On Reflection of Shock Waves from Boundary Layers

Measurements of the reflection characteristics of shock waves from a flat surface with a laminar and turbulent boundary layer are presented. The investigations were carried out at Mach numbers from about 1.3 to 1.5 and a Reynolds number of 0.9 x 10^4. THe difference in the shock-wave interaction with laminar and turbulent boundary layers, first found in transonic flow is confirmed and ,investigated in detail for supersonic flow. The relative upstream influence of a shock wave impinging on a given boundary layer has been measured for both laminar and turbulent layers. The upstream influence of a shock wave in the laminar layer is found to be of the order of 50 bounday-layer thicknesses as compared with about 5 in the turbulent case. Separation almost always occurs in the laminar boundary layer. The separation is restricted to a region of finite extent upstream of the the shock wave. In the turbulent case no separation was found. A model of the flow near the point of impingement of the shock wave on the boundary layer is given for both cases. The difference between impulse-type and step-type shock waves is discussed and their interactions with the boundary layer are compared. Some general considerations on the experimental production of shock waves from wedges and cones are presented, as well as a discussion of boundary layer in supersonic flow. A few exampies of reflection of shock waves from supersonic shear layers are also presented

Metallic and insulating behaviour of the two-dimensional electron gas on a vicinal surface of Si MOSFETs

The resistance R of the 2DEG on the vicinal Si surface shows an unusual behaviour, which is very different from that in the (100) Si MOSFET where an unconventional metal to insulator transition has been reported. The crossover from the insulator with dR/dT0 occurs at a low resistance of R_{\Box}^c \sim 0.04h/e^2. At the low-temperature transition, which we attribute to the existence of a narrow impurity band at the interface, a distinct hysteresis in the resistance is detected. At higher temperatures, another change in the sign of dR/dT is seen and related to the crossover from the degenerate to non-degenerate 2DEG.Comment: 4 pages, 4 figure

Heat Transfer on a Double Wedge Geometry in Hypervelocity Air and Nitrogen Flows

We investigate shock wave/boundary-layer interaction and resulting heat transfer in hypervelocity double wedge flows. An expansion tube is used to generate air and nitrogen flows with stagnation enthalpies ranging from 2.1-8.0 MJ/kg and Mach numbers from 4-7. The range of free stream conditions were selected to investigate the impact of thermochemical effects by i) systematically varying the chemical composition from nitrogen to air while maintaining constant the stagnation enthalpy or the Mach number, and ii) varying the stagnation enthalpy. Flow features are visualized with schlieren photography, and heat transfer is measured using fast response coaxial thermocouples. Data are presented for both nitrogen and air test conditions with eight cases in total. Current results indicate significantly different behavior in flows with enthalpies as low as 4 MJ/kg between air and nitrogen test conditions

Shot Noise in Mesoscopic Transport Through Localised States

We show that shot noise can be used for studies of hopping and resonant tunnelling between localised electron states. In hopping via several states, shot noise is seen to be suppressed compared with its classical Poisson value $S_I=2eI$ ($I$ is the average current) and the suppression depends on the distribution of the barriers between the localised states. In resonant tunnelling through a single impurity an enhancement of shot noise is observed. It has been established, both theoretically and experimentally, that a considerable increase of noise occurs due to Coulomb interaction between two resonant tunnelling channels.Comment: 7 pages, 5 figures; Proceedings of the 10th Conference on Hopping and Related Phenomena (Trieste 2003); requires Wiley style files (included

On Real Fluid Flow Over Yawed Circular Cylinders

The equations for both the boundary layer and the outer potential flow over a yawed cylinder can be resolved into equations for the crosswise and spanwise velocity components. These components of the boundary layer are evaluated using Sears’ method, and the separation point is found to be uninfluenced by the yaw angle. The potential-flow solutions for the spanwise and crosswise flows are added together to determine vortex patterns behind the cylinder. The approximate direct dependence of the Strouhal number upon the cosine of the yaw angle and/or the drag coefficient upon the square of the cosine, are verified. Experimental determinations of the Strouhal number and visualization of the flow pattern are consistent with the analysis

Changes in the Carpathian fauna of Malachiinae beetles (Coleoptera, Melyridae) in the context of temperature increase

