Numerical study of the droplet impact onto liquid film on the rough solid surface via lattice Boltzmann method
2017
The process of the droplet impact onto the liquid film on the rough
solid surface, as one of the basic multiphase problems, is very important
in many fields of science and engineering. On the other hand, the
problem is also very complicated since there are many parameters that
may influence the process of the droplet impact on the rough solid
surface with a liquid film. Up to now, there are still little research
on this problem, and to gain a better understanding on the physical
mechanics of the droplet impact onto the film on the rough solid surface,
it is desirable to conduct a detailed study. To clearly understand
the physical phenomena appearing in the process of droplet impact
on the liquid film, a parametric study on this problem is also carried
out based on a recently developed lattice Boltzmann method in which
a MRT lattice Boltzmann model is used to solve the Navier-Stokes equations,
and the other is adopted to solve the Cahn-Hilliard equation that
is used to depict the interface between different phases. In this
paper, the effects of the relative thickness of film ( h ), the relative width of cavity ( d * )
and the relative depth of cavity ( L * )
on the dynamic behavior of interface are investigated in detail, and
the velocity and pressure fields are also presented. In order to reduce
the influence of lattice, we fix the lattice to be 600×120 for
gas, which is fine enough to give accurate results. In addition, in
our simulations, We =500, Re =480,
viscosity ratio and density ratio are set to be 2:1. The numerical
results first show that, the phenomena of crown and entrainment can
be observed obviously during the process of droplet impact onto the
liquid film on the rough interface when We and Re are large. The radius of spray ( r ),
which is formed by the droplet impact onto liquid film, is related
to time through the relation r / 2 R ≈ α U t / 2 R when h is small, which is coincident
with the result of droplet impact onto the liquid film on smooth surface,
and additionally the coefficient α would decrease
with the increase of h . However, this relation seems
not accurate for the case with a large h , and simultaneously,
the splashing phenomenon has not been observed. Secondly, the relative
width of cavity d * plays an important
role on the phenomena of splashing. When d * =1, there will be two small droplets through the splashing phenomenon
(left half part), then with this parameter increase, the number of
small droplet and the point where the splashing occur will also change,
and there also are much difference in relation of spray radius and
time. Actually, if d * is small, the coefficient α would first decrease and then increase with the
increase of d * , while if d * >8, the cavity width would only have a little influence
on the behavior of spray. Finally, it is also found that the pressure
change near the cavity bottom is small at different L * , that is to say, the relative depth of cavity L * seems to has no apparent effect on the formation
of spray, but it brings a great influence on the splashing of spray
and the movement of the droplet which is produced in the process of
splashing.
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