Krishan K. Ahuja

Designing clean jet-noise facilities and making accurate jet-noise measurements †

The main objective of this paper is to provide guidelines for designing and calibrating a high quality, static, jet-noise research facility and making high-quality jet noise measurements. Particular emphasis is placed on methodology for determining if internal noise is dominant in the jet noise spectrum. A section of this document is devoted to clarifying the terminology associated with microphone frequency response corrections and providing a step-wise description of other corrections that must be applied to the measured raw spectra before the jet noise data can be considered accurate and ready for use for extrapolation to full-scale jet engine noise.

Noise Reduction Through Circulation Control

Circulation control technology uses tangential blowing around a rounded trailing edge or a leading edge to change the force and moment characteristics of an aerodynamic body. This technology has been applied to circular cylinders, wings, helicopter rotors, and even to automobiles for improved aerodynamic performance. Only limited research has been conducted on the acoustic of this technology. Since wing flaps contribute to the environmental noise of an aircraft, an alternate blown high lift system without complex mechanical flaps could prove beneficial in reducing the noise of an approaching aircraft. Thus, in this study, a direct comparison of the acoustic characteristics of high lift systems employing a circulation control wing configuration and a conventional wing flapped configuration has been made. These results indicate that acoustically, a circulation control wing high lift system could be considerably more acceptable than a wing with conventional mechanical flaps.

Noise Scaling for Unheated Low Aspect Ratio Rectangular Jets

A unique set of acoustic and fluid dynamic data was obtained for three converging rectangular jets of aspect ratios 1.5, 4.0, and 8.0 and a round nozzle. All four nozzles had the same exit area and were tested in the same facility for static and unheated conditions for a range of subsonic and slightly supersonic Mach numbers. Farfield noise measurements were made for polar angles ranging from 40° to 110° with respect to the jet axis and for azimuthal angles that corresponded to both a flyover and a sideline condition. On a spectral basis, it was found that the rectangular jets were typically louder in the high frequency range and quieter in the low frequency range. Attempts were made to collapse the acoustic data for all of the jets on a non-dimensional basis and thus determine the functional dependency of the noise upon the nozzle geometry. The process of collapsing the acoustic data mirrored a previously performed collapse for round jets. Upon discovering that existing methods for scaling round jet noise would not be sufficient to collapse the rectangular jet noise, additional scaling laws were adopted. Physical justification for the these scaling laws are provided.

Effect of Nozzle-Exit Boundary Layer on Jet Noise

A number of jet noise experiments have been performed over the years in different facilities and the measured lossless spectra do not always match. Some of the researchers have suggested this discrepancy is associated with rig noise. Another school of thought is that this discrepancy could be due to differing nozzle-exit boundary layers. One of the criticized datasets is the Tanna database, which was acquired at the Lockheed Georgia anechoic jetfacility, now operated by Georgia Tech Research Institute (GTRI), where the current authors work. In this paper, this rig noise contamination claim is examined by operating the facility at conditions and for nozzle geometries that would enhance upstream noise if it were dominant. The facility was found to produce jet mixing noise free of rig noise. A muffler installed upstream of the jet nozzle also showed that rig noise was nonexistent. Jet noise was acquired from two types of nozzles: ASME and conical. Boundary layer characteristics of both nozzle types were acquired and compared. It was found that a jet exhaust from an ASME nozzle produced more noise compared to a nozzle with a developed nozzle-exit boundary layer and that increase may be responsible for the effect that was blamed to be the result of contamination in some published studies.

Determination of Geometric Farfield for Ducted and Unducted Rotors

In this research, the requirements for microphones to be in the geometric farfield for ducted and unducted rotors have been established. The approach is to acquire acoustic data at various geometric locations relative to the source. This is performed for different rotor configurations and compared with the Inverse Square Law (ISL). This paper produced a measurement requirement that satisfies the ISL. It is presented in terms of rotor diameter and/or duct length. If this requirement is met then data for rotors with different dimensions acquired at any anechoic facility may be compared. Furthermore, the present work defines the shortest distance where farfield microphone should be placed for accurate extrapolation of the data to the rotor noise of a full-scale system by the ISL.

