An experimental analysis of the modal characteristics intrinsic to both the heated and cold jet

An experimental investigation of two ’acoustically matched’ Mach 0.6 jets, at temperature ratios of Tr=0.93 and Tr=1.7, is conducted to identify distinctions in ∞ow dynamics and contribution to the acoustic far-fleld spectrum. The ∞uctuating pressure fleld, sampled near the exit of the jet(x=D=2, r=D ’1), is shown to sense the dominant ∞ow features of the velocity fleld. The core region in the hot jet is seen to collapse at a more rapid rate, resulting in a shorter potential core length and larger ’high-speed’ shear layer thickness. The turbulence levels of the hot jet are also shown to increase. The hot jet exhibits an increase in OASPL at nearly all observer angles from `=15 o -75 o , largest increase of 2dB seen at `=15 o . There is a decease of nearly a half dB seen at `=90 o , consistent with the increase in the acoustic spectrum at low frequency, and decrease at high frequency seen at all locations. Directional dependency of the acoustic spectrum with frequency is highlighted. A snapshot POD analysis reveals the presence of larger scaled, more energetic structures in the hot jet, as well as a dominant Fourier-column mode-like structure, and a Fourier-helical mode-like structure in the cold jet. The continued environmental impact of aircraft engine exhaust noise has fueled research efiorts towards developing techniques aimed at controlling and/or mitigating this sound source. The primary mechanism of this noise can be linked to the turbulent mixing of the exhausted jet plume with the ambient air; thus creating pressure ∞uctuations in the near-fleld and acoustic-far fleld regions of the jet ∞ow. As such, numerous investigations have been conducted aimed at characterizing the near-fleld region of the turbulent axisymmetric jet. Much of this interest has been directed toward the identiflcation of turbulent coherent structures as sources of noise, and the mechanism by which it propagates to the far-fleld as acoustic energy. The downstream development and resultant pressure fleld generated near the jet exit, can be distinctly difierent based on nozzle exit geometry and ∞ow properties. This aeroacoustic efiect has been shown to be altered, particularly, at elevated ∞ow temperatures, at which realistic engine cycles occur. This important practical component is examined in an investigation of the aeroacoustic sources of an ’acoustically matched’ Mach 0.6 jet, at temperature ratios of Tr=0.93 and Tr=1.7, thus isolating the efiect of temperature in identifying the distinctions in contribution to the acoustic far-fleld spectrum. The turbulent motion of these shear ∞ows has long proved challenging to researchers in their attempts to ascertain ’order’ from what is seemingly ’chaos’. On the heels of the work of Sir James Lighthill, providing a description of the ’source’ by which sound is generated aerodynamically, 1 and Ffowc Williams & Kempton 2 reporting that jet noise production was biased towards the contribution of large-scale turbulence, many groups set out to identify orderly patterns in the ∞ow as ’sources’ of sound, in hopes of controlling it. The early work of researchers such as Crow & Champagne, 3 Lee & Ribner, 5 and Lau et al. 4 have contributed to much of our current understanding of the near-fleld region of the axisymmetric jet. Flow visualization aided in highlighting the presence of similar orderly patterns within the turbulent shear layer, and two-point