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Abstract: The local scale of rotating convection, ℓ, is a fundamental parameter in many turbulent geophysical and astrophysical fluid systems, yet it is often poorly constrained. Here we conduct rotating convection laboratory experiments analogous to convecting flows in planetary cores and subsurface oceans to obtain measurements of the local scales of motion. Utilizing silicone oil as the working fluid, we employ shadowgraph imagery to visualize the flow, from which we extract values of the characteristic cross‐axial scale of convective columns and plumes. These measurements are compared to the theoretical values of the critical onset length scale, ℓcrit, and the turbulent length scale, ℓturb. Our experimentally obtained length scale measurements simultaneously agree with both the onset and turbulent scale predictions across three orders of magnitude in convective supercriticality , a correlation that is consistent with inferences made in prior studies. We further explore the nature of this correlation and its implications for geophysical and astrophysical systems.

The gradients in surfactant distribution at a fluid-fluid interface can induce fluid flow known as the Marangoni flow. Fluid interfaces found in biological and environmental systems are seldom clean, where mixtures of various surfactants are present. The presence of multi-component surfactant mixtures introduces the possibility of interactions among constituents, which may impact Marangoni flows and alter flow dynamics. We employed flow visualization, surface tension and reaction kinetic measurements, and numerical simulations to quantitatively investigate the Marangoni flows induced by the reacting surfactant mixtures. Different binary surfactant mixtures were utilized for comparative analysis. The impact of surfactant interactions on Marangoni flows is confirmed through the observation of diverse complex flow patterns that result from the combination of oppositely charged surfactants in varying composition ratios and concentrations. Unique flow patterns originate from the composition-dependent interfacial phenomena upon mixing surfactants. Our findings provide vital insights that could be used to guide the development of effective oil remediation or the spreading of waterborne pathogens in contaminated regions. [ABSTRACT FROM AUTHOR]

Considered are interactive relationships between a normal shock wave and the downstream shock wave leg of the associated lambda foot, as well as between a normal shock wave and time-varying static pressure as measured along the bottom surface of the test section. Such relationships are investigated as they vary with two different magnitudes of inlet unsteady Mach wave intensity and are characterized using shadowgraph flow visualization data, as well as power spectral density, magnitude-squared coherence, and time lag data. Employed for the investigation is a specialty test section with an inlet Mach number of 1.54, as utilized within a transonic/supersonic wind tunnel. The resulting data provide evidence of distinct interactions over a wide range of frequencies between the normal shock wave and the downstream shock wave leg of the lambda foot for low inlet unsteady Mach wave intensity. Note that these are not present in the same form and over the same ranges of frequency with high inlet unsteady Mach wave intensity. These differences are partially due to the location where flow events originate. The most significant sources of flow unsteadiness within the present investigation are mostly associated with the normal and oblique shock waves (with low inlet unsteady Mach wave intensity), and mostly with inlet flow disturbances from unsteady Mach waves (with high inlet unsteady Mach wave intensity). The present experimental results additionally evidence important connections between the normal shock wave and unsteady flow events within lower portions of the lambda foot, especially near the adjacent boundary layer separation region. [ABSTRACT FROM AUTHOR]

This study presents an experimental and numerical investigation to characterize the plume pattern of a high-aspect-ratio rectangular convergent/divergent nozzle with an aft deck in under-expanded conditions. The function of an aft deck is to shield the infrared signal of an exhaust plume at its strongest intensity located at the immediate downstream region of the nozzle exit. However, this practice may cause undesirable plume deflection, which needs to be reduced as much as possible. The nozzle pressure ratios ranged from 2 to 4, and the effect of the nozzle exit aspect ratio was examined using wall static pressure measurements and schlieren visualization for cold flows. The experimental setup involved a 3D-printed aft deck nozzle made of acrylonitrile butadiene styrene material, which underwent surface smoothing using acetone vapor. Numerical simulations were conducted using the commercial STARCCM + software to analyze static pressure ratio variations at the aft deck. The investigation revealed that a nozzle pressure ratio of 3 induced a downward plume deflection at aspect ratio values of 6.77 and 7.54, while an increased aspect ratio of 8.35 resulted in the horizontal ejection of the plume. Moreover, at an aspect ratio of 8.35, the plume was ejected horizontally for nozzle pressure ratios ranging from 2 to 4. At a nozzle pressure ratio of 4, the flow separated from the deck without reattaching, and the plume moved horizontally with minimal deflection. The findings suggest that a combination of a high aspect ratio and sufficiently high nozzle pressure ratio can effectively reduce plume deflection. [ABSTRACT FROM AUTHOR]

A passive flow control device, Clark-Y airfoil-shaped vortex generator (VG) on NREL Phase VI turbine blade, which has s809 airfoil section, is investigated. Both qualitative oil flow visualization from wind tunnel experiments and quantitative measures of aerodynamic coefficients using steady-state CFD with OpenFOAM are reported. Airfoil-shaped VGs are proposed and compared with traditional rectangular and triangular VGs. The use of airfoil-shaped VGs to delay separation, improving aerodynamic efficiency, inducing local pressure peaks and augmenting vorticity in the flow field are reported in detail. Results show that blades equipped with airfoil-shaped VGs provide a 5 % lift coefficient increase and a 27.68 % drag coefficient reduction compared to clean blades at a stall angle of α = 11 ∘ . Airfoil-shaped VGs also generate more vorticity downstream compared to conventional VGs, contributing to maximum increase in peak vorticity inducing an additional momentum to the flow to delay separation without significant drag penalty. Thus, airfoil-shaped VGs offer a promising alternative to traditional VG designs. [ABSTRACT FROM AUTHOR]

