Hydrodynamic and hydromagnetic stability

Owls are commonly known for their quiet flight, enabled by three adaptions of their wings and plumage: leading edge serrations, trailing edge fringes and a soft and elastic downy upper surface of the feathers. In order to gain a better understanding of the aeroacoustic effects of the third property that is equivalent to an increased permeability of the plumage to air, an experimental survey on a set of airfoils made of different porous materials was carried out. Several airfoils with the same shape and size but made of different porous materials characterized by their flow resistivities and one non-porous reference airfoil were subject to the flow in an aeroacoustic open jet wind tunnel. The flow speed has been varied between approximately 25 and 50 m/s. The geometric angle of attack ranged from −16° to 20° in 4°-steps. The results of the aeroacoustic measurements, made with a 56-microphone array positioned out of flow, and of the measurements of lift and drag are given and discussed.

Measurement of the noise generation at the trailing edge of porous airfoils

This paper provides an experimental investigation into the use of leading edge (LE) serrations as a means of reducing the broadband noise generated due to the interaction between the aerofoil’s LE and impinging turbulence. Experiments are performed on a flat plate in an open jet wind tunnel. Grids are used to generate isotropic homogeneous turbulence. The leading edge serrations are in the form of sinusoidal profiles of wavelengths, λ, and amplitudes, 2h. The frequency and amplitude characteristics are studied in detail in order to understand the effect of LE serrations on noise reduction characteristics and are compared with straight edge baseline flat plates. Noise reductions are found to be insignificant at low frequencies but significant in the mid frequency range (500 Hz–8 kHz) for all the cases studied. The flat plate results are also compared to the noise reductions obtained on a serrated NACA-65 aerofoil with the same serration profile. Noise reductions are found to be significantly higher for the flat plates with a maximum noise reduction of around 9 dB compared with about 7 dB for the aerofoil. In general, it is observed that the sound power reduction level (ΔPWL) is sensitive to the amplitude, 2h of the LE serrations but less sensitive to the serration wavelength, λ. Thus, this paper sufficiently demonstrates that the LE amplitude acts as a key parameter for enhancing the noise reduction levels in flat plates and aerofoils.

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Airfoil noise reductions through leading edge serrations

This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto three-dimensional aerofoils. The influence on the noise reduction of the turbulence integral length scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length scale is approximately one-fourth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number , where , and are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce aerofoil self-noise. The mechanism for this phenomenon is explored through particle image velocimetry measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.

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Performance and mechanism of sinusoidal leading edge serrations for the reduction of turbulence–aerofoil interaction noise

Mostgeneraofowls(Strigiformes)havetheabilityto flysilently.Themechanismsofthesilent flightoftheowlhave been the subject of scientific interest for many decades. The results from studies in the past are discussed in detail in thispaperandtherationaleforthepresentresearchisgiven,whichincluded flyovernoisemeasurementsondifferent species of birds. Successful acoustic measurements were made on a Common Kestrel, a Harris Hawk, and a Barn Owl. Measurements on three other birds did not lead to reliable results. The setup and procedure used for the outdoor measurements are discussed. These include the estimation of the trajectory from dual video camera recordings and microphone-array measurements with a moving-focus beamforming technique. The main result from the 50 successful flyovers is that the owl flight produces aerodynamic noise that is indeed a few decibels below thatofotherbirds,evenif flyingatthesamespeed.Thisnoisereductionissignificantatfrequenciesabove1.6kHz.At frequencies above 6.3 kHz the noise from the owl remains too quiet to be measured.

