Turbulent jets with controlled initial conditions

Using carefully controlled initial conditions of varying intensity and spectral energy distribution, the sensitivity and nature of instabilities in the near region of axisymmetric free jets was examined. Simultaneous velocity and near-field pressure measurements revealed that the jet is initially unstable to axisymmetric (m = 0) and helical (m = + or - 1) modes. The occurrence of these modes is independent of initial conditions, but the initial disturbance characteristics determine the relative occurrence of the helical mode. A strong near-field pressure is found to be associated with each of these modes as well as their interactive modes and to be highly coherent with the velocity fluctuations in the jet.