Are Wind Tunnels For an Eternity in Automotive Design Testing

The development of testing methods for automotive design has evolved in line with significant technological advancements. In the past, these tests relied heavily on physical testing and prototyping, which were time-consuming and expensive. However, with the introduction of computer-aided design (CAD) and computer-aided engineering (CAE), the landscape of automotive testing has shifted to a more efficient and cost-effective process.

Automotive design currently relies on computational fluid dynamics (CFD) simulations and wind tunnel testing to evaluate a vehicle's aerodynamic characteristics. This blog post explores the intricacies of wind tunnel testing and clarifies the collaborative relationship between CFD and wind tunnels in automotive design, highlighting the precision and accuracy of these methods.

Wind Tunnels: Engineered for an Eternity

A wind tunnel is a large tube with a powerful fan or compressor at one end that blows air through the tube. Inside the tunnel, automotive models undergo testing with different wind speeds and directions to mimic real-world conditions. Engineers then use sensors to measure the forces acting on the model and collect data for analysis.

Aerodynamic testing in automotive design has shifted towards a more sophisticated interplay between CFD and wind tunnel testing, resulting in accelerated and more nuanced insights into vehicle performance. CFD, leveraging highly complex mathematical equations, simulates airflow around vehicle exteriors and components, permitting engineers to meticulously analyze aerodynamic properties and design efficacy without the immediate need for physical prototypes. This approach has significantly streamlined the initial design phases, offering detailed airflow visualization and enabling rapid iterative testing.

Despite the increasing prominence of CFD in the industry, wind tunnel testing remains indispensable. Wind tunnels provide an environment for empirical validation, which is essential for correlating and refining CFD simulations. They offer the unique ability to test under an array of real-world conditions that algorithms alone cannot fully replicate—such as rain, snow, and temperature variations. Furthermore, wind tunnels are crucial for evaluating cutting-edge concepts, including active aerodynamics and underbody airflow management, ensuring that designs perform well in simulations and meet the rigorous demands of real-world driving conditions. This underscores the irreplaceable value of wind tunnel testing in automotive design.

From Wood to Composites: How Wind Tunnel Models Have Evolved

Over the last 20-30 years, wind tunnel testing has evolved significantly due to advancements in manufacturing techniques used to create wind tunnel models. In the 1990s, automotive prototypes for aerodynamic testing were predominantly handcrafted using wood and metal components. This manual construction resulted in prolonged lead times and less precision. However, the early 2000s saw the rise of carbon fiber as a preferred material, which, while providing some advancements, still maintained similar lead times for full-size vehicle production.

On the other hand, modern wind tunnel models are primarily 3D printed directly from the CAD models used to manufacture full-size cars. This contemporary approach integrates some structural aluminum components, such as the rear wing, front wing mainplane, and suspension members, to ensure robust and accurate representations of the aerodynamic properties. The shift to these advanced manufacturing methods has greatly enhanced the precision and efficiency of wind tunnel testing, providing more reliable data and accelerating the overall design process.

Achieving dynamic similarity is crucial for accurate aerodynamic testing of scaled models. This means that the surface flow features of the scale tests must match those of the full-sized car. Larger model scales are preferred for improved accuracy as the dynamic similarity is related to the Reynolds number, which is a ratio of speed, length, and viscosity. If the model scale is 50%, then the airspeed must be twice as fast to match the Reynolds number and produce the same aerodynamic profile. When it comes to larger-scale testing, there are some drawbacks, primarily related to the cost of materials.

Understanding the Mechanics of Wind Tunnel Testing

Wind tunnels used in automotive and aerodynamics research are frequently constructed in a "closed return" configuration. This design circulates air in a continuous rectangular loop within a sealed system, ensuring controlled and repeatable test conditions. Various flow conditioners, such as honeycombs and screens, are installed within the tunnel to guide the air through the corners of the loop. These conditioners effectively straighten the airflow and mitigate turbulence generated by the powerful fans.

