Schlieren Optical System Research Articles

Experimental study on attenuation effect of liquid viscosity on shockwaves of cavitation bubbles collapse

How to precisely control and efficiently utilize the physical processes such as high temperature, high pressure, and shockwaves during the collapse of cavitation bubbles is a focal concern in the field of cavitation applications. The viscosity change of the liquid will affect the bubble dynamics in turn, and further affect the precise control of intensity of cavitation field. This study used high-speed photography technology and schlieren optical path system to observe the spatiotemporal evolution of shockwaves in liquid with different viscosities. It was found that as the viscosity of the liquid increased, the wave front of the collapse shockwave of the cavitation bubble gradually thickened. Furthermore, a high-frequency pressure testing system was used to quantitatively analyze the influence of viscosity on the intensity of the shockwave. It was found that the pressure peak of the shockwave in different viscous liquid was proportional to Lb (L represented the distance between the center of bubble and the sensor measuring point), and the larger the viscosity was, the smaller the value of b was. Through in-depth analysis, it was found that as the viscosity of the liquid increased, the proportion of the shockwave energy of first bubble collapse to the maximal mechanical energy of bubble gradually decreased. The proportion of the mechanical energy of rebounding bubble to the maximal mechanical energy of bubble gradually increased. These new findings have an important theoretical significance for the efficient utilization of ultrasonic cavitation.

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

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.

Schlieren imaging and video classification of alphabet pronunciations: exploiting phonetic flows for speech recognition and speech therapy

Speech is a highly coordinated process that requires precise control over vocal tract morphology/motion to produce intelligible sounds while simultaneously generating unique exhaled flow patterns. The schlieren imaging technique visualizes airflows with subtle density variations. It is hypothesized that speech flows captured by schlieren, when analyzed using a hybrid of convolutional neural network (CNN) and long short-term memory (LSTM) network, can recognize alphabet pronunciations, thus facilitating automatic speech recognition and speech disorder therapy. This study evaluates the feasibility of using a CNN-based video classification network to differentiate speech flows corresponding to the first four alphabets: /A/, /B/, /C/, and /D/. A schlieren optical system was developed, and the speech flows of alphabet pronunciations were recorded for two participants at an acquisition rate of 60 frames per second. A total of 640 video clips, each lasting 1 s, were utilized to train and test a hybrid CNN-LSTM network. Acoustic analyses of the recorded sounds were conducted to understand the phonetic differences among the four alphabets. The hybrid CNN-LSTM network was trained separately on four datasets of varying sizes (i.e., 20, 30, 40, 50 videos per alphabet), all achieving over 95% accuracy in classifying videos of the same participant. However, the network’s performance declined when tested on speech flows from a different participant, with accuracy dropping to around 44%, indicating significant inter-participant variability in alphabet pronunciation. Retraining the network with videos from both participants improved accuracy to 93% on the second participant. Analysis of misclassified videos indicated that factors such as low video quality and disproportional head size affected accuracy. These results highlight the potential of CNN-assisted speech recognition and speech therapy using articulation flows, although challenges remain in expanding the alphabet set and participant cohort.

Observation of gas flow around plants using Schlieren imaging system and high-refractive-index gas

The fruits of many plant are carried and flowers are also swayed by the wind. If the flow of air around plants can be visualized, the science behind it will be interestingly illustrated. In this study, the gas flow around cherry blossom fruits, clover flowers, maple seed propellers, and dandelion pappi as spherical and propeller-shaped samples is demonstrated using a Schlieren optical system and a high-refractive-index gas. The observed gas flow corresponding to the sample shape was well characterized by fluid dynamics features such as the Coandă effect. The results of experiments in which the flow of gas around plants is visualized are useful as a scientific education material for fluidics and optics.

