SHEAR FLUCTUATIONS IN A TURBULENT JET SHEAR-LAYER

The initial development of turbulence in the shear layer of a circular jet is observed to show disturbances of increasing scale at discrete frequencies in an approximate 4:2:1 sequence. These are completely confined within the laminar shear layer and convect at 0·59 of the jet velocity. Such relatively regular behaviour was not observed once the disturbances became larger in transverse dimension than the laminar shear layer. In the development of the subsequent turbulent shear layer it was found that the fluctuating turbulent shear did not scale with local mean shear, but rather scaled more closely with the minus half power of distance from the apparent origin of the laminar shear layer emanating from the nozzle. The scale of the shear fluctuations departed from growth in proportion to shear layer thickness, remaining almost constant. In these aspects it appears that the turbulent shear layer is not well described by a similar growth with axial distance. The shear fluctuations convected at a speed closer to the local mean velocity than do velocity fluctuations and showed a relatively more patchy distribution with a distinct rotational sense and no reversal of skewness across the layer. Velocity fluctuations showed phase lags approaching [formula ommited] relative to shear fluctuations suggesting that dominant velocity disturbances are those associated with entrainment behind concentrations of rotation. © 1979, Cambridge University Press. All rights reserved.