Stefan Haxter

Two-Dimensional Evaluation of Turbulent Boundary Layer Pressure Fluctuations at Cruise Flight Conditions

A distributed microphone array was installed on an Airbus model A320 airplane and measurements were performed under various cruise ight conditions. Turbulent boundary layer pressure fluctuations were recorded at several ight altitudes and velocities. The array was installed directly in front of the wing root by replacing three pax windows with aluminum dummy windows equipped with Kulite pressure sensors. Combining the signals via a frequency-domain array processing technique provided information about local flow phenomena. Narrow-band convectional direction was extracted from the measurement data and decay of coherence was determined for the predominant flow direction. Two dimensional coherence of turbulent structures was compared with the empirical models of Corcos and Efimtsov.

In-flight Sound Measurements: A First Overview

A series of flight tests were carried out in June 2011 with more than 250 sensors in a cabin cross section upstream of the wings. The main purpose of the flight test was to qualify and quantify the main sources of cabin noise as well as the transfer paths to the passenger under real flight condition. The sensor set consists of pressure transducers installed in three dummy windows, accelerometers on the fuselage and the cabin and microphones inside the cabin. While varying the flight conditions by changing flight altitude, thrust and speed the main noise sources were distinguished and qualified.

Contributions of Different Aeroacoustic Sources to Aircraft Cabin Noise

The turbulent boundary layer (TBL) on the fuselage, jet noise and the air conditioning system (ACS) are considered as three important aeroacoustic sources of aircraft cabin noise. To improve current cabin noise prediction approaches as well as to investigate the different noise sources and their respective noise transfer paths, flight tests with DLR’s A320-232 research aircraft ’D-ATRA’ were carried out within the German national (LuFo IV) project SIMKAB. Extensive measurement data were collected using microphones inside the cabin, unsteady surface-pressure sensors for the characterization of the external TBL- and jet noise induced fuselage excitation, and accelerometers mounted at the frame structure, fuselage skin fields and cabin panels. Flight speed and -level as well as engine and air conditioning system operating conditions were varied to separately evaluate their parametric effects on cabin noise. The analysis of this extensive data base is still ongoing; in the current paper the focus is set on the results from microphone measurements at various longitudinal positions inside the cabin. Both TBL- and jet noise induced contributions increase towards the rear, reflecting the natural growth of the TBL thickness and typical jet noise radiation characteristics. Contrary to that the air conditioning system noise is of minor importance.

Examination of the Influence of Flight Altitude and Speed on the Efimtsov Model Parameters

A flight test was conducted and the data was evaluated in order to investigate the influence of flight altitude and flight speed on the coherence length model by Efimtsov. An underestimation of the measured values by the model was found below a frequency of 1 kHz. Above this frequency, the predicted values matched the measured values very well. The Efimtsov ansatz was found to be suitable for the description of coherence length, when the parameters are adapted. Three further analyses were performed: the flight speed was varied with constant altitude, the altitude was varied with constant flight speed, and both speed & altitude were varied together. No considerable variation was measured when the flight speed was varied separately. A small variation was determined at low frequency when the flight altitude was altered separately. When varying the parameters together, a distinct variation was found for the cases of highest speed and altitude. The applicability of the exponential coherence decay was analyzed. The absolute deviation was found to increase with lower frequency. The characteristic of the deviation was found to collapse in different regions with different scaling of the strouhal number.

Experimental Investigation of Flow-Induced Panel Vibrations at Cruise Mach Number

Surface structures exposed to a fluctuating pressure field are excited best, if the pressure field contains characteristics that match the eigenmodes and wave speeds of the structural vibration. A pressure field can be caused either by acoustical sources, or a turbulent boundary layer, or both. The aim of this paper is the experimental determination of the excitation characteristic of the boundary layer and the corresponding vibration response of an underlying structure under like conditions. The motivation behind this is to obtain experimentally a joint model for the excitation and the related response of the vibration. Individual measurements of the vibration response of a generic aircraft panel exposed to the pressure field of a turbulent boundary layer have been conducted in the past. Individual characterization of a field of fluctuating pressure on a surface has been investigated as well. However, measurements of the excitation characteristics of the turbulent boundary layer of a generic aircraft panel at the same test setup are scarce. The excitation characteristics of the turbulent boundary layer have been measured individually in several speed ranges. A wavenumber decomposition is oftentimes used to display the features of the surface pressure. In the low-speed range, Arguillat^1 detected the convective propagation on a at plate in the wavenumber domain. Smith^2 used a generic setup of a at plate and a generic car side mirror in order to find turbulenceinduced acoustic waves propagating over the surface. These acoustic waves were believed to be responsible for a part of the excitation of the surface structure. In the high speed measurements by Ehrenfried & Koop^3 in the wind tunnel and Haxter & Spehr^4 in a flight test, the convective velocity of the turbulent structures in the boundary layer were considered to be the main cause for excitation of the surface structure. At high speeds, the modes of the structure match the convective speed of the turbulent vortices in the boundary layer, which is called aerodynamic coincidence. The vibration of airplane fuselage subject to excitation by a turbulent boundary layer has been measured by Wilby and Gloyna.^5 In their measurement, accelerometers were placed in lengthwise and streamwise direction on panels, stringers, and frames of a Boeing model 737 aircraft. The distribution was carried out with large distances in between the sensors in order to find correlations in between adjacent panels and structure.

