Low-velocity Zone Research Articles

Characterization of an earth dam in Brazil from seismic refraction tomography and multichannel analysis of surface waves

Effective methodologies to obtain information regarding the internal features in an earth dam are fundamental for rapid and technically correct decision making. Although several geophysical methods have potential applicability for investigations on dams, the contribution of seismic methods is well known, since the P and S-waves velocities can be associated with elastic modules of geotechnical interest. The study area was the Paranoá dam, the largest dam in Brazil's federal capital. We analyzed the internal structure of the dam and identified anomalies that could be associated with water saturation. We used traveltimes tomography to obtain Vp models and the MASW method to obtain Vs models. Four profiles of seismic data were acquired downstream of the dam. Borehole's information was used to corroborate with the geophysical interpretation of the profiles. The dam's abutments and the foundation ground could be identified within most models, and the velocity values obtained were mainly related to the clay material that forms the dam's massif, with higher values associated with the quartzite rocks. The rockfill material was well marked as a low velocity zone. Saturated zones were interpreted as local anomalies of high Vp/Vs ratio. An analysis of the depth range obtained within velocity models suggests that the depth of investigation from both methodologies are likely to be site specific, rather than exclusive matter of source and receiver instrumentation.

Ambient noise tomography of Gran Canaria island (Canary Islands) for geothermal exploration

Ambient noise tomography (ANT) has proved to be efficient in resolving crustal structures for different purposes. Despite the numerous studies carried out in Gran Canaria in the past for purposes of geothermal exploration, ANT had never been used before on this island. We applied this technique to understand Gran Canaria island’s shallow structure, determining the 3-D S-wave velocity model of the island and focusing on its implications for geothermal exploration. We used data from 30 seismic stations installed between October 2019 and February 2020. Our findings revealed five relevant velocity anomalies helpful in understanding the island’s geology and its geothermal potential. In particular, we identified two high-velocity anomalies in the central part of the island aligned with the primary northwest–southeast structural trends of Gran Canaria. We interpret them as evidence of intrusive volcanic bodies emplaced during the early stages of the island’s geological history. We also identified three low-velocity zones located in different parts of the island. We maintain that these anomalies could be associated with porous and highly fractured materials produced during the more recent volcanic episodes. In addition, we observed a spatial correlation of anomalies in the electric conductivity identified in previous studies and strong lateral gradients in our S-wave velocity model. We interpret them as evidence of hydrothermal circulation and thermal anomalies in correspondence of lateral contacts between different geological units and/or faults.

Low-degree melt metasomatic origin of heavy Fe isotope enrichment in the MORB mantle

Studies of mid-ocean ridge basalts (MORB) show a variable Fe isotope composition of the oceanic upper mantle. To test a recent hypothesis that heavy Fe isotope enrichment in the MORB mantle results from the same process of incompatible element enrichment, we conduct an Fe isotope study of well-characterized MORB samples from a magmatically robust segment (OH-1) of the Mid-Atlantic Ridge (MAR) at ∼ 35°N. The data show large Fe isotope variation (δ56Fe = +0.03 to +0.18‰) that correlates well with the abundances and ratios of more-to-less incompatible elements and with Sr-Nd-Hf isotopes. Our findings in support of the hypothesis can be detailed as follows: (1) the oceanic upper mantle has a heterogeneous Fe isotope composition on varying small spatial scales with isotopically heavy Fe (high-δ56Fe) preferentially associated with pyroxenite lithologies; (2) such lithologies, which are also enriched in the progressively more incompatible elements, are of low-degree (low-F) melt metasomatic origin; (3) with all the conceivable processes considered, the low-F melt metasomatism takes place at the lithosphere-asthenosphere boundary (LAB) beneath ocean basins through crystallization of incipient (Low-F) melt in the seismic low velocity zone (LVZ) at the base of the growing oceanic lithosphere (i.e., LAB) over the Earth's history since the onset of plate tectonics, forming composite lithologies with geochemically enriched pyroxenite veins dispersed in the depleted peridotite matrix; (4) such mantle of composite lithology when transported to beneath the present-day ocean ridges will undergo decompression melting and produce MORB melts with geochemical trends of “melting-induced mixing” as observed at the MAR and global MORB; (5) we predict all this to be a globally common process and widespread.

