Wall turbulence over acoustic liners: An aerodynamic perspective

This dissertation studies the aerodynamic behavior of turbulent flow over acoustic liners— permeable surfaces installed inside aircraft engine nacelles to reduce noise. While these liners are highly effective at attenuating sound, they are also known to increase drag. Most prior research has focused on their acoustic performance, often simplifying or overlooking their aerodynamic impact. This work shifts that focus, using fully resolved, high-fidelity direct numerical simulations (DNS) to study flow over realistic liner geometries. Unlike many earlier studies that rely on simplifying assumptions such as impedance boundary models, this study avoids those simplifications by directly resolving the geometry of the acoustic liners.

The study explores key questions: which geometric features of acoustic liners most influence their aerodynamic behavior, how do these surfaces compare to traditional rough walls surfaces, and what additional effects are introduced by acoustic excitation. Although acoustic liners are flush with the surface and lack protrusions, we find that they still behave like canonical rough surfaces due to their permeability. The aerodynamic impact is governed by the non-linear Forchheimer permeability—a parameter that we show is closely linked to strong wall-normal velocity fluctuations in the near-wall region. These fluctuations are the primary driver of the drag penalty: the higher the wall-normal velocity fluctutations, the higher is the drag penalty compared to the reference smooth wall case. Importantly, the findings show that by limiting these wall-normal motions through geometric modifications—such as tapered orifices, or alternative shapes like elliptical orifices—it is possible to reduce drag. Tapered holes in particular show potential, as they decrease permeability without significantly affecting sound absorption. More aggressive changes, like parallel slots, tend to degrade acoustic performance, highlighting a necessary trade-off. However, certain designs, such as perpendicular slots, appear to offer a favorable balance.

Using the first fully resolved spatially developing turbulent boundary layer simulation over an acoustic liner array, this dissertation further shows that, for the conditions studied, acoustic excitation—modeled as a planar upstream-propagating monochromatic wave—does not significantly affect aerodynamic behavior. However, this does not rule out more complex interactions under realistic engine conditions, where acoustic fields are broadband and multidirectional. Limitations in the numerical setup, particularly in acoustic modeling, mean that the full impact of sound waves remains an open question.

The work also touches on broadband acoustic liner geometries, which are becoming increasingly relevant. These designs are more permeable—not just in the wall-normal direction—but across multiple directions. Higher permeability typically correlates with higher drag, and this trend holds for acoustic liners as well. Still, the study shows that with careful design, broadband liners can be engineered to avoid additional drag penalties, achieving comparable aerodynamic performance to conventional designs.

In summary, this dissertation offers a detailed aerodynamic analysis of flow over acoustic liners, explaining the mechanisms behind drag increase and establishing the central role of permeability. It shows that aerodynamic optimization is possible without compromising acoustic effectiveness and highlights the need for fully resolved simulations when studying such complex surfaces. The findings lay the groundwork for the design of next-generation acoustic liners that better balance noise control and aerodynamic efficiency.