André M. Hall

A time-resolved estimate of the turbulence and sound source mechanisms in a subsonic jet flow

A dynamical estimate of the axial component of a Mach 0.60 axisymmetric jets turbulent velocity field is presented here using spectral linear stochastic estimation. The pressure field surrounding the exit of the jet is employed as the unconditional parameter in the estimation technique. A sub-grid interpolation method is used to improve the spatial resolution of the estimate. The model estimate is time-resolved and reconstructed using a purely experimental database. A decomposition of the model estimate using POD and Fourier-azimuthal techniques identifies the turbulent velocity modes that are responsible for driving the near-field pressure when compared with direct measurements of the jets modal features. In effect, the signatures left in the near pressure field by the turbulence are a result of the low-order structure, the higher azimuthal modes being inefficient in driving the hydrodynamic pressure. A direct calculation of the source field using a Lighthill approach is performed, from which the low-d...

TWO-POINT CORRELATIONS OF THE NEAR AND FAR-FIELD PRESSURE IN A TRANSONIC JET

To better understand the relationship between the near-field pressure and the acoustic far-field, multi-point measurements of the near-field pressure around the periphery of a cold Mach 0.85 round jet are compared to simultaneous multi-point farfield acoustic pressure measurements. The results indicate that the near-field pressure is low dimensional and the instantaneous contribution from both azimuthal mode 0 and 1 is sufficient to accurately recover the dynamics of the near-field pressure. Correlations of the acoustic far-field with the contribution of each azimuthal mode to the near field pressure, however, indicate that only azimuthal mode 0 is well correlated with the far-field pressure, suggesting that the acoustic source in the jet is predominantly axisymmetric. The correlation of the higher azimuthal pressure modes with the far-field acoustic pressure is extremely poor suggesting that the axisymmetric source is weakened by the presence of higher azimuthal modes in the near field of the jet.

Cross-Spectral Analysis of the Pressure in a Mach 0.85 Turbulent Jet

Measurements of the near-field pressure in an unheatedMach 0.85 round jet were performed simultaneously with far-field acoustic pressure measurements at Syracuse University’s Skytop Anechoic Chamber facility to directly quantify the strength and frequency content of the propagating portion of the jet’s near-field pressure. The largest contributions were determined to be at the lower wave numbers commonly associated with hydrodynamic pressure fluctuations. This indicates that large low-frequency hydrodynamic fluctuations are obscuring a significant source of acoustic fluctuations in the jet and dictates that care must be taken when using single point measurements of nearfield pressure spectral decay to differentiate between acoustic (propagating) and hydrodynamic (nonpropagating) fluctuations.

Experimental Investigation of the Pressure-Velocity Correlation of a M=0.6 Axisymmetric Jet

A quantitative measure of the strength of the pressure-velocity correlation of a Mach 0.6, axisymmetric jet, with an exit nozzle diameter of 50.8mm is examined. The exit ∞ow temperature is held constant at a temperature of 25 o C, and is pressure and temperature balanced with ambient conditions. The ∞uctuating pressure fleld is sampled by an azimuthal array of (15) dynamic transducers, evenly spaced at 24 o . These are held flxed and positioned just outside the shear layer near the jet exit at z/D=0.875, and 1.75R from the centerline, where the pressure fleld has been shown to be hydrodynamic. The instantaneous velocity measurements are simultaneously acquired using a multi-component LDA system who’s measurement volume is traversed along several radial and streamwise locations within the potential core, and mixing layer regions of the ∞ow. From this multi-point evaluation, the cross-correlation between the near-fleld pressure array, and streamwise component of the velocity fleld are examined as a function of radial, streamwise, and also azimuthal separation. The results illustrate a coherence on the order of 25% between the near fleld pressure and the velocity fleld. Analysis of the coherency spectra illustrates the frequency band of the correlations and suggests that the potential core and mixing layer regions of the ∞ow are, in general, governed by the high and low frequency motions of the ∞ow, respectively. The azimuthal modal distribution of the cross-correlation shows the dominance of the column mode of the jet, with no higher modes exhibited within the potential core region, and only modes 1 & 2 within the shear layer.

Low-dimensional signatures of the sound production mechanisms in subsonic jets: Towards their identification and control