• Effect of vortex generators on flow boiling oscillation are verified. • Vortex generators suppress the large bubble's formation and reverse flow. • The max increment of limiting oscillation heat flux by vortex generators is 21.1%. • A new correlation for vortex-enhanced limiting oscillation heat flux is developed. Subcooled flow boiling in micro-channels is an effective solution for high-heat-flux electronics thermal management, and the heat flux has a limitation value (i.e., limiting oscillation heat flux) due to flow boiling oscillation. The purpose of this study is to propose a new method to suppress the flow boiling oscillation (i.e., forcing bubbles to leave the channel by installing a vortex generator), experimentally verify the effect of vortex generators on suppression of flow boiling oscillation and increasing limiting oscillation heat flux, and develop a predicting correlation for this heat flux. In the experiments, the vortex generator is a helix wire with a pitch of 1.8 mm, a length of 22.5 mm, a wire diameter of 0.2 mm and a diameter of 1.8 mm; the inlet subcooling degree and mass flux range from 10 – 50 K and 750–1750 kg m−2 s−1, respectively. The results show that, the flow boiling oscillation occurs in the flow channel without vortex generators while it doesn't occur in the flow channel containing vortex generators at the same working condition; the vortex generators can raise the limiting oscillation heat flux with a maximum increment of 21.1%, which indicates that vortex generators can suppress the flow boiling oscillation. A new correlation predicting limiting oscillation heat flux in channels with vortex generators was developed, and its predictions match 95% of the experimental data within ± 20% while the average deviation is 7.6%. [ABSTRACT FROM AUTHOR]

Rainbow particle tracking velocimetry can be used to measure 3D3C flow velocity vectors with a single color camera. The particle depth position is calculated from the hue degree of a particle color. A liquid–crystal display (LCD) projector can be used conveniently as a light source of the color pattern in place of the spectral diffraction of white light. Another advantage of using an LCD projector is the ability to change the RGB components of the color pattern. In this study, the rainbow color pattern was modified to increase available color. Colors with the same hue degree but different saturations were used in the color pattern. Using color space enhanced positional resolution in the color change direction. The parameters of new color patterns were number of cycles, saturation continuity and range of color space. They were designed and their performances were compared. The effective resolution in color change direction enhanced 2.4 times from original patten (single cycle rainbow), and velocity vector acquisition rate improved, in particular, for the range of small amount movement per step. [ABSTRACT FROM AUTHOR]

Fluorescent tuft visualization technology, as a practical and convenient technique, has attracted widespread attention. In this study, a novel fluorescent tuft velocity measurement method is developed based on existing technologies. This method involves the recognition of tuft deflection angles, image transformation, block processing, temporal averaging, and post-visualization processing. Additionally, quantitative measurement of flow field velocity is achieved through tuft calibration. The study begins by comparing tufts of different materials, examining both fluorescent and deflection characteristics to determine optimal tuft parameters. The impact of tuft length on deflection characteristics is also investigated. Finally, the cotton tuft with a length of 7 mm and a diameter of about 0.1 mm was obtained as the best tuft sought in this experiment. Subsequently, at Reynolds number Re = 1.7 × 105, airfoil surface flow field visualization and quantitative analysis are conducted, exploring the relationship between airfoil surface flow structure and aerodynamics at different angles of attack. Finally, a comparative study is conducted between tuft velocity results and oil flow visualization experiments, as well as particle image velocimetry experiments, confirming the feasibility of the fluorescent tuft velocity measurement method. [ABSTRACT FROM AUTHOR]

Introduction Material and Methods Results Conclusions Detection and surveillance of fetal growth restriction (FGR) is well established, but there is still room for improvement. Animal studies indicate that compromised fetuses increase adrenal blood flow. Modern ultrasound equipment allows us to measure vascular impedance in the fetal adrenal arteries despite their modest size. However, extensive anatomical variance is a challenge to standardizing measurements. We set out to improve this.We included 75 low‐risk pregnant women in a prospective cross‐sectional study aiming to develop a reliable technique to visualize and measure flow velocity in human fetal adrenal arteries. We used commercially available ultrasound equipment: a GE Voluson 10 2019 with a C2‐9 probe (GE Healthcare, Zipf, Austria), and a Philips Epiq Elite with a V9‐2 probe (Philips Medical Systems International B.V., Best, The Netherlands), exploiting the modern sensitive power Doppler modes in both scanners to visualize small vessels.Among 72 fetuses, the inferior adrenal artery was the most consistently visualized and measured artery to the gland. Doppler velocimetry was achieved in 66 (92%) participants. We found the anatomical variation described previously but were able to develop visualization strategies to identify the common arteries and use a consistent Doppler technique for the second half of pregnancy.It is possible to visualize and measure flow velocity in the adrenal arteries of human fetuses. The success rate was highest for the inferior adrenal artery making this vessel a candidate for clinical studies. [ABSTRACT FROM AUTHOR]

