Triad interactions induced by vortex shedding in a free jet in air.

The non-linear processes caused by the convection term in the Navier-Stokes equation are of fundamental importance for both the understanding of turbulence and modelling turbulent flows. These so-called triad interactions in Fourier space were investigated by measuring a single Fourier mode injected into the initial part of a round, free jet in air. We studied the development of the measured velocity power spectrum as a function of the downstream distance. Furthermore, the development of the downstream velocity was also calculated by means of a simple, one-dimensional computer model. The comparison between the measured power spectra and the computational ones showed a good agreement between them, allowing us to draw some interesting conclusions regarding the fundamental non-linear processes in turbulence.