Multi-plane PIV using depth of field for in-cylinder flow measurements

Extending the planar Particle Image Velocimetry (PIV) technique to enable measurements on multiple planes simultaneously allows for some of the 3 dimensional nature of unsteady flow fields to be investigated. This requires less hardware and retains the typically higher spatial resolution of planar PIV compared to fully 3-dimensional PIV techniques. Performing multi-plane PIV measurements requires the light scattered from the different measurement planes to be distinguishable. This may be achieved by using different laser wavelengths which adds significantly to the expense and complexity of the system, by using different light sheet polarisations which is challenging for engine measurements through windows due to stress-induced birefringence, or by making alternating measurements of each plane which sacrifices the simultaneity of the flow measurement across multiple planes. In this work, simultaneous PIV measurements on two parallel offset planes are performed by modifying a standard two-dimensional two-component (2D2C) PIV system consisting of a single high-speed camera and a dual-cavity PIV laser. Image splitting optics with a variable optical path length were used to image two measurement planes onto the single camera sensor. The degree of defocus of the droplet images arising from the narrow depth of field was used to distinguish between the measurement planes. The robustness of the degree of defocus method for plane discrimination was evaluated using both representative test targets and experimentally obtained PIV images of the in-cylinder flow field of a spark ignition internal combustion engine (ICE). This depth of field based method enables simultaneous multi-plane PIV measurements of in-cylinder flows to be performed with only minor alterations to a standard 2D2C PIV system using off-the-shelf optics.