Christina Appel

RANS/CAA based prediction of NACA 0012 broadband trailing edge noise and experimental validation

The prediction quality of a fast Computational Aeroacoustics (CAA) approach is studied for noise generated at a NACA 0012 trailing edge. Broadband spectra, the power law underlying their Mach number scaling, and the effect of Reynolds Number on the spectra are juxtaposed against published data from measurements and results from a semi-empirical prediction tool. The CAA method rests on the use of Reynolds Averaged Navier-Stokes (RANS) solutions to describe the turbulent flow problem around the airfoil. Acoustic Perturbation Equations (APE) are solved in the time domain, using a vortex source term, which is a function of turbulent field quantities. The acoustic approach was sccessfully validated in other works by utilizing turbulence data from Large Eddy Simulation (LES) to prescribe the unsteady sound sources. For the fast CAA approach applied in this work unsteady vortex sound sources are determined by a stochastic method, which generates 4D spatio-temporal synthetic turbulence that very accurately accompishes a local realization of all statistical and mean-flow features provided by steady RANS. Based on these prerequisites it becomes now feasible to make an assessment as to which acoustic accuracy can be achieved with such a hybrid RANS / CAA prediction method, bearing in mind the approximative nature and limited turbulence resolution of RANS:

Trailing-Edge Noise Data Quality Assessment for CAA Validation

The paper provides a detailed reexamination of previously published trailing-edge (TE) noise experimental data as acquired in DLRs low-noise open-jet Acoustic Wind Tunnel Braunschweig (AWB). The objective is to set up a parametric reference data base to be later used for CAA validation purposes. A modular plate airfoil with variable chord length was used to investigate Reynolds number effects on both near field quantities and farfield sound. The original data base, including corrected farfield TE noise spectra as well as turbulent boundary-layer (TBL) characteristics, has been extended by unsteady surface pressure measurements in the TE region. Limitations and weaknesses of the applied measurement techniques are pinpointed by comparisons with independent data sets using alternative measurement approaches. Comparisons include also available semi-empirical prediction models for surface pressure spectra as well as for farfield TE sound emission. First RANS-based CAA prediction results using stochastic source models show essential agreement with the measurement data.

CAA-RPM prediction and validation of slat setting influence on broadband high-lift noise generation

Broadband sound generated at the slat of a three-element high-lift wing is simulated with a CAA method. Especially, the effect of different slat and gap settings is studied and the results are validated with measurements. The applied method rests on the use of steady Reynolds Averaged Navier-Stokes (RANS) simulation to prescribe the time-averaged motion of turbulent flow. By means of synthetic turbulence generated with the Random Particle-Mesh (RPM) method the steady one-point statistics (e.g. turbulent kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations with statistics that very accurately reproduce the spatial target distributions of RANS. The synthetic fluctuations are used to prescribe sound sources which drive linear acoustic perturbation equations. The whole approach represents a methodology to solve statistical noise theory with state-of-the-art Computational Aeroacoustics (CAA) tools in the timedomain. The comparison with experiments are conducted for four selected settings, chosen from a matrix comprising in total 43 individual slat-gap and overlap combinations. CAA simulations are performed for all matrix positions. The CAA computations are obtained as blind predictions prior to the measurements conducted in the AWB wind tunnel. Good agreement of the noise trends are found between CAA and experiments. The difference in level between the selected configurations is obtained qualitatively and quantitatively by CAA.

3D Computation of Broadband Slat Noise from Swept and Unswept High-Lift Wing Sections

In previous work a RANS based simulation technique for the simulation of broadband slat noise was established. Good agreement was found between predicted and measured slat noise spectra. These predictions were based on 2D CAA computations and a connection to 3D measured data is only possible assuming a certain functional behavior of the spanwise coherence of the essential slat noise source. For this purpose, results from trailing edge noise measurements were used. In this work the simulation strategy is extended to 3D CAA computations, resolving the spanwise slat noise coherence as part of the CAA computations. The considered wing span is one main-chord, which is large enough to establish a realistic 3D problem for the turbulence as well as for the sound radiation. The Fast Random Particle Mesh (FRPM) method is applied for this study to generate fluctuating sound sources from steady RANS turbulence statistics. The study is conducted for the 30P30N airfoil with 0.457m main chord. The Mach number is 0.17 and the angle of attack is 4∘. Good agreement is found between the previous 2D and the 3D results as well as with unsteady simulations published in the literature. The influence of sweep on slat noise generation is studied.

Numerical Study of Wall Pressure Fluctuations for Zero and Non-Zero Pressure Gradient Turbulent Boundary Layers

Turbulent boundary layers on a flat plate configuration are simulated using synthetic turbulence generated by the Fast Random Particle-Mesh Method. The averaged turbulence statistics needed for the stochastic realization is provided by a Reynolds averaged Navier-Stokes calculation. Wall pressure fluctuations are obtained by calculating a Poisson equation including both the mean-shear turbulence interaction source term and the turbulence-turbulence interaction source term. The Poisson equation is solved by means of Hockney’s method. Wall pressure fluctuations for zero and adverse pressure gradient boundary layers are calculated. The adverse pressure gradient is realized by placing an airfoil above the flat plate. Simulated one-point spectra and two-point statistics are analyzed. The results are compared to the experimental results, which were measured in the Acoustic Windtunnel Braunschweig for the same configurations. Good agreement with the experimental results is obtained.

