Local Flow Velocity Research Articles

Experimental and numerical investigation of seepage erosion in sandy cobbles under coupling hydraulic and dynamic load

The internal structure of sandy cobbles strata is sensitive to disturbances in the urban underground environment, but the structural evolution process under coupling hydraulic and dynamic loads remains unexplored. This paper presents a detailed investigation into the migration patterns and mechanisms of fine particles in sandy cobbles induced by coupled hydraulic and dynamic loading. A sandy cobble specimen with a typical particle size distribution (PSD) was designed and tested using an apparatus that included a constant inlet water head control system and an eccentric-vibrator-based dynamic loading system. Based on physical modeling tests, a numerical model was constructed to replicate the internal structural evolution under hydraulic and dynamic loading by calibrating the time history of local permeability. The test results indicate that the application of dynamic load can instantly disrupt the stable internal structure of sandy cobbles under static seepage, imparting kinetic energy to fine particles that detach from the skeleton structure and migrate along the seepage direction. Significant fine particle loss occurs near the seepage outlet, but due to energy loss during migration, fine particles far from the seepage outlet are recaptured by the skeleton pore throats and clogged again in the migration path. As the intensity of the dynamic loading increases, the migration path for fine particles becomes longer, and the amount and size of fine particles lost significantly increase. The changes in the internal structure of the soil are reflected in hydraulic parameters as a transient increase in local flow velocity, an increase in local pore water pressure due to clogging, and a decrease in the overall permeability coefficient with the loss of fine particles. The findings of this study will be useful for future geotechnical engineering design and analysis.

Investigation of aerodynamic processes in porous materials based on triply periodic minimal surfaces

RELEVANCE: The relevance of this work lies in the study of new porous materials for use in compact, highly efficient heat exchange devices. PURPOSE: To investigate the hydro-aerodynamic properties of flows passing through porous inserts based on triply periodic minimal surface (TPMS) topologies. To develop a methodology for studying porous materials with ordered structures. To identify potentially suitable TPMS-based porous materials for application in heat exchange equipment. METHODS: Numerical (CFD) and experimental methods were used to address the research objectives. Ansys Fluent 2019 R3 software was utilized for numerical modeling. Experimental samples for the physical experiments conducted on the VENT-08-7LR-01 laboratory setup were fabricated using SLA additive technologies. The porosity of the samples ranged from 0.73 to 0.89. The experiment was conducted with inlet velocities ranging from 0.3 to 4.5 m/s. RESULTS: New empirical dependencies of pressure drop on flow velocity were obtained for inserts based on the surfaces: Primitive (P), Fischer Koch S (FKS), Neovius (N), and Schoen's I-WP (IWP). The airflow through the N structure showed the highest pressure drop, while the P structure had 8 times less pressure drop at the same velocity. Stagnation zones, which can negatively impact heat transfer, were identified in the porous inserts. Changes in local flow velocity in the porous inserts were determined to correlate with the insert's transparency. CONCLUSION: The research results can be used for designing cooling systems with TPMS-based ribbing. Based on the analysis of the velocity vector field distribution and pressure drops, the FKS and IWP structures have potential applications in heat exchange equipment.

Characteristics of flow structure of lateral inlet/outlet in pumped storage power stations

Investigating the complex separated flows in lateral inlet/outlets of pumped-storage power stations(PSPS) is crucial for optimizing their design to ensure safe and efficient operation. In this study, Large Eddy Simulations (LES) are employed to reveal the relationship between turbulence intensity and the distribution of coherent structures in separated flows, as well as the evolution characteristics of hairpin vortices. The flow separation is notably influenced by the vertical diffusion angles α, which causes recirculation zones at the top of the inlet/outlet, leading to significant local flow velocities at the trash racks and increasing the risk of flow-induced vibration damage. Areas of high turbulence intensity exist at the top and bottom of the core flow, caused by the propagation of corresponding streams of vortical flow, which is identified as the primary cause of head loss in the inlet/outlet. Based on the average turbulence intensity, the inlet/outlet can be divided into zones of sharp turbulence increase, high turbulence intensity, and decreasing turbulence intensity. The vortical flow mainly includes three types of coherent structures: attached vortices, hairpin vortices, and quasi-streamwise vortices. Among these, the frequency of top hairpin vortices and the growth angle of hairpin vortex packets are highly susceptible to α. The growth direction of hairpin vortex packets closely aligns with the direction of turbulence intensity propagation. These findings contribute to optimizing the design of inlet/outlet of PSPS.

