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Propeller cavitation is one of the main causes of pressure fluctuation and noise around marine propellers, and tip vortex cavitation is one of the main causes of high-frequency underwater radiated noise. To predict tip vortex cavitation, it is necessary to correctly describe the radius of the vortex core and the vortex circulation of the tip vortex. Therefore, in this study, non-cavitating flow field measurements around the tip of model propellers were made for the 0.75 m diameter working section of the large cavitation tunnel at the National Maritime Research Institute, Japan (NMRI) using 2D-PIV. Identification procedures were investigated to develop a tip vortex model. The vortex properties (radius of vortex core and vortex circulation) of the tip vortex were obtained by applying by a Rankine vortex model and a Burgers vortex model. The measured velocity and vorticity values around the tip vortex as identified by the Burgers vortex model were in better agreement than those given by the Rankine vortex model. The Burgers vortex model was suitable for obtaining vortex properties from the measured flow field around the tip vortex. The vortex properties obtained from the identification using the Burgers vortex model showed the Reynolds number demonstrated a greater effect on the radius of the vortex core and vortex circulation. The higher the Reynolds numbers, the smaller the radius of the vortex core and the smaller vortex circulation tends to be. It is also shown that this Reynolds number effect differs depending on the blade shape of the propellers. [ABSTRACT FROM AUTHOR]

The objective of this study was to explore the higher-order turbulence statistics of flow and turbulent length scales around a complex pier (CP) and compare the results with those for a simple pier (SP). The velocity data were recorded in a laboratory flume using particle image velocimetry (PIV). The PIV data were analysed to estimate the velocity fluctuations of third order, turbulence production and dissipation rates, turbulent length scales and the contributions of burst–sweep events to the total Reynolds shear stress around the piers mounted on a rigid flat surface. The skewness and advection coefficients indicated a more asymmetric distribution of velocity fluctuations for the SP than for the CP. Ejection and sweep events illustrated the dominance of similar strengths for an extended period, downstream of the piers. The upstream turbulent kinetic energy production was similar for both piers, while the upstream dissipation rate was higher for the CP. The length scales were greater at the upstream for the case of the SP while, downstream, they were greater for the CP. The findings of this work demonstrate the importance of a pile cap in restricting downward-moving flow by showing a lower magnitude of scour-inducing turbulence parameters for the CP as compared with the SP. [ABSTRACT FROM AUTHOR]

Vacuum preloading combined with prefabricated vertical drains is a widely used system for improving soft clay-dredged slurry deposits. However, the influence of the initial water content on clogging is not yet fully understood. In this study, laboratory model tests were conducted on slurry with initial water contents of 106%, 133%, and 159% during vacuum preloading. The process of vacuum preloading was monitored from the perspectives of discharged water volume, pore water pressure, as well as the displacement and strain profile of soil patches by using the novel particle image velocimetry (PIV) technique. The results showed that the clogging formation process could be divided into three stages based on the water discharge rate: rapid formation, transition, and slow formation. An increase in initial water content resulted in a larger horizontal displacement, leading to a thinner and denser clogging zone with lower permeability coefficients and a reduced rate of pore pressure dissipation. Empirical equations for the thickness of the clogging zone versus time were proposed using the PIV technique, providing a reference for theoretical consolidation calculations and making the calculations more practical by considering the variation of the clogging zone during vacuum preloading. [ABSTRACT FROM AUTHOR]

This study investigates the performance of two types of multi-encapsulated electrode (MEE) plasma actuators, compared to typical dielectric barrier discharge (DBD) plasma actuators, in quiescent air. The objective is to determine whether the multiple encapsulated structure can enhance the performance of the plasma actuator. In the present paper, flow characteristics are investigated by using particle image velocimetry (PIV) and Schlieren visualisation. In addition, the distribution of body force over the gas volume based on the Navier–Stokes equations is calculated from velocity measurements. The obtained results demonstrate that the starting vortex behavior is influenced by electrode arrangement. Specifically, it can be observed that when the first encapsulated electrode is positioned closer to the exposed electrode, then a significantly higher induced velocity can be obtained compared to the baseline condition. In fact, the induced velocity can be increased by up to 1.5 times under this optimize configuration. These results highlight the importance of electrode arrangement in the plasma actuator design. Based on body force estimation, MEE plasma actuators exhibit a significantly higher momentum transfer, particularly in the wall normal direction. The investigation on the mechanical efficiency also reveals that the optimized configuration proposed in the present study can significantly enhance the efficiency. In fact, a four-fold increase in maximum efficiency compared to the typical configuration is observed. These results suggest that the proposed configuration could be considered a promising solution for improving the mechanical efficiency of plasma actuators. [ABSTRACT FROM AUTHOR]

Bioconvection is a fascinating phenomenon observed in various biological systems, where the motion of motile microorganisms generates fluid flow patterns. This article explores the occurrence and characteristics of bioconvection within the context of a jet flow. The study of bioconvection in jet flow involves the interaction between motile microorganisms and the fluid dynamics of the surrounding medium. Microorganisms such as bacteria and algae are known to exhibit directed swimming behavior, which can lead to the formation of dynamic flow structures. Investigating the mechanisms underlying bioconvection in jet flow requires a multidisciplinary approach encompassing fluid dynamics, microbial ecology, and mathematical modeling. Experimental techniques, such as microscopy and particle image velocimetry, along with computational simulations, are employed to analyze the complex interactions between microorganisms and the fluid flow. In this regard, a supportive mathematical model is designed using partial differential equations (PDEs) which are later transformed into ordinary differential equations using similarity transformations. The resulting system of equations is solved using the RKF-45 method and the outcomes are recorded in tables and graphs. The consideration of thermophoresis has shown a significant impact on the heat and mass transfer of the jet flow and both these profiles are observed to increase with thermophoresis. Meanwhile, the Schmidt number decrease their respective mass profiles. Furthermore, the porosity is found to create a drag force which tends to oppose the fluid flow. [ABSTRACT FROM AUTHOR]

Constitutive models in the literature for creep of frozen soil are based on the direct use of time counted from the onset of creep. An explicit time dependence in a constitutive equation violates the principles of rational mechanics. No change in stress or temperature is allowed for during creep, using the time-based formulations. Moreover, the existing descriptions need much verification and improvement on the experimental side as well. Creep behaviour of artificially frozen sand was evaluated experimentally. Novel testing methods were used, and new insights into the creep behaviour of frozen soil were gained. Creep rate under uniaxial compression was examined with different kinds of interruptions, like unloadings or overloadings. Experimental creep curves were presented as functions of creep strain. They were brought to a dimensionless form which describes the creep universally, despite changes in stress or temperature. Possible anisotropy of frozen soil was revealed in the creep tests on cubic samples with changes of the loading direction. Using the particle image velocimetry (PIV) technique, information on the lateral deformation and the uniformity of creep were obtained. Volumetric creep of unsaturated frozen soil under isotropic compression was demonstrated to be due to the presence of air bubbles only. [ABSTRACT FROM AUTHOR]

