A laser light beam is expanded in one direction only with the help of a cylindrical lens. This so called light sheet is then used to illuminate a flow seeded with tracer particles (smoke or tiny oil droplets in air, bubbles in water). In such a light sheet the light intensity detected in the plane of an observing camera will depend on the flow geometry and the concentration of the particles in the light sheet (if only one particle size is used for visualization). This technique provides a fast mean for real time flow visualization. At the Institute for Thermal Turbomachinery and Machine Dynamics a high-speed video camera (Kodak Ektapro 1000) might also be used for flow visualization. Depending on the laser power even larger areas can be illuminated and the flow visualized.
The picture below shows the principle setup used in the linear cascade at the Institute for Thermal Turbomachinery and Machine Dynamics. This type of Schlieren visualization is used for all flow investigations in the transonic test rig. Here, flow with Mach 1.2 through a VKI-LS59 profile is shown (Projekt S6801).
This movie below was recorded in a specially designed Mach 2 nozzle (flow from the right). Under continuous operation small sized particles can be fed into the flow to study material erosion and compare flow measurements (LDA, PIV) with numerical results for two phase flows (Project S6807). A truncated wedge with a fully developed shock system can be seen: Green indicates a positive density gradient from right to left (red indicates the opposite) and blue indicates a positive density gradient from the upper part to the lower one (yellow indicates the opposite).