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The noise and flow characteristics of combined backwards-facing and forward-facing steps (BFS and FFS) are experimentally investigated in an acoustic wind tunnel using a far-field microphone array and Particle Image Velocimetry (PIV), respectively. The step models considered have a step height H of 10∼40 mm and a step gap L of 10∼100 mm, and the Reynolds number is (Re H =) 2.1 × 105∼1.1 × 106. The incoming flow boundary layer is laminar, and the step height is approximately 3∼28 of the boundary layer thickness. The results show that the noise is closely related to the step type, step height H and the gap L. In the step-only configuration, the BFS noise does not vary with the step height H 1 and is much lower than the FFS noise, of which the sound pressure level depends on U 0 5.8 ∼ U 0 6.2 within the height range H 2 . As for the combined step model (BFS&FFS), the noise spectrum changes significantly with the step heights and gaps. When H 1 > H 2 , the noise spectrum resembles the BFS-only due to the flow shield of the FFS from the upstream BFS. When H 1 < H 2 , the noise spectrum is dominated by the FFS, especially at higher H 2 or longer gap L due to more face area of the FFS exposed to the incoming flow. As H 1 = H 2 , the noise resembles the BFS in the low-frequency range and the FFS at high frequencies, respectively. Generally, the step-only and BFS&FFS have broadband noise characteristics, except when the ratio L/H is small, the noise spectrum has the characteristics of a typical cavity dominated by tonal peaks. [ABSTRACT FROM AUTHOR]

Flooding in urban communities is an increasingly prevalent issue that causes significant financial loss, property damage, and long-term adverse health effects. Backwater valves can reduce the risk of basement flooding during sewer surcharge events at the lot-level scale. However, guidelines for installation and maintenance can be limited or inconsistent, with little underlying literature or research. Without proper installation and ongoing maintenance, solids can accumulate, resulting in the valve failing to close or being unable to form a watertight seal during a sewer surcharge event. This research provides insights to inform future design iterations or updates to best practices guidelines by characterizing flow patterns and velocities within the Mainline Fullport backwater valve. A series of laboratory experiments are described at two common flow rates (0.1 and 0.3 L/s) and various slopes (−2% , 0%, 2%, 5%, and 10%) using fluorescent particle tracers as a novel replacement for more traditional laser-based particle image velocimetry. Results revealed a complex flow environment influenced by slope, flow rate, initial water level conditions, and the fluid properties of water. Regions for potential solids accumulation leading to mechanisms of potential failure occurred near the inlet, at the downstream edge of the closing gate, and along the side channels. Increased slopes generally improved flow conditions, with least favorable outcomes below a 2% slope and diminishing returns above a 5% slope. Between 2% and 5% slope, conditions were the most complex but improved with increased flow rates. Fluorescent particle velocimetry shows promise as a powerful, affordable, and reliable tool to visualize flow and measure velocities in complex, shallow flow environments where other methods are unsuitable. Practical Applications: Floods are an increasingly common and highly damaging problem, during which wastewater can back up in sewage pipes into basements. A backwater valve reduces the risk of sewer back-up and basement flooding. The valve closes when sewage flows backward and seals shut until the event ends. A backwater valve must be properly installed and maintained to ensure intended performance, but the associated guidelines can be unclear. To help improve those guidelines, this paper clarifies performance for various flow patterns and slopes in a backwater valve at different flow rates using laboratory experiments. Faster-moving water flushes debris and helps keep the valve clean, but raising slopes to achieve that may be difficult or expensive. Experiments used particles glowing under a black light to visualize water movement. Results reveal complicated flow patterns, with the worst conditions for flushing debris developing below 2% slope (2-cm drop per 100-cm of length) and the best conditions at 5% or above. Between 2% and 5% slope, the benefits of increasing slope were complex and depended on other factors. Methods used in these experiments can be adapted for different applications because they are relatively inexpensive and can work in shallow-water environments. [ABSTRACT FROM AUTHOR]

Recent studies have revealed the large potential of vertical‐axis wind turbines (VAWTs) for high‐energy‐density wind farms due to their favorable wake recovery characteristics. The present study provides an experimental demonstration and proof‐of‐concept for the wake recovery mechanism of the novel X‐Rotor VAWT. The phase‐locked flowfield is measured at several streamwise locations along the X‐Rotor's wake using stereoscopic particle image velocimetry (PIV) with fixed‐pitch offsets applied to the blades. The streamwise vortex system of the upper half of the X‐Rotor is first hypothesized and then experimentally verified. The induced wake deformations of the vortex systems are discussed in comparison with previous studies concerning traditional H‐type VAWTs. The results suggest that positive blade pitch is more favorable for accelerated wake recovery due to the dominant tip‐vortex generated on the upwind windward quadrant of the cycle. Utilizing theoretical blade load variations along the span explains distinct unsteady flow features in the near wake generated at select quadrants of the rotor rotation, shedding light on the potential of the two pitch schemes. [ABSTRACT FROM AUTHOR]

Detailed investigation of pathogen transmission by respiratory droplets requires extensive experimental datasets with high spatial–temporal resolution in a wide range of ambient conditions. Respiratory simulators are attractive tools for those measurements, because they improve repeatability, endurance, and control of experimental conditions with respect to studies on human subjects. They also enable the use of powerful experimental techniques, which may raise health concerns if employed on humans. In this paper, we design and present a respiratory simulator, which is capable of accurately reproducing physiological flow rate profiles and allows the investigation of the spatial and temporal features of the exhaust flow by background‐oriented schlieren (BOS) and particle image velocimetry (PIV). We use laser interferometry and high‐magnification shadowgraphy to verify the size distributions of the emitted droplets, and we quantify the evolution of the droplet concentration during cough events by Mie scattering analysis. The experiments demonstrate the ability of the respiratory simulator to generate highly reproducible cough events with precise and controllable droplet size distributions over a wide range of flow rates. [ABSTRACT FROM AUTHOR]

Free-swimming polychaetes are common in marine habitats and exhibit a unique form of swimming whereby a metachronal wave occurs simultaneously with a bending body wave. This body wave is unusual among swimming animals because it travels in the same direction as the animal's swimming direction. However, we currently lack a mechanistic understanding of this unusual form of locomotion. In this study we use a combination of high-speed, high-resolution video and particle image velocimetry (PIV) to quantify kinematics and fluid dynamics for three species of swimming polychaetes, spanning two orders of magnitude in size. We find that in all species, flows generated by metachronal waves of parapodia dominate while typical flows associated with body bending is absent. However, the parapodia are less flexible than propulsive structures in other metachronal swimmers. This creates a localized, but substantial upstream flow during the recovery stroke. Using body bending, the recovery stroke can occur mostly beneath the bulk flow from the power strokes, resulting in minimal inference while the subsequent power stroke can benefit from the pressure field generated during recovery. These results may have implications for future vehicle designs that incorporate metachronal locomotion. [ABSTRACT FROM AUTHOR]

This paper conducts a fundamental study on the hydrodynamic performance of a floating offshore wind turbine (FOWT). This includes investigating the impacts of platform motion, turbine operation, and environmental conditions on hydrodynamic wake development behind the FOWT. Using underwater particle image velocimetry in a wave basin, the fluid flow behaviour behind the floating wind substructure model is compared for different test configurations. The FOWT is examined in three configurations: I. Zero degrees of freedom (fixed), II. Six degrees of freedom (floating), and III. Floating with turbine in operation (full system). The results reveal significant hydrodynamic wake turbulence differences between fixed and floating systems, with 95% and 89% increases in turbulent kinetic energy (TKE) and turbulence intensity (TI), respectively. Turbine loading in configuration III results in additional damping and reduces eddy shedding, resulting in a 26% and 31% decrease in TKE and TI, compared to the floating configuration II. It is found that neglecting turbine operation overestimates substructure drag terms by at least 30%, while fixing the model underestimates drag terms by 54%. This study highlights a trade-off between model accuracy and dynamic complexity when estimating viscous effects on FOWT. It also provides a valuable set of high-fidelity validation data for the development of numerical tools for FOWT analysis. [ABSTRACT FROM AUTHOR]

