Low Velocity Region Research Articles

Flow Control Around a Cylinder With a Perforated Cylinder

AbstractIn this work, periodic vortex shedding at both sides of a circular cylinder is aimed to be suppressed using a concentrically located perforated cylinder under laminar flow conditions at Re = 200. A code is developed in comsolmultiphysics, 5.3a, and validated using the outcomes of ansysfluent, and previous studies in the open literature, which exhibit a good agreement. In this study, the porosity, β is varied within 0.5 ≤ β ≤ 0.9, and the gap ratio, D/d is varied within 1.5 ≤ D/d ≤ 3.5. The results of the present numerical investigation are evaluated using instantaneous and time-averaged vorticity, streamwise, and transverse components of the velocity and pressure. The drag, CD and lift, CL coefficients are calculated. The Strouhal number, St from the pointwise spectral analysis of the streamwise velocity component is plotted for various cases. It is observed that porosity, β has a dominant effect rather than the gap ratio, D/d on the flow past a solid cylinder. The low-velocity and low-pressure regions are getting large in the transverse direction as the porosity, β increases. The separated layers from solid and perforated cylinders merge for low gap ratios, D/d. However, individual movement of these layers is evident for larger gap ratios, D/d with low porosity, β values. A perfect suppression of the periodicity of vortex shedding is obtained for the cases of D/d = 3.5 with β = 0.5, 0.6, and D/d = 3 with β = 0.5.

Control mechanism of vortex-generator jet on turbulent separation in a highly loaded compressor cascade

The active flow control technique of vortex-generator jet (VGJ) was used to control the turbulent separation of a highly loaded compressor cascade at a high incidence. VGJ schemes on suction surface with different jet locations, skewed angles, pitch angles were numerically performed and the control mechanism of VGJ parameters on the flow field of the cascade was revealed. For avoiding the effect of endwall boundary layer, both endwall surfaces of the cascade were set to translational periodicity boundary (TPB). Results show that VGJ significantly eliminates the turbulent separation and improves the performance of cascade. The overall loss coefficient of cascade is reduced by 52.3% at most. The optimal VGJ location in this study is not separation location but 7% axial chord downstream of it, which is probably due to the ultra-high loading of the cascade. The spanwise location and intensity of jet vortices are mainly affected by skewed angle, and will increase with the skewed angle. The VGJ case with a higher pitch angle features better control effect on suction surface but increases the pitchwise scale of the low-velocity region. In addition, as pitch angle increases, the mixing loss between mainstream and jet flow increases, which brings a negative effect on cascade performance. In order to enhance the momentum transporting between mainstream flow and separation flow, counter vortex-generator jet (CVGJ) schemes were designed and studied. Though CVGJ strengthens the jet vortices near the trailing edge and achieves minimum loss near TPB sections at 150% axial chord section, the interaction of jet vortices with opposite rotating direction brings extra loss, which leads to the control effect of CVGJ is weaker than that of VGJ.

Optical, flow, and thermal analysis of a phototherapy extracorporeal membrane oxygenator for treating carbon monoxide poisoning.

Extracorporeal membrane oxygenators (ECMO) are currently utilized to mechanically ventilate blood when lung or lung and heart function are impaired, like in cases of acute respiratory distress syndrome (ARDS). ARDS can be caused by severe cases of carbon monoxide (CO) inhalation, which is the leading cause of poison-related deaths in the United States. ECMOs can be further optimized for severe CO inhalation using visible light to photo-dissociate CO from hemoglobin (Hb). In previous studies, we combined phototherapy with an ECMO to design a photo-ECMO device, which significantly increased CO elimination and improved survival in CO-poisoned animal models using light at 460, 523, and 620 nm wavelengths. Light at 620 nm was the most effective in removing CO. The aim of this study is to analyze the light propagation at 460, 523, and 620 nm wavelengths and the 3D blood flow and heating distribution within the photo-ECMO device that increased CO elimination in CO-poisoned animal models. Light propagation, blood flow dynamics, and heat diffusion were modeled using the Monte Carlo method and the laminar Navier-Stokes and heat diffusion equations, respectively. Light at 620 nm propagated through the device blood compartment (4 mm), while light at 460 and 523 nm only penetrated 48% to 50% (~2 mm). The blood flow velocity in the blood compartment varied with regions of high (5 mm/s) and low (1 mm/s) velocity, including stagnant flow. The blood temperatures at the device outlet for 460, 523, and 620 nm wavelengths were approximately 26.7°C, 27.4°C, and 20°C, respectively. However, the maximum temperatures within the blood treatment compartment rose to approximately 71°C, 77°C, and 21°C, respectively. As the extent of light propagation correlates with efficiency in photodissociation, the light at 620 nm is the optimal wavelength for removing CO from Hb while maintaining blood temperatures below thermal damage. Measuring the inlet and outlet blood temperatures is not enough to avoid unintentional thermal damage by light irradiation. Computational models can help eliminate risks of excessive heating and improve device development by analyzing design modifications that improve blood flow, like suppressing stagnant flow, further increasing the rate of CO elimination.

