Methods and progress of experimental research on passive scalar turbulent mixing

Turbulent mixing plays a vital role in daily life and engineering applications. Extensive research in turbulent mixing is crucial for various subjects, including turbulent theories, turbulent combustion, and chemical engineering. For the experimental studies of turbulent mixing, the development of sensors and non-intrusive laser-based measurement techniques furthers exploring and understanding the multi-scale nonlinear process in turbulent mixing. This review focuses on the Rayleigh scattering technique and laser-induced fluorescence technique for passive scalar field measurements. Potential ways to improve the spatial resolution of laser-induced fluorescence are introduced and discussed for measurements of turbulent mixing in two dimensions. Then an experimental perspective on small-scale mixing is presented, covering topics of the scaling law of mixing in the viscous-convective regime, the probability density function of scalar fluctuation fields, and the fractal dimension of the mixing interface. This work aims to introduce fundamental experimental methods and progress on the passive scalar turbulent mixing, together with the corresponding theoretical basis.