Capacitance matrix technique for avoiding spurious eigenmodes in the solution of hydrodynamic stability problems by Chebyshev collocation method

We present a simple technique for avoiding physically spurious eigenmodes that often occur in the solution of hydrodynamic stability problems by the Chebyshev collocation method. The method is demonstrated on the solution of the Orr-Sommerfeld equation for plane Poiseuille flow. Following the standard approach, the original fourth order differential equation is factorised into two second-order equations using a vorticity-type auxiliary variable with unknown boundary values which are then eliminated by a capacitance matrix approach. However the elimination is constrained by the conservation of the structure of matrix eigenvalue problem, it can be done in two basically different ways. A straightforward application of the method results in a couple of physically spurious eigenvalues which are either huge or close to zero depending on the way the vorticity boundary conditions are eliminated. The zero eigenvalues can be shifted to any prescribed value and thus removed by a slight modification of the second approach.Comment: 10 pages, 1 figure, minor revision, to appear in J. Comp. Phy

Tweeting television between innovation and normalization: How Lebanese television and audiences are making use of Twitter in political talk shows

Traditional media have progressively integrated newer media practices, with the constant emergence of new digital technologies, without abandoning their former ones. The adoption of Twitter by TV channels and by other social actors during political talk shows is a case in point. This article aims to assess whether the hybridization of TV talk shows and Twitter has innovated or normalized existing patterns of communication. In the former case, by enabling more interaction between different actors and space for audience deliberation, or in the latter case, by reproducing a traditional one-way communication and a centralized network of information that remains controlled and oriented by the elite (journalists and politicians). The incorporation of older and emergent media logic has an impact on the construction and distribution of political information as well as on the power relationships between journalists, politicians, and TV audiences. Besides allowing political talk shows to expand their flow of information, and to promote their news online, hybridized practices have not only altered the way citizens consume and engage with political information but also how they counter-frame traditional political media content by producing new ones. The research methodology consists of descriptive, content, and network analyses of tweets collected from three Lebanese local TV political talk show “Sarelwa2et” (MTV), “Btefro’ aa Watan” (Al Jadeed), and “Vision 2030” (LBCI) between February and March 2022. Results revealed that TV talk shows are making use of Twitter as a top-down transmitter of information and resorting poorly to its interactive potential. Some newer media practices of Twitter, such as @mention and replies are being applied but only to interact with politicians and journalists, failing to engage with a larger array of voices and thus, leading to an elite-centric discourse within the network. Also, tweets are mostly used to inform audiences and promote TV programs. In addition, network analyses of talk shows via hashtags demonstrated the central and not monopolized role of politicians and journalists as influencers, bridges, and quick spreaders of information. Finally, content analysis of dual screeners’ tweets (n=6000) indicated very little space in a Habermasian public sphere. The total of subjective opinions, irony, and attack/insult tweets are still higher than the total of the introduction of new issues and counter-frames tweets

Oscillatory Thermocapillary Convection

Stability analysis of thermocapillary convection in rectangular cavities is performed using direct numerical simulation. The influence of the Reynolds number (Re), the fluid Prandtl number (Pr) and cavity aspect ratio (Ar) on the motion is investigated. Neutral stability curves for transition to time-dependent convection are delineated in the Re-Ar plane for fluids with Pr=1.0, 4.4, 6.78, and 10. Several interesting features of these diagrams are discussed. One important conclusion is that Ar(epsilon tau) increases as Pr decreases. Thus large values of both Ar and Re are necessary to induce thermocapillary oscillations for small Pr fluids such as liquid metals and semiconductor melts. Energy analysis is also performed for the oscillatory flow in the neighborhood of critical points in order to gain insight into the mechanisms leading to instability

Stabilization of Curcumin by Complexation with Divalent Cations in Glycerol/Water System

The purpose of present study was to stabilize curcumin food pigment by its complexation with divalent ions like (Zn2+, Cu2+, Mg2+, Se2+), in “green media” and evaluate its stability in vitro compared to curcumin alone. The curcumin complexes were prepared by mechanical mixture of curcumin and sulfate salts of each metal (metal : curcumin 1/1mol) into unconventional and nontoxic glycerol/water solvent. Two stoichiometry of complex were obtained, 1 : 1 and 1 : 2 (metal/curcumin), respectively. On evaluation of in vitro stability, all complexes were found to provide a higher stability from curcumin alone