Impact of ballast length on train aerodynamics for a wind tunnel layout via CFD analysis

Ground configurations are an essential part of rail vehicles during wind tunnel tests. Reasonable reproductions of the infrastructures are of significance for experimental accuracy. In this work, the effects of ballast length were numerically studied via improved delayed detached eddy simulation (IDDES). Three extension lengths of the ballast accompanied by a 1/8th three-unit train model were proposed, with a pseudo infinite scenario as the benchmark. The numerical algorithm was firstly validated through a wind tunnel test in terms of aerodynamic loads and pressure distribution. The results revealed that the extended length of ballast had a significant impact on the leading and tail cars while less affected the middle car, due to the relatively farther distance from ballast ends. Ignored the drag coefficient, the influence of the ballast length on the separate cars was that the aerodynamic coefficients predicted by much longer ballast got closer to the benchmark. The ballast-edge-induced unrealistic flow and the train-nose-tip-induced realistic flow could easily be merged and interacted with each other and reinforced, thus causing a noticeable low-velocity zone at the leeward side. With the increasing ballast length, those two separations gradually moved away from each other, decoupled, and receded. A minimum length of E/H = 2 is recommended considering the aerodynamic performance and practical constraints of wind tunnel width. This study provides potential guidance for the arrangement of ground configurations for rail vehicles during wind tunnel tests.

Marangoni flow patterns of molten pools in multi-pass laser cladding with added nano-CeO2

Adding rare-earth elements has been proven to modify the microstructure and enhance the mechanical properties of the laser cladding layer. However, the effect of such elements on the flow pattern in molten pools has not been extensively determined, especially for multi-pass laser cladding. The surface tension of T15 high speed steel and nano-CeO 2 mixed powders is measured using the sessile drop method. Comparing the results from the high-speed photography of the molten pool flow and the multicomponent multiphase flow model, various Marangoni flows driven by surface tension are elucidated, and the cross-section of the multi-pass cladding is further analyzed. The surface tension increases with increasing CeO 2 content. When CeO 2 is less than 0.6 wt%, the surface tension monotonically increases with increasing temperature, and the molten pool flows inward. When CeO 2 is greater than 0.6 wt%, the surface tension decreases at high temperatures, and the molten pool exhibits double flows. In multi-pass laser cladding, the inclination of the molten pool increases on adding CeO 2 . Consequently, the inclined angle of the cladding pass interface increases, and the fusion line on the substrate tends to be straightened. At 0.9 wt% CeO 2 , the added powders remain at the center of the molten pool because of a low velocity, creating a low-temperature zone. The temperature coefficient of surface tension demonstrates a positive–negative–positive distribution from the center to the surrounding pool surface, thus forming an outflow. The solidus point of the cladding alloys is lower than that of the substrate, resulting in a small indentation at the remelted region between passes, which increases the interfacial zone. • Flow in multi-pass molten pool changes from inward to outward with CeO 2 addition. • Surface tension curve drops at high temperature with CeO 2 composition above 0.6 wt%. • A low velocity and temperature zone forms in the center of molten pool at 0.9 wt% CeO 2 . • Fusion interface matching with experiments is dominated by flow and solidus point.

