Particle erosion and accretion rates can be monitored at wall boundaries. The erosion rate is defined as
 | (12–346) |
where 
is a function of particle diameter, 
is the impact angle of the particle path with the wall face, 
is a function of impact angle, 
is the relative particle velocity, 
is a function of relative particle velocity, and 
is the area of the cell face at the wall. Default values are 
, 
, and 
. Note that 
, 
, and 
are defined as boundary conditions at a wall, rather than properties of a material, so the default values are not updated to reflect the actual materials being used. Therefore, you should specify values appropriate to your problem at all walls as described in Setting Particle Erosion and Accretion Parameters in the Fluent User's Guide. Values of these functions for sand eroding both carbon steel and aluminum are given by Edwards et al. [160].
Functions for 
, 
, and 
must be expressed as piecewise-linear, piecewise-polynomial
or polynomial functions in order to define them as part of the wall
boundary conditions. Therefore, it may be necessary to approximate
functions found in the literature by one of these methods.
The erosion rate as calculated above is displayed in units of
removed material/(area-time), that is, mass flux, and can therefore
be changed accordingly to the defined units in Ansys Fluent. The functions 
and 
have to be specified in consistent
units to build a dimensionless group with the relative particle velocity
and its exponent. To compute an erosion rate in terms of length/time
(mm/year, for example) you can either define a custom field function
to divide the erosion rate by the density of the wall material or
include this division in the units for 
and/or 
. Note that the units given by Ansys Fluent when displaying the erosion rate are no longer valid in the
latter case.
A variety of erosion models [180], [424], [159], [479], [236], [568] containing model constants [236], [159] and angle functions can be easily implemented into Ansys Fluent. The equations describing some of the erosion models can be modified to appear in the form of the general equation describing the erosion rate, Equation 12–346.
For example, the Tulsa Angle Dependent Model [159] described by Equation 12–347
 | (12–347) |
can be rewritten in the form of Equation 12–346 with the following substitutions:
|
|
|
|
where 
is the erosion ratio (mass of surface eroded per mass of particles impinging), 
is the Brinell hardness, and 
is a particle shape coefficient. The Tulsa model [159] suggests values for 
of 1.0 for sharp/angular sand, 0.53 for semi-rounded sand, and 0.2 for fully rounded sand. As noted above, the impact angle function can be approximated by a piecewise-polynomial fit. The following is an approximation of the impact angle function for steel being eroded by sand.
User-defined functions can be used to describe erosion models of any form. For more complex models, such as those in which 
cannot be approximated by linear or polynomial functions of 
only, the default Erosion Model in the Wall boundary condition dialog box cannot be used. Hence, a user-defined function should be used instead. For information on how to apply user-defined functions for DPM erosion models, see DEFINE_DPM_EROSION in the Fluent Customization Manual, or contact your support engineer for further assistance.
The accretion rate is defined as
 | (12–348) |