The generation of two-dimensional thermal convection induced simultaneously by gravity and high-frequency vibration in a bounded rectangular enclosure or in a layer is investigated theoretically and numerically. The horizontal walls of the container are maintained at constant temperatures while the vertical boundaries are thermally insulated, impermeable and adiabatic. General equations for the description of the time-averaged convective flow and, within this framework, the generalized Boussinesq approximation are formulated. These equations are solved using a spectral collocation method to study the influence of vibrations (angle and intensity). Hence, a theoretical study shows that mechanical quasi-equilibrium (i.e., state in which the averaged velocity is zero but the oscillatory component is in general non-zero) is impossible when the direction of vibration is not parallel to the temperature gradient. In the other case, it is proved that the mechanical equilibrium is linearly stable up to a critical value of the unique stability parameter, which depends on the vibrational field. In this paper, it is shown that high-frequency vertical oscillations can delay convective instabilities and, in this way, reduce the convective flow. The isotherms are oriented perpendicular to the axis of vibration. In the case where the direction of vibration is perpendicular to the temperature gradient, small values of the Grashof number, the stability parameter, induce the generation of an average convective flow. When the aspect ratio is large enough, the character of the bifurcation is practically the same as in the limiting case of an infinitely long layer.

The concentrations of organochlorine pesticides (OCPs) in fish, shrimps, cattle fat and human serum samples from the Sene-Gambian region were measured using validated analytical methodologies. The results obtained were compared with those of other existing African studies and with data from other developing countries. In fish samples, p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) and HCB were detected with a frequency of 100%, whereas p,p'-dichlorodiphenyltrichloroethane (p,p'-DDT) was detected in eight, heptachlorepoxide in six and endosulfansulfate in five of the nine fish samples. Relatively low concentrations of OCPs were found in cattle and shrimp fat samples. p,p'-DDE and alpha-hexachlorocyclohexane (alpha-HCH) were the most frequently identified. The sum of HCHs ranged from undetected to 13.3 ng g-1 fat, the sum of DDTs from 11.1 to 199.2 ng g-1 fat and the sum of endosulfans from not detected to 49.7 ng g-1 fat in fish and shrimps. In serum samples, alpha-HCH, p,p'-DDE, o,p'-DDT and p,p'-DDT were detected in all 16 pooled serum samples, whereas endosulfansulfate, methoxychlor, mirex, heptachlorepoxide and endrin were detected in 15 samples with most of the concentrations below 10 ng mL-1. The concentrations of OCPs in human serum were given on a serum lipid and whole serum volume basis. The implications for the human diet of these OCP concentrations in serum were investigated by means of biomagnification factors related to the log Kow values of the targeted compounds. The current use of HCH mixtures was suggested to explain the unusually high alpha-HCH concentration. The distribution pattern of these OCPs in humans was also discussed and compared with that in other studies.

In Senegal road projects often traverse areas of lateritic soils which must be stabilised before reuse. This paper examines mineralogical properties of three laterites by SEM and XRD analysis. The composition is variable: mineral clays have an effect on behaviour. Analysis shows that the alteration to quartz and feldspar results in minerals which are reactive in the presence of lime. Hydroxides of iron and aluminium will also play an important role in the stabilisation. Development of hydration process of cement laterite mixes is studied and the products identified by SEM and XRD. The presence of CSH(I) is found principally in stabilised laterites that have a high kaolin content, this is attributed to the presence of amorphous silica formed in the kaolinisation of feldspar. (A) For the covering abstract see ITRD E121772.

A survey was carried out in urban areas of the Niayes zone in Senegal to explore grasslands development in agropastoral systems around towns. Rapid rural appraisal and farms multiple visits were done to describe the forage cropping system, identify constraints and opportunities of developing grasslands in urban areas. In Niayes urban areas, forage production system is being developed based on the cropping of several grass and tree species. Only a minority of farmers is involved in that activity. Level of grass production is rather low.Weak development of grasslands is observed. Main reasons are: farmer poor willingness to crop for livestock feeding, lack of water and production factors including seeds and fertiliser, land tenure system and lack of integration between livestock and horticulture, security and economical environment. There are opportunities that need to be developed for improvement of grassland production in the Niayes as water reserves could tolerate more forage cropping.Research and development programs should focus on programs that could generate recommendations on grasslands development in association with horticulture witch is a major activity in Niayes areas.

Carbon prepared from peanut husks (PHC) has been used for the adsorption of Pb2+, Zn2+, Ni2+ and Cd2+, over a range of initial metal ion concentration (0.15 mM). Chemical and physical characteristics of PHC were determined. The effects of particle size and of carbon doses were evaluated by batch experiments. The kinetics of sorption were followed, based on the amounts of metal sorbed at various time intervals. The modeling of kinetic and isotherm curves has also been investigated. The rate constant and the reaction order have been calculated. The results show that Pb2+ has best affinity to PHC than Cd2+, Ni2+, Zn2+. From these results an order of the sorption capacity of metal ions sorbed was derived.

Rice husk ash has been used in many countries as a low cost concrete admixture because of its role as a filler and a pozzolan. A study is made of an ash produced in Senegal to establish the existence and origin of silica. Two calcinated rice husk ashes were studied. X-Ray diffraction analysis revealed the presence of quartz whose origin may be attributed to contamination and/or re-crystallisation of silica in the calcination. Amorphous silica was detected by scanning electron microscopy and microanalysis, its presence is concentrated on the interior and exterior surfaces of the uncalcinated husk which may promote a pozzolanic action on the surface of the husk and therefore enable its use in lightweight concrete.

La plupart des pays tropicaux sont recouverts en grande partie de sols résiduels appelés latérites, pouvant avoir des dimensions granulométriques allant des fines aux enrochements. Les granulats à béton classiques utilisés jusqu'ici (basalte, grès, calcaire) existent au Sénégal en quantité limitée. Or, cette région est recouverte à plus de 80% de sols latéritiques qui, de par certaines de leurs caractéristiques autres que la dureté, peuvent servir très valablement de granulats à béton de densité normale. Dans cet article, les auteurs exposent les résultats d'une étude comparative de bétons à base de granulats divers (basalte, grès, calcaire, graveleux latéritiques) avec sable de plage ou sable des dunes. Les conclusions après plusieurs essais sont les suivantes: les graveleux latéritiques, malgré leur dureté relativement faible, ont des caractéristiques de surface (rugosité, coefficient d'absorption) qui en font de très bons granulats à béton courant, et les sables de dune peuvent valablement se substituer aux sables de plage qui sont moins accessibles et plus onéreux.