Designing a new Aircraft

Computational Fluid Dynamics (CFD) has increasingly become a vital tool in the aerospace industry, often seen as a replacement for traditional wind tunnel testing. Here are several reasons as to why we chose it over wind tunnels in the design of our new aircraft:

Flexibility and Speed

CFD allows for rapid iteration and testing of multiple design variations without the need to build new physical models. We can quickly adjust parameters and rerun simulations to explore different cases around the flight envelope. This flexibility accelerates the design process and enables more comprehensive exploration of design options.

Relevance of Existing Empirical Design Methods

Historically lots of valuable information has been extracted from wind tunnel testing and much of this can be found in ESDU, Datcom and NACA/NASA papers. Unfortunately though much of this data is only relevant to either typical commercial airliner configurations or those of older designs where blended body designs were not common. These are still great sources of information to frame up a preliminary design but during detailed design their relevance becomes less.

Validation and Limitations at High AoA

We have been involved in other projects where CFD has been used in parallel with wind tunnel testing, and the results have been remarkably consistent, demonstrating the reliability and accuracy of CFD in predicting aerodynamic performance.

However, despite its many advantages, CFD is not always reliable at high angles of attack where there are large regions of flow separation. At these conditions, the flow becomes highly turbulent and unsteady, making it challenging for CFD models to accurately predict the aerodynamic forces and moments.

Scaling

One of the most significant issues in aerodynamic performance is the difference in Reynolds number between scaled and full size aircraft. The Reynolds number, which is a dimensionless quantity representing the ratio of inertial forces to viscous forces, affects the flow characteristics around substantial parts of an aircraft structure and achieving the same Reynolds number as the full-scale aircraft in a wind tunnel is difficult because it requires large scale models and / or high speeds. Discrepancies in Reynolds number can lead to differences in boundary layer behaviour, flow separation, and transition from laminar to turbulent flow, which negatively impacts on accuracy.

Further, scaling down an aircraft model can introduce geometric distortions. Certain features, such as small protuberances, surface roughness, and gaps, may not scale proportionally or may have to be blended over completely. These geometric discrepancies can affect the aerodynamic characteristics of the model in the wind tunnel, leading to reduced accuracy in the results.

Detailed Flow Analysis

CFD can capture complex phenomena such as turbulence, boundary layer behavior, and flow separation with high resolution. This level of detail is often difficult to achieve with wind tunnel testing, where measurements are limited to specific points and require extensive instrumentation.

With CFD it is also easier to "zoom" into areas of the design and thoroughly interrogate them.

Simulation of Extreme Conditions

Wind tunnels have limitations in replicating extreme flight conditions, such as high-altitude, high-speed, or very low-speed scenarios. CFD, however, can simulate a wide range of conditions, including those that are impractical or impossible to recreate in a wind tunnel. This capability is crucial for designing aircraft that perform well in all possible flight regimes.

Integration with Modern Design Tools

CFD integrates seamlessly with modern computer-aided design (CAD) tools, allowing for a streamlined workflow from design to analysis. This facilitates concurrent engineering, where design and analysis occur simultaneously, reducing the overall development time. As an example, CFD can be used concurrently with FEA enabling the simulation of complex physical phenomena with one and two way fluid structure interaction, allowing us to predict how products will behave aerodynamically and structurally under transient conditions.

Swift currently makes extensive use of the ANSYS suite of products for both CFD, FEA and composite analysis.

Benefits of Recent Four-Equation Turbulence Models

Recent advancements in turbulence modeling, particularly the development of four-equation models, have significantly improved the ability of CFD to capture intermittency in transitional flows. These models, such as the Transition SST (Shear Stress Transport) model, provide a more accurate representation of the transition from laminar to turbulent flow. This is particularly beneficial in predicting flow behavior in regions where traditional two-equation models fall short. By incorporating additional transport equations for intermittency and transition onset, these models enhance the predictive accuracy of CFD simulations, making them more reliable for complex aerodynamic analyses.

Cost Efficiency and Environmental Considerations

Wind tunnel testing is notoriously expensive. Building and maintaining wind tunnels, along with the costs associated with running tests, can be prohibitively high. Each test requires physical models, which are costly to produce and modify. In contrast, CFD simulations only require computational resources and software, making them significantly cheaper in the long run. Additionally, wind tunnels consume a significant amount of energy and can have environmental impacts. CFD, on the other hand, primarily relies on computational power, which can be sourced from renewable energy. This not only reduces operational costs but also minimizes the environmental footprint.

Conclusion

While wind tunnels have been indispensable in the history of aeronautical engineering, CFD offers a modern, efficient, and versatile alternative. The ability to simulate complex flow phenomena, coupled with cost savings and flexibility, makes CFD an attractive option for aircraft design. As computational capabilities and turbulence models continue to advance, CFD is likely to play an even more significant role.