Reorganization of Coherent Structures Downstream a Circular Cylinder Located between Two Parallel Walls

Experiments were performed at low Reynolds numbers in the range 75 Re 275 in the wake of a circular cylinder of dc diameter placed symmetrically between two parallel walls of H height. 2D2C particle image velocimetry (PIV) was used to investigate the flow downstream the cylinder. In the unsteady flow regime downstream the cylinder, the detached primary vortices (Pi) interact with walls generating secondary ones (Pi’) and modify the cylinder wake dynamic. The kinematical properties (advection velocity, circulation, rotation kinetic energy, etc.) of the generated secondary vortices are studied and compared with the primary ones in order to show how the walls influence the von Kármán vortex street. The authors propose here a relation between the circulations and kinetic energies of primary and secondary vortices

Experimental Analysis on the Counter-Current Dumitrescu- Taylor Bubble Flow in a Smooth Vertical Conduct of Small Diameter

In this paper, an experimental investigation on the Dumitrescu-Taylor bubble in counter-current laminar downward flow in vertical pipe of a small internal diameter pipe is presented. The experimental design is realized to work for low and stable liquid flow rates. The Dumitrescu-Taylor bubble may be stationary or can be in motion with an ascending or descending velocity, and this displacement depends on the downward liquid flow rates. Consequently, the advantage of this device is to carry out the measurements of the velocities inside the gas Dumitrescu-Taylor bubble by Laser Doppler Velocimetry (LDV). Starting from the visual observations and image acquisitions with a fast camera, a qualitative description was brought on the hydrodynamic behavior of the liquid film and the ripples created at the bottom of the Dumitrescu-Taylor bubble. The experimental results show a presence of a long toroïdal vortex inside the gas bubble. It should be noted that previous work using a hot wire does not show the existence of this vortex. Additionally, other hydrodynamic magnitudes were measured as the liquid film thickness, the Dumitrescu-Taylor bubble rising velocity as well as the erosion bubble. Detailed descriptions are brought concerned this erosion. Strange phenomena have been observed primarily ahead of the nose of bubble and on the side of its end

Vortex Structure in the Wall Region of an Impinging Plane Jet

The work presented here comes within a research program dealing with vortex detection in the impingement region of a planar jet. In this study, experiments have been performed for a submerged turbulent water slot jet impinging normally on a flat plate, and an emphasis was put on the flow field characteristics. For this purpose, particle image velocimetry (PIV) have been employed. A comprehensive fluid mechanical data includes instantaneous and mean flow field, variance of normal and cross velocity fluctuations have been presented. The present work is also concerned with the flow structure in the impingement region where the transfers (heat/mass) occur. An attempt has been made to understand the flow structure by employing the vortex detection criteria on the instantaneous velocity vector field. Accordingly, the PIV measurements were carried out for four different Reynolds numbers: 3000, 6000, 11000 and 16000, and at three different planes: a plane parallel to the impingement plate, transverse plane of the jet and a plane perpendicular to the jet. A method of filtration, based on proper orthogonal decomposition (POD) technique was applied first to the instantaneous velocity and filtered velocity database is then used for vortex detection. Further, the results about the size, shape, spatial distribution and energy content of the detected vortices have been provided

Experimental Study of a Beta Stirling Thermal Machine Type Functioning in Receiver and Engine Modes

In this paper we studied a beta type Stirling machine. At first, we present the adopted theoretical quasi-stationary model. Then, we pass to the physical and geometrical presentation of this machine. The Latter was experimented according to two configurations: motor configuration and receiver configuration. For the first configuration, in order to improve the performances of the machine, we proceeded to the insulation of the machine hot room to reduce losses by radiation. For the second configuration, the machine is experimented as a heat pump and refrigerator. Comparisons between the theoretical and experimental results are also presented. We finally validated the results obtained by the model with experiments

Problématique de la régénération naturelle du Prunus avium au nord-ouest de la Tunisie: Influence des facteurs topographiques et édaphiques