Spatio-temporal evolution and precursory indicators of rock failure processes using acoustic emission tomography

The characterization of stress and crack development plays important part in monitoring disasters. To capture the instability enlightenment from the spatio-temporal evolution of rock damage, this paper constructs the velocity field of key nodes in the rock failure process with the active and passive collaborative acoustic emission (AE) tomography method. It realizes the exploration of the micro-crack propagation law and its potential connection with the stress environment, combining the velocity field, the AE density field, and the energy field. Furthermore, from the spatial integrity and directional evolution of the velocity field, the mutation trend coefficient (MTC) and deformation coefficient (DC) of the velocity field are proposed as precursor indicators of rock instability, respectively. Results show that the anisotropy of the velocity field increases with the aggravation of rock damage, and the stress environment controls the spatial heterogeneity of the velocity field. The most important feature of spatial heterogeneity is that there are the continuous decrease and extension of low-velocity regions and the transformation from the high velocity to the low velocity. Real-time acoustic emission monitoring of the velocity field with MTC and DC can early identify rock instability precursors and principal stress direction, which provides vital pre-alarm information for geotechnical engineering.

Vortex structure for reducing tip leakage flow of linear turbine cascade using dielectric barrier discharge plasma actuator

An axial-flow turbine is one of the main components of aircraft jet engines. It is important to suppress the leakage flow at the blade tip to improve the aerodynamic performance of the turbine blades. To achieve this, a unique ring-type plasma actuator was developed as an active flow control device. This plasma actuator consists of a high-frequency−high-voltage electrode installed in the outer casing of the turbine rotor. The metallic turbine rotor blades act as the grounded electrode. Dielectric barrier discharge (DBD) plasma is generated between the outer casing and tip of the turbine blade; this plasma interrupts the leakage flow. In this study, the effect of the plasma actuator in reducing the tip leakage flow of a linear turbine cascade was demonstrated using particle image velocimetry (PIV) to obtain velocity measurements at two planes, namely, the blade mid-passage and blade exit. The turbulence intensity and vorticity were also analyzed. The blade mid-passage measurements showed that the low-velocity region caused by the tip leakage flow was reduced by the plasma actuator and the blade exit measurements clarified the vortex structure. The operation of the plasma actuator successfully reduced or eradicated the tip leakage vortex. Contrarily, the passage vortex at the tip side became clearer and stronger. The plasma actuator reduced the tip leakage vortex even when the Reynolds number was changed.

Three-dimensional body wave velocity structure and seismogenic structure for the 2022 MS 6.8 Luding earthquake sequence in Sichuan, China