Experimental study of fuel distribution and combustion characteristics under subatmospheric pressure in an afterburner

The fuel distribution and ignition characteristics of an afterburner with typical components under subatmospheric inlet conditions are important references for improving the performance of the afterburner at high flight altitudes. In this work, lean ignition limits, outlet temperature distributions, fuel droplet size distributions and flame propagation processes in the afterburner were obtained experimentally. The results show that with increasing fuel injection angle, the fuel atomization characteristics improve, but the flame intensity fluctuation increases, and the ignition performance deteriorates; thus, a proper ratio of liquid-phase fuel to vapor-phase fuel is one of the key elements for successful ignition. On the one hand, the increased stabilizer blockage ratio results in a localized flow velocity increase near the trailing edge of the stabilizer, which effectively promotes fuel atomization in the shear layer, resulting in a fuel droplet size reduction of 25–30 μm. On the other hand, the extent of the recirculation zone increases, and a large low-pressure and low-turbulence recirculation zone is not conducive to fuel atomization in the flow direction. During the ignition process in an afterburner with all the typical components, the flame kernel first propagates along the flow direction for a certain distance and then propagates and anchors toward the shear layer. At subatmospheric pressures, the decrease in the intensity of the combustion reaction causes a reduction in the heat released by combustion, and the fuel droplet size increases by 10–25 μm, which requires more heat for evaporation. Therefore, the flame propagates an increased distance along the flow as the pressure decreases. Furthermore, the difficulty of ignition increases, and the ignition equivalence ratio increases by 0.1–0.15.

Flow-Mediated Dilatation: Learning Curve Study with a Novice Operator

BackgroundPerforming reproducible flow-mediated dilatation (FMD) measurements can be challenging, especially among inexperienced operators. Our aim was to present the detailed learning curve of a novice operator.MethodsFollowing a one-week basic training period, the operator performed duplicate measurements on 6–8 individuals per week, for six weeks. The operator followed the recommendations of the most recent guideline. Duplex ultrasound was used for the simultaneous and continuous measurement of brachial artery diameter and local blood flow velocity. Following a 1-min recording of baseline diameter (D), FMD was measured after a 5-min period of occlusion of the proximal forearm vessels. Inter-session coefficient of variation (CV) values for D and FMD were calculated for each week.ResultsThe number of volunteers assessed each week were the following: 8, 7, 7, 6, 7, 7 individuals. CV values for both D and FMD exhibited a decreasing trend over the training period, ending at 1.73% for D and 14.24% for FMD at week 6. These CV values are within the range outlined in the most recent FMD guideline for proficiency.ConclusionWithin a reasonable timeframe, and with careful adherence to measurement guidelines, the attainment of sound reproducibility in FMD measurements by a novice operator is feasible.

Haemodynamic study of left nonthrombotic iliac vein lesions: a preliminary report

Nonthrombotic iliac vein lesions (NIVLs) are significant causes of chronic venous insufficiency (CVI) in the left lower limb and symptom recurrence following left lower limb varicose vein treatment. The goal of this study was to explore the haemodynamic and morphological characteristics of iliac veins in patients with NIVLs. Pressure at the caudal end of the stenotic left common iliac vein (LCIV) segment, local blood flow velocity, and time-averaged wall shear stress in the stenotic segment exhibited positive correlations with the clinical CVI classification (R = 0.92, p < 0.001; R = 0.94, p < 0.001; R = 0.87, p < 0.001), while the relative retention time showed a negative correlation (R = -0.94, p < 0.001). The pressure difference (∆P) between the two ends of the stenotic segment and the velocity difference (∆V) between the stenotic segment and the caudal end were positively correlated with the clinical classification (R = 0.92, p < 0.001; R = 0.9, p < 0.001). The cross-sectional area stenosis rate and length of the stenotic LCIV segment were positively correlated with the clinical classification (R = 0.93, p < 0.001; R = 0.63, p < 0.001). The results suggest that haemodynamic assessment of the iliac vein could effectively portray blood flow disturbances in stenotic segments of the LCIV, potentially reflecting the degree of iliac vein stenosis. Haemodynamic indicators are correlated with the severity of clinical CVI symptoms.

