Search Results (648)

The integration of deep learning with optical flow estimation in Particle Image Velocimetry (PIV) represents an emerging solution. Extensive research indicates that deep learning has potential to match or outperform state-of-the-art classical algorithms in efficiency, accuracy, and spatial resolution. However, current learning-based methods, which rely on cost volumes and convolutions for flow regression, are limited to local correlations. This limitation hinders the capture of global information. While extensive iterative refinements enhance the quality of prediction flows, they also result in a linear increase in inference time. To enhance both efficiency and accuracy, we propose a global matching method for PIV. This method directly compares feature similarities to identify correspondences between images and generate estimated flows. The underlying idea is to first extract initial features of particle image pairs, then enhance these features through a Transformer specifically designed for PIV, and perform operations for feature correlation matching, followed by global optical flow propagation and optimization. Additionally, higher-resolution features are introduced for refinement. By employing both synthetic and experimental data, including benchmark sets and data from turbulent wave channel flow experiments, we demonstrate that global matching method in PIV achieves superior efficiency and accuracy compared to existing learning-based methods. Full article

This study investigates the flow field around a finite rectangular prism using both experimental and computational methods, with a particular focus on the influence of the turbulence approach adopted, the mesh resolution employed, and different subgrid length scales. Ten turbulence modelling and simulation approaches, including both ‘scale-modelling’ Reynolds-Averaged Navier–Stokes (RANS) models and ‘scale-resolving’ Delayed Detached Eddy Simulation (DDES), were tested across six different mesh resolutions. A case with sharp corners allows the location of the flow separation to be fixed, which facilitates a focus on the separated flow region and, in this instance, the three-dimensional interaction of three such regions. The case, therefore, readily enables an assessment of the ‘grey-area’ issue, whereby some DDES methods demonstrate delayed activation of the scale-resolving model, impacting the size of flow recirculation. Experimental measurements were shown to agree well with reference data for the same geometry, after which particle image velocimetry (PIV) data were gathered to extend the reference dataset. Numerical predictions from the RANS models were generally quite reasonable but did not show improvement with further refinement, as one would expect, whereas DDES clearly demonstrated continuous improvement in predictive accuracy with progressive mesh refinement. The shear-layer-adapted (SLA) subgrid length scale (ΔSLA) displayed consistently superior performance compared to the more widely used length scale based on local cell volume, particularly for moderate mesh resolutions commonly employed in industrial settings with limited resources. In general, front-body separation and reattachment exhibited greater sensitivity to mesh refinement than wake resolution. Finally, in order to correlate the observed DDES mesh requirements with the observations from the converged RANS solutions, an approximation for the Taylor microscale was explored as a potential tool for mesh sizing. Full article

Flame impingement heat transfer is implemented in many industrial applications. The laminar premixed Bunsen flame, impinging on a flat cold surface, represents a basic model for the validation of computational fluid dynamics (CFD) codes, used for the simulation of industrial processes. Meanwhile, as the present paper demonstrates, some features of basic flame configurations are not well-reviewed. The present paper reports on the direct numerical simulation of the thermofluidic field in a laminar premixed impinging Bunsen flame in comparison with advanced optical measurements. The results reveal the phenomenon of the central recirculation zone formation between the tip of the Bunsen flame cone and the cold surface. Cooled combustion products concentrate inside this zone, resulting in reduced heat transfer near the flow axis. All three tested chemical kinetic mechanisms (GRI-Mech 3.0, SanDiego, RMech1) provide reasonable predictions of the observed phenomenon, which explain previous experimental observations on the reduced heat transfer at the central axis of impinging flames. Moreover, the most detailed mechanism, GRI-Mech 3.0, predicts an elevated concentration of NO_X pollutants caused by the mentioned phenomenon. Full article

