An experimental study of the response of nearly isotropic turbulence to a spectrally local disturbance

A grid-generated, nearly isotropic turbulent flow was subjected to a spectrally local disturbance whose scale was close to but larger than the Kolmogorov scale. A nearly sinusoidal ripple in the velocity was introduced by placing a low-solidity screen of very fine wires (called a ‘zither’ by Kellogg & Corrsin 1980) downstream of a classical biplane grid. Measurements of the fluctuating velocity components normal to the screen wires show strong spatial oscillations of the second-order moments. Close to the screen the flow is quite inhomogeneous and anisotropic, but relaxes toward isotropy. After a short period of initial growth, the longitudinal one-dimensional spectrum E11(k1) exhibits an unusual decay behaviour, especially after the Reynolds stress created by the mean shear of the boundary layers and ‘laminar’ wakes of the screen wires has vanished. Bispectra and one-dimensional transfer terms do not show evidence of any preferred interaction between longitudinal wavenumbers, but an increased energy transfer to large wavenumbers is observed at small x1/d. The resulting increase of energy in the high-wavenumber end of the spectra is consistent with results of Townsend (1951). An energy budget downstream of the zither shows that the pressure–velocity correlations are important in the nonlinear interactions.