Introduction: The direct-contact condensation (DCC) of steam under water injection is the basic thermodynamic process of the bubble deaerator. In order to understand the complex coupling behavior of strong turbulence and fast phase-change heat transfer involved in the process. Methods: This study uses a visualized method and convective heat transfer model. Results: Since the contact area is affected by steam injection flow and sub-cooled degree is affected simultaneously, the trend of the condensation heat-transfer coefficient depends on the degree of their respective effects under each condition, and the maximum variation of the coefficient exceeds 104 W/m2.°C. Moreover, they still effect the period of steam plume, and the maximum variation of the period was beyond 80 ms. Discussion: Calculated the average condensation heat transfer coefficient and then produces the variation law of heat transfer coefficient under various conditions in one steam plume evolution period. [ABSTRACT FROM AUTHOR]

Contrast-enhanced ultrasonography (CEUS) has revolutionized diagnostic imaging by significantly improving the accuracy and capabilities of ultrasound technology. By utilizing contrast agents, CEUS offers enhanced visualization of blood flow, superior detection of vascular diseases, and delineation of pathological from healthy tissues. Real-time imaging capabilities afforded by CEUS have been shown to be instrumental in guiding interventions and assessing treatment efficacy, thereby contributing to personalized patient care and improved clinical outcomes. This document provides a comprehensive review of the current benefits, limitations, and future directions of contrast-enhanced ultrasonography. It acknowledges the potential for allergic reactions, the necessity for specialized training, high costs, and the occurrence of image artifacts as drawbacks, while also anticipating advancements in targeted agents and imaging techniques. Moreover, the document emphasizes the need for ongoing research to evaluate the long-term safety and efficacy of contrast agents and their impact across various medical disciplines. Despite the challenges, the integration of CEUS into clinical practice holds enormous potential to further refine diagnostic processes and enhance patient management. [ABSTRACT FROM AUTHOR]

In recent years, leading-edge tubercles have gained significant attention as an innovative biomimetic flow control technique. This paper explores their impact on the aerodynamic performance and flow patterns of an airfoil through wind tunnel experiments, utilizing force measurements and tuft visualization at Reynolds numbers between 2.7 × 105 and 6.3 × 105. The baseline airfoil exhibits a hysteresis loop near the stall angle, with sharp changes in lift coefficient during variations in the angle of attack (AOA). In contrast, the airfoil with leading-edge tubercles demonstrates a smoother stall process and enhanced post-stall performance, though its pre-stall performance is slightly reduced. The study identifies four distinct flow regimes on the modified airfoil, corresponding to different segments of the lift coefficient curve. As the AOA increases, the flow transitions through stages of full attachment, trailing-edge separation, and local leading-edge separation across some or all valley sections. Additionally, the study suggests that normalizing aerodynamic performance based on the valley section chord length is more effective, supporting the idea that leading-edge tubercles function like a series of delta wings in front of a straight-leading-edge wing. These insights provide valuable guidance for the design of blades with leading-edge tubercles in applications such as wind and tidal turbines. [ABSTRACT FROM AUTHOR]

Particle Tracking Velocimetry (PTV) is a Lagrange-based flow visualization technique that tracks the motion of multiple particles or granules simultaneously. With the widespread application of three-dimensional (3D) particle imaging systems, 3D PTV algorithms have attracted considerable interest, whereas many 3D algorithms are developed from the corresponding 2D algorithms; moreover, compared with 3D algorithms, 2D algorithms are more suitable for real-time flow monitoring in industry. This paper proposes a 2D PTV algorithm based on the Voronoi diagram (VD) that is optimized by the minimum enclosing ellipse (MEE); then a re-matching process based on a homemade method called Quasi-Parallel Correction (QPC) is developed to correct the abnormal results produced by PTV at large inter-frame particle displacement. This PTV is thereby named MQ-PTV. MQ-PTV is then employed for reconstructing a granular flow made of dense polypropylene particles along a declined chute, an aeolian sand flow over sand bed, the migration of a barchans swarm and the motion of stars, thus confirming its practicability in a wide variety of particle motion reconstruction. [ABSTRACT FROM AUTHOR]

Small hydro technology is playing a crucial role in advancing sustainable, clean energy policies as part of the global hydro development strategy. Its contribution to social and economic development is becoming more prominent, particularly in ensuring electricity access for rural communities and supporting industrial expansion. The main causes of bulb turbine failures under high operating conditions are frequently attributed to variations in pressure in the draft tubes, which are aggravated when a spinning vortex rope is formed under load operation. Different fluid injection techniques, namely compressed air and water jet injection, address these issues and reduce the negative results of cavitation. The investigation covers the flow visualization on the suction side of a single hydrofoil utilizing a cavitation tunnel and a bulb turbine. This study assesses the effectiveness of compressed air injection in reducing vibration generated by cavitation in bulb turbines. Positive results of the experimental studies suggest a decrease in noise and vibration by air injection that prevents oscillations of the vortex rope. This research also considers how the hydrofoil design of bulb runner blades influences flow characteristics. Hence, it provides knowledge on cavitation structures in diverse cavitation numbers. Different studies that compare the original and modified hydrofoil designs reveal remarkable improvements that may be due to changes in the key parameters of the hydrofoil, such as later cavitation initiation and reduced intensity. To obtain the optimal output of a bulb turbine by considering air injection for vibration reduction and hydrofoil design changes to limit the negative effect of cavitation, the Reynolds number and cavitation number are to be defined. This multidisciplinary approach possesses enormous potential to increase the reliability and efficiency of bulb turbines in challenging operating conditions. [ABSTRACT FROM AUTHOR]

