A Molecular Tagging Velocimetry Based Plenoptic Microscope for Measuring Wall Shear Stress*

In this contribution, we describe measurements using a plenoptic microscope that permit acquisition of the near-wall velocity field in an axi-symmetric stagnation jet using a single camera sensor. Three-dimensional, two-velocity component (3D-2C) measurements are acquired using Molecular Tagging Velocimetry, a spectroscopic technique relying on seeded molecular tracers to probe the local velocity field. Standard reconstruction algorithms based on geometrical optics are compared against advanced deconvolution algorithms that take diffraction into account. Convincing improvements in reconstructed probe resolution are obtained using the latter reconstruction. Plenoptic imaging is compared with a scanning confocal microscope which serves as a ground truth measurement, and good agreement is achieved in the near wall velocity profile. The flow facility was designed for a low friction Reynolds number, but with an adjustable viscous length scale comparable with higher Reynolds number flows. As such, it provides a proving ground for the instrument before deployment in higher Reynolds number flows of interest. Our instrument proves capable of resolving viscous length scales on the order of 30 micrometers, which may be improved by up to a factor of four increasing the numerical aperture of the microscope objective.