An MS 6.8 earthquake struck the Luding county in the southeastern section of the Xianshuihe fault on 5 September 2022, causing heavy casualties and serious property losses. Using the data of 61 seismic stations surrounding the focal area, we utilize the machine learning-based LOC-FLOW for phase picking and association, and obtain a total of 13,496 aftershocks. We also collate 2,331 preshocks near the mainshock. We then conduct event relocation and three-dimensional P- and S-wave velocity structure inversion by double-difference tomography. The catalog we obtained has higher accuracy and completeness than the manual catalog. The relocation results show that the mainshock struck the Moxi section of the Xianshuihe fault at 9.2 km depth. There is a southeast-striking seismic strip of 35 km long at 5–13 km depth, and two relatively shallow seismic clusters exist to the west and northwest of the mainshock. Furthermore, there is an obvious seismic gap to the northwest of the mainshock. Based on the spatial pattern of the aftershock sequence, we speculate that the Moxi section of the Xianshuihe fault is the seismogenic fault. The earthquakes are mainly located in the regions of low velocity and low Poisson’s ratio, which is consistent with the granite that is geologically distributed along the southeast section of the Xianshuihe fault. There are two high-velocity zones located to the northwest and southeast of the mainshock, which are speculated to have controlled the rupture scale of the MS 6.8 Luding earthquake.

Canopy elastic turbulence: Spontaneous formation of waves in beds of slender microposts

A microfluidic canopy flow device, formed from a large array of slender polymeric pillars within a glass microchannel, is subjected to viscoelastic flow in the regime of elastic turbulence. The system results in the spontaneous emergence of waves in the form of propagating regions of low flow velocity compared to the bulk, also inducing Monami-like waves in the canopy if the pillars are flexible. Due to the analogies with classical (inertial) canopy turbulence, this new phenomenon is named ``canopy elastic turbulence''.

The effect of drainage cannula tip position on risk of thrombosis during venoarterial extracorporeal membrane oxygenation

Background and objectivesVenoarterial extracorporeal membrane oxygenation (VA ECMO) is able to support critically ill patients undergoing refractory cardiopulmonary failure. It relies on drainage cannulae to extract venous blood from the patient, but cannula features and tip position may impact flow dynamics and thrombosis risk. Therefore, this study aimed to investigate the effect of tip position of single-stage (SS) and multi-stage (MS) VA ECMO drainage cannulae on the risk of thrombosis. MethodsComputational fluid dynamics was used to model flow dynamics within patient-specific geometry of the venous vasculature. The tip of the SS and MS cannula was placed in the superior vena cava (SVC), SVC-Right atrium (RA) junction, mid-RA, inferior vena cava (IVC)-RA junction, and IVC. The risk of thrombosis was assessed by measuring several factors. Blood residence time was measured via an Eulerian approach through the use of a scalar source term. Regions of stagnant volume were recognised by identifying regions of low fluid velocity and shear rate. Rate of blood washout was calculated by patching the domain with a scalar value and measuring the rate of fluid displacement. Lastly, wall shear stress values were determined to provide a qualitative understanding of potential blood trauma. ResultsThrombosis risk varied substantially with position changes of the SS cannula, which was less evident with the MS cannula. The SS cannula showed reduced thrombosis risk arising from stagnant regions when placed in the SVC or SVC-RA junction, whereas an MS cannula was predicted to create stagnant regions during all tip positions. When positioned in the IVC-RA junction or IVC, the risk of thrombosis was higher in the SS cannula than in the MS cannula due to both high and low shear flow. ConclusionTip position of the drainage cannula impacts cannula flow dynamics and, subsequently, the risk of thrombosis. The use of MS cannulae can reduce high shear-related thrombosis, but SS cannulae can eliminate stagnant regions when advanced into the SVC. Therefore, the choice of cannula design and tip position should be carefully considered during cannulation.