The wild cherry (Prunus avium) is a species very appreciated in cabinet making or as root stock for the cherry tree plantations in the Tunisian northwest. Prunus avium is endangered due to its excessive use by local population. It thus seemed necessary to study the process of the natural regeneration by seed of this species. The obtained results showed that the highest rates of regeneration in small plots were observed at medium altitude (400-600 m), with low slope, exposed northward and in loamy soils. This study allowed us to conclude on the importance of the role played by the environment parameters and other climatic factors such as the minimal and maximal temperature in the natural regeneration of this species in our study zone. To be able to conserve and develop Tunisian wild cherries, we recommend using other techniques of multiplication such as cuttings of root segments. Key words: Wild cherry, Tunisia, parameters of the medium, multiplication by sowingLe merisier (Prunus avium) est une espèce très prisée en ébénisterie ou comme porte greffe pour le cerisier dans le nord-ouest tunisien. Prunus avium est menacé de disparition due à son l’utilisation abusive par la population locale. Il est donc apparu nécessaire d’étudier le processus de la régénération naturelle par semis de cette espèce. Les résultats obtenus montrent que le taux de régénération par semis le plus élevé est observé sur les placettes à moyennes altitudes (400-600 m), à faible pente, exposé vers le nord-ouest et sur des sols limoneux. Cette étude nous a permis de conclure l’importance du rôle joué par les paramètres du milieu et d’autres facteurs climatiques comme la température minimale et maximale sur la régénération naturelle de cette espèce au niveau de notre zone d’étude. Pour pouvoir conserver et mettre en valeur les merisiers tunisiens, nous préconisons d’utiliser d’autres techniques de multiplication comme le bouturage de segments de racines. Mots clés: Merisier, Tunisie, paramètres du milieu, multiplication par semi

Experimental study of solar energy potential in the gulf of Tunis, Tunisia

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.This work carries out the availability of the global solar radiation over the site of Borj-Cedria in the gulf of Tunis (36°43’04” N latitude and 10°25’41” E longitude), Tunisia. Global solar radiation variability was assessed on hourly, daily, monthly and seasonal scales. Solar potential in the gulf of Tunis was evaluated using the solar radiation data collected by the meteorological NRG weather station installed in the Centre of Research and Technologies of Energy (CRTEn) in the Borj- Cedria area. The collected measurements during the last three years (2008, 2009 and 2010) were based on 10 minute time step. These data have allowed us to evaluate the global solar flux, the sun duration, the yearly and the seasonal frequency distribution of the global solar radiation. Moreover, a conventional model has been used to estimate the hourly solar radiation on a horizontal plane and it has been validated by experimental measurements in specific days. The results show that the global solar radiation predicted by the conventional model has a good agreement with the experimental data during the clear sky conditions with a relative error percentage of 4.1%. However, the limitation of the conventional model appears under the cloudy sky weather which is proved by the highest value of relative error percentage reaching 14.26% occurred during the autumnal equinox day.dc201

On the stability of standing waves of Klein-Gordon equations in a semiclassical regime

We investigate the orbital stability and instability of standing waves for two classes of Klein-Gordon equations in the semi-classical regime.Comment: 9 page

Can gold be used as a hedge against the risks of Sharia-compliant securities? Application for Islamic portfolio management

In this paper, we investigate whether gold hedges Sharia-compliant stocks and Sukuk during the period from September 2005 to October 2017. The inference is taken by using both the DCC-GARCH model and the wavelet coherence analysis. On the whole, our finding suggests that gold is not effective in hedging the fluctuations of Sharia-compliant securities. However, we find that combining gold with stocks (and Sukuk) is useful in diversification and portfolio optimization. These results imply that, while gold is an excellent hedge for plain vanilla securities, it is not for Islamic exposures. This is important in light of the increasing amount of assets that are managed according to Islamic screening

Oil prices and stock returns : nonlinear links across sectors

We present evidence of an asymmetric relationship between oil prices and stock returns. The two regime multivariate Markov switching vector autoregressive (MSVAR) model allow us to capture the state shifts in the relationship between regional stock markets and sectors. Results suggest that oil price risk is significantly priced in the sample used. The impact is asymmetric with respect to market phases, and regimes have been associated with world economic, social and political events. Our study also suggests asymmetric responses of sector stock returns to oil price changes and different transmission impacts depending on the sector analyzed. There is a high causality from oil to sectors like Industrials and Oil & Gas. Companies inside the Utilities sector were more able to hedge against oil price increases between 2007 and 2012. Historical crisis events between 1992–1998 and 2003–2007 do not seem to have affected the relationship between oil and sector stock returns, given the higher probability of remaining smoother. For all sectors there seems to be a turn back to stability from 2012 onwards. Finally, investors gain more through portfolio diversification benefits built across, rather than within sectors.info:eu-repo/semantics/publishedVersio