The presence of isolated stones in the soil layers of engineering sites has significantly increased. Currently, the existing methods for dealing with isolated stones are inadequate to meet engineering needs. This paper combines pile-planting technology with isolated stones to incorporate them into the load-bearing system, resulting in a new type of pre-drilled composite pile suitable for isolated stone sites. A visualization testing system for pile-soil deformation is developed using Particle Image Velocimetry (PIV) technology and transparent soil, conducting non-intrusive model tests on pile-planting and boulder-capped piles under different uplift load conditions, and comparing the results with a discrete-continuous coupled three-dimensional numerical model analysis. The results indicate that when an isolated stone with a cross-sectional area four times that of the pile exists at the pile tip, the ultimate pullout bearing capacity of the pile increases by a factor of two. Regarding the distribution of internal and external side friction resistances of the core and outer concrete of the piles, the internal friction resistance of piles without isolated stones is approximately 1.47 times that of the external friction resistance and about 0.8 times the ratio of the diameters of the pile and core. For piles with isolated stones at the tip, the internal friction resistance is approximately 1.37 times that of the external friction resistance. Under the ultimate load, the displacement field around the pile without an isolated stone exhibits an “inverted triangular” distribution; the displacement field around the pile with an isolated stone at the tip exhibits a “trapezoidal” distribution. This study investigates the bearing capacity and load transfer mechanisms of the new pre-drilled composite piles in isolated stone engineering sites, and the research findings may provide new solutions for similar construction projects involving rubble reclamation. Full article

This study investigates the transient behavior of a single-blade centrifugal pump under variable frequency speed regulation, with the objective of enhancing both pump efficiency and operational stability under variable frequency conditions. By integrating numerical simulations, external characteristic tests, and Particle Image Velocimetry (PIV) flow field experiments, the research provides a comprehensive analysis of the dynamic performance of the pump. The accuracy of the numerical simulations is first validated through a comparison between CFD results and experimental data, both at rated and variable speeds. This study then explores the transient external performance, internal flow patterns, and radial force characteristics of the pump under various speed-change schemes. In the process of acceleration, the variation trend of the centrifugal pump head and speed is basically the same, and Scheme 3 shows better stability; Scheme 2 minimizes the fluctuation of shaft power; with the increase in speed, the pressure and flow field in the pump will appear to be unstable. In the deceleration process, the Scheme 3 head fluctuates less, the change in shaft power is the most stable, and the more uniform pressure distribution and stable flow field can be maintained. The radial force increases with the increase in speed, but the degree of radial force fluctuation is different among different schemes. These findings offer valuable insights into the dynamic performance of centrifugal pumps under variable speed conditions and provide a foundation for optimizing both pump design and operational strategies. Full article

This article aims to highlight the importance of including quantitative measurements when conducting flow visualization tests, such as those performed in towing tanks, within fluid mechanics analysis. It investigates the possibility of measuring velocity fields with an economically accessible technique compared to other techniques that require large financial investments, such as traditional PIV. The development of a MATLAB R2024b code based on image recognition and the use of 3D-printed tracer particles is proposed. Code workflow and how to make a correct selection of the processing parameters and its activity are explained and demonstrated on artificial images, generated by a computer, as well as real images, obtained in a 2D-test in the tank, achieving an accuracy, in absolute values, of 95%. However, the proposed velocimetry system currently has one important limitation, the impossibility of distinguishing between particles in different planes, which limits the study to two-dimensional tests. Then, the opportunity to include this technique in the study of more complex tests requires further investigation. Full article

Passive flow control around airfoils, wind turbines, and submarines to enhance their aerodynamic properties is the subject of interest in several studies. Previous research provides different solutions, from basic changes in surface roughness and simple geometries to complex shapes and mechanical solutions. This article presents experimental research using the Particle Image Velocimetry (PIV) method on a NACA 0012 airfoil at a Reynolds number of 66,400. Initially, the airfoil was tested for three different angles of attack: 13°, 15°, 17°, and 19°. These tests revealed that angles of attack above 15° significantly increase boundary layer detachment, as shown in the normalized streamwise velocity fields Ux. In the second stage of the research, a different-shaped microcylinder with a characteristic dimension (d/c) of 0.01 was added to the leading edge of the airfoil at a high angle of attack of 17°. Unlike traditional vortex generators placed at the rear of the airfoil, this configuration aimed to reduce boundary layer detachment. The experiment demonstrated that the microcylinder effectively reduced boundary layer detachment at this angle of attack. Full article

