Eddy Convection in Developing Heated Supersonic Jets

An experimental study of the spatial distribution of integral eddy speeds in heated, supersonic free jets is presented. The physics of such jets are of immediate interest to noise-reduction approaches for tactical aircraft jet noise. In these flows, the noise produced has a strong influence from eddy speeds, particularly for radiation in peak noise directions, due to Mach wave contributions. The measurements obtained using a time-resolved Doppler global velocimetry instrument experimentally indicate a key mechanism of the role of heating in jet noise reduction relative to the classical Lighthill scaling. Measurements are presented in jets issuing from an axisymmetric biconic nozzle at two conditions: total temperatures of 1.6 and 2.0 times that of the ambient and both at approximately diameter Reynolds number and pressure-matched exit conditions. For the higher total temperature case, the scaled streamwise eddy convective velocities within the shear layer were measurably reduced compared to the lower total temperature case, especially as the axial distance from the nozzle exit increased. These findings confirm that core plume heating results in additional shear layer spreading and reduction of scaled eddy convective speeds, making acoustic radiation less efficient with heating. To accompany the eddy convection results, data obtained in the same facility using laser Doppler velocimetry are analyzed for second- and fourth-order moments, indicating the importance of the fourth-order correlation in the region of immediate interface between the potential core and the shear layer.