Measurements of wavenumber-celerity spectrum in plane and axisymmetric jets

The wavenumber-convection velocity spectra W(k, U) in the near and far fields of plane and axisymmetric free air jets have been computed from digitized hot-wire data. This spectrum, which is the double Fouriertransformation of the space-time correlation R(S, T) was obtained in a 3.18 X140 cm plane jet and in a 2.54-cmdiam axisymmetric jet. For both jets the exit mean velocity Ū_e was 30 m/s. The reference (upstream) probe was located at x/d= 8 for the near-field data and at x/d=30 for the far-field data, d denoting the exit diameter or slit width. The spectra indicate that the jets are characterized by energetic large-scale structures not only in the near field (slightly downstream of the potential core) but also in the self-preserving region. The stream wise length scale of the most energetic eddies increases in proportion with that of the jet width. The stream wise length scale of these eddies is about 3 times the local jet width in the circular jet and 3.5 times the local jet width in the plane jet. The convection velocity U of the dominant eddies decreases from about O.SŪ_e to Q.5Ū_e between the two regions in the plane jet and from about 0.73Ū_e to 0.25Ū_e between the two regions in the axisymmetric jet. With increasing eddy sizes, the dispersion in the convection velocity U increases as W(k, U) becomes progressively skewed towards (/valŪ_es higher than the local time mean velocity U.