Los Alamos National Lab
Los Alamos National, New Mexico, U. S. A.
Soap film flows that have attracted the attention of researchers since the mid-Eighties appear to present a convenient model for laboratory studies of two-dimensional (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, vorticity and thickness fields. These measurements yield the turbulent energy and enstrophy of the 2D flow with a microscale Reynolds number close to 100. We quantify the effective compressibility arising from variation in the film thickness. Despite the non-negligible influence of the compressibility, real-space correlations of velocity, vorticity and vorticity flux are consistent with theoretical predictions for 2D turbulence.