Tackling turbulence with holographic particle image velocimetry (HPIV)

Holographic Particle Image Velocimetry (Holographic PIV or HPIV) is an emerging technology that provides instantaneous 3D flow field information. Based on holographic imaging of particles-seeded or inherent in the flow-as well as advanced information processing technology, HPIV offers what appears to be the best solution to 3D velocimetry and particulate flow diagnostic tool. The development of HPIV has been rather challenging. The conflicting requirements of high spatial resolution, large measurement volume, and practicality have placed stringent limitations on the holographic and image processing techniques. Many methods have been explored to suppress speckle noise, control particle image depth of focus, acquire 3D information from holographic images, correlate or pair particles between two exposures in presence of large velocity gradients, compress the huge quantity of 3D data. At the Laser Flow Diagnostics Laboratory, we have been following two paths towards the fruition of HPIV: an off-axis technique to explore the high performance capabilities of HPIV including spatial resolution, and an in-line technique for increased user-friendliness. A fully automated off-axis HPIV system has been developed, characterized by 90-degree particle scattering, dual reference beams, in situ reconstruction/data processing, and 3D velocity extraction based on a fast “Concise Cross Correlation” (CCC) algorithm are utilized. The system is tested1 for an acoustically excited air jet and the wake of a surface-mounted tab in a water channel flow, giving instantaneous 3D velocity fields for both flows. Experimental data of instantaneously measured 3D flow structures using this technique show great promise. The establishment of HPIV is expected to generate a significant impact to the areas of turbulence and multiphase flow research.