Numerical Method to Generate and Evaluate Environmental Wind Over Hills: Comparison of Pedestrian Winds Over Hills and Plains

It is well known that the wind profile at altitudes below 10m from mean sea level (MSL) depends on the geometry of terrain, due to the boundary layer phenomenon. Hence, the profile of wind changes for hilly terrains and mountainous regions when compared with the plain regions. This phenomenon has become important to study due to the large-scale urbanisation taking place over hilly regions. The changing wind profile presents a challenge to evaluate the pedestrian winds, as depending on the aspect of the terrain an additional vertical velocity component is experienced due to the upwind climb of the winds. This creates a wind profile that is twisted in form. While wind tunnel studies have attempted to recreate this twisted wind profile (TWP), due to the inherent deficiency of wind tunnels to simultaneously map velocity and flow conditions, a lack of three-dimensional flow profile hinders pedestrian comfort evaluation. In the wind tunnel studies, it was also observed that small vertical eddies and wakes behind the interfering building were not identified which are an important factor to determine the pollution load dispersion. The authors have developed a numerical model to generate the twisted wind profile. The specialty of the numerical model lies in it’s unique boundary conditions that enable the visualization and quantification of the complete 3D wind profile, when the wind over a hilly terrain interacts with urban infrastructures. The developed model was validated with the wind tunnel experiments done previously by Tse and colleagues. The specialty of the model is that it ensures horizontal homogeneity while creating vertical heterogeneity. From the 3D flow profile hence generated the authors were able to deduce that the impact of twisted wind profile depends on the yaw angle of wind interacting with the structure and not on the wind attack angle. Also, the more the twist of the wind, more is the clockwise shifting of the far wakes behind the building. It was also seen that there are more low velocity zones in the pedestrian winds over a hill in comparison to that over the plains. The vertical eddies that aid in convective removal of pollutants were also missing in case of pedestrian winds over hilly terrains, which raises the risk of pollutant accumulation. The same was also observed in Hong-Kong during COVID 19, where due to the twisted nature of wind flow, the virus load increased and natural ventilation was inadequate in the removal of the viral load in the air near urban areas.

CCMOC: A new view of the Earth's outer core through the global coda correlation wavefield

Increasing seismic evidence has accumulated, suggesting that the Earth's outer core consists of distinct zones of low P-wave velocities in the top and bottom regions relative to the Preliminary Reference Earth Model (PREM). Seismically detected low velocities in the outer core could be linked with the stratification, essential for understanding the geodynamo and thermochemical evolution of the liquid core. However, a consistent globally-averaged radial structure of the outer core has not been obtained due to the incomplete coverage of sampling body waves. To remedy this problem, we explore the seismic structure of Earth's outer core by employing a new theoretical and observational concept termed coda correlation wavefield. We construct the global correlogram in the 15–50 s period range by stacking cross-correlations of the long-duration coda waves from the selected ten large earthquakes. We then assemble a dataset of prominent correlation features from the global correlogram that are sensitive to the outer core. The waveforms of these features are fit by computing synthetic correlograms through various outer core models. The obtained optimal model displays P-wave velocities in both the outer core's top and bottom, consistent with Coda Correlation Reference Earth Model (CCREM) and reduced relative to PREM. The P-wave velocity is ∼1% lower in the core's top than that in PREM, and the slow anomaly gradually approaches zero at about 800 km below the core-mantle boundary. The low seismic velocities in the top of the outer core could likely imply the formation of a thermal and/or compositional stratification.

