An investigation on the flow around three side-by-side square prisms can provide a better understanding of complicated flow physics associated with multiple, closely spaced structures in which more ...than one gap flow is involved. In this paper, the flow around three side-by-side square prisms at a Reynolds number
$Re=150$
is studied systematically at
$L/W=1.1{-}9.0$
, where
$L$
is the prism centre-to-centre spacing and
$W$
is the prism width. Five distinct flow structures and their ranges are identified, viz. base-bleed flow (
$L/W<1.4$
), flip-flopping flow
$(1.4<L/W<2.1)$
, symmetrically biased beat flow
$(2.1<L/W<2.6)$
, non-biased beat flow
$(2.6<L/W<7.25)$
and weak interaction flow
$(7.25<L/W<9.0)$
. Physical aspects of each flow regime, such as vortex structures, vortex dynamics, gap-flow behaviours, shedding frequencies and fluid forces, are discussed in detail. A secondary (beat) frequency other than the Strouhal frequency (primary frequency) is observed in the symmetrically biased and non-biased beat flows, associated with the beat-like modulation in
$C_{L}$
-peak or amplitude, where
$C_{L}$
is the lift force coefficient. Here we reveal the generic and intrinsic origin of the secondary frequency, establishing its connections with the phase lag between the two shear-layer sheddings from the two sides of a gap. When the two sheddings are in phase, no viscous force acts at the interface (i.e. at the centreline of the gap) of the two sheddings, resulting in the largest fluctuations in streamwise momentum, streamwise velocity and pressure; the maximum
$C_{L}$
amplitude thus features the in-phase shedding. Conversely, when the two sheddings are antiphase, a viscous force exists at the interface of the two sheddings and restricts the momentum fluctuation through the gap, yielding a minimum
$C_{L}$
amplitude. When the phase relationship between the two sheddings changes from in phase to antiphase, the extra viscous force acting at the interface becomes larger and causes the
$C_{L}$
amplitude to change from a maximum to a minimum.
Graphitic carbon nitride (g-C3N4) has recently emerged as a promising visible-light-responsive polymeric photocatalyst; however, a molecular-level understanding of material properties and its ...application for water purification were underexplored. In this study, we rationally designed nonmetal doped, supramolecule-based g-C3N4 with improved surface area and charge separation. Density functional theory (DFT) simulations indicated that carbon-doped g-C3N4 showed a thermodynamically stable structure, promoted charge separation, and had suitable energy levels of conduction and valence bands for photocatalytic oxidation compared to phosphorus-doped g-C3N4. The optimized carbon-doped, supramolecule-based g-C3N4 showed a reaction rate enhancement of 2.3–10.5-fold for the degradation of phenol and persistent organic micropollutants compared to that of conventional, melamine-based g-C3N4 in a model buffer system under the irradiation of simulated visible sunlight. Carbon-doping but not phosphorus-doping improved reactivity for contaminant degradation in agreement with DFT simulation results. Selective contaminant degradation was observed on g-C3N4, likely due to differences in reactive oxygen species production and/or contaminant-photocatalyst interfacial interactions on different g-C3N4 samples. Moreover, g-C3N4 is a robust photocatalyst for contaminant degradation in raw natural water and (partially) treated water and wastewater. In summary, DFT simulations are a viable tool to predict photocatalyst properties and oxidation performance for contaminant removal, and they guide the rational design, fabrication, and implementation of visible-light-responsive g-C3N4 for efficient, robust, and sustainable water treatment.
The flow around four identical side-by-side circular cylinders placed normal to the oncoming flow is numerically simulated using the finite volume method (FVM) at a low Reynolds number of 100 based ...on cylinder diameter D and freestream velocity. How the wake structure, forces, and vortex shedding patterns are contingent on the spacing ratio g* (=g/D, where g is the gap spacing between the cylinders) is studied systemically when g* varies from 0.0 to 2.0. Based on the intrinsic features of the flow, four distinct flow regimes are identified in the range of g* examined. The total time-averaged drag force acting on the four cylinders escalates exponentially with a decrease in g*, as does the lift force, repulsive, on the outer cylinders. The lift forces of the inner cylinders are also repulsive but very weakly sensitive to g*. The Strouhal number is identical for each of the four cylinders in single body flow, different for the outer and inner cylinders in flip-flopping and quasi-interlocked flows, and again identical for interlocked flow.