Polydimethylsiloxane (PDMS) has become a popular material in microfluidic and macroscale in vitro models due to its elastomeric properties and versatility. PDMS-based biomodels are widely used in blood flow studies, offering a platform for improving flow models and validating numerical simulations. This review highlights recent advances in bioflow studies conducted using both PDMS microfluidic devices and macroscale biomodels, particularly in replicating physiological environments. PDMS microchannels are used in studies of blood cell deformation under confined conditions, demonstrating the potential to distinguish between healthy and diseased cells. PDMS also plays a critical role in fabricating arterial models from real medical images, including pathological conditions such as aneurysms. Cutting-edge applications, such as nanofluid hemodynamic studies and nanoparticle drug delivery in organ-on-a-chip platforms, represent the latest developments in PDMS research. In addition to these applications, this review critically discusses PDMS properties, fabrication methods, and its expanding role in micro- and nanoscale flow studies. [ABSTRACT FROM AUTHOR]

In this paper, we propose a global modelling for vector field approximation from a given finite set of vectors (corresponding to the wind velocity field or marine currents). In the modelling, we propose using the minimization on a Hilbert space of an energy functional that includes a fidelity criterion to the data and a smoothing term. We discretize the continuous problem using a finite elements method. We then propose taking into account the topographic effects on the wind velocity field, and visualization using a free library is also proposed, which constitutes an added value compared to other vector field approximation models. [ABSTRACT FROM AUTHOR]

This study provides an in-depth simulation of the capillary underfill (CUF) process for flip-chip packages using Moldex3D, offering a comprehensive analysis that integrates material properties, process parameters, and structural factors within a unified framework. The material properties explored include viscosity and reaction kinetics, while the process parameters focus on controllable factors such as resin temperature, mold temperature, dispensing start time, dispensing weight, and dispensing end time. Structural parameters like bump pitch and gap height are also studied. The study further validates its findings with experiments conducted on a high bump count of 1530, showing that the experiment and simulation results are consistent. Key findings reveal that a larger die pitch accelerates the filling process, the incorporation of edge channels enhances filling efficiency, and optimizing the dispensing path improves flow uniformity. Adjusting the dispensing weight and intervals can mitigate edge effects and tongue spreading. Regarding material properties, higher surface tension and lower contact angle significantly increase filling speed, though they have a minimal impact on the overall flow pattern during the underfill filling process. These insights are crucial for optimizing the CUF process, ultimately improving the reliability and performance of flip-chip packages. [ABSTRACT FROM AUTHOR]

The article "Actuation manifold from snapshot data" in the Journal of Fluid Mechanics introduces a data-driven approach to analyze controlled flows using isometric mapping and neural networks. It focuses on a fluidic pinball configuration with three cylinders at Reynolds number 30, revealing a five-dimensional manifold with minimal error. The study uncovers key parameters related to vortex shedding and actuation, offering valuable insights into fluid mechanics. The document also discusses various research papers on wake analysis, vortex shedding stabilization, machine learning for fluid control, and nonlinear dimensionality reduction, providing a comprehensive overview for researchers in the field. [Extracted from the article]

Analyzing qualitative data from learning processes is considered "messy" and time consuming (Chi in J Learn Sci 6(3):271–315, 1997). It is often challenging to summarize and synthesize such data in a manner that conveys the richness and complexity of learning processes in a clear and concise manner. Moreover, qualitative data often contains patterns that are not immediately apparent. Consequently, visualization can be an effective tool for representing and unpacking the complexities and multidimensions of learning processes. Additionally, visualizations provide a time-efficient approach to analyzing data and a high-level view of the learning process over time for researchers to zoom in on intriguing moments and patterns (Huang et al. in Comput Human Behav 87:480–492, 2018). In this conceptual paper, we provide a broad overview of research in the field of visualizing qualitative data and discuss two studies (1) visualizing role-changing patterns in an interdisciplinary learning environment and (2) operationalizing collaborative computational thinking practices via visualization. By leveraging these studies, we aim to demonstrate a visualization processing flow along with qualitative research and methods. Particularly, the processing flow includes three critical elements: research subjectivity, complexity of visual encoding, and purpose of visual encoding. The discussion highlights the iterative and creative nature of the visualization technique. Furthermore, we discuss the benefits, challenges, and limitations of using visualization in the context of qualitative studies. [ABSTRACT FROM AUTHOR]

The cavitation cloud impingement of the jet in the rock breaking process was experimentally investigated to reveal the jet erosion mechanism in drilling of petroleum exploitation. Serial erosion tests and flow visualization were performed, where the cavitation cloud motion in the erosion crater was obtained with the designed transparent specimen. Various erosion patterns were identified in the whole erosion process based on the eroded specimen topography. The shallow eroded crater with a shrinking erosion area is generated by the combination of impinging and scattering cavitation clouds. The increase of ld promotes the development of cavitation cloud σc but reduces the impingement frequency fd, suggesting that the jet aggressive ability is enhanced when the balance between σc and fd is reached. The cavitation cloud motion in the erosion crater was investigated with the transparent specimen. The erosion in the crater at shorter exposure periods Te is generated by the combination of impingement and restricted scattering of cavitation clouds. With the continuous development of the erosion damage, the jet's aggressive ability is diminished due to the erosion expansion on sandstone, where the cavitation clouds impinge on the target and then collapse and vanish without restricted scattering. [ABSTRACT FROM AUTHOR]

