Interior noise considerations for advanced high-speed turboprop aircraft
Previously cited in issue 17, p. 2685, Accession no. A82-35018
Mixson, J. S.↗
Engineering topics
Farassat, F.
Publications and source records attributed to Farassat, F..
The prediction of the noise of supersonic propellers in time domain - New theoretical results
In this paper, a new formula for the prediction of the noise of supersonic propellers is derived in the time domain which is superior to the previous formulations in several respects. The governing equation is based on the Ffowcs Williams-Hawkings (FW-H) equation with the thickness source term replaced by an equivalent loading source term derived by Isom (1975). Using some results of generalized function theory and simple four-dimensional space-time geometry, the formal solution of the governing equation is manipulated to a form requiring only the knowledge of blade surface pressure data and geometry. The final form of the main result of this paper consists of some surface and line integrals. The surface integrals depend on the surface pressure, time rate of change of surface pressure, and surface pressure gradient. These integrals also involve blade surface curvatures. The line integrals which depend on local surface pressure are along the trailing edge, the shock traces on the blade, and the perimeter of the airfoil section at the inner radius of the blade. The new formulation is for the full blade surface and does not involve any numerical observer time differentiation. The method of implementation on a computer for numerical work is also discussed.
Farassat, F.↗
A note on the tip noise of rotating blades
A possible cause is suggested for the discrepancies that have arisen between measured results and linear acoustic predictions (based on thickness noise alone), at least at the lower tip Mach number range of the tests. The blade tip is shown to be an effective noise generation area when Isom's thickness noise formula is studied numerically (Farassat and Nystrom, 1980). It is found that neglecting the sources on the airfoil-shaped cut at the very tip of the blade causes serious errors in the predicted thickness noise. It is also found that the tip area becomes less important as the tip speed is increased. The effect of the tip surface on the thickness noise indicates that it should be included in the calculation of the loading noise.
Farassat, F.↗
Rotor noise prediction technology: Theoretical approach
Helicopter rotors which are the most complicated noise generators among rotating blade machinery are discussed. The main or the tail rotor can be the dominant source of noise depending on the range of frequencies and the observer position. Significant advances in noise prediction were made. Acoustic analysis is the most successful and general theoretical method to treat the acoustics of helicopter noise. The aerodynamic approach is used to study nonlinear acoustic effects. The results of acoustic analogy depend greatly on the input data.
Farassat, F.↗
Interior noise considerations for advanced high-speed turboprop aircraft
This paper describes recent research on noise generated by high-speed propellers, on noise transmission through acoustically treated aircraft sidewalls and on subjective response to simulated turboprop noise. Propeller noise discussion focuses on theoretical prediction methods for complex blade shapes designed for low noise at Mach = 0.8 flight and on comparisons with experimental test results. Noise transmission experiments using a 168 cm. diameter aircraft fuselage model and scaled heavy-double-wall treatments indicate that the treatments perform well and that the predictions are usually conservative. Studies of subjective comfort response in an anechoic environment are described for noise signatures having combinations of broadband and propeller-type tone components.
Mixson, J. S.↗
The prediction of helicopter rotor discrete frequency noise
An accurate prediction of the noise produced by helicopters requires a good understanding of the noise generating mechanisms involved. Such an understanding can lead to controlling the noise of existing helicopters by avoiding noisy regimes of flight or by redesigning the main and tail rotors. The present investigation is concerned with approaches which are suitable for the calculation of discrete frequency noise of helicopter rotors. The governing differential equation of acoustics used in a consideration of acoustic formulations is the Ffowcs Williams-Hawkings (FW-H) equation. Attention is given to a method reported by Farassat (1981), a method developed by Succi (1979), and a procedure discussed by Woan and Gregorek (1978).
