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Schlieren Imaging: A Technique to Visualize Supersonic Flow Features

Overview

Source: Jose Roberto Moreto, Jaime Dorado, and Xiaofeng Liu, Department of Aerospace Engineering, San Diego State University, San Diego, CA

Military jet fighters and projectiles can fly at incredible speeds that exceed the speed of sound, which means they are traveling at a supersonic speed. The speed of sound is the speed at which a sound wave propagates through a medium, which is 343 m/s. Mach numbers are used to gauge the flight speed of an object relative to the speed of sound.

An object traveling at the speed of sound would have a Mach number of 1.0, whereas an object traveling faster than the speed of sound has a Mach number greater than 1.0. The compressibility effects of air must be accounted for when traveling at such speeds. A flow is considered compressible when the Mach number is greater than 0.3. In this demonstration, Mach 2.0 supersonic flow over a cone will be analyzed by visualizing the formation of shock waves and compression waves in compressible flow using a Schlieren system.

Procedure

1. Visualizing shock waves using a schlieren imaging system

Activate the dryer towers to dehydrate the air. This will ensure that the air flow does not contain moisture, and will prevent ice formation when the local temperature in the test section drops due to the supersonic flow.
Open the test section and secure the 15° half-angle cone model to the support structure.
Check if the test section is clear of debris or other objects, then close test section.
Mak

Results

In this demonstration, a cone with a half angle of 15 degrees was subjected to a supersonic flow at Mach 2.0. In Figure 3, a shock wake and an expansion fan over the cone is observed. Theoretically, an oblique shock should form at the cone surface at an angle of 33.9°. The experimental angle was measured to be 33.6°, as shown by the red line in Figure 3B. Compared to the theoretical data, the percent error was found to be less than 1%. In addition, this flow visualization method

Application and Summary

The schlieren imaging technique is a classical optical flow visualization technique based on density changes in the fluid. It is a simple system built with concave mirrors, a knife-edge, and a light source. With this system, supersonic flow features, such as shock waves and expansion waves, can be visualized. This technique, however, has sensitivity limits to low-speed flows.

The schlieren imaging method may be used for a variety of applications, especially in the study of fluid mechanics and