The study investigates the evolution of flow instabilities in a magnetohydrodynamic (MHD) environment involving a shock-accelerated light cylindrical bubble. Numerical simulations were conducted using a cylindrical helium (He) bubble accelerated by a shock wave in nitrogen (N2) gas at various magnetic field strengths. The results highlight the impact of magnetic fields on flow morphology, vorticity generation, and enstrophy. The interaction between incident shock waves and the gas bubble revealed significant differences in flow patterns and interface features when magnetic fields were applied. Key findings include the quantification of shock trajectories and detailed visualizations of the evolving flow structure. The study provides insights into the dynamics of shock–bubble interactions under MHD conditions, contributing to the broader understanding of flow instability mechanisms in such complex environments. [ABSTRACT FROM AUTHOR]

Heat transfer characteristics of two types of jet impingement, confined and unconfined is investigated using air and water as the working fluids. The effects of volumetric quality are evaluated on the lateral variation of the Nusselt number at low nozzle-to-plate spacing. The results show that the lateral Nusselt number of the unconfined submerged air bubble jet is similar to that of the confined submerged air bubble jet in Region I (bubble collision region) due to the effect of bubble collision. In Region II (bubble sliding region), however, the lateral Nusselt number of the unconfined jet was found to be 30%–40% lower than that of the confined jet flow due to the buoyancy effect that causes air bubbles to rise in Region I. In addition, from the flow visualization, high velocity bubble movement was observed from the midpoint of the heating surface to the periphery of the confinement plate in the confined jet set-up. This movement enhances the lateral Nusselt number in Region II. In the unconfined jet set-up, air bubbles rise past the heating surface in Region I and thus neither slide across the heating surface nor increase the lateral Nusselt number in Region II. [ABSTRACT FROM AUTHOR]

We present an approach that supports the analysis of flow dynamics in the neighborhood of curved line-type features, such as vortex core lines, attachment lines, and trajectories. We achieve this with continuous deformation to the flow field to straighten such features. This provides "deformed frames of reference", within which qualitative flow dynamics are better observable with respect to the feature. Our approach operates at interactive rates on graphics hardware, and supports exploration of large and complex datasets by continuously navigating the additional degree of freedom of deformation. We demonstrate the properties and the utility of our approach using synthetic and simulated flow fields, with a focus on the application to vortex core lines. [ABSTRACT FROM AUTHOR]

Contact with infected individuals can lead to the spread of infectious diseases. During the COVID-19 pandemic, people were strongly urged to avoid the three Cs: closed spaces, crowded places, and close-contact settings. To hold large-scale events under such circumstances, reducing crowd congestion is key to preventing the further spread of infection. Therefore, identifying the pedestrian behaviors and walking patterns that pose a high risk of infection and utilizing them for effective crowd control is necessary. In this study, we propose an approach for visualizing walking paths while maintaining visibility from large-scale human flow data and representing both spatial and temporal features. The proposed method enables the visualization of the pedestrian proximity status as a network containing three components: a proximity network, proximity path, and pedestrian statistics that interact with each other. By operating the three components of this system interactively, we can observe the spatial and temporal features of situations with a high risk of infection during crowd congestion. An example of the operation of this system is presented by visualizing real-world human flow data measured at an event venue and identifying the proximity of the pedestrians. [ABSTRACT FROM AUTHOR]

This article reports about a wind‐tunnel experiment carried out in the ONERA F2 low‐speed wind tunnel on a model of the DU 97‐W‐300Mod airfoil designed for wind turbine application. The wind tunnel, the airfoil model, and experimental techniques used are presented, with special emphasis on the data processing and corrections required to derive airfoil forces and pressure distribution. To better document the flow physics at play, the results are illustrated by infrared thermography and surface oil flow visualization. The test allowed investigating Reynolds number effects between 1 and 3.8 millions. To ameliorate the understanding of the benefits and limitations of such airfoil testing, one section is devoted to the comparison of present results with previous experiments in other wind tunnels. Some of the difficulties arising in airfoil testing are evidenced and discussed to contribute to the improvement of test methods. [ABSTRACT FROM AUTHOR]

This article presents the results of an experimental study of the coolant flow in a fuel rod bundle of a nuclear reactor fuel assembly of a small modular reactor for a small ground-based nuclear power plant. The aim of the work is to experimentally determine the hydrodynamic characteristics of the coolant flow in a fuel rod bundle of a fuel assembly. For this purpose, experimental studies were conducted in an aerodynamic model that included simulators of fuel elements, burnable absorber rods, spacer grids, a central displacer, and stiffening corners. During the experiments, the water coolant flow was modeled using airflow based on the theory of hydrodynamic similarity. The studies were conducted using the pneumometric method and the contrast agent injection method. The flow structure was visualized by contour plots of axial and tangential velocity, as well as the distribution of the contrast agent. During the experiments, the features of the axial flow were identified, and the structure of the cross-flows of the coolant was determined. The database obtained during the experiments can be used to validate CFD programs, refine the methods of thermal-hydraulic calculation of nuclear reactor cores, and also to justify the design of fuel assemblies. [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]