Over last century in Europe a tendency of change in species ranges has occurred for insects of different taxonomic groups. We analyzed the changes that have taken place in the distribution of some soft-winged beetles (Malachiinae, Melyridae) species in the Carpathian region. The obtained data are based on a study of museum beetles’ collections and the authors’ collected materials. Data comparison relating to species distribution shows that the Carpathian fauna during the second half of the 20th century increased by 12 species. Some species have significantly changed their ranges. Species of Southern European and Mediterranean (Anthomalachius strangulatus, Clanoptilus spinipennis), Central-East European (Apalochrus femoralis, Clanoptilus falcifer) origin spread to the East Carpathian region. And vice versa, some species (e.g. Malachius scutellaris) from Central European and the Carpathian regions have spread beyond their borders of ranges to the East and North. Malachiinae species are anthophilic insects. Their life cycles, flight period, and distribution depend on the phenology of host plants. Acceleration of phenophases of the host plants and intensification of the metamorphosis processes due to increase in air temperature has caused an earlier appearance of soft-winged beetles in recent years and contributed to expansion of the ranges of some species of them. In our opinion, expansion of the species ranges and penetration of new beetle species into the Carpathian region from surrounding areas are related to the climate changes, in particular to warming. This thesis is confirmed by multiyear air temperature data of the Transcarpathian Centre of Hydrometeorology. On a background of increase in regional air temperature by about 0.8 °C an average air temperature in foothills of Ukrainian Carpathians has increased by 1.7 °C during last 50–60 years

Effect of sting size on the wake of a sphere at subcritical Reynolds numbers

Vortex shedding and turbulent motion in the wake of a sphere that is supported using a streamwise-aligned cylindrical sting are investigated at a subcritical Reynolds number of Re=3800, using high speed particle image velocimetry. The mechanism by which the presence of a sting of increasing diameter relative to the diameter of the sphere influences the wake, in terms of both the small-scale shear instability and the larger wake instability, is explored and brie y compared with the two-dimensional analog of the splitter plate introduced into a cylinder wake. The difficulties associated with obtaining converged statistics, along with the effect of free stream turbulence and sphere vibrations are detailed. An understanding of the mechanism by which the blockage, or interference, arising from the presence of the sting influences cross-wake communication and downstream development is a necessary precursor to studies of active control of the wake using surface actuation on a sting-mounted sphere

Universal behaviour of entrainment due to coherent structures in turbulent shear flow

I suggest a solution to a persistent mystery in the physics of turbulent shear flows: cumulus clouds rise to towering heights, practically without entraining the ambient medium, while apparently similar turbulent jets in general lose their identity within a small distance through entrainment and mixing. From dynamical systems computations on a model chaotic vortical flow, I show that entrainment and mixing due to coherent structures depend sensitively on the relative speeds of different portions of the flow. A small change in these speeds, effected for example by heating, drastically alters the sizes of the KAM tori and the chaotic mixing region. The entrainment rate and, hence, the lifetime of a turbulent shear flow, shows a universal, non-monotone dependence on the heating.Comment: Preprint replaced in order to add the following comment: accepted for publication in Phys. Rev. Let

Влияние структурной релаксации на температурную зависимость псевдощели в монокристаллах YВа₂Сu₃О₇₋δ с различным содержанием кислорода

Исследовано влияние отжига при комнатных температурах на избыточную проводимость монокристаллов YВа₂Сu₃О₇₋δ с различным отклонением от стехиометрии по кислороду. Обнаружено, что в процессе отжига при комнатных температурах в течение около четырех суток происходит общее относительное расширение линейного участка температурной зависимости ρab(Т). При этом избыточная проводимость подчиняется экспоненциальной температурной зависимости в широком интервале температур, а температурная зависимость псевдощели (ПЩ) удовлетворительно описывается в рамках теории кроссовера от механизма Бардина− Купера− Шриффера (БКШ) к механизму бозе-эйнштейновской конденсации (БЭК).Досліджено вплив відпалювання при кімнатних температурах на надлишкову провідність бездвійникового монокристала YВа₂Сu₃О₇₋δ з різним відхиленням від стехіометрії за киснем. Виявлено, що у процесі відпалювання при кімнатних температурах на протязі чотирьох діб відбувається загальне відносне розширення лінійної ділянки температурної залежності ρab(Т). При цьому надлишкова провідність підкоряється експоненціальній температурній залежності у широкому інтервалі температур, а температурна залежність псевдощілини (ПЩ) задовільно описується в рамках теорії кросовера від механізму Бардіна−Купера−Шрифера (БКШ) до механізму бозе-ейнштейнівської конденсації (БЕК).The effect of annealing at room temperatures on excess conductivity of the YВа₂Сu₃О₇₋δ single crystals with different deviation from oxygen stochiometry is investigated. It is shown that the process of annealing at room temperatures for about four days results in the total relative expansion of the linear area of the temperature dependence ρab(Т). The excess conductivity obeys exponential temperature dependence in the wide temperature interval. The temperature dependence of pseudogap is satisfactorily described by theoretical model of crossover from the Bardeen−Cooper−Schrieffer (BCSh) mechanism to the mechanism of Boze−Einstein condensate (BEC)