How a moving wall affects hypersonic flow separation

What is it about?

When a hypersonic vehicle—like a spacecraft or missile—flies at speeds over five times the speed of sound, the air rushing past it creates intense shock waves. These shock waves can interact with the thin layer of slower-moving air hugging the vehicle’s surface (called the boundary layer), causing the flow to separate and form a swirling “bubble” of recirculating air. This separation bubble can lead to extreme heating, pressure fluctuations, and loss of control—serious problems for high-speed flight. In this study, we explored a simple but clever idea: what if part of the vehicle’s surface could move? By sliding a flat plate forward at a steady speed beneath a fixed ramp in a wind tunnel running at Mach 5, we observed how this motion changes the size and behavior of the separation bubble. Our results show that the interaction evolves through three distinct stages, driven by changes in the local airflow structure induced by the moving wall. We also developed a physics-based model that predicts how the bubble’s size changes over time—and it works exceptionally well when the airflow is smooth (laminar). This work suggests that actively moving parts of a vehicle’s surface could be a practical way to control dangerous flow separations in hypersonic flight, potentially improving stability, reducing heat loads, and enhancing performance—without the need for complex jets or plasma devices.

Featured Image

Photo by Sergey Koznov on Unsplash

Photo by Sergey Koznov on Unsplash