A Building Design Environment CFD study is essential for making homes and offices healthy. People need fresh air to breathe and stay comfortable. If the air inside a building is not changed often enough, it gets stale and hot. This is where Air Change per Hour (ACH) becomes very important. ACH measures how many times the old air in a room is replaced by fresh outdoor air in one hour.

Architects use ACH ANSYS Fluent simulations to check their designs before building them. By performing a Building Design Environment CFD simulation, we can see invisible wind patterns. This helps us decide where to put windows and balconies. In this report, we use ACH CFD methods to compare four different building shapes. We want to find out which shape catches the wind best for natural ventilation. This Building Design Fluent analysis helps create energy-efficient buildings that don’t need expensive air conditioning all the time. For more details on ventilation systems, please explore our HVAC tutorials: https://cfdland.com/product-category/application/hvac-cfd-simulation/

• Reference [1]: Du, Yan, and Zhongcui Zhu. “Research on the form of energy-saving building under the influence of natural ventilation technology.” Journal of Physics: Conference Series. Vol. 2468. No. 1. IOP Publishing, 2023.

Figure 1: The four different building geometries (Form 1 to Form 4) used in the Building Design Environment CFD study to test ventilation performance.

Simulation Process: Atmospheric Boundary Layer and UDF Setup in Fluent

The simulation process for this Building Design Environment CFD project began with creating four distinct building models. All buildings are 12.8 meters high (about 4 stories), but they have different window and balcony layouts. To simulate the real world, we placed these models inside a large air domain. We generated a complex unstructured tetrahedral mesh. This type of grid is necessary for Building Design Fluent cases because it fits perfectly around the sharp corners of windows and doors. We refined the mesh near these openings to catch every bit of air entering the rooms.

Figure 2: The unstructured tetrahedral mesh generated for the buildings, showing refinement near windows for accurate ANSYS Fluent calculations.

Inside ANSYS Fluent, we needed to create a realistic wind. Wind is slow near the ground and fast at the top of the building. To mimic this, we wrote a script called a User Defined Function (UDF). This UDF creates a “logarithmic velocity profile” at the inlet. This setup allows the ACH CFD solver to simulate the true Atmospheric Boundary Layer. The wind blows into the domain, hits the building, and creates pressure differences that drive natural ventilation. The simulation calculates exactly how much air flows through each room to determine the Air Change per Hour.

Figure 3: The wind velocity profile applied at the inlet, defined by a User Defined Function (UDF) to match real atmospheric boundary layer conditions.

Post-processing: ACH Calculation and Ventilation Efficiency Analysis

The post-processing analysis provides a clear winner among the designs based on quantitative data. We must analyze the contours and the calculated Air Changes per Hour (ACH) to understand why one building is better than another. First, we look at the hard data in Table 1.

• Form 1 achieves an ACH of 72.33 with a mass flow rate of 37.77 kg/s.
• Form 2 improves this slightly to an ACH of 80.99.
• Form 3 is the superior design. It reaches the highest ACH of 83.02 with a massive flow rate of 51.24 kg/s.
• Form 4 performs poorly, dropping to an ACH of 67.55, despite having the same floor area (112 m²) as Form 3.

Table 1: Comparison of Air Changes per Hour (ACH) for four building design configurations from ANSYS Fluent CFD simulations.

Why does Form 3 win? The velocity contours in Figure 5 and Figure 6 explain the physics. In Form 3, the windows are aligned with the wind direction. This creates a “cross-ventilation” effect. The contours show green and yellow zones (0.5 – 1.5 m/s) penetrating deep into the rooms. The wind flows in one side and easily pushes the stale air out the other. In contrast, Form 4 is a failure for natural ventilation. The contours show mostly blue zones (low velocity) inside the rooms. This means the air is stagnant. The layout of Form 4 creates obstructions that block the wind path. Even though the wind hits the building, it cannot get inside effectively.

This Building Design Environment CFD simulation proves that simply having windows is not enough; their placement matters most. For a designer or architect, the lesson is clear: Form 3 provides the healthiest environment. It naturally replaces the air 83 times per hour, ensuring pollutants are removed quickly without needing expensive fans. The simulation successfully validates that maximizing the “flow path” is the key to high ACH values.

Figure 4: Streamlines showing the external wind path around Form 1 and Form 2, highlighting flow separation and entry into the building.

Figure 5: Velocity contours on horizontal planes at the first-floor level, visualizing the distribution of fresh air entering the rooms.

Figure 6: Velocity contours on horizontal planes at the fourth-floor level, comparing the ventilation effectiveness at higher elevations.

Figure 7: Velocity contours on vertical cut planes for Form 1 and Form 2, illustrating how wind penetrates through the different stories of the Building Design Environment.