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The fluid mechanics anomaly we can’t dismiss anymore


December 9, 2024

Professor Owen Williams is sounding the alarm on asymmetric flows, long dismissed as experimental errors, as critical to understand and predict for flight safety.

A widespread, underreported phenomenon: asymmetric flows

1/48 scale model of an F-18 aircraft in Flow Visualization Facility. Photo credit: NASA Dryden Flight Research Center Photo Collection, ECN-33298-03.

For decades, an unexpected phenomenon has puzzled fluid mechanics researchers: asymmetric flows that defy conventional predictions. Traditionally, researchers thought that symmetric objects would generate symmetric fluid flows around them. But a growing body of research reveals a more complex reality.

A&A’s Professor Owen Williams, along with Professor Alexander Smits of Princeton, are shedding light on this problem in an upcoming review paper for the Annual Review of Fluid Mechanics. Their work exposes a widespread yet underreported issue that has significant implications for vehicle design and safety.

Asymmetric flows occur in surprisingly diverse scenarios  – from ship and submarine wakes to airflows over cars, aircraft, hills and wings. These unpredictable flow patterns most frequently emerge at the edges of the flight envelope, when the flow can “separate” from the surface, creating complex recirculation regions that challenge existing predictive models.

"Historically, researchers often dismissed these asymmetries as experimental errors when they were observed,” Williams explains. “They were typically considered a problem with the test setup or methodology rather than a genuine scientific phenomenon.”

“Teasing together these threads of information is a crucial step in developing a unified theory to predict this behavior,” says Williams.

 

What is a flight envelope?

A flight envelope is the range of operational limits for an aircraft, including its maximum speed, altitude, and load factor.

Implications for design and safety

A resistance to report asymmetries has hindered progress in understanding and addressing these flow dynamics. Williams and Smits’s paper aims to change this by encouraging more open dialogue and reporting these “unexpected” results.

The implications are far-reaching, especially in high-stakes environments like aviation. In fighter jets, unexpected flow asymmetries can generate sudden side forces and yawing moments, potentially causing dangerous loss of control. As Williams vividly illustrates, "Imagine a pilot executing a tight turn, only to have the aircraft roll unexpectedly—the consequences could be catastrophic."

Unraveling the intricate dynamics

 

What are bifurcations?

In fluid mechanics and mathematics, bifurcations occur when small changes in conditions can drastically change the response of the underlying behavior. For example, a small change in velocity can cause a flow to change from steady to oscillating.

"The research reveals that these flow asymmetries are extraordinarily sensitive to minute disturbances or surface imperfections," explains Williams. "Slight variations in geometry, surface roughness, or even the presence of dust can trigger persistent asymmetric behaviors. But only if the conditions are right to produce this sensitivity."

The flows can become convectively unstable, with small disturbances growing downstream, or experience global instabilities that cause entire flow structures to simultaneously shift.

"It's a delicate dance between instabilities, bifurcations, and the inherent sensitivity of these flows," says Williams. "Unraveling these intricate dynamics is crucial for developing more reliable computational models and design tools for aviation."

Building on previous research

Wake of the “speed bump” investigated in the 3’x3’ Wind Tunnel at the University of Washington. Visualized using clay, kerosene and black-light powder.

This research stems from earlier studies conducted in the University of Washington's 3'x3' wind tunnel, where our researchers examined a speed-bump-like geometry. Funded by Boeing, the project aimed to shed light on why flow separation over smooth, curved surfaces like aircraft wings is so challenging to predict.

To tackle this puzzle, the team implemented new diagnostics and flow field measurements in our wind tunnels. Their findings revealed a surprising trend: the flow separating over these surfaces is far more dynamic than previously thought. It moves erratically and slowly, a behavior that most design tools fail to predict yet would significantly impact vehicle efficiency and control.

Inspired by recent discoveries at Virginia Tech involving asymmetric and bistable flow over hill-like geometries, the UW team investigated whether their bump flow exhibited similar characteristics. While they didn't observe bistability, likely due to the UW being proportionally wider, they did uncover a fascinating low-frequency side-to-side motion that is still under investigation.

These insights advance our understanding of aerodynamic performance at the edges of the flight envelope and pave the way for improved aircraft design and control.

Integrating AI and machine learning for better modeling

Madeline Samuell (MSAA ‘20) and associate research professor Owen Williams work to install new instrumentation onto the 3’x3’ Wind Tunnel for testing of the “speed bump.”

Looking to the future, Williams is exploring the sensitivities of modelling limitations of turbulent separated flows over high-lift wings with funding from Boeing. The goal will be to develop a better understanding of the physical processes that underpin the intricate multi-scale phenomena of turbulent flows, leading to more optimized aircraft designs. This effort will contribute to the international Common Research Model ecosystem, alongside researchers from the United States, Europe and Japan.

As the aviation industry continues to push the boundaries of performance and efficiency, the importance of documenting, understanding and mitigating asymmetric flows becomes increasingly crucial.