As hydrodynamic torque converter developing towards higher power density, leading to severe cavitation in the internal viscous oil, which adversely impacts performance and energy efficiency. This study introduces a novel borescope observation technology for visualizing cavitation in hydrodynamic torque converter. Utilizing this visualization approach alongside numerical calculation method, the research investigates transient cavitation phenomena within the torque converter, with a particular focus on the stator flow field. The development of a stress blended eddy simulation model for cavitation flow allows for an in-depth examination of the interaction between cavitation and vortex within the 3D enclosed flow field, elucidating the transient mass transfer process. Visualization successfully captures attached cavitation at the stator blade leading edge revealing a decrease in cavitation extent with increasing speed ratio. CFD simulations incorporating cavitation accurately predict the hydrodynamic performance, highlighting the significant impact of cavitation on performance metrics such as capacity factor and torque ratio. Identifying the critical cavitation number for torque converter underscores the adverse effects of cavitation on performance below this threshold. This comprehensive analysis, combining visualization and simulation, offers valuable insights into cavitation dynamics within hydrodynamic torque converter, laying a foundation for further optimization of flow mechanisms and performance improvement.

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ABSTRACT Three unsteady aerodynamic tools at different levels of fidelity and computational cost were used to investigate the unsteady aerodynamic behavior of a delta kite applied to airborne wind energy. The first tool is an in‐house unsteady panel method that is fast but delivers low to mid fidelity predictions. The second tool uses the open‐source CFD code SU2 to solve the unsteady Reynolds‐averaged Navier–Stokes equations with the k−ω SST turbulence model. At an intermediate level of fidelity, a semiempirical dynamic stall model that combines the panel method with a phenomenological dynamic stall module is proposed. The latter has free parameters that are fine‐tuned with CFD results from the second tool. The research on the dynamic stall model has been inspired by two flight test campaigns suggesting dynamic stall phenomena possibly driven by the periodic variation of the angle of attack (aerodynamic pitching motion) during crosswind maneuvers. The recorded inflow along the flight path was prescribed in the three aerodynamic tools. As expected, the price to pay for the low computational cost of the panel method is its inability to capture the dynamic stall phenomenon. The results from unsteady CFD qualitatively matched the experimental data identifying a leading‐edge vortex that forms and detaches cyclically during the pitching motion. Using RANS data, the semiempirical tool was fined‐tuned to reproduce the dynamic stall behavior, becoming an accurate and fast aerodynamic tool for coupling with any kite flight simulator. Further discussions on the effects of kite aerostructural deflections are included.

Lagrangian coherent structures (LCS) is an important concept for the visualization of unsteady flows. They describe the boundaries of regions for which material transport stays mostly coherent over time which can help for a better understanding of dynamical systems. One of the most common techniques for their computation is the extraction of ridges from the finite‐time Lyapunov exponent (FTLE) field. FTLE ridges are challenging to extract, both in terms of accuracy and performance, because they expose strong gradients of the underlying field, tend to come close to each other and are dynamic with respect to different time parameters. We present a new method for extracting FTLE ridges for series of integration times which is able to show how coherent regions and their borders evolve over time. Our techniques mainly build on a particle system which is used for sampling the ridges uniformly. This system is highly optimized for the challenges of FTLE ridge extraction. Further, it is able to take advantage of the continuous evolvement of the ridges which makes their sampling for multiple integration times much faster. We test our method on multiple 3D datasets and compare it to the standard Marching Ridges technique. For the extraction examples our method is 13 to over 300 times faster, suggesting a significant advantage. [ABSTRACT FROM AUTHOR]

The flow field serves as an important component of proton exchange membrane fuel cells (PEMFCs) for maintaining the hydration of the membrane and discharge of excessive water. In this study, a transparent polycarbonate plate was used as the cathode end plate of the PEMFC. The water management capacity of the PEMFCs with different cathode flow fields was evaluated. The movement and evolution patterns of water droplets, film, and columns in different flow fields were analyzed. The results show that liquid water is discharged faster as the cross-section of the flow channel becomes smaller. The performance of the PEMFC with a partially-narrowed flow field is higher due to better water management capacity and forced convection of gas reactant. Liquid water exists mostly in the form of liquid columns in the parallel flow channel, damaging the uniformity of gas distribution. The wavy flow field is likely to be flooded due to the difference of water movement velocity in different channel regions. In addition, a volume of fluid (VOF) model was developed to quantitatively evaluate the water management performance of each type of flow field. The water movement patterns in the different flow channels were concluded. This study provided real-time observations of water movement in the flow channel, revealing a correlation between water management capabilities and the performance of the PEMFC. [ABSTRACT FROM AUTHOR]

