Flow Visualization Experiments Research Articles

Colliding respiratory jets as a mechanism of air exchange and pathogen transport during conversations

Air exchange between people has emerged in the COVID-19 pandemic as the important vector for transmission of the SARS-CoV-2 virus. We study the airflow and exchange between two unmasked individuals conversing face-to-face at short range, which can potentially transfer a high dose of a pathogen, because the dilution is small when compared to long-range airborne transmission. We conduct flow visualization experiments and direct numerical simulations of colliding respiratory jets mimicking the initial phase of a conversation. The evolution and dynamics of the jets are affected by the vertical offset between the mouths of the speakers. At low offsets the head-on collision of jets results in a `blocking effect', temporarily shielding the susceptible speaker from the pathogen carrying jet, although, the lateral spread of the jets is enhanced. Sufficiently large offsets prevent the interaction of the jets. At intermediate offsets (8-10 cm for 1 m separation), jet entrainment and the inhaled breath assist the transport of the pathogen-loaded saliva droplets towards the susceptible speaker's mouth. Air exchange is expected, in spite of the blocking effect arising from the interaction of the respiratory jets from the two speakers.

Investigation on steady regimes in a X-shaped micromixer fed with water and ethanol

Microreactors are very attractive for intensifying chemical and biochemical reactions, as they enable a large heat transfer with high control of the operating conditions. These features allow improving the achievable reaction yield and selectivity. However, the mixing of reactants should be ensured, especially in simple designs, as the flow regime is laminar. This work aims at characterizing the flow regimes and degree of mixing in a X-shaped micromixer with two opposite inlets and at investigating how these regimes are affected by fluid properties. To this purpose we carry out numerical simulations of two different mixtures, i.e. Water-Water (W-W) and Water-Ethanol (W-E), for Reynolds numbers Re⩽200, to gain insight into the complex three-dimensional flow structures stemming from the impinging jets at the confluence. Besides, experimental flow visualizations from the W-W case provide a validation of the numerical model. The X-shaped reactor is observed to enable a superior mixing performance compared to T-, Y-, and Arrow-shaped microreactors, which are extensively employed.

Prediction of fiber rotation in an orifice channel during injection molding process

Additive alignment in polymer composites can enhance the physical properties of the constitutive material. Researchers have attempted to understand and predict the additive orientation during the fabrication process, such as injection molding. In this study, the rotation of carbon fibers embedded in uncured polydimethylsiloxane flowing in an orifice channel is analyzed via a flow visualization experiment and image processing of obtained images. The angular velocity of additives is correlated with the shear rates in the flow field calculated from the computational fluid dynamics simulation with the assumption of a single-phase and a non-Newtonian flow. The effective shear rate is found to have a larger effect on the rotation of fiber compared with the aspect ratio of the fiber and the initial alignment angle to the flow direction. A correlation between the effective shear rate and angular velocity of the fibers is developed and verified experimentally. This correlation can be used to predict the fiber rotation during polymer composite fabrication within an orifice channel and hence provide useful information regarding the anisotropic physical properties of the final product.

Exhaust rate for range hood at cooking temperature near the smoke point of edible oil in residential kitchen

The discharge capacity of the range hood for cooking-related pollution under current exhaust airflow rate (EAR) standard of 300–500 m3/h is limited when cooking temperature near the smoke point of edible oil. This study endeavored to determine appropriate EAR for cooking temperature varies from 200 to 300 °C, which comprised smoke points of different edible oils, considering whether the kitchen was ventilated with the adjacent room or not, and the cooking position of the left/right stove. Particles generated from a simplified cooking process of oil heating test, tracer gas experiment, and flow visualization experiment were conducted. CFD simulation with capture efficiency as an evaluation index was performed. Evidence is presented that when the cooking temperature is 200 °C, the pollutant control effect does not change much when the EAR is above 700 m3/h, while an EAR of 800 m3/h may be necessary at 300 °C. For open-door and closed-door cooking, a minimal difference exists in demand for a suitable EAR in this local ventilation system. However, the hood continues to exhaust air during post-cooking period, thereby showing that a higher EAR is necessary to control the pollutants in open-door cooking. The primary and secondary cross draft airflows caused by different air supply positions are observed, which play a vital role in selecting suitable left/right cooking position. The location of the left stove is conducive to the elimination of cooking-related pollutants due to primary cross draft, while it is the right stove due to secondary cross draft.

