Digital Frensel Reflection Holography for 3D high resolution near-wall flow measurement*

Fundamental study of wall-bounded turbulence requires measurement techniques that capture 3D near-wall flow at high spatial resolution. The typical resolution offered by commercial 3D PIV techniques is ~1 mm, insufficient to capture fine flow structures near the wall. Conventional Digital Inline Holography (DIH) can achieve substantially higher resolution but requires local injection of tracers upstream of the measurement region to minimize noise introduced through cross interference. This increases system complexity and can lead to temporally variable seeding, limiting the usage of DIH in large-scale facilities or over surfaces with complex geometries. Here, we demonstrate a new 3D holographic method that works with bulk flow seeding at a concentration ~1000 times that of conventional DIH, by capturing the interference between the backscattered light from particles (signal wave) and the reflection at the liquid-solid interface at the wall (reference wave). The technique is calibrated with brightfield imaging, and specific metrics relating measurement resolution with particle concentration are presented. Finally, we implement the approach to resolve near-wall flows in a flow channel, extracting the 3D flow field at ~50 μm resolution.