Development of a Self-Aligned Compact Focusing Schlieren System for NASA Test Facilities

A novel compact, inherently self-aligned focusing schlieren system is presented for use in experiments that require flow visualization. Unlike existing focusing schlieren systems that require a separate, and precisely aligned, source grid and cutoff grid to achieve sensitivity to density gradients, this system projects an image of a physical grid element onto a background and then reimages this projection back onto the physical grid element thereby achieving self-alignment. Further, unlike existing focusing schlieren systems where the image of the source grid is physically translated relative to the cutoff grid to achieve sensitivity to density gradients, a polarization prism in the system acts on the polarization state of the light of the projected grid element to provide sensitivity adjustment. By adjusting the position of the prism along the optical axis of the instrument, changes to the sensitivity can be realized. Since the outgoing and returning light used to project an image of the grid element are orthogonally polarized to one another, reflections from windows that may be present between the lens and background of the system can be eliminated. The system can also be implemented in test facilities where optical access is limited and/or exists on only one side. A discussion on different aspects of the development of this technology is provided, as are sample images from the system at the different stages of development. Images that demonstrate the ability to make on-the-fly adjustments to the instrument's field-of-view and working distance are also presented.