Flux Drop Research Articles

Deciphering the extreme X-ray variability of the nuclear transient eRASSt J045650.3−203750

Context. During its all-sky survey, the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spectrum-Roentgen-Gamma (SRG) observatory has uncovered a growing number of X-ray transients associated with the nuclei of quiescent galaxies. Benefitting from its large field of view and excellent sensitivity, the eROSITA window into time-domain X-ray astrophysics yields a valuable sample of X-ray selected nuclear transients. Multi-wavelength follow-up enables us to gain new insights into understanding the nature and emission mechanism of these phenomena. Aims. We present the results of a detailed multi-wavelength analysis of an exceptional repeating X-ray nuclear transient, eRASSt J045650.3−203750 (hereafter J0456−20), uncovered by SRG/eROSITA in a quiescent galaxy at a redshift of z ∼ 0.077. We aim to understand the radiation mechanism at different luminosity states of J0456−20, and provide further evidence that similar accretion processes are at work for black hole accretion systems at different black hole mass scales. Methods. We describe our temporal analysis, which addressed both the long- and short-term variability of J0456−20. A detailed X-ray spectral analysis was performed to investigate the X-ray emission mechanism. Results. Our main findings are that (1) J0456−20 cycles through four distinctive phases defined based on its X-ray variability: an X-ray rising phase leading to an X-ray plateau phase that lasts for abouttwo months. This is terminated by a rapid X-ray flux drop phase during which the X-ray flux can drop drastically by more than a factor of 100 within one week, followed by an X-ray faint state for about two months before the X-ray rising phase starts again. (2) The X-ray spectra are generally soft in the rising phase, with a photon index ≳3.0, and they become harder as the X-ray flux increases. There is evidence of a multi-colour disk with a temperature of Tin ∼ 70 eV in the inner region at the beginning of the X-ray rising phase. The high-quality XMM-Newton data suggest that a warm and hot corona might cause the X-ray emission through inverse Comptonisation of soft disk seed photons during the plateau phase and at the bright end of the rising phase. (3) J0456−20 shows only moderate UV variability and no significant optical variability above the host galaxy level. Optical spectra taken at different X-ray phases are constant in time and consistent with a typical quiescent galaxy with no indication of emission lines. (4) Radio emission is (as yet) only detected in the X-ray plateau phase and rapidly declines on a timescale of two weeks. Conclusions. J0456−20 is likely a repeating nuclear transient with a tentative recurrence time of ∼223 days. It is a new member of this rare class. We discuss several possibilities to explain the observational properties of J0456−20. We currently favour a repeating partial tidal disruption event as the most likely scenario. The long-term X-ray evolution is explained as a transition between a thermal disk-dominated soft state and a steep power-law state. This implies that the corona can be formed within a few months and is destroyed within a few weeks.

Gravity-driven membrane filtration with compact second-life modules daily backwashed: An alternative to conventional ultrafiltration for centralized facilities

Gravity-driven membrane (GDM) filtration is a strategic alternative to conventional ultrafiltration (UF) for the resilient production of drinking water via ultrafiltration when resources become scarce, given the low dependency on energy and chemicals, and longer membrane lifetime. Implementation at large scale requires the use of compact and low-cost membrane modules with high biopolymer removal capacity. We therefore evaluated (1) to what extent stable flux can be obtained with compact membrane modules, i.e., inside-out hollow fiber membranes, and frequent gravity-driven backwash, (2) whether we can reduce membrane expenses by effectively utilizing second-life UF modules, i.e., modules that have been discarded by treatment plant operators because they are no longer under warranty, (3) if biopolymer removal could be maintained when applying a frequent backwash and with second-life modules and (4) which GDM filtration scenarios are economically viable compared to conventional UF, when considering the influence of new or second-life modules, membrane lifetime, stable flux value and energy pricing. Our findings showed that it was possible to maintain stable fluxes around 10 L/m2/h with both new and second-life modules for 142 days, but a daily gravity-driven backwash was necessary and sufficient to compensate the continuous flux drop observed with compact modules. In addition, the backwash did not affect the biopolymer removal. Costs calculations revealed two significant findings: (1) using second-life modules made GDM filtration membrane investment less expensive than conventional UF, despite the higher module requirements for GDM filtration and (2) overall costs of GDM filtration with a gravity-driven backwash were unaffected by energy prices rise, while conventional UF costs rose significantly. The later increased the number of economically viable GDM filtration scenarios, including scenarios with new modules. In summary, we proposed an approach that could make GDM filtration in centralized facilities feasible and increase the range of UF operating conditions to better adapt to increasing environmental and societal constraints.