Farassat, F.↗
User's manual for a computer program to calculate discrete frequency noise of conventional and advanced propellers
A user's manual is presented for a computer program for the calculation of discrete frequency noise of conventional and advanced propellers. The structure of the program and the subroutines describing the input functions are discussed. Input variables and their default values and the variables in the output data sheet are defined. Two versions of the program are available. These differ only in the graphic output capability. One version has only printed output capability. A second version with extensive graphic output capability is available for the computer system at Langley. This Manual includes four detailed examples of both the printed and graphic outputs. These examples may be reproduced by users to check their code on their computer system.
Martin, R. M.↗
Noise prediction for jetstar prop-fan test
The acoustic calculations reported in this memorandum are for two model prop-fan designs (SR-2 and SR-3 blades) scheduled for test on top of Jetstar aircraft. The predicted acoustic pressure signatures and spectra for selected microphone positions on the fuselage and operating conditions are presented. A detailed presentation of the input data, the acoustic results, and the corrections for microphone fuselage reflection are included. The general trend observed in these calculations is that the acoustically optimized model (using SR-3 blades) is substantially quieter than the model with SR-2 blades. This latter design has conventional straight blades.
Farassat, F.↗
Isom's thickness noise formula for rotating blades with finite thickness at the tip
Explore the source record for details and available documents.
Farassat, F.↗
A review of propeller discrete frequency noise prediction technology with emphasis on two current methods for time domain calculations
A review of propeller noise prediction technology is presented which highlights the developments in the field from the successful attempt of Gutin to the current sophisticated techniques. Two methods for the predictions of the discrete frequency noise from conventional and advanced propellers in forward flight are described. These methods developed at MIT and NASA Langley Research Center are based on different time domain formulations. Brief description of the computer algorithms based on these formulations are given. The output of these two programs, which is the acoustic pressure signature, is Fourier analyzed to get the acoustic pressure spectrum. The main difference between the programs as they are coded now is that the Langley program can handle propellers with supersonic tip speed while the MIT program is for subsonic tip speed propellers. Comparisons of the calculated and measured acoustic data for a conventional and an advanced propeller show good agreement in general.
Farassat, F.↗
A numerical technique for calculation of the noise of high-speed propellers with advanced blade geometry
A numerical technique and computer program were developed for the prediction of the noise of propellers with advanced geometry. The blade upper and lower surfaces are described by a curvilinear coordinate system, which was also used to divide the blade surfaces into panels. Two different acoustic formulations in the time domain were used to improve the speed and efficiency of the noise calculations: an acoustic formualtion with the Doppler factor singularity for panels moving at subsonic speeds and the collapsing sphere formulation for panels moving at transonic or supersonic speeds. This second formulation involves a sphere which is centered at the observer position and whose radius decreases at the speed of sound. The acoustic equation consisted of integrals over the curve of intersection for both the sphere and the panels on the blade. Algorithms used in some parts of the computer program are discussed. Comparisons with measured acoustic data for two model high speed propellers with advanced geometry are also presented.
Nystrom, P. A.↗
A collection of formulas for calculation of rotating blade noise - Compact and noncompact source results
A unified approach is used to derive many of the current formulations for calculation of discrete frequency noise for helicopter rotors and propellers. Both compact and noncompact source formulations are derived. The compact formulations are obtained as the limit of noncompact source results. In particular, the linearized acoustic equations by Hawkings and Lowson, Farassat, Hanson, Woan and Gregorek, Succi, and Jou are derived in this paper. An interesting thickness noise formula by Isom and its recent extension to the near field by Ffowcs Williams are also presented. The paper includes some comparisons of measured and calculated acoustic pressure signatures and spectra for an advanced propeller. The theoretical results are obtained using a computer program developed by the author and P. A. Nystrom.
Farassat, F.↗
An evaluation of linear acoustic theory for a hovering rotor
Linear acoustic calculations are compared with previously reported data for a small-scale hovering rotor operated at high tip Mach numbers. A detailed calculated description of the distributions of blade surface pressure and shear stress due to skin friction is presented. The noise due to skin friction and loading, in the rotor disk plane, is small compared to thickness noise. The basic conclusions of Boxwell et al about the importance of nonlinear effects are upheld. Some approximations involved in the current theories for the inclusion of nonlinear effects are discussed. Using a model nonlinear problem, it is shown that to use the acoustic analogy, good knowledge of the flowfield is required.
