# Source Macro, UDF, Momentum Source Term CFD Simulation

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• The problem numerically simulates the porous medium with momentum source term using ANSYS Fluent software.
• We design the 3-D model with the Design Modeler software.
• We mesh the model with ANSYS Meshing software.
• The mesh is Structured, and the element number equals 256,000.
• We use the User-Defined Function (UDF) to define a pressure profile.
• We use the Source Macro for UDF.

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## Description

In this project, we performed a numerical simulation using ANSYS Fluent software’s User-Defined Function (UDF). For this CFD product, we used Source Macro to write UDF programming. We considered a simple channel in which water flows. We assume a porous medium is in the middle of this channel.

A porous medium consists of voids in which the fluid passes through its empty spaces. This porous material can have significant effects on fluid behavior. One of the uses of porous media is to create a pressure drop in the flowing fluid. To apply the porous medium, we considered a separate zone in the interior space of the channel to define the porous medium for that zone.

Fluent software has a porous option; we can activate it for our modeling. However, we aim to use another method to define the porous medium. We know that during the solution process by Fluent, various equations, such as momentum equations, are solved. We want to influence the behavior of the fluid by defining a source term to the momentum equation as if there is a porous medium in the fluid flow path. First, we modeled the geometry in 3D with Design Modeler software. Then, we meshed the model with ANSYS Meshing software. Meshing is Structured, and 256,000 cells are created. Finally, we numerically simulated the current model based on the CFD method by ANSYS Fluent software.

## Methodology (UDF)

In this project, we need to define a source term for the momentum equation in the z-direction. Since the direction of the fluid flow is along the z-axis, we use the source term for the momentum-z equation. Since this source term is added as an additional term to the solver’s momentum equation, we need to define a User-Defined Function (UDF) to define the source term.

We must use the Source Macro (DEFINE_SOURCE macro) for this UDF. The Source Macro has many applications in simulations. Source macro defines any source term in various equations, including momentum, energy, etc., which the Fluent solver solves.

The source term relation we use is defined as follows. According to this relation, the source term is defined by the velocity along the z-axis and the location (y-component). Also, when we define the term source macro, we must define the source term’s derivative to the transport equation’s dependent variable. ## Conclusion (UDF)

After completing the calculation, we will review the results. To analyze the results, we obtain some contours and some plots. We obtained the pressure and pressure gradient contour in 2D and 3D and the plot of pressure changes in the longitudinal direction of the channel. The results correctly show that the pressure in the longitudinal direction of the channel, and precisely after passing through the zone for which we have defined the source term, has a significant decrease. We conclude that we performed the current numerical simulation and our UDF correctly.

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