Tobias Berkefeld

Spectral broadening by shear layers of open jet wind tunnels

The presence of shear layers in open jet wind tunnels complicates aeroacoustic measurements. Tones from wind tunnel models are subject to spectral broadening (or ‘haystacking’) when propagating through the turbulent shear layer flow. For example, in measurements on contra-rotating propellers this obscures the identification and quantification of rotor tones. This paper describes a theoretical and experimental study into spectral broadening by the shear layers of open jet wind tunnels. A simple physical model is derived which predicts the amount of broadening as a function of a single parameter, which is proportional to wind speed, source frequency and shear layer thickness. The theory is compared against experimental data from five different wind tunnels. Especially for the smaller wind tunnels the agreement between theory and experiment is generally good, which makes it possible to retrieve the original level of a tone from a broadened spectrum.

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.

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

A Tomographic Directivity Approach to Frequency Domain Beamforming

Aeroacoustic measurements in wind tunnels are a common tool in the determination of sound sources on scaled models. Most of the algorithms used are based on the assumption that the unknown aeroacoustic sources radiate sound waves omnidirectionally, thus monopole sources are utilized. For the prediction of the noise footprint of aircrafts however, it is essential to have information on the directivity of the airframe sources. An approach to estimating this directivity is to use different array positions for measurement relative to the model which leads to different observation angles. The evaluation of measurements at several observation angles holds several issues, for example the difficult source localization due to the three-dimensionality of the point spread function and partial shadowing of sources at large observation angles. This paper therefore presents an approach to estimate the source positions taking into account information from all four observation angles at once. The positions estimated in this fashion are subsequently used for an improved prediction of source power and eventually lead to an estimation of directivity of the observed aeroacoustic sources.