Dynamic triade interactions by dual vortex shedding in a laminar jet*

Interactions between Fourier modes, so-called triad interactions, are widely seen as fundamental components in the physical process of turbulence. In this experiment, we study the triad interactions initiated by a single or a pair of near-sinusoidal eddies generated by vortex shedding from rods inserted into the core of a laminar jet in air. We measure the interactions as they develop downstream and plot the spectral components as they are created by the nonlinear effect of the convection. We analyze the combinations of frequency components and their cascading into higher frequency components and compare to the results of a simulation of Navier Stokes equation operating on the velocity signal measured close to the rods.

We relate our results to previous and current assumptions regarding the efficiency of various types of triad interactions whether they be local between wavenumbers of near equal size or whether they take place between modes far apart. We find that energy can couple efficiently both towards higher frequency modes and from high frequency modes to small difference modes, however with the well-known tendency for energy to couple predominantly towards high frequency. Our measurements also allow us to estimate the time constants in the cascade process.