Studying vorticity with mirror-embedded particles

The vorticity of a turbulent flow plays a key role in the flow’s dynamics. For many fluidic applications, a better understanding of vorticity is important but limited by current experimental techniques. In a new paper, Wu et al. expand on direct vorticity measurement techniques by encapsulating small mirrors in transparent microcapsules.

By using high-speed cameras to track the norm vector reflected by the mirrors, the authors were able to determine the trajectory of the microcapsules in a turbulent fluid. A concave mirror is placed ahead of the camera to converge all parallel beams and eliminate shifts in the beam reflection caused by particle translations, ensuring all movements indicated by the light spots are due to the rotation of the particles.

The researchers developed an algorithm that uses three measurements of the norm vector and a mapping of reflection spots to reconstruct the particle’s motion. The angular velocity of the particles and the vorticity of the flow can then be extracted from the trajectory data.

For the method to work, the microcapsules must be sufficiently small in order to prevent the particle motion from being influenced by strain. Additionally, the capsules must have the same density and refractive index as the fluid used – in this case, water. The authors demonstrated the method on Taylor-Couette flow, in which a fluid is confined between two concentric cylinders.

Compared to traditional techniques, this method of studying vorticity via particle tracking provides high spatial and temporal resolution.

Next, the group plans to integrate the method with particle tracking, so particle translation and rotation information can be obtained simultaneously.

Source: “Measuring vorticity vector from the spinning of micro-sized mirror-encapsulated spherical particles in the flow,” by Huixuan Wu, Haitao Xu, and Eberhard Bodenschatz, Review of Scientific Instruments (2019). The article can be accessed at https://doi.org/10.1063/1.5121016 .