Unsteady laminar compressible boundary-layer flow at a three-dimensional stagnation point

Unsteady laminar compressible boundary-layer flow with variable properties at a three-dimensional stagnation point for both cold and hot walls has been studied for the case when the velocity of the incident stream varies arbitrarily with time. The partial differential equations governing the flow have been solved numerically using an implicit finite-difference scheme. Computations have been carried out for two particular unsteady free-stream velocity distributions: (i) an accelerating stream and (ii) a fluctuating stream. The results indicate that the variation of the density-viscosity product across the boundary layer, the wall temperature and the nature of stagnation point significantly affect the skin friction and heat transfer.

A sufficient criterion for Rayleigh-Taylor instability of incompressible viscous three-layer flow

Using normal mode analysis Rayleigh-Taylor instability is investigated for three-layer viscous stratified incompressible steady flow, when the top 3rd and bottom 1st layers extend up to infinity, the middle layer has a small thickness δ. The wave Reynolds number in the middle layer is assumed to be sufficiently small. A dispersion relation (a seventh degree polynomial in wave frequency ω) valid up to the order of the maximal value of all possible Kj (j less-than-or-equals, slant 0, K is the wave number) in each coefficient of the polynomial is obtained. A sufficient condition for instability is found out for the first time, pursuing a medium wavelength analysis. It depends on ratios (α and β) of the coefficients of viscosity, the thickness of the middle layer δ, surface tension ratio T and wave number K. This is a new analytical criterion for Rayleigh-Taylor instability of three-layer fluids. It recovers the results of the corresponding problem for two-layer fluids. Among the results obtained, it is observed that taking the coefficients of viscosity of 2nd and 3rd layers same can inhibit the effect of surface tension completely. For large wave number K, the thickness of the middle layer should be correspondingly small to keep the domain of dependence of the threshold wave number Kc constant for fixed α, β and T.

Boundary-layer heights over the monsoon trough region during active and break phases

The thermodynamic structure and the heights of the boundary layer over the monsoon trough region of the Indian southwest monsoon are presented for the active and break phases of the monsoon. Results indicate significant and consistent variation in boundary-layer heights between the active and break phases.

Effect of interface friction on the mechanics of indentation of a finite layer resting on a rigid substrate

Copper strips of 2.5 mm thickness resting on stainless steel anvils were normally indented by wedges under nominal plane strain conditions. Inflections in the hardness-penetration characteristics were identified. Inflections separate stages where each stage has typical mechanics of deformation. These are arrived at by studying the distortion of 0.125 mm spaced grids inscribed on the deformation plane of the strip. The sensitivity of hardness and deformation mechanics to wedge angle and the interfacial friction between strip and anvil were investigated within the framework of existing slip line field models of indentation of semi-infinite and finite blocks.

Experimental investigation of three-layer electromagnetically coupled circular microstrip antennas

The broadband behaviour of a three-layer electromagnetically coupled circular microstrip antenna is investigated experimentally. The effects of interlayer spacings and the thickness of the parasitic layers on the impedance bandwidth, 3 dB beamwidth and pattern shape, are studied. Experiments show that this structure can provide a frequency bandwidth as high as 20% with a low crosspolarisation level and a moderately high gain.

A study on boundary-layer transition induced by free-stream turbulence

Boundary-layer transition at different free-stream turbulence levels has been investigated using the particle-image velocimetry technique. The measurements show organized positive and negative fluctuations of the streamwise fluctuating velocity component, which resemble the forward and backward jet-like structures reported in the direct numerical simulation of bypass transition. These fluctuations are associated with unsteady streaky structures. Large inclined high shear-layer regions are also observed and the organized negative fluctuations are found to appear consistently with these inclined shear layers, along with highly inflectional instantaneous streamwise velocity profiles. These inflectional velocity profiles are similar to those in the ribbon-induced boundary-layer transition. An oscillating-inclined shear layer appears to be the turbulent spot-precursor. The measurements also enabled to compare the actual turbulent spot in bypass transition with the simulated one. A proper orthogonal decomposition analysis of the fluctuating velocity field is carried out. The dominant flow structures of the organized positive and negative fluctuations are captured by the first few eigenfunction modes carrying most of the fluctuating energy. The similarity in the dominant eigenfunctions at different Reynolds numbers suggests that the flow prevails its structural identity even in intermittent flows. This analysis also indicates the possibility of the existence of a spatio-temporal symmetry associated with a travelling wave in the flow.

