
For granular flows in the compressible regime (i.e., where the solids volume fraction is less than its maximum allowed value), a solids pressure is calculated independently and used for the pressure gradient term, , in the granularphase momentum equation. Because a Maxwellian velocity distribution is used for the particles, a granular temperature is introduced into the model, and appears in the expression for the solids pressure and viscosities. The solids pressure is composed of a kinetic term and a second term due to particle collisions:
where is the coefficient of restitution for particle collisions, is the radial distribution function, and is the granular temperature. ANSYS FLUENT uses a default value of 0.9 for , but the value can be adjusted to suit the particle type. The granular temperature is proportional to the kinetic energy of the fluctuating particle motion, and will be described later in this section. The function (described below in more detail) is a distribution function that governs the transition from the "compressible'' condition with , where the spacing between the solid particles can continue to decrease, to the "incompressible'' condition with , where no further decrease in the spacing can occur. A value of 0.63 is the default for , but you can modify it during the problem setup.
Other formulations that are also available in ANSYS FLUENT are [ 343]
and [ 213]
(16.566) 
where is the average diameter, , are the number of particles, and are the masses of the particles in phases and , and is a function of the masses of the particles and their granular temperatures. For now, we have to simplify this expression so that it depends only on the granular temperature of phase
Since all models need to be cast in the general form, it follows that
(16.568) 
where is the collisional part of the pressure between phases and .
The above expression reverts to the one solids phase expression when and but also has the property of feeling the presence of other phases.
Radial Distribution Function
The radial distribution function, , is a correction factor that modifies the probability of collisions between grains when the solid granular phase becomes dense. This function may also be interpreted as the nondimensional distance between spheres:
where is the distance between grains. From Equation 16.569 it can be observed that for a dilute solid phase , and therefore . In the limit when the solid phase compacts, and . The radial distribution function is closely connected to the factor of Chapman and Cowling's [ 49] theory of nonuniform gases. is equal to 1 for a rare gas, and increases and tends to infinity when the molecules are so close together that motion is not possible.
In the literature there is no unique formulation for the radial distribution function. ANSYS FLUENT has a number of options:
This is an empirical function and does not extend easily to phases. For two identical phases with the property that , the above function is not consistent for the calculation of the partial pressures and , . In order to correct this problem, ANSYS FLUENT uses the following consistent formulation:
where
(16.572) 
and are solid phases only.
When the number of solid phases is greater than 1, Equation 16.571, Equation 16.573 and Equation 16.574 are extended to
It is interesting to note that Equation 16.573 and Equation 16.574 compare well with [ 3] experimental data, while Equation 16.575 reverts to the [ 46] derivation.