Experimental investigation on the characteristics of the shock wave emitted by the cavitation bubble near the air bubble

This study explores the mitigation of cavitation damage in hydraulic engineering through air entrainment. The primary aim is to experimentally analyze the shock wave characteristics emitted by cavitation bubbles adjacent to air bubbles affixed to a tube nozzle. The schlieren optical system is utilized to visualize the shock wave, while a hydrophone measures its pressure. Experiments are conducted on cavitation bubbles induced by the spark-generated method in the vicinity of air bubbles, varying the dimensionless distances and sizes of the air bubbles. The results indicate that (1) The introduction of an air bubble noticeably changes the morphology, kinematic behavior, and shock wave features of the cavitation bubble. (2) Four distinct shock wave patterns are identified based on the quantity and shape of the shock wave, with variations in the cavitation bubble's collapsing behavior and shock wave characteristics across different patterns. (3) The dimensionless distance γ and size δ exert significant influence on the shock wave's quantity, pressure peak, shape, and energy. With γ decreases or δ increases, the shock wave quantity increases while the shock wave intensity decreases. This investigation of the interaction between cavitation bubbles and air bubbles is essential for elucidating the mechanism through which air entrainment mitigates cavitation damage.

Combustion performance and flame front morphology of producer gas from a biomass gasification-based cookstove

The present work investigates the last phase of the biomass gasification-based cookstove, which is the combustion process of biomass producer gas (BGP). A constant volume combustion bomb and kinetic simulation are used to characterize the behavior of the biomass producer gas and its pollutant emissions under a conventional premixed combustion process. The BGP combustion processes of two types of biomasses available in Colombia (Pinus Patula and Cordia Alliodora) are studied under certain conditions of pressure, temperature, and equivalence ratio. To characterize the combustible mixture of gases obtained after biomass gasification, a combustion chamber with cylindrical geometry equipped with a Schlieren optical diagnostic system to visualize the combustion process, and a piezoelectric pressure transducer to register the instantaneous pressure are used. By visualizing the flame front, the flame propagation speed at which the flame propagates through the combustion chamber and the morphology of the flame are studied. Instantaneous pressure is the input of a two-zone diagnostic model through which variables, such as burning velocity, temperature, mass burned fraction, etc., are obtained to characterize the combustion process of the mentioned gasification gases. Experimental results are compared and complemented with kinetic modeling results obtained with the Cantera package using the Gri-Mech 3.0 and Aramko 1.3 kinetic mechanisms, in terms of laminar burning velocity and NO, NO2, CO, and CO2 exhaust emissions. Results show that Cordia Alliodora presents higher burning velocities than Pinus Patula BP. However, lower CO2 emissions are obtained during the Cordia Alliodora combustion, and NOx emissions are not influenced by the type of biomass considered.

Management of abdominal gas leakage from surgical trocars in laparoscopic surgery: a preclinical study

Introduction: There is an ongoing concern about the potential infectious risk due to pneumoperitoneal gas leakage from surgical trocars in laparoscopic surgery. We aimed to visually confirm the presence of leakage from trocars and investigate the changes in the leakage scale according to intra-abdominal pressures and trocar types. Material and methods: We established a porcine pneumoperitoneum model and performed experimental forceps manipulation using 5-mm grasping forceps with 12-mm trocars. The gas leakage, if any, was imaged using a Schlieren optical system, which can visualize minute gas flow invisible to the naked eye. For measuring the scale, we calculated the gas leakage velocity and area using image analysis software. Four types of unused and exhausted disposable trocars were compared. Results: Gas leakage was observed from trocars during forceps insertion and removal. Both the gas leakage velocity and area increased as the intra-abdominal pressure increased. Every type of trocar we handled was associated with gas leakage, and exhausted disposable trocars had the largest scale gas leakage. Conclusions: We confirmed gas leakage from trocars during device traffic. The scale of leakage increased with high intra-abdominal pressure and with the use of exhausted trocars. Current protection against gas leakage may not be sufficient and new surgical safety measures and device development may be needed in the future.

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < do < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < Tc < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.

Gas leakages from gastrointestinal endoscopy system-its visualization and semi-quantification utilizing schlieren optical system in the swine models.

