Bi-global stability analysis for understanding flow separation in aircraft

An airfoil can be the wing of an airplane, or the blade in a turbine. This area is susceptible to flow separation where the air stream close to the surface of the wing is slowed and does not interact with the higher velocity stream above. The separation of air flow causes increased drag and loss of lift, among other problems. Wang et al. developed a method to better understand these instabilities using bi-global stability analysis.

“Here, we identify what causes instabilities that can play a role in small aircraft tragedies,” said author Pierre Sullivan. “What we’re interested in looking at eventually is flight dynamics.”

In order to get a full picture of instability growth, the team decided on a bi-global stability approach because it considers a non-uniform flow in two spatial directions, which allowed them to investigate the entire airfoil during flow separation. Bi-global stability allows for a better look at the three dimensionalities of flow structure, as well as where these instabilities originate.

With their model, the researchers found wake instabilities to be one source of unstable modes. Additionally, they found that the upstream shear layer instability growth rate propagates downstream and interacts with the wake modes.

Sullivan believes that the research will not just be useful for smaller aircraft, but for drones and turbines as well. Experiments focused on applying the ideas presented in this research supplemented with full-field computer models in the future will add to a more well-rounded understanding of what occurs during flow separation.

Source: “Bi-global stability analysis in curvilinear coordinates,” by Jinchun Wang, Paul Ziadé, Guoping Huang, and Pierre E. Sullivan, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5118365 .