Advances in seismic imaging of magma and crystal mush

Seismic imaging methods have provided detailed three-dimensional constraints on the physical properties of magmatic systems leading to invaluable insight into the storage, differentiation and dynamics of magma. These constraints have been crucial to the development of our modern understanding of magmatic systems. However, there are still outstanding knowledge gaps resulting from the challenges inherent in seismic imaging of volcanoes. These challenges stem from the complex physics of wave propagation across highly heterogeneous low-velocity anomalies associated with magma reservoirs. Ray-based seismic imaging methods such as travel-time and surface-wave tomography lead to under-recovery of such velocity anomalies and to under-estimation of melt fractions. This review aims to help the volcanologist to fully utilize the insights gained from seismic imaging and account for the resolution limits. We summarize the advantages and limitations of the most common imaging methods and propose best practices for their implementation and the quantitative interpretation of low-velocity anomalies. We constructed and analysed a database of 277 seismic imaging studies at 78 arc, hotspot and continental rift volcanoes. Each study is accompanied by information about the seismic source, part of the wavefield used, imaging method, any detected low-velocity zones, and estimated melt fraction. Thirty nine studies attempted to estimate melt fractions at 22 different volcanoes. Only five studies have found evidence of melt storage at melt fractions above the critical porosity that separates crystal mush from mobile magma. The median reported melt fraction is 13% suggesting that magma storage is dominated by low-melt fraction crystal mush. However, due to the limits of seismic resolution, the seismological evidence does not rule out the presence of small (<10 km3) and medium-sized (<100 km3) high-melt fraction magma chambers at many of the studied volcanoes. The combination of multiple tomographic imaging methods and the wider adoption of methods that use more of the seismic wavefield than the first arriving travel-times, promise to overcome some of the limitations of seismic tomography and provide more reliable constraints on melt fractions. Wider adoption of these new methods and advances in data collection are needed to enable a revolution in imaging magma reservoirs.

Seismic imaging of the crust and uppermost mantle structure in the Qinling orogenic belt and its surroundings: Geodynamic implications

The Qinling orogenic belt and surrounding areas are a junction zone in Central China that connects the North China craton, Yangtze craton and northeastern Tibetan plateau. This area is in a key position within the more general framework where there is eastward extrusion of the Tibetan plateau and westward subduction of the Pacific plate. There is an ongoing scientific debate on how these two main processes are driving the geodynamic evolution of the Qinling orogenic belt and surrounding areas. Specific mechanisms have been proposed, such as lower crustal flow, crustal shortening and big mantle wedge tectonics. Knowledge of the seismic velocity structure helps us understand how these two driving forces affect the tectonic blocks of the study area, as well as test how well the proposed mechanisms to fit the regional velocity structure. For this purpose, a high-resolution P-wave velocity model of the crust and uppermost mantle was obtained by inverting arrival times from local earthquakes that occurred in the Qinling and surrounding region. Our three-dimensional (3D) model suggests that crustal channel flow cannot exist in the whole Qinling orogenic belt, and the localized low-velocity zone generated by the horizontal compressive stress caused by the collision of the Indian and Eurasian plates, is a possible explanation for the growth of the northeastern Tibetan plateau. The contrast in seismic velocities found under the east-Qinling and the west-Qinling orogenic belts implies a different response of these two terranes to the eastward extrusion of the Tibetan plateau. Our velocity model points to relatively stable structural features in the east-Qinling and the Weihe graben, while a low-velocity anomaly in the uppermost mantle of the southernmost Ordos plateau suggests that the lithosphere of the southern Ordos plateau is undergoing destruction.

Large Eddy Simulation of the cavitating flow around a Clark-Y mini cascade with an insight on the cavitation-vortex interaction

In this research, the Large Eddy Simulation method is utilized to predict the cavitation-vortex interaction in the flow around a Clark-Y mini cascade along with the dynamical processes of the U-type cavity, analyzed using the Lamb vector divergence transportation. To provide a novel perception of the development of the U-type cavity, various fundamental mechanisms of the periodic evolution of the U-type cavity are investigated thoroughly. The dynamical progress explicates the mechanisms that generate the high-pressure zone and low-velocity zone. This mechanism, combined with the re-entrant jet and vortex motions plays a significant role in the development of the U-type cavity. Since the Lamb vector curl associates the vorticity transport equation and is closely related to the source of vorticity of the vortex evolution, the Lamb vector curl captures the vortical structures which show good agreement with the Ω-method results. In addition, the distributions of high- and low-momentum flow in the wake of the cascade are delineated by the Lamb vector divergence. The transition from the maximum to the minimum of the lamb vector divergence in a region illustrates a quasi-cyclic behavior that is closely related to the momentum transport mechanisms and the cyclical motions of the vortex.

