Hydrodynamic dispersion relations at finite coupling

By using holographic methods, the radii of convergence of the hydrodynamic shear and sound dispersion relations were previously computed in the $$ \mathcal{N} $$ N = 4 supersymmetric Yang-Mills theory at infinite ’t Hooft coupling and infinite number of colours. Here, we extend this analysis to the domain of large but finite ’t Hooft coupling. To leading order in the perturbative expansion, we find that the radii grow with increasing inverse coupling, contrary to naive expectations. However, when the equations of motion are solved using a qualitative non-perturbative resummation, the dependence on the coupling becomes piecewise continuous and the initial growth is followed by a decrease. The piecewise nature of the dependence is related to the dynamics of branch point singularities of the energy-momentum tensor finite-temperature two-point functions in the complex plane of spatial momentum squared. We repeat the study using the Einstein-Gauss-Bonnet gravity as a model where the equations can be solved fully non-perturbatively, and find the expected decrease of the radii of convergence with the effective inverse coupling which is also piecewise continuous. Finally, we provide arguments in favour of the non-perturbative approach and show that the presence of non-perturbative modes in the quasinormal spectrum can be indirectly inferred from the analysis of perturbative critical points.

Influence of the Widespread Use of Corten Plate on the Acoustics of the European Solidarity Centre Building in Gdańsk

This paper describes the relationship between a strong architectural vision that is difficult to balance, and user expectations in terms of acoustics. The focus is on the use of corten steel as the dominant finishing material on façades and interiors to achieve an expressive, symbolic message through program-based design. The architectural premises justifying the adopted solutions are presented, especially the universality and homogeneity of the material. Against this background, the influence of corten steel on the acoustics of the two largest rooms of the European Solidarity Center, which are the winter garden and the multi-purpose hall, was discussed. Remedial steps have been taken to reduce the greatest acoustic inconveniences resulting from the widespread use of metal sheet as a finishing material in rooms, i.e., excessive reverberation and a low degree of sound dispersion. A positive result for the acoustic conditions achieved in the winter garden was the presentation of a large body of classical music in the building.

Reproductive activity, microhabitat use, and calling sites of Pristimantis bacchus (Anura: Craugastoridae)

Pristimantis bacchus is an endangered terrestrial frog, endemic to cloud forests of the northern part of the Cordillera Oriental in the Colombian Andes. The knowledge of its reproductive biology and microhabitat use can offer important information for future conservation programs. We evaluated the reproductive activity and microhabitat use for both sexes, and selection of calling sites by males during rainy and dry seasons. Individuals of all reproductive conditions were observed throughout the sampled months; however, we observed a higher abundance of frogs during the first rainy season, after the driest months of the year. Thus, this population seems to have continuous reproductive activity, with an apparent peak at the onset of the first rainy season of the year. There were differences in the microhabitat use between sexes, between adults and juveniles, and between non-reproductive and reproductive females. Males use a wider variety of substrates at higher perches, while females were mainly found on leaf litter and over leaves at low heights; however, ovigerous females mostly occupied substrates above 120 cm. Juveniles were less abundant than adults, and occupy low substrates, mostly at forest litter. Two selection models evaluated the preference for calling sites, related to physiological requirements of humidity and for vocal sound dispersion. Males preferred to vocalize on perches at heights among 41-120 cm and with a vegetal structure that maintains high humidity, avoiding desiccation, and reducing exposure to predators. Therefore, intrapopulation variation in microhabitat use in this species is related to age, sex, reproductive condition, and physiological requirements.

