The Venturi effect explains the increase or decrease in flow velocity, depending on the pressure difference and change in the cross-section of the flow area.
Using the Venturi effect, the flow phenomenon can be regulated in a building system to create natural ventilation.
Many structures make use of Venturi tubes, which optimize pressure differences inside and outside buildings to enforce ventilation and ensure thermal comfort for residents.
Natural ventilation, also known as passive ventilation, is an effective way to create a flow of air between indoor and outdoor spaces of building systems without the use of mechanical equipment. The physics behind this mechanism is the forced flow of air through a narrow passage, which can be achieved by creating a pressure difference—the phenomenon is also explained by the Venturi effect.
The Venturi effect explains the pressure difference around a building or in an interior and exterior of a structure and the natural flow prompted as a result of it. This mechanism can be exploited between structures to create natural ventilation during the design of buildings and walkways. This is an important consideration in modern structural design to ensure comfortable indoor conditions in an energy-efficient manner. The influence of the Venturi effect on natural ventilation can be analyzed in detail with CFD during the structural design phase of projects.
The Venturi Effect
The Venturi effect simply explains the flow of fluid as it passes through a narrow path.
In the pipe flow depicted in the diagram above, the volumetric flow rate is maintained through the inlet and outlet. To accommodate this flow, the velocity of fluids must increase at the constriction. However, when there is an increase in velocity, the static pressure decreases, according to Bernoulli’s principle. Mathematically, the principle can be represented as:
If we apply this statement to buildings and structures, the narrow air passage or opening where the cross-section is smaller acts as the constriction. The wind pressure creates high and low-pressure zones around the building; the low-pressure zone draws in the wind from the high-pressure zone, where the wind with high velocity creates a natural flow. This behavior of the wind can be employed in both vertical and horizontal directions of the building. One ideal way to do it is with the use of a Venturi tube.
Many buildings around the world have made use of Venturi tube structures to induce natural ventilation. Installed along the roof line, these tubes or vents create low pressure at openings, causing outdoor air to be sucked in and indoor air to be pushed out. The reduced cross-section due to the Venturi tube increases the velocity of airflow, which helps to manage the constant flow of a cool draft. This flow mechanism is also supported by the stack effect.
The stack effect explains the nature of the airflow as a result of heat transfer. When the air inside a room gets warmer, its density decreases and it rises towards the roof. The gap is then replaced by a gust of cold air with a higher density. The presence of a Venturi tube facilitates the constant circulation inside and outside the structure to provide thermal comfort to residents.
The building and roof design, Venturi tube placement, pressure difference, and flow velocity calculations are important analyses when studying the Venturi effect for natural ventilation. The relationship between these factors can be understood in detail with the help of CFD.
Simulating the Venturi Effect for Natural Ventilation
Optimizing the Venturi effect for natural ventilation involves a deeper analysis and understanding of fluid principles, flow behavior, and boundary conditions, alongside building and flow passage configurations. Different flow conditions for different structural designs can be numerically examined in multiple iterations with CFD simulation. Using RANS or LES modeling, accurate airflow details, especially near the solid boundaries, can be analyzed for steady and unsteady flow.
Furthermore, CFD analysis can be used to evaluate the ideal geometry, location, and spacing of the Venturi tube and establish its relationship with the flow velocity and pressure differences. With high-fidelity simulation, the ideal flow passage for natural ventilation can be designed, which simplifies engineering complexity while ensuring thermal comfort.