Background: Nasal airway obstruction (NAO) is characterised by high resistance in the nasal cavity with a collapsible and narrowed upper airway and is an integral part of OSA pathophysiology. The literature demonstrates that the identification of high-risk OSA in the young adult population leads to the prevention of later health consequences. A nasoorospirometer is a prototype device that measures nasal capacity during inspiration. The basis for measurement is a Wheatstone bridge and a thermal anemometer. The parameters are recorded via hot wire anemometry (HTA) with velocity measurements in the airflow field. Therefore, this pilot study aimed to test the feasibility of the device by examining a young adult sample. The secondary aim was to determine whether subjective NAO correlates with nasal capacity and whether NAO corresponds with anthropometric parameters and individual risk of OSA. Methods: A group of 31 participants (mean age 24.9 years) underwent a thorough laryngological examination. The nasoorospirometer was used to measure objective NAO (nasal capacity), the NOSE scale was used to gain subjective NAO evaluation, and the Berlin Questionnaire for the risk of OSA. Results: A correlation analysis confirmed no significant associations between the subjective and objective measures (p > 0.05). Higher BMI and neck circumference are associated with lower NAO and higher nasal patency in the population of young adults (r: 0.32–0.45; p < 0.05). The risk of OSA showed no statistically significant association (p > 0.05). Conclusions: We presented three methods of NAO assessment: subjective participant evaluation, objective nasoosopirometry, and objective laryngological assessment. However, the use of a nasoorospirometer with anthropometric measures in young adults needs to be verified in future studies. [ABSTRACT FROM AUTHOR]

The influence of apparent slip on oscillatory shear measurements of a viscoplastic microgel [0.6 wt. % of poly(acrylic acid)] is analyzed by Couette and parallel-plate rheometry and particle image velocimetry (Rheo-PIV). We first provide direct evidence of a critical shear stress for the onset of slip of the microgel under oscillatory (σos) and nonoscillatory measurements (σs). Afterward, we describe the effect of slip on oscillatory measurements via waveforms, Bowditch–Lissajous curves, Fourier transform (FT) rheology, PIV, and as a sequence of physical processes (SPP). The effect of slip is mainly observed at low oscillating frequencies. For amplitudes of the oscillating stresses σ0 ≤ σos, the microgel exhibits linear viscoelastic behavior with in-phase strain response. For σos< σ0 ≤ yield stress (σy), slip introduces a phase shift in the strain response with a forward-tilted waveform and "mango" shape Bowditch–Lissajous curves. Meanwhile, FT rheology shows negligible even harmonics. The strain measured by the rheometer does not match the true strain determined by PIV in the presence of slip, resulting in waveforms that depend on how the displacement distribution is interpreted. This result indicates a break in the symmetry of the flow, that is, the microgel response no longer follows the imposed oscillation, which makes any attempt to correct oscillatory data for slip complex. This behavior arises from recoil of the slipping microgel after reaching its maximum displacement in a cycle. Finally, we provide an overall picture of the kinematics of the process of yielding in the presence of slip as an SPP. [ABSTRACT FROM AUTHOR]

In this paper, granular flow is studied using particle image velocimetry (PIV) in two different geometries, that is, rectangular box and cylindrical bladed mixer. Effects of parameters like blade rake angle, bed height, moisture content, and the number of blades were studied. The shear rate and velocity magnitude of granular bed were calculated using PIV. The presence of moisture in the granular bed added additional cohesive forces and reduced shear rate between different layers of particles. Shear rate and shear forces were maximal at the tip of the blade. Temperature variations in the granular bed were measured experimentally and results were compared for static and mixing beds. [ABSTRACT FROM AUTHOR]

The curved surfaces widely exist in three-dimensional inward-turning inlets and variable sectional isolators, which determine the downstream evolution of the converged flow generated by the crossing shock wave/boundary layer interaction, thereby affecting the start performance and combustion efficiency of the hypersonic vehicles. In this work, the converged flow subjected to the curved surfaces is experimentally studied with the nanoparticle-based planar laser scattering method and particle image velocimetry. Results show that the velocity profiles of the converged flow on the transverse plane can be normalized at different angles. The upstream separation size is found to be decreased by the downstream convex surface due to the reduction of backpressure, especially in large-radius cases, whereas the converged flow gradually becomes smooth ought to the dependence of the acceleration effect on the velocity. The concave surface shows an opposite effect on the converged flow, which further indicates that a downstream convex surface is preferred for hypersonic inlets to weakens the flow non-uniformity to improve the inlet performance. • Converged flow generated by the crossing shock wave/boundary layer interaction on the transverse plane is visualized semi-elliptical. • Velocity profile of the converged flow on the transverse plane can be normalized at different angles and expressed in the logarithmic form. • The convex surface makes the converged flow dissipated under the momentum exchange and the expansion effect. • A downstream convex surface is preferred for inlet design. [ABSTRACT FROM AUTHOR]

Interparticle friction is an intrinsic property of particles, which plays an important role in the macroscopic and microscopic shear mechanical properties of granular materials. In this research, we investigate the shear behavior of granular materials with different friction coefficients using ring shear tests. The particle image velocimetry (PIV) technique was also used to analyze the shear flow characteristics. The results indicate that the peak shear strength of granular materials increases with the increase in shear rates, especially for granular materials with high friction coefficients. The shear stress fluctuation difference is smaller under low normal stress. Under high normal stress, the shear stress fluctuation of granular materials with high friction coefficient is higher than that of granular materials with low friction coefficient. In addition, the shear stress fluctuation shows a trend of increasing with the increase of shear rates. The range of the liquid phase flow region of granular materials decreases with the increase of friction coefficient and normal stress. This work reveals the shear flow characteristics of granular materials under different conditions, which can provide reference for the flow processes of geological disasters such as landslides and debris flows. [ABSTRACT FROM AUTHOR]

A series of shaking table tests were performed on reduced-scale models of helical soil-nailed walls (HSNWs) to evaluate the effect of the nail arrangement and nail inclination on the failure mechanisms and dynamic characteristics of this type of retaining structures under seismic conditions. The results of particle image velocimetry (PIV) showed that the potential failure surfaces in the helical soil-nailed walls were parabolic ones with an inflection point and the dimensions of failure wedge increased as the length and inclination of the nails increased. A combination of overturning and base sliding was identified as the predominant deformation mode in the HSNWs and that base sliding faded with an increase in the nail inclination. It was found that horizontal helical nails located in the lower half of the wall played a more effective role in reducing lateral displacement, but the opposite was true for HSNWs with inclined nails. The use of inclined nails instead of horizontal ones was found to be an efficient solution for increasing the shear modulus in HSNWs. The efficiency of this solution decreased with the use of shorter nails in the upper half of the walls and was eventually minimized by increasing the length of the nails across the wall height. It was found that, although the use of helical nails instead of grouted ones reduced wall damping, it could be a good solution for increasing the stiffness of the soil-nailed walls. [ABSTRACT FROM AUTHOR]

Flow inside the axial test turbine VT-400 is measured by using a standard methodology of Particle Image Velocimetry (PIV). The studied area lies between the stator and rotor wheel in meridional plane. This area covers both: the regular flow near hub and the endwall secondary structures as well. Regular structure of wakes and jets past stator blades is observed in terms of mean velocities and turbulence intensity. The fluctuations are close to isotropy and the energy distribution across length-scales exhibits almost Kolmogorov scaling. Near the hub, we observe secondary vortices. Despite the positive radial ejections in that regions, these secondary vortices drift towards the hub, they dissipate energy and circulation and their core grows in size. [Display omitted] • Particle Image Velocimetry measurement in Stereo configuration inside test axial turbine. • Meridional (i.e. axial × radial) plane past the stator wheel endwall at underloaded, nominal and overloaded regime. • Observation of wakes and jets; wake/jet amplitude do not follow velocity magnitude; periodicity corresponds to geometry of stator wheel. • Fluctuations in region of wakes and jets up to 10%; spatial spectra follows Kolmogorov law with depletion at the wake periodicity. • Secondary flow contain radial ejections and secondary vortices of single orientation; these vortices drift towards the hub. [ABSTRACT FROM AUTHOR]

