ABSTRACT
Particle image velocimetry (PIV) and other optical methods have been rapidly developed and advanced during the last two decades (1990s and 2000s), which have provided other means of access to the fundamental properties of turbulence that include assessment of the velocity gradient tensor (full or some components) and determination of the vorticity field (full vector or some of the components). This chapter briefly describes and highlights these remarkable developments and points out some of the most important things about the emerging methodology of turbulence measurements and the discovery of important features of coherent structures that develop in anisotropic turbulence due to shear, stratification, rotation, or electromotive forces. The essence of this methodology may be best expressed by the following citation:
And this experiment you will make with a square glass vessel, keeping your eye at about the center of one of these walls; and in the boiling water with small movement you may drop a few grains of panic-grass because by means of the movement of these grains you can quickly know the movement of the water that carries them with it. And from this experiment you will be able to proceed to investigate many beautiful movements which result from one element penetrating into another (i.e., ‘air’ into water). (Leonardo Da Vinci)
One of the most intriguing problems in measuring turbulent flows is determination of vorticity. Besides the multiple arrays probes that enable to assess the velocity gradient tensor and consequently strain tensor and vorticity by computing finite differences of the appropriate velocities at neighboring points measured by various arrays placed at different span-wise and lateral locations as explained in Chapter 33, other methods have been suggested. In particular, specially constructed Potential Difference Probe allows determining the vorticity component directed along the magnetic field. This approach has its roots in the field of Magnetohydrodynamics (MHD) and can be used in water when the latter is slightly salted.
