Kerwin R. Low

Properties of the far-field pressure signatures of individual jet noise sources

The detection of intermittent near-field noise sources is achieved by continuous wavelet transform of far-field acoustic data. The similarities between signals from three far-field microphones in the annular cone of acoustic radiation from coherent sources in a Ma = 0.6 cold jet are used to identify some of the loudest events. Cross-correlation of relevant band-passed excerpts identifies the most reliable matches, which occur at the rate of one every 20 dimensionless time units on average, and are intermittent in both time and frequency with no obvious pattern. Processing our database produced a catalog of over 9500 individual events with quantitative properties including the magnitude, frequency and time of detection at each microphone. The differences in detection times (lags) are distributed smoothly around the peak of cross-correlations for the raw data. We show how source magnitude and frequency depends on lags, and relate the lags to their relative source location.

Correlations and wavelet based analysis of near-field and far-field pressure of a controlled high speed jet

Discussed within are the experimental results obtained via simultaneous near-field and far-field measurements of a controlled jet. A large database of both open-loop and closedloop control cases are carried out. The two most interesting investigations from the openloop (OL) and closed-loop (CL) studies are highlighted here and compared to the uncontrolled jet. For the open-loop forcing, the shear layer is seeded with an axisymmetric sinusoidal input with a frequency of 1700 Hz. For the closed-loop case, the shear layer is seeded with a sinusoidal forcing amplitude modulated with the mode-filtered near-field pressure sampled from the stream-wise location at x/D = 6. Data analysis looks at both averaged and instantaneous quantities. In an averaged sense the introduction of control significantly modifies the characteristics of the developing shear layer. Open-loop control is shown to modify the phase lag between both near-field stations with a penalty in an increase in the overall sound pressure level. The closed-loop control imparts more subtle changes to the flow field but yields a slight reduction in the acoustic pressure at the microphone closest to the jet axis. Wavelet based filtering exposes the evolution of the broadband intermittent events seen at both stations.

Investigation of different active flow control strategies for high speed jets using synthetic jet actuators

We present in this paper a set of experiments geared towards characterizing the system response of the near fleld jet shear layer to difierent modes of forcing. Several open and closed loop control tests were conducted. The open loop control cases included simple sinusoidal forcing (with varying coe‐cient of momentum and frequency), phased forcing and amplitude modulated forcing. The closed loop cases included feeding back the Fourier flltered signals from 3 diameters and 6 diameters downstream. The correlations between the near fleld Fourier flltered pressure modes and the far fleld noise are changed signiflcantly for all control cases; demonstrating the ability to efiect control authority in the near fleld region. An examination of the far fleld noise spectra, however, demonstrates only minor changes from the control.

Reduced Order Models for a High Speed Jet with Time-Resolved PIV

In the current investigation, we examine a turbulent, compressible jet flow field in the high subsonic range (Mach 0.85). Experiments take place in the fully anechoic jet facility at Syracuse University, where the flow field is probed in order to measure near-field hydrodynamic pressure, far-field acoustic sound, and near-field velocity. High resolution 10 kHz time-resolved particle image velocimetry (TRPIV) is implemented to gain better insight into the structures formed in the region of the collapse of the potential core of the jet. By exploring these structures in conjunction with the near and far-field pressure, low-dimensional modeling techniques are implemented. With such techniques as proper orthogonal decomposition (POD) and observable inferred decomposition (OID), we seek to gain a better understanding of how jet noise created in the near-field propagates downstream, and how control can be implemented accordingly using such approaches. It has been found that through low-dimensional modeling techniques, “loud” modes in the flow have been identified, which will be utilized through a closed-loop control methodology.

Investigation of an axisymmetric transonic jet with high resolution time-resolved PIV

In the current investigation, we examine a turbulent, compressible jet flow field at Mach 1.0. In the fully anechoic jet facility at Syracuse University, the flow field is probed in order to measure near-field hydrodynamic pressure, far-field acoustic sound, and nearfield velocity. High resolution 10 kHz time-resolved particle image velocimetry (TRPIV) is implemented to gain better insight into the structures formed in the near region of the jet. By exploring those structures present just after the collapse of the potential core, in conjunction with the near and far-field pressure, low-dimensional modeling techniques are implemented in the form of proper orthogonal decomposition (POD). With these lowdimensional models, we seek to gain a better understanding of how jet noise created in the near-field propagates downstream, and how control can be implemented accordingly using such approaches.

Closed-loop control of a turbulent compressible Mach 0.6 Jet

column mode ) excitation, and a closed loop mode�1 ( helical mode ) excitation. A spatial Fourier azimuthal decomposition of the nearfield hydrodynamic pressure at 8D in the stream wise is used for state estimation and feedback. The farfield sound was measured with six microphones oriented along a boom at increasing polar angles from φ= 15°�90° at 75D downstream to evaluate the eof each controlled case. For φ = 15° (shallow angle), an overall increase in the sound at lower Strouhal numbers (0.04 - 0.2) of the jet was noticed for all controlled cases. The greatest reduction was noticed for the closed loop mode�0 case. Conversely, an overall dec rease in the sound was noticed at higher jet Strouhal numbers (0.2 - 3.7). At φ = 60° and 90°, there was an overall increase in far� field sound for all controlled cases.