Instabilities in Near Field of Turbulent Jets and Their Dependence on Initial Conditions and Reynolds Number

Abstract

Abstract : Near-field pressure and velocity measurements, along with multiple flow visualization techniques, were obtained at several azimuthal positions to establish the effect of initial conditions and Reynolds number on the development and nature of instabilitiues in the initial region of axisymmetric free jets. In cases where the exit boundary layer is laminar, the natural jet is found to be unstable alternately to axisymmetric and helical modes having Strouhal numbers of 0.013 and 0.016, respectively. The scaling of these modes is independent of Reynolds number and initial jet disturbance level. However, the disturbance level does not determine the sensitivity of the initial region to the helical mode. When the initial axisymmetric mode grows to a finite amplitude, a subharmonic resonance mechanism that leads to pairing develops two wavelengths from the jet exit. The axisymmetric and helical modes interact nonlinearly, generating numerous sum and difference modes. With the strong growth of the subharmonic mode, a secondary set of nonlinear interactions develops between the subharmonic mode and the above modes. A strong near-field pressure is associated with each of these instability modes and is coherent with the velocity fluctuations in the jet over the entire growth and decay regions of each mode. The details of this field reveal that pairing is not responsible for a feedback mechanism. This strong near-field pressure acts as a natural excitation to the jet. When the background disturbance is sufficiently low, a natural coupling is observed at special operating conditions where the initial modes are coupled to the mode observed near the end of the potential core of the jet.