Shot peening is a mechanical surface treatment process that enhances the fatigue performance of metallic components. Numerous studies have demonstrated the significant influence of peening particles' motion on the quality of treatment. However, previous research has not established a connection between particle motion characteristics and nozzle structure. The nozzle structure directly affects both airflow characteristics and shot movement, making it crucial to examine how it influences particle motion to optimize both nozzle design and process parameters. To accurately determine the motion characteristics of particles, this study constructs a high-speed camera system for capturing their movements and uses the Particle Image Velocimetry (PIV) method for analysis. This study thoroughly investigates how factors such as jet pressure, nozzle diameter, and length affect shot motion behavior. The results indicate positive correlations between particle velocity with jet pressure and nozzle length while showing a negative correlation with nozzle diameter. Modifying structural parameters can effectively regulate both the velocity and dispersion angles of particles during peening operations under specified jet pressure conditions. Furthermore, based on experimental findings, a new prediction model for particle velocity considering structural parameters associated with nozzles was proposed. [ABSTRACT FROM AUTHOR]

This study focused on the stabilization of lean premixed flames of n-butane/H 2 /air with swirl/bluff-body. The research explored four different hydrogen fractions (0, 20%, 40%, and 80%) at a constant equivalence ratio and Reynolds number. The turbulent reacting flow was resolved using a combination of Delayed Detached Eddy Simulation (DDES) and Flamelet Generated Manifold (FGM) model. The velocity fields and reaction zones were obtained through Particle Image Velocimetry (PIV) and OH* chemiluminescence measurements respectively. H 2 addition to n-butane significantly enhanced flame characteristics. The chemiluminescence imaging showed that the flame base became stronger with increasing hydrogen addition, reducing the risk of flame lift-off. Hydrogen addition not only increased the overall reaction rate but also changed the combustion intensity at the nozzle exit from weak to strong, which is crucial for flame stabilization. Interestingly, no flashback was observed even at 80% H 2 addition to n-butane at the same level of power rating. The flow strain rate analysis of the PIV measurements showed that the inclusion of hydrogen skewed the strain rate probability density functions towards the positive side, indicating an increase in the burning rate. The results demonstrate a progressive increase in preferential diffusion of H 2 from reactants to products as the H 2 enrichment in the fuel rises and resulted in a more intense flame. Also, the present simulation results were validated with the PIV data, and they showed good agreement. Specifically, an 80% H 2 blend exhibited a 16% peak enhancement in flame surface density compared to pure n-butane while concurrently reducing CO 2 emissions by 23.2%. The incorporation of H 2 also led to the increased vortex strength and strain rate within the flame. • H 2 addition changed the nature of the flame base from a weaker to a stronger one. • H 2 induced a double helical vortex structure in the flame with strong vorticity. • The preferential diffusion effect was dominant only at an 80% addition of H 2. • The CO 2 emissions were significantly reduced by 23.2% with 80% H 2. [ABSTRACT FROM AUTHOR]

We combine flow visualisation techniques and particle image velocimetry to experimentally investigate the higher-order transition to elastoinertial turbulence of Boger fluids ($El = 0.11\unicode{x2013}0.34$) in Taylor–Couette flows. The observed route to turbulence is associated with the appearance of chaotic inflow jets, termed flame patterns, for increasing inertia, and stable structures of solitons, known as diwhirls, for decreasing inertia. We also report for the first time spatially and temporally resolved flow fields in the meridional plane for the three characteristic viscoelastic flow regimes (diwhirls, flame patterns and elastoinertial turbulence). We observe in all cases coherent structures of dynamically independent solitary vortex pairs. The stability of these coherent structures is jet-dominated and can be mainly ascribed to the high extension of the polymer chains in the inflow boundaries in the $r$ – $z$ plane. Solitary pairs are self-sustained when created through random events and do not split; instead, they merge when moving sufficiently close and annihilate when hoop stresses are not sufficient to sustain them. The highly localised and random events result in highly fluctuating, chaotic flow states. We estimate the decay exponent of spatial power spectral density, illustrating a universal scaling of $-2.5$ for elastoinertial turbulence. Based on our observations and in an effort to unify and combine precedent theories with our results, we suggest a mechanism for the origins of elastoinertial instabilities, accounting for both the effect of elasticity on the vortex formation and the effect of increasing/decreasing inertia on the flow dynamics. [ABSTRACT FROM AUTHOR]

This paper presents a novel particle image velocimetry study of the effects of stationary upstream ice blocks on the turbulent flow dynamics around a single, fixed ice boom pontoon. The pontoon was modelled using a half-submerged horizontal circular cylinder. Different lengths, thicknesses, and undersurfaces of the ice blocks were examined. The Reynolds number was maintained at 25,500. The results show that the cylinder encounters an approach flow with lower momentum but higher turbulence intensity when placed behind a long ice block compared to a shorter one, and the presence of undersurface roughness reduces the momentum but enhances the turbulence intensity of the approach flow. The recirculation length, mean velocities, and turbulence levels around the cylinder reduce with increasing ice block length and subsequent roughening of its undersurface. However, a larger recirculation bubble forms from the ice block, and the mean velocities and turbulence levels increase when the ice block thickness increases. [ABSTRACT FROM AUTHOR]

Background: The Food and Drug Administration (FDA) blood pump is an open‐source benchmark cardiovascular device introduced for validating computational and experimental performance analysis tools. The time‐resolved velocity field for the whole impeller has not been established, as is undertaken in this particle image velocimetry (PIV) study. The level of instantaneous velocity fluctuations is important, to assess the flow‐induced rotor vibrations which may contribute to the total blood damage. Methods: To document these factors, time‐resolved two‐dimensional PIV experiments were performed that were precisely phase‐locked with the impeller rotation angle. The velocity fields in the impeller and in the volute conformed with the previous single blade passage experiments of literature. Results: Depending on the impeller orientation, present experiments showed that volute outlet nozzle flow can fluctuate up to 34% during impeller rotation, with a maximum standard experimental uncertainty of 2.2%. Likewise, the flow fields in each impeller passage also altered in average 33.5%. Considerably different vortex patterns were observed for different blade passages, with the largest vortical structures reaching an average core radii of 7 mm. The constant volute area employed in the FDA pump design contributes to the observed velocity imbalance, as illustrated in our velocity measurements. Conclusions: By introducing the impeller orientation parameter for the nozzle flow, this study considers the possible uncertainties influencing pump flow. Expanding the available literature data, analysis of inter‐blade relative velocity fields is provided here for the first‐time to the best of our knowledge. Consequently, our research fills a critical knowledge gap in the understanding of the flow dynamics of an important benchmark cardiovascular device. This study prompts the need for improved hydrodynamic designs and optimized devices to be used as benchmark test devices, to build more confidence and safety in future ventricular assist device performance assessment studies. [ABSTRACT FROM AUTHOR]

