Experimental evidence of 2D turbulence

Soap film flows that have attracted the attention of researchers since the mid-Eighties appear to present a convenient model for laboratory studies of 2D hydrodynamics including turbulence. However, two important issues associated with turbulence in soap films have not been resolved until recently. First, are there conditions under which direct evidence of enstrophy cascade can be presented? Second, in what flow regimes can the flow be considered incompressible? To address these issues, information about the flow velocity and thickness fields had to be acquired. Previous studies of soap films were either qualitative or relied upon velocity measurements at one or several points, typically acquired by laser Doppler velocimetry (LDV). We have developed a digital particle image velocimetry (DPIV) system that makes it possible to conduct simultaneous measurements of velocity and thickness fields. Thus, vorticity field can also be acquired and correlation between velocity, vorticity and film thickness can be evaluated. Our experimental setup is based on a gravity-driven soap film tunnel. The plane of the film can be tilted at an arbitrary angle with the vertical, making it possible to produce stable films with varying mean velocity and thickness. We present instantaneous velocity, thickness and vorticity fields, as well as real space and Fourier space statistics for the flow behind a comb inserted into the flow. It is of interest that for many flow regimes thickness and vorticity fluctuations are strongly coupled. Thickness and vorticity fluctuations decay with increase in downstream distance from the comb, and there is evidence that compressibility plays a role in the flow. Statistics of energy and enstrophy are consistent with the enstrophy cascade associated with 2D turbulence, although certain cautionary remarks have to be made about the reliability of the spectra.