Examination of the Influence of Flow Speed on the Coherence Lengths in Turbulent Boundary Layers at High Subsonic Mach Numbers

Wind tunnel measurements of wall pressure fluctuation were conducted at high subsonic Mach number. The data was analyzed in order to calculate the spatial decay of coherence of turbulent structures in the turbulent boundary layer as a function of the Strouhal number. The decay parameter determined from the measurements was compared with the predictions from the coherence length model of Efimtsov. The Mach number was varied in order to investigate the influence of a change in flow speed on the decay parameter for various Strouhal numbers.

Determining Flow Propagation Direction from In-Flight Array Surface Pressure Fluctuation Data

b wavenumber dirty map e steering vector f frequency H hermitian i imaginary unit kc convective wavenumber kx, ky wavenumber L window size lx, ly coherence lengths M number of signal averages N number of transducers R cross-spectral matrix s, ŝ inclination factors uφ phase velocity x, y transducer position α tilt angle of the coherence pattern α̂ tilt angle of the convective ridge β angle to the center of the convective ridge γ coherence φ spectrum ξ, η transducer separation ξ̂, η̂ rotated transducer separation ξ,′ η′ rotated transducer separation

Aeroacoustic Wind Tunnel Testing of a 1:6.5 Model Scale Innovative Regional Turboprop

The aeroacoustics of an innovative regional turboprop aircraft is experimentally investigated on a 6.5 scaled model. The measurement campaign was performed at the RUAG Large Subsonic Wind Tunnel in Emmen. The model was mounted on a ceiling strut in the 7-by-5 meters test section, not acoustically treated. The aeroacoustic noise generated by the aircraft model was evaluated analyzing the pressure fluctuations acquired through a phased array of 144 microphones installed on a flat-plate on the test section floor. Pressure fluctuations were acquired in different configurations the most important ones being: the baseline solution (no innovative device applied) with engine on and the model equipped with lined Flap. For the engine on configuration, take-off and approach settings were tested, whereas only the landing configurations was investigated for the baseline lined flap comparison. The main parameters varied during the tests were: the propeller thrust, the propeller revolutions per minute, the speed of the air-flow, the incidence angle of the aircraft and the position of the microphone array. Data were rocessed and then analyzed in the frequency domain and using a conventional beamforming algorithm to retrieve the sound

Up in the Air: In-Flight Wavenumber Characterization of Surface Pressure Fluctuations at Transonic Conditions

One way of representing surface pressure fluctuations is to display them in frequency-dependent wavenumber spectra. They can be used to feed structural vibration predictions model. However, mostly models and experimental data from wind tunnel and water tunnel experiments have been employed. Apart from possible scaling effects, only hydrodynamic pressure fluctuations are incorporated in these models. Some data from flight tests exists, but again this data is limited to sections of the aircraft where the hydrodynamic pressure fluctuations are dominant. In order to consider both, acoustic and hydrodynamic surface pressure fluctuations in flight a beamforming approach is used in the following to determine wavenumber spectra of pressure fluctuations in cruise flight. This allows for the simultaneous characterization of both, hydrodynamic and acoustic pressure fluctuations and for the discrimination between the two. The wavenumber spectra can be used directly as input for the prediction of structural vibration.

Microphone localization with self calibrating acoustic GPS

The exact positions of microphones are mandatory when using them as an array to apply beamforming algorithms. Nevertheless to obtain these positions in the three dimensional space is challenging. In this paper, further developments of the acoustic calibration unit, proposed in 2009 by Lauterbach are presented. The acoustic calibration unit consists of several speakers and a reference microphone to obtain the positions of an arbitrary amount of target microphones via triangulation. These calculated positions is not necessarily the geometric position of the microphone, it is the acoustic position, which includes possible phase shifts, directivity and mounting effects. In the new acoustic calibration unit a second reference microphone is installed to measure the actual speed of sound. A new triangulation algorithm was developed to reduce the uncertainty of the known reference positions. Furthermore the new acoustic calibration unit was tested on an traversed array with 16 microphones in an anechoic chamber and the open jet array of the ONW-LLF Windtunnel with an diameter of about 4 m. In the final paper a third measurement will be included with an array where the geometric microphone positions are known within a tolerance of 1/100 mm.