Flow Visualization Experiments of Inclined Slot Jets with Negative Buoyancy

Flow Visualization Experiments of Inclined Slot Jets with Negative Buoyancy

A Computational Framework for Pre-Interventional Planning of Peripheral Arteriovenous Malformations

PurposePeripheral arteriovenous malformations (pAVMs) are congenital lesions characterised by abnormal high-flow, low-resistance vascular connections—the so-called nidus—between arteries and veins. The mainstay treatment typically involves the embolisation of the nidus, however the complexity of pAVMs often leads to uncertain outcomes. This study aims at developing a simple, yet effective computational framework to aid the clinical decision making around the treatment of pAVMs using routinely acquired clinical data.MethodsA computational model was developed to simulate the pre-, intra-, and post-intervention haemodynamics of a patient-specific pAVM. A porous medium of varying permeability was employed to simulate the sclerosant effect on the nidus haemodynamics. Results were compared against clinical data (digital subtraction angiography, DSA, images) and experimental flow-visualization results in a 3D-printed phantom of the same pAVM.ResultsThe computational model allowed the simulation of the pAVM haemodynamics and the sclerotherapy-induced changes at different interventional stages. The predicted inlet flow rates closely matched the DSA-derived data, although the post-intervention one was overestimated, probably due to vascular system adaptations not accounted for numerically. The nidus embolization was successfully captured by varying the nidus permeability and increasing its hydraulic resistance from 0.330 to 3970 mmHg s ml−1. The nidus flow rate decreased from 71% of the inlet flow rate pre-intervention to 1%: the flow completely bypassed the nidus post-intervention confirming the success of the procedure.ConclusionThe study demonstrates that the haemodynamic effects of the embolisation procedure can be simulated from routinely acquired clinical data via a porous medium with varying permeability as evidenced by the good qualitative agreement between numerical predictions and both in vivo and in vitro data. It provides a fundamental building block towards a computational treatment-planning framework for AVM embolisation.

Increasing perpendicular alignment in extruded filament by an orifice embedded 3D printing nozzle

ABSTRACT The mixing of fibrous additives in the polymer base is being widely investigated for improving the mechanical properties of printed parts using fused filament fabrication (FFF). In this study, flow visualisation experiments and computational fluid dynamics (CFD) analysis were conducted on the flow channel of a nozzle with an orifice for enhancing the through-layer physical properties of the printed parts. Flow visualisation experiments were conducted on the flow channels of nozzles installed with orifices of various shapes. Using ball-milled carbon fibre in high-viscosity polydimethylsiloxane (PDMS) that imitates filament flow in FFF, the fibre angles were measured for different internal and external locations of the nozzle. The distribution of the fibre angles inside the nozzle was compared with the gradient of the flow field calculated for the same shape by CFD analysis. It was found that the orifice structure inside the nozzle increases the vertical alignment of the fibre. An abrupt increase in the extension rate at the exit of the orifice is considered to be the reason behind the rotation and vertical orientation of fibres inside the nozzle channel. Results presented in this work provide the basis for achieving improved through-plane physical properties of FFF printed parts through fibre alignment.

The World's First Test Facility That Enables the Experimental Visualization of Cavitation on a Rotating Inducer in Both Cryogenic and Ordinary Fluids

Abstract It is well known that cavitation breakdown, which is a phenomenon in which the pump head suddenly drops with a decrease in the inlet cavitation number, occurs in turbopumps. Especially in cryogenic pumps, cavitation breakdown occurs at a lower inlet cavitation number than that of ordinary fluids such as water. This phenomenon is referred to as a thermodynamic effect, as Stepanoff reported. The thermodynamic properties of the working fluid affect the sizes of the cavitation elements, which affect cavitation breakdown; therefore, experimental flow visualization is an effective approach to realize a more efficient and more reliable cryogenic pump. In 2010, the author and colleagues developed the world's first test facility to enable the visualization of cavitation on a rotating inducer in both cryogenic and ordinary fluids. At that time, only two reports on the flow visualization of a rotating cryogenic impeller had been published: one on flow visualization in liquid hydrogen by NASA in 1967 and the other on flow visualization in liquid nitrogen by the Japan Aerospace Exploration Agency (JAXA) in 2010. The present facility employs a triple-thread helical inducer with a diameter of 65.3 mm and a rotation rate of up to 8000 rpm, with both liquid nitrogen and water available as working fluids. Unsteady visualization experiments for cavitation on an inducer in liquid nitrogen and water have revealed the characteristics of tip vortex cavitation, backflow vortex cavitation, and cavitation element size based on comparisons between cryogenic fluids that exhibit a stronger thermodynamic effect and ordinary fluids such as water.

