This blog entry from Gaurav Upadhyay is one of two winning submissions to the Fluids Writing Competition, from the Journal of Fluid Mechanics and Flow: Applications of Fluid Mechanics. Gaurav is a graduate student at the Indian Institute of Technology Bombay, in the department of Mechanical Engineering focusing on fluid sciences.

Noise generated by the wings attached to an airplane or a wind turbine is a cause of serious concern for designers and environmentalists worldwide. With more and more cities switching to wind energy for their daily demands and having closer than ever airports built, the challenge is only expanding. The leading edge of a moving wing dispaces airflow downstream where it interacts with the structure, resulting in undesirable noise. Scientists have tried to invent novel solutions to mitigate this challenge for years, and they might have found an excellent one designed by nature itself.

Barn owls (Tyto alba), also known as one of the stealthiest predators in the wild, have amazed researchers for almost a century. Its silent flight leaves practically no time for its prey to realize the upcoming threat. Several past experiments on Barn Owl’s flight have attributed the reason for its excellent noise mitigation capability to structural features like canopies, serrations, leading-edge combs, and porosity. In the past, scientists have used the gained information to model individual features of the Barn Owl’s wings, which can further design low-noise wings.

In a recent study published in the Journal of Fluid Mechanics, Lorna Ayton and her colleagues from the United Kingdom and Germany have tried to understand and model the role of porosity present in the birds’ wing on the combined aerodynamic and acoustic performance. They chose to analyze the wings specimen of Barn Owl due to its lower noise generation and Common buzzards (Buteo buteo) for its better aerodynamic efficiency.

Firstly, to measure the wings’ porosity, the team devised an innovative solution where they pushed a movable head at the top of the wings, leaving no gap. Pressurized air was then passed through it at a known flow rate. The pressure at the bottom of the wing was then measured and used to gain information about its porosity. Specifically, more hollow wings will allow more air to pass without much pressure loss and hence, lower resistance.

The team observed that the porosity over the wing varied continuously from the leading edge to the trailing edge in both Barn Owl and Buzzard wings’ samples. The leading edge was less porous in both the cases and justifiably so as to provide more strength to the wing’s frontal part. However, the chordwise variation of porosity was more enhanced for the Owl wing, which might be a reason for its better acoustic performance.

Subsequently, to test the hypothesis, the researchers build a novel theoretical model based on Mathieu collocation, which enabled them to analyze varying porosity, unlike the past approaches. They first considered the experimentally obtained variation of porosity and found that the owl’s wing indeed gives a better acoustic performance. However, the buzzard’s wings were more aerodynamically favorable, as known from past observations. The model overpredicts the owl wings’ noise as it doesn’t take into account the intricate geometrical features present on the owl’s wings like serrations, canopies, camber, etc. Furthermore, they also tested the methodology against a generalized porosity variation and concluded that the varying porosity could provide better acoustic characteristics.

Though the model developed by the team is primitive and ignores the effect of other significant physical features present on the owl’s wings, it effectively provides a framework to understand the role of the wing’s chordwise varying porous structure. The design of a wing is based on several factors, including but not limited to strength, fuel carrying capacity in case of airplane wings, aerodynamics, and acoustic performances. Hence, the overall optimization of a wing is a complex problem that could be undertaken in the future, and this model can pave a path for such studies.

References:
Ayton, Lorna J., et al. “Reducing aerofoil–turbulence interaction noise through chordwise-varying porosity.” Journal of Fluid Mechanics 906 (2021). DOI: https://doi.org/10.1017/jfm.2020.746