Modified emulsion polyvinyl chloride membranes for enhanced antifouling and dye separation properties by introducing tungsten disulfide (WS2) nanosheets

In this study, the fabrication and modification of emulsion polyvinyl chloride (EPVC) membranes were studied using tungsten disulfide (WS2) nanosheets. The layered WS2 nanosheets were synthesized, characterized, and applied in the modification of EPVC membranes. The nonsolvent induced phase inversion method was utilized to fabricate new membranes by introducing the WS2 nanosheets into the polymeric solutions with different percentages. The performance of the fabricated membranes was characterized and evaluated using scanning electron microscopy, atomic force microscopy, attenuated total reflection-Fourier transform infrared, mean pore radius, porosity, contact angle analyses, pure water flux, fouling parameters, rejection tests for bovine serum albumin (BSA) and two dyes, and long-term and reusability tests. The results indicated that the use of WS2 nanosheets in the membrane structure up to the optimum value (1 wt%) increased the water flux and improved the antifouling properties. The pure water flux enhanced from 56.4 L/m2.h in the bare membrane to 191.2 L/m2.h in the 1 wt% WS2/EPVC membrane. In addition, the flux recovery ratio parameter increased from 64.7% to 83.5%. dye rejection rates of 94.7% and 97.8% were obtained for Reactive Red 195 and Reactive Green 19, respectively. Reusability tests for the 1 wt% WS2/EPVC membrane indicated a slight flux drop and negligible BSA rejection changes. Long-term tests showed very high stability in dye rejection and a negligible flux drop.

The photometric observation of the quasi-simultaneous mutual eclipse and occultation between Europa and Ganymede on 22 August 2021

Mutual events (MEs) are eclipses and occultations among planetary natural satellites. Most of the time, eclipses and occultations occur separately. However, the same satellite pair will exhibit an eclipse and an occultation quasi-simultaneously under particular orbital configurations. This kind of rare event is termed as a quasi-simultaneous mutual event (“QSME”). During the 2021 campaign of mutual events of jovian satellites, we observed a QSME between Europa and Ganymede. The present study aims to describe and study the event in detail. We observed the QSME with a CCD camera attached to a 300-mm telescope at the Hong Kong Space Museum Sai Kung iObservatory. We obtained the combined flux of Europa and Ganymede from aperture photometry. A geometric model was developed to explain the light curve observed. Our results are compared with theoretical predictions (“O-C”). We found that our simple geometric model can explain the QSME fairly accurately, and the QSME light curve is a superposition of the light curves of an eclipse and an occultation. Notably, the observed flux drops are within 2.6% of the theoretical predictions. The size of the event central time O-C’s ranges from −14.4 to 43.2 s. Both O-C’s of flux drop and timing are comparable to other studies adopting more complicated models. Given the event rarity, model simplicity and accuracy, we encourage more observations and analysis on QSMEs to improve Solar System ephemerides.

Fast flavor instability in hypermassive neutron star disk outflows

We examine the effect of neutrino flavor transformation by the fast flavor instability (FFI) on long-term mass ejection from accretion disks formed after neutron star mergers. Neutrino emission and absorption in the disk set the composition of the disk ejecta, which subsequently undergoes $r$-process nucleosynthesis upon expansion and cooling. Here we perform 28 time-dependent, axisymmetric, viscous-hydrodynamic simulations of accretion disks around hypermassive neutron stars (HMNSs) of variable lifetime, using a 3-species neutrino leakage scheme for emission and an annular-lightbulb scheme for absorption. We include neutrino flavor transformation due the FFI in a parametric way, by modifying the absorbed neutrino fluxes and temperatures, allowing for flavor mixing at various levels of flavor equilibration, and also in a way that aims to respect the lepton-number preserving symmetry of the neutrino self-interaction Hamiltonian. We find that for a promptly-formed black hole (BH), the FFI lowers the average electron fraction of the disk outflow due to a decrease in neutrino absorption, driven primarily by a drop in electron neutrino/antineutrino flux upon flavor mixing. For a long-lived HMNS, the disk emits more heavy lepton neutrinos and reabsorbs more electron neutrinos than for a BH, with a smaller drop in flux compensated by a higher neutrino temperature upon flavor mixing. The resulting outflow has a broader electron fraction distribution, a more proton-rich peak, and undergoes stronger radiative driving. Disks with intermediate HMNS lifetimes show results that fall in between these two limits. In most cases, the impact of the FFI on the outflow is moderate, with changes in mass ejection, average velocity, and average electron fraction of order $\sim 10\%$, and changes in the lanthanide/actinide mass fraction of up to a factor $\sim 2$.