Elastic energy storage in seahorses leads to a unique suction flow dynamics compared with other actinopterygians.

ABSTRACTSuction feeding is a dominant prey-capture strategy across actinopterygians, consisting of a rapid expansion of the mouth cavity that drives a flow of water containing the prey into the mouth. Suction feeding is a power-hungry behavior, involving the actuation of cranial muscles as well as the anterior third of the fish's swimming muscles. Seahorses, which have reduced swimming muscles, evolved a unique mechanism for elastic energy storage that powers their suction flows. This mechanism allows seahorses to achieve head rotation speeds that are 50 times faster than those of fish lacking such a mechanism. However, it is unclear how the dynamics of suction flows in seahorses differ from the conserved pattern observed across other actinopterygians, or how differences in snout length across seahorses affect these flows. Using flow visualization experiments, we show that seahorses generate suction flows that are 8 times faster than those of similar-sized fish, and that the temporal patterns of cranial kinematics and suction flows in seahorses differ from the conserved pattern observed across other actinopterygians. However, the spatial patterns retain the conserved actinopterygian characteristics, where suction flows impact a radially symmetric region of ∼1 gape diameter outside the mouth. Within seahorses, increases in snout length were associated with slower suction flows and faster head rotation speeds, resulting in a trade-off between pivot feeding and suction feeding. Overall, this study shows how the unique cranial kinematics in seahorses are manifested in their suction-feeding performance, and highlights the trade-offs associated with their unique morphology and mechanics.

Edge Race-Tracking during Film-Sealed Compression Resin Transfer Molding

Edge race-tracking is a frequently reported issue during resin transfer molding. It is caused by highly permeable channels and areas between the preform edge and cavity, which can significantly change the preform impregnation pattern. To date, information is scarce on the effect of edge race-tracking in compression resin transfer molding (CRTM). To close this gap, laboratory equipment was developed to study the CRTM preform impregnation via flow visualization experiments. The preform was thereby encapsulated in thin thermoplastic films sealing its impregnation. Film-sealed compression resin transfer molding (FS-CRTM) experiments of preforms with a small geometrical aspect ratio showed fast filling of the injection gap and a subsequent through-thickness preform impregnation. Creating an edge race-tracking channel, an additional lateral in-plane flow from the channel towards the preform center was observed, initiating soon after the injection started and caused by the spatial connection between the injection gap and the race-tracking channel. To diminish edge race-tracking, a passive flow control strategy was implemented via a split design of the upper tool to spatially isolate the injection gap from the channel and to pre-compact the preform edge. A delayed and reduced lateral race-tracking flow was observed, showing that the passive flow control strategy increases the process robustness of FS-CRTM regarding edge race-tracking effects.

Vortex interaction on a LEX configuration

Flow visualization experiments were conducted in a water channel and a low-speed wind tunnel at Reynolds number of 1.54 times 104 to 1.2 times 105 for a leading-edge extension model, which is referred to as a NASA TP-1803 model in this study. In addition, particle image velocimetry velocity measurements were taken in the water channel to obtain the quantitative information about the three-dimensional velocity field over the strake and wing surfaces. The results obtained at low, medium and high angles of attacks represent three distinct cases of interaction between the strake and wing vortices. Namely, at α = 5o and 10° the strake and wing vortices were developed over the wing surface without significant interaction noticed; at α = 20°, the strake vortex strongly interacted with the wing vortex in an intertwining manner, which was sensitive to Reynolds number; at α = 30°, the breakdown of the strake vortex took place close to the junction of the strake and the wing; thus, the interaction of the strake and the wing vortices appeared to be less significant than the case of α = 20°.Graphic abstract

Experimental analysis of fluid-structure interaction in flexible wings at low Reynolds number flows

