 
is a smoothing factor ranging from 0 to 1 (default = 0.5); and
is a weighting factor determined by the smoothing algorithm.
The weighted averaging is repeated as many times as specified by the Smoothing Level input parameter ranging from 1 to 4 (default = 2).