Since the air temperature rises due to the fan motors during recirculation, maintaining a constant temperature is crucial for obtaining precise measurements. Therefore, large heat exchangers, resembling those used in car radiators, are employed to cool the air and sustain a stable temperature throughout the tunnel.

The working section of a wind tunnel is the focal point for measurement and model testing. Here, the tunnel's cross-sectional area contracts to accelerate the airflow, creating a uniform current before entering the working section. To replicate realistic vehicle-road interactions, the ground section of the tunnel includes a rolling road system that matches the speed of the air and rotates the model's tires. When airflow interacts with a surface, a boundary layer—a region of slower-moving air close to the surface—forms due to viscosity. This boundary layer thickens as it travels along the surface, especially in areas where surface pressure increases rapidly.

For accurate underbody flow simulation, the rolling road system moves in conjunction with the vehicle model to prevent the merging of boundary layers on the ground and the car's underbody. This aspect is critical as vehicle underbodies create substantial low-pressure zones, necessitating the rolling road belt to be securely anchored to avoid lift and maintain aerodynamics under high-speed conditions. Advanced wind tunnels also feature turntables situated beneath the rolling road, facilitating cornering simulations to enhance the fidelity of aerodynamic testing further.

Current Demands in Automotive Testing

In response to the rising demand for energy-efficient vehicles, the automotive industry is intensively focusing on reducing aerodynamic drag to boost fuel efficiency and lower emissions. Accelerated design cycles and enhanced geometric variation exploration are essential for capturing incremental aerodynamic improvements and optimizing vehicle efficiency.

Central to this effort is applying high-fidelity CFD, particularly through Large Eddy Simulation (LES), for its precise aerodynamic behavior predictions. LES's detailed turbulence modeling around vehicles enables precise design optimizations, diminishing the dependency on wind tunnel testing and expediting the design process.

Moreover, GPU acceleration is crucial for maximizing the efficiency of high-fidelity CFD simulations. Utilizing GPUs' parallel processing capabilities significantly speeds up computational tasks, facilitating quicker iterations and complex aerodynamic studies. This advancement boosts simulation throughput and minimizes the need for physical prototypes, driving forward more efficient, innovative vehicle designs.

Millennium M1 CFD Supercomputer for Faster and Accurate Automotive Design Simulation

The Cadence Millennium M1 CFD Supercomputer is an all-in-one solution for tackling the challenges of lengthy design cycles and costly experiments in aerospace, automotive, power generation, and turbomachinery applications. Powered by high-performance computing hardware and graphic processing units, the Millennium M1 utilizes the Cadence Fidelity LES Solver for LES to deliver exceptional performance. This supercomputer generates synthetic data that enables generative AI to quickly identify optimal system design solutions without compromising accuracy. As a result, the turnaround time is reduced from days to hours, which expands the practical application of LES in various industries.

Recent advancements in computational and experimental aerodynamics, including the integration of high-fidelity CFD simulations with advanced wind tunnel testing, have significantly transformed automotive design. These developments have streamlined the design process and enhanced the accuracy and reliability of aerodynamic evaluations. With the ongoing improvement of these technologies and the advent of GPU acceleration, engineers can perform quicker, more detailed analyses, advancing vehicle performance and fuel efficiency. As sustainability becomes increasingly important in the automotive industry, CFD and wind tunnel testing are essential for optimizing vehicle aerodynamics.

Check out the blog posts and articles below to learn more about Cadence's Millennium platform and CFD solutions:

• Millennium M1 CFD Supercomputer
• Millennium Enterprise Multiphysics Platform
• Millennium Press Release
• Fidelity CFD Solver
• Fuel Economy and Aerodynamics of Automobiles
• Automotive CAFE 2032 Standards
• Millennium M1: For the Ultimate Balance of Vehicle Aerodynamics and Aesthetics!