Small-Scale BOS Measurement Using Background Projection

Abstract

Small-scale (smaller than 5 mm) flows were measured by the BOS technique using background projection. The BOS technique has been applied to many flow fields because of its ease of use, but there have been few cases of its application to small-scale flow fields. The standard BOS technique requires the installation of a background image far from flow field to prevent the distorting the flow by itself. Therefore, there is a problem of the spatial resolution being limited by the lens aperture when measuring small-scale flow fields. BOS measurement using background projection can achieve higher spatial resolution without any distorting flow by the background. In this case, we can project the background in or near the flow field while installing the background far from the measurement area. In this paper, we obtain the high spatial resolution of BOS measurement using background projection and apply this technique to focused ultrasound in water as small-scale flow. In the standard BOS technique, the distance between the background and the flow field was 20 mm due to the size of the ultrasound transducer, but the proposed method was installed at a position of 2 mm, achieving measurements with a spatial resolution more than six times higher. The displacement measured with the proposed method showed less distortion than the standard BOS, demonstrating the superiority of high spatial resolution