Particle Shadow Velocimetry

Particle Shadow Velocimetry (PSV) is a variant of Particle Image Velocimetry (PIV) where direct in-line illumination, combined with imaging optics, are utilized to produce a narrow depth-of-field (DOF) for two dimensional plane imaging. The narrow depth of field is produced by using a very large aperture as well as spacers or bellows placed between the camera body and the lens in use.

PSV employs a fundamentally different approach than PIV. Instead of relying on principles such as flourescence, scattering, Doppler frequecies, etc. it relies on the shadows cast by particles in the flow on a bright background. This is a direct result of the in-line, zero-degree-deviation direct-illumination experimental setup. The shadows cast by particles in the flow are recored by an imaging device, such as a high speed camera. The particles in the recorded image are observed as a result of light extinction caused by absorption and scattering, not the forward scatter of any light by the particle. In general, the ratio of extinction to forward scatter of the light depends on the size of hte particles, but it is not uncommon for extinction to be 10x larger than forward scatter [reference]. In a very small angular regions (the required setup for PSV) any forward scatter becomes extinction and the ratio changes from 10 to 1 to greater than 1000 to 1. The forward scatter only contributes to the bright background of the image. The narrow depth of field is required to ensure that only particles in the field of interest are used for correlation. Due to the nature of the setup, particles in the plane of view will cast the darkest shadows, allowing for threshholding techniques to eiminate the influence of other particles in processing. During processing, the image is inverted and processed in the way as with PIV.

 

Aside from a drastic reduction in cost of the overall system, there is a large advantage that PSV provides over PIV in bubbly flows. In cavitating wakes, it is extremely difficult to emply PIV to track the dissolved oxygen and water vapor (bubbles) that comes out of solution, especially in high void fraction flows. For PIV, when laser light comes in contact with the bubbles, laser light is scattered in numerous directions which has a high likelihood of illuminating bubbles that are not in the plane of interest. For high void fraction flows, bubbles can 'block' laser light from other bubbles in the plane of interest, resulting in an incomplete depiction of the area of interest when the results are processed. PSV's reliance on the shadows of the particles (bubbles for cavitating flows) used to track the flow eliminates the issues encountered with PIV. Direct in-line illunimation and the optical setup ensures that only bubbles in the plane of interest are considered during processing, elminating the influence of out of plane bubbles that might have been illuminated when using PIV. With edge detection algorithms and careful processing techniques, overlapping bubbles can still be tracked accurately. SAFL is equipped with both high speed PIV and PSV. For PSV, SAFL utilizes a custom LED array from ISSI (add information about LED light source). The cameras available are the same as those used for the TR-PIV system.

Schematic of Particle Shadow Velocimetry Setup

Top Left: Raw image obtained using PSV in a ventilated wake. Top Right: Inverted and squared image to prepare for processing. 
Lower Right: Sample correlation peaks for the field of interest. Bottom Left: Resultant vector field 
References: Estervadeordal J, Goss, L; PIV with LED: Particle Shadow Velocimetry (PSV), AIAA