Study of disturbed boundary layers using a hot-wire anemometer

One of the most important problems in fluid mechanics which has attracted the interest of investigators for many years is the transition from laminar to turbulent flow. This problem is of greatest importance in practice for flows over aerofoils and in closed channels, where the effects of transition are marked. Little is known, however, about the fundamental physical processes which cause a laminar flow to break down to turbulence. The particular case of boundary-layer flow has received considerable attention and has been treated with some measure of success. Linearized theories have been useful in predicting the nature of the mechanism of instability of a laminar flow; but the non-linear problem, namely, the transition from an unstable laminar to a fully developed turbulent flow, presents formidable mathematical difficulties, so that few theoretical results have been obtained. The development of air and water tunnels together with improved measuring techniques in fluid flow has led to attempts by engineers and experimental physicists to relate the unstable laminar to the turbulent flow regime. The aim of the work reported in this thesis was to gain further insight into the mechanism of transition in the boundary layer. The present experimental work was confined to the simplest example, the flow of an incompressible fluid along a smooth flat plate at zero angle of incidence with zero pressure gradient in the flow direction. The intention was to develop a hot -wire anemometer system to study the downstream development of disturbances introduced under controlled conditions into the boundary layer. As the work progressed emphasis was placed on one particular aspect of the non- linear development of the disturbances, the generation of higher harmonics of the fundamental wave

Evidence for the tongue of ionization under northward interplanetary magnetic field conditions

[1] The activities of the International Ionospheric Tomography Community open up new possibilities of simultaneously imaging the large-scale spatial structure of the ionosphere in different longitude sectors. In the study, tomography receiver chains in Scandinavia and Greenland were used to provide a wide view of the plasma density structure in the winter, magnetic postnoon sector under conditions of stable, positive interplanetary magnetic field B z component. The spatial distributions of the plasma are discussed in light of a high-latitude plasma convection pattern pertinent to the conditions, which is supported by DMSP flow measurements. The observations are consistent with a tongue of dayside photoionization being drawn antisunward by the convection pattern to form an arc of enhanced plasma density around the periphery of the polar cap

Multi-instrument probing of the polar ionosphere under steady northward IMF

International audienceObservations are presented of the polar ionosphere under steady, northward IMF. The measurements, made by six complementary experimental techniques, including radio tomography, all-sky and meridian scanning photometer optical imaging, incoherent and coherent scatter radars and satellite particle detection, reveal plasma parameters consistent with ionospheric signatures of lobe reconnection. The optical green-line footprint of the reconnection site is seen to lie in the sunward plasma convection of the lobe cells. Downstream in the region of softer precipitation the reverse energy dispersion of the incoming ions can be identified. A steep latitudinal density gradient at the equatorward edge of the precipitation identifies the general location of an adiaroic boundary, separating the open field lines of polar lobe cells from the closed field of viscous-driven cells. Enhancements in plasma density to the south of the gradient are interpreted as ionisation being reconfigured as it is thrust against the boundary by the antisunward flow of the viscous cells near noon. Each of the instruments individually provides valuable information on certain aspects of the ionosphere, but the paper demonstrates that taken together the different experiments complement each other to give a consistent and comprehensive picture of the dayside polar ionosphere

Total electron content - A key parameter in propagation:Measurement and use in ionospheric imaging

The paper reports on a series of studies carried out within the COST 271 Action relating to the measurement and use of Total Electron Content (TEC) of the ionosphere over North West Europe. Total electron content is a very important parameter for the correction of propagation effects on applied radio systems so that it is vital to have confidence in the experimental measurements and the resultant products derived as aids for the practical user. Comparative investigations have been carried out using TEC values from several different sources. It was found that in general there was broad statistical agreement between the data sets within the known limitations of the techniques, though discrepancies were identified linked to steep ionospheric gradients at the onset of geomagnetic storm disturbance and in the vicinity of the main trough. The paper also reviews recent progress in the development of tomographic inversion techniques that use total electron content measurements to image the ionosphere as an aid to various radio systems applications