In engineering systems, the presence of obstacles in the flow path provides resistance to flow accompanied by a pressure drop across the component. Often, the flow paths within such components are too complex to be resolved by a mesh (assuming it is known) and only the macro effects are of interest to design engineers.

Porous media routinely encountered in many engineering applications is a common example and refers to a multiphase matter consisting of a solid matrix with interconnected pore space allowing fluid flow (flow through membranes, filtration products, packed/fluidized beds, etc…). Similar characteristics can be exhibited by individual components of a large system (automobile radiators, showerheads of semiconductor process equipments, tubebanks in heat exchangers, screens/turning vanes in wind tunnels/HVAC etc…) and it is cheaper and desirable to represent them through a momentum loss model.

Momentum losses can be modeled by adding appropriate source (sink) terms to the momentum equations. In CFD-ACE+, this can be done using one of the following methods:

1. Momentum resistance model
   

   The pressure drop is characterized using lumped parameters (momentum resistance coefficients) and assigning them to the defined momentum resistance region. These coefficients are obtained from experiments or detailed numerical simulations of a representative section of the geometry. This model can be applied over volumes and defined geometries (cylindrical/polyhedral). This model would be appropriate for cases where a separate volume cannot be generated for the resistive region and you have access to the resistance coefficients through flow data such as pressure drop vs. velocity curves.

2. Full porous media model
   

   The overall effect of the pore space is represented through local volume averaging and the resulting momentum equation is solved. This allows a more physics based treatment of the problem and predicts the physical velocities and correct residence times within the porous region. It can be coupled with other physics including multiphase flows. This model can be applied only on separately meshed volumes and is useful for cases where you only have access to data related to the porous matrix such as porosity and permeability. It is appropriate to use this model when the problem involves other physics within the porous media. The user subroutine template ‘uporous’ can be used to control the porous media parameters.

3. Momentum sources
   

   Simple or complex momentum sources can be directly specified and added to the standard source terms in the momentum equations that are being solved. Sources can be applied only on separately meshed volumes. It is recommended for cases where you do not have access to much data about the porous matrix but able to estimate the source term based on empirical relations.

The following table provides a comparison of the three methods and can be used to identify and apply an appropriate method for the problem at hand.

Table 1.  Momentum loss models and corresponding parameters

If you have any questions about this tip or would like us to discuss other topics in the future, please let us know.

Regards,
Abraham Meganathan
ESI CFD Support Team