A Concise Review of Theoretical Models and Numerical Simulations of Membrane Fouling

Membrane fouling can cause severe flux drops and affect the quality of produced water, which is a major obstacle for membrane applications. Great efforts have been made to examine theoretical models and numerical simulations for fouling behavior and mechanisms in the past decades, but there is a lack of literature providing a systematic summary. This work aims to present a state-of-the-art review of the principles, applicability and advantages of fouling theoretical models (i.e., the concentration polarization, cake layer formation and blocking models), and numerical simulations (i.e., computational fluid dynamics, Monte Carlo simulation, and artificial neural networks) for fouling behavior and mechanisms. Through these models and simulations, the behaviors of foulant particles at the microscopic level are analyzed in detail from the perspective of force, energy, and particle trajectory during the fouling process. The concise summary of fouling modeling in this review gives guidelines for the selection and application of models to simulate the membrane fouling process accurately, and the optimization of the operation in membrane-based processes.

Multiwavelength Observations of the Obscuring Wind in the Radio-quiet Quasar MR 2251-178

Obscuring winds driven away from active supermassive black holes are rarely seen due to their transient nature. They have been observed with multiwavelength observations in a few Seyfert 1 galaxies and one broad absorption line radio-quiet quasar so far. An X-ray obscuration event in MR 2251-178 was caught in late-2020, which triggered multiwavelength (near-IR (NIR) to X-ray) observations targeting this radio-quiet quasar. In the X-ray band, the obscurer leads to a flux drop in the soft X-ray band from late-2020 to early-2021. X-ray obscuration events might have a quasi-period of two decades considering earlier events in 1980 and 1996. In the UV band, a forest of weak blueshifted absorption features emerged in the blue wing of Lyα λ1216 in late-2020. Our XMM-Newton, NuSTAR, and Hubble Space Telescope/COS observations are obtained simultaneously; hence, the transient X-ray obscuration event is expected to account for the UV outflow, although they are not necessarily caused by the same part of the wind. Both blueshifted and redshifted absorption features were found for He i λ10830, but no previous NIR spectra are available for comparison. The X-ray observational features of MR 2251-178 shared similarities with some other type 1 active galactic nuclei (AGNs) with obscuring wind. However, observational features in the UV to NIR bands are distinctly different from those seen in other AGN with obscuring winds. A general understanding of the observational variety and the nature of obscuring wind is still lacking.

Opposing comparable large effects of fine aerosols and coarse sea spray on marine warm clouds

Fine aerosols, by acting as cloud condensation nuclei, suppress rainfall and enhance the albedo and coverage of marine warm clouds, thereby partly counteracting the greenhouse-induced warming. While this is relatively well documented, the co-existing opposite effects of giant cloud condensation nuclei from coarse sea spray aerosols are poorly quantified. Here, satellite measurements show that the effects of coarse sea spray aerosols have comparable magnitudes with opposite sign to those of fine aerosols. For fixed cloud liquid water path and coarse sea spray aerosols, increasing fine aerosols decreased rainfall flux and cloud drop effective radius by a factor of 1/4 and 40%, respectively. Conversely, for fixed fine aerosols and cloud liquid water path, added coarse sea spray aerosols enhanced rainfall flux and cloud drop effective radius by a factor of 4 and 35%, respectively. These large and contrasting effects are independent on meteorological conditions. These processes must be fully incorporated into climate models to faithfully represent aerosol effects on clouds, precipitation, and radiative forcing.

Heat transfer analysis of a pulse-heated microwire in CO2 at supercritical pressures

This paper analyzes the characteristics of convective heat transfer of a pulse-heated platinum microwire cooling in CO2 under supercritical pressures based on experimental data. The microwire undergoes a rapid temperature rise of around 664 K within 0.35 ms. An inverse problem is formulated and numerically solved to extract heat transfer data from experimental measurements. In addition, a predictive model for the convective heat transfer coefficient is developed to fully close the equation set. Results are interpreted based on the bulk pressure from 7.38 to 9 MPa and bulk temperature from 295 to 325 K. The convective heat flux of CO2 generally decreases with time, and in the medium-term, the reduction is slightly decelerated owing to buoyancy-driven flows. This demonstrates that high-pressure and low-temperature bulk states generally exert larger convective heat flux to cool the microwire. During the early 10 ms, the time-averaged convective heat flux is of the order of 1 MW/m2, resulting in rapid cooling. This value shows a weak critical enhancement upon crossing the Widom line. During the remaining time, the time-averaged convective heat flux drops to the order of 0.1 MW/m2. Such a drop in heat flux is more obvious in low-bulk-density cases, leading to a relatively long time for sufficient W cooling.

