Flow Visualization Method Research Articles

Flow characteristics around juncture of a circular cylinder mounted normal to a flat plate

To study the flow characteristics in the upstream and downstream regions around the juncture of a long circular cylinder and a flat plate, experiments by particle-tracking flow visualization method and particle image velocimetry (PIV) were performed in a towing water tank. The Reynolds numbers were from 500 to 6000 and the inverse of non-dimensional boundary-layer displacement thickness were within 3.47–12.03. The temporal and spatial evolution processes of the vortical flow structures in the vertical and horizontal planes in the upstream and downstream regions around the juncture of a long circular cylinder and a flat plate were revealed. The flow patterns delineated by velocity vectors and streamlines were discussed for their topological flow characteristics. The unsteady features of the wake near the juncture were presented. Four characteristic flow modes (single-vortex, dual-vortex, triple-vortex, and unsteady-vortex) induced by the boundary-layer separation and shear effect of the counter flow near the juncture were observed in the upstream region near the juncture. The magnitude of the wall-shear stress under the vortex ring nearest the juncture presented the largest value. In the cylinder wake, no periodic vortex shedding was observed at the levels influenced by the upstream horse-shoe vortex rings. Above the region influenced by the upstream horse-shoe vortex rings, the vortex shedding presented periodic characteristics with a Strouhl number of approximately 0.2.

Acceleration statistics in thermally driven superfluid turbulence.

New methods of flow visualization near absolute zero have opened the way to directly compare quantum turbulence (in superfluid helium) to classical turbulence (in ordinary fluids such as air or water) and explore analogies and differences. We present results of numerical simulations in which we examine the statistics of the superfluid acceleration in thermal counterflow. We find that, unlike the velocity, the acceleration obeys scaling laws similar to classical turbulence, in agreement with a recent quantum turbulence experiment of La Mantia etal.

Unsteady flow motions of an oscillating jet in crossflow

The temporal and spatial variations of the flow behavior and velocity field of an oscillating jet in a crossflow were experimentally investigated. A pulsed jet issuing into a crossflow caused the deflected jet to oscillate. The instantaneous oscillating flow patterns in the symmetry plane at several specific phase angles were identified using smoke flow visualization method. The instantaneous flow velocities were measured using the high-speed particle image velocimetry (PIV) and presented as phase-averaged velocity vectors and streamlines. Owing to the variation of jet exit velocity, the near tube-tip jet column flapped up and down within one cycle of jet pulsation and induced a periodic wavy flow structure in the downstream area. By identifying the variations of near-wake velocity vectors and streamline patterns within one excitation cycle, four characteristic flow modes (downwash, crossflow-dominated, jet-dominated, and transitional) were found at different excitation phase angles. Around the mid-oscillation cycle, the wavy flow structure was characterized by two adjacent vorticity concentrated areas of opposite signs in the phase-averaged vorticity contour. The turbulence properties of the excited elevated transverse jet at large jet-to-crossflow momentum flux ratio were larger than those at low jet-to-crossflow momentum flux ratio.

Effects of Intraluminal Thrombus on Patient-Specific Abdominal Aortic Aneurysm Hemodynamics via Stereoscopic Particle Image Velocity and Computational Fluid Dynamics Modeling

The pathology of the human abdominal aortic aneurysm (AAA) and its relationship to the later complication of intraluminal thrombus (ILT) formation remains unclear. The hemodynamics in the diseased abdominal aorta are hypothesized to be a key contributor to the formation and growth of ILT. The objective of this investigation is to establish a reliable 3D flow visualization method with corresponding validation tests with high confidence in order to provide insight into the basic hemodynamic features for a better understanding of hemodynamics in AAA pathology and seek potential treatment for AAA diseases. A stereoscopic particle image velocity (PIV) experiment was conducted using transparent patient-specific experimental AAA models (with and without ILT) at three axial planes. Results show that before ILT formation, a 3D vortex was generated in the AAA phantom. This geometry-related vortex was not observed after the formation of ILT, indicating its possible role in the subsequent appearance of ILT in this patient. It may indicate that a longer residence time of recirculated blood flow in the aortic lumen due to this vortex caused sufficient shear-induced platelet activation to develop ILT and maintain uniform flow conditions. Additionally, two computational fluid dynamics (CFD) modeling codes (Fluent and an in-house cardiovascular CFD code) were compared with the two-dimensional, three-component velocity stereoscopic PIV data. Results showed that correlation coefficients of the out-of-plane velocity data between PIV and both CFD methods are greater than 0.85, demonstrating good quantitative agreement. The stereoscopic PIV study can be utilized as test case templates for ongoing efforts in cardiovascular CFD solver development. Likewise, it is envisaged that the patient-specific data may provide a benchmark for further studying hemodynamics of actual AAA, ILT, and their convolution effects under physiological conditions for clinical applications.