Combining different 3-D global and regional seismic wave propagation solvers towards box tomography in the deep Earth

SUMMARY In previous publications, we presented a general framework, which we called ‘box tomography’, that allows the coupling of any two different numerical seismic wave propagation solvers, respectively outside and inside a target region, or ‘box’. The goal of such hybrid wavefield computations is to reduce the cost of computations in the context of full-waveform inversion for structure within the target region, when sources and/or receivers are located at large distances from the box. Previously, we had demonstrated this approach with sources and receivers outside the target region in a 2-D acoustic spherical earth model, and demonstrated and applied this methodology in the 3-D spherical elastic Earth in a continental scale inversion in which all stations were inside the target region. Here we extend the implementation of the approach to the case of a 3-D global elastic earth model in the case where both sources and stations are outside the box. We couple a global 3-D solver, SPECFEM3D_GLOBE, for the computation of the wavefield and Green’s functions in a reference 3-D model, with a regional 3-D solver, RegSEM, for the computation of the wavefield within the box, by means of time-reversal mirrors. We briefly review key theoretical aspects, showing in particular how only the displacement is needed to be stored at the boundary of the box. We provide details of the practical implementation, including the geometrical design of the mirrors, how we deal with different sizes of meshes in the two solvers, and how we address memory-saving through the use of B-spline compression of the recorded wavefield on the mirror. The proposed approach is numerically efficient but also versatile, since adapting it to other solvers is straightforward and does not require any changes in the solver codes themselves, as long as the displacement can be recovered at any point in time and space. We present benchmarks of the hybrid computations against direct computations of the wavefield between a source and an array of stations in a realistic geometry centred in the Yellowstone region, with and without a hypothetical plume within the ‘box’, and with a 1-D or a 3-D background model, down to a period of 20 s. The ultimate goal of this development is for applications in the context of imaging of remote target regions in the deep mantle, such as, for example, Ultra Low Velocity Zones.

Two thin middle-crust low-velocity zones imaged in the Chuan-Dian region of Southeast Tibet and their tectonic implications

Two thin middle-crust low-velocity zones imaged in the Chuan-Dian region of Southeast Tibet and their tectonic implications

A crustal LVZ in Iceland revealed by ambient noise multimodal surface wave tomography

The crustal low-velocity zone (LVZ), an important anomaly found in some regional structures of Iceland, is still absent in the Icelandic average velocity structure due to limitations of tomography methods. Using stations from the HOTSPOT experiment and other supplemental stations throughout Iceland, we apply the frequency-Bessel transform method (F-J method) to extract the first two mode dispersion curves from ambient noise data. We obtain an average S-wave velocity (Vs) model of Iceland down to 120 km depth, where two LVZs at depths of 12–22 km and below 55 km are found. The shallow LVZ, whose rationalities are justified using theoretical dispersion curves of certain models to recover themselves, may improve the understanding of the Icelandic average crust. Furthermore, our model shows better representativeness by comparing travel time residuals of the primary wave between observed and synthetic data predicted using different average velocity models. Based on the variations of the Vs gradient, the Icelandic crust with an average thickness of 32 km is divided into the upper crust (0–10 km), middle crust (10–22 km), and lower crust (22–32 km). The asthenosphere starts from the deeper LVZ at 55 km depth, potentially indicating the relatively concentrated melt in this depth range. In this study, crustal LVZs are revealed both in a volcanic active zone and a non-volcanic zone, which may also suggest the LVZ in the average model has more complex origins than the high-temperature zone beneath the central volcanoes. The prevalent thick-cold crustal model of Iceland, considered to rule out the existence of a broad region of partial melt in the crust, also strengthens the possibility of diverse origins. The variations in petrology may also contribute to the crustal LVZ in the average model.

