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Flow Visualization: Experiments and Techniques

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A special issue of Fluids (ISSN 2311-5521). This special issue belongs to the section "Mathematical and Computational Fluid Mechanics".

Deadline for manuscript submissions: closed (9 September 2024) | Viewed by 16375

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Dr. Guangjian Zhang

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Research Center of Fluid Machinery Engineering and Technology, Jiangsu University, Zhenjiang, China
Interests: two-phase structure and dynamics of cavitation; flow visualization; particle image velocimetry (PIV); numerical simulation of turbulent cavitating flow
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Dr. Mingming Ge

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Guest Editor

Faculty of Science and Technology, Beijing Normal University-Hong Kong Baptist University United International College, Zhuhai, China
Interests: experimental fluid mechanics, cavitation and multiphase flow; aI for fluids; numerical simulation
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Dr. Xin-Lei Zhang

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Guest Editor

The State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China
Interests: turbulence modeling; data assimilation; machine learning
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Special Issue Information

Dear Colleagues,

Flow visualization is a technique used to observe and analyze the patterns and characteristics of fluid flow. It is a valuable tool in fluid mechanics research and engineering applications, as it allows researchers to gain insights into the behavior of fluids and identify various flow phenomena.

Flow visualization methods can be broadly categorized into two types:

Direct flow visualization: involves directly observing the flow using techniques such as dye injection, particle tracking, or smoke visualization;
Indirect flow visualization: involves using instruments and sensors to measure flow properties such as pressure, velocity, and temperature. These data are then processed to visualize the flow patterns and characteristics.

This Special Issue aims to gather recent advancements in the field of flow visualization techniques and present new findings in fluid mechanics using flow visualization techniques.

Suggested topics include, but are not limited to:

Multiphase flow measurement and instrumentation;
PIV/MicroPIV/Tomo-PIV/LIF-PIV techniques;
High-speed photography;
Experimental fluid mechanics;
Measurements of a two-phase structure and dynamics of cavitation;
Traditional and synchrotron X-ray imaging;
Pressure-sensitive paint (PSP) technique;
Multi-sensor data fusion;
Pressure field reconstruction;
AI techniques applied in experimental fluid mechanics.

We look forward to receiving your contributions, and hope this Special Issue will provide a platform for researchers to share their work and exchange knowledge and ideas.

Dr. Guangjian Zhang
Dr. Mingming Ge
Dr. Xin-Lei Zhang
Guest Editors

Manuscript Submission Information

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Keywords

flow visualization
multiphase flow
cavitation
high-speed photography
PIV
X-ray imaging
pressure field reconstruction
data assimilation
machine learning

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Published Papers (12 papers)

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Research

12 pages, 13062 KiB

Open AccessArticle

Modeling and Visualization of Coolant Flow in a Fuel Rod Bundle of a Small Modular Reactor

by Sergei Dmitriev, Tatiyana Demkina, Aleksandr Dobrov, Denis Doronkov, Daniil Kuritsin, Danil Nikolaev, Alexey Pronin, Anton Riazanov and Dmitriy Solntsev

Fluids 2024, 9(10), 235; https://doi.org/10.3390/fluids9100235 - 8 Oct 2024

Viewed by 618

Abstract

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. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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13 pages, 5322 KiB

Open AccessArticle

Improvement in the Number of Velocity Vector Acquisitions Using an In-Picture Tracking Method for 3D3C Rainbow Particle Tracking Velocimetry

by Mao Takeyama, Kota Fujiwara and Yasuo Hattori

Fluids 2024, 9(10), 226; https://doi.org/10.3390/fluids9100226 - 30 Sep 2024

Viewed by 566

Abstract

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). Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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14 pages, 14437 KiB

Open AccessArticle

Aeroacoustic Coupling in Rectangular Deep Cavities: Passive Control and Flow Dynamics

by Abdul Hamid Jabado, Mouhammad El Hassan, Ali Hammoud, Anas Sakout and Hassan H. Assoum

