Using Schlieren Imaging and a Radar Acoustic Sounding System for the Detection of Close-in Air Turbulence.

This paper presents a novel sensor for the detection and characterization of regions of air turbulence. As part of the ground truth process, it consists of a combined Schlieren imager and a Radar Acoustic Sounding System (RASS) to produce dual-modality "images" of air movement within the measurement volume. The ultrasound-modulated Schlieren imager consists of a strobed point light source, parabolic mirror, light block, and camera, which are controlled by two laptops. It provides a fine-scale projection of the acoustic pulse-modulated air turbulence through the measurement volume. The narrow beam 40 kHz/17 GHz RASS produces spectra based on Bragg-enhanced Doppler radar reflections from the acoustic pulse as it travels. Tests using artificially generated air vortices showed some disruption of the Schlieren image and of the RASS spectrogram. This should allow the higher-resolution Schlieren images to identify the turbulence mechanisms that are disrupting the RASS spectra. The objective of this combined sensor is to have the Schlieren component inform the interpretation of RASS spectra to allow the latter to be used as a stand-alone sensor on a UAV.

Shedding Light on Gas-Dynamic Effects in Laser Beam Fusion Cutting: The Potential of Background-Oriented Schlieren Imaging (BOS).

In laser beam fusion cutting of metals, the interaction of the gas jet with the melt determines the dynamics of the melt extrusion and the quality of the resulting cutting kerf. The gas-dynamic phenomena occurring during laser beam cutting are not fully known, especially regarding temporal fluctuations in the gas jet. The observation of gas and melt dynamics is difficult because the gas flow is not directly visible in video recordings and access to the process zone for observation is limited. In this study, the problem of imaging the gas jet from the cutting nozzle is addressed in a novel way by utilizing the striation pattern formed at the cutting kerf as a background pattern for background-oriented Schlieren imaging (BOS). In this first feasibility study, jets of different gas nozzles were observed in front of a solidified cutting kerf, which served as a background pattern for imaging. The results show that imaging of the characteristic shock diamonds of cutting nozzles is possible. Furthermore, the resulting shock fronts from an interaction of the gas jet with a model of a cutting front can be observed. The possibility of high-speed BOS with the proposed method is shown, which could be suitable to extend the knowledge of gas-dynamic phenomena in laser beam fusion cutting.

The spread of breathing air from wind instruments and singers using schlieren techniques.

The spread of breathing air when playing wind instruments and singing was investigated and visualized using two methods: (1) schlieren imaging with a schlieren mirror and (2) background-oriented schlieren (BOS). These methods visualize airflow by visualizing density gradients in transparent media. The playing of professional woodwind and brass instrument players, as well as professional classical trained singers were investigated to estimate the spread distances of the breathing air. For a better comparison and consistent measurement series, a single high note, a single low note, and an extract of a musical piece were investigated. Additionally, anemometry was used to determine the velocity of the spreading breathing air and the extent to which it was quantifiable. The results showed that the ejected airflow from the examined instruments and singers did not exceed a spreading range of 1.2 m into the room. However, differences in the various instruments have to be considered to assess properly the spread of the breathing air. The findings discussed below help to estimate the risk of cross-infection for wind instrument players and singers and to develop efficacious safety precautions, which is essential during critical health periods such as the current COVID-19 pandemic.

Qualitative and quantitative schlieren optical measurement of the human thermal plume.

A new large-field, high-sensitivity, single-mirror coincident schlieren optical instrument has been installed at the Bauhaus-Universität Weimar for the purpose of indoor air research. Its performance is assessed by the non-intrusive measurement of the thermal plume of a heated manikin. The schlieren system produces excellent qualitative images of the manikin's thermal plume and also quantitative data, especially schlieren velocimetry of the plume's velocity field that is derived from the digital cross-correlation analysis of a large time sequence of schlieren images. The quantitative results are compared with thermistor and hot-wire anemometer data obtained at discrete points in the plume. Good agreement is obtained, once the differences between path-averaged schlieren data and planar anemometry data are reconciled.

Optimized Schlieren imaging for real-time visualization of high-intensity focused ultrasound waves.

OBJECTIVES: This article presents an low-cost experimental setup for visualizing refraction anomalies caused by high-intensity focused ultrasound (HIFU). The technique is based on Schlieren imaging, commonly used to visualize temperature and pressure differences in a medium. With this setup, double images of the Schlieren or their shadows to be investigated occur, so that the experimental setup is modified to avoid these double image artifacts. METHODS: The optical setup mainly consists of a point light source, a parabolic mirror, and a camera. Birefringence artifacts are avoided by placing the point light source at a certain vertical distance to the camera, so that the light beam passes through the medium only once. The soundfield is generated by a HIFU transducer in a water tank placed in the beam path of the optical setup. RESULTS: The experimental setup is capable of capturing Schlieren or shadow images. These images show the soundfield without disturbing double images and enable further analysis and qualitative assessment of the soundfield. CONCLUSIONS: The presented setup provides a reliable and efficient method for visualizing refraction anomalies caused by the sonic field of a HIFU transducer and allows for accurate depiction of the refraction anomalies. The double images that usually occur are avoided.

