A Portable, Projection Focusing Schlieren System

John H. Glenn Research Center, Cleveland, Ohio

A new type of projection schlieren camera system (schlierenscope) is more portable, easier to align, and more versatile than existing systems. The schlierenscope is a projection focusing schlieren camera system that can acquire images of shock waves, vortices, gas jets, and other disturbances that create gradients in the refractive index of a transparent medium. These gradients appear as streaks (called schlieren in German) in the resulting image. Thus, a schlierenscope is an apparatus for looking at disturbances in transparent media. The schlierenscope constructed in the project utilizes fast strobes that freeze motion and capture images with a scientific CCD (charge-coupled device) camera. The schlierenscope is unique among schlieren instruments because all of the critical controls are contained within the instrument housing.

Figure 1. Schlieren image of air rifle pellet traveling Mach 1.3, showing shock waves reflected by a barrier.

The schlieren technique has been used for flow diagnostics in wind tunnels since the beginning of aerospace research, due to its ability to make airflow, especially shock waves and turbulence, visible. Convection currents in air can also be visualized using the schlieren technique, which makes it a useful research tool for a wide range of commercial applications such as heating and ventilation systems, the spread of fires, transmission of diseases from coughing and sneezing, and detecting gas leaks. A major drawback of previous schlieren systems is that they require exact alignment between a pair of widely separated mirrors or grids. This limits the portability of schlieren systems and restricts their widespread commercial use. If a facility is not already equipped for schlieren imaging, the costs of acquiring large mirrors and aligning the system are often prohibitive.

Figure 2. Schlieren image of heat rising from a tea kettle.

An innovative feature of the schlierenscope is that the positions of the source and cutoff grids can be adjusted, which allows changing the screen distance to accommodate different fields of view. The optical elements that require precise adjustment (the Ronchi grids and lenses) are mounted on a single, compact platform. A retro-reflective screen fulfills a role similar to the source mirror in a classical schlieren system; however, the retro-reflective property of the screen makes it far less sensitive to disturbances in the alignment. This increases the robustness and portability of the system. The schlierenscope excels as a system for visualizing large fields of view, which can be especially costly for classical schlieren systems. The effort required to operate a schlierenscope is comparable in difficulty to operating a focused shadowgraph system, which is frequently employed in measurements where a quick setup is needed.

Compared to conventional schlieren systems, the schlierenscope is compact and easier to use, which makes it possible to photograph airflows in industrial settings where traditional laboratory instruments are too delicate and complex. To improve ventilation systems and blowers, the engineers who design them require portable camera systems that allow them to visualize airflow. When the schlierenscope is equipped with strobes and high-speed cameras, it can visualize shock waves from explosions, which makes it applicable for many safety applications.

This work was done by Drew L’Esperance of MetroLaser, Inc. and Leonard Weinstein of the National Institute of Aerospace Associates (NIAA) for Glenn Research Center.

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