Boundary Layer

The present study reports the effect of momentum injection at the inlet on the performance of an S-shaped rectangular diffuser having an aspect ratio of 6 at inlet. A cylinder of diameter 3 cm was placed at the inflexion plane across the width of the diffuser and rotated at different speeds to inject momentum to the decelerating flow as a result of boundary layer separation. It was found that pressure recovery and flow distribution improved significantly. Improvement in coefficient of pressure recovery was about 24% for duct having an area ratio of 2 and nearly 22% for duct with an area ratio of 3. Simultaneously the values of the coefficient of total pressure loss decreased by 32% and 51% for ducts with area ratio of 2 and 3 respectively.

Keywords: S-shaped rectangular diffuser; rotating cylinder; momentum injection; boundary layer; CFD; pressure recovery coefficient

We carry out an analytical study of laminar circular hydraulic jumps, in generalized-Newtonian fluids obeying the two-parametric power-law model of Ostwald-de Waele. Under the boundary-layer approximation we obtained exact expressions determining the flow, an implicit relation for the jump radius is derived. Corresponding results for Newtonian fluids can be retrieved as a limiting case for the flow behavior index n=1, predictions are made for fluids deviating from Newtonian behavior.

Observations of the tropical atmosphere are fundamental to the understanding of global changes in air quality, atmospheric oxidation capacity and climate, yet the tropics are under-populated with long-term measurements. The first three years (October 2006–September 2009) of meteorological, trace gas and particulate data from the global WMO/Global Atmospheric Watch (GAW) Cape Verde Atmospheric Observatory Humberto Duarte Fonseca (CVAO; 16° 51′ N, 24° 52′ W) are presented, along with a characterisation of the origin and pathways of air masses arriving at the station using the NAME dispersion model and simulations of dust deposition using the COSMO-MUSCAT dust model. The observations show a strong influence from Saharan dust in winter with a maximum in super-micron aerosol and particulate iron and aluminium. The dust model results match the magnitude and daily variations of dust events, but in the region of the CVAO underestimate the measured aerosol optical thickness (AOT) because of contributions from other aerosol. The NAME model also captured the dust events, giving confidence in its ability to correctly identify air mass origins and pathways in this region. Dissolution experiments on collected dust samples showed a strong correlation between soluble Fe and Al and measured solubilities were lower at high atmospheric dust concentrations. Fine mode aerosol at the CVAO contains a significant fraction of non-sea salt components including dicarboxylic acids, methanesulfonic acid and aliphatic amines, all believed to be of oceanic origin. A marine influence is also apparent in the year-round presence of iodine and bromine monoxide (IO and BrO), with IO suggested to be confined mainly to the surface few hundred metres but BrO well mixed in the boundary layer. Enhanced CO2 and CH4 and depleted oxygen concentrations are markers for air-sea exchange over the nearby northwest African coastal upwelling area. Long-range transport results in generally higher levels of O3 and anthropogenic non-methane hydrocarbons (NMHC) in air originating from North America. Ozone/CO ratios were highest (up to 0.42) in relatively fresh European air masses. In air heavily influenced by Saharan dust the O3/CO ratio was as low as 0.13, possibly indicating O3 uptake to dust. Nitrogen oxides (NOx and NOy) show generally higher concentrations in winter when air mass origins are predominantly from Africa. High photochemical activity at the site is shown by maximum spring/summer concentrations of OH and HO2 of 9 × 106 molecule cm−3 and 6 × 108 molecule cm−3, respectively. After the primary photolysis source, the most important controls on the HOx budget in this region are IO and BrO chemistry, the abundance of HCHO, and uptake of HOx to aerosol.

We present an exact solution of Einstein's field equations in toroidal coordinates. The solution has three regions: an interior with a string equation of state; an Israel boundary layer; and an exterior with constant isotropic pressure and constant density, locally isometric to anti-de Sitter space-time. The exterior can be a cosmological vacuum with negative cosmological constant. The size and mass of the toroidal loop depend on the size of Lambda.

Laboratory experiments were performed to study the influence of density and viscosity layering on the formation and stability of plumes. Viscosity ratios ranged from 0.1 to 6400 for buoyancy ratios between 0.3 and 20, and Rayleigh numbers between 105 and 2.108. The presence of a chemically stratified boundary layer generates long-lived thermochemical plumes. These plumes first develop from the interface

We report on sizable antiferromagnetic interlayer exchange coupling (AFC) of Fe(001) layers across epitaxial Si spacers, for which epitaxial growth of a pseudomorphic phase stabilized by the interface is confirmed by low-energy electron diffraction and high-resolution transmission electron microscopy. The coupling strength decays with spacer thickness on a length scale of a few Å and shows a negative temperature coefficient. Transport measurements of lithographically structured junctions in current-perpendicular-to-plane geometry show the validity of the three “Rowell criteria” for tunneling: (i) exponential increase of resistance R with thickness of the barrier, (ii) parabolic dI/dV-V curves, and (iii) slight decrease of R with increasing temperature. Therefore, AFC is mediated by non-conductive spacers, which in transport experiments act as tunneling barriers with a barrier height of several tenths of an eV. We discuss our data — in particular the strength, thickness and temperature dependence — in the context of two previously proposed models for AFC across non-conducting spacers. We find that neither the molecular-orbital model for heat-induced effective exchange coupling nor the quantum interference model extended to insulator spacers by introducing complex Fermi surfaces can account for the strong AFC across epitaxial Si spacers and its negative temperature coefficient. The recently proposed defect-assisted interlayer exchange coupling model, however, yields qualitative agreement with the enhanced AFC and the temperature dependence.

In the present paper different types of scramjet (supersonic combustion ramjet) combustors are investigated. Thereby the main difference between the combustors is the way of injecting the fuel into the combustion chamber. The first investigated concept of fuel injection is the injection by strut injectors. Here the injection of fuel is realized by a lobed strut that is located in the middle of the combustion chamber. The second concept for fuel supply is the wall injection of hydrogen. Here the fuel is injected by several holes in the wall of the combustor. Both concepts of fuel injection have different advantages and disadvantages which are explained in detail. Although different performance parameters for both scramjet combustors are introduced this paper will not compare the different techniques among each other. Because of the high Reynolds numbers in scramjet combustors, the need to resolve the boundary layers and the necessity of detailed chemistry, the simulation of scramjets is extremely CPU time demanding.