• A series of novel chitosan-based magnetic flocculants was prepared by molecular structure control. • Fe 3 O 4 @SiO 2 @CTS-P(AM-DMC) can effectively removal turbidity from 200 to 2000 NTU. • Magnetic flocs form eddy currents and experience acceleration, stabilization, and deceleration stages. • Charge neutralization was dominant under acidic and neutral conditions, while adsorption and bridging effects play an important role in alkaline environments. A series of novel chitosan-based magnetic flocculants FS@CTS-P(AM-DMC) was prepared by molecular structure control. The characterization results showed that FS@CTS-P(AM-DMC) had a uniform size of about 21.46 nm, featuring a typical core-shell structure, and the average coating layer thickness of CTS-P(AM-DMC) was about 5.03 nm. FS@CTS-P(AM-DMC) exhibited excellent flocculation performance for kaolin suspension, achieved 92.54% turbidity removal efficiency under dosage of 150 mg/L, pH 7.0, even at high turbidity (2000 NTU) with a removal efficiency of 96.96%. The flocculation mechanism was revealed to be dominated by charge neutralization under acidic and neutral conditions, while adsorption and bridging effects play an important role in alkaline environments. The properties of magnetic aggregates during flocculation, breakage, and regeneration were studied at different pH levels and dosages. In the process of magnetophoretic, magnetic particles collide and adsorb with kaolin particles continuously due to magnetic and electrostatic attraction, transform into magnetic chain clusters, and then further form three-dimensional network magnetic aggregates that can capture free kaolin particles and other chain clusters. Particle image velocimetry confirmed the formation of eddy current of magnetic flocs and experienced three stages: acceleration, stabilization, and deceleration. [Display omitted] [ABSTRACT FROM AUTHOR]

The flow field of a hydrocyclone was investigated using both computational fluid dynamics (CFD) and particle image velocimetry (PIV). A refractive index matching method was employed to improve the precision of the PIV measurements. The CFD results are in good agreement with PIV measurements. Detailed analysis reveals significant axial asymmetry in the velocity components, with the radial velocity component exhibiting notable disparities. This observation is supported by quantitative data comparing different sections of the hydrocyclone. It is further found that the asymmetry might be mainly attributed to the secondary vortexes with the single inlet of the hydrocyclone. And the secondary vortexes, superimposed on the primary flow rather than existing on its own, spiral downwards from near the inlet towards the underflow orifice. It is hypothesized that specific boundary effects and pressure gradients play a pivotal role in the formation of secondary flows. This assumption is grounded on both theoretical considerations and empirical observations, suggesting that these factors significantly influence the flow dynamics within the hydrocyclone. The insights gained from our measurement methodology and enhanced understanding of secondary flows within hydrocyclones are not only poised to serve as valuable references for fellow researchers but also have the potential to inform the design and operational optimization of hydrocyclones for improved efficiency and performance. [ABSTRACT FROM AUTHOR]

In recent years, morphing wings have become not only a concept, but an aerodynamic solution for the aviation industry to take a step forward toward future technologies. However, continuously morphing airfoils became an interesting answer to provide green energy solutions. In this paper, the authors conducted experimental research on a continuously camber-morphing airfoil using the Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) methods. The main objective of this work was to research a variety of morphing airfoils with different camber deflections. An average velocity distribution and turbulence distribution were compared and are discussed. The two-dimensional PIV results were compared to the CFD simulations to validate the numerical method's accuracy and obtain the aerodynamic coefficient's trends. A further comparison revealed that morphing airfoils have better aerodynamic performance than conventional airfoils for very low camber deflections and create substantial amounts of drag for significant camber deflections. [ABSTRACT FROM AUTHOR]

This paper proposes a light field (LF) three-dimensional (3D) particle image velocimetry (PIV) method based on a digital refocused algorithm and 3D U-Net neural network for 3D three-component (3D-3C) velocity measurement. A digital refocused algorithm is used to generate a stack of LF-refocused images of tracer particles for establishing the 3D U-Net. The 3D U-Net is then used for the 3D particle field reconstruction. Based on a pair of 3D particle fields, the 3D-3C velocity field is obtained through a 3D cross correlation algorithm. Numerical simulations and experiments are conducted to analyze the accuracy and efficiency of the proposed method. The simulation results show that the elongation along the depth direction and the efficiency of the 3D particle field reconstruction are improved by the 3D U-Net. The 3D U-Net also provides a better correlation coefficient. The experimental results show that the reconstruction time of the proposed method is ∼220 s which is 10 times faster than the LF tomographic PIV. This further demonstrates that the proposed method improves the reconstruction efficiency without affecting the accuracy of velocity measurement. [ABSTRACT FROM AUTHOR]

This article details the construction of an experimental visualization platform for observing cavitation. The platform uses high-speed photography and particle image velocimetry (PIV) techniques to conduct experimental research into the flow pattern and vortex field of cavitation in Venturi tubes. Dynamic mode decomposition is employed to extract the energy distribution characteristics of the cavitation flow field. Cavitation occurs at an exit-to-inlet pressure ratio of 0.595, and the length of the cavitation zone increases as this ratio decreases. When the pressure ratio reaches 0.280, the flow rate remains almost constant and the flow becomes chocked. The cavitation shape evolves periodically in the chocking flow, and the cavitation zone can be divided into three parts: an initiation and development area, a fusion area, and a collapse area. The fusion area exhibits periodic changes in the form of contraction, expansion, and re-contraction. Near the wall, the collapse area exhibits complex boundary conditions, with re-entrant jet causing bubble aggregation, rolling, and shedding. PIV and energy extraction reveal that vortices primarily appear near the wall, where they undergo a periodic process of fragmentation and fusion. The strength of the vortices exhibits a small–large–small pattern that is related to the cloud aggregation, rolling, and shedding caused by the re-entrant jet. [ABSTRACT FROM AUTHOR]

This study employs time-resolved particle image velocimetry technology to investigate the flow field characteristics of a double-wall jet structure under both stationary and rotating conditions. The jet rotation number range from 0 to 0.08, covering both clockwise and counterclockwise rotations, with a jet Reynolds number of 4000. The dimensionless jet-to-target spacing is 2. In order to elucidate the flow mechanisms based on experimental results and provide complementary insight, validated numerical simulations under conditions identical to the experiments were conducted. This paper considers average velocity and Reynolds stress and utilizes the proper orthogonal decomposition method to study flow characteristics. The results indicate that the rotation-induced Coriolis force, centrifugal force, and radial pressure gradient in the radial direction influence the jet to deflect. The pressure gradient generated by the centrifugal force weakens its effect, making the jet deflection primarily dominated by the Coriolis force. The deflection direction and degree of the jet differ with the rotation direction and increase with the rotation number. When the rotation direction is reversed, differences in the direction of the force acting on the fluid lead to variations in the secondary flow structures of the jet; the secondary flow structures within the impingement holes exhibit two-vortex or four-vortex structures under counterclockwise or clockwise rotation, respectively. At the highest rotation number, jet deflection intensifies mixing with the surrounding fluid, resulting in a reduction of about 30% in the velocity peak compared to the stationary state but an increase of approximately 0.6 times in turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