PurposeThe purpose of this exhaustive experimental study is to investigate the fluid-structure interaction in the flexible membrane wings over a range of angles of attack for various Reynolds numbers.Design/methodology/approachIn this paper, an experimental study on fluid-structure interaction of flexible membrane wings was presented at Reynolds numbers of 2.5 × 104, 5 × 104 and 7.5 × 104. In the experimental studies, flow visualization, velocity and deformation measurements for flexible membrane wings were performed by the smoke-wire technique, multichannel constant temperature anemometer and digital image correlation system, respectively. All experimental results were combined and fluid-structure interaction was discussed.FindingsIn the flexible wings with the higher aspect ratio, higher vibration modes were noticed because the leading-edge separation was dominant at lower angles of attack. As both Reynolds number and the aspect ratio increased, the maximum membrane deformations increased and the vibrations became visible, secondary vibration modes were observed with growing the leading-edge vortices at moderate angles of attack. Moreover, in the graphs of the spectral analysis of the membrane displacement and the velocity; the dominant frequencies coincided because of the interaction of the flow over the wings and the membrane deformations.Originality/valueUnlike available literature, obtained results were presented comparatively using the sketches of the smoke-wire photographs with deformation measurement or turbulence statistics from the velocity measurements. In this study, fluid-structure interaction and leading-edge vortices of membrane wings were investigated in detail with increasing both Reynolds number and the aspect ratio.

Experimental validation of a new type of direct-connect facility for scramjet studies

Combustion chamber design has been important to the development of scramjet engines for a long time. Conventional experimental facilities of scramjet could be divided into two types: free-jet and direct-connect facilities. Free-jet facility can provide real incoming conditions to the combustion chamber, especially the real shock wave train structures formed in inlet and isolator. Unfortunately, it is relatively expensive. While direct-connect facility has a distinct advantage in experimental cost and period, so it has been used widely in relevant researches. However, traditional direct-connect facility usually can only provide a uniform incoming flow, which is much different from the distorted flow field in real scramjet, and this is a non-negligible disadvantage to relevant design. An inverse design method based on method of characteristics (MOC) is presented to solve this problem. The facility designed by this new method can produce the pre-assigned distorted flow field without encountering unstart phenomenon. In this paper, experimental investigation of the new direct-connect facility, which is designed to reproduce a specific distorted flow field with Mach number more than 2, is further presented. Test data of particle image velocimetry (PIV), flow visualization experiments and corresponding numerical simulation results are used to verify the reliability and accuracy of this new facility. Qualitative flow visualization and quantitative measurement results further prove that this new type of direct-connect facility can generate the target distorted flow field as pre-defined.

A study of drag reduction with textile roughness on a cyclist model

Efforts were made to examine the effect of textile roughness for the reduction of aerodynamic drag experienced on a cyclist model. First of all, flow visualization experiments were conducted in a water channel with a 1/5 scale cyclist model to identify the flow separation lines on the contoured surface. Subsequently, based on the information learned, the idea of delaying flow separation on the model surface was implemented by means of sport suits. Seven sport suits featuring different textile materials and local roughness patterns were tested on a full-scale cyclist model in wind tunnel experiments, in addition to a reference case without a sport suit. From the CD data deduced, we identified a critical condition in most of the cases studied, featuring a least drag coefficient occurred at a Reynolds number within the flow speed range studied. For the best case found, the drag coefficient is amounted 7.5% less than that of the reference case at the same Reynolds number. Furthermore, the Cp and Cprms values reduced from the pressure measurements on the cyclist model with the sport suits unveil a trend that the reduction of drag actually infers the retreat of the local flow separation lines toward the rear side of the body accompanied with less severe the flow unsteadiness.

Study of flow and heat transfer characteristics around a flat plate located between two symmetrical cylinders

This study was conducted to investigate the flow and heat transfer characteristics of a flat plate located between two symmetrical cylinders. Flow visualization experiments were conducted based on the open-loop fluid test bench to verify the accuracy of numerical simulation. The effects of nondimensional vertical gap space (C/D) between the cylinder and the plate ranging from 0.1 to 0.8 were considered. Simulations were carried out at a fixed Reynolds number of 500. The numerical results show that the gap space is a key parameter of the flow and heat transfer characteristics. In cases of small gap space, a pair of large-scale vortices and different shapes of vortices are formed along with the plate. The heat transfer effect is improved in the upstream and downstream areas of the plate but deteriorated in the middle area. In cases of large gap space, large-scale vortices disappear and a single vortex is formed near the middle area of the plate, which improves the heat transfer effect there. For special cases of C/D = 0.3 and 0.6, the heat transfer enhancement caused by flow instability is discussed in detail. These results could provide theoretical guidance for the design of heat exchanger equipment.