Silent Owl Flight: Bird Flyover Noise Measurements

Owls are known for their silent flight. Even though there is some information available on the mechanisms that lead to a reduction of noise emission, neither the morphological basis, nor the biological mechanisms of the owl's silent flight are known. Therefore, we have initiated a systematic analysis of wing morphology in both a specialist, the barn owl, and a generalist, the pigeon. This report presents a comparison between the feathers of the barn owl and the pigeon and emphasise the specific characteristics of the owl's feathers on macroscopic and microscopic level. An understanding of the features and mechanisms underlying this silent flight might eventually be employed for aerodynamic purposes and lead to a new wing design in modern aircrafts. A variety of different feathers (six remiges and six coverts), taken from several specimen in either species, were investigated. Quantitative analysis of digital images and scanning electron microscopy were used for a morphometric characterisation. Although both species have comparable body weights, barn owl feathers were in general larger than pigeon feathers. For both species, the depth and the area of the outer vanes of the remiges were typically smaller than those of the inner vanes. This difference was more pronounced in the barn owl than in the pigeon. Owl feathers also had lesser radiates, longer pennula, and were more translucent than pigeon feathers. The two species achieved smooth edges and regular surfaces of the vanes by different construction principles: while the angles of attachment to the rachis and the length of the barbs was nearly constant for the barn owl, these parameters varied in the pigeon. We also present a quantitative description of several characteristic features of barn owl feathers, e.g., the serrations at the leading edge of the wing, the fringes at the edges of each feather, and the velvet-like dorsal surface. The quantitative description of the feathers and the specific structures of owl feathers can be used as a model for the construction of a biomimetic airplane wing or, in general, as a source for noise-reducing applications on any surfaces subjected to flow fields.

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Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia

The aerodynamics of a newly constructed wing model the geometry of which is related to the wing of a barn owl is experimentally investigated. Several barn owl wings are scanned to obtain three-dimensional surface models of natural wings. A rectangular wing model with the general geometry of the barn owl but without any owl-specific structure being the reference case for all subsequent measurements is investigated using pressure tabs, oil flow pattern technique, and particle-image velocimetry. The main flow feature of the clean wing is a transitional separation bubble on the suction side. The size of the bubble depends on the Reynolds number and the angle of attack, whereas the location is mainly influenced by the angle of attack. Next, a second model with a modified surface is considered and its influence on the flow field is analyzed. Applying a velvet onto the suction side drastically reduces the size of this separation at moderate angles of attack and higher Reynolds numbers.

Experimental analysis of the flow field over a novel owl based airfoil

A model wing based on the geometry of the wing of a barn owl was designed, in which the feather structure of the barn-owl wing is approximated by a velvet-like surface. The first objective of this paper is to investigate the impact of artificial surface filaments on the overall flow field of a quasi-2D configuration of the model 3D wing. Two velvet-like surfaces are used and the velocity field is measured by particle-image velocimetry in a chord-length based Reynolds number range 20 , 000 ≤ R e c ≤ 60 , 000 at angles of attack 0 ° ≤ α ≤ 6 ° . An explanation of the mechanism that leads to the change in the near-wall flow field due to the surface structures is given. The second objective of the paper is the comparison of the 2D and the 3D results and the analysis of the impact of the three-dimensionality on the flow field. The first surface structure (“velvet 1”) mimics the length and density of the hairs and the softness of the natural owl-wing surface. It diminishes the size of the separation bubble or completely prevents separation. However, at three-dimensional flow the effect of the “velvet 1” surface is clearly reduced. The “velvet 2” surface consists of longer and thinner filaments than the “velvet 1” surface. At the lower Reynolds numbers ( R e c ≤ 40 , 000 ), the “velvet 2” surface structure does not alter the near-wall flow field significantly. However, at R e c > 40 , 000 the “velvet 2” surface structure serves as a distributed field of moving roughness elements such that the size of the separation bubble is reduced and becomes nearly independent of the angle of attack. When the three-dimensional flow field at the highest Reynolds number ( R e c = 60 , 000 ) is considered it is evident that the “velvet 2” surface yields the aerodynamically more stable flow field.

Stephan Klän

Papers

Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia

Experimental analysis of the flow field over a novel owl based airfoil

Surface structure and dimensional effects on the aerodynamics of an owl-based wing model

Particle-Image Velocimetry and Force Measurements of Leading-Edge Serrations on Owl-Based Wing Models

The Influence of Leading Edge Serrations on The Flow Field of an Artificial Owl Wing