Fluids 2024, 9(8), 187; https://doi.org/10.3390/fluids9080187 - 17 Aug 2024

Viewed by 639

Abstract

Deep cavity configurations are common in various industrial applications, including automotive windows, sunroofs, and many other applications in aerospace engineering. Flows over such a geometry can result in aeroacoustic coupling between the cavity shear layer oscillations and the surrounding acoustic modes. This phenomenon can result in a resonance that can lead to significant noise and may cause damage to mechanical structures. Flow control methods are usually used to reduce or eliminate the aeroacoustic resonance. An experimental set up was developed to study the effectiveness of both a cylinder and a profiled cylinder positioned upstream from the cavity in reducing the flow resonance. The cavity flow and the acoustic signals were obtained using particle image velocimetry (PIV) and unsteady pressure sensors, respectively. A decrease of up to 36 dB was obtained in the sound pressure levels (SPL) using the passive control methods. The profiled cylinder showed a similar efficacy in reducing the resonance despite the absence of a high-frequency forcing. Time-space cross-correlation maps along the cavity shear layer showed the suppression of the feedback mechanism for both control methods. A snapshot proper orthogonal decomposition (POD) showed interesting differences between the cylinder and profiled cylinder control methods in terms of kinetic energy content and the vortex dynamics behavior. Furthermore, the interaction of the wake of the control device with the cavity shear layer and its impact on the aeroacoustic coupling was investigated using the POD analysis. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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18 pages, 2766 KiB

Open AccessArticle

Flowfield and Noise Dynamics of Supersonic Rectangular Impinging Jets: Major versus Minor Axis Orientations

by Yogesh Mehta, Vikas N. Bhargav and Rajan Kumar

Fluids 2024, 9(8), 169; https://doi.org/10.3390/fluids9080169 - 24 Jul 2024

Viewed by 802

Abstract

The current study explores the flowfield and noise characteristics of an ideally expanded supersonic (Mach 1.44) rectangular jet impinging on a flat surface. The existing literature is primarily concentrated on axisymmetric jets, known for their resonance dominance, pronounced unsteadiness, and acoustic signatures. In contrast, non-axisymmetric jets remain relatively less understood, particularly those impinging on a ground surface. By employing Schlieren imaging, high-frequency pressure measurements using high-bandwidth transducers, and particle image velocimetry (PIV), this research comprehensively examines the flow-acoustic phenomena. Schlieren imaging revealed distinct, coherent structures and strong acoustic waves, while pressure measurements at the impingement surface exhibited high-amplitude fluctuations, peaking at approximately 186 dB. Acoustic analysis identified multiple high-amplitude tones with unique directional characteristics, suggesting the potential for multiple simultaneous modes in rectangular jets. Furthermore, the PIV data elucidated differences in the jet shear layer and wall jet development attributed to the nozzle orientation. These findings contribute to a deeper understanding of non-axisymmetric jet behavior, offering insights relevant to fundamental flow physics and practical applications such as vertical takeoff and landing aircraft. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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19 pages, 6530 KiB

Open AccessArticle

Visualization and Quantification of Facemask Leakage Flows and Interpersonal Transmission with Varying Face Coverings

by Xiuhua Si, Jensen S. Xi, Mohamed Talaat, Jay Hoon Park, Ramaswamy Nagarajan, Michael Rein and Jinxiang Xi