The article explores the use of triglobal resolvent analysis to control separated flows around low-aspect-ratio wings, aiming to reduce separation bubbles and wing tip vortices. Direct numerical simulations and resolvent analysis identify optimal perturbations to enhance aerodynamic performance, particularly at high angles of attack. By strategically choosing actuation frequencies and locations, the study demonstrates the effectiveness of flow control in improving aerodynamic performance by modifying wake dynamics and reducing reversed-flow bubbles. The research provides valuable insights into the mechanisms of flow control and its impact on aerodynamics, emphasizing the importance of strategic actuation for optimal outcomes. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into the stability of different steady flows in an electrolyte layer driven by the Lorentz force in an annular channel. It identifies a new flow state, type 3, which sustains free-surface vortices and undergoes transitions as the Reynolds number increases. The study highlights the role of the type 3 flow in the development of free-surface vortices and contributes to understanding electromagnetically driven flows in fluid dynamics. Other researchers have also explored fluid behavior in rotating systems and spherical Couette flows, offering valuable insights into vortices and shear layers in diverse fluid environments. [Extracted from the article]

The document delves into the complexities of co-rotating suspension Taylor-Couette flow, highlighting the transitions from TVF to SVF and RIB states with increasing particle loading. It identifies the unique features of particle-induced spiral vortices and their impact on pattern dynamics in suspension TCF. The research by Majji, Morris, Moazzen, and others sheds light on the destabilizing effects of particle loading and co-rotation on primary states, offering valuable insights into fluid dynamics in complex geometries. [Extracted from the article]

This study explores the flow dynamics and sediment entrainment mechanisms around a circular array of emerged cylinders in a shallow channel. Eddy resolving simulations reveal the formation of vortex street-type shallow wakes, influenced by cylinder diameter and submergence ratio. The research emphasizes the impact of flow conditions on wake instabilities, coherent structures, and sediment transport, shedding light on the complex interactions in shallow water environments. [Extracted from the article]

A comparative application of major dynamic light scattering (DLS)‐based image methodologies applied to transcranial cerebral blood flow imaging is presented. In particular, the study delves into assessing capability of Laser Doppler Flowmetry (LDF), Laser Speckle Contrast Imaging (LSCI), and Diffuse Correlation Spectroscopy (DCS) in enhancing the spatial and temporal resolution of transcranial blood flow imaging. An integral part of the study is focused on the modulation of blood flow through the administration of the vasodilator drug, Sodium Nitroprusside (SNP). This pharmacological intervention facilitated a direct observation of cerebral vasculature's responsiveness to external stimuli, illuminating the physiological adaptations within the brain's microvascular architecture. Advanced LSCI processing techniques are incorporated, notably entropy and principal component analysis (PCA). Entropy is providing a quantifiable measure of the randomness and complexity within the speckle patterns of transcranial blood flow images, revealing remarkably similar outcomes with DSC approach in terms of blood flow dynamics and its quantitative evaluation. The application of PCA approach is provided a more nuanced understanding of blood flow dynamics, facilitating the identification of subtle changes induced by drug administration. This method proved instrumental in enhancing the visualization and detection of nuanced blood flow dynamics, thereby allowing for a more detailed examination of cerebral circulation alterations induced by SNP administration. The study seeks to offer a wider‐ranging insight into comprehending the translating further the concept of DLS into transcrainial blood flow vizualization and explore its practical applications, considering hardware, advanced quantitative image processing, and data acquisition. [ABSTRACT FROM AUTHOR]

Magnesium is a combustible metal that poses various safety risks, including fires and explosions. However, there are limited safety measures available to prevent and respond to potential fires and explosion incidents in the metal industry. In this study, the combustion process of Mg fires was closely examined using infrared thermal imaging, focusing on the effects of Mg powder size. For the experiment, Mg powder was burned by increasing the temperature to approximately 967.4 K using an ignition unit and controller equipped with a tungsten heater. Moreover, combustion velocity measurement experiments for Mg particle sizes of 75, 100, and 150 μm were conducted using the combustion velocity measurement device presented in the NFPA 484 standard. On combustion of Mg, flames are observed; smoke is emitted as demonstrated by thermal and flow visualization experiments. The combustion velocity measurement experiment results demonstrated that the greater the slope value (combustion velocity) for the combustion length over time, the faster is the combustion velocity, with the 75 μm particle size having the fastest combustion velocity. The results of this experiment can be utilized as references for Mg fire control design and to gain a better understanding of the scope of smoke and fire hazard investigation measures. [ABSTRACT FROM AUTHOR]

This paper investigates the application of Schlieren flow visualization for detecting leaks in pipelines carrying high-temperature fluids. Two experimental setups were constructed: one with a 25 mm PTFE tube featuring a 2 mm diameter perforation, and another with a 100 mm diameter pipe insulated with an aluminum jacket and featuring a 12 mm leak gap. A single-mirror-off-axis Schlieren system, employing a 150 mm diameter parabolic mirror, was used to visualize the leaks. The temperature of the leaking air varied between 20 and 100 °C, while the ambient temperature was maintained at 14 °C. To quantify the leaks, the coefficient of variation for pixel intensity within the leak region was calculated. Results showed that for the PTFE tube, leaks became detectable when the temperature difference exceeded 34 °C, with the coefficient of variation surpassing 0.1. However, in the insulated pipe, detecting clear leak patterns was challenging. This research demonstrates the potential of Schlieren visualization as a valuable tool in enhancing pipeline leak detection. [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]

