In this paper, we consider the generation of the Tollmien–Schlichting waves in the boundary layer on the surface of a wing exposed to entropy waves. It is well known that the free-stream turbulence ...is composed of two perturbation modes: the vorticity waves and the entropy waves. The receptivity of the boundary layer to the vorticity waves has been studied extensively by various authors. The entropy waves have not attracted such attention. We show that, in high speed subsonic flows, the entropy waves are as important for the receptivity as the vorticity waves. Methodologically, our work relies on the asymptotic analysis of the Navier–Stokes equations at large values of the Reynolds number, which results in the formulation of a suitably modified triple-deck theory. The entropy waves produce oscillations of the gas temperature and density, but the velocity and the pressure remain unperturbed to the leading order. This precludes the entropy waves from penetrating the boundary layer, as happens, for example, with the acoustic waves. Our analysis reveals that the entropy waves decay rapidly in the transition layer that forms near the outer edge of the boundary layer. We find that an entropy wave alone cannot generate the Tollmien–Schlichting waves. However, when the boundary layer encounters a wall roughness, the flow near the roughness appears to be perturbed not only inside the boundary layer but also in the inviscid region outside the boundary layer. The latter comes into the interaction with the density perturbations in the entropy wave. As a result, a localised ‘forcing’ is created that produces the Tollmien–Schlichting waves. In this paper we present the results of a linear and nonlinear receptivity analysis. We find that the nonlinearity enhances the receptivity significantly, especially when a local separation region forms on the roughness.
Owing to the presence of valence shells for charge transfer, a high theoretical specific capacitance, and variable redox properties, MnO2 appears as a promising agent in the realm of energy storage. ...Crystallographic structures of manganese oxide (MnO2), a transition metal oxide, play a crucial role in determining its capacitive behavior by controlling the ion intercalation and double-layer formation. In this work, two different phases of MnO2 were synthesized using the facile chemical reduction method and biological method, say α-MnO2 and λ-MnO2. The phases were incorporated with different carbonaceous additives including GO and CNT while maintaining a constant weight ratio of 8:1:1 between active materials (MnO2), additive, and PVDF binder. Among different composites formed, the best electrode performance is demonstrated by λ-MnO2/CNT/PVDF composite with an excellent specific capacitance of 356 F/g at a scan rate of 1 A/g. Moreover, the best-performing electrodes are investigated with a symmetrical two-electrode system yielding a wide potential window of 1.8V with an outstanding power density of 13.5 KW/Kg at 5 A/g and an energy density of 53.78 Wh/Kg at 1A/g having a specific capacitance of 190 F/g.
The commented review of the volcanological research in Antarctica misses links to geoheritage. Interpretation of Antarctic volcanism in the terms of geoheritage is reasonable both theoretically and ...practically. This task cannot be left to only specialists in geoconservation and geotourism, and the published geological information is not enough to make geoheritage interpretations. Principally, it is suggested that the latter should be considered among the major directions of the volcanological research in Antarctica.
In this paper we study the generation of Tollmien–Schlichting waves initiated by vibrations of a wall where the wall is coated with a thin liquid film in a transonic flow regime. Motion of fluids are ...described by the two-dimensional Navier–Stokes equations assuming the Reynolds number is large. To find asymptotic solutions of the transonic boundary layer, we conduct an inspection analysis on the affine transformations of the triple-deck model for a subsonic flow and the unsteady full potential equations, with the intention of obtaining the order quantity of the free-stream Mach number in the transonic flow. We construct a modified triple-deck model for the transonic flow by considering the scalings of the perturbations that lead to the viscous–inviscid interaction problem for the flow in a subsonic regime. In particular, we are interested in the region where the subsonic scalings become invalid as the flow approaches transonic regime. We assume the wall oscillates in the vertical direction to the oncoming flow and these vibrations are periodic in time. We outline the process where the flow in the boundary layer converts the wall vibration perturbations into the instability modes which are measured by the receptivity coefficient. The viscous–inviscid interaction problem describes the stability of the boundary layer on the lower branch of the neutral curve. We show that the governing equations for the air viscous sublayer and the film flow are quasi-steady. The equation describing the inviscid layer of the airflow is unsteady and its referred to as the unsteady Kàrmàn–Guderley equation. The influence of the film surface tension is expressed through normal shear stress condition at the interface. We present an analytic formula for the amplitude of the Tollmien–Schlichting waves that are formed in the boundary layer. We analysed our model with different values of surface tension, initial film thickness and Kàrmàn–Guderley parameter. Depending on the value of these parameters, the initial amplitude of the instability waves may grow or decay.
In this work, nanostructure FeMnO
3
perovskite material was synthesis by sol–gel auto combustion method using the glycine as a chelating agent for the first time in the literature. The synthesized ...materials were investigated structural, electrical, and magnetic properties by various characterization techniques. The X-ray diffraction reveals that the density and specific surface value are 2.54 g/cm
3
and 1.468 m
2
/g, respectively. FE-SEM investigated surface morphology in the nanoscale region. The FeMnO
3
material is existing antiferromagnetism at room temperature. Electrical property such as dielectric constant, tangent loss, and impedance spectrum, Nyquist plot with an equivalent circuit, AC conductivity, and electric modulus was analysed. The real part of permittivity, dielectric tangent loss with varying with temperature shows that the behaviour of Maxwell–Wagner relaxation in the FeMnO
3
might be observed oxygen vacancy. The Nyquist plot gives more information-related electrical conduction process and individual contribution of grain and grain boundaries. The resistance (Ω) and capacitance (farads) values are varied with increasing temperature. An electric modulus value is varied under suppression electrode polarization and shows the effect of bulk and grain boundaries.
