A momentum source is implemented in Ansys CFX as a force per unit volume acting on the fluid(s). Momentum sources can be used to model isotropic losses in porous regions, directional losses in porous regions, or other processes. These situations are described further in the following sections. More information on how to use the momentum source models is available in Momentum Sources in the CFX-Solver Modeling Guide.
The momentum loss through an isotropic porous region can be formulated using permeability and loss coefficients as follows:
 | (1–186) |
where 
is the permeability
and 
is the quadratic
loss coefficient. The linear component of this source represents viscous
losses and the quadratic term represents inertial losses.
Note:
may be thought
of as a 'pressure gradient per dynamic head'. An engineering handbook
may provide data in the form of a 'pressure drop per unit head'. To
calculate 
, divide by
the distance over which the pressure drop occurs (which could be the
thickness of a perforated plate, for example).
The source may alternatively be formulated using linear and
quadratic resistance coefficients, substituting two coefficients 
and 
as follows:
 | (1–187) |
 | (1–188) |
The momentum source through an anisotropic porous region (such
as a honeycomb or perforated plate) may be modeled using the directional
loss model. With this model, the streamwise direction (which is permitted
to vary in space), must be specified. Consider a streamwise-oriented
coordinate system (
) such that the 
-axis is aligned with the streamwise
direction and the 
axes lie on the transverse plane. The momentum
losses in these directions are:
 | (1–189) |
where 
and 
are the streamwise and transverse permeabilities,
and 
and 
are the streamwise and transverse quadratic loss
coefficients. These quantities may also be expressed in terms of linear
and quadratic resistance coefficients.
In addition, the transverse loss may be modeled by multiplying the streamwise coefficients by some factor. If this option is used together with a streamwise permeability, the implied transverse permeability is equal to the streamwise permeability divided by this factor.
Note: Each of 
and 
may be thought of as a 'pressure gradient per dynamic
head'. An engineering handbook may provide data in the form of a 'pressure
drop per unit head'. To calculate 
or 
, divide by the distance over which the pressure
drop occurs (which could be the thickness of a perforated plate, for
example).
The general momentum source is available for specifying momentum sources that are not covered by the isotropic or directional loss models. A different source can be specified for each direction as follows:
 | (1–190) |
 | (1–191) |
 | (1–192) |
where the 
quantities are the specified momentum components.
In Ansys CFX 2024 R2, a representative scalar linearization coefficient
based on the derivative:
 | (1–193) |
may be specified to provide robust convergence when a general momentum source is present.
The presence of an immersed solid in the flow field is modeled through a body force similar to the general momentum source. For details, see Immersed Solid Control in the CFX-Pre User's Guide and Momentum Sources.