Effect of short–time variations of wind velocity on mass transfer rate between street canyons and the atmospheric boundary layer

Abstract 2D URANS CFD simulations were conducted to study the effect of short–time variations of wind velocity on mass transfer rate between street canyons and the atmospheric boundary layer (ABL). A street canyon with a height–to– width ratio (aspect ratio) of three was considered as a case study. The study is of practical interest since it illustrates a skimming flow regime, the regime where pollutants are less effectively exchanged between the canyon and the above atmosphere, typically found in many urban areas in Mediterranean countries. Short–time variations of wind velocity magnitude were simulated assuming a sinusoidal function with average magnitude = 4 m s −1 ; amplitude ±2 m s −1 and period from 1 to 40 s, and subsequently with short–time averaged (0.1 s, 1 s and 10 s) real world data measured with an ultrasonic anemometer (50 Hz). Mass transfer rate between the canyon and the ABL was evaluated as the rate of reduction of spatially averaged concentration of a passive pollutant, carbon monoxide (CO), in the street canyon. Results show that mass transfer rate increases with the frequency of short–time variations. In CFD studies pertaining to pollutant dispersion in street canyons, wind hourly average velocity is usually assumed as a reference value to simulate real world cases. Our results show that this input data must be completed with additional information about the extent of variation in wind intensity and its frequency in the hour

Electrodeposition of a Au-Dy 2 O 3 Composite Solid Oxide Fuel Cell Catalyst from Eutectic Urea/Choline Chloride Ionic Liquid

In this research we have fabricated and tested Au/Dy2O3 composites for applications as Solid Oxide Fuel Cell (SOFC) electrocatalysts. The material was obtained by a process involving electrodeposition of a Au-Dy alloy from a urea/choline chloride ionic liquid electrolyte, followed by selective oxidation of Dy to Dy2O3 in air at high temperature. The electrochemical kinetics of the electrodeposition bath were studied by cyclic voltammetry, whence optimal electrodeposition conditions were identified. The heat-treated material was characterised from the morphological (scanning electron microscopy), compositional (X-ray fluorescence spectroscopy) and structural (X-ray diffractometry) points of view. The electrocatalytic activity towards H2 oxidation and O2 reduction was tested at 650 °C by electrochemical impedance spectrometry. Our composite electrodes exhibit an anodic activity that compares favourably with the only literature result available at the time of this writing for Dy2O3 and an even better cathodic performance

Mapping the spatial variation of soil moisture at the large scale using GPR for pavement applications

The characterization of shallow soil moisture spatial variability at the large scale is a crucial issue in many research studies and fields of application ranging from agriculture and geology to civil and environmental engineering. In this framework, this work contributes to the research in the area of pavement engineering for preventing damages and planning effective management. High spatial variations of subsurface water content can lead to unexpected damage of the load-bearing layers; accordingly, both safety and operability of roads become lower, thereby affecting an increase in expected accidents. A pulsed ground-penetrating radar system with ground-coupled antennas, i.e., 600-MHz and 1600-MHz center frequencies of investigation, was used to collect data in a 16 m × 16 m study site in the Po Valley area in northern Italy. Two ground-penetrating radar techniques were employed to non-destructively retrieve the subsurface moisture spatial profile. The first technique is based on the evalu¬ation of the dielectric permittivity from the attenuation of signal amplitudes. Therefore, dielectrics were converted into moisture values using soil-specific coefficients from Topp’s relationship. Ground-penetrating-radar-derived values of soil moisture were then compared with measurements from eight capacitance probes. The second technique is based on the Rayleigh scattering of the signal from the Fresnel theory, wherein the shifts of the peaks of frequency spectra are assumed comprehensive indi¬cators for characterizing the spatial variability of moisture. Both ground-penetrating radar methods have shown great promise for mapping the spatial variability of soil moisture at the large scale

A non-invasive approach to monitor variability of soil water content with electromagnetic methods

An accurate and high-resolution description of the spatial variability of soil water content at the field scale and the distribution of water in the unsaturated zone requires a large number of measurements. Financial and time constraints limit the numbers of measurement locations; thus the standard approach for monitoring could lead to a large degree of uncertainty in spatial predictions. We test in a parcel of bare soil an alternative approach based on ground-based geophysical techniques, by comparing the monitoring of the soil water content obtained from the Electrical Resisitivity Imaging and the Ground Penetrating Radar with the variability maps estimated from the interpolation of soil water contents measured in different locations with capacitance probes. The agreement is good and the integration of the techniques is promising

Amélioration des performances d'un avon régional par l'utilisation de riblets et de la technologie NLF