Background and aimsWith the global epidemic of SARS-CoV-2, there has been a growing concern regarding the risk of aerosol exposure to healthcare workers and patients during medical/surgical interventions. The Schlieren device is capable of visualizing fine gas-flows by using refractive index differences in the medium. We aimed to reveal the existence of gas leakage from gastro-intestinal endoscopy system by utilizing Schlieren device and to clarify the factors which relates to the amount of gas leakage.MethodsThe experiments were performed on the excised swine stomach while maintaining a constant pressure environment in the stomach. The System Schlieren (SS100,KatoKoken) was used to visualize possible gas leakages from forceps plugs of endoscopy. We attempted to semi-quantify the leakage by referring to the image of the gas from the forceps plug and by measuring the initial velocity and diffusion area of the leakage.ResultsRegardless of the type of forceps plugs, a certain amount of leakage was detected during both insertion and removal of forceps. The initial velocity and the diffusion area of the leakage increased with the increase in intragastric pressure. Semi-quantitative comparison showed that there was a difference in the amount of gas leakage among various forceps plugs. Furthermore, the amount of gas leakage was significantly greater in the forceps plugs that were used repeatedly.ConclusionIt was possible to visualize gas leakages from the gastrointestinal endoscope system using the Schlieren optical device. Avoiding too high intragastric pressure and not using deteriorated plugs may reduce the risk of aerosol exposure.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00464-022-09668-y.

Effect of inhomogeneous methane-air mixtures on combustion characteristics in a constant volume combustion chamber

The inhomogeneous mixture can improve the combustion performance under certain conditions. In this study, an experimental study of turbulent stratified combustion under different background equivalent ratio conditions (0.6, 0.8, 1.0) was carried out based on a constant volume combustion chamber. Injection of pure methane to achieve stratification of gas mixture concentration, and the ignition timing was adjusted to control the mixture stratification degree. Optical diagnosis of the flame using a high-speed camera and the optical schlieren system to understand the morphological changes in flame development. Based on the combustion pressure and ion current signals to explore the effect law of stratification degree on the turbulent stratified combustion characteristics. The results of the study show that the concentration stratification of the mixture can accelerate the flame propagation speed. The leaner the background mixture concentration, the stronger the promotion effect of the mixture concentration stratification on the propagation speed. The appropriate degree of stratification is conducive to improving the stability of the flame. Turbulent stratified combustion can increase the maximum combustion pressure, shorten the combustion duration. The degree of stratification has a large effect on the ion current signal curve during the background equivalent ratio of 0.6 mixture combustion, and almost no effect when the background equivalent ratio is 1.0. The peak moment of the combustion pressure rises rate and the peak moment of the ion current signal occur almost simultaneously under different combustion conditions. The correlation between the maximum of the combustion pressure and the maximum of the ion current is approximately linear and the correlation is greater than 95%.

Dynamics of liquid film rupture under local heating

Rupture dynamics of a horizontal liquid film non-uniformly heated from below was experimentally studied in a wide range of liquid viscosity. To visualize the liquid surface deformations and disruption of the film, an optical schlieren system was used. The instantaneous local film thickness was measured using a chromatic confocal sensor. In order to measure the thickness of thin liquid films, the substrate was covered with a black coating eliminating reflection of the incident light. Angular and frequency characteristics of the confocal sensor were studied in detail. The process of film rupture can be divided into three stages: 1) film thinning down to a residual film on the heater; 2) existence of a stable residual film for some time; 3) rupture and dry out of the residual film. The thickness of the residual film was found to strongly depend on the liquid viscosity: for water it is about 10 µm, whereas for silicone oil PMS-200 it is about 275 µm. The data obtained for various working liquids were generalized by a dimensionless correlation.

Visualization of ultrasonic wave fields and demonstration of sound wave property in undergraduate physics experiment

By building a schlieren optical system, the visualization of a 40 KHz sound wave is realized. By using the schlieren system, the demonstration of acoustic reflection, interference and diffraction can be easily realized. At the same time, the visual acoustic field provides another simple way for the measurement of sound velocity. In this paper, the experimental device is simple in structure and low in cost. The experimental content combines the optical and acoustic content, which can be used as the acoustic experimental content for undergraduate students.

Experimental Investigation of Lean Methane-Air Laminar Premixed Flames at Engine-Relevant Temperatures.

