Abstract: In an experiment on a turbulent jet seeded with fluorescent dye we detect the turbulent- nonturbulent interface in a frame that moves, on average, with the interface. This significantly prolongs the observation time of scalar- and velocity structures and enables the measurement of two coherent structures. We apply the finite-time Lyapunov field (FTLE) to investigate the barriers of diffusive momentum transport. We discuss a novel approach to finding the turbulent-nonturbulent interface from the measured fluorescence field.

Conditional averages teach that the turbulent-nonturbulent interface is not associated with the barrier of diffusive momentum transport, but that there is a relation with the large-scale ridges of the FTLE. Inside the turbulent jet other interfaces exist between two regions with approximately uniform dye concentration. It appears that these turbulent-turbulent interfaces discriminate even stronger between the coherent structures.

Curriculum Vitae: My interests are turbulence and instationary flows around objects, among others. My main expertise is in the development and application of experimental methods. I have been an editor for Experiments in Fluids since 2003.