Cavity flows have a wide range of low-speed applications (M≤0.3), such as aircraft wheel wells, ground transportations, and pipelines. They induce strong flow oscillations which can substantially increase noise, drag, vibration, and lead to structural fatigue. In the current study, a steady jet was forced from the cavity trailing edge with different momentum fluxes (J = 0.11 kg/m·s², 0.44 kg/m·s², and 0.96 kg/m·s²). The aim of this study was to investigate the impact of the steady jet on the time-averaged flow field and the cavity separated shear layer oscillations for an open cavity with a length-to-depth ratio of L/D=4 at Reθ=1.28×103. Particle image velocimetry, surface oil flow visualisation, constant temperature anemometry, and pressure measurements were performed. The study found that increasing the jet momentum flux caused a significant increase in thickness and deflection of the cavity separated shear layer. Due to the counterflow interaction between the jet and cavity separated shear layer, the growth rate (dδω/dx) of the cavity separated shear layer increased significantly from 0.193 for the no-jet case to 0.273 for the J = 0.96 kg/m·s² case. As a result, the return flow rate increased, causing the separation point on the cavity floor to shift upstream from x/L≈0.2 for the no-jet case to x/L≈0.1 for the J = 0.96 kg/m·s² case. Furthermore, increasing the jet momentum flux increased the broadband level of the cavity separated shear layer oscillations. Full article

A series of experiments were performed using multiple configurations of hydrofoils to assess the energy harvesting capabilities present within the wake of streamlined bodies. The experiments were performed in a low-speed water tunnel, with energy harvesting assessed using a piezoelectric eel and imaging equipment. Half-sinusoidal undulations were introduced in different combinations on the leading and trailing edges of the hydrofoil. All hydrofoils utilized a NACA 0012 cross-sectional profile. A piezoelectric eel was placed at a variable distance downstream of the hydrofoil’s trailing edge, and the hydrofoil’s angle of attack (α) was varied in order to assess the variation in power generation. The maximum power output was achieved at x/c = 1–1.5 downstream of the trailing edge in all configurations. It was observed that harvested energy is dependent on the oscillation of the eel, α, the streamwise distance between the trailing edge of the hydrofoils and the eel, as well as the geometry of the hydrofoils. Particle image velocimetry was also performed on selected cases for which the recorded energy harvest was high. The results showed that the NACA 0012 base profile has a higher extractable energy capacity in its wake than do the serrated hydrofoils, which confirms the results found in the literature. Full article

Image velocimetry has become an effective method of mapping flow conditions in rivers, but this analysis is typically performed in a post-processing mode after data collection is complete. In this study, we evaluated the potential to infer flow velocities in approximately real time as thermal images are being acquired from an uncrewed aircraft system (UAS). The sensitivity of thermal image velocimetry to environmental conditions was quantified by conducting 20 flights over four days and assessing the accuracy of image-derived velocity estimates via comparison to direct field measurements made with an acoustic Doppler current profiler (ADCP). This analysis indicated that velocity mapping was most reliable when the air was cooler than the water. We also introduced a workflow for River Velocity Measurement in Approximately Real Time (RiVMART) that involved transferring brief image sequences from the UAS to a ground station as distinct data packets. The resulting velocity fields were as accurate as those generated via post-processing. A new particle image velocimetry (PIV) algorithm based on staggered image sequences increased the number of image pairs available for a given image sequence duration and slightly improved accuracy relative to a standard PIV implementation. Direct, automated geo-referencing of image-derived velocity vectors based on information on the position and orientation of the UAS acquired during flight led to poor alignment with vectors that were geo-referenced manually by selecting ground control points from an orthophoto. This initial proof-of-concept investigation suggests that our workflow could enable highly efficient characterization of flow fields in rivers and might help support applications that require rapid response to changing conditions. Full article