Suppression of vortex–airfoil interaction noise from a rod-airfoil model by virtual serrations and surface morphing formed by the leading-edge (LE) blowing was investigated experimentally in an anechoic wind tunnel. The control efficiency of the virtual serrations and surface morphing was evaluated and analyzed by setting different air flow rates (Q), deflection angles (α), and orifice number (M). Noise measurements by far-field microphones show that both can effectively reduce the peak tonal noise generated by the vortex–airfoil interaction, with the control efficiency increasing with the increment of flow rate Q. As for the LE serration, the noise reduction increases with the virtual serration amplitude ratio Av (=Ub/U∞, Ub: blowing velocity; U∞: wind speed), but decreases slightly with the deflection angle α. A reduction of 12–14 dB is obtained when Av = 3.7 and α = 0° or 10°, and there exists a critical amplitude of Av = 1.5, under which no noise reduction is achieved. Compared to the serration at the same flow condition, the virtual surface morphing has much lower control efficiency, with a maximum noise reduction of 3–5 dB. The flow visualization by the particle image velocimetry technique reveals that both build buffer zones in the front of the airfoil LE, preventing the vortices from directly impinging upon the solid LE, thus reducing the intensity of vortex–airfoil interaction. In particular, the virtual serration breaks up the large-scale vortex structure into small-scale vortices, manifesting its high noise control efficiency. [ABSTRACT FROM AUTHOR]

With the micro-miniaturization of offshore wireless sensors, signal lights, and other devices and the emergence of the problem of self-powering in the distant sea, how to harvest energy from low-speed currents has become a hot spot of research nowadays. To improve the energy output power and conversion efficiency of low-speed water flow, we propose a vertical cantilever beam circular cylinders fitted with a rigid splitter plate piezoelectric energy harvester (CSPPEH). In this paper, the influence of the length and the attack angle of the splitter plate on CSPPEH has been experimentally investigated. The vibration response mechanism involving the mutual transition between vortex-induced vibration and galloping was analyzed through particle image velocimetry flow field visualization. The experimental results indicate that the vibration and piezoelectric characteristics of the CSPPEH increase initially and then decrease with the length of the splitter plates (L/D = 0–2.4) at the attack angle of 0°, which can be explained by the theoretical model of the energy harvester. It is found that the optimal vibration and piezoelectric characteristics occur at a rigid splitter plate length of 1.40D with an attack angle of 90°. The maximum values for amplitude, vibration swing angle, voltage, power, and power density are 4.96D, 21.7°, 42.68 V, 910.81 μW, and 1.94 mW/cm3, respectively. Efficiency was up to 2.2% at 0.4D length and 90° attack angle of the splitter plate. Compared to the bare circular cylinder energy harvester, the output power and efficiency are significantly improved. The demonstration of continuous charging and discharging of capacitors and light emitting diode lights is performed to show the practicability of the designed CSPPEH. Overall, the present study enables the applications of CSPPEH for realizing self-powered wireless sensing and signal lights under low-water-speed environments. [ABSTRACT FROM AUTHOR]

Jets arrays have become a promising technology for industrial applications, including heat transfer and combustion processes. This paper presents the first experimental results of arrays of bidirectional swirling jets, including their flow structures and specific turbulent interactions. Particle image velocimetry of non-reacting linear arrays showed the formation of recirculation zones within each jet of the array. These regions are the most stable, with fluctuations less than 0.002 of the bulk inlet velocity Vin. In addition, jets merging locations also show a significant decrease in velocity fluctuations, with values V′/Vin < 0.005, which is most clearly seen at a smaller pitch distance. However, the highest turbulent fluctuations occur in shear layers, reaching values of V'/Vin ≈ 0.02 for linear arrays and V'/Vin ≈ 0.03 for planar configurations. The flow structure of planar arrays is more complex, leading to the formation of secondary vortex structures at locations of jet interaction. This results in a significant decrease in the flow swirl number, from an initial value of Sin = 2.1 in the vortex chamber, to S1 = 0.36–0.49 at z/dout2 = 1 and S2 = 0.19–0.27 at z/dout2 = 2 beyond the outlet nozzle. Strong cross-sectional motions are observed in planar arrangements, leading to the development of additional regions of negative axial velocity between the outlet nozzles. The boundaries of these regions have nearly zero axial velocity values and can provide reliable ignition and flame stabilization in the case of reactive bidirectional jets. Therefore, the flow structure in planar arrays forms a large stabilization zone, and the obtained results can be used to develop a new multipoint combustion technique. [ABSTRACT FROM AUTHOR]

In this work, we studied the wave-induced vortex generation and shedding from a semi-submerged vertical square cylinder interacting with an upcoming undular bore wave train under a shallow water configuration. This unsteady and rapid process was investigated by means of numerical and experimental approaches. A numerical simulation, solving the full turbulent viscous Navier–Stokes equations, was carried out in order to study and characterize both the undular bore wave properties and the vortex dynamics triggered during this unsteady interaction. Starting with fluid at rest, the undular bore was generated by the impulsive translational motion of a piston wavemaker at laboratory scale in both a numerical and an experimental wave tank. When the undular bore impinges on the cylinder, filamentary vortex structures were formed at the four cylinder's edges synchronized with the propagating wave motion, leading to the vortex shedding phenomena at a frequency that matched the wave instantaneous frequency. These vortices extended along the entire cylinder span under the water column, from the free surface to the seabed. At the trailing edge of the cylinder, a pairing process of two shed vortices was observed, similar to a Lamb–Oseen vortex pair. These vortices were present during the whole undular bore wave train dynamic forcing. An overall agreement was found with the experimental version of the bore–cylinder interaction, carried out in a physical wave tank. Laser sheet bore profiling and particle image velocimetry measurements of the velocity field confirmed undular bore properties, the onset of vortex formation, subsequent shedding, and pairing in the experiments performed in similar conditions with the numerical approach. [ABSTRACT FROM AUTHOR]

The interaction between liquid spray and the surrounding air is crucial in fluid research, especially in the study of fuel spray and combustion. However, the fuel spray–air interaction is a complex process influenced by multiple factors, including fuel type, fuel injection pressure, and fuel temperature. These factors are coupled together, making it challenging and time-consuming to accurately capture the spray–air data using traditional experimental methods alone. The current study proposes a hybrid physics-based and machine learning model for utilizing spray images to reconstruct ambient flow fields. The novelty of this work lies in leveraging the spatial characteristics of spray and airflow data to optimize feature extraction and reduce unnecessary nonlinearity in the model. Consequently, the model offers complementary advantages, improving model interpretability and reducing its reliance on massive data. The training dataset is collected using a combined diagnostic approach, utilizing Mie-scattering imaging and fluorescence particle image velocimetry. The liquid spray and the ambient air velocity field are measured simultaneously under a wide range of experimental conditions, including different fuel types, fuel injection pressures, and fuel temperatures. The reconstruction results are validated against unseen experimental data. In general, the reconstruction results indicate that the model is accurate, fast, and robust for different fuel conditions and injector types. It provides an innovative way to reconstruct airflow fields based on spray images (spray density distribution). These findings highlight the potential of integrating physics-based and machine learning methods for multiphase flow diagnostics, paving the way for broader data-driven applications in fluid research. [ABSTRACT FROM AUTHOR]

Off-gas systems are implemented in molten salt reactor designs to control the release of gaseous fission products. Two-phase flow in molten salt must be studied to understand how the system will behave in comparison to traditional working fluids like water. Flow visualization experiments and particle image velocimetry measurements were performed for three argon bubble sizes injected into a co-current stream of molten salt in a natural circulation loop facility. Similar bubble sizes were injected in experiments with water to compare the bubble shape, trajectory, and wake flow behavior of the fluids. The bubble region of interest was used to calculate the equivalent diameter and terminal velocity. Results for water showed a wobbling bubble surface and less stable bubble trajectory due to lower surface tension and viscosity compared with molten salt. Particle image velocimetry results demonstrated the increased viscosity of salt dampens turbulent fluctuations for the smaller bubble size. For a cap bubble, turbulent fluctuations were larger and longer lasting than in results for the wake flow of an argon cap bubble in water. [ABSTRACT FROM AUTHOR]