Global-local state analysis and monitoring-based velocity measurement of oil–gas–water three-phase flow

The mixture fluids of oil–gas–water metamorphose through the change of phase, properties and structure. This poses a challenge to decipher their dynamic entanglement for phase velocity measurement. It is evident that progressively grasp of process states provides valuable prior information for further flow velocity measurement. As a significant flow characteristic, multiscale gives unique inspiration for flow state monitoring and velocity measurement from a global–local view. In this work, empirical wavelet transform-based Hilbert-Huang transform (EWT-based HHT) and multiscale robust empirical permutation entropy (Ms-rePE) are employed for the analysis of CWUD and conductance signals respectively, achieving the joint characterization of oil–gas–water flow structure by acoustic-electrical dual-mode information. On this basis, a data-mechanism dual-driven model for flow velocity measurement is established under a unified multiscale framework. And a novel approach of hypergraph-based global–local structure analysis (HGLSA) is proposed to simultaneously extract global–local information embodied in the complex flow process. The proposed scheme of global–local state monitoring-based flow velocity measurement is verified by the dynamic experiment of oil–gas–water three phase flow.

High-speed synchrotron X-ray imaging of melt pool dynamics during ultrasonic melt processing of Al6061

Ultrasonic processing of solidifying metals in additive manufacturing can provide grain refinement and advantageous mechanical properties. However, the specific physical mechanisms of microstructural refinement relevant to laser-based additive manufacturing have not been directly observed because of sub-millimeter length scales and rapid solidification rates associated with melt pools. Here, high-speed synchrotron X-ray imaging is used to observe the effect of ultrasonic vibration directly on melt pool dynamics and solidification of Al6061 alloy. The high temporal and spatial resolution enabled direct observation of cavitation effects driven by a 20.2 kHz ultrasonic source. We utilized multiphysics simulations to validate the postulated connection between ultrasonic treatment and solidification. The X-ray results show a decrease in melt pool and keyhole depth fluctuations during melting and promotion of pore migration toward the melt pool surface with applied sonication. Additionally, the simulation results reveal increased localized melt pool flow velocity, cooling rates, and thermal gradients with applied sonication. This work shows how ultrasonic treatment can impact melt pools and its potential for improving part quality.

Stereoscopic Micro-PIV measurement of the flow dynamics in a spherical dimple

One way to increase the thermal efficiency of heat exchangers is to structure the heat transfer surfaces with dimples, resulting in an enlarged surface area and intensified turbulence in the fluid flow. The increased turbulence also causes higher wall shear stress, which potentially suppresses the deposition of particles and supports a self-cleaning of the surface. For a deeper understanding of these phenomena, the flow dynamics inside the dimple were observed experimentally with Stereoscopic Micro-Particle Image Velocimetry (Stereo µPIV). The formation of an unsteady oscillating vortex, which leads to an asymmetric trail downstream of the dimple, is visualized. The significant influence of the dimple geometry on heat transfer enhancement is shown, and the most beneficial geometric ratio of the spherical dimple regarding its ability to increase turbulence is identified. A comparison of the local flow velocities with the results of the numerically and experimentally observed patterns of the deposited particles caused by the dimple’s self-cleaning effect shows a good match.

Behavior of hydrofoil cavitation in a slit channel

The paper presents the results of a cavitation in a slit channel study and offers an analytical description of cavity development. Special emphasis was placed on examining partial cavitation near a NACA 0012 hydrofoil (NACA – the National Advisory Committee for Aeronautics) inside slit channels of different geometries. Experimental investigation was carried out via high-speed imaging and the laser Doppler anemometry (LDA) method. The experimental data showed that the local flow velocity in hydrofoil leading edge area increased abruptly under arising cavitation. In addition, occurrence of cavitation raised flow velocity pulsation by 20%. In the case of a shorter channel, cavity growth occurred at higher cavitation numbers than for a longer channel. The cavity growth velocity was higher for a shorter channel. We showed that the tendency of partial cavitation development in the slit channel can be described as follows: L/C ∼ σ−1, where L is the cavity length; C is the hydrofoil chord; σ is the cavitation number; and parameter A changes as the slit channel length is varied. Comparison of cavitation development near hydrofoil at different attack angles α inside the slit channel with a three-dimensional (3D) cavitation tunnel was conducted. Cavitation in the slit channel occurred at lower σ/2α values compared to 3D cavitation flow around the hydrofoil. To directly compare lengths of the attached cavities arising in slit channels and 3D cavitation tunnels, an additional parameter is proposed, taking into account friction of the slit channel: K = λ·l/D. This parameter allowed us to quantitatively compare the characteristics of cavitating hydrofoils in slit channels and 3D tunnels. The paper provides the governing criteria of the cavitation in the slit channel. Our results propose the physical foundations for the development of cavities in the slit channel.