Fluoroquinolone antibiotics attract increasing attention in the water treatment field because of the potential adverse effects on aquatic ecosystems and human health. The graphitic carbon nitride ...(g-C
3
N
4
) based photocatalysis has been demonstrated as an economically feasible and environmentally benign process to control these persistent contaminants. In this study, a new visible-light-driven of reduced graphene oxide (rGO) and nanoscale zero-valent iron (nZVI) co-modified g-C
3
N
4
-based photocatalyst was synthesized
via
ultrasonication-assisted chemisorption method. The optimized nZVI-loaded rGO/g-C
3
N
4
(10% IGCN) showed a reaction rate enhancement of 2.12∼3.69-fold and 1.20∼1.68-fold for the degradation of ofloxacin (OFL), norfloxacin (NOR), and ciprofloxacin (CIP) compared to that of carbon-doped g-C
3
N
4
(MCB
0.07
) and rGO-supported g-C
3
N
4
(7.5% GCN) under the irradiation of simulated visible light, respectively. The enhanced photocatalytic activity can be ascribed to the synergistic effect of nZVI and rGO to improve the separation of charge carriers and boost the harvest of visible light. The degradation mechanisms were explored by scavenger tests and X-ray photoelectron spectroscopy (XPS), indicating that holes (h
+
) played a dominant role in the decomposition of OFL, NOR, and CIP. The piperazine ring and C–N between the piperazine ring and benzene were the primary attack sites of h
+
. In addition, the ring-opening oxidation of benzene (C=C bond) connected by the C–F bond may also be an essential step. This study shed light on the degradation mechanism of OFL, NOR, and CIP under visible light irradiation of the 10% IGCN and provided theoretical support for the practical application of photocatalysis in treating antibiotics-containing water.
Although water quality has extensively improved over the last decade, recreational uses of the canal network in Amsterdam are limited by variations in water quality associated with stormwater runoff ...and episodic harmful algal blooms. The current systems for monitoring water quality are based on a stationary network of sampling points, offline testing methods, and online measurements of conventional water quality parameters on board a boat that continuously navigates the urban canal network. Here we describe the development and deployment of online algal sensors on board the boat, including a prototype LED-induced fluorescence instrument for algal identification and quantification. We demonstrate that by using only a single patrol vessel, we are able to achieve enough sampling coverage to observe spatiotemporal heterogeneity of algal and chemical water quality within the canal network. The data provide encouraging evidence that opportunistic measurements from a small number of mobile platforms can enable high-resolution mapping and can be used to improve the monitoring of water quality across the city compared to the current network of fixed sampling locations. We also discuss the challenges of operating water quality sensors for long-term autonomous monitoring.
Enstrophy production and flow topology are numerically investigated for statistically stationary compressible isotropic turbulence in vibrational non-equilibrium with a large-scale thermal forcing. ...The net enstrophy production term is decomposed into solenoidal, dilatational and isotropic dilatational terms based on the Helmholtz decomposition. From the full flow field perspective, the net enstrophy production mainly stems from the solenoidal term. For the dilatational and isotropic dilatational terms, although their local magnitudes can be considerable, the positive values in the compression region and the negative values in the expansion region cancel out on average. For the solenoidal component of the deviatoric strain-rate tensor, the statistical properties of its eigenvalues and alignments between vorticity and its eigenvectors are nearly independent of the local dilatation and vibrational relaxation. The solenoidal components of enstrophy production along three eigendirections are thus mainly affected by the vorticity. For the dilatational component of deviatoric strain-rate tensor, the statistical properties of its eigenvalues and alignments between vorticity and its eigenvectors closely relate to the local dilatation and vibrational relaxation. The dilatational components of enstrophy production along three eigendirections are therefore affected by the vorticity, eigenvalues and alignments between the vorticity and eigenvectors. The topological classification proposed by Chong et al. (Phys. Fluids, vol. 2, issue 5, 1990, pp. 765–777) is employed to decompose the flow field into various flow topologies. In the strong compression and strong expansion regions, the relaxation effects on the volume fractions of flow topologies and their relative contributions to the local enstrophy production are significant.