Lean premixed combustion is one of the most effective methods to constrain pollutant emissions for modern industrial gas turbines. An experimental study was performed on its propagation speed and internal structure at engine-relevant temperatures. A Bunsen burner was employed for the measurement with an optical schlieren system. The results show that the increase of preheating temperature dramatically accelerates the propagation of methane flames. The numerical results predicted by GRI-Mech 3.0, FFCM-1, and USC Mech II were also compared. The GRI-Mech 3.0 seems to overestimate the laminar flame speed at high operating conditions, while FFCM-1 underestimates the laminar flame speed compared to the present experimental data. The prediction by FFCM-1 shows good agreement with the overall existing data. The USC Mech II seems to overestimate the laminar flame speed at fuel-lean conditions while shows good agreement with present experimental measurements at stoichiometric conditions when the inlet temperature increases. It is also indicated that the flame is thinned at high-temperature conditions and the importance of CO production to the propagation speed increases. Finally, based on the experimental data, an empirical correlation of the laminar flame speed was developed in the range of Tu = 300–800 K and ϕ = 0.7–1.0, the maximum deviation of which was less than 8%. The results of this study may contribute to the optimization of advanced gas turbine combustors.

Exhalation Spreading During Nasal High-Flow Therapy at Different Flow Rates.

Severe acute respiratory syndrome coronavirus 2 is transmitted through aerosols and droplets. Nasal high-flow therapy could possibly increase the spreading of exhalates from patients. The aim of this study is to investigate whether nasal high-flow therapy affects the range of the expiratory plume compared with spontaneous breathing. Interventional experiment on single breaths of a healthy volunteer. Research laboratory at the Bauhaus-University Weimar. A male subject. Videos and images from a schlieren optical system were analyzed during spontaneous breathing and different nasal high-flow rates. The maximal exhalation spread was 0.99, 2.18, 2.92, and 4.1 m during spontaneous breathing, nasal high-flow of 20 L/min, nasal high-flow of 40 L/min, and nasal high-flow of 60 L/min, respectively. Spreading of the expiratory plume in the sagittal plane can completely be blocked with a surgical mask. Nasal high-flow therapy increases the range of the expiratory air up to more than 4 meters. The risk to pick up infectious particles could be increased within this range. Attachment of a surgical mask over the nasal high-flow cannula blocks the expiratory airstream.

The effect of modulated electric field on characteristic of SDBD‐like plasma jet for surface modification

AbstractIn this paper, the dynamics and flow behaviour of an atmospheric argon plasma jet was studied in the new nozzle structure similar to the surface dielectric barrier discharge (SDBD) using the Schlieren imaging method. The effect of plasma jet driven by repetitive high‐voltage microsecond pulses with low‐frequency sinusoidal bias was measured qualitatively in a single mirror Schlieren optical system. The enhancement of plasma jet length and cross‐section of plasma jet with surface in this condition is due to highly turbulent flow of argon plasma jet in this structure. This study revealed the important role of SDBD structure and modulated electric field on the behaviour of plasma jet in a high diameter nozzle. In practice, this technique allows us to increase the jet length of the nozzle output to 5 cm and under these conditions, the diameter of the plasma jet cross‐section is increased to 8 mm, without increasing the electrical power consumption. Eventually, the hydrophilicity of the surface is also measured by the contact angle of a water droplet that decreases from 78° to 8° after surface treatment, implying we were able to reach a super‐hydrophilic surface with this plasma jet structure.

An ultraviolet dyegraph for measuring the chemical disturbances of sinking particles and swimming plankton

AbstractSinking particles and swimming plankton may deform ambient chemical gradients, leaving long trails of chemical disturbance that dissipate on a diffusive timescale. Imaging this phenomenon in a laboratory setting requires a large field of view, a broad depth of field, and a passive chemical substance that does not influence the hydrodynamics. These requirements obviate the use of traditional schlieren optical systems, which rely on density stratification and their consequent buoyancy forces, and modern planar laser‐induced fluorescence, which has a narrow depth of field. Further, the light used to induce plankton movement must be kept separate from the recorded signal. Here, we describe a new ultraviolet “dyegraph” technique that quantifies dye disturbance caused by sinking spheres and swimming zooplankton. An experimental tank is filled with seawater and a gradient of an ultraviolet‐absorbing pigment. Using modified shadowgraph optics, ultraviolet light spreads from a pinhole to a collimating lens, passes through the tank to a telecentric lens, and is recorded by a camera fitted with an ultraviolet bandpass filter. The dye is transparent in visible light but appears dark in the imaging system. The 3D chemical disturbance is reconstructed from images using tomographic inversion. After outlining the method, we present examples of chemical deformation trails generated by sinking spheres and live, swimming copepods.