Small-scale kinematics of two-phase flows: identifying relaxation processes in separated- and disperse-phase flow models

We propose here to investigate the impact of small-scale effects on the bulk evolution of a two-phase flow system. More precisely, we choose to examine the sole influence of a small-scale (with respect to the bulk velocity) off-equilibrium velocity on the system. In order to narrow our analysis and avoid complex well-posedness issues, we choose to examine a simple barotropic 5-equation two-phase flow model that accounts for an equilibrium common bulk velocity and a small-scale off-equilibrium velocity. A full derivation of the model is presented: it is based on a variational principle which allows us to insert the two-scale kinematics into the model by considering two different kinetic energies. Additional entropy dissipation requirements allow us to add dissipative structures to the model. This system is neutral with respect to the topology of the flow structure and is equipped with parameters that can be connected to relaxation processes. When considering instantaneous relaxations, we obtain two limit systems of the literature that are used for the simulation of separated-phase flows. In this sense we obtain a hierarchy of models. We show that the parent 5-equation model is also compatible with the description of a bubbly fluid that allows small-scale vibrations for the disperse phase. This identification is verified and discussed through comparisons with experimental measurements of sound dispersion (Silberman, J. Acoust. Soc. Am., vol. 29, 1957, pp. 925–933; Leroy et al., J. Acoust. Soc. Am., vol. 123, 2008, pp. 1931–1940) and with the dispersion relations of a reference model for bubbly flows by Cheng et al. (Trans. ASME J. Heat Transfer, vol. 107, 1985, pp. 402–408). The present work is a first contribution to a larger effort that aims at unifying models that can describe both separated and disperse two-phase flows, coupling small-scale modelling with large-scale resolution.

Peculiarities of ultra-sound dispersion of thermally expanded graphite

Thermally expanded graphite (TEG) is one of the best precursors for receiving of plane nano-dimensional particles. Crucible graphite of Taigin deposit was used as a basic material to receive TEG. It is established that repeated thermo-chemical, as well as additional mechanical TEG dispersion does not lead to formation of nano-dimensional graphite particles. In order to receive graphite particles with several nano-meters of thickness, TEG was treated by ultra-sound dispersion in four liquid mediums. Different influence of chosen liquid mediums on the process of ultra-sound dispersion of TEG particles is conditioned by peculiarities of liquids’ behavior in cavitation process, it is described integrally by a notion “erosion activity”. Erosion activity, in the first turn, depends on the superficial tension. The quantity of the received broken material is defined by the depth of penetration of the liquid into the body volume, which depends on liquid wettability. The used liquids: water, acetone, benzene and toluene had different polarity and superficial tension that allowed to put into reality different physical and chemical effects at TEG dispersion. Benzene has the lowest meaning of superficial tension, it moistens the graphite surface very well and penetrates easily into its micropores. Despite of the relatively low erosion capacity, non-polar organic liquids, penetrating to the big depth into the graphite particle’s volume are capable to provoke a visible breaking influence at ultra-sound dispersion on inner graphite structures and its breakage into plates. The best results of breakage are reached at use of benzene that has zero molecule polarity and the best wettability of graphite surface. At ultra-sound dispersion in all environments, plane graphite particles are received that are several dozens of nano-meters thick. However, some particles broken in ultra-sound have visible fluctuations from ideal plane form and can form complicated spatial figures of irregular structure. The received results may be a base for creation of a method for receiving of thin graphite plates with geometrical parameters that are necessary for making of composites and objects of nano-techniques.

Acoustic Signatures of the Phases and Phase Transitions in the Blume Capel Model with Random Crystal Field

Sound propagation in the Blume Capel model with quenched diluted single-ion anisotropy is investigated. The sound dispersion relation and an expression for the ultrasonic attenuation are derived with the aid of the method of thermodynamics of irreversible processes. A frequency-dependent dispersion minimum that is shifted to lower temperatures with rising frequency is observed in the ordered region. The thermal and sound frequency (ω) dependencies of the sound attenuation and effect of the Onsager rate coefficient are studied in low- and high-frequency regimes. The results showed that ωτ≪1 and ωτ≫1 are the conditions that describe low- and high-frequency regimes, where τ is the single relaxation time diverging in the vicinity of the critical temperature. In addition, assuming a linear coupling of sound wave with the order parameter fluctuations in the system and ε as the temperature distance from the critical point, we found that the sound attenuation follows the power laws α(ω,ε)~ω2ε-1 and α(ω,ε)~ω0ε1 in the low- and high-frequency regions, while ε→0. Finally, a comparison of the findings of this study with previous theoretical and experimental studies is presented and it is shown that a good agreement is found with our results.