Impact of Aspect Ratio on Airflow Dynamics

The study of how objects interact with airflow is important for understanding various applications, especially in urban environments. One such object is a square cylinder placed in the airflow, which can create different patterns or "Wakes" behind it. This understanding is crucial for designing buildings, planning drone deliveries, and addressing pollution issues in cities. Here, we focus on how the aspect ratio (the height compared to the width) of the square cylinder influences the wake and Turbulence in the surrounding airflow.

#The Importance of Wake Analysis

Wake analysis refers to observing the flow patterns that develop behind an object in the airflow. The shape and size of an object can change these patterns, affecting how air moves around it. In urban areas, where many buildings are present, knowing these patterns can help predict how pollutants disperse, assist in planning drone routes for deliveries, and contribute to urban safety.

#Turbulent Boundary Layer

When wind flow interacts with a surface, like the ground or a building, it can create a boundary layer. This layer refers to the region where the airflow is influenced by friction against the surface. Within this boundary layer, turbulence, which is chaotic and irregular airflow, plays a significant role. Understanding how turbulence behaves close to buildings and structures is essential for predicting how air will flow in cities.

#Square Cylinder and Aspect Ratio

In our study, we look at a square cylinder placed against the airflow. The aspect ratio (AR) is the measurement that defines the height of the cylinder divided by its width. For example, a square cylinder has an AR of 1. When the height increases while keeping the width constant, the AR increases. Different ARs can lead to various wake patterns and impact how turbulence develops.

#Methodology

To analyze the wakes behind the wall-mounted square cylinder, we used a high-resolution computer simulation method known as Large Eddy Simulation (LES). This method allows us to observe the flow characteristics in detail. We set the airflow speed and cylinder height to specific values while varying the AR from 1 to 4.

#Wake Description and Configuration

The wake behind the cylinder typically consists of several features, including tip vortices (swirling air at the top of the cylinder), base vortices (swirling air at the base), and spanwise vortices (structures stretching from side to side). The wake configuration changes depending on the AR of the cylinder.

• Low Aspect Ratios (AR 1 and 2): At lower ARs, the wake tends to be more stable and exhibits a "dipole" configuration, meaning the air moves symmetrically on both sides of the cylinder with a single recirculation zone behind it.
• High Aspect Ratios (AR 3 and 4): As the AR increases, the wake becomes more complex and can exhibit an "antisymmetric" configuration known as Kármán vortex shedding. This means that the airflow behind the cylinder loses its symmetry and has more chaotic patterns.

#The Role of Tip and Base Vortices

Tip vortices are created at the top edge of the cylinder and can lead to strong downward flow in the wake. Base vortices, on the other hand, are generated at the base of the cylinder and can create an upward flow. Both types of vortices contribute to the turbulence seen in the wake:

• For low AR: The tip vortices are weak, and the base vortices are less prominent, leading to a larger and more stable wake.
• For high AR: The tip vortices become stronger, causing the wake to shrink in size and create two distinct recirculation zones.

#Turbulent Kinetic Energy

Turbulent Kinetic Energy (TKE) is a measure of the energy present in turbulent flows. In our analysis, we observed how TKE varies across different ARs.

• At lower ARs: TKE tends to be concentrated above the cylinder, primarily due to the tip vortices.
• At higher ARs: TKE distribution changes, with notable peaks forming in the wake area, attributed to stronger base vortices and complex interactions between the turbulent structures.

#Anisotropy and Turbulence Distribution

Turbulence does not distribute evenly in different directions. By studying the anisotropy of the turbulence, we can visualize how turbulence varies across the wake.

• Three-Dimensional Features: As AR increases, the turbulence distribution in the wake becomes more three-dimensional. This means that the energy in the turbulent flow is not uniform but varies in strength depending on the location relative to the cylinder.

#Simulations and Observations

The simulations involved several configurations at different ARs. For each configuration, we collected data on velocity distributions and turbulence strengths.

• Visualizations: We created visual representations of the flow, helping to illustrate how the changes in AR affect the wake structure. These observations confirmed the shift from a symmetric dipole wake at low ARs to the more chaotic and unstable structures at higher ARs.

#Impacts of Aspect Ratio on Pollution Dispersion

Understanding the wake behind a square cylinder is particularly relevant for cities with high pollution levels.

• Pollutant Distribution: The way in which airflow and turbulence interact with buildings can significantly influence how pollutants spread. For example, configurations that create stronger downwash may trap pollutants closer to the ground, impacting air quality.
• Urban Design: Insights gained from these observations can inform better urban planning and building designs, minimizing pollution exposure for residents.

#Conclusion

In summary, the aspect ratio of a wall-mounted square cylinder plays a critical role in shaping the airflow and turbulence patterns around it. The relationship between AR and the wake configuration helps us understand how turbulent flows behave, which is essential for applications in urban environments.

Moving forward, further studies will add to our knowledge of how other geometric features and airflow conditions influence wake dynamics. This understanding is crucial for enhancing urban safety, improving pollution management, and optimizing drone delivery routes.