During the underwater movement of a submarine, cooling water at a specific temperature is discharged into the surrounding water through nuclear reactor secondary loop circulation, creating a thermal jet. Thermal jets are characterized by initial velocity and temperature properties that allow for complete mixing with the surrounding water through a combination of mixing and heat transfer processes. This paper aims to investigate the movement and diffusion of underwater thermal jets, specifically examining the temperature stratification of the ambient water, the initial velocity of the jet, and the effect of temperature on the velocity field and temperature field of the underwater thermal jet. This study utilizes particle velocity measurements and the laser-induced fluorescence method to measure the velocity field and temperature field of the thermal jet, as well as simulation methods to validate conclusions. The experimental and simulation conditions in this paper are mainly categorized into two types: uniform water body and thermally-stratified water body. Upon analysis and comparison of the experimental and simulation results, it has been observed that an increase in jet velocity will hinder the upward diffusion of jet temperature, decrease the floating height of the jet, and slow down the rate at which the jet temperature decays. Furthermore, as the difference between the jet temperature and the ambient water temperature increases, the upward diffusion of the jet temperature becomes predominant, resulting in a 40–50% increase in its floating rate. It is evident that the stratification conditions of the background environment have a significant impact on the jet temperature diffusion. When the jet temperature diffuses to the thermally-stratified interface of water in the tank, it ceases to float due to density differences; consequently, its temperature cannot diffuse further towards or reach the water surface. Full article

This study investigates the movements of particles in an accelerated toroidal flow channel filled with water, with specific applications for a particle imaging velocimetry gyroscope (PIVG). We used computational fluid dynamics (CFD) to simulate particle behavior under different angular accelerations. These angular accelerations were 4 rad/s², 6 rad/s², and 8 rad/s² for particles densities of 1100 kg/m³, 1050 kg/m³, and 980 kg/m³. An examination was performed on the particles’ concentration distribution, velocity profiles, and displacement patterns with respect to the toroidal geometry, which had a volume fraction of 1.5% and was sized at 50 microns. Our results show that particle density significantly affects behavior and displacement within the toroidal flow, with heavier particles (1100 kg/m³) settling more quickly and concentrating near the lower z values over time, while lighter particles (980 kg/m³) maintain a more uniform distribution. This understanding is crucial for optimizing PIVG accuracy and reliability. Full article