Study on pore behavior of laser welding process between steel and CFRP based on numerical simulation

In order to understand the pore behavior during laser welding of metal and polymer, the laser welding of steel and carbon fiber reinforced plastics (CFRP) was studied. A three-dimensional transient model of multi-physical field couple with heat, flow and force was constructed to simulate the temperature distribution, fluid flow, stress change and solidification under the thermal decomposition of CFRP based on the experiments. The formation, function and evolution mechanism of pores in welded joints were discussed, and the relationship between heat input and pore morphology at the interface was clarified. The results show that the model error is lower than 9 %, and the model is reliable. In laser welding process, bubbles generate, expand and fuse in the center of CFRP molten pool, which changes the local flow velocity and direction, but most of the fluid still flows to the molten pool edge under the action of temperature gradient, and forms pores of stable size with the cooling process. When the heat input is high, the large pores are formed at the interface due to the violent thermal decomposition of CFRP and big stress difference. When the heat input is low, there is still a large stress gradient in the center of the interface, and it is difficult for the insufficiently melted polymer to provide sufficient resistance to deformation, resulting in obvious pore phenomenon in the center of the interface.

Study on the liquid metal flow transitions behind a circular cylinder under the axial magnetic field

We study the magnetohydrodynamic flow around a circular cylinder confined in a rectangular duct. In this configuration, both the circular cylinder and the walls of the rectangular duct are electrically insulating, while the magnetic field aligns with the axial direction of the cylinder. The experimental measurements are performed by controlling two parameters Re and N (N is the ratio of electromagnetic forces to inertial forces) in the ranges of (180–722) and (0.8–264), respectively. Utilizing the electrical potential method, we employ both movable and wall probes to obtain the local flow velocities in the wake of the cylinder. By analyzing the space correlation of the signals obtained from the wall probes, a distinct transition in the flow behavior is observed, transitioning from a three-dimensional state to a quasi-two-dimensional (Q2D) state when the external magnetic field reaches a sufficient strength (N &amp;gt; 3). Additionally, the Q2D state allows for a further subdivision based on the scaling relationship Re/Ha∼(0.41–0.42) in the stability map, thereby distinguishing between steady and unsteady flow states, which is consistent with findings from previous studies.

The Effect of Deflector Angle on the Performance of Turbine Combination Darrieus-Savonius

The Savonius and Darrieus are vertical-axis wind turbines. This turbine has resistance on one side of the blade that makes it difficult for the shaft to rotate resulting in reduced efficiency. However, the vertical shaft turbine type provides the advantage of being able to accept wind speeds from any direction without an additional tail. Savonius and Darrieus turbines show different characteristics in terms of their performances. Savonius turbine has a high power coefficient (CP) in the range of low tip speed ratio (TSR), when its TSR increases the CP will fall. In contrast to Darrieus, high CP is achieved if the TSR is also high but the CP achievement will fall at a low TSR. If it is compared to Savonius, Darrieus has higher power efficiency although it is difficult to self-start. In contrast to Savonius, Darrieus has a better self-starting ability but a lower efficiency. This study aims to improve the performance of vertical shaft wind turbines by creating a new design combination of Darrieus and Savonius turbines with deflectors to produce CP achievement on a wide TSR. The combination of the Darrieus-Savonius turbine is to improve efficiency to make self-start easier. The research method uses numerical simulation by employing CFD Ansys software. On the airfoil with a deflector angle of 70 deg, it shows that there is an increase in speed in several parts of the Darrieus blade airfoil. The increase in speed causes the decrease of static pressure in the area. The pressure difference between the sides of the airfoil surface causes a force. The direction of the force causes the turbine shaft to rotate. The deflector acts as a directional headwind, increasing the local flow velocity to counter the resistance on one side of the rotor blades The average torque produced at an angle of 70 deg is 0.5 Nm. Whereas, at an angle of 90 deg, the average torque generated is lower by around 0.15 Nm.

