Description

In this project, we performed a numerical simulation using ANSYS Fluent software's User-Defined Function (UDF). For this CFD product, we used Profile Macro to write UDF programming.

This product is the 9th chapter of the User-Defined Function (UDF) Training Course.

We considered a sample part of a simple urban area. In this sample area, we considered some volumes as apartments and assumed the space between two rows of apartments was a street and traffic area.

We defined the two sides of this urban area as the inlet and outlet boundaries. We used a pressure condition for both inlet and outlet boundaries. However, air pressure decreases as the height above the ground increases. So, it is better not to use a constant value to define the inlet air pressure. So, we must define a relation according to which the pressure varies with height. For this purpose, we use a profile describing the pressure regarding height.

We modeled the geometry in 3D with Design Modeler software. Meshing the model with ANSYS Meshing software produced 118,400 Structured cells. 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 function of air pressure variations in terms of position. The position we are considering is the height (Y-component of Cartesian coordinates). Since the pressure value at the inlet boundary is not constant, we need to define a User-Defined Function (UDF) to define the pressure function. We must use the Profile Macro (DEFINE_PROFILE macro) for this UDF. The PROFILE Macro has many applications in simulations.

One of the main applications of this type of macros is to use them in model boundaries to define the value of velocity, temperature, flow rate, pressure, etc., in terms of spatial coordinates or time. The pressure profile that we defined is as follows. According to this relation, the base air pressure is considered equal to the atmospheric pressure, and as the height increases, the air pressure decreases.

Conclusion (UDF)

After completing the calculation, we will review the results. To analyze the results, we obtain some contours and some plots. We obtain the pressure contour at the inlet boundary and two passing planes of the model domain. We obtained the plot of pressure changes in terms of the y-component of the position (i.e., height) at the inlet boundary and a central plane of the model domain.

All the results correctly show that air pressure decreases significantly with increasing air height. We conclude that we performed the current numerical simulation correctly, and our UDF worked correctly.