Wavefield distortion imaging of Earth's deep mantle

The seismic wavefield, as recorded at the surface, carries information about the seismic source and Earth's structure along the seismic path, essential for the understanding of the interior of our planet. For 40 years seismic tomography studies have resolved the 3D seismic velocity structure in growing detail using seismic traveltimes and waveforms. These studies have been driving our understanding of the dynamics and evolution of the planet, but are limited in their spatial resolution to imaging scales of a few 100 s to 1000 km due to the constraints of the tomographic inversion. Detailed studies of seismic waveforms can resolve finer scale structure but are often reliant on serendipitous source-receiver combinations and provide very uneven coverage of the planet. Therefore, we often lack an understanding of the fine scale structure of the Earth that is important to understand structures and processes such as mantle plumes or details of slab recycling. Here we show evidence that we can exploit slowness vector deviations of the seismic wavefield to extend our knowledge of Earth structure to smaller scales using large datasets. Analysing seismic array data, we show strong and measurable focussing and defocussing effects of the teleseismic P and Pdiff wavefield sampling the deep Earth. We compare the P-wave results to additional S and Sdiff data and find good agreement between both wavetypes. We can link the wavefield deviations to strong velocity variations assuming sharp boundaries are sampled along the path in the deep mantle. The dataset samples the Pacific and Gulf of Mexico well and shows strong horizontal incidence (backazimuth) deviations in the Pacific (up to 14° westwards) and beneath the Gulf of Mexico (up to 5 to 8° east- and west-ward). The backazimuth deviations are also reflected in slowness deviations in the range of ± 0.8 s/° relating to velocity variations in the range of ± 9 km/s. Using 3D raytracing we are able to forward model the detected backazimuth variations of the P and Pdiff dataset. The high frequencies of the P-waves, density of the ray-paths, and low computational cost of our forward calculation allow us to construct a higher resolution and more detailed model of velocity anomalies under Hawaii than was possible with previous methods. The best-fitting velocity model for the Pacific contains two low-velocity regions located at N25°/W155° and N25°/W165° beneath the tip of the Hawaii Emperor chain. The Pacific anomalies have diameters (D) of 6° and 2° with velocity reductions (dVP) of 8% and 4% with heights (H) above the CMB of 70 km and at least 200 km, respectively. We also detect a fast region of 3% velocity increase in the North Pacific rising at least 300 km above the CMB with a diameter of 12° at N60°/W175°. Beneath the Gulf of Mexico we find ambiguous results with either a slow region (N25°/W85°, H = 200 km, dVP=-3%, D = 2°) or a fast region (N15°/W75°, H = 200 km, dVP = 3%, D = 4°) able to explain the data. We thus show that the directivity information of the seismic wavefield - largely underexploited - can be used to resolve the fine scale velocity structure of the Earth's interior with great accuracy and can deliver additional insight into deep Earth dynamics.

Experimental investigation of the distorted flow field in a new type of direct-connect facility for scramjet studies

A supersonic distorted flow field generated by a new type of direct-connect facility at Mach 2.1 is investigated experimentally via particle image velocimetry (PIV). This facility is designed to accurately reproduce the real flow field in scramjet especially the flow distortion within the isolator, while avoiding the unstart problem in traditional direct-connect facility. The time-averaged velocity fields reveal the principle of flow distortion generated by this facility and show the detailed flow field structures. The mean results suggest that the choose of target parameters distribution will directly affect the performance of this facility. Then, the distorted flow fields in the isolator with a specific non-uniform incoming flow condition are investigated in detail, with the method of proper orthogonal decomposition (POD). The POD analysis of the fluctuating velocity fields finds that there are complex flow field structures within the isolator, including the large-scale vortex structure and the small-scale structure near the wall surface. And these structures in the eigenmodes are associated with the shock wave and the low-velocity region close to the wall surface. Meanwhile, the distorted main flow is also considered to contribute to the flow field instability. Finally, according to the analysis results, the practical guidance of this new type of direct-connect facility is proposed for the subsequent experimental research about scramjet.

Temperature-induced densification in compressed basaltic glass revealed by in-situ ultrasonic measurements