Exploring the stabilization mechanism of NH3/CH4 non-premixed flames under gradient magnetic fields

This study explores the stabilization of non-premixed NH3/CH4 flames using gradient magnetic fields, overcoming challenges like low flame speed and instability inherent in ammonia—a carbon-free fuel and efficient hydrogen carrier. The action of the magnetic field modified the flame flow field, resulting in the suppression of emissions. By applying magnetic fields, the blending ratio of ammonia in the diffusion flame was elevated, resulting in an increase of 69% in the limit concentration to a near-pure level of 0.914. Measurements with Particle Image Velocimetry and schlieren system, alongside computational fluid dynamics, reveal how gradient magnetic fields mitigate flame detachment, lift-off, and blowout by altering flame root stabilization mechanisms. Specifically, gradient magnetic fields reduce localized flow velocity at the flame root and improve mass transfer. This indicates that gradient magnetic fields can significantly enhance the stability of NH3/CH4 flames, suggesting a promising route toward cleaner energy production and environmental sustainability.

Efficient survival strategy for zooplankton in turbulence

Zooplankton in a quiescent environment can detect predators by hydrodynamic sensing, triggering powerful escape responses. Since turbulent strain tends to mask the hydrodynamic signal, the organisms should avoid such regions, but it is not known how they accomplish this. We found a simple, robust, and highly efficient strategy that relies on measuring the sign of gradients of squared strain. Plankton following this strategy show very strong spatial clustering, and align against the local flow velocity, facilitating mate finding and feeding. The strategy has the potential to reconcile competing fitness pressures. Published by the American Physical Society 2024

Spatiotemporal patterns corresponding to phase synchronization and generalized synchronization states of thermoacoustic instability.

Thermoacoustic instability in turbulent combustion systems emerges from the complex interplay among the flame, flow, and acoustic subsystems. While the onset of thermoacoustic instability exhibits a global order, the characteristics of local interactions between subsystems responsible for this order are not well understood. Here, we utilize the framework of synchronization to elucidate the spatiotemporal interactions among heat release rate fluctuations in the flame, velocity fluctuations in the flow, and acoustic pressure fluctuations in a turbulent combustor, across the bluff-body stabilized flame. We examine two forms of thermoacoustic instability, characterized by phase synchronization and generalized synchronization of the acoustic pressure and global heat release rate oscillations. Despite the presence of global synchrony, we uncover a coexistence of frequency synchrony and desynchrony in the local interaction of these oscillations within the reaction field. In regions of frequency-locked oscillations, various phase-locking patterns occur, including phase synchrony and partial phase synchrony. We observe that the local formation of small pockets of phase synchrony and strong amplitude correlation between these oscillations is sufficient to trigger the state of global phase synchronization. As the global dynamics approach generalized synchronization, these local regions of synchrony expand in the reaction field. Additionally, through coupled analysis of acoustic pressure and local flow velocity fluctuations, we infer that the spatial region of flow-acoustic synchrony plays a significant role in governing thermoacoustic instabilities. Our findings imply that, in turbulent combustors, an intrinsic local balance between order, partial order, and disorder within the coupled subsystems sustains the global order during thermoacoustic instability.

Experimental and numerical investigation of the aerodynamic characteristics of high-performance vehicle configurations under yaw conditions

This study investigates the impact of yaw conditions on the aerodynamic performance and flow field of three high-performance vehicle model configurations by means of wind tunnel testing and unsteady Reynolds-Averaged Navier–Stokes-based computational fluid dynamics simulations. While yaw effects on automotive vehicles have been explored, the effects on far more complex flow fields of high-performance vehicles remain insufficiently researched. This paper reveals that yaw conditions have a significant negative influence both downforce and drag performance. Spoiler and rear wing devices enhance downforce but increase the vehicle's sensitivity to yaw. Furthermore, yaw conditions significantly alter vortex structures and local flow velocities, affecting downstream flow behavior. Surface pressure measurements on the slant confirm these findings and highlight notable yaw effects and upstream effects from spoiler and rear wing devices. Wake analyses through total pressure measurements show that yaw induces a substantial deviation from straight-line wake characteristics, which become dominated by an inboard rotating vehicle body vortex. Overall, this research enhances the understanding of the effects of yaw conditions on high-performance vehicle aerodynamics and provides valuable data for future vehicle aerodynamics research in real-world operating conditions.