This study investigates the complex flow field across a spanwise vegetative model canopy edge focusing on turbulent transport processes. Utilizing stereoscopic particle image velocimetry, the three velocity components were measured in wall-parallel planes at various elevations within canopy across the spanwise canopy edge. Conventional ensemble averaged results were contrasted with those obtained by conditionally averaged flow properties across instantaneous internal interfaces in the flow to understand their contribution to the ensemble average. The conditional average captured the strong gradients in mean velocities, Reynolds stresses, vorticity, swirling strength, and turbulent kinetic energy production across the dynamically changing instantaneous interface. In contrast, the conventional ensemble average smeared out the strong gradients. Small magnitudes of advective terms in the turbulent kinetic energy transport equation suggested weak secondary transverse flows in the present model canopy. The turbulent flow structure across the spanwise canopy edge was further investigated using Quadrant-Hole analysis for both averaging approaches. Conventional ensemble averaged results indicated a shift from sweep to ejection dominance when moving from canopy into the open patch, while the conditional average showed only sweep dominated transport. In contrast to a homogeneous canopy layout, below canopy height at the canopy edge, sweeps and ejections lose their dominance in vertical turbulent transport. The present results show that the dynamics of internal interfaces govern the ensemble averaged results and a possible implementation into existing models is proposed. The present results are expected to increase understanding of spanwise turbulent transport and aid in developing strategies to mitigate desertification. [ABSTRACT FROM AUTHOR]

The evolution of in-cylinder flow involves large- and small-scale structures during the intake and compression strokes, significantly influencing the fuel-air mixing and combustion processes. Extensive research has been conducted to investigate the flow evolution in medium- to large-sized engines using laser-based diagnostic methods, computational fluid dynamics (CFD) simulations, and zero-dimensional (0D) based modeling. In the present study, we provide a detailed analysis of the evolution of flow fields in a small-bore spark ignition (SI) engine with a displacement volume of 110 cm³. This analysis employs a unique methodology, where CFD simulation is performed and validated using measured particle image velocimetry (PIV) data. Subsequently, the validated CFD results are utilized to develop and validate a 0D-based model as it is computationally more efficient. The validated CFD simulation and 0D-based model are then used to evaluate the quantified strength of the flow by calculating the tumble ratio and turbulent kinetic energy (TKE). The streamlines and velocity vectors of the flow fields obtained from CFD simulations are utilized to explain the evolution of these parameters during intake and compression strokes. The study is further extended to analyze the effect of engine speed on the evolution of flow fields. With an increase in engine speed, relatively higher values of tumble ratio and TKE at the end of the compression stroke are observed, which is expected to improve the fuel-air mixing and combustion efficiency. [ABSTRACT FROM AUTHOR]

Penn State University is developing a pediatric total artificial heart (TAH) as a bridge-to-transplant device that supports infants and small children with single ventricle anomalies or biventricular heart failure to address high waitlist mortality rates for pediatric patients with severe congenital heart disease (CHD). Two issues with mechanical circulatory support devices are thrombus formation and thromboembolic events. This in vitro study characterizes flow within Penn State's pediatric total artificial heart under physiological operating conditions. Particle image velocimetry (PIV) is used to quantify flow within the pump and to calculate wall shear rates (WSRs) along the internal pump surface to identify potential thrombogenic regions. Results show that the diastolic inflow jets produce sufficient wall shear rates to reduce thrombus deposition potential along the inlet side of the left and right pumps. The inlet jet transitions to rotational flow, which promotes wall washing along the apex of the pumps, prevents flow stasis, and aligns flow with the outlet valve prior to systolic ejection. However, inconsistent high wall shear rates near the pump apex cause increased thrombogenic potential. Strong systolic outflow jets produce high wall shear rates near the outlet valve to reduce thrombus deposition risk. The right pump, which has a modified outlet port angle to improve anatomical fit, produces lower wall shear rates and higher thrombus susceptibility potential (TSP) compared to the left pump. In summary, this study provides a fluid dynamic understanding of a new pediatric total artificial heart and indicates thrombus susceptibility is primarily confined to the apex, consistent with similar pulsatile heart pumps. [ABSTRACT FROM AUTHOR]

Zero-plane is an important reference level included in the logarithmic velocity profile. The prediction of the zero-plane displacement remains challenging, especially in open-channel flows with submerged vegetation. In this study, we first review the existing formulas for estimating the zero-plane displacement. Then, we conducted a series of laboratory experiments of open-channel flows with submerged rigid vegetation, by varying flow rate, water depth, vegetation density, and stem diameter. By utilizing particle image velocimetry (PIV) technique, the spatio-temporal averaged (double-averaged) flow velocity and Reynolds shear stress profiles were obtained. The data analysis shows that the zero-plane displacement is almost proportional to the surface layer depth for the shallow flow cases. As the flow depth increases, the zero-plane displacement tends to be constant and increases with decreasing vegetation density. This result substantiates the hypothesis that the zero-plane displacement is related to large-scale eddies. Lastly, a heuristic model is developed to explain possible variations of the large-scale eddies, which yields a formula for predicting the zero-plane displacement for flows subjected to submerged rigid vegetation. [ABSTRACT FROM AUTHOR]

The circular synthetic jet (SJ) for different dimensionless stroke lengths and at Reynolds number Re = 5000 was investigated in this paper. Particle image velocimetry (PIV) was used. The flow was measured at a distance of 240 mm from the orifice, and this area was divided into two fields of view (FOV). The parameter fields were created by the injunction of these two FOVs. The time-averaged velocity, turbulent kinetic energy (TKE), turbulence intensity, vorticity field, centerline, and profiles of SJ were presented and discussed. Additionally, the jet half-width of SJ was investigated. The data discontinuity at a line of the FOVs was discussed. The impact of the dimensionless stroke lengths on the parameters of SJ at Re = 5000 was discussed. [ABSTRACT FROM AUTHOR]

In this work, a large-scale mockup of a compact complex system integrating a steam generator (SG) and a reactor coolant pump (RCP) is considered. The three-dimensional turbulent flow in the steam generator channel head (SGCH) is measured in detail. Dual-orthogonal-plane particle image velocimetry (PIV) is employed to extract high-resolution flow information in two orthogonal planes. Two separate measurements are first made to see the three-dimensional time-mean flow dynamics and the statistical quantities in the two planes. These measurements highlight two distinct flow phenomena: jet arrays and massive turbulent separation bubbles (TSBs). These patterns are attributed to mass flow redistribution in the U-shaped tubes. Proper orthogonal decomposition (POD) identifies the first POD mode as corresponding to the TSB breathing-like motion, which significantly intensifies the side view streamwise velocity fluctuations, leading to them reaching 370% of the local mean velocity. To examine the unsteady behavior of massively separated regions, the dual-orthogonal-plane PIV system is then synchronized to simultaneously measure variations in the flow fields, and the missing data due to illumination interference are reconstructed using gappy POD. The synchronized analysis reveals a direct relationship between the low-frequency fluctuations in the side and front views. These fluctuations are in phase across both views, indicating a synchronized behavior that spans the entire field. This large-scale low-frequency breathing motion has critical implications for numerical simulations and sheds light on the unsteady behavior of the RCP system within the SGCH. [ABSTRACT FROM AUTHOR]