Abstract Acoustic velocities of a model basalt glass (64 mol% CaMgSi2O6 + 36 mol% CaAl2Si2O8) were measured along different pressure-temperature (P-T) paths. One set of experiments involved isothermal compression-decompression cycles, performed at temperatures of 300, 641, 823, and 1006 K and pressures up to 12.2 GPa. The other set of experiments involved constant-load heating-cooling cycles at temperatures up to 823 K and pressures up to 7.5 GPa. Both sets of experiments were performed in a multi-anvil apparatus using a synchrotron-based ultrasonic technique. Our results show that the glass compressed isothermally at 300 K (cold-compression) displays anomalously decreasing compressional (VP) and shear (VS) wave velocities with increasing pressure until ~8 GPa. Beyond 8 GPa, both VP and VS start to increase sharply with pressure and irreversible densification of the glass occurred, producing large hysteresis loops of velocities upon decompression. However, for the glass compressed isothermally at increasingly higher temperatures (hot-compression), the velocity minima gradually shift to lower pressures. At temperature close to the glass transition temperature Tg, the velocity minima disappear completely, displaying a monotonic increase of velocities during compression and higher VP and VS during decompression. In addition, constant-load heating-cooling experiments show that velocities generally decrease slightly with increasing temperature, but start to increase once heated above a threshold temperature (~650 K). During cooling the velocities increase almost linearly with decreasing temperature, resulting in higher velocities (~1.5–2.5% higher) when returned to 300 K. This implies that a temperature-induced densification may have occurred in the glass at high pressures. Raman spectra on recovered samples show that the hot-compressed and high-P heated glasses contain distinctly densified and depolymerized structural signatures compared to the initial glass and the cold-compressed glass below the velocity transition pressure PT (~8 GPa). Such densification may be attributed to the breaking of bridging oxygen bonds and compaction in the intermediate-range structure. Our results demonstrate that temperature can facilitate glass densification at high pressures and point out the importance of P-T history in understanding the elastic properties of silicate glasses. Comparison with melt velocity suggests that hot-compressed glasses may better resemble the pressure dependence of velocity of silicate melts than cold-compressed glasses, but still show significantly higher velocities than melts. If the abnormal acoustic behaviors of cold-compressed glasses were used to constrain melt fractions in the mantle low-velocity regions, the melt fractions needed to explain a given velocity reduction would be significantly underestimated at high pressures.

Study on the geometric characteristics effect of contact area on fracture seepage

Seepage in rock fractures is influenced by the geometry of the contact area. A parallel model with two circular cylindrical contact areas is studied by numerical simulation. The variation of the seepage field in the fracture model is analyzed by changing the size (radius r) and location (mutual distance s and upstream angle θ) of the two contact areas. Changing r, s, and θ is essentially changing the width and angle of the flow channel on both sides of the contact area. The contact area compresses the flow channel so that there is an obvious pressure drop around the contact area, and the reduced pressure potential energy is mainly converted into the kinetic energy of flow around the contact area and the water flow obtains a larger velocity. The resulting inertial effect controls the generation and development of the low-velocity region and eddy behind the contact area. θ is about 30°, which is the critical point of eddy current generation behind the contact area, the changing trend of −∇P and s and −∇P and sinθ. Two conceptual models are proposed that there is a quadratic function relationship between −∇P and s, and an exponential function relationship between −∇P and sinθ. The parameter N is proposed to represent the joint action of two kinds of the flow channels (the flow channel between contact area and contact area and the flow channel between contact area and lateral wall) and the relationship between −∇P and N is a quadratic function.

Inlet Gap Influence on Low-Frequency Flow Unsteadiness in a Centrifugal Fan

In this study, unsteady low-frequency characteristics in a voluteless low-speed centrifugal fan operating at a high mass flow rate are studied with improved delayed detached eddy simulation (IDDES). This study is motivated by a recent finding that the non-uniformly distributed pressure inside this type of fan could be alleviated by improving the gap geometry. The present simulation results show that the velocity magnitudes of the gap have distinct low and high regions. Intensive turbulent structures are developed in the low-velocity regions and are swept downstream along the intersection between the blade and shroud, on the pressure side of the blade. Eventually, the turbulence gives rise to a high-pressure region near the blade’s trailing edge. This unsteady flow behavior revolves around the fan rotation axis. Additionally, its period is 5% of the fan rotation speed, based on the analysis of the time history of the gap velocity magnitudes and the evolution of the high-pressure region. The same frequency of high pressure was also found in previous experimental measurements. To the authors’ knowledge, this is the first time that the trigger of the gap turbulence, i.e., the unsteady local low velocity, has been determined.