A numerical study of microtube geometry effect on flow boiling using the volume of fluid method

Other than pursuing high heat flux, pressure drop also plays a key role in microtube applications as the power and efficiency of a pump are determined in terms of pressure drop. In the literature, only a few studies focus on how tube geometry influences pressure drop. This study numerically investigates the geometry effect on microtube flow boiling and pressure drop using the volume of fluid (VOF) method. Three types of geometries are used in this study: divergent, normal, and convergent microtubes with different lengths and hydraulic diameter ratios. The transient developments of vapor formation, heat flux, and pressure drops are presented within 200 ms. It is shown that the total pressure drop in the divergent microtube is the lowest. The heat flux is decreased as the massive vapor bubbles clog the tube. However, the shortened divergent microtube can increase the heat flux up to 38713.1 W/m2 and reduce the pressure drop down to 214.24 Pa, compared to the normal one under certain flow conditions, while the tube-clogging effect is not dominant. A larger inflow mass flux can delay the vapor bubble formation on the heated wall, which is beneficial to prevent the tube clogging and leads to a higher heat flux. On the other hand, higher heat flux can be achieved in the convergent microtube due to the increasing velocity inside the tube, although it induces the highest total pressure drop. This study provides a better fundamental understanding of the microtube geometry effect on flow boiling and relevant physical insight for the future design of microscale heat transfer applications.

Application of a hybrid ultrasonic stripping-membrane distillation (US-MD) system for the treatment of mariculture wastewater

Membrane distillation (MD) is presently regarded as a superior technique for the advanced purification of mariculture wastewater, while membrane fouling and unsatisfactory removal efficiency of NH4+-N are two major barriers to its massive application. Hence, a hybrid ultrasonic stripping-membrane distillation (US-MD) system was established in this work for efficient purification of mariculture wastewater. Compared to sole MD (71.7 %), a considerable removal ratio of 86.8 % for NH4+-N was obtained in US-MD system without pH adjustment. US pretreatment obviously promoted protein denaturation and salt crystallization before the MD treatment, contributing to the fouling mitigation in subsequent MD unit. With a period of standing treatment (40 min) between US and MD unit, these denatured protein floccules and salt crystals could be effectively removed outside sewage, with a considerable removal efficiency of 58.7 % and 8.6 % for protein and phosphate, respectively. Hence, a retarded fouling behavior (NF of 0.38, 384 h) was seen for US-MD system compared to sole MD (0.17, 384 h). Without setting treatment, the flux drop of US-MD system (0.11, 288 h) was even sharper than that of sole MD, suggesting that a period of standing treatment was essential in US-MD system.

The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk–Corona System

We present X-ray, UV, optical, and radio observations of the nearby (≈78 Mpc) tidal disruption event AT2021ehb/ZTF21aanxhjv during its first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxy hosting a≈107 M ⊙ black hole (M BH inferred from host galaxy scaling relations). High-cadence Swift and Neutron Star Interior Composition Explorer (NICER) monitoring reveals a delayed X-ray brightening. The spectrum first undergoes a gradual soft → hard transition and then suddenly turns soft again within 3 days at δ t≈272 days during which the X-ray flux drops by a factor of 10. In the joint NICER+NuSTAR observation (δ t = 264 days, harder state), we observe a prominent nonthermal component up to 30 keV and an extremely broad emission line in the iron K band. The bolometric luminosity of AT2021ehb reaches a maximum of when the X-ray spectrum is the hardest. During the dramatic X-ray evolution, no radio emission is detected, the UV/optical luminosity stays relatively constant, and the optical spectra are featureless. We propose the following interpretations: (i) the soft → hard transition may be caused by the gradual formation of a magnetically dominated corona; (ii) hard X-ray photons escape from the system along solid angles with low scattering optical depth (∼a few) whereas the UV/optical emission is likely generated by reprocessing materials with much larger column density—the system is highly aspherical; and (iii) the abrupt X-ray flux drop may be triggered by the thermal–viscous instability in the inner accretion flow, leading to a much thinner disk.