Neareld pressure and turbulence velocity data from measurements performed in subsonic jets with Mach numbers of 0.3 and 0.85 are analysed in the context of feedback control of jet noise. The low-dimensional building-blocks of the pressure and velocity elds are extracted, by means of joint Proper-Orthogonal, Fourier- azimuthal Decompositions, and studied with a view to understanding their respective roles in the production of jet noise. These building blocks exhibit many of the characteristics commonly associated with instability waves in forced jets, and it is thus possible to identify their pertinent properties where the production of sound is concerned, properties which will serve as input for an eventual feedback controller. for a reduced order description of complex dynamical systems. At both Syracuse and Poitiers these techniques have recently been applied to free jets with the focus, traditionally turned towards the dynamic of the turbulence, now shifted to consider the mechanism by which this turbulence generates sound. This is a highly complex problem, the science of which, although now over fty years old, has struggled to come up with reliable modelling and reduction strategies. The crux of the problem is to be found in the fact that a subsonic jet constitutes an acoustic source with an efcienc y the order of 0.01 . So, we are faced with a problem where the dynamic we must understand and learn to control is a random function of space and time and has an efcienc y of approximately zero! All however is perhaps not lost. We know that free jets present a certain underlying determinism - the coherent structures, the Kelvin Helmholtz instability, the structured azimuthal modes. These phenomena underlie the global turbulence characteristics of the o w, and have been shown to be important in the sound generation process. If their dynamic, and that of the mechanism by which they produce sound can be understood and modelled, then feedback control of this component of the jet noise may be feasible. The experiments described in this paper aim to do just that. Work has been focused on ascertaining the relationship between the deterministic component of the o w and its sound

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

Identifying the most energetic modes of the pressure near-field region of a Mach 0.85 axisymmetric jet

The near-field pressure region of a Mach 0.85 axisymmetric jet with an exit nozzle diameter of 50.8mm, is examined experimentally using an azimuthal array of 15 equidistantly spaced (24) transducers positioned just outside the jet shear layer. The exit flow temperature is held constant at a temperature of 27C, and is pressure and temperature balanced with ambient conditions. The transducer array is traversed downstream through the end of the potential core. Examination of the Fourier-azimuthal decomposition reveals the presence of only the first three modes (0, 1, &2), with a downstream contribution of lowfrequency energy only. A second set of experiments fixes 7 transducers, spaced at (48) in azimuth, near the jet exit at z/D=0.875. A separate azimuthal array is then fitted with another 7 transducers, positioned in similar fashion, and again traversed downstream through the end of the potential core. The cross-correlations between the array fixed at the jet lip and the downstream array, exhibit magnitudes on the order of 60%, extending through the end of the potential core where the correlation falls to 20%. The modal decomposition of the cross-correlations suggest a column mode dominance. The decay in magnitude is in direct relation with the decay in the contribution of mode-0.

A spatio-temporal decomposition of the acoustic source in a mach 0.85 jet

Multi-point measurements of the near-field pressure around the periphery of a cold Mach 0.85 round jet were performed simultaneously with far-field acoustic pressure measurements to examine the spatial and temporal character of the acoustic source in the jet. A Fourier decomposition of the near-field pressure indicates that the majority of the energy is relatively evenly split between azimuthal modes 0 and 1. Only azimuthal mode 0, however, made a significant contribution to the near-field/far-field pressure cross-spectrum indicating that the acoustic source in the jet is axisymmetric. The largest contribution of azimuthal mode 0 was for low wavenumbers commonly associated with hydrodynamic pressure fluctuations, indicating that large, low frequency hydrodynamic fluctuations are hiding the dominant acoustic source in the jet.

A time-resolved estimate of the turbulence and source mechanisms in a subsonic jet flow

The primary focus of this paper is to highlight the research activities that have been underway at two research institutions: (1) Syracuse University, in Syracuse, New York, USA, and (2) Laboratoire d’Etudes Aérodynamiques, in Poitiers, France. In recent years, a subset of the research activities at these institutions have been rooted in developing lowdimensional turbulence models to improve our existing understanding of the sound source mechanisms in high speed jet flows. The investigation discussed here involves a pressurefiltered volume reconstruction of the turbulence and source mechanisms of a Mach 0.60 jet. The model is time-resolved and is reconstructed using a purely experimental database.

An experimental investigation of the pressure-velocity cross-correlation in an axisymmetric jet

This study investigates the strength of the pressure-velocity correlations of a Mach 0.6, axisymmetric jet, with an exit nozzle diameter of 50.8mm. Experiments are conducted at a constant exit temperature of 25°C, and exit pressure and temperature are balanced with ambient conditions. The instantaneous velocity measurements are acquired using a multi-component LDA system who’s measurement volume is traversed along several radial and streamwise locations within the potential core, and mixing layer regions of the flow. The fluctuating lip pressure is simultaneously sampled by an azimuthal array of (15) dynamic transducers, evenly spaced at 24°. These are positioned just outside the shear layer near the jet exit at z/D = 0.875, and 1.75R from the centerline, where the pressure field has been shown to be hydrodynamic. From this multi-point evaluation, the cross-correlations between the near-field pressure array (fixed), and streamwise component of the velocity field (traversed) are examined as a function of radial, streamwise, and also azimuthal separation. The results illustrate a remarkable coherence between the near field pressure and the velocity field, on the order of 25%. Streamwise convection velocities of 0.77Uj and 0.73Uj are calculated within the potential core and shear layer, respectively. Analysis of the coherency spectra illustrates the frequency band of the correlations and suggest that the potential core and mixing layer regions of the flow are, in general, governed by the high and low frequency motions of the flow, respectively. The azimuthal modal distribution of the cross-correlation shows the dominance of the column mode of the jet, with no higher modes exhibited within the potential core region, and only modes 1 & 2 within the shear layer.Copyright