David B. Stephens

Acoustic Shielding for a Model Scale Counter-rotation Open Rotor

The noise shielding benefit of installing an open rotor above a simplified wing or tail is explored experimentally. The test results provide both a benchmark data set for validating shielding prediction tools and an opportunity for a system level evaluation of the noise reduction potential of propulsion noise shielding by an airframe component. A short barrier near the open rotor was found to provide up to 8.5 dB of attenuation at some directivity angles, with tonal sound particularly well shielded. Predictions from two simple shielding theories were found to overestimate the shielding benefit. Nomenclature f Frequency, Hz Bf Number of blades on the front rotor Ba Number of blades on the aft rotor Nf Front rotor shaft rotation frequency, Hz Na Aft rotor shaft rotation frequency, Hz ps Complex acoustic pressure when the acoustic shield is in place, Pa pu Complex acoustic pressure in the unshielded configuration, Pa ˜ ps Measured RMS acoustic pressure spectrum when the acoustic shield is in place, Pa ˜ pu Measured RMS acoustic pressure spectrum in the unshielded configuration, Pa SO Propulsor Shaft Order, 2f/(Nf + Na) �SPL Decibel reduction in sound pressure level due to the acoustic shield

Nearfield Unsteady Pressures at Cruise Mach Numbers for a Model Scale Counter-Rotation Open Rotor

An open rotor experiment was conducted at cruise Mach numbers and the unsteady pressure in the nearfield was measured. The system included extensive performance measurements, which can help provide insight into the noise generating mechanisms in the absence of flow measurements. A set of data acquired at a constant blade pitch angle but various rotor speeds was examined. The tone levels generated by the front and rear rotor were found to be nearly equal when the thrust was evenly balanced between rotors.

Shear Layer Driven Acoustic Modes in a Cylindrical Cavity

This paper describes the interior acoustic pressure of a cylindrical cavity driven by a shear layer. Existing cavity flow literature is generally focused on rectangular cavities, where the resonance is either longitudinal or depth modes inside the cavity. The design of the present circular cavity is such that azimuthal duct modes can be excited in various combinations with depth modes depending on free stream velocity. An acoustic simulation of the system was used to identify the modes as a function of frequency when the system is driven by an acoustic point source. With appropriate manipulation of the free stream flow, abrupt mode switching and mode oscillation were both observed, and a condition with a dominant azimuthal mode was found. The strength of the lock-on was documented for the various resonance conditions, and the effects of the cavity opening size and location were studied. A Area of cavity opening, m 2

Resonant mode characterisation of a cylindrical Helmholtz cavity excited by a shear layer

This paper investigates the interaction between the shear-layer over a circular cavity with a relatively small opening and the flow-excited acoustic response of the volume within to shear-layer instability modes. Within the fluid-resonant category of cavity oscillation, most research has been conducted on rectangular geometries: generally restricted to longitudinal standing waves, or when cylindrical: to Helmholtz resonance. In practical situations, however, where the cavity is subject to a range of flow speeds, many different resonant mode types may be excited. The current work presents a cylindrical cavity design where Helmholtz oscillation, longitudinal resonance, and azimuthal acoustic modes may all be excited upon varying the flow speed. Experiments performed show how lock-on between each of the three fluid-resonances and shear-layer instability modes can be generated. A circumferential array of microphones flush-mounted with the internal surface of the cavity wall was used to decompose the acoustic pressure field into acoustic modes and has verified the excitation of higher order azimuthal modes by the shear-layer. For azimuthal modes especially, the location of the cavity opening affects the pressure response. A numerical solution is validated and provides additional insight and will be applied to more complex aeronautical and automotive geometries in the future.

Sound Generation by a Rotor Ingesting a Casing Turbulent Boundary Layer

A new formulation for predicting the noise generated by a ducted rotor interacting with a casing boundary layer has been developed. The method accounts for the streamwiseelongated turbulent structures that have been recently observed in ∞at-plate boundary layers. These structures have been measured to have streamwise lengths on the order of 20‐, which is long enough to cause unsteady lift that is correlated between rotor blades. An analytical relation between the rotor unsteady lift and the two-point correlation of upwash velocity provides a link between the elongated turbulent structures and the sound radiated by the ducted rotor. A simple model for the true two-point correlation function in the duct boundary layer allows for the unsteady lift to be estimated. The predicted unsteady lift spectrum matches an experimentally determined spectrum within … §4 dB up to about flve times the blade passing frequency.