Morris, C. E. K., Jr.↗
A comparison of linear acoustic theory with experimental noise data for a small-scale hovering rotor
Linear acoustic calculations based on full aerodynamic data as input are presented and compared with measured cases reported by Boxwell et al. (1978). The full aerodynamic data are obtained using three programs giving radial loading, chordwise loading, and chordwise position of transition. It is shown that in the theoretical results the most significant noise source mechanism is due to blade thickness. Thus the conclusions of Boxwell et al. as to the importance of nonlinearities around the blades are upheld. These conclusions concern the width, shape and the level of the acoustic pressure calculated from linear acoustic theory. Some of the approximations involved in the application of acoustic analogy using quadrupole sources are discussed. It is necessary that the near- and far-field problems of rotating blades be treated together as shown for the case of an oscillating sphere.
Farassat, F.↗
Acoustic analysis of the propfan
A review of propeller noise prediction technology is presented. Two methods for the prediction of the noise from conventional and advanced propellers in forward flight are described. These methods are based on different time domain formulations. Brief descriptions of the computer algorithms based on these formulations are given. The output of the programs (the acoustic pressure signature) was Fourier analyzed to get the acoustic pressure spectrum. The main difference between the two programs is that one can handle propellers with supersonic tip speed while the other is for subsonic tip speed propellers. Comparisons of the calculated and measured acoustic data for a conventional and an advanced propeller show good agreement in general.
Farassat, F.↗
Bounds on thickness and loading noise of rotating blades and the favorable effect of blade sweep on noise reduction
The maxima of amplitudes of thickness and loading noise harmonics are established when the radial distribution of blade chord, thickness ratio, and lift coefficient is specified. It is first shown that only airfoils with thickness distribution and chordwise loading distributions which are symmetric with respect to midchord need be considered for finding the absolute maxima of thickness and loading noise. The resulting chordwise thickness and load distributions for these maximum noise conditions require infinite slope at some points along the chord but otherwise are uniform. It is shown that sweeping the blades reduces the thickness and loading noise, but there is no optimum sweep which generates the lowest noise.
Farassat, F.↗
A new capability for predicting helicopter rotor and propeller noise including the effect of forward motion
The governing equation and computing technique for the prediction of helicopter rotor and propeller noise are described. The method which gives both the acoustic pressure time history and spectrum of the noise includes the thickness and the loading noise. It was adapted to computers resulting in a new capability in noise prediction by removing many of the restrictions and limitations of previous theories. The capability results from the fact that the theory is developed entirely in the time domain. The formulation and the technique used are not limited to compact sources, steady level flight or to the far-field. In addition, the inputs to the computer program are normally available or are amenable to experimental measurements. This program can be used to study rotor and propeller noise with the aim of minimizing the radiated noise to reduce annoyance to the public. Several examples demonstrating the features and capability of the computer program are presented.
Farassat, F.↗
Some measured and calculated effects of forward velocity on propeller noise
Test flights using a twin-engine, high-wing, light STOL transport plane powered by free turbine engines driving three-bladed propellers, were conducted at the NASA Wallops Flight Center. A ground-based acoustic measuring range was set up, and aircraft tracking information was supplied by a radar system. The noise of the unshrouded propeller was measured under static and forward flight conditions, and the measurements were compared with theory. Under static conditions, the principal noise is related to the unsteady loading associated with the interaction of the propeller with persistent turbulent eddies passing through the propeller disk. Through the use of existing prediction techniques, it was shown that changes in the geometry such as different airfoil thickness distribution can affect the radiated acoustic pressure signatures. This test program confirmed that lower propeller noise levels are produced in forward flight than under static conditions and that the most significant reductions occur at the midfrequencies which dominate perceived and A-weighted noise levels.
Pegg, R. J.↗