Holographic focusing schlieren imaging (HFSI) for three-dimensional flow visualization

Flow field visualization techniques, e.g., schlieren and shadowgraphy, are indispensable in fluid mechanics research and application. In this paper, we present a novel technique, named holographic focusing schlieren imaging (HFSI), which takes only a single-camera and single-shot configuration to achieve three-dimensional (3D) flow visualization. The essence of this technique is that a coherent reference wave is introduced to interfere with the wavefront yielded by the traditional focusing schlieren (FS) method, forming a hologram. The reconstruction of the hologram directly yields the FS results along the test volume slice by slice with adjustable intervals, achieving 3D visualization. To demonstrate the capability of HFSI, a proof-of-concept setup was established, and experiments were performed using a compressed air jet, with a comparison to the FS method. The result shows that HFSI image reconstruction remarkably refocuses the out-of-focus jet flow, yielding similar schlieren effect observed in the FS images. The proposed HFSI holds significant practical value in some scenarios, such as in a wind tunnel, as it requires only one pair of parallel windows to achieve 3D flow visualization.