Lagrangian 3D particle tracking in high-speed flows: Shake-The-Box for multi-pulse systems

The Shake-The-Box (STB) particle tracking technique, recently introduced for time-resolved 3D particle image velocimetry (PIV) images, is applied here to data from a multi-pulse investigation of a turbulent boundary layer flow with adverse pressure gradient in air at 36 m/s (Re _τ  = 10,650). The multi-pulse acquisition strategy allows for the recording of four-pulse long time-resolved sequences with a time separation of a few microseconds. The experimental setup consists of a dual-imaging system and a dual-double-cavity laser emitting orthogonal polarization directions to separate the four pulses. The STB particle triangulation and tracking strategy is adapted here to cope with the limited amount of realizations available along the time sequence and to take advantage of the ghost track reduction offered by the use of two independent imaging systems. Furthermore, a correction scheme to compensate for camera vibrations is discussed, together with a method to accurately identify the position of the wall within the measurement domain. Results show that approximately 80,000 tracks can be instantaneously reconstructed within the measurement volume, enabling the evaluation of both dense velocity fields, suitable for spatial gradients evaluation, and highly spatially resolved boundary layer profiles. Turbulent boundary layer profiles obtained from ensemble averaging of the STB tracks are compared to results from 2D-PIV and long-range micro particle tracking velocimetry; the comparison shows the capability of the STB approach in delivering accurate results across a wide range of scales.