Effects of Laser Surface Texturing and Lubrication on the Vibrational and Tribological Performance of Sliding Contact

Various textures are fabricated by a picosecond laser machine on the surfaces of circular stainless steel specimens. Vibrational and tribological effects of laser surface textures are investigated by means of a tribometer and a data acquisition and signal processing (DASP) system. Experimental results show that surface textures can reduce the coefficients of friction (COFs), enhance the wear resistance, and improve the dynamical performance of frictional surfaces. In this study, the surface with micro circular dimples in diameter of 150 μm or textured area density of 25% has the best tribological and dynamical performance. Compared with the non-textured surface, the surface with circular dimples in diameter of 150 μm and 15% textured area density has 27% reduction of COFs, 95% reduction of frictional vibrations, and 66% reduction of frictional noise. The frictional vibrations and noise in the sliding contacts can be effectively reduced by adding graphene to the lubrication oil, and the surface textures enhance the frictional noise reduction performance of lubrication.

Analysis of Ship Noise Characteristics Generated According to Sailing Conditions of the G/T 1000-ton Stern Fishing Trawler

In this study, we conducted onboard noise measurement experiments under the conditions of anchoring, sailing, casting, and hauling to determine whether noise generated in the G/T 1000-ton fishing trawler triggers zone-specific effects upon vessel operation. It was shown that most accommodation areas of the trawler comfortably met the IMO acceptance criteria regardless of the sailing condition, but most of the stern area, where the fishing actually occurs, exceeded the permitted limit of 75 dB (A). Furthermore, the statistical analysis revealed a significant difference (p < 0.05) only in the bow and the stern, which are both open areas. In the case of the former, improvements were deemed possible due to the influence of the fluid emission noise from the seawater piping in the bow, and the acceptance criteria were also appropriate. However, in the case of the latter, a significant difference was seen in hauling conditions, and on-site analysis confirmed frictional noise from hydraulic oil in the trawl winch and between the chains and the metal hull, leading to the conclusion that various improvements are required, such as the mandatory wearing of safety equipment by workers and stricter legal standards for permitted noise levels.

Interdependence of friction, wear, and noise: A review

Phenomena of friction, wear, and noise in mechanical contacts are particularly important in the field of tribomechanics but equally complex if one wants to represent their exact relationship with mathematical models. Efforts have been made to describe these phenomena with different approaches in past. These efforts have been compiled in different reviews but most of them treated friction, wear mechanics, and acoustic noise separately. However, an in-depth review that provides a critical analysis on their interdependencies is still missing. In this review paper, the interdependencies of friction, wear, and noise are analysed in the mechanical contacts at asperitical level. The origin of frictional noise, its dependencies on contact’s mechanical properties, and its performance under different wear conditions are critically reviewed. A discussion on the existing mathematical models of friction and wear is also provided in the last section that leads to uncover the gap in the existing literature. This review concludes that still a comprehensive analytical modelling approach is required to relate the interdependencies of friction, noise, and wear with mathematical expressions.

Evidence that the Dorsal Velvet of Barn Owl Wing Feathers Decreases Rubbing Sounds during Flapping Flight

Synopsis Owls have specialized feather features hypothesized to reduce sound produced during flight. One of these features is the velvet, a structure composed of elongated filaments termed pennulae that project dorsally from the upper surface of wing and tail feathers. There are two hypotheses of how the velvet functions to reduce sound. According to the aerodynamic noise hypothesis, the velvet reduces sound produced by aerodynamic processes, such as turbulence development on the surface of the wing. Alternatively, under the structural noise hypothesis, the velvet reduces frictional noise produced when two feathers rub together. The aerodynamic noise hypothesis predicts impairing the velvet will increase aerodynamic flight sounds predominantly at low frequency, since turbulence formation predominantly generates low frequency sound; and that changes in sound levels will occur predominantly during the downstroke, when aerodynamic forces are greatest. Conversely, the frictional noise hypothesis predicts impairing the velvet will cause a broadband (i.e., across all frequencies) increase in flight sounds, since frictional sounds are broadband; and that changes in sound levels will occur during the upstroke, when the wing feathers rub against each other the most. Here, we tested these hypotheses by impairing with hairspray the velvet on inner wing feathers (P1-S4) of 13 live barn owls (Tyto alba) and measuring the sound produced between 0.1 and 16 kHz during flapping flight. Relative to control flights, impairing the velvet increased sound produced across the entire frequency range (i.e., the effect was broadband) and the upstroke increased more than the downstroke, such that the upstroke of manipulated birds was louder than the downstroke, supporting the frictional noise hypothesis. Our results suggest that a substantial amount of bird flight sound is produced by feathers rubbing against feathers during flapping flight.