Coastal hydrodynamics and morphodynamics integrate the effects of small‐scale fluid‐sediment interactions; yet, these small‐scale processes are not well understood. To investigate sediment trapping by turbulent coherent structures or vortices, the transport of coarse sand over ripples was analyzed in a small‐oscillatory flow tunnel with phase‐separated Particle Image and Tracking Velocimetry. Results from one of the first direct measurements of vortex‐trapped sand grains under oscillatory flows are presented. The vortices mobilized sand grains along the ripple slopes just prior to flow reversal and transported the suspended sediment grains. During several flow cycles, some sand grains were temporarily trapped in the vortex, prescribing semi‐circular trajectories off‐center from the vortex core in quadrants of the vortex that were closest to the ripple slope, as illustrated by Nielsen (1992, https://doi.org/10.1142/1269). Comparisons of the horizontal sediment grain velocity with the horizontal fluid velocity yielded a linear relationship with a slope of 0.87. The vertical grain velocities also varied linearly with the vertical fluid velocity with a slope of approximately 1 and an offset of −0.08 m s−1 ${\mathrm{s}}^{-1}$. The offset is close to the still water settling velocity for coarse sand grains, as hypothesized during vortex trapping. Additionally, estimates of the off‐center distance, between the centers of the semi‐circular sediment paths and vortex cores, compared well with the ratio of the settling velocity to the radian frequency of the vortex yielding a linear regression slope of 0.99. Improved understanding of vortex trapping effects on sediment dynamics may decrease uncertainty in model predictions of large‐scale coastal hydrodynamics and sediment transport. Plain Language Summary: Large‐scale coastal processes integrate the effects of small‐scale fluid‐sediment interactions; yet, these small‐scale processes are not well understood. Nielsen (1992, https://doi.org/10.1142/1269) hypothesized that suspended sand grains over ripples could get caught in a vortex under certain hydrodynamic conditions, in a process called vortex trapping. In this article, we present results from one of the first direct measurements of vortex‐trapped sand grains under oscillatory flows. Vortices along the ripple slope mobilized sand grains prior to flow reversal and transported the suspended grains. During several flow cycles, some sand grains were temporarily trapped in the vortex and moved along semi‐circular paths off‐center from the vortex core in sections of the vortex that were closest to the ripple slope, as illustrated by Nielsen (1992, https://doi.org/10.1142/1269). Comparisons of the sediment grain velocity with the fluid velocity yielded a linear relationship. Additionally, the distance between the center of the semi‐circular sediment paths and the center of the vortex cores agreed with the theory. Improved understanding of the effects of vortex trapping on sediment dynamics may decrease uncertainty in the predictions of large‐scale coastal hydrodynamics and sediment transport models, which do not typically account for this small‐scale process. Key Points: Observations of vortex‐trapped grains suggest delayed settling of advected grains, as well as delayed advection of grains mobilized from the bedQuantitative comparisons of vortex‐trapped sand grains compared well with theoretical formulations by Nielsen (1992, https://doi.org/10.1142/1269) for a forced vortexImproved understanding of vortex trapping effects on sediment dynamics may decrease uncertainty in large‐scale coastal model predictions [ABSTRACT FROM AUTHOR]

Particle image velocimetry and particle tracking velocimetry (PTV) have developed from two-dimensional two-component (2D2C) velocity vector measurements to 3D3C measurements. Rainbow particle tracking velocimetry is a low-cost 3D3C measurement technique adopting a single color camera. However, the vector acquisition rate is not so high. To increase the number of acquired vectors, this paper proposes a high probability and long-term tracking method. First, particles are tracked in a raw picture instead of in three-dimensional space. The tracking is aided by the color information. Second, a particle that temporarily cannot be tracked due to particle overlap is compensated for using the positional information at times before and after. The proposed method is demonstrated for flow under a rotating disk with different particle densities and velocities. The use of the proposed method improves the tracking rate, number of continuous tracking steps, and number of acquired velocity vectors. The method can be applied under the difficult conditions of high particle density (0.004 particles per pixel) and large particle movement (maximum of 60 pix). [ABSTRACT FROM AUTHOR]

Debris flows, highly destructive and rapidly moving mixtures of water, sediment, and rock, pose significant threats to human settlements and infrastructure in mountainous regions. Predicting the deposition patterns of debris flows is crucial for hazard assessment and mitigation. Smoothed Particle Hydrodynamics (SPH) has emerged as a valuable numerical simulation technique for modeling debris flow deposition due to its ability to capture complex fluid-solid interactions and particle dynamics. This study focuses on employing SPH simulation to investigate debris flow deposition patterns and assess their accuracy through Particle Image Velocimetry (PIV) validation. The methodology encompasses the discretization of fluid and solid phases into particles, governed by hydrodynamic and constitutive equations, respectively. The coupling of fluid and solid interactions enables the representation of entrainment, transport, and settling of particles, yielding insights into deposition patterns. Twelve case studies are discussed to validate the accuracy and applicability of SPH simulations in reproducing deposition patterns. Additionally, challenges such as numerical stability, parameter sensitivity, and computational efficiency are addressed, along with potential enhancements in modeling techniques. Notably, it was observed that around three specific numerical instances demonstrated an extraordinary level of similarity (2.98%, 7.51% & 9.83%) to the deposition pattern that was observed during the experimental phase. As SPH continues to advance, it holds promise as a reliable tool for assessing debris flow hazards and guiding land-use planning and disaster management efforts in vulnerable regions. [ABSTRACT FROM AUTHOR]

Detailed, accurate information on flow patterns in river channels can improve understanding of habitat conditions, geomorphic processes, and potential hazards to help inform water management. Data describing flow patterns in river channels can be obtained efficiently via image‐based techniques that have become more widely used in recent years as the number of platforms for acquiring images has expanded and the number of algorithms for inferring velocities has grown. Image‐based techniques have been incorporated into various software packages, including the Toolbox for River Velocimetry using Images from Aircraft (TRiVIA). TRiVIA is a freely available, standalone computer program that provides a comprehensive workflow for performing particle image velocimetry (PIV)‐based analyses within a graphical interface. This paper summarizes major enhancements incorporated into the latest release of TRiVIA, version 2.1. For example, a new Tool for Input Parameter Selection (TIPS) provides guidance for specifying key inputs to the PIV algorithm by allowing users to explore relationships between flow velocity, pixel size, output vector spacing, and frame interval. Improved visualization capabilities include the ability to create streamlines and display PIV output on an interactive web map. The program now provides greater flexibility for importing field data in various formats and selecting which observations to use for accuracy assessment. The most substantial additions to TRiVIA 2.1 are the ability to integrate bathymetric information with image‐derived velocity estimates to calculate river discharge and to use images acquired from moving aircraft to efficiently map long segments of large rivers to support habitat assessment, contaminant transport studies, and a range of other applications. [ABSTRACT FROM AUTHOR]

To date, there is a lack of in-situ turbulence data in natural aquatic systems. Adapting particle image velocimetry (PIV) to field use is of interest to provide such data. Past field PIV systems have been successful, but suffer from high costs, lack of portability, and flow disruption. We present a new field PIV system designed to address these limitations and capture turbulence metrics significant to aquatic biota: velocity, turbulent kinetic energy (TKE), vorticity, eddy size, and eddy circulation. Velocity measurements by the novel PIV system were compared to measurements by an acoustic doppler velocimeter (ADV) and an electromagnetic flowmeter for validation, and its results agreed with these other two methods. The PIV's TKE results were compared to those of an ADV, and they agreed as well. Vorticity, eddy size, and eddy circulation were validated in the field through repeating past measurements. Each of these data sets generated by the novel PIV fell within the range of the validation measurements. The novel system thus provides accurate turbulence and velocity measurements; further, its design is low-cost, uses widely available materials, is minimally disruptive to the flow, and is highly portable and adjustable, offering an accessible method for acquiring meaningful field turbulence data. [ABSTRACT FROM AUTHOR]

Landslides are common geological hazards that cause significant losses. Anti-slide piles are commonly used in landslide engineering, and model testing is one of the means to study pile-supported structures. However, model tests face several challenges, including difficulty in controlling the experimental process, challenges in repeated tests, and difficulty in monitoring soil deformation around piles. To address these issues, this study presents a model test method using particle image velocimetry (PIV), transparent soil, and 3D printing technology. Using this method, a series of model tests were conducted, including single-row and double-row anti-slide piles. The experimental results indicate that, compared with single-row piles, double-row piles exhibit better supporting effects. In the pile‒soil interaction, the displacement of the extrusion of soil between piles was controlled under the combined action of the front and back rows of piles. The inclination angle of a single-row pile after the test was 8°, whereas that of a double-row pile was reduced by 62.5%. With respect to the displacement of the soil behind the piles, the phenomenon of a "displacement triangle" behind the piles was observed. An analysis of the change process in this area revealed that the relative displacement caused by pile‒soil interactions is mainly distributed in the surface layer of the soil. The experiments demonstrate that this system is suitable for pile-supported structure model tests. [ABSTRACT FROM AUTHOR]