Crustal structure beneath the Cyclades Metamorphic Core Complex, Aegean Sea, Greece

The velocity structure of the crust and sediments of the Cyclades Metamorphic Core Complex (CMCC), central Aegean Sea, Greece, was obtained from active seismic observations. The CMCC comprises blueschists resulted from subduction during Eocene-Oligocene, and greenschists formed during exhumation and magmatic/volcanic intrusions during Miocene. Crustal thickness below the Myrtoon Sea is 23 km, thinning below Andros Island to 20 km and, gradually thickening towards the Chios Basin to 23 km, resulting to a dome structure. The upper crust is inhomogeneously structured with high Vp-velocity of 6.4 to 6.8 km/s, exposed to surface levels, followed by a velocity inversion of 5.8 to 5.5 km/s. Geometry of the velocity units indicates that, during crustal shortening, delamination of crustal layers involved in the subduction was significant. The high velocity units exposed on the islands correspond to pelagic volcano-sedimentary rocks and ophiolites of the Pindos-Cyclades oceanic domain, overthrusting low velocity units of the External Carbonate Platform of the Hellenides developed above continental crust that followed into the subduction. Internal thrusting and isoclinal folding have produced thickening of the Cycladic blueschists bearing nappes and duplication of the nappe pile with younger thrusting beneath Andros Island, resembling the structure of Northern Attica. Below the Chios Basin, velocity units are associated with the Internal Carbonate Platform of the Hellenides developed above the Pelagonian continental basement. Below the crust/mantle boundary we mapped reflectivity separating low velocity zones (7.5 to 7.9 km/s) up to 65 km depth. This low velocity upper mantle, associated also with low density, contributes to the isostatic equilibrium of the CMCC. Sediments of 2.6 to 3.6 km thickness are observed within post tectonic basins underlain by extensional detachments. The high velocity (4.8 to 5.3 km/s) sediments at the lower part of the surficial basins are most probably klippen of the Internal Carbonate Platform.

Fingerprinting secondary mantle plumes

Many vertical seismic velocity anomalies observed below different parts of the Eurasian plate are rooted in the transition zone between the upper and lower mantle (410–660 km), forming so-called secondary plumes. These anomalies are interpreted as the result of thermal effects of large-scale thermal upwelling (primary plume) in the lower mantle or deep dehydration of fluid-rich subducting oceanic plates. We present the results of thermo-mechanical numerical modelling to investigate the dynamics of such small-scale thermal and chemical (hydrous) anomalies rising from the lower part of the Earth's upper mantle. Our objective is to determine the conditions that allow thermo-chemical secondary plumes of moderate size (initial radius of 50 km) to penetrate the continental lithosphere, as often detected in seismo-tomographic studies. To this end, we examine the effect of the following parameters: (1) the compositional deficit of the plume density due to the presence of water and hydrous silicate melts, (2) the width of the weak zone in the overlying lithosphere formed because of plume-induced magmatic weakening and/or previous tectonic events, and (3) a tectonic regime varied from neutral to extensional. In our models, secondary plumes of purely thermal origin do not penetrate the overlying plate, but flatten at its base, forming “mushroom”-shaped structures at the level of the lithosphere-asthenosphere boundary. On the contrary, plumes with enhanced density contrast due to a chemical (hydrous) component are shown to be able to pass upwards through the lithospheric mantle to shallow depths near the Moho when (1) the compositional density contrast is ≥ 100 kg m−3 and (2) the width of the lithospheric weakness zone above the plume is ≥ 100 km. An extensional tectonic regime facilitates plume penetration into the lithosphere but is not mandatory. Our findings can explain observations that have long remained enigmatic, such as the “arrow”-shaped zone of low seismic velocities below the Tengchong volcano in south-western China and the columnar (“finger”-shaped) anomaly within the lithospheric mantle discovered more than two decades ago beneath the Eifel volcanic fields in north-western Germany. It appears that a chemical component is a characteristic feature not only of conventional hydrous plumes located over presently downgoing oceanic slabs, but also of upper mantle plumes in other tectonic settings.

