Fundamental Issues and Latest Development in Holographic Particle Image Velocimetry

Abstract

Holographic PIV (HPIV) is currently the most promising technique for truly instantaneous, three-dimensional (3D), three-component (3C) velocity field measurements for complex flows including turbulent and multiphase flows. This paper reports new understanding on some fundamental issues and challenges in HPIV including the complex 3D imaging characteristics, the extraction of full particle information (intensities, sizes, and locations) in 3D space, the extraction of particle displacements, and the huge data volume to process. The latest off-axis HPIV system will be presented, which incorporates the new understanding of imaging characteristics of particle holography, careful development of data processing algorithms, and a well-designed distributed parallel processing system. We will demonstrate capabilities of HPIV by a semi-time-series measurement of instantaneous 3D, 3C velocity fields in highly 3D vortical flow.

Light-in-flight holographic particle image velocimetry for wind-tunnel applications

“…For further wind-tunnel measurements the production of seeding particles needs to be optimized in terms of size distribution and density. For this purpose, investigations have been made to understand the physical processes of particle generation (Kähler et al 2002) and the influence of particle parameters on the holographic imaging process (Pu et al 2002). Increasing seeding densities could allow for smaller ICs, with a 64 pixel or even 32 pixel side length; non-cubic ICs in the pixel domain are also feasible.…”

“…To guarantee a given SNR the number density n s of the seeding particles must not exceed a maximum value which grows with the limiting imaging aperture and decreases with the depth L of the recorded volume (Pu et al 2002):…”

Advances in Light-in-Flight HPIV for the Study of Wind Tunnel Flows

“…The most important part in LiFH is the near-backscattering regime (180° -210°) which shows strongly modulated intensities for all particles -especially pronounced, however, for those particles commonly used to probe turbulent air flows (diameter d ≤ 1 µm). The scattering spectrum is recorded only partly by the hologram due to its finite size and limited dynamical range which results in a variety of spu-rious effects in the reconstructed image (Pu et al 2002). For LiFH-PIV, however there are no detailed experimental results yet to quantify such influences.…”

“…Meanwhile it has been used even as a validation-tool for numerical simulations (Tao et al 2000) as well as for investigations of quasi timeseries by phase-locked measurements (Pu et al 2002). HPIV is advancing to become a versatile and feasible technique -still with huge demands on computing power and technical equipment, yet.…”

“…Finally, the relevant flow structures are often in the sub-millimeter range and require dense seeding to sample with sufficient accuracy. To preserve a given signal-to-noise ratio, however, the number density s n of the seeding particles must not exceed a maximum value which grows with the imaging aperture and decreases with the depth of the recorded volume (Pu et al 2002). Thus, there are two possibilities to properly record such flows:…”