The practical aspects of an advanced schlieren technique, which has been presented by Meier (1999) and Richard et al (2000) and in a similar form by Dalziel et al (2000), are described in this paper. The application of the technique is demonstrated by three experimental investigations on compressible vortices. These vortices play a major role in the blade vortex interaction (BVI) phenomenon, which is responsible for the typical impulsive noise of helicopters. Two experiments were performed in order to investigate the details of the vortex formation from the blade tips of two different helicopters in flight: a Eurocopter BK117 and a large US utility helicopter. In addition to this, simultaneous measurements of velocity and density fields were conducted in a transonic wind tunnel in order to characterize the structure of compressible vortices. The background oriented schlieren technique has the potential of complementing other optical techniques such as shadowgraphy or focusing schlieren methods and yields additional quantitative information. Furthermore, in the case of helicopter aerodynamics, this technique allows the effect of Reynolds number on vortex development from blade tips to be studied in full-scale flight tests more easily than through the use of laser-based techniques.

https://iopscience.iop.org/article/10.1088/0957-0233/12/9/325/pdf

Principle and applications of the background oriented schlieren (BOS) method

This article gives an overview of the back- ground-oriented schlieren (BOS) technique, typical appli- cations and literature in the field. BOS is an optical den- sity visualization technique, belonging to the same family as schlieren photography, shadowgraphy or interferometry. In contrast to these older techniques, BOS uses correlation techniques on a background dot pattern to quantitatively characterize compressible and thermal flows with good spatial and temporal resolution. The main advantages of this technique, the experimental simplicity and the robust- ness of correlation-based digital analysis, mean that it is widely used, and variant versions are reviewed in the arti- cle. The advantages of each variant are reviewed, and fur- ther literature is provided for the reader. List of symbols

/pdf/background-oriented-schlieren-bos-techniques-3m41owd4oz.pdf

Background-oriented schlieren (BOS) techniques

Density fields of the blade tip vortices from a helicopter in hover flight were visualized by a technique which does not require any installation on the helicopter or close to it. The results illustrate an encouraging prospect for the applicability of the technique. It offers the capability of at least qualitative investigations of unsteady density fields even in full-scale flight tests. The underlying principle is briefly described in this article and an extension to a three-dimensional quantitative technique by using multiple cameras is outlined.

On the applicability of background oriented optical tomography for large scale aerodynamic investigations

A new microscope imaging system, modulation contrast, has been devised that reveals phase gradients; the image intensity is proportional to the first derivative of the optical density in the object. The modulator, a special filter, is placed in the Fourier plane, a plane conjugate with a slit aperture. The image of the slit aperture is registered within a gray region of the modulator; on one side of the gray region is a region of low transmittance and on the other side, a region of maximum transmittance. The modulator processes opposite gradients to produce opposite intensities, creating an optical shadowing effect. The dark region may be outside the optical system when the gray region is offset to the edge of the Fourier plane, to achieve maximum resolution. Modulation contrast is directional and capable of optical sectioning, revealing details without obscuring effects of structures above and below the plane of focus. The imaging theory of microscope optics has been extended to include effects of phase gradients. Phase gradients distribute the zero order across the Fourier plane. Intensity of the gradient's image is controlled by the zero order of the gradient diffraction pattern.

Modulation contrast microscope.

Speckle photography is used to obtain full-field gas-phase temperature measurements which are relatively insensitive to system motion. A cylindrical propane flame is used to test the accuracy and sensitivity of the method. Results are compared with thermocouple measurements.

Temperature measurement in gases using speckle photography.

With the help of a simple ground glass and a photographic plate the phase variations of a transparent object can be studied. The object is interposed between the source and the ground glass. The photographic plate, kept behind the ground glass, is exposed successively twice, with and without the object. When properly illuminated the negative gives the variation in the derivative of the phase of the object, and this observation can be made in white light.

C.P. Grover

Papers

Ground glass differential interferometer.

Detection of differences between two images

Detection of the difference between two images: An improved method

Correlation of light beams scattered at different angles by a ground glass

Encoding and decoding of colour signals