Flow visualization and characterization for optimized MQL machining of composites

Limited information is available on the effect of Minimum Quantity Lubrication (MQL) parameters (oil flow rate OFR, air flow rate AFR, nozzle orientation and distance from the cutting zone) on flow characteristics. ‘Particle Image Velocimetry’ and ‘Phase Doppler Anemometry’ flow visualization methods were used to define the optimal MQL jet for better penetration and cooling/lubrication; coherent, small magnitude/number of vorticities, and small droplets of high velocity. Effect of flow characteristics on cutting forces, temperature, tool wear and geometric errors was examined in CFRP milling. Optimum AFR, OFR and nozzle distance from the cutting zone were established and compared to flood, pressurized air, and dry machining.

Long-Term Visualization of Micro Vortices in Bacillus Subtilis Bacterial Biofilm on Agar Plate using Particle Image Velocimetry

The collective dynamics of bacteria has a crucial role in the development of biofilm. Although the behavior of the fluidic biofilm was studied by many research groups, detailed mechanics of its motion still remains elusive. Previously, we developed a method for visualization of vortical flow in the Bacillus Subtilis (B. subtilis) colony using 200-nm fluorescent microbeads, which were initially embedded in the agar plate and distributed spontaneously at the upper surface of the growing colony. Here, we conducted a long-term live imaging of B. subtilis colony with the fluorescent beads and obtained high-resolution velocity maps of microscale vortices in the biofilm using particle image velocimetry (PIV). At the tip of the colony, a distinct periodic fluctuation of average speed and vorticity revealed by Micro-PIV analysis was correlated with switching between bacterial swarming and growth phases. Our advanced imaging tool helps to explain the effect of micro vorticies in the biofilm on the collective dynamics of bacteria.

Flow and Leakage Characteristics of a Sashless Inclined Air-Curtain (sIAC) Fume Hood Containing Tall Pollutant-Generation Tanks

In many fume hood applications, pollutant-generation devices are tall. Human operators of a fume hood must stand close to the front of the hood and lift up their hands to reach the top opening of the tall tank. In this situation, it is inconvenient to access the conventional hood because the sash acts as a barrier. Also, the bluff-body wake in front of the operator's chest causes a problem. By using laser-assisted smoke flow visualization and tracer-gas test methods, the present study examines a sashless inclined air-curtain (sIAC) fume hood for tall pollutant-generation tanks, with a mannequin standing in front of the hood face. The configuration of the sIAC fume hood, which had the important element of a backward-inclined push-pull air curtain, was different from conventional configurations. Depending on suction velocity, the backward-inclined air curtain had three characteristic modes: straight, concave, and attachment. A large recirculation bubble covering the area—from the hood ceiling to the work surface—was formed behind the inclined air curtain in the straight and concave modes. In the attachment mode, the inclined air curtain was attached to the rear wall of the hood, about 50 cm from the hood ceiling, and bifurcated into up and down streams. Releasing the pollutants at an altitude above where the inclined air curtain was attached caused the suction slot to directly draw up the pollutants. Releasing pollutants in the rear recirculation bubble created a risk of pollutants’ leaking from the hood face. The tracer-gas (SF6) test results showed that operating the sIAC hood in the attachment mode, with the pollutants being released high above the critical altitude, could guarantee almost no leakage, even though a mannequin was standing in front of the sashless hood face.

