Unexcited Jet Research Articles

Coherence between large-scale jet-mixing structure and its pressure field

A schlieren system has been arranged to sense the total fluctuation over a cross-section of the flow and thus becomes very sensitive to large-scale azimuthally coherent structures in the flow. For a natural unexcited jet it is found that there is a concentration of the large-scale structure at a characteristic Strouhal number which is not sensitive to the beam thickness and which reduces progressively with distance from the nozzle. This large-scale structure exhibits a coherence of over 70 % with the near-field pressure and convects at between 75 % and 95 % of the jet velocity. The coherence between the potential core-pressure field and the large-scale structure downstream increases rapidly with distance from the nozzle exit plane, rather limited coherence being found at the exit plane for these observations at a jet-exit Mach number Mj = 0·7. Movement of a central microphone from x = 0 to x = 2D introduced a solid centre body over the first 2·5 diameters of flow and gave rise to a set of discrete components in the flow structure in the range 0.6 < S < 1·4.With harmonic excitation at S = 1·12 a subharmonic at S = 0·55 occurs at x/D = 3 and a second at S = 0.26, x/D = 6. The flow cross-sectional-average sensing thus appears to show up the vortex-pairing mechanism at greater distances from the nozzle than is easily detectable by other means. Under strong impulse excitation a set of discrete components was observed in a transient response extending over times of 400D/Uj. These had a strongest component which decreases more rapidly in Strouhal number with distance than that associated with natural or harmonically excited conditions.

The self-excited axisymmetric jet

The characteristics of a self-excited axisymmetric air jet driven by a whistler (i.e. pipe-collar) nozzle have been explored experimentally for various choices of the controlling parameters: namely, pipe length, jet diameter, collar length, step height, and jet speed. By appropriate choices of these parameters as well as the stage and mode (half- and full-wave), the self-sustained excitation has been induced at specific values of the excitation amplitude and the Reynolds number RD. The jet characteristics up to RD ≃ 3·1 × 105 and x/D ≃ 60 have been documented for both laminar and turbulent flows at the pipe exit. Comparison with corresponding unexcited-jet data reveals that self-excitation produces a large increase in the fluctuation intensity in the near field of the jet, while it increases the jet spread and decay rate for the entire x-range of measurement. The dependence of the jet structure on the initial condition is stronger when self-excited than when unexcited. The first stage of excitation always produces the highest turbulence augmentation and the spectral evolution is significantly modified by self-excitation up to x/D ≃ 6. The excitation produces a significant increase in the broad-band turbulence level over that of the unexcited jet. The broad-band amplification is maximized at x/D ≃ 4 and is the highest at the largest RD studied.These data suggest interesting possibilities for the self-excited jet in the augmentation or control of entrainment, mixing and aerodynamic noise production.

Simulation of Instabilities and Sound Radiation in a Jet

The inflow and near field of a jet that is excited by an axial mass source located in the potential core are simulated numerically. The simulation takes into account the spreading of the jet. Comparison is made with experimental results for both excited and unexcited jets. It is shown that many of the features observed experimentally are due to the instability of the mean profile (i.e., the large-scale structures) and not due to the turbulence. The instability is shown to generate low-frequency sound. The terms identified by Ribner as the shear noise terms are shown to be responsible for this sound generation.

Vortex pairing in a circular jet under controlled excitation. Part 1. General jet response

Hot-wire and flow-visualization studies have been carried out in three air jets subjected to pure-tone acoustic excitation, and the instability, vortex roll-up and transition as well as jet response to the controlled excitation have been investigated. The centreline fluctuation intensity can be enhanced by inducing stable vortex pairing to a level much higher than even that at the ‘preferred mode’, but can also be suppressed below the unexcited level under certain conditions of excitation. The conditions most favourable to vortex pairing were determined as a function of the excitation Strouhal number, the Reynolds number (ReD), and the initial shear-layer state, i.e. laminar or turbulent. It is shown that the rolled-up vortex rings undergo pairing under two distinct conditions of excitation: ‘the shear layer mode’ when the Strouhal number based on the initial shear-layer momentum thickness (Stθ) is about 0·012, and ‘the jet column mode’ when the Strouhal number based on the jet diameter (StD) is about 0·85. The former involves pairing of the near-exit thin vortex rings when the initial boundary layer is laminar, irrespective of the value of StD. The latter involves pairing of the thick vortex rings at x/D ≅ 1·75, irrespective of Stθ or whether the initial boundary layer is laminar or turbulent. For laminar exit boundary layer, pairing is found to be stable, i.e., occurring regularly in space and time, for ReD < 5 × 104, but becomes intermittent with increasing ReD or fluctuation intensity in the initial boundary layer.The trajectories of the vortex centres and their convection velocities during a pairing event have been recorded through phase-locked measurements. In the presence of stable vortex pairing, the time average profiles of fluctuation intensities and Reynolds stress show noticeable deviations from those in the unexcited jet. The vortex pairing phenomenon produce considerably larger excursions of the $\widetilde{uv}(t)$ signal than the time-average Reynolds stress reveals, suggesting that only certain phases of the pairing process may be important in entrainment, and production of Reynolds stress and jet noise.