Quantitative Schlieren Image-Noise Reduction Using Inverse Process and Multi-Path Integration

This report deals with introducing two new techniques based on a novel concept of complex brightness gradient in quantitative schlieren images, “inverse process” and “multi-path integration” for image-noise reduction. Noise in schlieren images affects the projections (density thickness) images of computerized tomography (CT). One spot noise in the schlieren image appears in a line shape in the density thickness image. Noise effect like an infectious disease spreads from a noisy pixel to the next pixel in the direction of single-path integration. On the one hand, the noise in the schlieren image reduces the quality of the image and quantitative analysis and is undesirable; on the other it is unavoidable. Therefore, the importance of proper noise reduction techniques seems essential and tangible. In the present report, a novel technique “multi-path integration” is proposed for noise reduction in projections images of CT. Multi-path integration is required the schlieren brightness gradient in two orthogonal directions. The 20-directional quantitative schlieren optical system presents only images of schlieren brightness in the horizontal gradient and another 20-directional optical system seems necessary to obtain vertical schlieren brightness gradient, simultaneously. Using the “inverse process”, a new technique enables us to obtain vertical schlieren brightness gradient from horizontal experimental data without the necessity of a new optical system and can be used for obtaining any optional directions of schlieren brightness gradient.

Multi-Schlieren CT Measurements of Supersonic Microjets from Circular and Square Micro Nozzles

Instantaneous three-dimensional (3D) density distributions of a shock-cell structure of perfectly and imperfectly expanded supersonic microjets escaping into an ambient space are measured. For the 3D observation of supersonic microjets, non-scanning 3D computerized tomography (CT) technique using a 20-directional quantitative schlieren optical system with flashlight source is employed for simultaneous schlieren photography. The 3D density distributions data of the microjets are obtained by 3D-CT reconstruction of the projection’s images using maximum likelihood-expectation maximization. Axisymmetric convergent-divergent (Laval) circular and square micro nozzles with operating nozzle pressure ratio 5.0, 4.5, 4.0, 3.67, and 3.5 have been studied. This study examines perfectly expanded, overexpanded, and underexpanded supersonic microjets issued from micro nozzles with fully expanded jet Mach numbers Mj ranging from 1.47 - 1.71, where the design Mach number is Md = 1.5. A complex phenomenon for free square microjets called axis switching is clearly observed with two types “upright” and “diagonal” of “cross-shaped”. The initial axis-switching is 45° within the first shock-cell range. In addition, from the symmetry and diagonal views of square microjets for the first shock-cells, two different patterns of shock waves are viewed. The shock-cell spacing and supersonic core length for all nozzle pressure ratios are investigated and reported.

THERMOCAPILLARY RUPTURE AND CONTACT LINE DYNAMICS IN THE HEATED LIQUID LAYERS

The experimental study of the rupture of a horizontal liquid layer placed on a stainless-steel substrate non-uniformly heated from below was conducted. Deformation profiles in the liquid film were measured using the confocal Micro-Epsilon sensor. Propagation of a dry spot over the substrate surface was studied using an optical schlieren system coupled with a high-speed camera. The initial thickness of the liquid film varied from 300 to 1100 μm. It was found that with an increase in the liquid layer thickness, the threshold substrate temperature for rupture increases. At the initial stage of film rupture, the contact line velocity has a maximum increasing with the initial film thickness.

The effect of viscosity on rupture dynamics in the non-uniformly heated horizontal liquid film

In the present work, we study the rupture dynamics of a horizontal liquid film non-uniformly heated from below in a wide range of liquid viscosity. To visualize the liquid surface deformations and disruption of the film, we use a Nikon digital camera (shot at 60 fps) coupled with an optical schlieren system. In order to measure the instantaneous film thickness, we use the confocal Micro-Epsilon chromatic sensor. It was found that the process of film rupture can be divided into three stages: 1) film thinning down to a residual film on the heater; 2) existence of a stable residual film for some time; 3) rupture and dryout of the residual film. The thickness of the residual film was found to strongly depend on the liquid viscosity: for water, it is about 10 µm, whereas for PMS-200 it is about 275 µm.