Klaus Ehrenfried

Comparison of Iterative Deconvolution Algorithms for the Mapping of Acoustic Sources

The DAMAS2 algorithm is compared with several other Fourier-based deconvolution approaches. One is the Richardson-Lucy method, which is widely used for the deconvolution of astronomical images. The second is a modified gradient-type NNLS approach, where spectral procedures are implemented to accelerate the computations. Both methods require a computational effort similar to the DAMAS2 algorithm. All three algorithms use an approximate shift invariant point-spread function. %do not take the variation of the %point-spread function into account. %They solve the deconvolution problem only approximately. Furthermore it is described how the DAMAS2 and the Fourier-based NNLS algorithms can be embedded in an outer iteration loop to take the variation of the point-spread function into account. The resulting methods have two nested iterations, which require much more numerical effort than a single DAMAS2 iteration loop. All methods are tested with synthetic data. At first an example with a simple linear array and a small opening angle is considered, where the variation of the point-spread function in the source region is negligible. In this test case the results of the DAMAS2, the modified gradient-type NNLS, and the Richardson-Lucy algorithm are compared. It is shown that these algorithms more or less introduce oscillations in the reconstructed source distribution. A second test case with a planar array and a large opening angle is presented, to demonstrate the influence of a strong variation of the point-spread function. It is shown that the approximate methods lead to distorted results, while the methods with nested iterations give a significantly better reconstruction of the source distribution.

Investigation of the unsteady flow field inside a leading edge slat cove

An experimental investigation of the unsteady flow field inside a slat cove was performed to identify possible sound source generation mechanisms. Tests were carried out on a swept constant chord half-model with a stored chord length of 0.45 m. Experiments were conducted in the 2.0 m x 1.4 m wind tunnel facility of the Technical University of Berlin at an angle of attack of 4°, 8°, 12° and 16° and free stream velocities ranging from 20 m/s up to 35 m/s. The slat cove flow field was investigated by Particle Image Velocimitry (PIV) while the radiated sound was recorded with a single microphone. Both measurements were conducted in a synchronized manner to allow a correlation between the flow field and the sound pressure. An inspection of instantaneous PIV images shows a free shear layer emanating from the slat cusp with discrete vortices further downstream. The shear layer impinges on the inner slat surface and some vortices get trapped inside the recirculation area. Based on a simplified form of the Lighthill-equation sound source terms are calculated. A dipole type sound source distribution is found along the shear layer with decreasing strength further down the shear layer. In addition the vortex shedding frequency of the shear layer vortices is estimated from the instantaneous PIV data. The shedding frequency has a Poisson-like probability distribution with center frequencies ranging from 4 kHz to 10 kHz. The mean shedding frequency scales linearly with the mean velocity of the shear layer. It could not clearly be verified that vortex shedding from the slat cusp is the cause for low frequency broad band noise. For a particular configuration several discrete tones can be observed. It is suspected that this tonal noise originates from the junction between the slat and the fuselage.

Particle Image Velocimetry in Aerodynamics: Technology and Applications in Wind Tunnels

Particle image velocimetry (PIV) is increasingly used for aerodynamic research and development. The PIV technique allows the recording of a complete flow velocity field in a plane of the flow within a few microseconds. Thus, it provides information about unsteady flow fields, which is difficult to obtain with other experimental techniques. The short acquisition time and fast availability of data reduce the operational time, and hence cost, in large scale test facilities. Technical progress made in the last years allowed DLR to develop a reliable, modular PIV system for use in industrial wind tunnels. The features of this system are summarized and results of recent PIV applications are presented.

Microphone-array processing for wind-tunnel measurements with strong background noise

During the last decade, microphone arrays became a standard tool to study aeroacoustic sources. Nowadays, arrays are frequently applied in fly-over measurements 1 and wind tunnel testing. 2 To improve the resolution of the arrays and to reduce disturbing side lobes in the measured source maps, sophisticated deconvolution methods were proposed 3-8 in recent years. First, these methods were tested using synthetic data or data from experiments with generic configurations. 8 In a next step the methods have to be applied to real experiments. This is done in the present paper. It is tested, if the deconvolution works also under difficult conditions, and if the findings of the simulations using artificial background noise 7 can be confirmed in case of real data.

Reduction of Flap Side-Edge Noise: Passive and Active Flow Control

Microphone array and particle image velocimetry measurements have been performed to investigate the potential of passive and active flow control methods for flap noise reduction. The extended wing flap of the swept constant chord half-model was equipped with either a blowing facility as an active flow control device or with other flap side-edge modifications such as wing tip fences, microtabs and winglets. The flap side edge noise is reduced with the blowing configuration between 2 kHz and 5 kHz. The maximum noise reduction of 15:9 dB is achieved at 2:9 kHz. Varying the diameter of the blowing orifices shows that the noise reduction is governed by the momentum rather than the flow rate of the blowing. The winglets and the suction side fence are most effective in reducing the flap side-edge noise. The reduced level with these configurations is even lower than with the active blowing. The vortex generators and the pressure side fence turn out to be the least effective flow control devices. PIV-measurements at an isolated unswept flap model show that the main vortex on the suction side exhibits a very inhomogeneous vorticity distribution which can be attributed to the unstable shear layer that separates at the lower corner of the flap side-edge and to the negative vorticity production on the suction surface. With blowing the vorticity of the shear layer is concentrated in several small vortices. The distance between those vortices and the solid surface increases with higher blowing momentum and because of this the negative vorticity production due to recirculation is disappears. These distinct modifications of the side-edge flow field explain the noise reduction shown in the aeroacoustic measurements.

