Image processing methods for particle image velocimetry (PIV)
are reviewed. The discussion focuses on iterative methods aimed
at enhancing the precision and spatial resolution of numerical
interrogation schemes.
Emphasis is placed on the efforts made to overcome the
limitations of the correlation interrogation method with
respect to typical problems such as in-plane loss of pairs,
velocity gradient compensation and correlation peak locking.
The discussion shows how the correlation signal benefits from
simple operations such as the window-offset technique, or the
continuous window deformation, which compensates for the
in-plane velocity gradient.
The image interrogation process is presented within the
discussion of the image matching problem and several algorithms
and implementations currently in use are classified depending
on the choice made about the particle image pattern matching
scheme.
Several methods that differ in their implementations are found
to be substantially similar. Iterative image deformation
methods that account for the continuous particle image pattern
transformation are analysed and the effect of crucial choices
such as image interpolation method, displacement
prediction correction and correlation peak fit scheme are
discussed. The quantitative performance assessment made through
synthetic PIV images yields order-of-magnitude improvement on
the precision of the particle image displacement at a sub-pixel
level when the image deformation is applied. Moreover, the
issue of spatial resolution is addressed and the limiting
factors of the specific interrogation methods are discussed.
Finally, an attempt for a flow-adaptive spatial resolution method
is proposed. The method takes into account the local velocity
derivatives in order to perform a local increase of the
interrogation spatial density and a refinement of the local
interrogation window size. The resulting spatial resolution is
selectively enhanced. The method's performance is analysed and
compared with some precursor techniques, namely the conventional
cross-correlation analysis with and without the effect of a
window discrete offset and deformation.
The suitability of the method for the measurement in turbulent
flows is illustrated with the application to a turbulent
backward facing step flow.
A survey is given of the major developments in three-dimensional velocity field measurements using the tomographic particle image velocimetry (PIV) technique. The appearance of tomo-PIV dates back seven years from the present review (Elsinga et al 2005a 6th Int. Symp. PIV (Pasadena, CA)) and this approach has rapidly spread as a versatile, robust and accurate technique to investigate three-dimensional flows (Arroyo and Hinsch 2008 Topics in Applied Physics vol 112 ed A Schröder and C E Willert (Berlin: Springer) pp 127–54) and turbulence physics in particular. A considerable number of applications have been achieved over a wide range of flow problems, which requires the current status and capabilities of tomographic PIV to be reviewed. The fundamental aspects of the technique are discussed beginning from hardware considerations for volume illumination, imaging systems, their configurations and system calibration. The data processing aspects are of uppermost importance: image pre-processing, 3D object reconstruction and particle motion analysis are presented with their fundamental aspects along with the most advanced approaches. Reconstruction and cross-correlation algorithms, attaining higher measurement precision, spatial resolution or higher computational efficiency, are also discussed. The exploitation of 3D and time-resolved (4D) tomographic PIV data includes the evaluation of flow field pressure on the basis of the flow governing equation. The discussion also covers a-posteriori error analysis techniques. The most relevant applications of tomo-PIV in fluid mechanics are surveyed, covering experiments in air and water flows. In measurements in flow regimes from low-speed to supersonic, most emphasis is given to the complex 3D organization of turbulent coherent structures.
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