Evolutionary and Ecological Correlates of Quiet Flight in Nightbirds, Hawks, Falcons, and Owls

Synopsis Two hypotheses have been proposed for the evolution of structures that reduce flight sounds in birds. According to the stealth hypothesis, flying quietly reduces the ability of other animals (e.g., prey) to detect the animal’s approach from its flight sounds. This hypothesis predicts that animals hunting prey with acute hearing evolve silencing features. The self-masking hypothesis posits that reduced flight sounds permit the animal itself to hear better (such as the sounds of its prey, or its own echolocation calls) during flight. This hypothesis predicts that quieting features evolve in predators that hunt by ear, or in species that echolocate. Owls, certain hawks, and nightbirds (nocturnal Caprimulgiformes) have convergently evolved a sound-reducing feature: a velvety coating on the dorsal surface of wing and tail feathers. Here we document a fourth independent origin of the velvet, in the American kestrel (Falco sparverius). Among these four clades (hawks, falcons, nightbirds, and owls), the velvet is longer and better developed in wing and tail regions prone to rubbing with neighboring feathers, apparently to reduce broadband frictional noise produced by rubbing of adjacent feathers. We tested whether stealth or self-masking better predicted which species evolved the velvet. There was no support of echolocation as a driver of the velvet: oilbird(Steatornis caripensis) and glossy swiftlet (Collocalia esculenta) each evolved echolocation but neither had any velvet. A phylogenetic least squares fit of stealth and self-masking (to better hear prey sounds) provided support for both hypotheses. Some nightbirds (nightjars, potoos, and owlet-nightjars) eat flying insects that do not make much sound, implying the velvet permits stealthy approach of flying insects. One nightbird clade, frogmouths (Podargus) have more extensive velvet than other nightbirds and may hunt terrestrial prey by ear, in support of self-masking. In hawks, the velvet is also best developed in species known or suspected to hunt by ear (harriers and kites), supporting the self-masking hypothesis, but velvet is also present in reduced form in hawk species not known to hunt by ear, in support of the stealth hypothesis. American kestrel is not known to hunt by ear, and unlike the other falcons sampled, flies slowly (kite-like) when hunting. Thus the presence of velvet in it supports the stealth hypothesis. All owls sampled (n = 13 species) had extensive velvet, including the buffy fish-owl (Ketupa ketupu), contrary to literature claims that fish-owls had lost the velvet. Collectively, there is support for both the self-masking and stealth hypotheses for the evolution of dorsal velvet in birds.

Evolution and Ecology of Silent Flight in Owls and Other Flying Vertebrates

Synopsis We raise and explore possible answers to three questions about the evolution and ecology of silent flight of owls: (1) do owls fly silently for stealth, or is it to reduce self-masking? Current evidence slightly favors the self-masking hypothesis, but this question remains unsettled. (2) Two of the derived wing features that apparently evolved to suppress flight sound are the vane fringes and dorsal velvet of owl wing feathers. Do these two features suppress aerodynamic noise (sounds generated by airflow), or do they instead reduce structural noise, such as frictional sounds of feathers rubbing during flight? The aerodynamic noise hypothesis lacks empirical support. Several lines of evidence instead support the hypothesis that the velvet and fringe reduce frictional sound, including: the anatomical location of the fringe and velvet, which is best developed in wing and tail regions prone to rubbing, rather than in areas exposed to airflow; the acoustic signature of rubbing, which is broadband and includes ultrasound, is present in the flight of other birds but not owls; and the apparent relationship between the velvet and friction barbules found on the remiges of other birds. (3) Have other animals also evolved silent flight? Wing features in nightbirds (nocturnal members of Caprimulgiformes) suggest that they may have independently evolved to fly in relative silence, as have more than one diurnal hawk (Accipitriformes). We hypothesize that bird flight is noisy because wing feathers are intrinsically predisposed to rub and make frictional noise. This hypothesis suggests a new perspective: rather than regarding owls as silent, perhaps it is bird flight that is loud. This implies that bats may be an overlooked model for silent flight. Owl flight may not be the best (and certainly, not the only) model for “bio-inspiration” of silent flight.