Kenneth S. Brentner

Modeling aerodynamically generated sound of helicopter rotors

Abstract A great deal of progress has been made in the modeling of aerodynamically generated sound of rotors over the past decade. Although the modeling effort has focused on helicopter main rotors, the theory is generally valid for a wide range of rotor configurations. The Ffowcs Williams–Hawkings (FW–H) equation has been the foundation for much of the development. The monopole and dipole source terms of the FW–H equation account for the thickness and loading noise, respectively. Blade–vortex-interaction noise and broadband noise are important types of loading noise, hence much research has been directed toward the accurate modeling of these noise mechanisms. Both subsonic and supersonic quadrupole noise formulations have been developed for the prediction of high-speed impulsive noise. In an effort to eliminate the need to compute the quadrupole contribution, the FW–H equation has also been utilized on permeable surfaces surrounding all physical noise sources. Comparisons of the Kirchhoff formulation for moving surfaces with the FW–H equation have shown that the Kirchhoff formulation for moving surfaces can give erroneous results for aeroacoustic problems. Finally, significant progress has been made incorporating the rotor noise models into full vehicle noise prediction tools.

COMPUTATIONAL AEROACOUSTIC ANALYSIS OF SLAT TRAILING-EDGE FLOW

An acoustic analysis based on the Ffowcs Williams and Hawkings equation was performed for a high-lift system. As input, the acoustic analysis used unsteady flow data obtained from a highly resolved, time-dependent, Reynolds-averaged Navier-Stokes caclulation. The analysis strongly suggests that vortex shedding from the trailing edge of the slat results in a high-amplitude, high-frequency acoustic signal, similar to that which was observed in a corresponding experimental study of the high-lift system.

High-accuracy algorithms for computational aeroacoustics

This paper presents an analysis of high-bandwidth operators developed for use with an essentially nonoscillatory (ENO) method. The spatial operators of a sixth-order ENO code are modified to resolve waves with as few as 7 points per wavelength (PPW) by decreasing the formal order of the algorithm. Numerical and analytical solutions are compared for the model problems of plane-wave propagation and sound generation by an oscillating sphere. These problems involve linear propagation, wave steepening, and shock formation. An analysis of the PPW required for sufficient accuracy shows that low-order algorithms need an excessive number of grid points to produce acceptable solutions. In contrast, high-order codes provide good predictions on relatively coarse grids. The high-bandwidth operators produce only modest improvements over the original sixth-order operators for nonlinear problems in which wave steepening is significant ; however, they clearly outperform the original operators for long-distance linear propagation. Because the high-bandwidth operators have the same stencil as the original sixth-order operators, these gains are achieved with no additional computational work.

Acoustic Scattering in the Time Domain Using an Equivalent Source Method

A numerical method to solve acoustic scattering in the time domain is presented in the present paper. Equivalent sources are embedded within a scattering surface and their strengths are determined as a function of time by the pressure-gradient boundary condition on a scattering surface. Once the strengths are determined, the equivalent sources are used to predict the scattered pressure. Linear shape functions are used to discretize the strength of the equivalent sources in time, and singular value decomposition is used to find the least-squares solution and to overcome potential numerical instabilities. The predictions are found to be in excellent agreement with the exact solutions for sound from a point monopole source and band-passed broadband sound. The method works well even at the irregular frequencies at which internal resonance modes occur. Finally, the method is used to predict the scattering of sound from a moving source. It is shown that the method has the capability to capture aperiodic characteristics very well.

Assessment of Time-Domain Equivalent Source Method for Acoustic Scattering

Equivalent source methods have been developed in the frequency and time domain to provide a fast and efficient computation for acoustic scattering. Although the advantages and capabilities of the method have been demonstrated, the limitations and drawbacks of the method have not yet been explored in detail. A detailed understanding of the equivalent source method is needed to use the method in a wide range of applications with more confidence. This paper presents an assessment of the time-domain equivalent source method for the prediction of acoustic scattering. The sensitivity of the method to numerical parameters, including the number of the surface collocation points, the number and position of the equivalent sources, the time step, and the cut-off singular value, is investigated and suggestions for these parameters are given for accurate predictions. A numerical instability issue is shown and a way to stabilize the solution with a time-averaging scheme is introduced. The sound power is calculated using the equivalent source strength to demonstrate the redistribution of the sound intensity by a scattering body and the conservation of the total power. Finally, scattering of sound from a source in a short duct is tested to demonstrate the utility of the tool for a more complicated shape of the scattering surface.

