Ray tracing-based PIV of turbulent flows in roughened circular channels

Particle image velocimetry (PIV) is more and more used as a reference method for the measurement of velocity fields. However, this technique requires optical access and the current distortion correction methods are efficient only for small optical distortions. The motivation of the present study is to properly determine the velocity field of the turbulent flow in a channel with optically complex-shaped obstacles, designed for heat transfer enhancement. A ray tracing-based image correction method is employed to eliminate high-level image distortions on PIV images induced by heart-shaped dimples. To reduce the uncertainties in the application of the method, an optimization algorithm is built for artificially recreating the PIV calibration image using rendering software. The positions, material properties and dimensions of the objects in the experimental setup, which construct the 3D model, are considered as the design parameters. The artificial image was obtained with a standard deviation of 0.13 pixels from the actual calibration image in 4–5 h. In the calibration process, the ray tracing-based correction with the optimized artificial image provided a standard deviation of 0.32 pixels from the reference grid while the third-order polynomials had provided 9.6 pixels. To illustrate the approach, measurements were acquired on the center plane of a circular channel with the heart-shaped dimples in the streamwise direction. The 2D velocity and turbulent kinetic energy field obtained at a Reynolds number of around 20,000 showed that the flow separates as it reached the leading edge of this dimple whereas the reattachment point was captured at the trailing edge. The highest amounts of turbulent kinetic energy were found just downstream of the dimple where the best heat transfer was expected.