Fluids 2024, 9(7), 166; https://doi.org/10.3390/fluids9070166 - 22 Jul 2024

Cited by 2 | Viewed by 739

Abstract

Although mask-wearing is now widespread, the knowledge of how to quantify or improve their performance remains surprisingly limited and is largely based on empirical evidence. The objective of this study was to visualize the expiratory airflows from facemasks and evaluate aerosol transmission between two persons. Different visualization methods were explored, including the Schlieren optical system, laser/LED-particle imaging system, thermal camera, and vapor–SarGel system. The leakage flows and escaped aerosols were quantified using a hotwire anemometer and a particle counter, respectively. The results show that mask-wearing reduces the exhaled flow velocity from 2~4 m/s (with no facemask) to around 0.1 m/s, thus decreasing droplet transmission speeds. Cloth, surgical, and KN95 masks showed varying leakage flows at the nose top, sides, and chin. The leakage rate also differed between inhalation and exhalation. The neck gaiter has low filtration efficiency and high leakage fractions, providing low protection efficiency. There was considerable deposition in the mouth–nose area, as well as the neck, chin, and jaw, which heightened the risk of self-inoculation through spontaneous face-touching. A face shield plus surgical mask greatly reduced droplets on the head, neck, and face, indicating that double face coverings can be highly effective when a single mask is insufficient. The vapor–SarGel system provided a practical approach to study interpersonal transmission under varying close contact scenarios or with different face coverings. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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18 pages, 26838 KiB

Open AccessArticle

Experimental Study of Oil–Water Flow Downstream of a Restriction in a Horizontal Pipe

by Denghong Zhou, Kanat Karatayev, Yilin Fan, Benjamin Straiton and Qussai Marashdeh

Fluids 2024, 9(6), 146; https://doi.org/10.3390/fluids9060146 - 20 Jun 2024

Viewed by 893

Abstract

This work presents an experimental study on oil–water flow downstream of a restriction. The flow pattern, volumetric phase distribution, and their impacts on pressure drop are discussed. We employed two techniques to visualize the oil–water flow patterns, a high-speed camera and an Electrical Capacitance Volume Tomography (ECVT) system. The ECVT system is a non-intrusive device that measures the volumetric phase distribution at the pipe cross-section with time, which plays a critical role in determining the continuous phase in the oil–water flow, and therefore the oil–water flow pattern. In this study, we delved into the oil–water flow pattern and volumetric phase distribution for different valve openings, flow rates, and water cuts, and how they impact the pressure drop. The experimental results have demonstrated a strong relationship between the oil–water flow pattern and the pressure gradient, while the oil–water flow pattern is significantly influenced by the flowing conditions and the valve openings. The impacts of water cuts on the oil–water flow pattern are more obvious for smaller valve openings. For large valve openings, the oil and water phases tend to be more separated. This results in a moderate variation in the pressure gradient as a function of water cuts. However, it becomes more complex as the valve opening decreases. The pressure gradient generally increases with decreasing valve openings until the flow pattern becomes an oil-in-water dispersed flow. The impact of the valve on the pressure gradient is more pronounced in water-dominated flow when the water cut is above the inversion point, while it seems to be most obvious for medium water cut conditions. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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24 pages, 26677 KiB

Open AccessArticle

Wind Tunnel Experiments on Parallel Blade–Vortex Interaction with Static and Oscillating Airfoil

by Andrea Colli, Alex Zanotti and Giuseppe Gibertini

Fluids 2024, 9(5), 111; https://doi.org/10.3390/fluids9050111 - 10 May 2024

Cited by 1 | Viewed by 1388

Abstract

This study aims to experimentally investigate the effects of parallel blade–vortex interaction (BVI) on the aerodynamic performances of an airfoil, in particular as a possible cause of blade stall, since similar effects have been observed in literature in the case of perpendicular BVI. A wind tunnel test campaign was conducted reproducing parallel BVI on a NACA 23012 blade model at a Reynolds number of 300,000. The vortex was generated by impulsively pitching a second airfoil model, placed upstream. Measurements of the aerodynamic loads acting on the blade were performed by means of unsteady Kulite pressure transducers, while particle image velocimetry (PIV) techniques were employed to study the flow field over the blade model. After a first phase of vortex characterisation, different test cases were investigated with the blade model both kept fixed at different incidences and oscillating sinusoidally in pitch, with the latter case, a novelty in available research on parallel BVI, representing the pitching motion of a helicopter main rotor blade. The results show that parallel BVI produces a thickening of the boundary layer and can induce local flow separation at incidences close to the stall condition of the airfoil. The aerodynamic loads, both lift and drag, suffer important impulsive variations, in agreement with literature on BVI, the effects of which are extended in time. In the case of the oscillating airfoil, BVI introduces hysteresis cycles in the loads, which are generally reduced. In conclusion, parallel BVI can have a detrimental impact on the aerodynamic performances of the blade and even cause flow separation, which, while not being as catastrophic as in the case of dynamic stall, has relatively long-lasting effects. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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25 pages, 23569 KiB

