Analysis of a turbulent boundary layer subjected to a strong adverse pressure gradient

We define two non-dimensional parameters $\text{Λ =}\text{τ}{}_{\mathrm{w}}\text{p}{}_{\mathrm{x}}\text{δ}$ and $\text{R}{}_{\mathrm{p}}\text{=}\text{U}{}_{\mathrm{p}}\text{δ}\text{ν}$ where τ_w is the wall stress, p_x(?0) is the pressure gradient to which the turbulent boundary layer (of thickness δ) is subjected, ν is the kinematic viscosity, $\text{U}{}_{\mathrm{p}}\text{= (}\text{νp}{}_{\mathrm{x}}\text{p}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ is a characteristic velocity and p is the density. The limit corresponding to the strong adverse pressure gradient is formulated as Λ → 0, R_p → ∞, ΛR_p finite. Using appropriate inner and outer asympcotic expansions, both above a wall layer possibly scaling with τ_w and ν, it is found by an application of Millikan's argument that there is an inertial sublayer where the streamwise velocity distribution obeys a half-power law, whose slope depends on Λ, and intercept on ΛR_p. Indeed comparison with available experimental data shows the inner law to be well represented by $\text{u}\text{Up}\text{= (3.5 + 19Λ)(}\text{yU}{}_{\mathrm{p}}\text{ν}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{+ 2.5ΛR}{}_{\mathrm{p}}$. The outer flow obeys a generalized defect law; use of constant eddy viscosity closure yields results in good agreement with experiment.     

Tracing post-limit-point paths with reduced basis technique

A reduced basis technique and a problem-adaptive computational algorithm are presented for predicting the post-limit-point paths of structures. In the proposed approach the structure is discretized by using displacement finite element models. The nodal displacement vector is expressed as a linear combination of a small number of vectors and a Rayleigh-Ritz technique is used to approximate the finite element equations by a small system of nonlinear algebraic equations.To circumvent the difficulties associated with the singularity of the stiffness matrix at limit points, a constraint equation, defining a generalized arc-length in the solution space, is added to the system of nonlinear algebraic equations and the Rayleigh-Ritz approximation functions (or basis vectors) are chosen to consist of a nonlinear solution of the discretized structure and its various order derivatives with respect to the generalized arc-length. The potential of the proposed approach and its advantages over the reduced basis-load control technique are outlined. The effectiveness of the proposed approach is demonstrated by means of numerical examples of structural problems with snap-through and snap-back phenomena.     

Recombination effects with high-pressure Langmuir probes

With increasing ion density, the ion current collected by a negatively biased electric probe is eventually dominated by ions produced by reactive processes in the vicinity of the probe. Here, we derive an expression for the probe current to a cylindrical probe when all the ions collected to the probe are assumed to be generated within the sheath which surrounds the probe. Measurements of probe current in a seeded atmospheric-pressure flame are in good agreement with the theory. The significance of reaction processes is reinforced by the fact that the maximum probe current is found to be approximately three times the total current that the flame would produce with frozen chemistry, i.e. the saturation current.     

Islam and Online Imagery on Malaysian Tourist Destination Websites

In December 1996, TIME magazine published an article titled "Finding God on the Web," which projected how the Internet would shape information seeking on faith and religion. A decade later, the Web has become a popular tool for gathering religious information as well as information and imagery related to religious travel. Yet there is little research on religious destination imagery online. Imagery—positive or negative, pictographic or narrative—influences the selection of tourism destinations. This study explored online Muslim images in Malaysia via interviews and content analyses of pictures and text on tourism destination websites' homepages. The results reveal minimal portrayal of Muslim images by Malaysian tourism destination organizations. This study adds to the small body of research, especially in Muslim countries, on online religious imagery and suggests avenues for tourism operators in Malaysia and elsewhere to improve their online image with both Muslim and non-Muslim travelers.     

Power Law Velocity Profile in Fully Developed Turbulent Pipe and Channel Flows

The power law velocity profile has been analyzed in terms of the envelope of the friction factor which gives the friction factor log law. The power law index α and prefactor C are shown as the function of the friction Reynolds number, as well as the function of the alternate variable the nondimensional friction velocity. The fully developed turbulent superpipe flow data of McKeon et al. and fully developed channel flow data of Zanoun et al. have been analyzed and the power law index α and prefactor C data have been estimated, first as a function of the friction Reynolds number and second as function of the nondimensional friction velocity. Based on analysis, several correlations have been proposed that have been supported by the data.     

A Carrier-Based Unipolar PWM Current Controller That Minimizes the PWM-Cycle Average Current-Error Using Internal Feedback of the PWM Signals

A carrier-based unipolar pulsewidth modulation (PWM) current controller is described using a one-phase PWM rectifier bridge as a test circuit. The controller PWM signal generator uses a sawtooth carrier signal to obtain both synchronized PWM waveforms and a constant switching frequency. A current-error signal and an amplitude modulation depth signal are used to create the input reference signal to the PWM signal generator. The amplitude modulation depth signal is obtained from the PWM-cycle average of the PWM signal generator output signals. This internal feedback is illustrated using a second-order low pass Butterworth filter. With an appropriately designed low-pass filter, the current-error signal is forced to be centered on zero over a PWM cycle. The maximum peak-peak magnitude of the current-error signal is used as a design guideline for deciding an appropriate gain constant in the controller current feedback loop. The low pass filter design is examined to determine a suitable gain and 20-dB attenuation frequency to minimize the current-error signal. The steady-state and transient operation of the proposed current controller is compared with a proportional-integral controller and an average current-error controller. SPICE simulations and experimental results are used to demonstrate the characteristics of the controller.     