Solar-driven photocatalytic generation of H2O2 over metal-free catalysts is a sustainable approach for value-added chemical production. Here, we synthesized chlorine-doped graphitic carbon nitride ...(Cl-doped g-C3N4) through a solvothermal method to effectively produce H2O2 with a rate of 1.19 ± 0.06 µM min−1 under visible light irradiation, which was improved by 104 times compared to pristine g-C3N4. Continuous net production of H2O2 was realized at a rate of 2.78 ± 0.10 µM min−1 up to 54 h with isopropanol as the hole scavenger, whereas H2O2 production was only sustained for ~ 6 h without scavengers. Both molecular simulations and advanced spectroscopic characterizations elucidated that the Cl dopant increased the charge transfer rate, decreased the bandgap, and reduced the activation energy of the rate-limiting step of O2 reduction, all of which favored H2O2 production. This work implemented a novel metal-free photocatalyst for sustainable H2O2 production and elucidated the mechanism for promoting H2O2 production that can guide future photoreactive nanomaterial design.
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•Cl-doped g-C3N4 was synthesized via a solvothermal method.•Cl dopant decreased the g-C3N4 bandgap and promoted charge transfer.•Cl dopant lowered the activation energy of the rate-determining step in H2O2 formation.•In the presence of isopropanol H2O2 was continuously produced on Cl-doped g-C3N4.
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•g-C3N4/chitosan hydrogel beads (GCHBs) were developed for water purification.•A compound parabolic collector photoreactor was fabricated via 3D printing.•The photocatalytic system is ...promising for micropollutant removal.•Global drinking water production was estimated to show system practicality.•Research outcome provides a sustainable platform for small-scale water treatment.
Solar-energy-enabled photocatalysis is promising for sustainable water purification. However, photoreactor design, especially immobilizing nano-sized photocatalysts, remains a major barrier preventing industrial-scale application of photocatalysis. In this study, we immobilized photocatalytic graphitic carbon nitride on chitosan to produce g-C3N4/chitosan hydrogel beads (GCHBs), and evaluated GCHB photoreactivity for degrading phenol and emerging persistent micropollutants in a 3D printed compound parabolic collector (CPC) reactor. The CPC photocatalytic system showed comparable performance with slurry reactors for sulfamethoxazole and carbamazepine degradation under simulated sunlight, and it maintained the performance for contaminant removal in real water samples collected from water/wastewater treatment plants or under outdoor sunlight irradiation. Global drinking water production was estimated for the CPC system, and it holds promise for small-scale sustainable water treatment, including, but not limited to, the production of high-quality potable water for single houses, small communities, rural areas, and areas impacted by natural disasters in both developed and developing countries.
Trihalomethanes (THMs) and haloacetonitriles (HANs), most common disinfection by-products in drinking water, pose adverse environmental impacts and potential risks to human health. There is a ...pressing need to develop innovative, economically feasible, and environmentally benign processes to control these persistent contaminants. In this paper, visible-light-responsive graphitic carbon nitride (g-C3N4) samples were synthesized to degrade the THMs and HANs and the photocatalytic degradation mechanism was explored. The results indicated that a carbon-doped g-C3N4 with an optimum dopant content (MCB0.07) displayed the best photocatalytic activity for the total trihalomethanes (TTHM) and total haloacetonitriles (THAN), with the reaction rate constant of 11.6 and 10.4 (10−3 min−1), respectively. MCB0.07 demonstrated a high THMs and HANs removal efficiency under visible light irradiation and could be reused. According to scavenger tests of the selected reactive species and X-ray photoelectron spectroscopy, holes play a dominant role for both THMs and HANs degradation on the MCB0.07. The degradation of HANs by holes proceeded mainly through breakage of the CC bond in the CCN group. The THMs degradation was achieved through hydrogen abstraction or/and dehalogenation. The brominated-THMs/HANs were more photosensitive than their chlorinated analogous and were less stable than bromo-chloro-THMs/HANs. This study sheds light on the mechanism of the photocatalytic degradation of THMs and HANs under visible light irradiation by carbon-doped g-C3N4. Furthermore, it could provide insights for engineering applications and contaminant control in drinking water purification.
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•A carbon-doped g-C3N4 displayed high photocatalytic activity for TTHM and THAN.•Holes played dominant roles for THMs and HANs degradation.•THMs degradation was achieved through hydrogen abstraction or/and dehalogenation.•The photocatalysis removed HANs mainly through CC bond breakage.
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