If the particle impinging on the surface has a sufficiently high energy, the particle splashes and several new particles are created. The number of particles created by each impact is explicitly set by the user in the
DPM tab in the
Boundary Conditions panel, as in Figure
22.4.3. The number of splashed parcels may be set to an integer value between zero and ten. The properties (diameter, magnitude, and direction) of the splashed parcels are randomly sampled from the experimentally obtained distribution functions described in the following sections. Setting the number of splashed parcels to zero turns off the splashing calculation. Bear in mind that each splashed parcel can be considered a discrete sample of the distribution curves and that selecting the number of splashed drops in the
Boundary Conditions panel does not limit the number of splashed drops, only the number of
parcels representing those drops.
Figure 22.4.3: The
Discrete Phase Model Panel and the
Film Model Parameters
Therefore, for each splashed parcel, a different diameter is obtained by sampling a cumulative probability distribution function (CPDF), which is obtained from a Weibull distribution function and fitted to the data from Mundo, et al. [
255]. The equation is
(22.4-4)
and it represents the probability of finding drops of diameter
in a sample of splashed drops. This distribution is similar to the Nakamura-Tanasawa distribution function used by O'Rourke [
272], but with the peak of the distribution function being
. To ensure that the distribution functions produce physical results with an increasing Weber number, the following expression for
from O'Rourke [
272] is used. The peak of the splashed diameter distribution is
(22.4-5)
where the expression for energy is given by Equation
22.4-1. Low Weber number impacts are described by the second term in Equation
22.4-5 and the peak of the minimum splashed diameter distribution is never less than 0.06 for very high energy impacts in any of the experiments analyzed by O'Rourke [
272]. The Weber number in Equation
22.4-5 is defined using the relative velocity and drop diameter:
(22.4-6)
The cumulative probability distribution function (CPDF) is needed so that a diameter can be sampled from the experimental data. The CPDF is obtained from integrating Equation
22.4-4 to obtain
(22.4-7)
which is bounded by zero and one. An expression for the diameter (which is a function of
, the impingement Weber number
, and the impingement energy) is obtained by inverting Equation
22.4-7 and sampling the CPDF between zero and one. The expression for the diameter of the
splashed parcel is therefore given by,
where
is the
random sample. Once the diameter of the splashed drop has been determined, the probability of finding that drop in a given sample is determined by evaluating Equation
22.4-4 at the given diameter. The number of drops per parcel can be expressed as a function of the total number of splashed drops:
(22.4-8)
where the
is for the
sample. The values of
are then normalized so that their sum is one. Both the number per parcel (
) and the total number of splashed drops (
) is unknown, but an expression for
can be obtained from the conservation of mass if the total splashed mass is known.
The amount of mass splashed from the surface is a quadratic function of the splashing energy, obtained from the experimental data from Mundo [
255]. The splashed mass fraction
is given by
The authors (O'Rourke et al. [
272]) noted that nearly all of the impacts for typical diesel sprays are well above the upper bound and so the splashing event nearly always ejects 70% of the mass of the impinging drop. To obtain an expression for the total number of drops, we note that overall conservation of mass requires that the sum of the total mass of the splashed parcel(s) must equal the splashed mass fraction, or
(22.4-9)
where
is the total mass of the impinging parcel. The expression for the total number of splashed drops is
The number of splashed drops per parcel is then determined by Equation
22.4-8 with the values of
given by Equation
22.4-4.
To calculate the velocity with which the splashed drops leave the surface, additional correlations are sampled for the normal component of the velocity. A Weibull function, fit to the data from Mundo [
255], is used as the PDF for the normal component. The probability density is given by
(22.4-10)
where
(22.4-11)
and
(22.4-12)
where
is the angle at which the parcel impacts the surface, or the impingement angle. The tangential component of the velocity is obtained from the expression for the reflection angle
:
(22.4-13)
combined with
(22.4-14)
Finally, an energy balance is performed for the new parcels so that the sum of the kinetic and surface energies of the new drops does not exceed that of the old drops. The energy balance is given by
where
is the threshold energy for splashing to occur. To ensure conservation of energy, the following correction factor is computed:
(22.4-15)
This correction factor is needed due to the relatively small number of sampled points for the velocity of the splashed drops (see Stanton [
355] for more detail). The components of the splashed parcel are multiplied by the square root of
in Equation
22.4-15 so that energy will be conserved. The normal and tangential velocity components of the splashed parcels are therefore given by
FLUENT will limit the velocity of the splashed parcels so that they do not exceed the impact velocity of the original parcel. It is important to note that splashing events are inherently transient, so the splashing submodel is only available with unsteady tracking in
FLUENT. Splashing can also cause large increases in source terms in the cells in which it occurs, which can cause difficulty in convergence between time steps. Thus, it may be necessary to use a smaller time step during the simulation when splashing is enabled.