Malte Siefert

Sweeping Sound Generation in Jets Realized with a Random Particle-Mesh Method

the sound spectrum in a manner that cannot be explained based on the classical Kolmogorov-theory of turbulence. But because of the lack of appropriate methods these findings could not be incorporated into the common statistical modeling approaches for realistic aeroacoustic problems using for example the acoustic analogy. In this article we develop an ecient Random Particle-Mesh method which enables us to model successfully the sweeping mechanism. We use it to simulate the noise radiation of a realistic subsonic jet. The results show that sweeping dynamics of turbulence is an important noise production mechanism in jets. Noise from the fine-scale turbulence can be predicted. There are furthermore indications that also Mach waves are generated.

Handling of Non-Periodic Contra Rotating Open Rotor Data

In this contribution, we present two different concepts to handle the non-periodic nature of Contra Rotating Open Rotor (CROR) if the front and the aft rotor rotate with a slightly different rotational speed. The first procedure that is presented consists of a correction matrix applied to the source data used in the DLR FWH-code APSIM+. For periodic data the correction matrix reduces to the identity matrix, thereby recovering the standard Fourier transformation. The second method is based on the Vanicek approximation, and consists of a successive least-square approximation of non-periodic data. The developed methods are tested with artificially generated data, illustrating the ability to accurately representing non-periodic data. A comparison between the two methods shows that the first method is more accurate than the Vanicek approximation. Preliminary results on actual non-periodic CROR data reveals the influence of the non-periodic correction as compared to uncorrected data, i.e., differences ranging up to 10 dB are seen for the considered cases.

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.

Refraction and Scattering in High Mach Number Boundary Layers

A significant part of the cabin noise of airplanes comes from the engines. The noise field of the engines is considerable changed due to the fuselage’s boundary layer before it hits the fuselage skin. In this work we study the influence of the mean boundary layer as well as of the instationary turbulent fluctuating boundary layer on the sound field on the fuselage using a point noise source. The flow field is calculated with the Reynolds-averaged Navier-Stokes (RANS) equation and the turbulent fluctuations are reconstructed stochastically by means of a Random-Particle-Mesh method (RPM). The sound propagation is calculated with a computational aeroacoustic (CAA) approach solving the linearized Euler equations. At high Mach numbers we find a strong refraction effect due to the mean boundary layer and a substantial scattering effect of the turbulent vortices. The surface pressure completely differs from the free acoustic field: The frequency is increased, the pressure field is spatially incoherent and a considerable part of the original signal is reflected by the boundary layer. According to our knowledge, the combined effect of refraction and scattering of high Mach numbers boundary layers was not studied before. We think that the results are very important for the assessment of the structural response of the fuselage skin to the acoustic surface pressures.

Anisotropic Synthetic Turbulence with Sweeping Generated by Random Particle-Mesh Method

We develop an efficient method to generate convecting synthetic turbulence in complex mean flows. In this article we present the extension to anisotropic turbulence, realistic spectra and sweeping effects, i.e. the advection of inertial range structures by the energy containing large scales. We show that due to this formulation sweeping effects can be included and the spatial-temporal field shows the similarity of Kraichnan’s sweeping hypothesis.

A synthetic wall pressure model for the efficient simulation of boundary layer induced cabin noise

This work presents a complete numerical procedure for the simulation of the vibroacoustic response of a structure to a turbulent boundary layer. As a paradigm for the sound transmission into an aircraft cabin, a generic set-up is studied that is composed out of a closed cavity which is separated from a turbulent boundary layer by an elastic flat plate. The focus is on a new stochastic model for synthesizing the instationary surface pressure fluctuations. Due to the use of a random Particle mesh method with recursive filters, this method is very efficient. The spatial-temporal pressure field is used as input in a finite element method to calculate the vibroacoustic response of the elastic plate. The sound entrainment into the cabin is simulated with the finite element method as well. Results encompass the calculation of surface pressure characteristics, acceleration spectra of the surface, and sound spectra of the closed cavity. The results are compared with experimental data taken from the EC founded project ENABLE.

A stochastic source model for turbulent noise prediction including sweeping time dynamics

We extend a low‐cost computational aeroacoustic approach by taking into account temporal effects of the modeled turbulent flow. As the noise production is determined by the turbulent frequency spectrum rather than the wavenumber spectrum, temporal properties of the turbulent flow play a role in many cases. We focus on the most dominant temporal effect, on sweeping, i. e. the advection of inertial range structures by the energy containing large scales. By introducing a feedback‐mechanism of the turbulent field onto itself, we are able to incorporate this effect in the fast random particle method, which was successfully applied to different cases such as slat noise, jet noise and others. It is shown that sweeping reproduces important properties of spatio‐temporal correlations of the turbulent flow, which are not grasp by most turbulence‐models. The influence on the sound generation will be discussed for aeroacoustic simulations of jet and trailing edge noise.

Active CFRP-Panels for Reduction of Low-Frequency Turbulent Boundary Layer Noise

The increasing use of modern lightweight materials in the aircraft industry raises the demand for innovative and lightweight-compliant noise abatement techniques. Smartstructures technology is able to overcome the passive constraints in the low-frequency range (<500Hz) by augmenting the light-weight structure with structurally integrated transducers and a suitable control scheme. In these investigations, an active carbon fiber reinforced plastic (CFRP) panel was designed, manufactured and experimentally investigated in an acoustic wind tunnel. Measurement data of turbulent wall pressure fluctuations, structural vibration and active sound power provide a database for the verification and validation of the developed simulation models. Active control of the smart CFRP-panel showed a broadband reduction of third-octave band sound power level up to 6dB(A).