For decades, studies have been conducted on the efficiency of gas purification processes with wet scrubbers, including the Venturi scrubbers, and this is the most commonly addressed issue in the field literature. The Venturi scrubber consists of a Venturi nozzle and a cyclone. The article addresses the empirical and analytical studies on the annular–mist flow regime that exists in the throat of the Venturi nozzle with a square cross-section. The uniform distribution of droplets over the cross-section area of the Venturi’s throat strongly correlates with the efficiency of the gas cleaning process using Venturi scrubbers. Due to the above, studies on the physics of the phenomena that affect the quantity of small droplets present in the core of the flow are highly justified. The influence of body forces and diffusive mechanisms impacting the number of droplets in the core flow were investigated to tackle the problem in question. Consequently, the fractions of droplets susceptible to turbulent or inertial–turbulent diffusion mechanisms can now be predicted using the outcomes of the research carried out. The droplets were divided into three fractions that differed by their sizes as follows: airborne droplets I confirm thar italic can be removed in all cases. (dd≤ 10 µm), medium-sized droplets (dd≤ 20 µm), and largest droplets (dd = (50–150) µm). The estimation of diffusion coefficients εd,M,εd,ref and stopping distances sM,sref of all fractions of droplets was carried out with the inclusion εd,M,sM and exclusion εd,ref,sref of the Magnus lift force M in equations of both the droplet’s stopping distance and its diffusion coefficient. The outcomes revealed that the inclusion of the M force translates significantly to the growth in values of εd,M,sM compared to εd,ref,sref. Hence, it was concluded that the M force impacts the increase in the speed of the diffusion of the droplets with dd≥ 16.45 µm, which is favorable. Hence, the inertial–turbulent diffusion of larger droplets and the turbulent diffusion of medium ones seem to be supported by the M force. The local velocity gradient, which varied within the region of the flow’s hydraulic stabilization also impacted the mass content of droplets with diameter dd≤ 10 µm in the core of the flow. As the flow development progressed, the number of droplets measured at n = 5 Hz varied nonlinearly up to the point where the boundary layer thickness reached the channel radius. The quantity of small droplets in the main flow was significantly influenced by turbulence intensity (Tu). The desired high number of small droplets in the core of the flow (mist flow) was estimated empirically, and it was achieved when gas flows at high speed and has a mean value of Tu. The former benefits the efficiency of gas purification. Investigations on the effects of body forces of inertia of the continuous phase on the separation of droplets with diameters of a few microns and sub-microns from the flow were performed by employing two channel elbows, namely e4 and e1. The curved channels were subsequently mounted at the end of the straight channel (SCh2). The curvature angle (α) of the e4 and e1 equaled 90 °C and 30 °C, respectively. The number of droplets existing in the mist flow was higher in value, as desired, when the e4 was used, unlike e1. Two-dimensional flow fields of the mist have been obtained using the Particle Imaging Velocimetry (PIV) technique and analyzed further. Topas LAP 332 Aerosol Spectrometer was used for the determination of droplet (dd≤ 40 µm) size distribution (DSD) and particle concentrations, while the Droplet Size Analyzer D Kamika Instruments (DSA) was exploited to ascertain DSD of droplets with diameter dd>40 µm. Full article

At present, research on aerial spraying operations with UAVs mainly focuses on the deposition outcomes of droplets, with insufficient depth in the exploration of the movement process of droplet deposition. The movement characteristics of droplet deposition as the most fundamental factors affecting the effectiveness of pesticide application by UAVs are of great significance for improving droplet deposition. This study takes flat spray nozzles as the research object, uses the Particle Image Velocimetry (PIV) technique to obtain movement data of water droplet deposition under the influence of rotor flow fields, and investigates the variation characteristics of droplet deposition speed under different influencing factors. The results show that the deposition speed and the distribution area of high-speed (>12 m/s) particles increase with the increase of rotor speed, spraying pressure, and nozzle size. When the rotor speed increases from 0 r/min to 1800 r/min, the average increase in maximum droplet deposition speed for nozzle models LU120-02, LU120-03 and LU120-04 is 33.26%, 19.02%, and 7.62%, respectively. The rotor flow field significantly increases the number of high-speed droplets, making the dispersed droplet velocity distribution more concentrated. When the rotor speed is 0, 1000, 1500, and 1800 r/min, the average decay rates of droplet deposition speed are 36.72%, 20.00%, 15.47%, and 13.21%, respectively, indicating that the rotor flow field helps to reduce the decrease in droplet deposition speed, enabling droplets to deposit on the target area at a higher speed, reducing drift risk and evaporation loss. This study’s results are beneficial for revealing the mechanism of droplet deposition movement in aerial spraying by plant protection UAVs, improving the understanding of droplet movement, and providing data support and guidance for precise spraying operations. Full article

This study presents the experimental validation of a remote sensing method for river flow velocity measurement, from which discharge is calculated, using Particle Image Velocimetry (PIV) combined with Unmanned Aerial Vehicles (UAVs). The case study focuses on the Antisana River in the Ecuadorian Andes, a region with challenging geography where accurate flow measurement is crucial for hydroelectric projects. The validation results demonstrate that the velocity measurements obtained through PIV closely align with those from standardized traditional methods. Furthermore, integrating technologies such as LiDAR for cross-sectional measurements, along with UAVs, would enable the accurate estimation of discharge in difficult-to-access areas. This approach has the potential to significantly enhance hydrological studies and water resource management in remote regions, especially for hydroelectric projects in the Ecuadorian Andes. Full article