Oscillatory thermocapillary convection

We study thermocapillary and buoyant thermocapillary convection in rectangular cavities with aspect ratio A = 4 and Pr = 0.015. Two separate problems are considered. The first is combined buoyant thermocapillary convection with a nondeforming interface. We establish neutral curves for transition to oscillatory convection in the Re-Gr plane. It is shown that while pure buoyant convection exhibits oscillatory behavior for Gr is greater than Gr(sub cr) (where Gr(sub cr) is defined for the pure buoyant problem), pure thermocapillary convection is steady within the range of parameters tested. In the second problem, we consider the influence of surface deformation on the pure thermocapillary problem. For the range of parameters considered, thermocapillary convection remained steady

Political communication through the prism of social media : how are Lebanese political parties using Facebook in electoral campaigns?

The integration of social media in political communication has expanded significantly over recent years. This article focuses on how politicians are using Facebook in electoral campaigns. The main objective is to analyse the different functions ascribed to this digital platform in electoral communication. The present research studies the usage of Facebook by political parties in Lebanon during the parliamentary election of 2018 based on quantitative content analysis. All Facebook posts of three major political parties published during the electoral campaign were collected and examined. The results of the study reveal first that most parties were using Facebook in a unidirectional way; that is, only to disseminate information without giving any real value to users’ feedback. A prime example of this was how the Future Movement, a popular political party, made significant use of external links, and effective forms of controlled Facebook tools. This demonstrated how politicians aimed to contain and direct users to their preferred information flow. Second, emerging parties from the opposition tended to publish more negative and attack posts than incumbents. Third, the interactive tools of Facebook were used poorly. Fourth, there was, however, a relative difference with regards to political groups outside the formal institution of power, such as the Political Alliance “Kollouna Watani” who made better use of the interactive function. The main factors that could explain the findings are the historical background and path of the political parties as well as their position in or outside the government, the partisan and costly coverage of traditional media outlets, and the hybridisation of political communication

Spectrally selective AlXOY/Pt/AlXOY solar absorber coatings for high temprature solar-thermal applications

Philosophiae Doctor - PhDThe limited supply of fossil hydrocarbon resources and the negative impact of CO2 emission on the global environment dictate the increasing usage of renewable energy sources. Concentrating solar power (CSP) systems are the most likely candidate for providing the majority of the renewable energy. For efficient photo-thermal conversion, these systems require spectrally selective solar absorber surfaces with high solar absorbance in the solar spectrum region and low thermal emittance in the infrared region. In this thesis, a spectrally selective AlxOy/Pt/AlxOy multilayer solar absorber was designed and deposited onto copper substrate using electron beam evaporation at room temperature. The employment of ellipsometric measurements and optical simulation was proposed as an effective method to optimize and deposit the multilayer solar absorber coatings. The optical constants measured using spectroscopic ellipsometry, showed that both AlxOy layers, which used in the coatings, were dielectric in nature and the Pt layer was semi-transparent. The optimized multilayer coatings exhibited high solar absorptance ~ 0.94±0.01 and low thermal emittance ~ 0.06 ± 0.01 at 82oC.The structural and optical properties of the coatings were investigated. It was found that the stratification of the coatings consists of a semitransparent middle Pt layer sandwiched between two layers of AlxOy. The top and bottom AlxOy layers were nonstoichiometric with no crystalline phases present. The Pt layer is in the fcc crystalline phase with a broad size distribution and spheroidal shape in and between the rims of AlxOy. The surface roughness of the stack was found to be comparable to the inter-particle distance. To study the thermal stability of the multilayer solar absorber coatings, the samples were annealed at different temperatures for different duration in air. The results showed changes in morphology, structure, composition, and optical properties depend on both temperature and duration of annealing. The XRD pattern showed that the intensity of Pt decreased with increasing annealing temperature and therefore, disappeared at high temperature. With increasing annealing temperature, an increase in the size of Pt particles was observed from SEM. The AlxOy/Pt/AlxOy multilayer solar absorber coatings deposited onto Cu substrate were found to be thermally stable up to 500oC in air for 2 h with good spectral selectivity of 0.951/0.09. At 600oC and 700oC, the spectral selectivity decreased to 0.92/0.10 and 0.846/0.11 respectively, which is attributed to the diffusion of Cu and formation of CuO and Cu2O phases. Long term thermal stability study showed that the coatings were thermally stable in air up to 450oC for 24 h. To elucidate the degradation mechanism beyond 500oC, HI-ERDA has been used to study depth-dependent atomic concentration profiles. These measurements revealed outward diffusion of the copper substrate towards the surface and therefore, the decrease in the constituents of the coating. Hence, to prevent copper from diffusing towards the coatings, a thin Tantalum (Ta) layer was deposited between the base AlxOy layer and the copper substrate.The effect of a thin Ta layer on the thermal stability of AlxOy/Pt/AlxOy multilayer solar absorber coatings was investigated. The Cu/Ta/AlxOy/Pt/AlxOy multilayer solar absorber coatings were found to be thermally stable up to 700oC in air for 2 h with good spectral selectivity of 0.937/0.10. At 800oC, the spectral selectivity decreased to 0.870/0.12, which is attributed to the diffusion of Cu and formation of CuO phase. The formation of CuO phase was confirmed by XRD, EDS and Raman spectroscopy. Long term thermal stability study showed that the coatings were thermally stable in air up to 550oC for 24 h. Therefore, the Cu/Ta/AlxOy/Pt/AlxOy spectrally selective solar absorber coatings can be used for high temperature solar-thermal applications

