Schlieren photography

Schlieren photography is a visual process used to photograph the flow of fluids of varying density. It is widely used in aerodynamics to capture the flow of air around objects, such as aircraft wings, and to visualize shock waves and other phenomena in fluid dynamics.

Principle

Schlieren photography is based on the principle that light rays are deflected when they pass through regions of different refractive index. This occurs when there are variations in the density of a fluid, such as air, due to temperature or pressure changes. The technique makes these variations visible by converting them into intensity variations in an image.

History

The technique was first developed by August Toepler in the 19th century to study supersonic motion. Since then, it has been refined and adapted for various applications in both scientific research and industrial testing.

Setup and Equipment

A typical schlieren system consists of a light source, a set of lenses or mirrors, a knife edge, and a camera. The light source is collimated, meaning the light rays are parallel, and directed through the test area. As the light passes through the fluid, it is deflected by changes in the refractive index. A knife edge is placed at the focal point of the light, blocking some of the deflected light and creating a shadowgraph image that highlights the density variations.

Single mirror schlieren setup

Single Mirror Setup

In a single mirror setup, a parabolic mirror is used to collimate the light. The light source is placed at the focal point of the mirror, and the light is reflected through the test area. After passing through the test area, the light is focused back to a point where the knife edge is placed.

Double Mirror Setup

A double mirror setup uses two mirrors to achieve the same effect. This setup can be more compact and is often used in smaller laboratories.

Applications

Schlieren photography is used in a variety of fields:

• Aerodynamics: To visualize airflow over wings and around vehicles.
• Ballistics: To study the shock waves produced by bullets and other projectiles.
• Combustion: To observe the mixing of gases and the propagation of flames.

Schlieren image of a wing at Mach 1.2

Advanced Techniques

Color Schlieren

Color schlieren photography uses colored filters to enhance the visibility of the flow patterns. This technique can provide more information about the direction and magnitude of the density gradients.

Color schlieren image of a candle plume

Background-Oriented Schlieren (BOS)

BOS is a modern variant that uses a patterned background instead of a knife edge. This method is more flexible and can be used in outdoor environments.

Shockwave pattern around a T-38C observed with BOS

Related Pages

• Shadowgraph
• Interferometry
• Flow visualization