In this present study, we have discussed about the synthesis of spinel type ZnCo2O4 by sol-gel combustion method using glycine as a chelating agent. Structural and morphological verifications are ...carried out using XRD, SEM and the functional group confirmation by using FT-IR. The temperature dependent dielectric relaxation and conduction mechanism have been analyzed by using dielectric measurements, impedance spectroscopy and electric modulus studies in the frequency range of 50 Hz to 5 MHz between the temperatures 303 K and 593 K. Influence of oxygen vacancy on the behaviour of dielectric relaxation is due to the hopping mechanism between Co2+ and Co3+ sites due to the structural defect in the material. XPS measurement confirms the different oxidation state in Co-site and the existence of oxygen vacancy in the material. The conductivity of the material is studied with Jonscher Power law.
•Spinal ZnCo2O4 is synthesized by microwave assisted sol-gel method.•Grain effect and grain boundary effect are analyzed from impedance and modulus spectra.•Activation energy and XPS measurements revealed the oxygen vacancy.•Conductivity is influenced by the hopping mechanism at higher temperatures.
Geological tourism (geotourism) is a global activity that represents an important research direction. The latter is very “young”, but has spread rapidly since the 2000s. The geographical pattern of ...geotourism research can be examined through a bibliographical survey. A total of 165 journal articles were selected on geotourism published by 417 specialists from 45 countries during the 2012–2014 term. Authors' affiliations and the focus of regional research were analyzed. The results demonstrate that geotourism research concentrates in Europe, East Asia, the Middle East, and South America. The largest research communities are active in Italy, Brazil, China, and Poland. Overall, geotourism studies are conducted on all continents (except for Antarctica). The results demonstrate the global scale of geotourism research. The spread of this concept also is shown by the evidence of the growth of national and international networks of specialists. There is no good explanation for the documented world distribution of geotourism research that can be fully confirmed. Such an outcome implies that the geographical pattern of this rising scientific discipline is determined by a set of puzzling factors that may be equally important.
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•417 specialists published 165 journal articles on geotourism in 2012–2014.•Large geotourism research communities in Italy, Brazil, China, Poland•Geotourism research concentrates in Europe, East Asia, Middle East, and South America.•Geotourism research is of a global scale.•No single explanation of world distribution of geotourism research is possible.
In this present work we discussed the synthesis of pure Ca3Co4O9 ceramic powder by a starch assisted sol–gel combustion method. The products were characterized by powder X-ray diffraction (XRD), ...thermogravimetric and differential thermal analyses (TGA–DTA), Fourier transformation infrared spectroscopy (FTIR), scanning electron microscope (SEM) and UV–visible diffuse reflectance spectroscopy (DRS). X-ray diffraction pattern confirmed the formation of single phase Ca3Co4O9 at a sintering temperature of 1073K, and it is also confirmed in the thermal analysis. SEM images indicate the presence of diffused microporous sphere like morphology and the grain sizes are in the range of 150–300nm. Optical properties of Ca3Co4O9 ceramic show a band gap at an energy level of 2.10eV. A maximum electrical resistivity of 0.002mΩcm was exhibited by Ca3Co4O9 that was decreased to 0.0012mΩcm, when the temperature increased from 300K to 473K. Dielectric studies were conducted at various temperatures from room temperature to 673K and the results indicate that the space charge polarization contributes to the conduction mechanism. It also shows that the dielectric relaxation with activation energy is 0.96eV. The magnetic properties as a function of temperature represent the ferri-paramagnetic phase transition at above 50K. M–H curve shows the hysteresis loop with saturation magnetization (Ms) and confirms the presence of soft magnetic materials.
•Ca3Co4O9 has not yet been reported by this starch assisted sol–gel combustion method.•SEM image shows microporous sphere like morphology.•The optical and dielectric properties of Ca3Co4O9 sample were studied.•Temperature dependent magnetic property has been studied for Ca3Co4O9. It behaves as a soft magnetic material at 5K.
We consider the motion at large Reynolds number of an incompressible fluid around a thin circular disk of finite radius rotating in its plane. The disk is placed in a large tank filled with an ...initially stagnant fluid. Then it is brought into rotation about its centre with constant angular velocity. Due to viscosity, a layer of fluid adjacent to the disk gets involved in the circumferential motion. This activates centrifugal forces; the fluid particles start to deviate in the radial direction. When they cross the edge of the disk, a thin jet is formed. Firstly, we solve the classical boundary-layer equations for the flow in the boundary layer in the direct neighbourhood of the disk surface and in the jet. The solution is found to develop a discontinuity at the ‘head’ of the jet where the radial and circumferential velocity components experience a jump. This type of discontinuity, called a pseudo-shock, was previously observed by Ruban & Vonatsos (J. Fluid Mech., vol. 614, 2008, pp. 407–424). Then, we investigate the internal structure of the pseudo-shock. We find that the fluid motion is described by the Euler equations in the leading-order approximation. Their solution shows that, as the jet penetrates the stagnant fluid, it ejects the fluid from the boundary layer into the surrounding area. Analysis of the inviscid region outside the boundary layer reveals that the ejected fluid returns back to the boundary layer through the ‘entrainment process’. Finally, we conclude this paper with the study of the wake in the vicinity of the disk rim.