The Spread of Exhaled Air and Aerosols during Physical Exercise.

Physical exercise demonstrates a special case of aerosol emission due to its associated elevated breathing rate. This can lead to a faster spread of airborne viruses and respiratory diseases. Therefore, this study investigates cross-infection risk during training. Twelve human subjects exercised on a cycle ergometer under three mask scenarios: no mask, surgical mask, and FFP2 mask. The emitted aerosols were measured in a grey room with a measurement setup equipped with an optical particle sensor. The spread of expired air was qualitatively and quantitatively assessed using schlieren imaging. Moreover, user satisfaction surveys were conducted to evaluate the comfort of wearing face masks during training. The results indicated that both surgical and FFP2 masks significantly reduced particles emission with a reduction efficiency of 87.1% and 91.3% of all particle sizes, respectively. However, compared to surgical masks, FFP2 masks provided a nearly tenfold greater reduction of the particle size range with long residence time in the air (0.3-0.5 µm). Furthermore, the investigated masks reduced exhalation spreading distances to less than 0.15 m and 0.1 m in the case of the surgical mask and FFP2 mask, respectively. User satisfaction solely differed with respect to perceived dyspnea between no mask and FFP2 mask conditions.

Computational design and experimental analysis of a novel visor for COVID-19 patients receiving high-flow nasal oxygen therapy.

The Covid-19 global pandemic has reshaped the requirements of healthcare sectors worldwide. Following the exposure risks associated with Covid-19, this paper aims to design, optimise, and validate a wearable medical device that reduces the risk of transmission of contagious droplets from infected patients in a hospital setting. This study specifically focuses on those receiving high-flow nasal oxygen therapy. The design process consisted of optimising the geometry of the visor to ensure that the maximum possible percentage of harmful droplets exhaled by the patient can be successfully captured by a vacuum tube attached to the visor. This has been completed by deriving a number of concept designs and assessing their effectiveness, based on numerical analysis, computational fluid dynamics (CFD) simulations and experimental testing. The CFD results are validated using various experimental methods such as Schlieren imaging, particle measurement testing and laser sheet visualisation. Droplet capturing efficiency of the visor was measured through CFD and validated through experimental particle measurement testing. The results presented a 5% deviation between CFD and experimental results. Also, the modifications based on the validated CFD results improved the visor effectiveness by 47% and 38% for breathing and coughing events, respectively.

Aerosol release, distribution, and prevention during aerosol therapy: a simulated model for infection control.

Aerosol therapy is used to deliver medical therapeutics directly to the airways to treat respiratory conditions. A potential consequence of this form of treatment is the release of fugitive aerosols, both patient derived and medical, into the environment and the subsequent exposure of caregivers and bystanders to potential viral infections. This study examined the release of these fugitive aerosols during a standard aerosol therapy to a simulated adult patient. An aerosol holding chamber and mouthpiece were connected to a representative head model and breathing simulator. A combination of laser and Schlieren imaging was used to non-invasively visualize the release and dispersion of fugitive aerosol particles. Time-varying aerosol particle number concentrations and size distributions were measured with optical particle sizers at clinically relevant positions to the simulated patient. The influence of breathing pattern, normal and distressed, supplemental air flow, at 0.2 and 6 LPM, and the addition of a bacterial filter to the exhalation port of the mouthpiece were assessed. Images showed large quantities of fugitive aerosols emitted from the unfiltered mouthpiece. The images and particle counter data show that the addition of a bacterial filter limited the release of these fugitive aerosols, with the peak fugitive aerosol concentrations decreasing by 47.3-83.3%, depending on distance from the simulated patient. The addition of a bacterial filter to the mouthpiece significantly reduces the levels of fugitive aerosols emitted during a simulated aerosol therapy, p≤ .05, and would greatly aid in reducing healthcare worker and bystander exposure to potentially harmful fugitive aerosols.

Effectiveness of different types of mask in aerosol dispersion in SARS-CoV-2 infection.

