Enhanced PIV Measurement Methods Using Polarized Light in Porous Media Flows

Experimental measurements in a porous media flow are extremely challenging due to the highly three dimensional nature of the flow, probe access, the wide range of flow channel characteristic lengths and the wide dynamic range of velocities, especially at relatively high Reynolds number flow conditions. One method that has been used and is being further developed is refractive index matching (RIM) coupled with particle image velocimetry (PIV). One of the challenges is the ability to achieve high signal to noise ratio due to inherent noise generated by scattering at solid/liquid interfaces that can result from even slight mismatches in refractive index (on the order of 0.001). In this study the use of polarized light source (p-polarization versus s-polarization) is evaluated for flow in a porous bed. P-polarization is known to reduce scattered light and thereby improve signal to noise. In this case the seed scattering intensity is shown not to significantly decrease, thereby maintaining signal strength, as well as reducing bead scattering, thereby reducing noise. This study shows the ability to obtain high spatial resolution data near bead surfaces. The flow chamber used is a 20 mm square channel with a random packing of 6 mm beads using an aqueous solution of ammonium thiocyanate such that the index of refraction matches that of the beads and channel walls (1.4695). The seed particles were selected to be 10 μm silver coated hollow glass spheres due to their high signal intensity. A cross correlation technique is used coupled with adaptive grid interrogation. Results demonstrate improved velocity vector resolution and a decrease in PIV vector errors using p-polarized light.