Chemical Composition of the Essential Oil ofSatureja myrtifolia(Boiss. & Hohen.) from Lebanon

Satureja myrtifolia (Boiss. & Hohen.) Greuter & Burdeta medicinal plant belonging to the Lamiaceae family was collected from south of Lebanon and hydro-distilled by Clevenger method. Essential oil composition from aerial parts was analyzed by GC-MS technique. The odor of essential oil is characteristic, and clear yellow liquid oil was obtained after hydro-distillation. The yield of the essential oil was 1.25±0.02 % of dry matter (w/w). Thirty nine volatile components were identified in the Satureja myrtifolia oil, which shows a high amount of hydrocarbons class (57.82±0.1 %). Other classes were also identified such assesquiterpene hydrocarbons (12.96±0.1 %), oxygenated sesquiterpenes (10.65±0.2 %), phenolic compounds (10.32±0.1 %), acids (5.53±0.1 %), and monoterpenes hydrocarbons (2.21±0.1 %).In addition, a comparison with the unique study performed on Satureja myrtifolia was also carried out

Autocatalytic plume pinch-off

A localized source of buoyancy flux in a non-reactive fluid medium creates a plume. The flux can be provided by either heat, a compositional difference between the fluid comprising the plume and its surroundings, or a combination of both. For autocatalytic plumes produced by the iodate-arsenous acid reaction, however, buoyancy is produced along the entire reacting interface between the plume and its surroundings. Buoyancy production at the moving interface drives fluid motion, which in turn generates flow that advects the reaction front. As a consequence of this interplay between fluid flow and chemical reaction, autocatalytic plumes exhibit a rich dynamics during their ascent through the reactant medium. One of the more interesting dynamical features is the production of an accelerating vortical plume head that in certain cases pinches-off and detaches from the upwelling conduit. After pinch-off, a new plume head forms in the conduit below, and this can lead to multiple generations of plume heads for a single plume initiation. We investigated the pinch-off process using both experimentation and simulation. Experiments were performed using various concentrations of glycerol, in which it was found that repeated pinch-off occurs exclusively in a specific concentration range. Autocatalytic plume simulations revealed that pinch-off is triggered by the appearance of accelerating flow in the plume conduit.Comment: 10 figures. Accepted for publication in Phys Rev E. See also http://www.physics.utoronto.ca/nonlinear/papers_chemwave.htm

Satureja myrtifolia (Boiss. & Hohen.) Lebanese wild plant, as a resource of natural antioxidants

This study aims to evaluate total phenolic content (TPC) and in vitro radical scavenging activity (RSA) of Satureja myrtifolia collected from Lebanon. TPCs of Satureja myrtifolia were extracted by two polar solvents, namely methanol and water and subsequently determined spectrophotometrically. The RSA of both crude extracts were evaluated in vitro by 2,2'-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity assay. According to our findings, the obtained extracts exhibit relatively high levels of phenolic compounds which range from 343.12 ± 7.53 to 441.09 ± 10.70 mg GAE g-1 in terms of dry weight (DW) depending on the solvent used. Moreover, both extracts exerted hydrogen-donating abilities in the presence of DDPH stable radical. However, the aqueous extract from the aerial parts of S. myrtifolia showed higher RSA capability. This study revealed that S. myrtifolia is a potential resource of biological active compounds which can reduce the risk of diseases and their noxious effects correlating with the antioxidant compounds