Tone and Broadband Noise Separation from Acoustic Data of a Scale-Model Counter-Rotating Open Rotor

Renewed interest in contra-rotating open rotor technology for aircraft propulsion application has prompted the development of advanced diagnostic tools for better design and improved acoustical performance. In particular, the determination of tonal and broadband components of open rotor acoustic spectra is essential for properly assessing the noise control parameters and also for validating the open rotor noise simulation codes. The technique of phase averaging has been employed to separate the tone and broadband components from a single rotor, but this method does not work for the two-shaft contra-rotating open rotor. A new signal processing technique was recently developed to process the contra-rotating open rotor acoustic data. The technique was first tested using acoustic data taken of a hobby aircraft open rotor propeller, and reported previously. The intent of the present work is to verify and validate the applicability of the new technique to a realistic one-fifth scale open rotor model which has 12 forward and 10 aft contra-rotating blades operating at realistic forward flight Mach numbers and tip speeds. The results and discussions of that study are presented in this paper.

The Acoustic Environment of the NASA Glenn 9- by 15-foot Low-Speed Wind Tunnel

The 9- by 15-Foot Low Speed Wind Tunnel is an acoustic testing facility with a long history of aircraft propulsion noise research. Due to interest in renovating the facility to support future testing of advanced quiet engine designs, a study was conducted to document the background noise level in the facility and investigate the sources of contaminating noise. The anechoic quality of the facility was also investigated using an interrupted noise method. The present report discusses these aspects of the noise environment in this facility.

A High-Resolution, Continuous-Scan Acoustic Measurement Method for Turbofan Engine Applications

Detailed mapping of the sound field produced by a modern turbofan engine, with its multitude of overlapping noise sources, often requires a large number of microphones to properly resolve the directivity patterns of the constituent tonal and broadband components. This is especially true at high frequencies where the acoustic wavelength is short, or when shielding, scattering, and reflection of the sound field may be present due to installation effects. This paper presents a novel method for measuring the harmonic and broadband content of complex noncompact noise sources using continuously moving (referred to here as continuous-scan) microphones in conjunction with a state-of-the-art phase-referencing technique. Because the microphones are moving through the sound field produced by the noise sources, they effectively provide infinite spatial resolution of the sound directivity over the scan path. In this method, harmonic (i.e., shaft-coherent) content at the integer multiples of the instantaneous shaft rotational frequency is first extracted from the time signal using a tachometer signal and the Vold-Kalman filter. The residual broadband signal is then filtered in the time domain in fractional octave bands. The broadband spectra of the signals from the moving microphones are then computed at arbitrary positions along their scan paths using weighted averages (based on Chebyshev polynomial zero-crossings) and the assumption of a complex envelope that varies slowly over a spatial scale whose lower bound is set by the acoustic wavenumber. A benefit of this method is that the decomposition of the total measured sound field into a stochastic superposition of components preserves a meaningful phase definition for each “partial field” associated with a given shaft order. This preservation of phase data enables the forward or backward projection of each of these partial fields using acoustical holography. The benefits of the continuous-scan method are demonstrated using acoustic data acquired for a 22-inch scale-model fan stage run at the NASA Glenn Research Center’s 9-foot by 15-foot wind tunnel. Two key outcomes of the work include (1) significant improvement in the spatial resolution of the measured sound field and (2) reduction in the overall data acquisition time. Additionally, the methods described here lead to new opportunities for noise source diagnostics and visualization.Copyright

Turbulence and Acoustics in Propulsion

The prediction of the far fleld acoustic signature in propulsion systems is complicated by the physical complexity of the acoustic sources and the transfer of energy to the far fleld. In this study, a low speed ducted rotor was used to provide experimental data in a simplifled system. Careful measurements of the approach turbulence allowed for detailed modeling of the unsteady lift on the blades. This and other source models have been combined in order to provide an estimate of the net or blocked dipole sources. An algorithm was developed to use the model source functions to separate the source from the acoustic transfer function. The results indicate a very complex acoustic source. The models adequately represented the broadband sources, but did not capture the sources related to the blade passing frequency. The duct transfer function showed large local maxima at integer multiples of the resonant organ pipe mode, and complex behavior in between these modes.

NASA aeronautical noise research

This paper presents a broad historical overview of NASAs major aircraft noise research program, and the advances and contributions made to the understanding, prediction, and reduction of aeronautical noise. Major programs including Quiet Nacelle, Quiet Engine, Refan, and the Aircraft Noise Prediction Program (ANOPP) are described and their role in the technology transfer process is discussed. It outlines projected future activities and discusses prospects for further noise reduction on conventional turbofan transports. New directions and focus in aeroacoustics research and technology for advanced turboprops, rotorcraft, and supersonic transports are also discussed.