Vehicle Aerodynamics: Impact of Road Conditions

This research examines how changes on the road affect the Airflow around a vehicle. The goal is to understand how these changes, or disturbances, impact the Pressure Distribution at the back of the vehicle. This is important because knowing this can help in making vehicles more efficient and reduce their impact on the environment.

Vehicles are tested using several methods: in a controlled wind tunnel, on tracks built specifically for testing, and on regular roads. This mix allows for a better understanding of how real-world conditions affect vehicle aerodynamics. In the tests, researchers focus on how the airflow becomes uneven, which can lead to increased drag or wind resistance.

#Importance of Aerodynamics

Reducing drag is crucial because it directly affects how much fuel a vehicle uses. Studies show that around 70% of Energy loss in vehicles at highway speeds comes from Aerodynamic Drag. This highlights the need for vehicle manufacturers to continually search for ways to improve car shapes to minimize aerodynamic resistance.

However, cars do not always travel in a straight line or at constant speeds when on the road. Factors like wind direction and different road environments can influence how a car moves through the air and changes its drag. For instance, when cars pass other vehicles, or drive near trees and buildings, the airflow changes, affecting the car's performance.

#Research Focus

This study centers on examining how the airflow, or wake, behind a vehicle responds to the different conditions it encounters while driving. The focus is mainly on how pressure distribution at the back of the car changes, which can lead to either vertical or horizontal imbalances in the airflow.

To do this, researchers equipped a Citroën C4 Cactus with special tools to measure the airflow and pressure at the back of the vehicle. They used various testing methods to collect this data, including wind tunnel testing, driving on a track, and taking the vehicle on a real-world driving route.

#Testing Methods

1. Wind Tunnel Testing: In this controlled environment, researchers can simulate different wind conditions and Yaw Angles (the angle between the vehicle's heading and the wind direction) while keeping other variables constant. This helps in understanding the baseline aerodynamic performance of the vehicle.

2. Track Testing: On closed tracks, researchers can control many variables but still mimic real-world conditions. Tests here allow for a controlled driving experience while examining how airflow around the vehicle changes over different speeds and road conditions.

3. On-Road Testing: This method provides the most realistic data. The vehicle is driven on various roads to see how it performs in everyday driving conditions. This helps researchers understand how external factors affect the wake behind the vehicle.

#Measurement Setup

The vehicle used for testing was equipped with multiple sensors to gather data on airflow and pressure distribution. This includes:

• Prandtl Antenna: Helps measure wind direction and speed.
• Conrad Probe: Measures pressure differences.
• Pressure Taps: Located at the base of the vehicle to provide detailed pressure data.

The data from these tools helps researchers analyze how the pressure distribution at the back of the vehicle changes under different conditions.

#Key Findings from Wind Tunnel Tests

Wind tunnel tests showed that the flow of air around the vehicle is affected by the yaw angle. At a yaw angle of zero, the airflow remains largely predictable. However, the distribution of pressure is not perfect and can shift due to small misalignments during testing.

The analysis of the airflow using a method called Proper Orthogonal Decomposition (POD) demonstrated that two main patterns of airflow emerge, which are linked to vertical and horizontal changes in the vehicle's wake. These patterns together account for a significant portion of the total energy in the system.

#Findings from On-Road Testing

On-road tests revealed that the airflow behaves differently compared to wind tunnel tests. The pressure distribution showed much greater variability, which is representative of real-world conditions. The yaw angles were found to be normally distributed, meaning most of the time, the vehicle experienced yaw angles close to zero, with occasional larger angles caused by various environmental factors.

The analysis again showed two main modes in the airflow patterns, similar to the wind tunnel findings, but with increased energy contribution in the on-road testing. This indicates that the vehicle experiences more dynamic changes in airflow while on the road compared to controlled environments.

#Role of Low Frequencies

An important aspect identified in both wind tunnel and on-road tests is the impact of low-frequency changes in the air. Low frequencies indicate gradual changes in airflow that can occur when a vehicle moves through different environments or encounters obstacles. This suggests that the vehicle’s wake takes longer to adjust, affecting drag and efficiency.

Research showed that more than 70% of the energy in the airflow patterns occurred at these low frequencies. Understanding this can help in designing vehicles that maintain better aerodynamic efficiency across varying driving conditions.

#Comparison of Results

When comparing results from wind tunnel tests to those from on-road tests, researchers found significant differences. Wind tunnel tests with fixed yaw angles did not capture the same variety of conditions that the vehicle encountered on the road. The energy associated with the main modes of airflow was much higher in on-road tests, indicating that real-world driving presents more challenges and variations for vehicle aerodynamics.

Additionally, the data suggests that incorporating variable yaw angles into wind tunnel testing can help better simulate real-world conditions and consequently improve the design and performance of vehicles.

#Conclusion

The findings of this research emphasize the vital role of aerodynamics in vehicle efficiency, particularly in real driving conditions. Testing under a variety of conditions provides crucial data that helps in understanding how vehicles interact with the surrounding environment.

By focusing on the asymmetry of the vehicle wake, researchers can develop more accurate models of vehicle performance and contribute to efforts aimed at reducing drag and improving fuel efficiency. The study showcases that vehicles must be understood not only in controlled environments but also in the diverse conditions they experience daily.

Future research could explore how other factors, such as pitch angle and different vehicle types, influence aerodynamic performance. Overall, this work lays the groundwork for more effective design strategies that can lead to better performance and lower emissions in the long run.