Post-Processing of Schlieren Images

In general, the Schlieren visualization method is used to qualitatively describe phenomena. However, recent studies have attempted to convert the classical Schlieren system into a quantitative method to describe certain flow parameters. This paper aims at analysing pictures from a qualitative and a quantitative point of view. The post-processing of images for both situations is described based on different applications. Real examples are used and both methodologies and logical schemes are explained. The article focuses on image processing, and not on the studied phenomena.

Visualization of Acoustic Waves and Cavitation in Ultrasonic Water Flow

Visualization experiments were performed to examine whether acoustic bubbles play a role in ultrasonic water flow cleaning, as in convention cleaning with ultrasonic baths. Schlieren visualization confirmed the standing-wave-like acoustic field in ultrasonic water flow that collides with a glass surface. Backlight visualization showed that cavitation bubbles appear in the water flow spreading over the glass surface. These bubbles are found to oscillate in volume and move inside film flow and thus expected to play a role as active cleaning agents.

A Comparison Between Numerical and Experimental High Reynolds Number Supersonic Jets Generated by Millimeter Scale Converging-Diverging Nozzles

In thermal spray applications, such as cold spray, an inert gas jet (typically helium or nitrogen) is used to accelerate micron scale particles to supersonic velocities. The complex gas dynamics of these supersonic jets are critical to understand via computational methods for the control of the spray. This work compares supersonic jet waveforms visualized by schlieren imaging with those predicted by computational fluid dynamics (CFD) simulations. A supersonic nitrogen jet is produced by a millimeter scale converging-diverging nozzle with inlet pressures as high as 50 bars. The jet Reynolds numbers based on the nozzle exit diameter and stagnation gas properties range between 60,000 to 325,000. A schlieren visualization setup has been built which shows the first spatial derivative of densities within the flow field. The strong density gradients across the oblique shock waves in the jets allow for clear photographs of the flow pattern of the jets using this schlieren visualization setup. Comparisons between the experiments and the CFD results act as a validation technique for the accuracy of the simulations in terms of the positions and orientations of the oblique shock waves. Through this study, the nozzle internal surface roughness is determined to be a critical parameter in millimeter scale nozzles for the development of the boundary layer. The CFD surface roughness parameters inside the nozzle are incremented until the geometry of the oblique shock waves matches the schlieren images. This work validates the simulation techniques which will be used for future jet simulations, in which shock wave locations and orientations are important, such as jet impingement on a flat plate and particle-shock interactions.

Effects of the Thickness of Boundary Layer on Droplet’s Evaporation Rate

The evaporation rate of a droplet was explained in relation to the thickness of the boundary layer and the condition near the droplet’s surface. However, the number of results obtained from experiments is very limited. This study aims to investigate the thickness of the boundary layer of an ethanol‐water mixture droplet and its effect on the evaporation rate by Z-type Schlieren visualization. Single and double droplets are tested and compared to identify the effect of the second droplet on the average and instantaneous evaporation rate. The double droplet’s lifetime is found to be longer than the single droplet’s lifetime. The formation of a larger vapor region on the top of the droplet indicates a higher instantaneous evaporation rate. The thickness of the boundary layer is found to increase with increase in ethanol concentration. Furthermore, a larger vapor distribution area is found in the case of higher ethanol concentration, which explains the faster evaporation rate at higher ethanol concentration.