OBJECTIVE: To compare the effectiveness of different mask types in limiting the dispersal of coughed air. METHOD: The Schlieren method with a single curved mirror was used in this study. Coughed air has a slightly higher temperature than ambient air, which generates a refractive index gradient. A curved mirror with a radius of curvature of 10 m and a diameter of 60 cm was used. The spread of the cough wavefront was investigated among five subjects wearing: (1) no mask; (2) a single surgical mask; (3) a double surgical mask; (4) a cloth mask; (5) a valveless N95 mask; and (6) a valved N95 mask. RESULTS: All mask types reduced the size of the contaminated region significantly. The percentage reduction in the cross-sectional area of the contaminated region for the same mask types on different subjects revealed by normalized data suggests that the fit of a mask plays an important role. CONCLUSIONS: No significant difference in the spread of coughed air was found between the use of a single surgical mask or a double surgical mask. Cloth masks may be effective, depending on the quality of the cloth. Valved N95 masks exclusively protect the user. The fit of a mask is an important factor to minimize the contaminated region.

Investigation of the Powder Aerosol Deposition Method Using Shadowgraph Imaging.

The powder aerosol deposition method (PAD) is a vacuum-based spray coating technology. It allows for production of highly dense coatings at room temperature, especially of brittle-breaking materials. This yields new options for coating substrate materials that even melt at low temperatures. The film formation mechanism is called room temperature impact consolidation (RTIC). The occurrence of this mechanism is strongly linked to the gas jet used in the process. The velocity and direction of the particles in the gas jet forming between the nozzle orifice and the substrate are the main factors influencing the quality of the coating. This dependency aimed to be elaborated with a measurement setup and coating experiments and is shown in this work. We investigated the gas jet formation using a shadow optical imaging system. Regions of different gas density are visualized by this technique. Several parameter sets, in particular gas flow rates and chamber pressures, were investigated. In addition, coatings were produced on glass substrates with the same parameters. As a coating material, the superconducting ceramic-like magnesium diboride (MgB2) was chosen. A correlation between shadow images and thickness profiles of the coatings shows how the gas jet formation affects the uniformity of thickness. Shadow optical images provide valuable information on the flight direction of the particles and allow validation of simulation results.

An in vitro visual study of fugitive aerosols released during aerosol therapy to an invasively ventilated simulated patient.

COVID-19 can cause serious respiratory complications resulting in the need for invasive ventilatory support and concurrent aerosol therapy. Aerosol therapy is considered a high risk procedure for the transmission of patient derived infectious aerosol droplets. Critical-care workers are considered to be at a high risk of inhaling such infectious droplets. The objective of this work was to use noninvasive optical methods to visualize the potential release of aerosol droplets during aerosol therapy in a model of an invasively ventilated adult patient. The noninvasive Schlieren imaging technique was used to visualize the movement of air and aerosol. Three different aerosol delivery devices: (i) a pressurized metered dose inhaler (pMDI), (ii) a compressed air driven jet nebulizer (JN), and (iii) a vibrating mesh nebulizer (VMN), were used to deliver an aerosolized therapeutic at two different positions: (i) on the inspiratory limb at the wye and (ii) on the patient side of the wye, between the wye and endotracheal tube, to a simulated intubated adult patient. Irrespective of position, there was a significant release of air and aerosol from the ventilator circuit during aerosol delivery with the pMDI and the compressed air driven JN. There was no such release when aerosol therapy was delivered with a closed-circuit VMN. Selection of aerosol delivery device is a major determining factor in the release of infectious patient derived bioaerosol from an invasively mechanically ventilated patient receiving aerosol therapy.

Schlieren imaging for the visualization of particles entrapped in bubble films.

Bubbles are one of the primary sources of transferring substances from water to air. When bubbles burst, small droplets containing microparticles and microorganisms previously suspended in water disperse. Hence, visualizing small objects in bubble films can provide a new methodology for investigating the material transfer from water to the environment and the dynamic behavior of objects in the films. We used Schlieren imaging of bubbles to visualize small objects such as bacteria and microplastics. Remarkably, black spots (Schlieren spots) appeared when light rays passed parallel to bubbles formed on the water containing microparticles and bacteria. The simulation method of Schlieren imaging of bubbles was developed to clarify the underlying mechanism and experimentally validated with different sizes and concentrations of microparticles. We found that a specific water meniscus is formed around a particle when the bubble film thickness is smaller than the particle diameter, and the meniscus plays an important role in enlarging the Schlieren spots. The Schlieren spots are forty times larger than the bubble film thickness in this work. To understand the magnification rule, we illuminated the correlation between bubble film thickness, particle diameter, and Schlieren spot diameter.

Long-distance shift of ultrasonic beam using a thin plate with periodic gratings.

We achieved the shift of ultrasonic beam over a long distance by guided waves through a brass plate with two groups of periodical gratings on the surface. Using Schlieren imaging, we experimentally observed the propagation of ultrasonic waves through this structure. In addition, simulations were performed using the finite-element method. Both the experimental and simulation results revealed that the shift of ultrasonic beam can be realized in this structure. We further investigated the effect of the shift distance and the size of the gratings on the shift efficiency, and discussed the mechanism. The proposed structure has potential applications in non-destructive evaluation.