Denitrification in intertidal sediments of a tropical estuary subject to increasing development pressures

Darwin Harbour is a working port and the most populated city in the Northern Territory of Australia. This macrotidal estuary is located in the wet–dry tropics of Northern Australia and notwithstanding mounting development pressures in the region, is largely unmodified. The prevailing oligotrophic condition of estuarine waters suggest that biogeochemical cycling in sediments remain active, buffering the influence of anthropogenic inputs. We tested the hypothesis that nutrient hotspots exist in depositional low-velocity zones, with a gradient of high to low nitrogen processing from the upper to outer reaches of the estuary. A number of factors were examined for their influence on the effectiveness of denitrification in these depositional zones, a putative key process driving nitrogen removal, with particular emphasis on carbon-loading extremes in tidal creeks, spatial gradients along the estuary and the influence of seasonality. There were significant differences in process rates between hypereutrophic/eutrophic tidal creeks that receive the largest proportion of treated sewage loads in the region and the mesotrophic/oligotrophic tidal creeks that were comparatively undisturbed. Net benthic nutrient fluxes and dinitrogen efflux rates were more than an order of magnitude higher and lower, respectively, in degraded (hypereutrophic/eutrophic) tidal creek systems where denitrification efficiency (DE%) was < 40%. Denitrification (Dinitrogen efflux) rates in tidal creeks (mesotrophic/oligotrophic) and broader estuarine sites were high (~ 8 mmol N m−2 day−1) and denitrification efficiency remained > 65%, particularly during the wet season. On a whole-of-estuary basis, denitrification in conjunction with mechanisms such as burial could feasibly make a substantial impact, abating the influence of anthropogenic inputs. Although considerable variability was encountered, particularly across seasons, the hypothesis of elevated denitrification rates as nutrient hotspots in depositional zones along the estuary was not convincing. More influential are tidal creeks as potential ‘reactors’ for N cycling and removal, but their capacity can be degraded by overloading with nutrients.

Numerical Study on Cubic Particle Solid–Liquid Two-Phase Flow in Multistage Pump

To investigate the movement characteristics of cubic particles in a pump, a deep-sea mining lift model pump with a specific speed of 94 is used as the research object in this study. The discrete element method is coupled with the computational fluid dynamics method to simulate the solid–liquid two-phase flow of cubic particles with different densities in the pump while the effect of particle shape on the solid–liquid two-phase flow in the pump is considered. Results show that the cubic particle movement rules for the same flow component are the same. The cubic particle density imposes a more significant effect on the number of particles in the low-velocity zone than in other zones. The number of particles in the low-velocity zone increases with the increase of density. The cubic particle velocity gradient in the impeller decreases as the particle density increases, and the particles exhibit unsatisfactory following performance in the fluid. As the density increases, the collision exhibited by the cubic particles is primarily particle-to-particle collisions, (i.e., more than 37%), and the collision rate between the cubic particles and first-stage guide vane decreases significantly. Compared with cubic particles, spherical particles are likely to obstruct the flow channel in the guide vane. The collision exhibited by the spherical particles in the pump is primarily particle-to-guide vane collision, and the collision rate between the spherical particles decreases by 15.92%.

Experimental Study on Damage Characteristics of Coal Samples under True Triaxial Loading and Dynamic Unloading

Abstract Coal bursts are one of the formidable hazards in underground coal mines, yet it is still not fully explored due to the complex stress environment that exists during mining. To better understand the bursting mechanism of coal under real-time mining conditions, it is necessary to develop an experimental method capable of reproducing in situ stress and loading-unloading paths of coal in in situ conditions. In this study, a self-developed true triaxial testing system was used to investigate the damage and failure characteristics of coal samples under true triaxial loading and dynamic unloading conditions. Acoustic Emission (AE) monitoring was used to capture the fracturing of the loaded coal. Passive Velocity Tomography (PVT) and Cumulative AE Energy Density (CAEED) were used to analyse damage evolution characteristics of the coal samples under true triaxial loading conditions. A high-speed camera was used to record the failure of the coal samples when the minimum principal stress σ3 was suddenly unloaded. It was found that continuous coal damage occurred primarily during the true triaxial loading period. The peak number of AE events in the coal samples increased and then dropped as σ1 levels increased. High and low wave velocity zones in the coal samples represent regenerations of the high-density zone and fracture emergence, respectively. Significant energy release zones transferred and expanded as the triaxial loading level increased. Under significant triaxial loads, the coal samples failed and were severely damaged, and the dynamic unloading of σ3 caused ejections of coal fragments at low velocities. The outcome of this study provides in-depth understanding of the failure mechanism of coal under in situ conditions.

