Being able to cross half the planet in barely more time than it takes to watch an episode of your favorite series... What was once pure fiction could soon become our daily reality, thanks to recent advances in the field of very high-speed flight.
Intercontinental travel could be radically transformed by the advent of hypersonic flight. Currently, a journey between Sydney and Los Angeles takes about fifteen hours of flight time, but this duration could be reduced to just sixty minutes. Professor Nicholaus Parziale states that this technology would have the effect of "shrinking the planet," making travel faster and more pleasant.
To achieve such speeds, an aircraft would need to fly at Mach 10, which is ten times the speed of sound. The main obstacle lies in the intense turbulence and extreme heat generated when an aircraft cuts through the air at such speeds. Air does not behave the same way at low and high speeds. Below Mach 0.3, the flow is said to be incompressible: the density of air varies little, which simplifies design calculations. At the speed of sound (Mach 1), the flow is said to be compressible, as the gas compresses under the effect of pressure and temperature.
Compressibility profoundly changes how air interacts with the aircraft, influencing essential parameters like lift, drag, and the thrust required for flight. Engineers have a good grasp of these phenomena at the subsonic speeds of current airliners, but the conditions encountered between Mach 5 and Mach 10 remain largely misunderstood. A guiding idea, known as Morkovin's hypothesis, postulates that the turbulent behavior of air at these high speeds would become similar again to that observed at lower speeds, below Mach 1.
To test this hypothesis, Parziale's team developed an ingenious experiment using lasers. They ionized krypton, a gas injected into the air of a wind tunnel, creating a fluorescent line that deforms under the effect of turbulence. By observing how this line twists and undulates, the researchers were able to analyze the structure of the flow at Mach 6. The results, obtained after eleven years of development, indicate that the turbulent behavior at this speed is indeed close to that of incompressible flows.
Nicholaus Parziale believes hypersonic aircraft could one day connect Los Angeles to Sydney in one hour.
Credit: Stevens Institute of Technology
This discovery would significantly simplify the design of hypersonic aircraft. Currently, numerically simulating all the details of a flight at Mach 6 would require colossal computing resources, perhaps even impossible to mobilize. Morkovin's hypothesis would allow for reasonable approximations, making these calculations much more accessible. Applications could also extend to space transportation, by enabling the development of vehicles capable of reaching low Earth orbit without using traditional launchers.
Morkovin's hypothesis and turbulence
Morkovin's hypothesis, formulated in the mid-20th century, proposes a unifying vision of turbulence at different speeds. It suggests that the disordered motion of air, characterized by its vortices and fluctuations, retains fundamental properties even when the speed far exceeds that of sound. This idea challenges the intuition that very fast flows would be radically different from slower flows.
In compressible flows, the density of air varies significantly with pressure and temperature, which complicates the analysis. Yet, Morkovin postulates that the structure of the turbulence, meaning how vortices form and interact, remains largely unchanged. This means that the mathematical models and concepts used to describe turbulence at low speed could be adapted rather than replaced.
The experimental validation of this hypothesis opens up major prospects for aerodynamics. If turbulence behaves similarly at different speeds, engineers can rely on established knowledge to design hypersonic vehicles. This reduces uncertainty and accelerates the development of technologies capable of operating in extreme conditions, where design errors can have catastrophic consequences.
Beyond aeronautics, this understanding of turbulence could benefit other fields, such as meteorology or wind energy, where fast and turbulent flows are common. A unified approach would simplify modeling and improve the accuracy of forecasts, whether for the flight of an aircraft or the path of a storm.