4D Neutron Imaging of Solute Transport and Fluid Flow in Sandstone Before and After Mineral Precipitation

AbstractIn many geological systems, the porosity of rock or soil may evolve during mineral precipitation, a process that controls fluid transport properties. Here, we investigate the use of 4D neutron imaging to image flow and transport in Bentheim sandstone core samples before and after in‐situ calcium carbonate precipitation. First, we demonstrate the applicability of neutron imaging to quantify the solute dispersion along the interface between heavy water and a cadmium aqueous solution. Then, we monitor the flow of heavy water within two Bentheim sandstone core samples before and after a step of in‐situ mineral precipitation. The precipitation of calcium carbonate is induced by reactive mixing of two solutions containing CaCl2 and Na2CO3, either by injecting these two fluids one after each other (sequential experiment) or by injecting them in parallel (co‐flow experiment). We use the contrast in neutron attenuation from time‐resolved tomograms to derive three‐dimensional fluid velocity field by using an inversion technique based on the advection‐dispersion equation. Results show mineral precipitation induces a wider distribution of local flow velocities and leads to alterations in the main flow pathways. The flow distribution appears to be independent of the initial distribution in the sequential experiment, while in the co‐flow experiment, we observed that higher initial local fluid velocities tended to increase slightly following precipitation. The outcome of this study contributes to progressing the knowledge in the domain of reactive solute and contaminant transport in the subsurface using the promising technique of neutron imaging.

Modeling Aircraft Fuel Jettison Using Smoothed Particle Hydrodynamics on Finite-Volume Meshes

Weakly compressible smoothed particle hydrodynamics is used to investigate aircraft fuel jettison using a single-phase model. Fuel simulations are coupled to the aircraft computational fluid dynamics flowfield using cell containment checks on the finite-volume mesh to locate any smoothed particle hydrodynamics (SPH) particle within the mesh, after which the local flow velocity vector is retrieved and then used to apply an approximate aerodynamic force to the SPH particle based on a continuum correction to discrete droplet calculations. Further downstream, the SPH simulation may be continued, or a switch may be made to implicit particle tracking (IPT) in order to accelerate the simulation. Comparison to IPT results shows that a fluid model of the initial continuum breakup of the jet is required, but following this transition to IPT is reasonable to reduce computation time. Models are validated against volume of fluid (VOF) simulations, with the runtime of the proposed model being approximately 100 times less than the VOF, and good qualitative agreement is also found compared to recorded flight results.

Solar Wind Current Sheets: MVA Inaccuracy and Recommended Single‐Spacecraft Methodology

AbstractWe present the analysis of 1,831 current sheets (CS) observed aboard four Cluster spacecraft in a pristine solar wind. Four‐spacecraft estimates of the CS normal and propagation velocity are compared with different single‐spacecraft estimates. The Minimum Variance Analysis (MVA) of the magnetic field is shown to be highly inaccurate in estimating the normal. The MVA normal often differs by more than 60° from the normal obtained by multi‐spacecraft timing method, likely due to ambient turbulent fluctuations. In contrast, the cross‐product of magnetic fields at the CS boundaries delivers the normal with an uncertainty of less than 15° at the confidence level of 90%. The CSs are essentially frozen into plasma flow, since their propagation velocity is consistent with local ion flow velocity within 20% at the confidence level of 90%. The single‐spacecraft methodology based on the cross‐product method and frozen‐in assumption delivers the CS thickness and current density amplitude within 20% of their actual values at the confidence level of 90%. The CSs are kinetic‐scale structures with half‐thickness λ from a few tenths to tens of local proton inertial length λp and scale‐dependent shear angle and current density amplitude, and . The classification of the CSs in terms of tangential and rotational discontinuities remains a challenge, because even the four‐spacecraft normal has too large uncertainties to reveal the actual normal magnetic field component. The presented results will be valuable for the analysis of solar wind CSs, when only single‐spacecraft measurements are available.