The implementation of advanced low-emission aircraft engine technologies and the reduction of noise from airframe, fan, and jet exhaust have made noise contributions from an engine core increasingly important. Therefore, meeting future ambitious noise-reduction goals requires the consideration of engine-core noise. This article reviews progress on the fundamental understanding, experimental analysis, and modeling of engine-core noise; addresses limitations of current techniques; and identifies opportunities for future research. After identifying core-noise contributions from the combustor, turbomachinery, nozzles, and jet exhaust, they are examined in detail. Contributions from direct combustion noise, originating from unsteady combustion, and indirect combustion noise, resulting from the interaction of flow-field perturbations with mean-flow variations in turbine stages and nozzles, are analyzed. A new indirect noise-source contribution arising from mixture inhomogeneities is identified by extending the ...

Combustion and Engine-Core Noise

Vibration in the microgravity environment is with the characteristics of low frequency, small amplitude, and randomness. Control method of an active vibration isolation system with parallel mechanism applied to space application, which is effective for disturbance suppression, is proposed. The dynamics model of active vibration isolation system with payload is represented via Kane’s method, thereafter the description in state-space linearization is introduced. System properties and step responses of the systems in open loop are evaluated in detail. Controllability and observability of the system are checked by state-space equations of the system. The state feedback decoupling with double-loop proportional-integral-derivative (PID) control method is adopted as the system controller to design the decoupling matrix and the PID controller. For improving the properties of the system, a control system with H-infinity method is also designed and evaluated. Finally, various types of simulation results are demonstrated to verify the effectiveness of the active vibration damping system proposed.

Dynamics and control of a parallel mechanism for active vibration isolation in space station

The core noise components of a dual spool turbofan engine were separated by the use of a coherence function. A source location technique based on adjusting the time delay between the combustor pressure sensor signal and the far-field microphone signal to maximize the coherence and remove as much variation of the phase angle with frequency as possible was used. The discovery was made that for the 130o microphone a 90.027 ms time shift worked best for the frequency band from 0 to 200 Hz while a 86.975 ms time shift worked best for the frequency band from 200 to 400 Hz. Hence, the 0 to 200 Hz band signal took more time than the 200 to 400 Hz band signal to travel the same distance. This suggests the 0 to 200 Hz coherent cross spectral density band is partly due to indirect combustion noise attributed to entropy fluctuations, which travel at the flow velocity, interacting with the turbine. The signal in the 200 to 400 Hz frequency band is attributed mostly to direct combustion noise. Results are presented herein for engine power settings of 48, 54, and 60 percent of the maximum power setting

/pdf/time-delay-analysis-of-turbofan-engine-direct-and-indirect-5bsqiynkzc.pdf

Time Delay Analysis of Turbofan Engine Direct and Indirect Combustion Noise Sources

The infinite line pressure probe provides a means for measuring high frequency fluctuating pressures in difficult environments. A properly designed infinite line probe does not resonate; thus its frequency response is not limited by acoustic resonance in the probe tubing, as in conventional probes. The characteristics of infinite line pressure probes are reviewed and some applications in turbine engine research are described. A probe with a flat-oval cross section, permitting a constant-impedance pressure transducer installation, is described. Techniques for predicting the frequency response of probes with both circular and flat-oval cross sections are also cited.

https://ntrs.nasa.gov/api/citations/19840011719/downloads/19840011719.pdf

The infinite line pressure probe

Fluctuating pressure measurements at several locations within the core of a turbofan engine were made simultaneously with far field acoustic measurements. Correlation and coherence techniques were used to determine the relative amplitude and phase relationships between core pressures at these various locations and between the core pressures and far field acoustic pressure. The combustor is a low frequency source region for acoustic propagation through the core nozzle and out to the far field. The relation between source pressure and the resulting sound pressure involves a 180 degree phase shift and an amplitude transfer function which varies approximately as frequency squared. This is consistent with a simplified model using fluctuating entropy as a source term.

/pdf/core-noise-source-diagnostics-on-a-turbofan-engine-using-45rtc2qlwi.pdf

Core noise source diagnostics on a turbofan engine using correlation and coherence techniques

Core noise from a YF-102 high bypass ratio turbofan engine was investigated through the use of simultaneous measurements of internal fluctuating pressures and far field noise. Acoustic waveguide probes, located in the engine at the compressor exit, in the combustor, at the turbine exit, and in the core nozzle, were employed to measure internal fluctuating pressures. Spectra showed that the internal signals were free of tones, except at high frequency where machinery noise was present. Data obtained over a wide range of engine conditions suggest that below 60% of maximum fan speed the low frequency core noise contributes significantly to the far field noise.

/pdf/core-noise-measurements-on-a-yf-102-turbofan-engine-kezsrc89kt.pdf

Core Noise Measurements on a YF-102 Turbofan Engine

Microgravity environment countermeasures - Panel discussion

As part of a program to investigate combustor noise, simultaneous measurements were made with a YF‐102 engine of combustor internal fluctuating pressure and farfield noise. As the dependence of farfield noise on engine internal measurement becomes understood, an important question becomes the connection between combustor internal measurements obtained in an engine and those obtained in a component test facility. To explore this question, a YF‐102 combustor, instrumented identically with that of the engine tests, was operated in a component test facility over a range of conditions encompassing engine operation, and the fluctuating internal measurements were repeated. A comparison of the directly measured spectra at corresponding locations in the two tests shows significant differences. However, the results of two‐point signal analyses within each combustor such as coherence function, transfer function, and phase relationships are similar for both tests. This indicates that the internal dynamics of the combustor as an acoustic source are preserved in a component test facility.

/pdf/combustor-fluctuating-pressure-measurements-in-engine-and-in-1hn52q8v42.pdf

Combustor fluctuating pressure measurements in engine and in a component test facility: A preliminary comparison

As part of a program to investigate core and combustor noise, simultaneous measurements were made of internal fluctuating pressure and farfield noise on a JT‐15D turbofan engine. Acoustic waveguide probes, located in the engine at the combustor, in the turbine exit and in the core nozzle, were used to measure internal fluctuating pressures. Low‐frequency acoustic power determined at the core nozzle exit corresponds in level to the farfield acoustic power at engine speeds below 70% of maximum, the approach condition. At engine speeds above 70% of maximum, the jet noise dominates in the farfield, greatly exceeding that of the core. From coherence measurements, it is shown that the combustor is the dominant source of the low‐frequency core noise. The results obtained from the JT‐15D engine compare favorably with those obtained previously from a YF‐102 engine, both engines having reverse flow annular combustors and being in the same size class.

/pdf/core-noise-measurements-on-a-small-general-aviation-class-ekx93lm96m.pdf

Allen Karchmer

Papers

Core noise source diagnostics on a turbofan engine using correlation and coherence techniques

Core Noise Measurements on a YF-102 Turbofan Engine

Microgravity environment countermeasures - Panel discussion

Combustor fluctuating pressure measurements in engine and in a component test facility: A preliminary comparison

Core noise measurements on a small, general aviation class turbofan engine