The colonization of the pelagic realm by the vertebrates represents one of the major transitions in the evolutionary success of the group and in the establishment of modern complex marine ecosystem. It has been traditionally related with the Devonian rise of jawed vertebrates, but new evidences indicate that first active swimmers, invading the water column, occurred within earlier armoured jawless fishes ("ostracoderms"). These "primitive" fishes lacked conventional fish control surfaces and the precise mechanism used to generate lift and stabilizing forces still remains unclear. We show that, because of their shape, the rigid cephalic shield of Pteraspidiformes, a group of Silurian-Devonian "ostracoderms", generate significant forces for hydrodynamic lift. Particle Image Velocimetry and force measurements in a water channel shows that the flow over real-sized Pteraspidiformes models is similar to that over delta wings, dominated by the formation of leading-edge vortices resulting in enhanced vortex lift forces and delayed stall angles of attack. Additionally, experiments simulating ground effect show that Pteraspidiformes present better hydrodynamic performance under fully pelagic conditions than in a benthic scenario. This suggests that, lacking movable appendages other than the caudal fin, leading-edge vortices were exploited by earliest vertebrates to colonize the water column more than 400 Mya. Digital particle image velocimetry and force measurements in a water channel provide evidence that leading-edge vortices could be exploited by earliest vertebrates to colonize the water column more than 400 Mya. [ABSTRACT FROM AUTHOR]

Armour has been present in vertebrates for millions of years and has evolved independently several times in the ray finned fishes. While armour is typically considered defensive, it is a multifunctional trait with many plausible alternative functions. We explore the hydrodynamic function of armour in nine species of poachers (Agonidae) by visualizing flow to assess drag reduction. We used microcomputed tomography scans to categorize armour morphology into morphotypes, and compared the rugosity (surface roughness) across these morphotypes. We then used digital particle image velocimetry to visualize boundary layer separation along 3D-printed whole-body models made using the scans. Poacher armour comprises eight rows that, some distance behind the second dorsal fin, merge into six. We found four morphotypes, with higher rugosity observed for the prominent spine morphotype and the lowest rugosity in the no-spine morphotype. Principal component analysis revealed that much of the variation in armour morphology is driven by plate shape, spine size, and protrusion. The boundary layer was retained longer in species with larger spines. Overall, the presence of a spine increased boundary layer retention, decreasing the drag, which is advantageous for these benthic fish. [ABSTRACT FROM AUTHOR]

The mechanism of surface subsidence induced by shallow buried excavation construction and the failure mode under ultimate state were not unclear. In view of this situation, a scaled model experiment based on particle image velocimetry(PIV)was conducted to simulate the construction process of shallow buried tunnels in sandy soil and Ⅴ-grade surrounding rock. PIV technology was used to analyze the deformation images of the surrounding rock during the construction process of the tunnel, and to obtain displacement cloud maps of geological deformation induced by construction disturbance. By analyzing the characteristics and gradual evolution of surface deformation induced by underground excavation construction disturbance, the shape and range of the failure surface and collapse induced by underground excavation tunnel construction in different strata under the limit state were summarized. The equation of the failure surface induced by the collapse of the arch top strata during underground excavation construction was derived by using the upper limit theorem of limit analysis and variational principle, and the collapse surface graph was drawn. The validity of the model test results was verified by comparing the range of collapse induced by underground excavation construction in the arch crown strata obtained from the model test with the theoretical calculation results. [ABSTRACT FROM AUTHOR]

The influence of circulating liquid velocity on bubble size distribution (BSD) and turbulence characteristics of an external loop airlift reactor was studied in this paper. The instantaneous and time‐averaged velocity in the riser were studied using particle image velocimetry (PIV), and BSDs were measured by digital image analysis technique based on machine learning. Then turbulence kinetic energy (TKE) and energy dissipation rate (EDR) were calculated through the velocity field. The results indicate that as the circulating liquid velocity increased, the peak value of BSD rapidly decreased from nearly 6 mm to approximately 2 mm. The radial velocity of the liquid gradually decreased and changed direction, eventually increasing again. TKE first decreased and then increased. Compared with bubble flow with a BSD peak of 2–6 mm, bubble flow with a BSD peak of 2 mm had larger TKE. The radial movement of bubbles had great influence on the turbulence characteristics. This study demonstrates that selecting an appropriate circulating liquid velocity can reduce the diameter of bubbles while obtaining greater TKE, thereby improving the mass transfer and reaction efficiency in external loop airlift reactors (EL‐ALR). [ABSTRACT FROM AUTHOR]

In the present study, the response of a hypersonic turbulent boundary layer at an inflow of Ma∞ = 6 and Re∞ = 16·106 1/m to a smooth and rough surface along a sharp cone is examined. The model consisted of three segments with exchangeable parts to consider smooth and rough surfaces with a roughness topology of square bar elements with a nominal wavelength of four times the height of the elements. In selected regions of interest, the flow field was measured by particle image velocimetry (PIV) which enabled analysis of mean velocity fields and Reynolds stresses. Van Driest transformed smooth wall mean velocity profiles showed the expected incompressible behavior and compared well to previous investigations. A combination of an integral and fitting approach is discussed to enable inner scaling of the rough wall profiles, which showed the expected shift below the smooth wall profile. The smooth wall turbulence profiles from PIV agreed to artificially filtered DNS in case of the streamwise component. Turbulence profiles above the smooth and rough wall agreed to within measurement accuracies. Additionally, two−point correlations were used to investigate turbulent structures above the smooth and rough wall. Both, length scales and orientations of the correlations, showed high level of agreement between smooth and rough walls, with only differences close to the wall. Furthermore, uniform momentum zones could be identified with similar behavior along both smooth and rough walls. Information from turbulence data support outer layer similarity, whereas mean velocity profiles show an increase in Coles wake parameter for the rough wall data. This might be influenced by transitional roughness effects. [ABSTRACT FROM AUTHOR]

Particle streak velocimetry (PSV) is a Lagrangian velocity measurement method based on streak imaging of moving particles and is regarded as the origin of particle image velocimetry (PIV) and particle tracking velocimetry (PTV). Recently, the PSV technique has undergone further developments, realizing measurements of three velocity components in three dimensions (3D3C), by combining with stereoscopic observation, defocused imaging, light field photography and /or holography. Moreover, image processing algorithms based on deep learning have been successfully applied to PSV. Compared with PIV and PTV, the PSV technique can exhibit several advantages, including extending the upper limit of the velocity measurement range under the same equipment conditions, measuring with lower illumination intensity, often an overall lower equipment complexity and cost for the same measuring requirement, as well as avoiding the particle matching problems of PTV. However, the PSV method also has obstacles to overcome, such as directional ambiguity and the difficulty in identifying streak crossings. For the directional ambiguity problem, there are currently time-coding, color-coding, brightness-coding and determination methods using additional image frames that can be employed. The main application areas of PSV currently include microfluidics, high-velocity flows and large-scale flow field measurements. This review presents the state of the art of PSV and summarizes advantages, disadvantages, accuracy, complexity and application of various configurations. The configurations discussed are focused on those measuring three velocity components and several examples are described in which PSV can be advantageously applied. The review concludes with some future developments that can be anticipated. [ABSTRACT FROM AUTHOR]

