Color schlieren methods in shock wave research

Abstract

Abstract. Schlieren methods are widely known and well established to visualize refractive index variations in transparent media. The use of color allows one to obtain more data and previously inaccessible information from a picture taken with this technique. In general, a hue can be related to a certain strength or a certain direction of a refractive index gradient. While the first case essentially corresponds to the usual black-and-white system the latter correlation cannot be made adequately evident without the use of color. Two color schlieren techniques are presented here, which reach or even exceed the quality and sensitivity range of conventional black-and-white methods. Using a powerful short duration light source these methods are applied to visualize transient flow phenomena in a shock tube.

Visualization of developing high temperature supersonic impulse jet induced by blast wave simulator

“…The DInCS (Kleine and Grönig, 1991) was used for identifying the density field. Figure 2(a) shows the optical alignment of the DInCS used for this experiment.…”

Flash lighting instrumentation for color schlieren shock wave photography

“…The tricolor filter was assembled of three filter foils, each being identical with the types used for the streak records. Otherwise, the arrangement of the filter foils was similar to the setup described by Kleine and Gr6nig (1991).…”

“…Beginning witl) a conventional blackand-white schtieren setup, they used first an enlarged version of the Nanospark with a fl ash duration of 60 ns FWHM. Subsequently, for color schlieren photography the light source was a xenon-filled flash lamp with a flash duration of about 1200 ns FWHM and a light output sufficiently high to expose a regular ASA 400 slide film (Kleine and Gr6nig 1991). Later on selfmade flash lamps, filled with Xe and small amounts of H2 and N2, were also used to reduce the flash duration below 500 ns FWHM .…”

“…Hitherto it has been applied on the following shock Schematic of a 2-D color schlieren method, using Settles' modified dissection method. A uniform illumination of the diffuser/tricolor-filter package can be achieved by using: a a point light source in combination with a condenser system, or b a closely positioned small area light source and flow phenomena: (i) supersonic airflow over a blunt object or a wedge (Stong and Settles 1971); (ii) bullet shock wave interactions with soap bubbles, candle flames and solid objects, a vortex produced by a fan blade, and the vapor expanding from an underwater spark (Stong and Vandiver 1974); (iii) air flow around the teeth of high-speed circular saws, in an attempt to reduce noise generation in lumber mills (Vandiver and Edgerton 1974); (iv) aerofoil tests in transonics (Ruiyi and Zhoufang 1988); (v) broad applications to various high-speed shock and flow phenomena (Kleine and Gr6nig 1990, 1991; and (vi) shock interactions (Seitz et al 1993). Recently, it has also been introduced at the Ernst-Mach-Institut (EMI) with the intention of visualizing shock precursor and decursor phenomena.…”

“…Kleine who gained much practical experience in this field by taking hundreds of high-speed color schlieren photos recently stated: "The main limiting factor at least for short duration applications appears to be the light source. Some improvements and optimizations in this field may even enhance the performance of the color techniques" (Kleine and Gr6nig 1991).…”