Adaptive Control of Axisymmetric Jets by Cavities

Actuators capable of producing large amplitude oscillations at desired frequencies are needed in many flow control applications. In this dissertation, turbulent jets were actuated using self-sustaining oscillations in axisymmetric cavities. Tested geometries include baseline turbulent jets passing through axisymmetric cavities, with and without peripheral walls. Cavity length to depth ratio (Lc/Dc) was varied from 1 to 5 and data were collected at several intermediate steps. Tests were performed with air over a Reynolds number, based on jet exit diameter, range of 40,000 to 225,000 in low to high subsonic jet flow conditions. Pressure signals, 2D velocity data obtained using Particle Image Velocimetry (PIV) and 2D axisymmetric Computational Fluid Dynamics (CFD) modeling were concurrently used to analyze and understand the problem. Measurements on baseline turbulent jets showed that the near field coherent structures and their residence time are two important factors that could be manipulated to influence the initial growth rate of a jet. Frequency of oscillation of the pressure field within the cavity was primarily dependent on the length scale (length to depth ratio and the pipe diameter) of the cavity and the Mach number of the flow. Axisymmetric cavities with cavity length to depth ratio 1.5 –2.0, preferably 1.75, placed immediately after the exit of the jet exhibited a resonant condition between the axial shallow cavity mode and the radial acoustic mode of the pipe) with very high amplitude oscillations (in excess of 180 dB within the cavity) at moderate to high Reynolds number range of this study. Comparison with baseline jets based on centerline velocity decay and lateral jet spread rate showed that mixing characteristics had improved significantly. The potential core length (location of 90% jet exit velocity) for the best case was at 1D as compared to 5D for the baseline jet. Ensemble averages of different phases of an oscillation cycle (along the jet flow direction and in the cross-section) were used to recreate the oscillation cycle (flow visualization images, velocity and vorticity fields) and study the flow structures in the near field. Mode of oscillation (axial / helical) of the jet exiting the cavity was found to be dependent on the inlet and exit boundary conditions of the cavity. Cavities with sharp orifices at the inlet and exit boundaries had complex helical mode structures in the near field of the jet. When the exit boundary of the cavity had a short pipe instead of a sharp orifice, only axial mode vortex rings remained in the near field. However the jet entrainment was not as high, as in the case of sharp orifices