In this work, micro air vehicles (MAVs) equipped with bio-inspired wings are investigated experimentally in wind tunnel. The starting point is that insects such as dragonflies, butterflies and locusts have wings with rigid tubular elements (corrugation) connected by flexible parts (profiling). So far, it is important to understand the specific aerodynamic effects of corrugation and profiling as applied to conventional wings for the optimization of low-Reynolds-number aerodynamics. The present study, in comparison to previous investigations on the topic, considers whole MAVs rather than isolated wings. A planform with a low aperture-to-chord ratio is employed in order to investigate the interaction between large tip vortices and the flow over the wing surface at large angles of incidence. Comparisons are made by measuring global aerodynamic loads using force balance, specifically drag and lift, and detailed local velocity fields over wing surfaces, by means of particle image velocimetry (PIV). This type of combined global–local investigation allows describing and relating overall MAV performance to detailed high-resolution flow fields. The results indicate that the combination of wing corrugation and profiling gives effective enhancements in performance, around 50%, in comparison to the classical flat-plate configuration. These results are particularly relevant in the framework of low-aspect-ratio MAVs, undergoing beneficial interactions between tip vortices and large-scale separation. [ABSTRACT FROM AUTHOR]

This study utilized particle image velocimetry (PIV) to analyze the kinematic and hydrodynamic characteristics of juvenile goldfish across three swimming modes: forward swimming, burst and coast, and turning. The results demonstrated that C-shaped turning exhibited the highest speed, enabling rapid and agile maneuvers for predator evasion. Meanwhile, forward swimming was optimal for sustained locomotion, and burst-and-coast swimming was suited for predatory behaviors. A vorticity analysis revealed that vorticity around the tail fin was the primary source of propulsive force, corroborating the correlation between vorticity magnitude and propulsion found in previous research. The findings emphasize the crucial role of the tail fin in swimming efficiency and performance. Future research should integrate ethology, biomechanics, and physiology to deepen the understanding of fish locomotion, potentially informing the design of efficient biomimetic underwater robots and contributing to fish conservation efforts. [ABSTRACT FROM AUTHOR]

This study experimentally optimizes the mixing of a turbulent jet at Reynolds number 10 000 with the surrounding air by targeted shear layer actuation. The forcing is composed of superposed harmonic signals of different azimuthal wavenumber m generated by eight loudspeakers circumferentially distributed around the nozzle lip. Amplitudes and frequencies of the individual harmonic contributions serve as optimization parameters, and the time-averaged centerline velocity downstream of the potential core is used as a metric for mixing optimization. The actuation is optimized through Bayesian optimization. Three search spaces are explored—axisymmetric forcing, m = 0, superposed axisymmetric and helical forcing, m ∈ { 0 , 1 } , and axisymmetric actuation combined with two counter-rotating helical modes, m ∈ { − 1 , 0 , 1 }. High-speed particle image velocimetry (PIV) is employed to analyze the jet response to the optimized forcing. The optimization processes are analyzed by persistent data topology. In the search space of axisymmetric excitation, the routine identifies an actuation at the natural frequency of the flow to be most efficient, with the centerline velocity being decreased by 15%. The optimal solutions in both the two-mode and three-mode search space converge to a similar forcing with one axial and one helical mode combined at a frequency ratio of around 2.3. Spectral analysis of the PIV images reveals that for the identified optimal forcing frequencies, a non-linear interaction between forced and natural structures in the jet flow is triggered, leading to a reduction in centerline velocity of around 35%. The topology of the most complex search space from the discrete data reveals four basins of attractions, classified into three forcing patterns including axisymmetric, axisymmetric-helical, and axisymmetric-flapping. Two deep basins are related to the optimal pattern found as axisymmetric-helical, and the others are shallower. [ABSTRACT FROM AUTHOR]

The effects of microperforated ring fairings on the flow and noise of a circular cylinder were investigated experimentally at Reynolds number based on the cylinder diameter (D) ranging between 1.3 and 2.6 × 104. The fairings were installed either concentrically or eccentrically, and the parameters investigated are the perforation rate σ (11.8%–34.6%), the clearance ratio δ /D (0.1–0.3), and the deflection angle θ (0°–180°). The noise was measured using far-field microphones, and flow characteristics were tested by the particle image velocimetry (PIV) and a dynamic balance. The acoustic results showed that the aerodynamic noise of the cylinder generally decreases with the increase in the perforation rate σ and the clearance δ. The maximum noise reduction at the fundamental vortex shedding frequency can reach 25 dB when arranged concentrically at δ /D = 0.3 and σ = 34.6%. Under the same perforation and clearance, the eccentric arrangement at θ = 60°–120° significantly improves the noise control performance. Flow visualization by PIV test demonstrated that the perforated fairing effectively controls the unsteady flow downstream of the cylinder and attenuates the large-scale vortex shedding, resulting in the noise reduction. The force measurement results showed that unsteady lift coefficient is significantly reduced, but the mean drag coefficient of the cylinder together with the fairings is generally higher than the bare cylinder. Nevertheless, the increment in drag coefficient can be lessened by eccentrically arrangement of fairings at deflection angle between θ = 60°–120°. [ABSTRACT FROM AUTHOR]

The results of particle image velocimetry in the near-wake of a rectangular wing section with the leading edge tubercles of various lengths and an airfoil, developed by the National Advisory Committee for Aeronautics (NACA 0012), are presented. The main attention is paid to the study of the influence of the tubercle length on the features of the formation of the flow in the near-wake of the wing model. Based on the reference case of a wing model with a chord of 100 mm and a span–chord ratio of 2, the influence of four tubercle lengths (5, 15, 25, and 35 mm) of tubercles, located symmetrically relative to the chord of the wing airfoil, was studied within the interval of chord-based Reynolds number Reb = 3.65 × 104–2.36 × 105 and for two angles of attack α = 0° (no separation case) and α = 9.5° (massive laminar stall). In order to understand the effect of the regular structure of counter-rotating vortex pairs, formed by the tubercles and its further interaction with the boundary layer of the wing, measurements were performed in two sections of the near-wake perpendicular to the wingspan: in the plane of symmetry of the tubercle and in the middle plane between the pair of neighbor tubercles. It has been experimentally confirmed that even at α = 0°, the tubercles are the source of additional drag, estimated at 3%–18%. It was also found that even the shortest tubercles of 5 mm are, apparently, the most rational in terms of their both aerodynamic and weight efficiency. [ABSTRACT FROM AUTHOR]

In this work, laser-induced florescent particle image velocimetry was performed to measure simultaneously the liquid and vapor velocity fields at the mid-span of a small-scale Venturi type section to determine the presence of a slip velocity between the phases. Various dynamic behavior and Kelvin–Helmholtz (K-H) instability involved in the cloud cavity shedding regime are discussed at four different cavitation numbers. The velocity, vorticity, and turbulence field information of the two phases are analyzed. The liquid–vapor mixture in a cavitating flow is usually considered a homogeneous medium in currently used computational models, but it is shown in this study that the two phases have very different dynamics. The measurements of the time-averaged velocities highlight the existence of a noteworthy slippage between the liquid and the vapor phases, especially in the upstream part of the cavitation region, where the slippage between the two phases can reach about 50% of the liquid velocity. Using phase-locked average, it is shown that the slip velocity in the upstream region is mainly located at the upper liquid–vapor interface, while the slip velocity in the closure area is near the bottom wall, due to the reentrant jet. These results contradict a primary assumption of the current models, where the medium is usually considered as a homogeneous mixture with a unique velocity field, thus providing a reference for future computational model improvement. [ABSTRACT FROM AUTHOR]