REDUCTION OF FLAP SIDE EDGE NOISE BY ACTIVE FLOW CONTROL

One dominant airframe noise source is situated at the side edge of the extended wing flap. The objective of the present study is to reduce this noise by blowing air into the flap side-edge vortex to displace or destroy the vortical structure and thus reduce the emission of sound. PIV measurements without blowing yield a rather complicated unsteady vortical structure at the flap side-edge which confirms the assumption of a noise source. This is verified by microphone array measurements. They show that the flap side-edge noise, besides other noise sources, is present over a broad frequency range. The flight parameters such as angle of incidence and slat and flap angles, however, determine which noise source is dominant. PIV measurements with blowing show that the vortical structure can be almost completely dispersed and that the maximum vorticity in the vortex core is reduced. Consequently, a reduction of the flap side‐edge noise can be seen in the microphone array measurement. In addition, the sound pressure level in the acoustic far field is reduced by 3 to 4 dB above 1.25 kHz.

Investigation of the systematic phase mismatch in microphone-array analysis

The paper investigates phase deviations as they occur in aeroacoustic array measurements in open jet wind tunnels. The accuracy of an approximative zero-thickness shear layer model, which is used in practice to predict the phase shift between source and microphones, is investigated numerically. The exact wave propagation through a generic shear layer is calculated by solving the full linearized Euler equations using a finite element method and by means of geometrical acoustics. The results are taken to determine the phase deviations between the simplified model and the exact case. It is demonstrated that the position of the zero-thickness shear layer is important for the accuracy at high frequencies. Additionally measurements are made to analyze the phase fluctuations which are generated by the scattering at turbulent structures in the open-jet shear layer. A procedure is proposed to compensate these phase fluctuations and to improve the array efficiency.

Microphone array wind tunnel measurements of Reynolds number effects in high-speed train aeroacoustics

The present study focuses on the Reynolds number dependence of high-speed train aeroacoustic sound sources. To cover a wide range of Reynolds numbers the experimental investigations are carried out on a 1: 25 scale-model of the high-speed train Inter City Express 3 by conducting microphone array measurements in two wind tunnels. The latter are the Aeroacoustic Wind tunnel (AWB) of the German Aerospace Center (DLR) in Brunswick, providing nearly perfect acoustical conditions, and the Cryogenic wind tunnel (DNW-KKK) of the DNW (German -Dutch wind tunnels) in Cologne, allowing measurements at higher Reynolds numbers. Two types of sound sources with different characteristics at Reynolds numbers of up to Re = 0.46 × 106 have been identified by measurements in the AWB. It was found, that the aeroacoustic noise from the bogie section is dominant for frequencies f < 5 kHz and can be characterised by cavity mode excitation. Further, the pantograph is the dominant sound source above f = 5 kHz with an Aeolian tone characteristic. Additional aeroacoustic measurements have been conducted in the cryogenic wind tunnel DNW-KKK in order to analyse the Reynolds number dependence of the noise generated at the first bogie, for higher Reynolds numbers of up to Re = 3.70 × 106. The DNW-KKK admits varying the Mach and Reynolds numbers independently. These measurements reveal only a weak Reynolds number dependence of the noise source generated at the first bogie.

Simultaneous Particle Image Velocimetry and Microphone Array Measurements on a Rod-Airfoil Configuration

The aeroacoustic sound generation process on a rod-airfoil configuration has been investigated by means of simultaneous particle image velocimetry in the near field and phased-microphone-array measurements in the far field. Up to 20,000 particle image velocimetry snapshots per field of view have been recorded. A coplanar multiplane particle image velocimetry system has been used, providing statistically independent samples of the temporal derivative of the velocity field. Both measurements were conducted in a synchronized manner so as to enable the calculation of the cross-correlation between the acoustic pressure and flow quantities derived from the measured velocity fluctuations. The main idea of the concept used in the study presented here was to use the coefficient matrix obtained from the aforementioned correlation to identify regularities in the near-field fluctuations that are related to the radiated sound field. Additionally, a technique to localize flow structures responsible for the far-field sound by means of the calculation of the near-field intensity has been tested.

Simultaneous Multiplane PIV and Microphone Array Measurements on a Rod-Airfoil Configuration

The influence of the elastic rotor blade deformation and the aerodynamic interference from the fuselage on the rotor aerodynamics as well as rotor noise characteristics was studied. A BO105 Main Rotor /Tail Rotor/Fuselage (FUS) configuration is chosen for the numerical simulations. A coupling of unsteady free wake 3-D panel method (UPM) and Airbus Helicopters’ (formerly: Eurocopter) rotor code HOST was conducted to account for the effect of elastic blade deformation as well as compressibility correction. The effect of fuselage is simulated by using two fuselage models in UPM, (1) potential theory in form of a panelized fuselage and (2) an analytic fuselage influence formulation derived from isolated fuselage simulation based on (1).Measurement techniques based on microphone-arrays are well-known and common practice on scaled models in wind tunnels with closed test section. Usually, full-scale Reynolds numbers are not achieved. To increase the Reynolds number measurements are performed in cryogenic and/or pressurized wind tunnels. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. In order to use a microphone array in a cryogenic environment, coming to grips with cold hardiness and ensuring long term stability of the array fairing and the electronic devices, especially the microphones, are the primary challenge. In a first step measurements of the radiated noise from a single rod configuration have been conducted in the cryogenic wind tunnel DNW-KKK using a prototype microphone array designed for cryogenic environment. Measurements were carried out with a wide range of operational flow parameters. The measured sound radiation results showed a very good agreement with theory and a Reynolds number dependency of the measured and predicted sound power was shown.