Study of Half-Metallic Ferromagnetism in Be0.75Ti0.25Y (Y = S,Se, and Te) Using Ab Initio Calculations: Potential Candidate for Spintronic Devices

The structural, elastic, electronic, and magnetic properties of Be_0.75Ti_0.25Y (Y = S, Se, and Te) have been investigated to understand their potential applications in spintonic devices. Crystals of BeS, BeSe, and BeTe, individually doped with Ti with a dopant concentration of x = 0.25, have been evaluated by using full-potential linearized augmented plane-wave plus local orbital method within the framework of density functional theory. We employed the Wu–Cohen generalized gradient approximation for optimizing the crystal structure and evaluating elastic properties. In order improve bandgap values and optical parameters, the modified Becke and Johnson (mBJ) potential has been employed. The theoretical investigation of band structure and density of states confirms the half-metallic ferromagnetic nature of these compounds. The elastic constants are calculated by the charpin method which shows that the compounds under consideration are brittle and anisotropic. Moreover, it is noted that tetrahedral crystal field splits the 3d state of Ti into triple degenerate t_2g and double degenerate e_g states. The exchange splitting energies Δ _x (d) and Δ _x (pd) and exchange constants (N ₀ α) and (N ₀ β) are predicted from triple degenerate t_2g states, and negative values of N ₀ β justify that the nature of effective potential is more attractive in spin down case rather than that in the spin up case. We also find the crystal field splitting (ΔE _crystal = E _t2g?E _eg) energy and reduction of the local magnetic moment of Ti from its free space charge value and creation of small local magnetic moments on the non-magnetic Be, S, Se, and Te sites by p–d hybridization.     

Microwave-assisted urea-modified sorghum biomass for Cr (III) elimination from aqueous solutions

The present study concentrated on the use of an agro-waste biodegradable sorghum biomass in its simple and modified forms for the binding of Cr (III) ions. A relatively new method of modification was adopted using urea under microwave irradiation. FTIR analysis showed the presence of oxygen and nitrogen bearing functional groups in unmodified (UMS) and modified (MS) sorghum biomass. The appearance of new bands and shifts in the peaks confirmed the modification. The influence of different process parameters such as the adsorbent dose, solution pH, contact time, agitation speed and initial metal ion concentration was studied thoroughly to evaluate optimum conditions for adsorption. Maximum adsorption for Cr (III) ions occurred at pH 5.0–6.0 using UMS and MS. Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models in a non-linear fashion were used to explain the phenomenon. Maximum adsorption capacity was 7.03 and 16.36 mg of Cr (III) per gram of UMS and MS, respectively. Adsorption mechanism was explored by pseudo-first- and pseudo-second-order kinetic models, and it was found that the process followed pseudo-second-order kinetics. Thermodynamic study indicated the process favorability. The study concluded that urea modification under microwave irradiation produces a non-toxic and more effective adsorbent for Cr (III) remediation by inducing new nitrogen bearing functional groups to sorghum biomass.     

Effects of Pressure on the Performance of CNTs-Supported Catalyst in a Fischer-Tropsch Reaction

Fischer-Tropsch （FT） reaction involves conversion of syngas （a mixture of carbon monoxide and hydrogen） into higher hydrocarbons in the presence of an active catalyst. The syngas can be derived from non-petroleum feedstocks such as coal, biomass and natural gas, thus the FT reaction provides an alternative route for production of clean fuels. The FT process has received growing interest in recent years due to uncertainty in the Middle East, fast depletion of fossil fuel and environmental concern. This paper reports the synthesis, physicochemical properties and catalytic performance of cobalt-based catalyst in the FT reaction. The catalysts comprised metal nanoparticles supported on carbon nanotubes （CNTs） which were synthesized via a wet impregnation method. The catalysts were characterized using transmission electron microscopy （TEM）, temperature-programmed reduction/desorption （TPR/TPD） and X-ray photoelectron spectroscopy （XPS）. The performance of the cobalt-based catalyts in a FT reaction was evaluated in a fixed bed microreactor equipped with an on-line gas chromatograph for analyses of hydrocarbon products. The catalysts investigated in this work were Co/CNTs, 70Co30Mn/CNTs, 0.06%K/70Co30Fe/CNTs and 0.04%Nb/70Co30Fe/CNTs. TEM analyses revealed that the average sizes of the metal nanoparticles were 4-5 nm. Based on TPD analyses, the dispersion of these nanoparticles on CNTs were greater than 90%. The presence of both Co2＋ and Co3＋ ions were confirmed by XPS analysis. The 0.04% Nb/70Co30Fe/CNTs catalyst performed better than other catalysts in the FT reaction where it resulted in CO conversion of 35% and 16% C5＋ selectivity at pressure of 1 bar, 220 ~C and H2：CO of 2：1. Using the same catalyst, the CO conversion and C5＋ selectivity increased to 60% and 57%, respectively when the pressure was increased to 20 bar.