Understanding the flow physics behind fish schooling poses significant challenges due to the difficulties in directly measuring hydrodynamic performance and the three-dimensional, chaotic, and complex flow structures generated by collective moving organisms. Numerous previous simulations and experiments have utilized computational, mechanical, or robotic models to represent live fish. And existing studies of live fish schools have contributed significantly to dissecting the complexities of fish schooling. But the scarcity of combined approaches that include both computational and experimental studies, ideally of the same fish schools, has limited our ability to understand the physical factors that are involved in fish collective behavior. This underscores the necessity of developing new approaches to working directly with live fish schools. An integrated method that combines experiments on live fish schools with computational fluid dynamics (CFD) simulations represents an innovative method of studying the hydrodynamics of fish schooling. CFD techniques can deliver accurate performance measurements and high-fidelity flow characteristics for comprehensive analysis. Concurrently, experimental approaches can capture the precise locomotor kinematics of fish and offer additional flow information through particle image velocimetry (PIV) measurements, potentially enhancing the accuracy and efficiency of CFD studies via advanced data assimilation techniques. The flow patterns observed in PIV experiments with fish schools and the complex hydrodynamic interactions revealed by integrated analyses highlight the complexity of fish schooling, prompting a reevaluation of the classic Weihs model of school dynamics. The synergy between CFD models and experimental data grants us comprehensive insights into the flow dynamics of fish schools, facilitating the evaluation of their functional significance and enabling comparative studies of schooling behavior. In addition, we consider the challenges in developing integrated analytical methods and suggest promising directions for future research. [ABSTRACT FROM AUTHOR]

Dielectric barrier discharge plasma actuators, capable of creating a quasi-steady wall jet, are well suited for flow control over the microair vehicles at low Reynolds number. The plasma actuator is usually excited by a sinusoidal high-voltage power with a frequency of a few to tens of kilohertz. However, the investigations on the flowfield produced by a plasma actuator with a frequency below 200 Hz remain limited. Motivated by this demand, the formation and characterization of the vortices generated by a plasma actuator with a high-voltage frequency of 125 Hz are studied in detail. In addition to the starting vortex, it is of great importance that a train of vortices that shed periodically from the junction between the two electrodes and are quite different from the starting vortex are first observed. The shedding frequency of the periodic vortices is the same as the high-voltage frequency of 125 Hz. Combining the acoustic with the flow characteristics of the plasma actuator, the formation mechanism of these periodic vortices is discussed. Finally, a criterion for generating the periodic vortices is proposed based on the relationship between the scale of vortices and the separation distance between the two neighboring periodic vortices. [ABSTRACT FROM AUTHOR]

Acoustic spectra of rotor noise yield frequency distributions of energy within pressure time series. However, they are unable to reveal phase relations between different frequency components while these play a role in the fundamental understanding of low-frequency intensity modulation of higher-frequency rotor noise. A methodology to quantify interfrequency modulation is applied to a comprehensive acoustic dataset of a fixed-pitch benchmark rotor, operating at a low Reynolds number and at advance ratios ranging from 𝐽=0 to 0.61. Our findings strengthen earlier observations in case of a hovering rotor, in which the modulation of the high-frequency noise is strongest around an elevation angle of 𝜃=-20° (below the rotor plane). For the nonzero advance ratios, modulation becomes dominant in the sector -45°≲𝜃≲0° and is most pronounced at the highest advance ratio tested (𝐽=0.61). Intensity modulation of high-frequency noise is primarily the consequence of a far-field observer experiencing a cyclic sweep through the noise directivity pattern of relatively directive trailing-edge noise. This noise component becomes more intense with increasing 𝐽 and is associated with broadband features of the partially separated flow over the rotor blades. [ABSTRACT FROM AUTHOR]

Trailing edge serration is an effective method to reduce broadband noise generation by an airfoil. However, the noise reduction performance can be significantly reduced when there is flow misalignment at the serration. This experimental study investigates the impact of a shift in the serration position downstream of the airfoil trailing edge on the noise reduction performance, referred to in this paper as serration extension. Experiments were performed on a 100-mm-chord NACA 0012 wing model with sawtooth trailing edge serrations. The serration performance was studied at 0° and 7° flap-down configurations at various angles of attack. The serrations with three different extension lengths of 5, 10, and 15 mm were tested and compared with the baseline case without extension. The emitted noise was measured with a phased microphone array. The results show a significant reduction in broadband high-frequency noise by extension under loading conditions. Particle image velocimetry measurements of root flow along the wall-normal plane reveal diminished crossflow across the serration after extension. Furthermore, the extensions cause an up-to-25% increase in the maximum lift coefficient and improved or unaltered lift-to-drag ratios. [ABSTRACT FROM AUTHOR]

The local interactions between the flame-front and turbulence control the dynamics, morphology, and propagation of a premixed turbulent flame. To investigate such complex dynamics of a flame-turbulence interaction, we present an experimental exposition of a premixed turbulent Bunsen flame. Several quantities have been evaluated to assess the flame-turbulence interaction. We first measured the statistics of the flowfield adjacent to the flame and compared it with the cold flow. This allowed us to evaluate the effect of the flame on the upstream turbulence. Subsequently, we performed statistical analyses of the local values of various stretch rates and quantified how their distribution changes with turbulence intensity and flame temperature. We also evaluated the pairwise relation among various stretch rates to assess their dependence on each other. Finally, we used flame particles to evaluate the Lagrangian evolution of stretch rates conditioned on flame-fronts. All the analyses presented in this work point out Karlovitz number as a key factor in determining the flame-turbulence interaction. Specifically, we observe a stronger influence of turbulent eddies on flames with increasing Karlovitz number, as evidenced by the reduced effect of flame on upstream flow, wider probability distribution functions of stretch rates, and increased persistence timescales for stretches. [ABSTRACT FROM AUTHOR]

A comprehensive analysis of the evolutionary dynamics of soil arches is crucial for accurately predicting soil deformations above sinkholes and assessing stability of underground structures. In this study, a series of trapdoor tests were conducted to investigate the progressive development of soil arching in dense sand. The particle image velocimetry (PIV) technique was utilized to capture soil deformation patterns, while fiber optic strain sensing cables were used to validate the displacement influence zone of soil by measuring strain profiles of the foundation. The ground reaction curves, derived from the measurements of earth pressure cells, shed light on the evolution process of stress redistribution and the rotation of principal stresses. The test results reveal that the formation of soil arching alters the overlying pressure on the trapdoor, transferring loads from yielding soil to adjacent stationary soil. The development of soil failure surfaces corresponded with vertical stress variations on the trapdoor. The strain profiles exhibited a characteristic trough above the trapdoor, along with double peaks at its edges. The measurements of micro-anchored strain sensing cables with small anchor spacings provided more accurate distributions of soil shear deformation. Furthermore, the orientation and magnitude of soil arching was inferred from principal stress rotations. The insights gained in this study are valuable for understanding the propagation of soil arching, offering potential implications for the execution of rational geotechnical design and the mitigation of related geological hazards. [ABSTRACT FROM AUTHOR]

Flow-type landslides, including subaerial and submarine debris flows, have poor spatiotemporal predictability. Therefore, researchers rely heavily on experimental evidence in revealing complex flow mechanisms and evaluating theoretical models. To measure the velocity field of experimental flows, conventional image analysis tools for measuring soil deformation and hydraulics have been borrowed. However, these tools were not developed for capturing the kinematics of fast-moving soil–water mixtures over complex terrain under non-uniform lighting conditions. In this study, a new framework based on deep learning was used to automatically digitalize the kinematics of experimental flow-type landslides. Captured images were broken into sequences and binarized using a fully convolutional neural network (FCNN). The proposed framework was demonstrated to outperform classic image processing algorithms (e.g., particle image velocimetry, trainable Weka segmentation, and thresholding algorithms) over a wide range of experimental conditions. The FCNN model was even able to process images from consumer-grade cameras under complex shadow, light, and boundary conditions. This feature is most useful for field-scale experimentation. With fewer than 15 annotated training images, the FCNN digitalized experimental flows with an accuracy of 97% in semantic segmentation. [ABSTRACT FROM AUTHOR]

