Simultaneous synthetic schlieren and PIV measurements for internal solitary waves

Large-amplitude internal solitary waves in a stratification comprising a thick, lower, homogeneous layer separated from a thin, upper, homogeneous layer by a broad gradient region are studied using simultaneous measurements of the density and velocity fields. Density field measurements are achieved through synthetic schlieren, operating in an absolute mode to allow efficient and accurate measurements of density in systems with strong curvatures and large perturbations to the density field. The images used for these density measurements are interleaved with images used for particle image velocimetry by phase locking two video cameras (one configured for the density measurements and the other for the velocity measurements) with a computer-driven LCD monitor, allowing the background texture required for synthetic schlieren to be turned off for the particle image velocimetry measurements on the mid-plane of the experimental tank. The simultaneous measurements of both density and velocity fields not only allow greater insight into the internal wave dynamics, but also allow the velocity measurements to be corrected for the normal errors associated with the refractive index variations. As an illustration of the power of this technique, we determine for the first time in an internal solitary wave the spatial structure of the local gradient Richardson number, finding regions where this falls below the limit for linear stability.