Improvement of Bidirectional Schlieren Images Using Wavelet Transform for Optical Flow Application in Fluid Flows

The Schlieren technique is a powerful tool used in its early years for flow visualization. However, over time, the optical system has been modified for measuring temperature and velocity fields in fluid flows. A tool utilized for measuring velocity fields in Schlieren images is the optical flow method. A requirement for successfully applying optical flow to Schlieren images is that the images contain sufficient contrast to satisfy the mathematical defining condition. On the other hand, a common Schlieren technique uses a single knife-edge cutoff to create contrast images, allowing refractive-index gradients to be imaged in only one direction. However, fluid movement occurs in both directions, leaving the flow information incomplete to study. In this work, we use a bidirectional Schlieren method to calculate velocity fields of a low-speed unstable fluid flow. The Schlieren images are processed using the continuous wavelet transform to enhance their contrast for optical flow applications. To generate the flow at low speed, we use a flapping plate at a fixed frequency. The results show that the continuous wavelet transform improves the image characteristics of Schlieren images for optical flow applications, and the bidirectional Schlieren method provides more information about the behavior of the unstable flow than the classical Schlieren system. On the other hand, to validate some experimental results, we compared them with a numerical simulation in ANSYS Fluent 2024r. The comparison results are in good agreement.