The swirl cup airblast fuel injector is widely used in modern low-emission gas turbine combustors. It is important to understand the underlying physics in the interaction between fuel spray and complex swirling airflow. The present study investigates a swirl cup airblast fuel injector's swirling flow and spray atomization characteristics. Various laser-based diagnostics, including planar Mie scattering, phase-Doppler particle analyzer, and high-speed particle imaging velocimetry, were employed for this purpose. The swirl cup airblast fuel injectors, featuring different outer swirler swirl numbers (0.8 < S N < 1.8) and inner–outer relative swirling directions, were investigated in an air Reynolds number ranging from 0.77 × 106 to 1.16 × 106 and a 2 kg/h constant kerosene mass flow rate. The results showed that, since the swirler relative direction showed a significant influence on the inner swirler induced central toroidal recirculation zone and outer swirler induced swirling jet zone, the interaction between spray and swirling airflow was changed. Therefore, spray angle, Sauter mean diameter, and droplet velocity were changed by the swirler relative direction. Moreover, the effect of swirl number on the spray characteristics was found to be different for co- and counter-direction swirler configurations. [ABSTRACT FROM AUTHOR]

A three-dimensional and three-component velocity measurement on the flow field in a 45° end-to-side anastomosis model is conducted to investigate the hemodynamics, which is an important factor to the intimal hyperplasia formation and graft failure after surgery. Thanks to the advanced volumetric measurement technology of tomographic particle image velocimetry, the recirculation zone, low-speed region, and the spiral flow structures can be visualized. As a result, the flow field of three cases with the local maximum velocity of 0.15, 0.8, and 1.4 m/s are visible and the inlet velocity profile tends to be skewed as the flow rate increases. The mean vorticity contours indicate that the positive vortex center rotates 6.47°, 50.23°, and 90.4° and the negative vortex center rotates 20.44°, 15.73°, and 68.47°, respectively, in three cases. The instantaneous vortex structures identified by the λ c i criterion demonstrate two large-scale vortex structures in the distal section. The two vortices have the tendency to intertwine while one of them decays earlier. The wall shear stress (WSS) distributions on the entire model with the local maximum of 0.8, 5.8, and 13.8 Pa in three cases have been quantitatively achieved. The abnormal WSS and WSS gradient can help localize risk areas and understand the intimal hyperplasia formation. A detailed illustration of hemodynamics inside the 45° end-to-side anastomosis model has been provided, which demonstrates more comprehensive large-scale flow structures and abnormal WSS regions. Combined with the information of flow structures and WSS distribution, the understanding of the hemodynamics in the anastomosis can be strengthened. [ABSTRACT FROM AUTHOR]

This study measured Rayleigh and Schlichting streaming velocities via particle image velocimetry (PIV) in a standing wave field. Emphasis was placed on balancing high measurement accuracy with high spatial resolution in the boundary layer region. We aimed to achieve high resolution and enhanced measurement accuracy, typically a trade-off, by significantly increasing the aspect ratio of the interrogation window in the flow direction during PIV post-processing. The experiment operated under conditions with a duct axis streaming velocity of approximately 8.3 × 10 − 3 m/s in the measurement area. Increasing the aspect ratio from 1 to 21 reduced the standard deviation across the entire radial direction, which served as the accuracy index, from 8.05 × 10 − 3 m/s to 5.23 × 10 − 4 m/s (94% reduction). Additionally, the method's validity was confirmed through synthetic images with particles of known velocity. This verification demonstrated a decrease in the standard deviation comparable to the experimental images (97% reduction), aligning the analyzed velocity's radial distribution with the known streaming velocity distribution. [ABSTRACT FROM AUTHOR]

The algorithm of three-dimensional (3D) particle cloud reconstruction is a building block for 3D particle tracking velocimetry (3D-PTV). In the present study, a new 3D particle cloud reconstruction algorithm, i.e., neural-network-enhanced line of sight (NN-LOS), is proposed to update the traditional method based on Line of Sight (LOS) algorithm. The essence of NN-LOS is to use a matching neural network (Matching-NN) to evaluate whether or not one set of candidate matching being recorded by different cameras with various viewing perspectives is valid. Such a Matching-NN is in situ trained from virtual datasets that are synthetically generated by taking into account both the spatial calibration information and the actual seeding density in one particular experiment setup. This makes NN-LOS be self-adaptive to the measurement configuration, and thus avoids the problem of properly selecting a filtering threshold for the reprojection error in LOS. Both a series of synthetic tests and one surface morphology measurement are taken to prove that comparing to LOS, NN-LOS provides a significant improvement of the matching accuracy at high seeding density. A 3D-PTV measurement of a vortex ring in a synthetic jet is experimentally performed to demonstrate the advantage of NN-LOS. Comparing to tomographic particle image velocimetry, NNLOS-PTV enhances the spatial resolution of the velocity-field measurement and reduces the uncertainty of instantaneous velocity. The performance improvement is further empirically explained by a semiempirical test. [ABSTRACT FROM AUTHOR]

The lift and drag of spinning spheres roughened with macro-roughness elements are examined. The velocity field of these same spheres in flight is measured with particle image velocimetry (PIV). Several spheres with varying roughness are examined at various spin rates and fixed Reynolds number. Unlike previous studies, where the roughness height is varied, in the present work, the number of roughness elements is varied. The PIV datasets are used to determine the boundary layer separation points for each case. Comparing the lift and drag to the separation points reveals that (1) the separation points become more asymmetric with spin (the Magnus effect), (2) The drag increases with the size of the wake, and (3) the drag increases with the asymmetry of the separation points, meaning that lift on spheres is accompanied by increased drag. Scant evidence of this third effect has been reported previously. Additionally, it is shown that, counter to smooth spheres, the force transmitted to the surface through the roughness elements leads to significant drag. The drag is shown to increase with the number of roughness elements while the lift decreases. Results have implications for understanding aerodynamic forces on bluff bodies with roughness and passive control of aerodynamic forces through roughness element frequency rather than the traditional roughness height. [ABSTRACT FROM AUTHOR]

This work describes the development of a particle tracking velocimetry (PTV) algorithm designed to improve three-dimensional (3D), three-component velocity field measurements using a single plenoptic camera. Particular focus is on mitigating the longstanding depth uncertainty issues that have traditionally plagued plenoptic particle image velocimetry (PIV) experiments by leveraging the camera's ability to generate multiple perspective views of a scene in order to assist both particle triangulation and tracking. 3D positions are first estimated via light field ray bundling (LFRB) whereby particle rays are projected into the measurement volume using image-to-object space mapping. Tracking is subsequently performed independently within each perspective view, providing a statistical amalgamation of each particle's predicted motion through time in order to help guide 3D trajectory estimation while simultaneously protecting the tracking algorithm from physically unreasonable fluctuations in particle depth positions. A synthetic performance assessment revealed a reduction in the average depth errors obtained by LFRB as compared to the conventional multiplicative algebraic reconstruction technique when estimating particle locations. Further analysis using a synthetic vortex ring at a magnification of − 0.6 demonstrated plenoptic-PIV capable of maintaining the equivalent of 0.1–0.15 voxel accuracy in the depth domain at a spacing to displacement ratio of 5.3–10.5, an improvement of 84–89% compared to plenoptic-PIV. Experiments were conducted at a spacing to displacement ratio of approximately 5.8 to capture the 3D flow field around a rotor within the rotating reference frame. The resulting plenoptic-PIV/PTV vector fields were evaluated with reference to a fixed frame stereoscopic-PIV (stereo-PIV) validation experiment. A systematic depth-wise (radial) component of velocity directed toward the wingtip, consistent with observations from prior literature and stereo-PIV experiments, was captured by plenoptic-PTV at magnitudes similar to the validation data. In contrast, the plenoptic-PIV did not discern any coherent indication of radial motion. Our algorithm constitutes a significant advancement in enhancing the functionality and versatility of single-plenoptic camera flow diagnostics by directly addressing the primary limitation associated with plenoptic imaging. [ABSTRACT FROM AUTHOR]