Performance intensification and anti-fouling of the two-phase flow enhanced direct contact membrane distillation for seawater desalination

Membrane distillation is a promising approach to the fresh water crisis due to the low operating temperature and high rejection of non-volatile components. However, it suffers from the polarization effect and membrane fouling. This paper aims at developing a novel two-phase flow enhanced direct contact membrane distillation (TP-DCMD) system and investigating the effect of two-phase flow behaviors on the heat and mass transfer and membrane fouling comprehensively. First, a novel experimental system of TP-DCMD is developed and well validated using the mass and energy conservations. Then, the effect of flow parameters on the transmembrane permeate flux is discussed. The introduction of gas could improve the membrane distillation performance by as high as 27 %. The relationship between the performance enhancement and two-phase flow regimes is discussed. Both permeate flux and multiplier clouds are proposed based on two-phase flow regime map. The slug flow demonstrates superior enhancing effect to the other flow regimes due to the low possibility of gas penetration. Finally, the effect of two-phase flows on the membrane fouling is investigated. After 420 min of accelerating fouling test, the transmembrane permeate flux drops by 38.2 % in the conventional DCMD system, while it just decreases by 6.6 % in TP-DCMD system.

High performance and sustainable CNF membrane via facile in-situ envelopment of hydrochar for water treatment

In this study, cellulose nanofiber (CNF) membranes for water treatment are prepared via in-situ hydrothermal carbonization of glucose in a CNF suspension. The in-situ carbonized CNFs were fabricated into the membranes via dead-end filtration under hydraulic pressure (1 bar). The in-situ carbonized CNF membranes showed high pure water flux (56.25 L·h−1·m−2) without critical flux drop for 12 h of membrane fabrication. The hydrochar chemically bonded with CNF enhanced the durability of CNF during membrane fabrication. Owing to the strong and stable electrostatic interaction between the target dye and hydrochar, the in-situ carbonized membrane also displayed excellent dye rejection for dilute and concentrated solutions, with high selectivity and good reusability. This study provides a successful strategy for fabricating an all-carbohydrate-based high-performance water treatment membrane.

The bright side of the light curve: A general photometric model of non-transiting exorings

Rings around exoplanets (exorings) are one of the most expected discoveries in exoplanetary research. There is an increasing number of theoretical and observational efforts for detecting exorings, but none of them have succeeded yet. Most of those methods focus on the photometric signatures of exorings during transits, whereas less attention has been paid to light diffusely reflected: what we denote here as the bright side of the light curve. This is particularly important when we cannot detect the typical stellar flux drop produced by transiting exoplanets. Here, we endeavour to develop a general method to model the variations on the light curves of both ringed non-transiting and transiting exoplanets. Our model (dubbed as Pryngles) simulates the complex interaction of luminous, opaque, and semitransparent objects in planetary systems, discretizing their surface with small circular plane discs that resemble sequins or spangles. We perform several numerical experiments with this model, and show its incredible potential to describe the light curve of complex systems under various orbital, planetary, and observational configurations of planets, moons, rings, or discs. As our model uses a very general approach, we can capture effects like shadows or planetary/ring shine, and since the model is also modular we can easily integrate arbitrarily complex physics of planetary light scattering. A comparison against existing tools and analytical models of reflected light reveals that our model, despite its novel features, reliably reproduces light curves under common circumstances. Pryngles source code is written in PYTHON and made publicly available.

Comparison of disinfection-residual-bacteria (DRB) after seven different kinds of disinfection: Biofilm formation, membrane fouling and mechanisms

Membrane fouling is the Achilles' heel of the reverse osmosis (RO) system for high-quality reclaimed water production. Previous studies have found that after the significant selection effect of traditional disinfection, the remaining disinfection-residual bacteria (DRB) may possess more severe biofouling potentials. To provide more constructive advice for the prevention of biofouling, we compared the RO membrane fouling characteristics of DRB after using five commonly used disinfection methods (NaClO, NH2Cl, ClO2, UV, and O3) and two novel disinfection methods (K2FeO4 and the flow-through electrode system (FES)). Compared with the control group (undisinfected, 21.1 % flux drop), the UV-DRB biofilm aggravated biofouling of the RO membrane (23.4 % flux drop), while the FES, K2FeO4, and NH2Cl treatments showed less severe biofouling, with final flux drops of 6.9 %, 8.1 %, and 8.1 %, respectively. Adenosine triphosphate (ATP) was found to be a capable indicator for predicting the biofouling potential of DRB. Systematic analysis showed that the thickness and density of the DRB biofilms were most closely related to the different fouling degree of RO membranes. Moreover, the relative abundance of bacteria with higher extracellular polymeric substance (EPS) secretion levels, such as Pseudomonas and Sphingomonas, was found closely related with the biofouling degree of RO membranes.