This paper reports the findings of an investigation on self-mitigation of the vortex breakdown phenomenon in a high-head model Francis turbine draft tube diffuser during the transition from high load to design operating conditions. The transient operating sequence is achieved by closing the flow regulating guide vanes assuming a linearly proportional decrease in flow rate. Scale-adaptive simulation shear stress transport turbulence model is used to ensure that the large-scale structures of the unsteady flow are resolved delivering a higher accuracy compared to complete averaging. The simulation is validated through a comparison between numerical and experimental axial velocity profiles on a radial line in the draft tube near its inlet. At high load, the numerical results agree satisfactorily with experiment, excepting slightly increased deviation in the central region due to the presence of vortex breakdown. However, at best efficiency point, a close agreement between numerical and experimental velocity profiles is seen in the central region as well. At high load, the vortex core is swollen, has sharp twists, encloses zones of flow stagnation and intermittent flow reversal, and is wrapped by a well-sped outflow through strong shear layers. Commencement of the transient sequence results in a gradual reorganization of the velocity field, leading to purge of major part of the vortex breakdown, like flow reversals and stagnation, within 50% of the time of load rejection. Onward, the flow is gradually restored to a streamlined, defect-free form. A comprehensive analysis and visualization of the evolving flow field is disseminated by this article. [ABSTRACT FROM AUTHOR]

Insight into the seepage patterns of fluid flow within fractured rocks can lead to a better understanding of many challenging issues, such as oil or gas exploitation, heat extraction from hot dry rock (HDR), and controlling water inflow through cement grouting. However, these can only be assessed via some macroscopic phenomena like inlet or outlet flow properties or numeral simulation with synthetic rock fractures. This situation is because the real-time seepage diffusion features within rock fractures cannot be directly observed via conventional techniques, including the CT scanning technique, even if the colored solution is used as a developer. Therefore, we remove the obstacles that prevent fluid flow visualization within rock fractures by developing a transparent rock replica, and introducing Particle Image Velocity (PIV) technology. In addition to overcoming the opacity of rock, this method can help analyze the distribution patterns of velocity and vorticity fields, as well as the main seepage path by capturing the migration of particles and the fluid flow. The results show that the fluid transports along the low part of the rough fractures. Meanwhile, the flow direction is less affected by the fracture geometry if the flow rate exceeds a certain value. When the number of vortices on the fractures increases, they are mainly distributed in areas with large surface fluctuations. Interestingly, the maximum vorticity first grows slowly and then increases rapidly with increasing pumping rates. This study contributes to a more comprehensive understanding of the seepage patterns of fluid flow within rock fractures and their related engineering applications. Highlights: Based on PIV technology, a set of visual test equipment for seepage in rock fractures is developed. The distribution information of velocity vector, flow field and vorticity field on rock fractures can be obtained in real time. The digital extraction of fracture surface of rock samples is realized. [ABSTRACT FROM AUTHOR]

The lifting behavior of turbulent nonpremixed coaxial jet flames was investigated experimentally as a function of the center jet Reynolds number, annular-to-center jet velocity ratio, acoustic frequency, and acoustic amplitude, all with and without transverse acoustic disturbances acting on the flames situated at a pressure antinode. Global lifting regimes were mapped and consisted of attached flames, periodically lifted flames, and permanently lifted flames. With one exception, all flames were attached in the absence of acoustics and lifted only because of the applied acoustic excitation. The exception occurred at the highest Reynolds number and velocity ratio, where the flames were unconditionally lifted in all cases. Between these two extremes, a range of lifting regimes was observed. Lower applied frequencies and smaller amplitudes were found to generally promote attachment to the burner. Flow visualizations using high-speed Schlieren and OH* chemiluminescence imaging revealed a lateral spreading effect near the jet exit. The lateral spreading was hypothesized to be caused by an axially oriented counterflow collision between the downstream flowing jets and presumed local upstream portion of the acoustic velocity fluctuations. Physical mechanisms were hypothesized based on this observation, which appear to consistently explain most of the observed behavior. [ABSTRACT FROM AUTHOR]

The unsteady pressure field over an axisymmetric backward-facing step was investigated experimentally at transonic freestream Mach numbers of 1.05, 1.2, and 1.4. The study was aimed at examining the influence of transonic freestream Mach numbers on the spatio-temporal character of the unsteady pressure field and on the dominant modes/mechanisms driving it. Surface flow visualization, schlieren, and unsteady pressure measurements were carried out as part of the experimental investigation. From oil flow visualization and schlieren, the reattachment region was identified, and consequently, the mean reattachment length was estimated. The mean reattachment length shows an increase with the increase in freestream Mach number. The coefficient of mean pressure along the rearbody imitates a classical backward-facing step flow profile and can be divided into three distinct regions. The peak values of the coefficient of mean pressure and the coefficient of root mean square of the fluctuation are seen to decrease with an increase in the freestream Mach number. Conventional spectral analysis reveals that as the freestream Mach number increases, the dominant peak in the spectra shifts to lower frequencies. From the spectra, three dominant fluid dynamic mechanisms depending on the freestream Mach number have been identified. Proper Orthogonal Decomposition (POD) analysis shows that 79–84 % of the total energy contribution comes from the first six modes. The temporal dynamics of the POD modes indicate three prominent mechanisms are responsible for the unsteady pressure field. Spectral analysis of POD modes indicates that the spectra are primarily driven by the first three POD modes for freestream Mach number of 1.05 and the first two modes for freestream Mach numbers of 1.2 and 1.4. Moreover, it reveals the presence of three dominant modes, and the freestream Mach number strongly dictates the dominant mode that is driving the pressure field. [ABSTRACT FROM AUTHOR]