Application of a Novel Projection Focusing Schlieren System in NASA Test Facilities

A portable projection focusing schlieren system for flow visualization has been designed and built by Metrolaser, Inc. with support from NASA’s Small Business Innovative Research Program. The system is based on a lens and grid technique where a source grid is projected onto a retro-reflective screen and imaged onto a cut-off grid creating the schlieren effect. The light source is a 5 Watt Xenon flashlamp with a 1-s pulse duration, which enables instantaneous snapshots of the flow field. Density gradients or refractive index gradients in the flow cause the light rays to bend or refract. The cut-off grid, which is analogous to the “knife-edge” in a classical schlieren system, causes these disturbances to be seen as light or dark regions in the image of the flow that is captured on a camera sensor located somewhere behind the cut-off grid. Two system designs are presented: one that relays the image of the object to the CCD sensor by way of relay lenses, enabling very large fields of view but at the expense of image quality, and another that directly images the object on the CCD sensor by way of the same lens that images the projected grid onto the cut-off grid, which results in a limited field-of-view but higher quality images. A discussion of the two optical systems and images from various NASA experiments demonstrating the system capabilities are presented.