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Improvement of measurement accuracy in micro PIV by image overlapping

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Abstract

Micro PIV uses volume illumination; therefore, the velocity measured at the focal plane is a weighted average of the velocities within the measurement volume. The contribution of out-of-focus particles to the PIV correlation can generate significant measurement errors particularly in near wall regions. We present a new application of image overlapping, which is shown to be very effective in improving the accuracy of time-averaged velocity measurements by effectively reducing the measurement depth. The performance of image overlapping and correlation averaging were studied using synthetic and experimental images of micro channel flow, both with and without image pre-processing. The results show that for flows without particle clumping, image overlapping provides the best measurement accuracy without any need for image pre-processing. For flows with particle clumping, image overlapping combined with band-pass filtering provides the best measurement accuracy. When overlapped images are saturated with particles due to a large number of image pairs, image overlapping measurement still does not show any visible pixel-locking effect. Image overlapping was found to have comparable or slightly reduced pixel-locking effects compared to correlation averaging. In addition, image overlapping utilizes significantly fewer computational resources than the other techniques.

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Abbreviations

δx, δy :

Horizontal and vertical displacements in pixels

δz corr :

Depth of correlation

λ:

Fluorescent wavelength

\(\Upphi_k(\delta x,\delta y) \) :

Correlation function from an image pair

\(\Upphi_{\rm ens}(\delta x,\delta y) \) :

Averaged correlation function

\(\Upphi_{\rm ovl}(\delta x,\delta y) \) :

Correlation function from an overlapped image pairs

NA :

Numerical aperture

ɛ:

Signal threshold where depth of correlation ends

d p :

Particle diameter

I k (i, j), I′ k (i, j):

First and second image exposures

M :

Lens magnification

N p :

Number of particles per image

P, Q:

Image width and height

R :

Half of channel height

U * :

Normalized velocity

\(U_c^* \) :

Normalized velocity at the center, z = 0

\(U_w^* \) :

Normalized velocity near the wall, z = 0.9R

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Authors and Affiliations

  1. Fluid Laboratory for Aeronautical and Industrial Research (FLAIR), Monash University, Melbourne, VIC, 3800, Australia

    Chuong Vinh Nguyen, Andreas Fouras & Josie Carberry

  2. Division of Biological Engineering, Monash University, Melbourne, VIC, 3800, Australia

    Andreas Fouras

Authors
  1. Chuong Vinh Nguyen
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  2. Andreas Fouras
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  3. Josie Carberry
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Correspondence to Andreas Fouras.

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Nguyen, C.V., Fouras, A. & Carberry, J. Improvement of measurement accuracy in micro PIV by image overlapping. Exp Fluids 49, 701–712 (2010). https://doi.org/10.1007/s00348-010-0837-9

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  • DOI: https://doi.org/10.1007/s00348-010-0837-9

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