Boundary layer swirling flow through a conical hydrocyclone

The paper deals with the flow and heat-transfer problem of a steady axisymmetric laminar incompressible boundary layer swirling flow of a fluid through a conical hydrocyclone. The implicit finitedifference scheme is used to solve the partial differential equations governing the flow. The effect of swirl is found to be more pronounced on the longitudinal skin friction than on the tangential skin friction and heat transfer. The skin friction and heat transfer increase with swirl or with longitudinal distance. Swirl also gives rise to velocity overshoot in the longitudinal velocity profiles and the magnitude of the velocity overshoot increases as the swirl parameter increases. The results are found to be in good agreement with those of the local nonsimilarity and momentum integral methods but they differ appreciably from those of the local similarity method except for the longitudinal skin friction which is fairly in good agreement with that of the local similarity method.Die Arbeit beschäftigt sich mit der Strömung und dem Wärmeübergang in einem konischen Zyklon unter der Voraussetzung stationärer, achsensymmetrischer, laminarer, inkompressibler Grenzschichtströmung. Ein implizites Differenzenverfahren wird benutzt, um die partiellen Differentialgleichungen zu lösen. Der Einfluß des Dralls ist besonders ausgeprägt auf die longitudinale Komponente der Oberflächenreibung, weniger dagegen bei der tangentialen Komponente und beim Wärmeübergang. Die Oberflächenreibung und der Wärmeübergang nehmen zu mit dem Drall, sowie mit dem longitudinalen Abstand. Der Drall erzeugt ein Überschießen der Geschwindigkeit in der longitudinalen Abstand. Der Drall erzeugt ein Überschießen der Geschwindigkeit in der Längsrichtung. Die Größe des Überschusses nimmt mit wachsendem Drallparameter zu. Die Resultate stimmen gut mit den Ergebnissen der Theorie der lokalen Nichtähnlichkeit und der Impulsintegralmethode überein. Dagegen weichen sie mit Ausnahme der longitudinalen Komponente der Oberflächenreibung beträchtlich von den Resultaten der Theorie der lokalen Ähnlichkeit ab.

Dielectric relaxation in bismuth layer-structured BaBi4Ti4O15 ferroelectric ceramics

The dielectric properties of BaBi4Ti4O15 ceramics were investigated as a function of frequency (10(2)-10(6) Hz) at various temperatures (30 degrees C-470 degrees C), covering the phase transition temperature. Two different conduction mechanisms were obtained by fitting the complex impedance data to Cole-Cole equation. The grain and grain boundary resistivities were found to follow the Arrhenius law associated with activation energies: E-g similar to 1.12 eV below T-m and E-g similar to 0.70 eV above T-m for the grain conduction; and E-gb similar to 0.93 eV below T-m and E-gb similar to 0.71 eV above T-m for the grain boundary conduction. Relaxation times extracted using imaginary part of complex impedance Z `'(omega) and modulus M `'(omega) were also found to follow the Arrhenius law and showed an anomaly around the phase transition temperature. The frequency dependence of conductivity was interpreted in terms of the jump relaxation model and was fitted to the double power law. (C) 2010 Elsevier B. V. All rights reserved.

Atmospheric boundary-layer across Hadley and Ferrel cells over the Indian Ocean

A Southern Ocean Pilot cruise covering the latitudes from 10 degrees N to 56 degrees S in the open Indian Ocean was carried out during January February 2004. Surface and upper air data collected during this cruise are reported here. It is shown that the broad features of the atmosphere, in particular that of temperature, follow the tropical and mid-latitude weather expected during January February in this region. However, the atmospheric boundary-layer shows large variations, both in its height and structure between tropics and high latitudes. Strong influence of the surface heat flux on boundary layer structure is clearly seen. Humidity field reveals several local maxima and minima, suggesting a laminated atmosphere with air from different sources moving almost unmixed in adjacent layers.

On the possibility of an alpha-sq omega-type dynamo in a thin layer inside the sun

If the solar dynamo operates in a thin layer of 10,000-km thickness at the interface between the convection zone and the radiative core, using the facts that the dynamo should have a period of 22 years and a half-wavelength of 40 deg in the theta-direction, it is possible to impose restrictions on the values which various dynamo parameters are allowed to have. It is pointed out that the dynamo should be of alpha-sq omega nature, and kinematical calculations are presented for free dynamo waves and for dynamos in thin rectangular slabs with appropriate boundary conditions. An alpha-sq omega dynamo is expected to produce a significant poloidal field which does not leak to the solar surface. It is found that the turbulent diffusity eta and alpha-coefficient are restricted to values within about a factor of 10, the median values being eta of about 10 to the 10th sq cm/sec and alpha of about 10 cm/sec. On the basis of mixing length theory, it is pointed out that such values imply a reasonable turbulent velocity of the order 30 m/s, but rather small turbulent length scales like 300 km.