A Numerical Performance Analysis of a Rim-Driven Turbine in Real Flow Conditions

The tidal turbines represent a new frontier for extracting energy from tides source. Despite the technology being mature, new solutions aimed at improving performance, reliability with reduced environmental impact, manufacturing and installation costs are currently under investigation. The Rim-driven turbine (abbreviated as RDT) was recently proposed. A RDT resembles a ducted turbine (abbreviated as DT), as both contain blades and a duct. The present study aims at investigating the detail performance and flow field of a RDT in a real flow based on the China Zhaitang Island’s tidal current data. To show the difference between the RDT and DT, simulations are also performed on the corresponding DT. It is found that the power and thrust for the two configurations exhibit time-periodic behavior that is consistent with the wave frequency. At axial flow, the fluctuation amplitude on the power and thrust increase with the increase of tip speed ratio. The RDT has higher power output when operating at lower tip speed ratio and has a potential reduction in flow resistance and disturbance with respect to the DT. At yawed flow, the fluctuation amplitude on the power and thrust decrease with the increase of yaw angle. The RDT has less capable of compensating the effect of yawed inflow in reducing the power than the DT at larger yaw angle. In addition, the power and thrust generates micro-amplitude fluctuation integrated into the main waveform, which the frequency is consistent with the turbine rotation frequency. The wake characteristics analysis reveals that the yawed flow field is more turbulent, and the two configurations suffer strong unsteady flow separation along the whole span. Strong interactions are observed between the rotor’s main wake and the duct’s upper wake. The yaw angle primarily determines the downstream wake deflection direction and significantly changes the wake shape and vortex structures. Meanwhile, the wake flow is found to recover more quickly at larger yaw angle. Besides, due to the open-center of RDT, a part free-stream flow is allowed to travel through and forms an obvious high velocity zone. The presence of open-center of RDT has avoided the low velocity zone, improved the wake structure and accelerated wakes recover, which seems to give an advantageous effect in operating a RDT.

Seismic tomography of the mantle and primary hydrogen deposits in the Dnieper-Donetsk basin

According to the three-dimensional P-velocity model of the mantle under Eurasia obtained by the Taylor approximation method, the analysis of the velocity structure of the mantle (to depths of 2500 km south of 50° N and 1700 km north) in the territory of the Dnieper-Donetsk depression was carried out in order to determine the possible areas of primary hydrogen release. Primary hydrogen is formed in the core and lower mantle, can be transferred to the surface (according to I.L. Gufeld). According to seismotomography, nine superdeep mantle fluids are isolated on the territory of Ukraine, the routes of which are defined as subvertical columns of alternation of high-speed and low-speed anomalies. In addition to the presence of superdeep mantle fluids in the study area, the following characteristics were analyzed: the depth of the main geodynamic boundary, the influence of the high-velocity transition zone of the upper mantle (propagating northward into the low-velocity transition zone of the upper mantle of the East European platform), the depth of the Moho boundary, gravity mantle anomalies and heat flow. The totality of the studies performed allowed us to conclude that, according to seismic tomography data, the Izyumsky and the eastern part of the Lokhvitsky segment of the Dnieper-Donetsk depression are promising areas for detecting primary hydrogen in the territory of the Dnieper-Donetsk depression.