International audienceThe application of riblets on a typical regional turboprop configuration is discussed in this paper. The effect of the riblets is modeled as a singular roughness problem by a proper boundary condition at the wall. The model, already proposed in a previous paper, is briefly described. The drag prediction capabilities are verified by showing some airfoil flow applications. Then a typical wing-body of a regional aircraft is considered. The configuration has been designed to have extended natural laminar flow in cruise conditions. Riblets are applied at flow specifications representative of cruise in combination with the natural laminar flow technology and in climb/descent conditions. A comparison of the two technologies in terms of drag reduction is presented. Their combined application can result in a cruise drag reduction of more than 20%. The resulting fuel savings during a typical operational day are evaluated

Multilevel, room-temperature nanoimprint lithography for conjugated polymer-based photonics.

We demonstrate the multilevel patterning of organic light-emitting polymers by room-temperature nanoimprint lithography (RT-NIL), which is impossible to obtain by conventional hot embossing. In particular, we realize one- and two-dimensional photonic crystals with 500 nm periodic features and investigate the changes in the optical properties (luminescence and quantum yield) of the organic active layer. An increase of the quantum yield by 2.4% for the patterned film with respect to the untextured one and the enhancement of the output light emitted at a particular angle (ϑ = 69°) are observed for gratings whose Bragg periodicity matched the emission wavelength of the polymer. The employment of RT-NIL to pattern polymer semiconductors without degradation of their optical properties represents a strategic route for the realization of novel nanopatterned optoelectronic devices

Development of a Methodology for the Identification of High Emitting Mobile Sources in Narrow and Deep Street Canyons

In urban areas transport represents a significant source of atmospheric pollutants and greenhouse gases (GHG). In the case of vehicular transport, a significant contribution to total emissions is given by a category of vehicles with excessively high emissions of one or more pollutants defined as high emitting vehicles (high-emitters). High emitters can contribute a disproportionally way to total emissions of many airborne pollutants (NOx, COV, PM and GHGs). Remote sensing (RS) techniques have been developed with the aim to identify high emitterss but until now they have found only few practical applications. Among RS technologies, point sampling (PS) is the most promising for implementation in narrow and deep street canyons due to the limited impact on both pedestrians and architecture and the small space occupancy. In this paper we present results of preliminary monitoring campaigns carried out in a narrow and deep street canyon in Naples (Italy) in low-traffic conditions. Fine particles (FPs) concentration (20-1000 nm) were monitored using a condensation particle counter (CPC). Time patterns of FPs concentration have been analyzed by a code developed in MATLAB to identify FP concentration peaks and successively to attribute each identified peak to a specific vehicle. To study the effect of operating conditions (wind speed and direction) on the plume formed by vehicle exhausts, CFD simulations have been also carried out. Results show good performances of the code in the identification of FPs peaks and a limited effect of ambient parameters on the dispersion of the plumes inside the street canyon studied

Drag Reduction by Riblets on a Commercial UAV

Riblets are micro-grooves capable of decreasing skin-friction drag, but recent work suggests that additional benefits are possible for other components of aerodynamic drag. The effect of riblets on a fixed-wing, low-speed Unmanned Aerial Vehicle (UAV) on the total aerodynamic drag are assessed here for the first time by means of RANS simulations. Since the microscopic scale of riblets precludes their direct representation in the geometric model of the UAV, we model riblets via a homogenised boundary condition applied on the smooth wall. The boundary condition consists in a suitably tuned partial slip, which assumes riblets to be locally aligned with the flow velocity, and to possess optimal size. Several configurations of riblets coverage are considered to extract the potential for drag reduction of different parts of the aircraft surface. Installing riblets with optimal size over the complete surface of the UAV leads to a reduction of 3% for the drag coefficient of the aircraft. In addition to friction reduction, analysis shows a significant additional form of drag reduction localised on the wing. By installing riblets only on the upper surface of the wing, total drag reduction remains at 1.7%, with a surface coverage that is only 29%, thus yielding a significant improvement in the cost–benefit ratio

Polymer nanofibers by soft lithography

The fabrication of polymeric fibers by soft lithography is demonstrated. Polyurethane, patterned by capillarity-induced molding with high-resolution elastomeric templates, forms mm-long fibers with a diameter below 0.3 mu m. The Youngs modulus of the fabricated structures, evaluated by force-distance scanning probe spectroscopy, has a value of 0.8 MPa. This is an excellent example of nanostructures feasible by the combination of soft nanopatterning and high-resolution fabrication approaches for master templates, and particularly electron-beam lithography