Revolutionary Physics-Based Design Tools for Quiet Helicopters

Abstract : A computational research program was performed at Georgia Institute of Technology, Penn State University, and at Northern Arizona University to develop a set of first-principles based computational modeling tools for analyzing and designing advanced helicopter configurations. The approach involved incorporation of advanced numerical algorithms and turbulence models in OVERFLOW 2, development of advanced comprehensive analyses (DYMORE and RCAS) that are seamlessly coupled to the flow analysis, modeling of rotor noise characteristics using an advanced acoustics prediction tool (PSU-WOPWOP) that is seamlessly coupled to the flow analysis and the comprehensive analyses, and validation and application of the integrated suite of tools for current generation (UH-60, BO105) and next generation configurations. Under the Phase I-B extension, assessment of this suite of tools is being performed by Ga Tech and Penn State for the Boeing MD-900 model rotor (MDART), an actively controlled rotor (SMART), and the Comanche rotor blade (as an option).

Numerical Algorithms for Acoustic Integrals with Examples for Rotor Noise Prediction

The accurate prediction of the aeroacoustic field generated by aerospace vehicles or nonaerospace machinery is necessary for designers to control and reduce source noise. Powerful computational aeroacoustic methods, based on various acoustic analogies (primarily the Lighthill acoustic analogy) and Kirchhoff methods, have been developed for prediction of noise from complicated sources, such as rotating blades. Both methods ultimately predict the noise through numerical evaluation of an integral formulation. We consider three generic acoustic formulations and several numerical algorithms that have been used to compute the solutions to these formulations. Algorithms for retarded-time formulations are the most efficient and robust, but they are difficult to implement for supersonic-source motion. Collapsing-sphere and emission-surface formulations are good alternatives when supersonic-source motion is present, but the numerical implementations of these formulations are more computationally demanding. New algorithms--which utilize solution adaptation to provide a specified error level-are needed.

Near Real-Time Simulation of Rotorcraft Acoustics and Flight Dynamics

In this paper, a near-real-time rotorcraft flight dynamics-acoustics prediction system is presented. The highfidelity PSU-WOPWOP rotor noise prediction code is coupled with the GENHEL flight simulation code, which provides low-fidelity blade loading and motion. This system is an initial step intended to investigate the feasibility of real-time rotorcraft noise prediction and to demonstrate the utility of such a system. Limited acoustic validation is shown for a contemporary-design four-bladed main rotor in level flight. A complex 80-s maneuver was used to demonstrate the potential of the coupled system. This realistic maneuver includes a climb, coordinated turn, and level flight conditions. The noise predictions show changes in main rotor noise radiation strength and directivity caused by maneuver transients, aircraft attitude changes, and the aircraft flight—but do not include the effect of blade-vortex-interaction noise. A comparison of the total noise with the thickness and loading noise components helps explain the noise directivity. The computations for a single observer were very fast, although the algorithm is not currently organized as a real-time computation.

Noise Prediction for Maneuvering Rotorcraft

This paper presents the initial work toward first-principles noise prediction for maneuvering rotors. Both the aeromechanical and acoustics aspects of the maneuver noise problem are discussed. The comprehensive analysis code, CAMRAD 2. was utilized to predict the time-dependent aircraft position and attitude, along - with the rotor blade airloads and motion. The major focus of this effort was the enhancement of the acoustic code WOPWOP necessary to compute the noise from a maneuvering rotorcraft. Full aircraft motion, including arbitrary transient motion, is modeled together with arbitrary rotor blade motions. Noise from a rotorcraft in turning and descending flight is compared to level flight. A substantial increase in the rotor noise is found both for turning flight and during a transient maneuver. Additional enhancements to take advantage of parallel computers and clusters of workstations, in addition to a new compact-chordwise loading formulation, are also described.

AN AEROACOUSTIC ANALYSIS OF WIND TURBINES

This paper describes computational aeroacoustic methods that are being applied to predict the noise radiated by wind turbines. Since the wind turbine noise problem is very challenging, only some of the important noise sources and mechanisms are being considered. These are airfoil self-noise, the effects of blade rotation, and the propagation of sound over large distances. Two aspects of airfoil self-noise are being studied. The first is the relatively low frequency noise generated by deep stall and the second is trailing edge noise. The noise associated with blade rotation includes the effects of blade rotation on the blade aerodynamics, incoming gusts, incoming atmospheric turbulence and wind shear. The unsteady flow simulations are coupled to the radiated noise field with the permeable surface Ffowcs Williams – Hawkings formulation. For longrange noise propagation predictions, methods based on solutions of the linearized Euler equations or the Parabolic Equation approximation are discussed. Alternative methods for the implementation of boundary conditions for the propagation studies are also included.