
Particle Force Balance
FLUENT predicts the trajectory of a discrete phase particle (or droplet or bubble) by integrating the force balance on the particle, which is written in a Lagrangian reference frame. This force balance equates the particle inertia with the forces acting on the particle, and can be written (for the direction in Cartesian coordinates) as
where is an additional acceleration (force/unit particle mass) term, is the drag force per unit particle mass and
Here, is the fluid phase velocity, is the particle velocity, is the molecular viscosity of the fluid, is the fluid density, is the density of the particle, and is the particle diameter. Re is the relative Reynolds number, which is defined as
Inclusion of the Gravity Term
While Equation 22.21 includes a force of gravity on the particle, it is important to note that in FLUENT the default gravitational acceleration is zero. If you want to include the gravitational force, you must remember to define the magnitude and direction of the gravity vector in the Operating Conditions panel .
Other Forces
Equation 22.21 incorporates additional forces ( ) in the particle force balance that can be important under special circumstances. The first of these is the "virtual mass'' force, the force required to accelerate the fluid surrounding the particle. This force can be written as
and is important when
. An additional force arises due to the pressure gradient in the fluid:
Laws for Drag Coefficients
The drag coefficient, , can be taken from either
where , , and are constants that apply to smooth spherical particles over several ranges of Re given by Morsi and Alexander [ 253], or
(22.27) 
where
(22.28) 
which is taken from Haider and Levenspiel [
132]. The shape factor,
, is defined as
where is the surface area of a sphere having the same volume as the particle, and is the actual surface area of the particle. The Reynolds number is computed with the diameter of a sphere having the same volume.

The shape factor cannot exceed a value of 1.

For submicron particles, a form of Stokes' drag law is available [ 274]. In this case, is defined as
The factor is the Cunningham correction to Stokes' drag law , which you can compute from
where is the molecular mean free path.
A highMachnumber drag law is also available. This drag law is similar to the spherical law (Equation 22.26) with corrections [ 62] to account for a particle Mach number greater than 0.4 at a particle Reynolds number greater than 20.
For unsteady models involving discrete phase droplet breakup, a dynamic drag law option is also available. See Section 22.6 for a description of this law.
Instructions for selecting the drag law are provided in Section 22.11.4.
Forces in Rotating Reference Frames
The additional force term, , in Equation 22.21 also includes forces on particles that arise due to rotation of the reference frame. These forces arise when you are modeling flows in rotating frames of reference (see Section 10.2). For rotation defined about the axis, for example, the forces on the particles in the Cartesian and directions can be written as
where and are the particle and fluid velocities in the Cartesian direction, and
where and are the particle and fluid velocities in the Cartesian direction.
Thermophoretic Force
Small particles suspended in a gas that has a temperature gradient experience a force in the direction opposite to that of the gradient. This phenomenon is known as thermophoresis. FLUENT can optionally include a thermophoretic effect on particles in the additional acceleration (force/unit mass) term, , in Equation 22.21:
where is the thermophoretic coefficient. You can define the coefficient to be constant, polynomial, or a userdefined function, or you can use the form suggested by Talbot [ 367]:
This expression assumes that the particle is a sphere and that the fluid is an ideal gas.
Brownian Force
For submicron particles, the effects of Brownian motion can be optionally included in the additional force term. The components of the Brownian force are modeled as a Gaussian white noise process with spectral intensity given by [ 205]
where is the Kronecker delta function, and
is the absolute temperature of the fluid, is the kinematic viscosity, and is the Boltzmann constant. Amplitudes of the Brownian force components are of the form
where are zeromean, unitvarianceindependent Gaussian random numbers. The amplitudes of the Brownian force components are evaluated at each time step. The energy equation must be enabled in order for the Brownian force to take effect. Brownian force is intended only for nonturbulent models.
Saffman's Lift Force
The Saffman's lift force, or lift due to shear, can also be included in the additional force term as an option. The lift force used is from Li and Ahmadi [ 205] and is a generalization of the expression provided by Saffman [ 313]:
where and is the deformation tensor. This form of the lift force is intended for small particle Reynolds numbers. Also, the particle Reynolds number based on the particlefluid velocity difference must be smaller than the square root of the particle Reynolds number based on the shear field. Since this restriction is valid for submicron particles, it is recommended to use this option only for submicron particles.