A MTV Based Plenoptic Microscope to Measure Small-Scale Velocity Gradients

Determination of small-scale velocity gradients and frictional drag in near-wall regions is fundamental for understanding overall resistance in ships moving at low to moderate speed.  We present a novel measurement technique based on Molecular Tagging Velocimetry (MTV), used to determine the two wall-parallel velocity components in three dimensions (3D-2C) within the viscous sublayer of a turbulent channel flow using a single plenoptic camera.  The Talbot Effect Structured Illumination (TESI) method creates an array of small diameter (30 um)  beamlets extending in the wall-normal direction with 300 um)  periodicity.  The pattern is photobleached in rhodamine 6G dye with a frequency-tripled Nd:YAG laser (355 nm); the write pulse.  A frequency-doubled Nd:YLF laser (527 nm) excites the remaining fluorescent dye (the read pulse). The laser induced fluorescent signal permits visualizing the initial pattern and its deformation by local flow structures.  A plenoptic microscope (3x) resolves the wall-parallel velocity components in the wall-normal direction, giving the velocity gradient.  Due to the continuous nature of the photobleached signal, the resolution of the velocity gradient is as good as the 3D reconstruction algorithm.  Novel algorithms tailored to TESI-MTV are presented which improve the resolution of a non-opaque object.