Effect of attack angle on flow around the adjacent circular and rectangular prism

An experimental investigation has been carried out to clarify the flow structure around adjacent circular and rectangular prisms. The measurements of hot-wire for Re= 4.1 x 10³, 9.0 x 10³ and 1.5 x 10⁴, lift and drag force for Re = 1.0 x 10⁴, and in addition the flow visualization experiments for Re = 2.3 x 10³ have been performed in the range of 0° ≤ α ≤ 180°. The adjacent bodies consist of a circular cylinder having 9.5 mm diameter and a rectangular prism having 6 mm x 10 mm cross-section attached to the circular cylinder from its short side in all the cases. The hot wire measurement results showed that the Strouhal number has Reynolds number independence for Re = 4.1x10³, 9.0x10³, and 1.5x10⁴. Sudden peaks in St and lift coefficients are obtained for a similar reason that is reattachment of the separated shear layer in the vicinity of α = 60° and 174°. For the adjacent bodies compared with the bare circular cylinder, 40%, and 25% drag reduction is obtained at α = 0° and 174°, respectively. The obtained results show that the variations of lift coefficient, drag coefficient, and Strouhal number are strictly subjected to the attack angle.

How Fast It Can Stick: Visualizing Flow Delivery to Microtoroid Biosensors.

Sensitive and rapid biosensors are of critical importance for a variety of applications including infectious disease detection and monitoring as well as medical diagnostics and drug discovery. Whispering gallery mode microtoroid optical resonators are among the most sensitive biochemical sensors in existence. When combined with frequency-locking and data-processing techniques, these sensors have been shown to be capable of single-molecule detection in under 30 s. The sensitivity of these sensors is affected by how a concentration of analyte molecules is transported to the surface of the sensors and the average time it takes the molecules to bind at that concentration. Currently, one question in the field is that at these low concentrations, how these microsensors achieve such rapid response times. Here, we reconcile theory and experiment and demonstrate through flow visualization experiments and finite-element simulations that the total analyte arrival and binding time can be on the order of seconds. This fast response time provides an advantage over nanoscale sensors such as nanowires or nanorods. We anticipate that these results can help us to control, with confidence, when and how many molecules bind to these sensors, thus enabling the building of faster and more sensitive sensors.

Estimation of Vortex-Induced Vibration Based on Observed Wakes Using Computational Fluid Dynamics-Trained Deep Neural Network

Abstract The vibration of a circular cylinder due to fluid forces is of interest in various engineering fields. In this study, we investigate an approach to estimate the fluid forces acting on a circular cylinder in a flow field based on experimental flow visualizations using a deep neural network (DNN). Specifically, the wake patterns and fluid forces are correlated in a computational fluid dynamics (CFD) simulation, and the forces in the experiment are estimated by comparing experimental and computational wake patterns using a DNN. The approach is tested via dye-ink visualization around a circular cylinder at a Reynolds number of 560, referring to Seyed-Aghazadeh et al. (2015, “An Experimental Investigation of Vortex-Induced Vibration of a Rotating Circular Cylinder in the Crossflow Direction,” Phys. Fluids, 27(6), p. 067101). First, the CFD simulation of a circular cylinder with forced vibration in the crossflow direction is conducted with various vibration frequencies. Subsequently, the visualized wake images of the resulting flow fields and corresponding fluid forces are used as training data for the DNN. In the estimation, the images from the experiment are detected by the CFD-trained DNN. Thus, we can recall the correlated fluid forces using CFD simulation. The average drag coefficient and peak value of the lift coefficient estimated using streaming experimental images, have standard deviations of 2.1–13.7% and 6.6–18.6%, respectively, depending on the number of training images. The root-mean-square value of the lift coefficient obtained from the estimation is 0.82, which is comparable to the experimental value of 0.8, under the same flow and oscillation conditions.

Flow visualization experiment of jet mixing enhancement using pulsed jets

Flow visualization experiment of jet mixing enhancement using pulsed jets

Forced Convection Teat Transfer from a Cross Circular Yawed Tube.(Dept.M)

Complementary heat transfer and flow visualization experiments were performed lo investigate the effect of yaw on tube. Wind tunnel experiments encompassing seven angles of yaw between ᶱ= o( crossflow ) and 28° yielded Nusselt numbers extending over the range of free stream Reynolds numbers from 9000 to 25000. It was found that as the cylinder was yawed with respect to the crossflow orientation, the Nusselt number at first increased, attained a maximum at a yaw angle o 5o and then decreased to a broad minimum which occurred in the range of yaw angles between 20 and 25o and then start to somewhat increase at yaw angle equal 28o. As a further supplement to the heat transfer experiments, the pattern of fluid flow adjacent to the cylinder surface was visualized by means of the oil lampblack technique. The results were compared with the available experimental data. The values of the average Nusselt numbers were fitted in a correlation. This correlation indicates that the Nusselt number is a significant function of Reynolds number and angle of yaw.