Self-aligned focusing schlieren in optically constrained environments

Abstract

A method is presented to acquire self-aligned focusing schlieren (SAFS) images when the optical axis of the instrument is positioned at a non-normal incidence angle to the background plane on which the grid pattern is projected, or to the measurement plane on which gradients in density are visualized, or both. This is accomplished by applying the Scheimpflug principle and properly orienting the planes on which the physical grid pattern and image sensor reside within the instrument so that focused schlieren images on the measurement plane can be captured. Focused schlieren images from laboratory tests, where the background plane and measurement plane were separately and then simultaneously oriented at non-normal incidence to the instrument’s optical axis, were acquired with and without the appropriate Scheimpflug adjustments. The results with the appropriate Scheimpflug adjustments provided the highest quality focused schlieren images. However, for the scale of the experiments in this work, it was determined that the Scheimpflug adjustment to the image sensor affected focused schlieren image quality more than adjustment to the grid pattern plane. Based on the results of the initial experiments, two demonstration tests were performed to highlight the applicability of the technique in optically constrained environments. In the first test, relatively good focused schlieren images were acquired using a curved background plane. In the second test, two synchronized high-speed (100 kHz) SAFS systems acquired images of a jet flow with overlapping fields-of-view where their optical axes were at non-normal incidence to the measurement plane, along which the jet propagated. The resulting images were then stitched together in post-processing so that an extended field-of-view could be imaged at a resolution and framing rate that exceeded that of a single high-speed SAFS system. In general, the results of this work show that high-quality focused schlieren images can be obtained with the background plane and measurement plane oriented over a large range of angles relative to the SAFS instrument’s optical axis.