Broadband ultrasound-trapping barrier based on hollow cylinder with a periodic grating.

It is important to modulate ultrasonic waves in some ultrasonic application fields. We formed an ultrasound-trapping barrier through a hollow cylinder with a periodic grating on the surface and simulated the barrier using the finite element method. Using the Schlieren imaging technique, we visualized the ultrasound field distribution in the structure. Both the experimental and simulation results revealed that the structure can work in a broadband frequency range, demonstrating the effectiveness and practicability of the structure as a virtual ultrasound barrier. We also describe how the silent region and the ultrasound intensity can be adjusted in the hollow cylinder.

Impact of Drug Formulation Variables on Silicone Oil Structure and Functionality of Prefilled Syringe System.

Use of prefilled syringes to self-administer biologics via subcutaneous administration provides convenience to patients. The barrel interior of prefilled syringes is typically coated with silicone oil for lubrication to aid plunger movement at the time of administration. This study intended to evaluate the impact of formulation variables on the silicone oil on the barrel interior surface. Characterization techniques including syringe glide force, break loose force, Schlieren imaging, contact angle, inductively coupled plasma spectrometry, and thin film interference reflectometry were used in assessing the interactions. Data indicated that formulation variables such as pH, buffer/tonicity agent type and concentration, and surfactant present in the formulation can effect silicone oil lubrication of prefilled syringes, leading to changes in functional properties of the syringe over time. Syringe samples containing acetate and histidine buffers showed an increase in glide force at accelerated storage temperature conditions, but the change was minimal at 5 °C. The samples with the highest glide force correlated with the presence of mannitol in combination with sodium acetate buffer. Sodium chloride had lesser impact on glide force than mannitol. Samples with higher glide force exhibited a substantial change in the silicone oil layer of the syringe, as observed with Schlieren imaging, as well as a significant reduction in surface hydrophobicity, as demonstrated through contact angle measurement. These data indicated that the structure of the siliconized surface can change over time in contact with different formulations. During formulation development of drug products in prefilled syringes, in addition to potential impact on molecule stability, the selection of formulation variables should also be guided by assessing the impact to syringe functionality with the glide force as one of the key parameters.LAY ABSTRACT: Self-administering drug products packaged in prefilled syringes provides convenience to patients. The interior of a prefilled glass syringe is typically lubricated with silicone oil for easy plunger movement during injection. This article discusses the impact of formulation excipients on silicone oil coating inside the syringe. Characterization techniques were used to assess the ease of plunger movement and structure of the silicone coating. Data indicate formulation excipients can affect silicone oil distribution of prefilled syringes, leading to an increase in plunger glide force at accelerated storage temperature conditions. The increase in glide force within a prefilled syringe with or without an auto-injector can have an impact on dose accuracy and user experience. Syringes with a higher plunger glide force appeared to exhibit a change over time in surface energy and structure of the silicone oil layer in contact with particular formulations.

A flat acoustic lens to generate a Bessel-like beam.

We report a flat acoustic lens with a periodical structure to transform a divergent beam into a Bessel-like beam. Using the Schlieren imaging technique, the propagation process of acoustic wave in the periodical structure was experimentally observed. The pressure distribution in the main lobe is in good agreement with the Bessel function and the positions of the side lobes are close to the peak of the Bessel function. To observe the directivity of the beam, simulations were performed using the finite-element method. The simulation results indicated that the transmitted acoustic intensity at the central axis was several times greater with the lens than without it. The applicability of the lens for detecting the location of an acoustic source was also investigated.

Development of a multichannel microfluidic system with Schlieren imaging microscopy for online chip-based moving boundary electrophoresis.

The concentration gradient detection method based on the Schlieren optics employed for electrophoresis analyses by extending the technology to a multi-channel system using a prototyped microfluidic chip (thinXXS Micro-technology, Germany). The results prove that coupling a chip-based microfluidic device with Schlieren detection is an appropriate approach to improve the electrophoretic separations. The effects of channel's geometry and dimension were investigated by conducting the experiments in channels with different cross sectional areas. Fast kinetic data acquisition of the charge-coupled device (CCD) camera facilitated recording of a time sequence of optical images, demonstrating the potential of the CCD camera as a powerful tool for studying dynamic processes such as diffusion. Diffusion coefficients of sample proteins were measured under static and dynamic conditions, where the static mode demonstrated more accurate results. Furthermore, the Fourier transformation was employed to improve the Schlieren images for quantitative analysis of the diffusion coefficient measurement.