A novel method is introduced to estimate phase-averaged pressure and acceleration of the coherent structures. It is applied and validated in the near wake of a two-dimensional bluff body. The method is validated using synthetic data generated by simulating an unsteady laminar flow around a D-shaped bluff body; the effect of noise is investigated to simulate an experimental scenario. The acceleration terms, required for the closure of flow-governing equations, are accurately estimated using this approach. The findings demonstrate that the methodology outputs consistent results for the pressure field while combined with a divergence-correction scheme, based on Helmholtz decomposition, and an optimal control method using sparse sensor measurements. The approach is later validated on experimental data obtained in the turbulent wake of a bluff body by means of "Particle Image Velocimetry" (PIV). Triple-decomposed flow fields are provided as input, where triple-decomposition refers to the instantaneous flow fields split into a mean flow, coherent and incoherent structures. The coherent structures are extracted using the method of "Proper Orthogonal Decomposition" (POD). Coherent lift coefficients, estimated using integral budgets of momentum, remain within the sensor-measured range, whereas coherent drag coefficients show excellent agreement. Moreover, a significant reduction in the residuals, arising from the balance of the flow-governing equations, is achieved. Although the temporal information is lost when computing POD modes, the proposed method is able to recover it and estimate the pressure. Accordingly, the method can be extended to any time-dependant modal decomposition, thereby providing a low-cost alternative to expensive "time-resolved" PIV (tr-PIV) measurements. [ABSTRACT FROM AUTHOR]

This study reports the development and characterization of a multiple-supersonic-jet wind tunnel designed to investigate the decay of nearly homogeneous and isotropic turbulence whose generation process is strongly influenced by fluid compressibility. The interaction of 36 supersonic jets generates turbulence that decays in the streamwise direction. The velocity field is measured with particle image velocimetry by seeding tracer particles with ethanol condensation. Various velocity statistics are evaluated to diagnose decaying turbulence generated by the supersonic jet interaction. The flow is initially inhomogeneous and anisotropic and possesses intermittent large-scale velocity fluctuations. The flow evolves into a statistically homogeneous and isotropic state as the mean velocity profile becomes uniform. In the nearly homogeneous and isotropic region, the ratio of root-mean-squared velocity fluctuations in the streamwise and vertical directions is about 1.08, the longitudinal integral scales are also similar in these directions, and the large-scale intermittency becomes insignificant. The turbulent kinetic energy per unit mass decays according to a power law with an exponent of about 2, larger than those reported for incompressible grid turbulence. The energy spectra in the inertial subrange agree well with other turbulent flows when normalized by the turbulent kinetic energy dissipation rate and kinematic viscosity. The non-dimensional dissipation rate C ε is within a range of 0.56–0.87, which is also consistent with incompressible grid turbulence. The dependence of C ε on the turbulent Reynolds number aligns with the scaling of non-equilibrium turbulence, leading to the large decay exponent. [ABSTRACT FROM AUTHOR]

The effects of vertical Lorentz forces generated by an external magnetic field applied perpendicular to the inherent electric field on the oxygen evolution reaction conducted on a Ni wire are investigated using cyclic voltammetry, impedance measurements, and particle image velocimetry (PIV). Both downward and upward Lorentz forces provide smaller overpotentials than that generated in their absence. Based on a dual-bubble layer model for reactant transfer, we find that the diffusion resistance of the hydroxide ions and increased ohmic resistance (after iR correction) induced by the layer of bubbles attached to the electrode surface are most effectively alleviated by the downward Lorentz force, while the charge-transfer resistance is retained. Furthermore, the generated bubbles have smaller average diameters. By using PIV measurements, we find that stronger shear stresses induced by the faster flow of the electrolyte in the vicinity of the working electrode facilitate the detachment of bubbles from the electrode surface. [Display omitted] • Downward Lorentz force by an external magnetic field promoted the OER the most. • Downward Lorentz force disturbed the upward flow induced by charged bubbles. • Chaotic convection flow increased the shear stress acting on the electrode surface. • Shear stress drastically reduced the mass transfer resistance due to bubbles. • OER catalytic properties are not affected by the Lorentz force. [ABSTRACT FROM AUTHOR]

Unsteady characteristics of pinhole-ejected ultrahigh-pressure hydrogen jets in the near field were investigated by performing particle image velocimetry and fast Fourier transform–inverse fast Fourier transform (IFFT) analysis of time-series shadowgraph images. Hydrogen stored at a maximum pressure of 99 MPa was ejected through pinholes of varying diameters. The fluctuating components caused by turbulence were removed to identify the barrel shock within the high-pressure hydrogen jets. The relationship between the Mach disk height and pressure ratios obtained through time-averaging aligned with the Mach disk height measured in experiments involving non-hydrogen gases under low ambient pressure. The obtained results exhibited similarities to the time-averaged velocity observed in the far field, suggesting the feasibility of utilizing Schlieren image velocimetry to analyze jet velocity characteristics in the near field. By applying IFFT analysis, the spatiotemporal information of these components was reintegrated into the field, enabling separate identification and demonstration of the turbulent components comprising the high-pressure hydrogen jet. [Display omitted] [ABSTRACT FROM AUTHOR]

The reuse of scrap tires to save storage space and lower traditional disposal risks is an environmental challenge. Currently, recycled tires are considered low-cost, lightweight materials that can be used to improve soil, retaining walls, backfills and roadbeds. The present study investigated the effect of the tire-shred aspect ratio and weight percentage on the bearing capacity and displacement of a stabilized mechanical retaining wall with multi-plate anchors under static loading. To this end, sandy soil containing tire shreds with widths of 1 and 2 cm, aspect ratios of 1, 2, 3, 4 and 5 at 5%, 10%, 15% and 20% contents (weight percentage of soil) was examined. Direct shear tests were conducted to determine the shear strength parameters of the mixtures. It was found that an increase in the tire-shred content in the mixture at a fixed aspect ratio, the bearing capacity of the mixture increased. Also at a fixed aspect ratio, the horizontal wall displacement increased with an increase in the tire-shred content. The wall bearing capacity was determined to increase at a fixed tire-shred content with an increase in the aspect ratio of the tire shreds. The results of the particle image velocimetry showed that an increase in the tire-shred aspect ratio increased the strain on the particle slip surface. [ABSTRACT FROM AUTHOR]

