Experimental investigation of the turbulent cascade development by injection of single large‑scale Fourier modes

The current work presents an experimental investigation of the dynamic interactions between flow scales caused by repeated actions of the nonlinear term of the Navier–Stokes equation. Injecting a narrow band oscillation, representing a single Fourier mode, into a round jet flow allows the measurement of the downstream generation and development of higher harmonic spectral components and to measure when these components are eventually absorbed into fully developed turbulence. Furthermore, the dynamic evolution of the measured power spectra observed corresponds well to the measured cascaded delays reported by others. Closely matching spectral development and cascade delays have also been derived directly from a one-dimensional solution of the Navier–Stokes equation described in a companion paper. The results in the current work provide vital information about how initial conditions influence the development of the shape of the spectrum and about the extent of the timescales in the triad interaction process, which should be of significance to turbulence modelers