Open AccessArticle

Analyzing the Influence of Dean Number on an Accelerated Toroidal: Insights from Particle Imaging Velocimetry Gyroscope (PIVG)

by Ramy Elaswad, Naser El-Sheimy and Abdulmajeed Mohamad

Fluids 2024, 9(5), 103; https://doi.org/10.3390/fluids9050103 - 25 Apr 2024

Viewed by 1095

Abstract

Computational Fluid Dynamics (CFD) simulations were utilized in this study to comprehensively explore the fluid dynamics within an accelerated toroidal vessel, specifically those central to Particle Imaging Velocimetry Gyroscope (PIVG) technology. To ensure the robustness of our simulations, we systematically conducted grid convergence studies and quantified uncertainties, affirming the stability, accuracy, and reliability of our computational grid and results. Comprehensive validation against experimental data further confirmed our simulations’ fidelity, emphasizing the model’s fidelity. As the PIVG is set up to address the primary flow through the toroidal pipe, we focused on the interaction between the primary and secondary flows to provide insights into the relevant dynamics of the fluid. In our investigation covering Dean numbers (D_e) from 10 to 70, we analyzed diverse aspects, including primary flow, secondary flow patterns, pressure distribution, and the interrelation between primary and secondary flows within toroidal structures, offering a comprehensive view across this range. Our research indicated stability and fully developed fluid dynamics within the toroidal pipe under accelerated angular velocity, particularly for low D_e. Furthermore, we identified an optimal Dean number of 11, which corresponded to ideal dimensions for the toroidal geometry with a curvature radius of 25 mm and a cross-sectional diameter of 5 mm. This study enhances our understanding of toroidal fluid dynamics and highlights the pivotal role of CFD in optimizing toroidal vessel design for advanced navigation technologies. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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30 pages, 6875 KiB

Open AccessArticle

Application of a Combinatorial Vortex Detection Algorithm on 2 Component 2 Dimensional Particle Image Velocimetry Data to Characterize the Wake of an Oscillating Wing

by Mathew Bussière, Guilherme M. Bessa, Charles R. Koch and David S. Nobes

Fluids 2024, 9(3), 53; https://doi.org/10.3390/fluids9030053 - 22 Feb 2024

Cited by 2 | Viewed by 1961

Abstract

To investigate the vortical wake pattern generated by water flow past an oscillating symmetric airfoil, using experimental velocity fields from particle image velocimetry (PIV), a novel combinatorial vortex detection (CVD) algorithm is developed. The primary goal is to identify and characterize vortices within the wake. Experimental flows introduce complexities not present in numerical simulations, posing challenges for vortex detection. The proposed CVD approach offers a more robust alternative, excelling in both vortex detection and quantification of essential parameters, unlike widely-used methods such as Q-criterion, λ₂-criterion, and Δ-criterion, which rely on subjective and arbitrary thresholds resulting in uncertainty. The CVD algorithm effectively characterizes the airfoil wake, identifying and analyzing vortices aligning with the Burgers model. This research enhances understanding of wake phenomena and showcases the algorithm’s potential as a valuable tool for vortex detection and characterization, particularly for experimental fluid dynamics. It provides a comprehensive, robust, and non-arbitrary approach, overcoming limitations of traditional methods and opening new avenues for studying complex flows. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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15 pages, 6572 KiB