A generic computational study of broadband high-liftnoise generation for simplified slat and flap problems

In previous work a RANS based simulation technique for the simulation of broadband slat noise was established. The computational approach is based on acoustic perturbation equations forced by stochastic sound sources to simulate sound generation and radiation in space and time. Good agreement was found between predicted and measured slat noise spectra and for trailing edge noise generated at a NACA 0012 airfoil. In this work the simulation approach is applied to generic high-lift noise problems defined in the framework of the European project VALIANT. The problems considered are sound generation at the slat and the flap of simplified high-lift geometries. Results of the time-averaged mean flow and turbulence statistics from RANS are evaluated and compared to measurements. A comparison of the statistics of the stochastic fluctuations computed with the stochastic turbulence model FRPM with flow data is provided. Sound generated by the stochastic source model is evaluated and compared to measurements. As a general outcome of the study, the agreement between the RANS flow fields to measurements is quite good. The reconstruction of the turbulence statistics by FRPM is in general accurate, but in some reagions it differes from the target statistic. The agreement of the acoustic far field predictions to the measured data is satisfyings. The overall procedure is with a computational effort of around two days quite short.

Fuselage Excitation During Cruise Flight Conditions:Measurement and Prediction of Pressure Point Spectra

In the last �fifty years many semi-empirical models to predict surface pressure fluctuations beneath turbulent boundary layers (TBL) have been developed for a large variety of test conditions. Nowadays, the relevance of the TBL as a source of cabin interior noise is steadily increasing, due to quieter aircraft engines. The possibility of predicting surface pressure auto-spectra with the various publicly available semi-empirical models at several positions on the fuselage of DLRs Advanced Technology Research Aircraft (ATRA) is investigated. A large validation database was used, involving in-flight measurements at different flight levels (FL) and Mach numbers, applying two different sensor types. Predictions were performed based on semi-empirically estimated TBL parameters (partly included in the different models) and, additionally, based on TBL properties that were extracted from CFD simulations. This procedure served to identify different sources of error in the prediction. Overall, it is shown that todays models provide a large (> 10 dB) scatter among the predicted spectra. Even the most suitable approaches are not generally applicable to all relevant positions at the fuselage. Particularly in regions with strong pressure gradients and high turbulence kinetic energy measured auto-spectra cannot be reproduced with su�fficient accuracy. This indicates the need for more universally applicable CFD- and CAA-based surface pressure prediction methods.

Experimental Synthesis of Sound Pressure Fields for Active Structural Acoustic Control Testing

For next generation aircraft, contra rotating open rotor propulsion systems are currently discussed. Their economic advantages are in conflict with their high noise emission, which concentrates in distinct frequency bands. To bring contra rotating open rotor engines into operation at commercial aircraft and to maintain the passenger comfort level, active systems for noise reduction are considered. In this article, a contra rotating open rotor noise simulator for the future testing of active systems is presented. The simulator consists of a 14 × 8 loudspeaker array, which is placed close to a test fuselage. Driving each loudspeaker by a particular signal, complex sound pressure fields can be synthesized on the fuselage. The algorithm presented here calculates loudspeaker signals to synthesize the target sound pressure spectra on the fuselage. These target spectra were derived from numerical contra rotating open rotor engine simulations, coupled with a Ffowcs Williams–Hawkings solver to propagate the calculated sound pressure field toward virtual surface microphone positions on the test fuselage. Requirement for such synthesis is the knowledge of the transfer paths from all loudspeakers to all surface microphones. In experiments, these paths are measured, and the loudspeaker signals are synthesized by the algorithm. Finally, measurements with the loudspeaker array and a microphone array are shown which prove the concept.

Aircraft and technology for low noise short take-off and landing

This paper discusses characteristic multi-disciplinary issues related to quiet short take-off and landing for civil transport aircraft with a typical short to medium range mission. The work reported here is focussing on the noise aspects and is embedded in the collaborative research centre CRC880 in Braunschweig, Germany. This long term aircraft Research initiative focusses on a new transport aircraft segment for operation on airports with shorter runway length in commercial air transport. This calls for a community-friendly aircraft designed for operations much closer to the home of its passengers than today. This Scenario sets challenging, seemingly contradictory aircraft technology requirements, namely those for extreme lift augmentation at low noise. The Research Centre CRC880 has therefore devised a range of technology projects that aim at significant noise reductions and at the generation of e�cffient and flexible high lift. The research also addresses flight Dynamics of aircraft at takeoff and landing. It is envisaged that in general significant noise reduction -compared to a reference turbofan driven aircraft of year 2000 technology- necessarily requires component noise reduction in combination with a low noise a/c concept. Results are presented from all the acoustics related projects of CRC880 which cover the aeroacoustic simulation of the source noise reduction by flow permeable materials, the characterization, development, manufacturing and operation of (porous) materials especially tailored to aeroacoustics, new UHBR turbofan arrangements for minimum exterior noise due to acoustic shielding as well as the prediction of jet noise vibration excitation of cabin noise by UHBR engines compared to conventional turbofans at cruise.

Fuselage Excitation During Cruise Flight Conditions: From Flight Test to Numerical Prediction

In the context of aircraft cabin interior noise, the fuselage structural excitation by turbulent boundary layer (TBL) flows is an important noise source for aircraft manufacturers to deal with. During cruise flight, it is the dominant source of cabin noise for state-of-the-art aircraft. Aircraft at cruise conditions is flying at high Mach numbers, typically between Ma = 0.78...0.85, dependent on the type and mission of the aircraft. The vortices within the TBL cause pressure fluctuations on the fuselage and therefore, its structure receives energy and starts to vibrate. In this paper, methods to estimate the aerodynamic TBL sources by numerical and semi-empirical tools are presented. Besides, also the induced vibration of real aircraft structures is calculated with existing industrial tools. Furthermore, measured flight test data used for validation are presented and finally measurements and predictions are compared.