Tomography images to analyze the deformation of the cavern in the continuous‐flow mixing of non‐Newtonian fluids

Tomography, an efficient nonintrusive technique, was employed to visualize the flow in continuous‐flow mixing and to measure the cavern volume (Vc) in batch mixing. This study has demonstrated an efficient method for flow visualization in the continuous‐flow mixing of opaque fluids using two‐dimensional (2‐D) and 3‐D tomograms. The main objective of this study was to explore the effects of four inlet‐outlet configurations, fluid rheology (0.5–1.5% xanthan gum concentration), high‐velocity jet (0.317–1.660 m s−1), and feed flow rate (5.3 × 10−5−2.36 × 10−4 m3 s−1) on the deformation of the cavern. Dynamic tests were also performed to estimate the fully mixed volume (Vfully mixed) for the RT, A310, and 3AM impellers in a continuous‐flow mixing system, and it was found that Vfully mixed was greater than Vc. Incorporating the findings of this study into the design criteria will minimize the extent of nonideal flows in the continuous‐flow mixing of complex fluids and eventually improve the quality of end‐products. © 2013 American Institute of Chemical Engineers AIChE J, 60: 315–331, 2014

Phase-resolved and time-averaged puff motions of an excited stack-issued transverse jet

The dynamics of puff motions in an excited stack-issued transverse jet were studied experimentally in a wind tunnel. The temporal and spatial evolution processes of the puffs induced by acoustic excitation were examined using the smoke flow visualization method and high-speed particle image velocimetry. The temporal and spatial evolutions of the puffs were examined using phase-resolved ensemble-averaged velocity fields and the velocity, length scales, and vorticity characteristics of the puffs were studied. The time-averaged velocity fields were calculated to analyze the velocity distributions and vorticity contours. The results show that a puff consists of a pair of counter-rotating vortex rings. An initial vortex ring was formed due to a concentration of vorticity at the lee side of the issuing jet at the instant of the mid-oscillation cycle. A vortex ring rotating in the opposite direction to that of the initial vortex ring was subsequently formed at the upwind side of the issuing jet. These two counter-rotating vortex rings formed a “mushroom” vortex pair, which was deflected by the crossflow and traveled downstream along a time-averaged trajectory of zero vorticity. The trajectory was situated far above the time-averaged streamline evolving from the leading edge of the tube. The velocity magnitudes of the vortex rings at the upwind and the lee side decreased with time evolution as the puffs traveled downstream due to momentum dissipation and entrainment effects. The puffs traveling along the trajectory of zero vorticity caused large velocities to appear above the leading-edge streamline.

Mass‐Dependent Integral Curves in Unsteady Vector Fields

AbstractRecent research in flow visualization is focusing on the analysis of time‐dependent, but mass‐less particles. However, in many application scenarios, the mass of particles – and their resulting inertia – is essential in understanding fluid mechanics. This includes critical processes, such as dust particles interacting with aircraft (e.g., brown‐or white‐out effects) and particle separation based on density variation. In this paper, we contribute a generalized description of mass‐dependent particle trajectories and apply existing unsteady flow visualization methods to the mass‐dependent case. This comprises the extension of common concepts, i.e., path lines, streak lines, and time lines. Furthermore, we introduce a new class of integral curves, called mass lines that effectively visualizes mass separation and captures mass‐related features in unsteady flow fields that are inaccessible using traditional methods. We demonstrate the applicability of our method, using a number of real‐world and artificial data sets, in which mass is a crucial parameter. In particular, we focus on the analysis of brown‐out conditions, introduced by a helicopter in forward flight close to the ground.

기체-액체 이젝터 유동의 가시화와 광섬유 탐침에 의한 기포분율 측정

Gas/liquid two-phase ejector is a device without moving parts, in which liquid is used to drive gas of a low-pressure source. In this paper, the hydrodynamic characteristics of a vertical down type two-phase ejector were studied using an air-water loop system. Entrained air flow rates were measured with inlet and outlet pressures of the ejector with varying water flow rate. Homogeneous bubbly flows in the discharge pipe were confirmed by the high speed flow visualization method. Quantitative measurements of void fraction were made using a newly developed fiber optic probe system.

Heat flow visualization analysis on natural convection in rhombic enclosures with isothermal hot side or bottom wall

