Quantitative Shadowgraphy Research Articles

Modeling and correction of image drift in dynamic shadowgraphy experiments

The study of phoretic transport phenomena under non-stationary conditions presents several challenges, mostly related to the stability of the experimental apparatus. This is particularly true when investigating with optical means the subtle temperature and concentration fluctuations that arise during diffusion processes, superimposed to the macroscopic state of the system. Under these conditions, the tenuous signal from fluctuations is easily altered by the presence of artifacts. Here, we address an experimental issue frequently reported in the investigation by means of dynamic shadowgraphy of the non-equilibrium fluctuations arising in liquid mixtures under non-stationary conditions, such as those arising after the imposition or removal of a thermal stress, where experiments show systematically the presence of a spurious contribution in the reconstructed structure function of the fluctuations, which depends quadratically from the time delay. We clarify the mechanisms responsible for this artifact, showing that it is caused by the imperfect alignment of the sample cell with respect to gravity, which couples the temporal evolution of the concentration profile within the sample with the optical signal collected by the shadowgraph diagnostics. We propose a data analysis protocol that enables disentangling the spurious contributions and the genuine dynamics of the fluctuations, which can be thus reliably reconstructed.Graphic The imposition of a thermal gradient across a liquid mixture results in a time-dependent refractive index distribution. In the presence of a misalignment of the confining cell with respect to gravity, this leads to a deflection of the optical probe beam used to monitor concentration fluctuations within the sample in quantitative shadowgraphy experiments. If not properly accounted for, this effect can introduce a significant bias in the optical signal.

Quantitative shadowgraphy of aerosol and droplet creation during oscillatory motion of the microkeratome amid COVID-19 and other infectious diseases.

To quantify the atomization of liquid over the cornea during flap creation using microkeratome using high-speed shadowgraphy. Laboratory study. Laboratory investigational study. In an experimental setup, flap creation was performed on enucleated goat's eyes (n = 8) mounted on a stand using One Use-Plus SBK Moria microkeratome (Moria SA) to assess the spread of aerosols and droplets using high-speed shadowgraphy. Two conditions were computed. A constant airflow assumed uniform air velocity throughout the room. A decaying jet assumed that local air velocity at the site of measurements was smaller than the exit velocity from the air duct. With the advancement of the microkeratome across the wet corneal surface, the atomization of a balanced salt solution was recorded on shadowgraphy. The minimum droplet size was ∼90 μm. The maximum distance traversed was ∼1.8 m and ∼1.3 m assuming a constant airflow (setting of refractive surgery theater) and decaying jet condition (setting of an operating theater with air-handling unit), respectively. The microkeratome-assisted LASIK flap creation seemed to cause spread of droplets. The droplet diameters and velocities did not permit the formation of aerosols. Therefore, the risk of transmission of the virus to the surgeon and surgical personnel due to the microkeratome procedure seemed to be low.

Combustion behavior of pulverized sponge iron as a recyclable electrofuel

In this work, the combustion behavior of pulverized sponge iron (PSI), a practical-grade iron product that was proposed as a potential candidate in the metal fuel cycle, was observed directly using high-magnification shadowgraphy and other optical diagnostics techniques. The PSI was combusted in a laboratory-scale, McKenna flat-flame burner. Results suggest that, in agreement with theoretical models, PSI combusted heterogeneously, with most of the particle mass converting to an intact, solid oxide. However, in contrast with previous hypotheses, the formation of a microflame of combusting aerosol that was attached to the particle surface was observed. Results from quantitative shadowgraphy indicated near-instantaneous melting and complex behavior—we attempted to explain these based on the FeO phase diagram. The analysis of micron- and nano-sized combustion products confirmed that the PSI combusted heterogeneously and a gaseous sub-oxide was formed. Combustion under high excess oxygen was hypothesized to reduce the formation of these oxides.

Quantitative shadowgraphy and proton radiography for large intensity modulations.

Shadowgraphy is a technique widely used to diagnose objects or systems in various fields in physics and engineering. In shadowgraphy, an optical beam is deflected by the object and then the intensity modulation is captured on a screen placed some distance away. However, retrieving quantitative information from the shadowgrams themselves is a challenging task because of the nonlinear nature of the process. Here, we present a method to retrieve quantitative information from shadowgrams, based on computational geometry. This process can also be applied to proton radiography for electric and magnetic field diagnosis in high-energy-density plasmas and has been benchmarked using a toroidal magnetic field as the object, among others. It is shown that the method can accurately retrieve quantitative parameters with error bars less than 10%, even when caustics are present. The method is also shown to be robust enough to process real experimental results with simple pre- and postprocessing techniques. This adds a powerful tool for research in various fields in engineering and physics for both techniques.

Quantitative shadowgraphy for laser–plasma interactions

A new all optic non-perturbative diagnostic for measuring densities of long plasma structures is presented. A model based on the refraction of a laser beam crossing a cylindrical column of plasma with a Gaussian density profile is derived and used to measure plasma electronic densities. The model shows good agreement with the performed ray tracing simulations. Experimental measurements of the electronic density of plasma columns ranging from 2 to cm−3 are presented and show good agreement when compared to interferometry measurements.

Quantitative shadowgraphy made easy

This paper is dedicated to the learning of the shadowgraphy technique at graduate and undergraduate levels. It presents an experiment that allows measurement of the refractive index of butane with the help of an affordable quantitative shadowgraphy bench. The paper is constituted of two distinctive parts. The first presents the theory and data processing involved in shadowgraphy, introducing the concept of Abel inversion and modern data computer processing for graduate students. The second focuses on the experimental set-up and results; here, a qualitative interpretation of shadowgrams suitable for undergraduate students is given, as well as a quantitative explanation using a butane gas jet. The refractive index of butane measured with our simple experimental set-up is in close agreement with values available in the literature.

Quantitative shadowgraphy on a laminar argon plasma jet at atmospheric pressure

This paper deals with the diagnostics of a dc laminar argon plasma jet operating at atmospheric pressure in ambient air using three techniques. Through the pumping effect of ambient air by the laminar jet, it is possible to observe the UV OH spectrum at 306.357 nm (transition A 2Σ, ν = 0 → X 2Π, ν′ = 0) and to perform emission spectroscopy in order to find the OH rotational temperature close to the thermodynamic temperature of the gas. In addition, measurements of the refractive index are made by considering two different methods: optical interferometry and quantitative shadowgraphy. It is shown that the temperatures obtained by the three diagnostics techniques are very close.

Temperature measurements in a laser-heated gas by quantitative shadowgraphy

A whole-field quantitative shadow method was used to determine the temperature in a laser ignition experiment. An ethylene-air mixture was heated by a CO(2) laser, and the hot gas was illuminated with a collimated laser beam. The resulting shadow image was recorded digitally and analyzed to determine the temperature. Very good spatial and temporal resolutions (15 microm and 10 ns) were achieved using a simple optical arrangement. A computer simulation was used to determine the sensitivity of the shadow temperatures to blurring due to diffraction and uncertainties in parameters such as the ambient pressure. Temperatures determined from Rayleigh light scattering agreed with the shadow measurements to within 1-4%.