In this study, ultrasonic sensors were used to measure the vertical vortex at flood discharge outlets in real time, and numerical simulations and model experiments were conducted. When a sound signal passes through a vortex, its propagation characteristics will change, which helps to determine the existence of the vortex. Moreover, its characteristic parameters can be obtained through inversion. In this paper, first, the theories of acoustic measurement methods were introduced and their feasibility was verified through a comparison between Particle Image Velocimetry (PIV) measurement and numerical simulation results. Then, the Computational Fluid Dynamics (CFD) method was used to simulate the vertical vortex at the flood discharge outlets of hydraulic structures and the simulation data were restored to the actual size at scale. Finally, acoustic numerical simulations of actual vortex data were conducted, and ultrasonic sensors were used to measure the velocity of a simplified vertical vortex model under laboratory conditions. The research results indicate that the acoustic measurement method proposed in this article is effective in the measurement of the characteristic parameters of vertical vortex with a core radius of 0.03~0.05 m and a maximum tangential velocity of 0.5 m/s, the measurement error of the maximum tangential velocity is within 10%. [ABSTRACT FROM AUTHOR]

The work focuses on developing a detailed understanding of the effects of initial composition (Cin) and bottom cooling temperatures (TB) on different characteristics of double-diffusive convection during unidirectional solidification of water-NH4Cl solution in a complete non-intrusive manner. The qualitative investigation, simultaneous quantification of transported parameters (composition and temperature) and fluid velocities associated with flow patterns are carried out using rainbow schlieren deflectometry, dual-wavelength interferometry and PIV (particle image velocimetry) technique respectively. The dependence of the characteristics of double-diffusive convection for different Cin and TB is explained on the basis of real-time whole-field investigation using a combination of aforementioned imaging techniques. [ABSTRACT FROM AUTHOR]

This experimental study investigates the effect of surface waves that are superimposed on a unidirectional current around a bridge pier in a gravel bed, which employs particle image velocimetry (PIV). A comprehensive examination of various flow characteristics, which include the intermittency factor (IF), turbulent kinetic energy (TKE), and vorticity analysis, is conducted to elucidate the variations around the pier with respect to the Keulegan–Carpenter (KC) number. The findings reveal a correlation between the KC number and the flow dynamics in combined conditions: specifically, due to combined wave–flow conditions, escalation in the frequency of vortices near the pier and the increment in turbulent strength, in terms of the TKE. Of note, this augmentation in turbulent strength is associated with an increase in the KC number. At a low KC number, the maximum scour depth occurred downstream near the pier, which shifted to upstream with an increase in the KC number. In addition, the results could be utilized for the development and validation of numerical models, which accurately estimate the local scour under combined flow conditions. [ABSTRACT FROM AUTHOR]

This study aims to investigate the wake of two tandem square cylinders based on the Proper Orthogonal Decomposition (POD) analyses of the PIV and hotwire data. The cylinder centre-to-centre spacing ratio L/w examined is from 1.2 to 4.2, covering the four flow regimes, i.e., extended body, reattachment, transition and co-shedding. The Reynolds number examined was 1.3 × 104. A novel Proper Orthogonal Decomposition (POD) technique (hereafter referred to as PODHW) is developed to analyse data from single point hotwire measurements, offering a new perspective compared to the conventional POD analysis (PODPIV) based on Particle Image Velocimetry (PIV) data. A key finding is the identification of two distinct states, reattachment and co-shedding, within the transition flow regime at L/w = 2.8, which PODPIV fails to capture due to the limited duration of the PIV data obtained. This study confirms, for the first time, the existence of these states as proposed by Zhou et al. (2024), highlighting the advantage of using PODHW for capturing intermittent flow phenomena. Furthermore, the analysis reveals how the predominant coherent structures contribute to the total fluctuating velocity energy in each individual regime. Other aspects of the flow are also discussed, including the Strouhal numbers, the contribution to the total fluctuating energy of the flow from the first four POD modes, and a comparison between different regimes. [ABSTRACT FROM AUTHOR]

To gain a better understanding of the complex flow dynamics and stealth characteristics of submarines under maneuvering conditions, flow field experiments were conducted on the SUBOFF submarine model in the large low-speed wind tunnel at the China Ship Scientific Research Center (CSSRC). The three-dimensional velocity field above the hull at 6° and 9° yaw angles was captured using the stereo particle image velocimetry (SPIV) system. The experimental Reynolds numbers were selected as ReL = 0.46 × 107 and ReL = 1.08 × 107. The wake of the sail and the junction between the sail root and the hull were analyzed in detail, focusing on the core flow of the sail-tip vortex. The results revealed that at a larger yaw angle, the vorticity magnitude and turbulent kinetic energy (TKE) of the wake increased, and the downwash effect of the sail-tip vortex center became more pronounced. Furthermore, a higher Reynolds number resulted in an even more significant downwash of the vortex center, accompanied by a slight deviation towards the suction side. These experimental findings can contribute to the enrichment of the benchmark database for validating and improving numerical simulations of submarine wakes. [ABSTRACT FROM AUTHOR]

Turbulent flows are investigated upstream of a bar rack system that is recommended as optimum in recent literature from tests with several fish species of different morphology, swimming ability, and behavior. Both two‐dimensional two‐component and two‐dimensional three‐component state‐of‐the‐art particle image velocimetry were used to quantify and analyze hydrodynamic metrics important for downstream migrating species. The inclination angles of the bar and rack were 45° and 30°, respectively, and the thickness of the bottom overlay was 13% of the water depth. The two Reynolds numbers investigated, based on incoming velocity and bar thickness, were 4,000 and 6,000. The statistical and structural characteristics of turbulent flows in the streamwise‐spanwise plane at 5% water depth, and the streamwise‐vertical plane at channel mid‐span are discussed. Upstream of the bottom overlay, the mean flow is deflected and accelerated toward the bypass, leading to an increase in the Reynolds stresses, while the turbulence eddies become smaller. For effective fish guidance, it is recommended that sweeping velocity (Vp) be larger than normal velocity (Vn), with Vp parallel and Vn perpendicular to the bar rack and bottom overlay. In the downstream half of the bar rack, Vn may increase sufficiently to surpass Vp near the bypass, possibly reducing effective guidance for some species and sizes. Upstream of the bars, the levels of streamwise mean velocity vary abruptly, which may deter fish from contacting the bars. Although inferences on passage effectiveness are made based on previous studies, tests with different species and sizes are needed to confirm fish responses. Key Points: Turbulent flows upstream of a bar rack system with bottom overlay are characterized using particle image velocimetryUpstream, levels of the mean velocity and flow acceleration are significant and vary abruptly, which may deter fish from contacting the barsFlow conditions upstream of the bottom overlay may disorient fish and may affect guidance toward the bypass for safer downstream passage [ABSTRACT FROM AUTHOR]

Micropile groups (MPGs) are typical landslide resistant structures. To investigate the effects of these two factors on the micropile–soil interaction mechanism, seven sets of transparent soil model experiments were conducted on miniature cluster piles. The soil was scanned and photographed, and the particle image velocimetry (PIV) technique was used to obtain the deformation characteristics of the pile and soil during lateral loading. The spatial distribution information of the soil behind the pile was obtained by a 3D reconstruction program. The results showed that a sufficient roughness of the pile surface was a necessary condition for the formation of a soil arch. If the surface of the pile was smooth, stable arch foundation formation was difficult. When the roughness of the pile surface increases, the soil arch range behind the pile and the load-sharing ratio of the pile and soil will increase. After the roughness reaches a certain level, the above indicators hardly change. Pile spacing within the range of 5–7 d (pile diameters) was suitable. The support effect was poor when the pile spacing was too large. No stable soil arch can be formed, and the soil slips out from between the piles. [ABSTRACT FROM AUTHOR]