Double field of view digital sideband holography as an optimized method to measure velocity fields in a large fluid volume

Abstract

Digital in-line holography is a technique that allows the measurement of the three velocity components of a three dimensional fluid flow. The application of digital in-line holography in fluid velocimetry is mainly limited by three factors: the sensor size that limits the transversal area that can be recorded, the low optical aperture that reduces the spatial resolution along the optical axis and introduces aliasing, and the noise coming from the twin image that hinders the particle position and velocity measurements. These factors do not affect in the same way when characterizing the movement of the fluid, and require different solutions.In this work we are going to show how to overcome those limitations. We applied digital sideband holography for the measurement of the particle position and velocity in a fluid volume with a cross-section twice the area allowed by the camera sensor, with magnification M = 1, and a very large the dimension along the optical axis. Digital sideband holography configuration, that keeps the simplicity of the classical in-line holographic set-up, consists of a camera, a lens and a frequency filter. This frequency filter is the key element that allows us to measure in such a large volume while maintaining spatial resolution, as well as accuracy: not only removes the twin image but also prevents the recording of unwanted frequencies that causes aliasing. We have found that the Signal to Noise Ratio depends mainly on the noise introduced by the twin image, the aliasing and the particle concentration rather than on parameters such as field of view, depth of field or the intensity of the particles.This technique is applied for the quantitative characterization of the three-dimensional flow in a lid-driven squared-sectioned cuvette. The introduction of a prism in the optical set-up allowed us to double the field of view. This is achieved by illuminating two volumes of the cuvette (22 × 22 × 100 mm3, each) with the same beam that crosses the fluid twice before reaching the camera sensor. These two fluid volumes are analyzed independently while keeping the same spatial resolution in the axial component along the whole volume than the expected for a shorter one. The experimental 3D data show a very good agreement with numerical 3D simulation, which proves the very good performance of our method.