Fluid Dynamics of a High Aspect-Ratio Jet

Circulation control wings are a type of pneumatic high-lift device that have been extensively researched as to their aerodynamic benefits. However, there has been little research into the possible airframe noise reduction benefits of a circulation control wing. The key element of noise is the jet noise associated with the jet sheet emitted from the blowing slot. High aspect-ratio jet acoustic results (aspect-ratios from 100 to 3,000) from a related study showed that the jet noise of this type of jet was proportional to the slot height to the 3/2 power and slot width to the 1/2 power. Fluid dynamic experiments were performed in the present study on the high aspect-ratio nozzle to gain understanding of the flow characteristics in an effort to relate the acoustic results to flow parameters. Single hot-wire experiments indicated that the jet exhaust from the high aspect-ratio nozzle was similar to a 2-d turbulent jet. Two-wire space-correlation measurements were performed to attempt to find a relationship between the slot height of the jet and the length-scale of the flow noise generating turbulence structure. The turbulent eddy convection velocity was also calculated, and was found to vary with the local centerline velocity, and also as a function of the frequency of the eddy.

JET MIXING ENHANCEMENT

The application of a combustion-driven, pulsed fluidic actuator to jet mixing enhancement is described. An array of 14 combustion actuators was distributed around a 2.4˝ diameter nozzle. A sequential actuation scheme is described to achieve higher actuation frequencies. Fluidic and acoustic measurements were performed on the actuated jet. The jets were tested at speeds up to Mach 1. Enhanced mixing was shown at speeds up to Mach 0.55. Acoustic measurements showed increased jet noise at these low subsonic speeds.

Effect of Orifice Shape on Acoustic Impedance

The effect on normal incidence acoustic impedance of a non-circular orifice shape is examined relative to a circular orifice. The impedance of an adjustable porosity perforate, formed from two identical perforates sliding over each other, is measured. As the orifice shape becomes more non-circular, the measured impedance is found to deviate from the predicted results for a circular orifice of the same area. Several isolated orifices of the same open area but different shapes are tested and compared with a circular orifice. Both low incident sound pressure levels using broadband noise and high incident sound pressure levels using sinusoidal tones are used to evaluate the impedance performance of these isolated orifices. One orifice mimics the unique shape produced by the adjustable perforate and results in a smaller attached mass (or mass end correction) compared with a round orifice. This is consistent with the perforate impedance results. The unique orifice shape does not appear to have measurable differences in acoustic normal resistance at high incident sound pressure levels. However, since the attached mass plays a key role in determining the peak absorption frequency of resonant liners, the reduction in attached mass relative to a circular orifice has implications where these types of orifices are used.

Measuring Jet Noise Source Locations with Acoustic Beamforming

Numerous methods have been used by the aeroacoustics community to locate noise sources of different frequencies in subsonic jets. These include: acoustic mirrors, microphone arrays, two-microphone methods, causality correlation and coherence techniques, acoustic beamforming, and automated source breakdown. Except for the acoustic beamforming, most of these methods are quite time consuming. No one seems to have ever compared the performance of these techniques. This paper compares the performance of various existing techniques. As the beamforming technique is the least time consuming, emphasis has been given to examining the performance of this technique in measuring source location in a subsonic jet for a range of Reynolds numbers. A 48-microphone acoustic beamformer was used to locate the noise sources for several subsonic Mach numbers from a one-inch and a half-inch nozzle. The effects of tabs on source location are also examined for the one-inch nozzle. Once set up, this method allows for faster and thus more numerous tests. It is shown that jet noise source locations may be a strong function of either Reynolds number or nozzle exit boundary layer. In general, there is a shift of all the noise sources away from the nozzle exit as Reynolds number decreases.

On Coherence of Jet Noise

Cross-correlation measurements and related coherence spectra for a range of nozzle geometries and jet Mach numbers were studied in an attempt to understand the coherent nature of jet mixing noise from the small-scale structure and that from the large-scale structure using a number of nozzle geometries. Nozzle geometries include: (1) three conical round nozzles; (2) an ASME nozzle; (3) a two-tabbed conical round nozzle configuration; (4) a four-tabbed conical round nozzle configuration (5) a rectangular nozzle with an aspect ratio of 8, and (6) a Mach 1.67 converging-diverging round nozzle. Extensive coherence measurements between a number of microphone pairs both in the side line and in the downstream direction of the jets were analyzed. For one nozzle geometry, data was also acquired at a microphone located at the jet centerline and its correlation measured with the noise at other microphones. All of the data indicates that jet noise is highly coherent in the downstream direction and with the exception of very low frequencies is incoherent in the sideline and upstream direction as long as the microphones are separated by 10 to 20 degrees. At Strouhal numbers of less than about 0.1, the jet noise is found to be coherent at all microphones and further understanding is needed at