Numerical study to predict optimal configuration of wavy fin and tube heat exchanger with various tube shapes

This paper aims to investigate the influence of tube shapes on thermal-flow characteristics of sinusoidal wavy finned-tube heat exchangers. Two row staggered bundle with six geometries of tubes (four flat tube geometries, one oval tube and a circular tube) are analyzed for a range of (1600 Re 4800). The inspection revealed that the heat flux and the pressure drop decrease with the tube flatness for all Reynolds values. However, the oval tube O1 reaches, for all Reynolds values, the lowest values of heat flux and pressure drop. Regarding the global performance criterion, the sinusoidal wavy fins with O1 shaped tubes reached the highest global performance values, being 14.8–24.4% and 31.6–36.3% higher than the fin with F1 and O2 tube geometry, respectively.

Computational study on the flame and extinction behavior of a high enthalpy air slab

A solid fuel ramjet has been recognized as a viable contender for atmospheric supersonic flight due to its performance and simple construction. However, the static temperature increases due to shock compression at the inlet that results in high temperature air entering the combustor. As such, the goal of this study is to analyze how the kinetics and flame sustainment properties are altered with this higher enthalpy air exposure. To examine these effects, a slab burner is modeled with gaseous using OpenFOAM© software. The reaction processes are modeled using complete University of California San Diego chemical-kinetic mechanism for burning in a hydrogen–air environment. Temperatures in the air intake range from 300 to 1000 K in 100 K increments, with the hydrogen fuel temperature kept at 300 K. In examining the conductive and convective heat flux data at the fuel surface, two distinct temperature regimes appeared for each respective heat flux mechanism. The results showed a large drop in conductive heat flux seen only between inlet temperatures of 300 and 500 K. Conversely, convective heat flux saw an increase in heat flux as a fully turbulent flow was approached for temperatures between 600 and 1000 K. In analyzing the Damköhler numbers of involved reactions, it was found that higher temperatures activated an endothermic chain branching reaction, which contributed significantly to the OH radical pool. This activation prevented a large drop in conductive heat flux for high inlet air temperatures. The results showed the impact of inlet temperatures on reaction pathways, which allows improved flame holding at higher air velocities.

Influence of gap size between aluminium radiant sheets on the efficiency of lightweight floor heating

For environmental and economic reasons, it is necessary to search for heating systems solutions that are efficient and provide thermal comfort at a low investment cost. In this study, the optimum gap between aluminium radiant sheets in lightweight floor heating is determined, ensuring high system efficiency and low material consumption. Numerical calculations were performed using the ANSYS. The structures analysed consist of thermal insulation covered with a 0.3 mm thick aluminium sheet, wood-like board and ceramic tiles or panels. The average surface temperature and the temperature amplitude are determined for the construction with no gap between the sheets and with gaps of 10, 20, 30 and 40 mm. Increasing the gap size results in a decrease in the surface temperature and heat flux and an increase in temperature amplitude. For a 10–20 mm gap, the decrease in temperature (<0.47 K), relative heat flux drop (<4.4%) and increase in surface temperature amplitude (<1K) can be considered negligible at a water temperature of 25–40°C, pipe spacing of 0.075–0.150 m and the thermal resistance of the layers above the pipes ≤0.118 m2K/W. A gap of 10–20 mm compared to no gap reduces aluminium consumption, which is environmentally friendly while ensuring high system efficiency.

Refined three-dimensional numerical simulation of velocity field and seston flux within a scallop-fishing net cage

With the expansion of aquaculture, its impacts have become increasingly important. The environmental impact of suspended aquaculture is dependent upon the farmed species, production intensity, and farming conditions. A refined three-dimensional lattice Boltzmann model was established and validated, and was then used to simulate flow field and seston distribution within a net cage used for scallop fishing. Simulations were carried out in order to analyze the influences of the number of scallops and the current velocity on the local environment (flow velocity, seston flux, and local seston consumption). The results indicated that the reduction of flow velocity and seston flux are related to the number of scallops in the net cage such that a higher velocity/seston flux drop results from a larger amount of scallops. The results also implied that the seston flux and scallop number lead to variable local seston consumption. This study provides fundamental information on the hydrodynamic and seston conditions in net-cage scallop fishing, which have potential application in managing the scale of scallop-fishing and the environmental impacts of suspended-cage aquaculture.