The heat flow visualization method based on heatfunction has been adopted to analyze natural convection via differential heating (case 1) and Rayleigh–Benard convection (case 2) in fluid filled (Prandtl number, Pr=0.015–1000) rhombic enclosures with various inclination angles, φ, using the Galerkin finite element method for the range of Rayleigh number, Ra=103–105. An accurate prediction of the flow structure and heat distribution in such configurations is of great importance due to various engineering applications such as solar energy systems, cooling of electronic devices, combustion, etc. The strength of fluid and heat flow increases with φ and Ra due to an enhanced convection effect and the maximum magnitude of streamfunction (ψmax) and heatfunction (Πmax) is observed at φ=90° in both cases. Conduction based static fluid solutions are also found at φ=90° associated with convection based dynamic fluid solutions in case 2 for all Pr. Heating strategies are compared via heatlines and also based on local (Nu) and average Nusselt numbers (Nu¯). It is also shown that φ=30° shows higher Nu¯ in case 2 compared to case 1, whereas φ=90° shows higher Nu¯ in case 1 compared to case 2 for all Pr. There exists a critical rhombic angle (φ=50°) such that differential heating may be the profound heating situation at φ≥50° for materials involving Pr=1000. Also, the rhombic cavity may be an alternative geometrical design in convective thermal processing of fluids with vertical thermal gradient (case 2) as it establishes only convection based dynamic solutions for all possible angles (φ<90°).

Mass transfer and fluid flow visualization for single cylinder by the adsorption method

Mass transfer coefficient in single phase flow around short cylinder, has been studied experimentally using adsorption method. Experiments were conducted in “two-dimensional” column 140×10×200mm. The mass transfer data were obtained by studying transfer for flow past single cylinder, 30mm in diameter.This paper, also, discusses the possibilities of application of the adsorption method for fluid flow visualization. Local and average mass transfer coefficients were determined from color intensity on the surface of foils of silica gel. Experimental correlations for Sh=f (Re) and jD=f (Re) were established using mass transfer coefficients data.

E122 流れの可視化技術を用いた原子炉・格納容器のデブリ・水漏れ検知手法の開発(OS8 軽水炉・新型炉・原子力安全(3))

It is the important issue to fill up each nuclear reactor/containment vessel with water and to take out debris of damaged fuel from them for decommissioning of Fukushima Daiichi nuclear power plants. It is necessary to detect the debris and water leaks at a nuclear reactor/containment vessel for the purpose However, the method is not completely developed in the present stage. Accordingly, we have developed a method for detecting debris and water leaks at a nuclear reactor/containment vessel by flow visualization Experiments of the flow visualization were conducted using two types of water tanks An optical fiber and a collimator lens were employed for modifying a straight laser beam into a sheet projection Some visualized images were obtained through the experiments. Particle Image Velocimetry, i.e. PIV, analysis was applied to the images for quantitative flow rate analysis. Consequently, it is considered that the flow visualization method has a possibility for the practical use.

S114016 熱硬化油膜法による静圧気体軸受すきま内流れの可視化 : 油膜のレオロジー特性に及ぼす油膜成分の影響([S114-01]トライボロジーの基礎と応用(1))

The heat curing oil film method using thermosetting property oil film is a new oil film method. The heat curing oil film method, in which not only the plane flow pattern in wall surface but also oil film thickness of the steady flow can be measured, is newly developed. The heat curing oil film method is a very useful method for visualization of an air flow in the clearance of an externally pressurized gas-lubricated circular thrust bearing with a central supply hole. The effect of oil component on the rheological characteristics of the heat curing oil without hardener is investigated in this paper.

Experimental Investigation of Operating Room Air Distribution in a Full-Scale Laboratory Chamber Using Particle Image Velocimetry and Flow Visualization

Room air distribution in hospital operating rooms (OR) is critical to the effective functioning of surgical procedures, but the air distribution patterns are governed by complex physics that are currently not well understood. Both qualitative and quantitative flow visualization techniques were used to evaluate the room air distribution in a full-scale chamber designed to simulate a hospital operating room. A laser sheet illumination technique was used to identify key features of the room air distribution, and particle image velocimetry (PIV) was used to measure the velocity field in a plane crossing the surgical site. Hospital operating rooms require the use of ASHRAE Group E diffusers in an array above the surgical table, providing downward, unidirectional, non-aspirating air flow across the sterile region of the room. The supply air jet is characterized by complex physics, including annular shape, impingement, buoyancy, a large jet to room aspect ratio, and recirculation. The large diameter of the jet relative to the room size makes the overall room air distribution highly sensitive to the parameters of the supply air. The air distribution pattern in the room was found to have relatively low velocity and turbulence near the supply air diffuser, but increasing velocity and turbulence in the shear region at the edge of the supply air jet. Flow visualization and PIV methods both demonstrated an angle of the shear layer inwards towards the center of the jet. This flow feature reduces the overall coverage area for the sterile air flow and may pose a risk to the protection of the surgical patient.

