Optical Flow for Flight and Wind Tunnel Background Oriented Schlieren Imaging

Background oriented schlieren images have historically been generated by calculating the observed pixel displacement between a wind-on and wind-off image pair using normalized cross-correlation. This work uses optical flow to solve the displacement fields which generate the schlieren images. A well established method used in the computer vision community, optical flow is the apparent motion in an image sequence due to brightness changes. The regularization method of Horn and Schunck is used to create schlieren images using two data sets: a supersonic jet plume/shock interaction from the NASA Ames Unitary Plan Wind Tunnel, and a transonic flight test of a T-38 aircraft using a naturally occurring background, performed in conjunction with NASA Ames and Armstrong Research Centers. Results are presented and contrasted with those using normalized cross-correlation. The optical flow schlieren images are found to provided significantly more detail. We apply the method to an historical data set to demonstrate both the broad applicability and limitations of the technique.