Structural optimization and hydraulic performance analysis of bionic pit flow channels based on a genetic algorithm

Orthogonal experiments have mostly been used in the structural optimization of drip irrigation emitter flow channels. To further improve the efficiency of the optimal design, this study used a genetic algorithm to optimize the structure of the bionic pit flow channel. Based on the structural similarity and performance optimization of the torus-margo bordered pit structure, the constitutive equation of the flow channel unit was constructed. The selection, crossover and mutation operators were set by the genetic algorithm, and the objective function value was calculated. The design variables and known variables that met the requirements were put into the computational domain model, and the pit flow channel structure was simulated and optimized. The results showed that there were large low-velocity regions at the junctions and corners of the pit flow channel units at a working pressure of 50 kPa, and no complete low-velocity vortices were observed, indicating that the flow channels had good anti-clogging performance. The distribution of flow velocity on the same cross-section was quite different, which made the flow layers collide and mix, which intensified the loss of energy, indicating that it had a good energy dissipation effect. The multivariate linear regression analysis showed that the four variables of tooth stagger value (j), flow channel angle (θ), tooth spacing (l) and inner and outer boundary spacing (h) had a decreasing degree of influence on the flow index (x). The flow index (x) was negatively correlated with the tooth stagger value (j), flow channel angle (θ) and tooth spacing (l), and positively correlated with the inner and outer boundary spacing (h). The test results of physical samples showed that the average error between the simulation results and the real values was 3.4%, indicating that the accuracy was high, which can provide a basis for the structural optimization design of related pit drip irrigation emitters.

Multi-Parameter Optimization Design of Axial-Flow Pump Based on Orthogonal Method

The axial-flow pump is a widely used piece of general machinery which consumes large amounts of energy. In this study, an axial-flow pump with the specific speed of 536 is firstly designed and experimentally measured; then, the orthogonal method is employed to conduct the energy performance optimization. Five optimization parameters, including hub control point, hub stagger angle, shroud stagger angle, camber angle and centroid position were set with four levels. Sixteen individual pumps were designed according to the orthogonal method; then, a numerical simulation was implemented to obtain the energy performance and flow pattern. Results showed that the shroud stagger angle has the maximum influence on the pump head and efficiency, and the hub stagger angle and camber angle are also very important. At a design point of flow rate 70 kg/s, the efficiency of the optimal pump is 86.29%, which improved by 2.05% in comparison with the baseline pump. The pressure gradient of the optimal pump from blade inlet to outlet becomes more fluent than that of baseline pump, and the low-velocity region of the optimal pump at the blade head shrinks, compared to that of the baseline pump.

Prediction of submicron particle dynamics in fibrous filter using deep convolutional neural networks

This study developed a data-driven model for the prediction of fluid–particle dynamics by coupling a flow surrogate model based on the deep convolutional neural network (CNN) and a Lagrangian particle tracking model based on the discrete phase model. The applicability of the model for the prediction of the single-fiber filtration efficiency (SFFE) for elliptical- and trilobal-shaped fibers was investigated. The ground-truth training data for the CNN flow surrogate model were obtained from a validated computational fluid dynamics (CFD) model for laminar incompressible flow. Details of fluid–particle dynamics parameters, including fluid and particle velocity vectors and contribution of Brownian and hydrodynamic forces, were examined to qualitatively and quantitatively evaluate the developed data-driven model. The CNN model with the U-net architecture provided highly accurate per-pixel predictions of velocity vectors and static pressure around the fibers with a speedup of more than three orders of magnitude compared with CFD simulations. Although SFFE was accurately predicted by the data-driven model, the uncertainties in the velocity predictions by the CNN flow surrogate model in low-velocity regions near the fibers resulted in deviations in the particle dynamics predictions. These flow uncertainties contributed to the random motion of particles due to Brownian diffusion and increased the probability of particles being captured by the fiber. The findings provide guidelines for the development of data science-based models for multiphysics fluid mechanics problems encountered in fibrous systems.

