Near Field Pressure and Associated Coherent Structures of Excited Jets

Abstract

Large Eddy Simulations (LES) were performed for Mach 0.9 and 1.3 cold jets to associate the structures of the shear layer with near field pressure fluctuations. The jets were excited by Localized Arc Filament Plasma Actuators (LAFPAs) arranged around the periphery of the nozzle with the axisymmetric (m = 0) mode. Excitation frequencies of St = fD/Uj = 0.05 to 0.25 (close to the column mode frequency) were computed for each Mach number. The St = 0.05 produces one pulse that propagates downstream without interacting with previously emitted pulses. This is referred to as the the impulse response. The St = 0.25 frequency exhibits subsequent pulse interactions. Simulation data for both Mach numbers was collected along three arrays at different radial locations. Strong agreement was found for the near field response to excitation and the mean center-line axial velocity between the subsonic simulations and the experiments. The experiment and simulations depict a large hydrodynamic wave downstream of the exit moving at the speed of convection near the shear layer consisting of a large peak followed by a large trough after the actuator pulse. For the highest excitation frequency, the interaction between structures yields an almost sinusoidal wave in the near field. These hydrodynamic waves are associated to the phase-averaged flow structure which includes a series of rollers and ribs and the associated dilatation field. The structure interactions from subsequent pulses results in a quasi-linear superposition of the impulse jet response (St = 0.05) to actuation. Auto-correlations and two-point correlations describe the development and interaction between adjacent structures in time and space.

Finite-Time Lyapunov Exponent Analysis of Intermittent Acoustic Events in a Round Jet

“…The passage of this structure appears to have the same effect as the phase-averaged large-scale structures studied in Ref. [32] (i.e. a modulation of the local pressure) and shown in Fig.…”

“…Measurements on 3 different rays have been described in Refs. [32,57,58]. Three representative arrays covering regions of the jet containing hydrodynamic and acoustic pressure components are shown in Fig.…”

“…For the present data, such an analysis has been presented in Ref. [32]: for ready reference, a summary is presented in Section III. The method uses phase-averaging (at the excitation frequency) to connect single and multi-point correlations to large coherent structures.…”