A dynamic masking technique for combined measurements of PIV and synthetic schlieren applied to internal gravity waves

We present a novel technique for dynamically masking images for synthetic schlieren, a recently developed technique for measuring density gradients in fluids. Synthetic schlieren uses pattern matching algorithms to determine the apparent displacement of a background pattern. In the present case this pattern is displayed on a regular LCD monitor. By changing what is displayed on the monitor, foreground features (such as particles or optical defects in the apparatus) can be identified separately from the background pattern. The influence of these features can then be locally masked out from the synthetic schlieren calculations. The technique also has the benefit of enabling the simultaneous use of particle image velocimetry or particle tracking velocimetry, and hence it may produce combined measurements of both density gradients and velocity. The masking techniques introduced here significantly reduce the errors in the measured density gradient that would otherwise be introduced by the presence of particles in the flow. The mask itself will introduce errors in the pattern matching, but we show that these are smaller than the benefits.