Schlieren

SCHLIEREN

Schlieren imaging is used as a means of visualizing changes in pressure, temperature and shock waves in a transparent medium such as air. In applications such as wind-tunnels and pressure chambers schlieren imaging provides clear and detailed information on changes of pressure and density.

Schlieren imaging was developed in the 1800’s to detect flaws or ‘schliere’ in glass. The technique is frequently used today as a means of visualizing shock waves in wind-tunnels and temperature gradients around objects. Schlieren imaging relies on the refractive index, the ability to ‘bend’ light, of a transparent medium changing with density to produce an image. In a simple schlieren system a parallel beam of light is passed through the subject and is focused on to a knife edge using lenses or spherical mirrors. A change in density in part of the subject causes that part of the light beam to be refracted and to fall above or below the knife edge creating lighter or darker areas in the image. Color filters are sometimes used in place of a knife edge to produce an image using different colors to denote different areas of density.

Optical Set up – Illumination for schlieren imaging must be produced from a point light source. For high speed photography the intensity of the light source and sensitivity of the imager should allow recording at the desired frame rate. A parallel beam of light is created using a pair of lenses or mirrors. The diameter of the lenses or mirrors determines the size of the working area, or measurement volume, in which the image is produced. A knife edge or specially designed color filter is mounted on a Vernier adjustment close to the image plane. The schematic diagram above shows the light path and position of the camera sensor.

Optical-flow-based background-oriented schlieren technique for measuring a laser-induced underwater shock wave

The background-oriented schlieren (BOS) technique with the physics-based optical flow method (OF-BOS) is developed for measuring the pressure field of a laser-induced underwater shock wave. Compared to BOS with the conventional cross-correlation method that is also applied for particle image velocimetry (here called PIV-BOS), by using the OF-BOS, the displacement field generated by a small density gradient in water can be obtained at the spatial resolution of one vector per pixel. The corresponding density and pressure fields can be further extracted. It is demonstrated in particular that the sufficiently high spatial resolution of the extracted displacement vector field is required in the tomographic reconstruction to correctly infer the pressure field of the spherical underwater shock wave. The capability of the OF-BOS method is critically evaluated based on synchronized hydrophone measurements. Special emphasis is placed on direct comparison between the OF-BOS and PIV-BOS methods.

Visualization of the aerodynamic vortices by Background Oriented Schlieren

Investigating of toroidal vortices (vortex rings) currently arouses growing interest. Their presence reveals in various fields of science and technology: the effect of a vortex ring while helicopter decreasing, vortex rings in the ventricle of the heart, the internal structure of ball lightning and more. Background Oriented Schlieren (BOS) – one of the relatively young methods of diagnostic of optical inhomogeneities based on the use of the reference and distorted images of a background pattern. This method is commonly used in aerodynamics, hydrodynamics and heat transfer. The main advantages of this method are the simplicity of implementation and an almost unlimited field of view. In this work laboratory testing of the applicability of the Background Oriented Schlieren method for aerodynamic problems was carried out. For result verifying simultaneous registration via shearing interferometer was used. Field of view of the camera for the Background Oriented Schlieren and the interferometer were situated close and at a small angle to each other. Toroidal vortex was recorded directly at the output of the generator. Background pattern for Background Oriented Schlieren and interferometer mirror were located in the same plane. For achieving better contrast background pattern was illuminated by LED spotlight.

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Whole-field optical measurements of sound wave propagation from high-speed exhaust jets

It is the goal of this paper to advance the field of noise measurement techniques to better understand the fundamental guiding principles of noise generation. This is accomplished in this study by demonstrating the capabilities of ultrahigh speed Rainbow Schlieren Deflectometry (UHS-RSD) technique to visualize and quantify, in real-time, sound waves propagating from a supersonic cold air jet. Basic optical theory states that light rays passing through varying density transparent medium undergo deviation from their original path because of refraction. Therefore, an experimental setup was developed to direct parallel white light rays through a supersonic air jet. The variation in density field created in the jet stream causes light rays to deviate from their original path. The UHS-RSD technique employs aforementioned technique and enables mapping of the light deflection angle, a measure of deviation of a light ray from its original path due to refraction. Deflection mapping process is realized through variation in color (hue) between an image without and with test medium.