To deepen the understanding of flow mechanisms related to corner separation and associated control techniques, a passive control scheme based on a vortex generator (VG) installed on the end wall of the cascade passage was adopted. Detailed particle image velocimetry investigations were performed at different attack angles and flow velocities in a low-speed wind tunnel. At a 5° attack angle, the VG control cascade can effectively suppress the corner separation for chord Reynolds numbers (Rec) of 2.1 × 104 and 3.1 × 104. As Rec increases to 3.8 × 104 and 4.8 × 104, the original separation zone is relatively small, and the strong trailing vortex generated by the VG fails to intersect it, instead producing excessive interference to the main flow, resulting in additional flow loss. The separation zone is generally small at a 0° attack angle, and the VG control cascade performs similarly to that at the 5° attack angle. Through analysis of the instantaneous velocity and vorticity, it is discovered that the primary mechanism by which the VG suppresses corner separation is the unsteady disturbance of the trailing vortex to the separation, which increases the kinetic energy in the separation zone, lowers the accumulation of low-energy fluid, thereby suppressing the corner separation. [ABSTRACT FROM AUTHOR]

Background: X-ray imaging addresses many challenges with visible light imaging in extreme environments, where optical diagnostics such as digital image correlation (DIC) and particle image velocimetry (PIV) suffer biases from index of refraction changes and/or cannot penetrate visibly occluded objects. However, conservation of intensity—the fundamental principle of optical image correlation algorithms—may be violated if ancillary components in the X-ray path besides the surface or fluid of interest move during the test. Objective: The test series treated in this work sought to characterize the safe use of fiber-epoxy composites in aerospace and aviation industries during fire accident scenarios. Stereo X-ray DIC was employed to measure test unit deformation in an extreme thermal environment involving a visibly occluded test unit, incident surface heating to temperatures above 600oC, and flames and soot from combusting epoxy decomposition gasses. The objective of the current work is to evaluate two solutions to resolve the violation of conservation of intensity that resulted from both the test unit and the thermal chamber deforming during the test. Methods: The first solution recovered conservation of intensity by pre-processing the path-integrated X-ray images to isolate the DIC pattern of the test unit from the thermal chamber components. These images were then correlated with standard, optical DIC software. The second solution, called Path-Integrated Digital Image Correlation (PI-DIC), modified the fundamental matching criterion of DIC to account for multiple, independently-moving components contributing to the final image intensity. The PI-DIC algorithm was extended from a 2D framework to a stereo framework and implemented in a custom DIC software. Results: Both solutions accurately measured the cylindrical shape of the undeformed test unit, recovering radii values of R = 76.20 ± 0.12 mm compared to the theoretical radius of R theor = 76.20 mm. During the test, the test unit bulged asymmetrically as decomposition gasses pressurized the interior and eventually burned in a localized jet. Both solutions measured the heterogeneous radius of this bulge, which reached a maximum of approximately R = 91 mm, with a discrepancy of 2–3% between the two solutions. Conclusions: Two solutions that resolve the violation of conservation of intensity for path-integrated X-ray images were developed. Both were successfully employed in a stereo X-ray DIC configuration to measure deformation of an aluminum-skinned, fiber-epoxy composite test unit in a fire accident scenario. [ABSTRACT FROM AUTHOR]

The nodular diaphragm wall (NDW) is a novel foundation that can provide significant uplift load resistance compared to a typical diaphragm wall foundation. With the advantages of low noise, cost‐effectiveness, high work efficiency, and construction abilities close to existing buildings, NDW has excellent engineering application prospects in urbanization construction. However, the scarce knowledge on the uplift resistance, failure mode, and calculation method of the NDW has hindered its application in practice. In this paper, indoor model tests based on particle image velocimetry (PIV) technology are carried out to investigate the uplift behavior of three NDW models with different amounts and locations of nodular parts. The load transfer mechanisms of the three NDW models are discussed in detail, and special attention is given to the failure mode analysis. In addition, a theoretical analysis is implemented based on the load transfer approach. The transfer function and associated critical parameters are discussed in detail. Accordingly, an iterative procedure is proposed to interpret the load transfer information of an uplift NDW in multilayered soils under different loading levels. The findings derived in this paper are helpful to the uplift deformation, capacity calculation, and design of NDW in practical engineering. [ABSTRACT FROM AUTHOR]

Turbulent boundary layer (TBL) flows over various spanwise-heterogeneous rough walls exhibit spanwise variations in turbulence statistics, implying modifications of the turbulent/non-turbulent interface (T/NTI). In this work, based on the datasets of TBL flows over a spanwise-alternating converging/diverging riblets wall, we investigate behavior of T/NTI, properties of 'bubble' and 'drop' that are associated with T/NTI, as well as connections between T/NTI and large-scale structures within the boundary layer. The flow datasets were obtained through stereoscopic particle image velocimetry measurements on the cross-stream plane of the TBL flow at a Reynolds number R e θ = 1.3 × 104. The T/NTI and associated 'bubble' and 'drop' are identified using a kinetic energy criterion, while the large-scale structures by a streak detection algorithm. It is observed that the T/NTI height and the occurrence of 'bubble' and 'drop' display a spanwise-heterogeneous feature, with lower T/NTI height and higher occurrence probability of 'bubble' over the diverging section of riblets than over the converging section of riblets. Results of conditional average show that the 'bubble' is associated with positive streamwise fluctuations, common-down flow and a pair of counter-rotating streamwise vortices, while the 'drop' with negative streamwise fluctuation, common-up flow and a pair of counter-rotating streamwise vortices. Probability density functions of the 'bubble' size and fractal dimensions of the T/NTI are found similar for both the riblets and smooth wall flows. Furthermore, it is observed that the large-scale low-speed structures occurring preferentially over the converging section of riblets play a role in elevating T/NTI while the large-scale high-speed ones over the diverging section of riblets lowering down T/NTI. [ABSTRACT FROM AUTHOR]

Accurate simulation of nonlinear acoustic waves is essential for the continued development of a wide range of (high-intensity) focused ultrasound applications. In this article, we explore mixed finite element formulations of classical strongly damped quasilinear models of ultrasonic wave propagation, the Kuznetsov and Westervelt equations. Such formulations allow simultaneous retrieval of the acoustic particle velocity and either the pressure or acoustic velocity potential , thus characterizing the complete ultrasonic field at once. Using non-standard energy analysis and a fixed-point technique, we establish sufficient conditions for the well-posedness, stability, and optimal a priori errors in the energy norm for the semi-discrete equations. For the Westervelt equation, we also determine the conditions under which the error bounds can be made uniform with respect to the involved dissipation parameter. Additionally, we discuss convergence in the maximum error norm of the involved scalar quantities. Finally, computer experiments for Raviart–Thomas (RT) and Brezzi–Douglas–Marini (BDM) elements are performed to confirm the theoretical findings. [ABSTRACT FROM AUTHOR]

A series of reduced-scale model tests was conducted on helical soil-nailed walls (HSNWs) under vertical surface loading to evaluate the effect of length, pattern, and inclination of nails on lateral pressure distribution in this innovative retaining structure. The particle image velocimetry (PIV) technique was also used to trace shear bands and identify the failure mechanism. PIV results showed that increasing the nail length, as well as using a square pattern and a 15° angle to install the nails, could be three effective ways to reduce the penetration depth of the slip surface and, consequently, to limit wedge failure dimensions. A linear relationship formed between the lateral pressure induced on the wall facing and the footing pressure at q < qu. This relationship showed that a greater portion of the footing pressure was converted to lateral pressure using longer nails and installing them in a square pattern and at an angle of 15°. Considering the inverse relationship between lateral pressure behind the facing and wall displacement in HSNWs under vertical surface loading, it was concluded that both pressure and displacement criteria should be considered simultaneously to optimally select the arrangement, pattern, and inclination of helical nails. [ABSTRACT FROM AUTHOR]