Open AccessArticle

Quantitative Color Schlieren for an H₂–O₂ Exhaust Jet Developing in Air

by Emilia-Georgiana Prisăcariu and Tudor Prisecaru

Fluids 2024, 9(1), 19; https://doi.org/10.3390/fluids9010019 - 8 Jan 2024

Cited by 3 | Viewed by 1793

Abstract

Throughout many decades, the Schlieren visualization method has been mainly used as means to visualize transparent flows in a qualitative manner. The images recorded provide data regarding the existence of the flow, or illustrate predicted flow geometries and details. The colored Schlieren method has been developed in the late 1890s and has always had the intent to provide quantitative data rather than qualitative pictures of the studied phenomena. This paper centers on applying a quantitative color Schlieren method to help determine the gasodynamic parameters of an H₂–O₂ exhaust jet, developing in air. A comparison between the parameters obtained through calibrating the color filter for the Schlieren method and the results from a CFD simulation is performed to assess the range of the CS (color Schlieren) measurement. This paper’s findings address the issues of calibrated color filter Schlieren encounter during its implementation and discusses possible errors appearing when the method is applied to a 3D flow. While the qualitative Schlieren images are still impressive to observe, the quantitative Schlieren presents challenges and a low measurement accuracy (75%) when applied to 3D flows and compared to 2D cases found in the literature (97–98%). Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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14 pages, 3906 KiB

Open AccessArticle

Volumetric Flow Field inside a Gas Stirred Cylindrical Water Tank

by Yasmeen Jojo-Cunningham, Xipeng Guo, Chenn Zhou and Yun Liu

Fluids 2024, 9(1), 11; https://doi.org/10.3390/fluids9010011 - 28 Dec 2023

Cited by 3 | Viewed by 1821

Abstract

Ladle metallurgy serves as a crucial component of the steelmaking industry, where it plays a pivotal role in manipulating the molten steel to exercise precise control over its composition and properties. Turbulence in ladle metallurgy influences various important aspects of the steelmaking process, including mixing and distribution of additives, alongside the transport and removal of inclusions within the ladle. Consequently, gaining a clear understanding of the stirred flow field holds the potential of optimizing ladle design, improving control strategies, and enhancing the overall efficiency and steel quality. In this project, an advanced Particle-Tracking-Velocimetry system known as “Shake-the-Box” is implemented on a cylindrical water ladle model while compressed air injections through two circular plugs positioned at the bottom of the model are employed to actively stir the flow. To mitigate the particle images distortion caused by the cylindrical plexi-glass walls, the method of refractive matching is utilized with an outer polygon tank filled with a sodium iodide solution. The volumetric flow measurement is achieved on a 6 × 6 × 2 cm domain between the two plugs inside the cylindrical container while the flow rate of gas injection is set from 0.1 to 0.4 L per minute. The volumetric flow field result suggests double gas injection at low flow rate (0.1 L per minute) produce the least disturbed flow while highly disturbed and turbulent flow can be created at higher flow rate of gas injection. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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23 pages, 6256 KiB

Open AccessArticle

An Enhanced Python-Based Open-Source Particle Image Velocimetry Software for Use with Central Processing Units

by Ali Shirinzad, Khodr Jaber, Kecheng Xu and Pierre E. Sullivan

Fluids 2023, 8(11), 285; https://doi.org/10.3390/fluids8110285 - 27 Oct 2023

Cited by 2 | Viewed by 2553

Abstract

Particle Image Velocimetry (PIV) is a widely used experimental technique for measuring flow. In recent years, open-source PIV software has become more popular as it offers researchers and practitioners enhanced computational capabilities. Software development for graphical processing unit (GPU) architectures requires careful algorithm design and data structure selection for optimal performance. PIV software, optimized for central processing units (CPUs), offer an alternative to specialized GPU software. In the present work, an improved algorithm for the OpenPIV–Python software (Version 0.25.1, OpenPIV, Tel Aviv-Yafo, Israel) is presented and implemented under a traditional CPU framework. The Python language was selected due to its versatility and widespread adoption. The algorithm was also tested on a supercomputing cluster, a workstation, and Google Colaboratory during the development phase. Using a known velocity field, the algorithm precisely captured the time-average flow, momentary velocity fields, and vortices. Full article

(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)

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