1214 STATE OF THE ART IN IDENTIFICATION OF TWO-PHASE TRANSONIC FLOW PHENOMENA IN TRANSCRITICAL CO2 EJECTORS

The paper presents current developments in both experimental and numerical identification of two-phase flow phenomena in transcritical CO2 ejectors. Neither explicit identification/classification of the flow patterns nor determination of velocity field for the CO2 ejector passages, based on experimental methods of flow visualization, was reported so far. First trials for determination of spatial fields for the Mach number, pressure, and density inside the CO2 ejector passages, modelled with a validated CFD tool, were reviewed. The influence of the phase transition relaxation on the converging-diverging motive nozzle performance was analysed. Insufficient qualitative and quantitative identification of the phenomena influencing the mass, momentum, and energy transfer was discussed.

Role of distributed heating on enhancement of thermal mixing for liquid food processing with heat flow visualization method

Abstract Thermal treatment is an important step in food processing. The conventional methods of heating of an enclosure may result in inadequate thermal mixing and poor temperature distribution leading to energy wastage. In this work, an alternative, energy-efficient method of distributed heating of the cavity is studied and compared with the isothermal bottom wall heating case in enhancing the thermal mixing and improving the temperature distribution in the cavity. Steady laminar natural convection of liquid food materials with a representative Prandtl number, Pr = 3.14 is studied in the range of Rayleigh number, Ra = 103–105 in differentially and discretely heated square cavities. Detailed analysis is carried out by visualizing the heat flow by heatlines. Further, the thermal mixing and the temperature uniformity are analyzed based on the cup-mixing temperature and root-mean square deviation. It is found that the thermal management policy of distributed heating significantly influences the thermal mixing and the temperature uniformity within liquid food. In a case with multiple discrete heat sources, a remarkable uniformity in temperature across the cavity is achieved with moderate thermal mixing. Based on heatlines, cup-mixing temperature and root-mean square deviation, optimal heating policy for food processing may be chosen. Industrial Relevance Three types of discrete heating situation are considered. Numerical simulations were carried out to predict the temperature and flow characteristics. Numerical simulation results were benchmarked or validated with literature data. Thus, current numerical simulation results are useful to select the suitable heating strategy for efficient thermal processing of liquid food. The proposed methodology with the numerical data would be useful to practicing industries for efficient juice heating.

Buoyancy-driven flows by a heat source at different levels

This study applies thermal plume theory and laboratory water tank experiments to investigate the effect of a localized heat source at different levels on buoyancy-driven flows in a naturally ventilated space. The space has an inlet and outlet openings connecting to the exterior environment respectively at the floor and ceiling levels, and keeps a constant heat source at several levels inside it. In the laboratory experiments, the thermal structures were measured by a series of thermocouples and the flow fields were captured by a laser light sheet flow visualization method. The research results show that a different heat source level changes the mass flux in the space, and also the temperature field and the stratification interface of the buoyant layer. When the same heat strength source is raised to a higher level, the mass flux decreases, but the temperature increment and the interface of the buoyant layer both rise in the steady state. In the same space, an elevated heat source level always leads to a higher interface level regardless of the input heat strength, and the stratification interface changes more linearly with the heat source level for the case of a smaller effective opening area. The stratification performance of buoyancy-driven displacement ventilation by a heat source at an elevated level is different from that of mechanical displacement ventilation with the same heat source condition.

Momentum transfer and flow induction in a dielectric barrier discharge plasma actuator

The flow induction process resulting from a dielectric barrier discharge plasma actuator was experimentally investigated by a flow visualization method. This method enables a detailed examination of the flow induction process. The overall behavior as well as the detailed temporal and spatial behavior of the flow induction process was clarified. The induced flow originated from the vicinity of an exposed electrode edge, and similar to a wall jet, it smoothly spread over the surface until a vortex was generated at the tip of the wall jet. Even after the vortex disappeared, the wall jet continued to extend. This wall jet was classified into two distinct regions. The first region contained the wall jet extending from the electrode edge to the location where the vortex was generated. In this region, active momentum coupling occurred; therefore, the flow was actively induced. During active momentum transfer in the acceleration region, the periodic inflation of the wall jet thickness was synchronized with the decreasing phase of the applied high-voltage input, which suggested different mechanisms of momentum transfer during the increasing and decreasing phases of the high-voltage input. The second region was an extension of the first one and was formed through the inertia of the flow originating from the first region.