### 1.8.1. Momentum Sources

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.

#### 1.8.1.1. Isotropic Loss Model

The momentum loss through an isotropic porous region can be formulated using permeability and loss coefficients as follows:

| (1–182)

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:

| (1–183)

| (1–184)

#### 1.8.1.2. Directional Loss Model

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:

| (1–185)

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).

#### 1.8.1.3. General Momentum Sources

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:

| (1–186)

| (1–187)

| (1–188)

where the quantities are the specified momentum components. In ANSYS CFX 18.2, a representative scalar linearization coefficient based on the derivative:

| (1–189)

may be specified to provide robust convergence when a general momentum source is present.

#### 1.8.1.4. Immersed Solid Sources

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.

Release 18.2 - © ANSYS, Inc. All rights reserved.