The time-averaged particle image velocimetry technique has been applied to measure flow mixing in the Michigan Multi-jet Gas-mixture Dome (MiGaDome) facility, a 1/12th, scaled-down model of the high-temperature gas-cooled reactor upper plenum. Measurements were first conducted with one jet injection into the upper plenum for various Reynolds numbers (Re = 1022, 2038, 4097, and 6021). The experimental region of interest includes a plane within the dome located above one of the jet inlets of interest. First- and second-order statistics are presented and discussed to analyze the local mixing process and turbulent characteristics under the effects of jet spreading and jet impingement. Results have shown that the normalized statistics of the jet reach asymptotic behavior as the inlet Reynolds number is increased. By investigating the two-dimensional budgets for the momentum equation on the measurement plane, it was concluded that the contribution of turbulent diffusion is minor near the enclosure surface where strong convection is present due to impingement. An additional measurement on a triple-jet injection case has shown that jet spreading is suppressed by a recirculation zone, which causes a redistribution of turbulent fluctuations. The detailed local fluctuation patterns/coherent structures have been examined through a proper orthogonal decomposition analysis. [ABSTRACT FROM AUTHOR]

Gas fluxes from aquatic ecosystems are a significant component of the carbon cycle. Gas exchange across the air-water interface is regulated by near-surface turbulence and can be controlled by different atmospheric forcing conditions, with wind speed and surface buoyancy flux being the most recognized drivers in empirical studies and modeling approaches. The effect of rainfall on near-surface turbulence has rarely been studied and a consistent relationship between rain rate and near-surface turbulence has not yet been established. In this study, we addressed some limitations still present in the quantitative understanding of the effect of rain rate on near-surface turbulence and on the resulting gas transfer velocity in freshwater. We performed controlled laboratory experiments over a wide range of rain rates (7 to 90 mm h-1) and estimated gas transfer velocities from high-resolution measurements of O2 concentration, while rain-induced turbulence was characterized based on particle image velocimetry. We found that the rain-induced dissipation rates of turbulent kinetic energy declined with depth following a consistent power-law relationship. Both energy dissipation rates and gas transfer velocity increased systematically with the rain rate. The results confirm a causal relationship between rainfall, turbulence, and gas exchange. We propose a power-law relationship between near-surface turbulent dissipation rates and rain rate. In combination with surface renewal theory, we derived a direct relationship between gas transfer velocity and rain rate, which can be used to assess the importance of short-term drivers, such as rain events, on gas dynamics and biogeochemical cycling in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Supra-aortic extra-anatomic debranch (SAD) are prosthetic surgical grafts used to revascularize head and neck arteries that would be blocked during a surgical or hybrid procedure used in treating ascending and arch of the aorta pathologies. However, bypassing the supra-aortic arteries but not occluding their orifice might introduce potential for competitive flow that reduces bypass patency. Competitive flow within the bypasses across the supra-aortic arteries has not previously been identified. This research identified haemodynamics due to prophylactic inclusion of bypasses from the brachiocephalic artery (BCA) to the left common carotid artery (LCCA), and from the LCCA to left subclavian artery (LSA). Four model configurations investigated the risk of competitive flow and the necessity of intentionally blocking the proximal LSA and/or LCCA. Particle image velocimetry (PIV) was used to assess haemodynamics in each model configuration. We found potential for competitive flow in the BCA-LCCA bypass when the LSA was blocked, in the LSA-LCCA bypass, when the LCCA alone or LCCA and LSA were blocked. Flow stagnated at the start of systole within the RCCA-LCCA bypass, along with notable recirculation zones and reciprocating flow occurring throughout systolic flow. Flow also stagnated in the LCCA-LSA bypass when the LCCA was blocked. There was a large recirculation in the LCCA-LSA bypass when both the LCCA and LSA were blocked. The presence of competitive flow in all other configurations indicated that it is necessary to block or ligate the native LCCA and LSA once the debranch is made and the thoracic endovascular aortic repair (TEVAR) completed. [ABSTRACT FROM AUTHOR]

Ultrasonic irrigation during root canal treatment can enhance biofilm disruption. The challenge is to improve the fluid flow so that the irrigant reaches areas inaccessible to hand instrumentation. The aim of this study is to experimentally investigate how the flow field and hydrodynamic forces induced by ultrasonic irrigation are influenced by the ultrasound power and file insertion depth. A root canal phantom was 3D printed and used as a mold for the fabrication of a PDMS channel. An ultrasonic instrument with a #15K-file provided the irrigation. The flow field was studied by means of Particle Image Velocimetry (PIV). The time averaged velocity and shear stress distributions were found to vary significantly with ultrasound power. Their maximum values increase sharply for low powers and up to a critical power level. At and above this setting, the flow pattern changes, from the high velocity and shear stress region confined in the vicinity of the tip, to one covering the whole root canal domain. Exceeding this threshold also induces a moderate increase in the maximum velocities and shear stresses. The insertion depth was found to have a smaller effect on the measured velocity and shear stresses. Due to the oscillating nature of the flow, instantaneous maximum velocities and shear stresses can reach much higher values than the mean, especially for high powers. Ultrasonic irrigation will benefit from using a higher power setting as this does produce greater shear stresses near the walls of the root canal leading to the potential for increased biofilm removal. [ABSTRACT FROM AUTHOR]

By conducting a two-dimensional experimental study, this paper aims to enhance the understanding of the mechanism of sand convective motions in the vicinity of a wall subjected to long-term cyclic lateral loadings. The experimental tests were conducted in a rectangular sandbox with a transparent front-wall, through which the process of sand particle motions could be recorded by using a high-resolution digital camera. The images were processed with a high time-resolved PIV (Particle Image Velocimetry) system. Based on the experimental data, this work (1) presents the sand flow field in the convective zones; (2) provides means to describe the convection mechanism; (3) proposes the relationships between the loading conditions and dimensions of the region with intense sand movement; and (4) elaborates the similarity of the sand flow velocity structure within the sand convective zones. [ABSTRACT FROM AUTHOR]

Aquatic organisms have evolved exceptional propulsion and even transoceanic migrating capabilities, surpassing artificial vessels significantly in maneuverability and efficiency. Understanding the hydrodynamic mechanisms of aquatic organisms is crucial for developing advanced biomimetic underwater propulsion vehicles. Underwater tetrapods such as sea turtles use fins or flippers for propulsion, which exhibit three rotational degrees of freedom, including flapping, sweeping, and pitching motions. Unlike previous studies that often simplify motion kinematics, this study employs a specially designed experimental device to mimic sea turtle fins' motion and explore the impact of pitching amplitude, asymmetric pitching kinematics, and pausing time on lift and thrust generation. Force transducers and particle image velocimetry techniques are used to examine the hydrodynamic forces and flow field, respectively. It is found that boosting the fin's pitching amplitude enhances both its lift and thrust efficiency to a certain extent, with a more pronounced effect on thrust performance. Surprisingly, the asymmetrical nature of the pitching angle's pausing time within one flapping cycle significantly influences the lift and thrust characteristics during sea turtle swimming; extending the pausing time during the forward and upward flapping process improves lift efficiency; and prolonging the pausing time during the downward flapping process enhances thrust efficiency. Furthermore, the mechanism for high lift and thrust efficiency is revealed by examining the vortices shed from the fin during different motion kinematics. This research contributes to a more comprehensive understanding of the fin's hydrodynamic characteristic, providing insights that can guide the design of more efficient biomimetic underwater propulsion systems. [ABSTRACT FROM AUTHOR]