Bidirectional Schlieren Imaging for Flow Visualization and Velocity Measurement of Convective Fluid Flows

The Schlieren technique is a compelling technique for visualizing variable-density fluid flows. Recently, this technique has also been used to calculate velocity fields for such fluid flows. The Schlieren technique is useful in cases where particle tracers are difficult to introduce into the fluid flow to measure velocity. In these studies, it is common to use the classical Schlieren method, which is sensitive to density gradients in the horizontal or vertical direction. However, fluid flow behavior requires a Schlieren technique sensitive to both directions simultaneously. In this way, the bidirectional Schlieren technique is used to determine the velocity field of low-velocity flow. Schlieren images sensitive to the horizontal and vertical directions are obtained in a single snapshot. The velocity field is obtained utilizing the optical flow method. In this method, a weight factor, α, is used to compensate for the intensity difference between consecutive Schlieren images. Therefore, different α values were tested to find the proper one for each case of fluid flow under analysis. The convective fluid flows used in this study were generated by a candle flame and a hot metal plate. The velocity fields obtained show more information than those obtained with the classic Schlieren system.