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21.
A new formula for the coefficient of dielectronic recombination has been derived by substituting excitation cross-section extrapolated below threshold for capture cross-section in Equation (7) of Burgess (1964). In this case the excitation cross-section modified by Mewe (1972b) through a fitted gaunt factor has been used. The dielectronic recombination coefficients calculated by the new formula are found to be about an order of magnitude smaller than those obtained by Burgess simplified formula. The results for coronal ions of silicon and iron are presented here as our sample calculations. A comparative study of the dielectronic recombination with radiative recombination is also made. 相似文献
22.
RUDOLF O. WEBER PETER TALKNER INGEBORG AUER REINHARD BÖHM MARJANA GAJIĆ-ČAPKA KSENIJA ZANINOVIĆ RUDOLF BRÁZDIL PAVEL FAŠKO 《Climatic change》1997,36(3-4):327-344
Daily maximum and minimum temperatures from 29 low-lying and mountain stations of 7 countries in Central Europe were analyzed. The analysis of the annual variation of diurnal temperature range helps to distinguish unique climatic characteristics of high and low altitude stations. A comparison of the time series of extreme daily temperatures as well as mean temperature shows a good agreement between the low-lying stations and the mountain stations. Many of the pronounced warm and cold periods are present in all time series and are therefore representative for the whole region. A linear trend analysis of the station data for the period 1901–1990 (19 stations) and 1951–1990 (all 29 stations) shows spatial patterns of similar changes in maximum and minimum daily temperatures and diurnal temperature range. Mountain stations show only small changes of the diurnal temperature range over the 1901–1990 period, whereas the low-lying stations in the western part of the Alps show a significant decrease of diurnal temperature range, caused by strong increase of the minimum temperature. For the shorter period 1951–1990, the diurnal temperature range decreases at the western low-lying stations, mainly in spring, whereas it remains roughly constant at the mountain stations. The decrease of diurnal temperature range is stronger in the western part than in the eastern part of the Alps. 相似文献
23.
H.M. Badr 《Ocean Engineering》1994,21(4)
The problem of oscillating viscous flow over a stationary cylinder of elliptic cross-section is investigated. The free stream direction is horizontal and is always normal to the cylinder axis and oscillations are only allowed in the magnitude of the stream velocity. The flow is incompressible and two-dimensional and the free steam oscillations are harmonic. The elliptic cylinder considered has a major-minor axis ratio of 0.6 and is inclined to the free stream direction at angles of incidence of either 30 or 60°. The investigation is based on the solution of the time-dependent Navier-Stokes equations together with the mass conservation equation. The parameters involved are the cylinder axis ratio, angle of incidence, Reynolds number, and Strouhal number. The Reynolds number ranges between 102 and 103 and the Strouhal number ranges between π/4 and π/2. The time variation of the flow field is presented in the form of streamline patterns as well as surface vorticity distribution. The surface pressure distribution and the time variation of the in-line and transverse force coefficients are also presented and compared with an inviscid flow solution for the same problem. The comparison between viscous and inviscid flow results shows a better agreement for higher values of Reynolds and Strouhal numbers. 相似文献
24.
Omran?E.?FrihyEmail author Moheb?M.?Iskander Abd?El?Moniem?A.?Badr 《Geo-Marine Letters》2004,24(4):195-211
Beach-nearshore profiles combined with beach and surficial sediment samples were analyzed in conjunction with wave, current, littoral drift and sea-level data to determine the effect of bedrock on morphodynamic processes within the littoral zone of Alexandria on the Mediterranean coast of Egypt. This 14.5-km-long littoral cell is bounded by pronounced embayments and pocket beaches separated by headlands which prevent bypassing of beach sands, in effect making this cell a large, semi-closed basin. The compartmented nature of this cell acts together with the rough irregularity of the rocky seafloor to trap a thin veneer of sediment (<3 m thick), showing proportional mixing between two sedimentary provinces. A modern fine-grained sediment facies consisting of mixed carbonate/siliciclastic sand flanks most of the nearshore zone down to a depth of 8–10 m. Beyond this depth, considered to be the depth of closure, a relict late Pleistocene to mid-Holocene coarse-grained facies composed of biogenic carbonate sand is found. Along a short section of the coastline (km 3–6), the coarser sediment also occupies the nearshore zone. Over most of the study area the two sediment types are mixed in various proportions, largest mixing coinciding with poorest sorting. Profile analyses revealed seasonal changes in sediment volume along the coast which closely follow the cyclicity of seasonal changes in wave climate. The present shoreline orientation, headlands and rough, irregular rocky seabed are reflected in the erosion/accretion pattern, sediment characteristics, and the reversibility of longshore currents and littoral drift. Although there is a marked deficiency in the sediment balance, the sand budget for this cell, including artificial material (2.339*106 m3) has increased slightly by 0.041*106 m3 year–1
as a result of engineering works carried out to widen the coastal road (Corniche). In addition to the physical properties of the bedrock (degree of induration), the accelerating sea-level rise during the Holocene and human influences, the modern morphology of the coast, the erosional seabed features in the nearshore zone, and the texture of seabed sediments are all controlled by the original geometry of the coast which consisted of an elevated subaerial ridge. 相似文献
25.
Intensities of resonance lines in X-ray region for Si ix–Si xiv ions are calculated by considering total excitation, i.e., excitation due to electron impact and dielectronic recombination. It is found that the contribution of the latter is quite significant. For consistency, the electron density effect in our calculations is not only accounted for in the ionization equilibrium but also in the total rate of excitation. It is also found that the contribution of electron density effect is pronounced with the inclusion of dielectronic recombination as an excitation mechanism. The computed average line intensities are compared with the available observations and a table of line flux for various wavelengths of the above-mentioned ions at different temperatures is also given. 相似文献
26.
The present work studies the environmental isotopes assess groundwater characteristics of the different parts of the main aquifer in the northeast Missan Province in south of Iraq.Water samples of groundwater and surface water were collected for two dry and wet seasons during the water year of 2011–2012.The study shows that most of the groundwater in the aquifer falls above the global meteoric water line,and all the samples fall below the Mediterranean meteoric water line,indicating that these samples are a mixture of two water types.The tritium content of these samples supports this conclusion.The overall conclusion of this study indicates that there are two sources of groundwater recharge in the studied area:the ephemeral streams(Teeb and Dewerge) and major precipitation sources.According to the tritium levels at or below one tritium unit(TU) obtained from the water,supply wells are highly confined or "not vulnerable".Overall,the 3H results imply that recent recharge has taken place during the last four to five decades.In the recharge area,the high tritium content in the southern part of the Teeb area suggests that the recharge originates from rapid infiltration of surface runoff.Therefore,the groundwater resources in the study area should be protected from contamination,because it will influence the aquifer in a relatively short period of time if any contamination enters the recharge areas of the aquifer. 相似文献