Sind elektrisch angetriebene Flugzeuge effizient und leise(r)?

The noise of electrically driven aircraft is discussed including efficiency aspects. The presentation provides a survey over all sources of sound on aircraft and identifies those which directly or indirectly depend on the driving system of the propulsor. It is shown that little benefit may be expected by replacing conventional drive systems by electric ones. However, potential for noise benefits is seen in three concepts specific to electric drives: i) distributed propulsors, ii) non-uniform rotation, synchro phasing of propellers. Considerably noise reduction in line with a slight improvement of the efficiency is shown for distributed propellers. Noticeable benefit is projected for the use of controlled oscialltory non-uniformity of the rotation. The concept of synchrophasing (many propellers) may reduce noise in defined (and choosable) directions, while leaving noise increase elsewere, at the same time being acompanied by high system complexity

CAA-Simulation of Flap Noise Generation using Perturbation Equations

The interaction of vorticity with high lift systems of airfoils is an important source of airframe noise. In the framework of the SWING project, noise generation measurements at a wing section of a modern transport aircraft with deployed flap have been performed, where the influence of a flap cove cover on the radiated sound was examined. In the present paper, these experiments will be studied numerically using the DLR CAA-code PIANO. In PIANO, the interaction of a single vortex with a flap is calculated by the solution of nonlinear perturbation equations with a RANS mean flow. Non-linearities are taken into account up to first order. Self excited oscillations of the disturbance quantities, which are a consequence of the unstable mean flow field are tried to suppress by introduction of a wall damping term in the momentum equations which shall enforce a no slip boundary condition also for the disturbance flow field. It will be shown that the success of this procedure depends on the details of the mean flow field. Qualitative agreement with the experimental results has been found

Active Flow Control for Interaction Noise Reduction of Contra-Rotating Open Rotors

A possible way to decrease interaction tone noise of a contra-rotating open rotor is through the application of trailing-edge blowing by reducing the momentum deficit of the front rotor wake, and therefore its interaction with the aft rotor. In this contribution, an assessment of front-rotor trailing-edge blowing is presented for the reduction of contra-rotating open-rotor interaction noise. For this purpose, the German Aerospace Center designed a generic contra-rotating open rotor that has been modified to include trailing-edge blowing at the front rotor blades. With the German Aerospace Center computational fluid dynamics code TAU, unsteady Reynolds-averaged Navier–Stokes simulations have been made of the baseline and the trailing-edge blowing configuration. Subsequently, an aeroacoustic analysis has been performed with the Ffowcs-Williams/Hawkings tool APSIM+ for both configurations. The results show negligible differences of the aerodynamic performance, with significantly lower unsteady loading of the aft rotor when the trailing-edge blowing technique is active. The aeroacoustic results illustrate that the interaction tones are notably ameliorated; that is, a reduction over almost the complete polar angle is seen when trailing-edge blowing is active. With respect to the A-weighted overall sound pressure level, a reduction of approximately 2.5 dBA in the maximum level for the trailing-edge blowing configuration is achieved for the considered geometry. Noise reductions up to 5 dBA are displayed in the up- and downstream directions, which mainly arise from reduced interaction tones caused by the trailing-edge blowing technique. The decreased noise radiation with trailing-edge blowing mainly stems from a reduction in the first two interaction tones

Perturbation Approaches to the Simulation of Airframe Noise

The application of perturbation approaches to simulate noise mechanisms of aircraft during landing approach is discussed. Euler-perturbation approaches that simulate sound generation at obstacles due to upstream injected test-vorticity allow to estimate the efficiency of a geometry to transform vortical energy into acoustic energy. Acoustic analogies with sources determined either by an unsteady flow simulation or described statistically yield hybrid two-step methods that in addition take into account the turbulent source strength. A hybrid method is presented that predicts trailing edge noise based on a Large Eddy Simulation (LES) of the compressible flow problem in the immediate acoustic source region and acoustic perturbation equations (APE) for the simulation of the acoustic radiation problem in space in time. The mean flow convection and refraction effects are part of the simulation of wave propagation such that the computational domain of the flow simulation in general has to comprise only the significant acoustic source region. Using linearized acoustic perturbation equations the unbounded growth of hydrodynamic instabilities in critical mean flows is prevented completely

Simulation Of Multigeometry Scattering Problems And The Radiation And Refraction of Acoustic Waves Through A Shear Layer With Instability Waves Suppressed

The scattering of harmonic acoustic waves at multiple rigid cylinders is simulated applying a Chimera technique and a body-fitted grid approach. The Chimera technique uses a Cartesian background mesh in conjunction with body-fitted O-grids in the immediate vicinity of the solid surfaces to resolve the geometry. The body-fitted grid approach applies a multiblock topology with O-grids wrapping the cylinders while an H-type topology is used away from the cylinders that converges against a Cartesian mesh in the far field. Both grid approaches are compared using a similar far-field resolution of about seven points per wavelength. The refraction of acoustic waves in an unstable jet is simulated using a modification of the linearized Euler equations that can be proven to be stable for arbitrary mean flow fields. The simulations confirm the modified perturbation equations to suppress the excitation of unstable modes. The wave operator encoded in the perturbation equations is known to be exact for irrotational mean flows, that is, in the high-frequency limit of the Strouhal number based on frequency and mean-flow vorticity tending to infinity. It is shown that even for a Strouhal numbers of 0(1), the modified acoustic perturbation equations resolve mean flow refraction and scattering effects accurately. For the jet, larger differences to the reference solution appear, in particular, downstream of the harmonic source position due to the small Strouhal number in the jet shear layer