Study of inverse natural convection-conduction heat transfer for in-line tube heat exchanger in a hot box with experimental data

BackgroundThe accuracy of the chosen flow model and near-wall treatment requires detailed experimental verification. It can affect the accuracy of the numerical results obtained. Thus, the selection of an appropriate flow model is important. However, a review of the literature found that few researchers have engaged in the inverse convection-conduction conjugate heat transfer problem. MethodsThis study uses an inverse computational fluid dynamics (CFD) method combined with the least squares method and overdetermined experimental data to predict the unknown heat transfer rate Q and the average Nusselt number Nuk on each tube. The conservation equations of the air outside and inside the hot box and the heat conduction equation of the hot box walls are also solved by coupling. Then, in order to select a suitable flow model, a comparison of the root mean square errors between the CFD results of temperature obtained from all selected flow models and the experimental data is required. Significant FindingsThe significant findings are that the RNG k-ε model is selected as the appropriate flow model for this study through the validation tests of various flow models. The vortex ring appears in the central low-velocity region of the four tubes with StD=2 and gradually disappears with the decrease of StD. The two thermal plumes above the upper two tubes with StD=4/3 are slightly inclined inward and merge with each other. But these two plumes above the upper tubes are inclined slightly outward as StD increases. The obtained velocity patterns and Nuk agree with the interferometric images and the modified correlation, respectively.

Hydrodynamic CFD-DEM model validation in a gas–solid vortex unit

Process intensification in gas–solid fluidization processes can be achieved by working in a centrifugal rather than a gravitational field. In this regard, the gas–solid vortex unit (GSVU) is an ideal candidate for heterogeneously catalyzed processes. A four-way coupled CFD-DEM model describing the hydrodynamics in the GSVU with an unprecedented level of detail is validated using 2D particle image velocimetry (PIV) experimental data on both azimuthal and radial particle velocity components. It captures high and low velocity regions, both qualitatively and quantitatively. Gas-solid slip velocities several times higher than those obtainable in a gravitational field are achieved, greatly enhancing heat and mass transfer rates. Furthermore, the gas-phase residence time distribution in the GSVU is shown to be narrow. This developed model presents a powerful tool for a better understanding and a detailed design aimed at enhancing the non-reactive and reactive process intensification capabilities of the gas–solid vortex technology.

Effect of bubble trailing vortex on coal slime motion in flotation

Understanding the coal slime motion around the trailing vortex is an important precursor for predicting the entrainment probability of the bubble trailing vortex in coal flotation. Using a particle image velocimetry (PIV) system, we investigated the length of low-velocity region and the separation behavior of liquid from the cylinder surface under different fluid flow velocities. As the average fluid flow velocity increased, the height of the low-velocity region firstly increased and then gradually decreased, while the separation angle firstly decreased rapidly and then be almost constant at 94.2°. The relative height of low-velocity region and the separation angle fitted well with the fluid flow velocity, respectively. The trajectory of the coal particle having a diameter of 0.1875 mm moved around the cylinder was explored via a high-speed camera system. The coal particle first moved against the cylinder surface and then entered the trailing vortex region, where the centrifugal force and dynamic pressure provided sufficient energy. After a similar circular motion, the coal particle left the region relying on the dynamic pressure. Our results can provide a valuable insight into the development of technology for cleaner mineral flotation.

Nature of mantle discontinuities beneath the Ontong Java Plateau

We determined the depths of the mantle discontinuities with a receiver function method using broadband data recorded on the seafloor and islands of the Ontong Java Plateau (OJP) and its vicinity. The 410-km discontinuity is broadly elevated by 10–20 km beneath the OJP. The elevation is observed in the low-velocity region above a stagnant paleo-Pacific slab that had been subducted until 25 Ma. The elevated 410-km discontinuity and the slow anomalies may be caused by partial melting induced by hydration with water released from the paleo-Pacific slab. The 660-km discontinuity is close to the global average beneath the OJP, suggesting that the stagnant Pacific slab is bottoming in the mantle transition zone. The 410-km discontinuity is depressed by 10–20 km beneath the Caroline volcanic chain, where a low-velocity sheet has been observed by tomographic studies, implying that a sheet-like upwelling occurs at least from the top of the mantle transition zone. The 660-km discontinuity is also depressed by 10–30 km beneath the Caroline volcanic chain, which may be attributed to the majorite-bridgmanite phase change in a high-temperature environment and/or an underestimated velocity correction in the moveout correction of receiver functions. It is thus unclear whether the sheet-like mantle upwelling has its roots in the lower mantle.