Open Rotor Noise Prediction Methods at NASA Langley: A Technology Review

Abstract

Open rotors are once again under consideration for propulsion of the future airliners because of their high efficiency. The noise generated by these propulsion systems must meet the stringent noise standards of today to reduce community impact. In this paper we review the open rotor noise prediction methods available at NASA Langley. We discuss three codes called ASSPIN (Advanced Subsonic-Supersonic Propeller Induced Noise), FW H pds (Ffowcs Williams-Hawkings with penetrable data surface) and the FSC (Fast Scattering Code). The first two codes are in the time domain and the third code is a frequency domain code. The capabilities of these codes and the input data requirements as well as the output data are presented. Plans for further improvements of these codes are discussed. In particular, a method based on equivalent sources is outlined to get rid of spurious signals in the FW H pds code.https://ntrs.nasa.gov/search.jsp?R=20090020657 2018-05-10T21:55:46+00:00Z 1-IntroductionThe high fuel prices of recent years have caused the operating cost of the airlines to soar. In an effort to bring down the fuel consumption, the major aircraft engine manufacturers such as GE, Rolls-Royce and CFM International are now taking a fresh look at open rotors for the propulsion of future airliners. Open rotors, also known as propfans or unducted fans, can offer up to 30 per cent improvement in efficiency compared to high bypass engines of 1980 vintage currently in use in most civilian aircraft. The lower fuel consumption of open rotors also contributes to the reduction of carbon emission into the atmosphere by airliners. Open rotor propulsion system can, therefore, be considered a Green engine. The single and contra-rotating open rotors are both under consideration for powering future aircraft. An example of a contra-rotating open rotor is shown in Figure 1 below. Figure 1-An example of contra-rotating open rotor design by GEOpen rotors operate at transonic to supersonic helical tip speeds that can go as high as Mach 1.3. The noise of these propulsion systems can cause both passenger and community concerns. The ability to predict the noise of open rotors at the design stage and to reduce the noise to an acceptable level are, therefore, of great importance to engine manufacturers. In 1980s, a comprehensive study of open rotors by NASA and the U.S. aircraft engine industry was carried out involving theory development, model and full scale tests. An impressive array of technology development in aeroelasticity, composite materials, mechanical design, aerodynamics and aeroacoustics resulted from this study. A large number of technical reports and publications in open literature were written by the engineers and researchers involved in this effort. These reports and publications are of great value to those involved in the resurgence of interest in open rotors around the world. NASA Langley researchers have contributed significantly to the development of aeroacoustic technology of open rotors. The present authors feel that it is...

Conceptual design study of a geared turbofan and an open rotor aero engine with intercooled recuperated core

“…However, it is important to mention that one of the major problems of OR configuration is the high noise penalty. Currently, in USA 13 and Europe 14 many researchers are focusing on reducing the noise level of OR engines.…”

Aeroacoustic Analysis of a High-Speed Open Rotor

“…While, in principle, eqn (1) can be computed by carrying out the indicated spatial and temporal integrals numerically using appropriate quadrature schemes (see, for example, Farassat et al [10] and Brès et al [11]), most often this equation is tackled in the frequency domain and by assuming that the observer is in the farfield. This assumption results in considerable simplification of the integrands in eqn (1) and leads to analytic expressions involving the Bessel function for the integrals over the source time τ.…”

“…This assumption results in acoustic energy being distributed amongst a subset of shaft orders. The blades in actual hardware, however, are rarely identical 10 and, as a result, do not experience exactly the same time histories. More importantly, the setting angles for the blades in an actual rotor disc vary within a small range (in the ± 0.25°range for F31/A31) in a random fashion around the disc thus slightly modifying the loading distribution on each blade.…”

“…In the interest of brevity, for the remainder of this section, we shall focus on comparisons for the tone overall sound pressure level (OASPL). This is defined as the sum of all of the theoretically predicted sum tones up to the 66 th shaft order which, according to the relations mB 1 Ω 1 + kB 2 Ω 2 for the front rotor and mB 2 Ω 2 + kB 1 Ω 1 for the aft rotor, include the shaft orders 10,12,20,22,24,30,32,34,36,40,42,44,46,48,50,54,56,60,66. Note that the difference tones are not included since the theory predicts their levels as being negligible.…”

Prediction of Near-And Far-Field Noise Generated by Contra-Rotating Open Rotors

“…has developed its own FW-H solver which has been extensively tested against various analytical test cases; see Ghorbaniasl et al (2006Ghorbaniasl et al ( , 2007. The module is based on Farassat's formulation 1A of the FW-H equations; see Farassat (2007) and Farassat et al (2009). In this formulation, monopole and dipole sources are evaluated by an integration performed on a 3D source surface (Σ e ) and then radiated into free space.…”