R.A.D. Akkermans

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.

Installation Effects of a Propeller Mounted on a High-Lift Wing with a Coanda Flap. Part I: Aeroacoustic Experiments

In this contribution, we present aeroacoustic experiments concerning installation effects of propellers. Such installation effects are important as they can significantly alter the sound radiation as compared to an isolated propeller. For this purpose, detailed experiments have been conducted in the NWB aeroacoustic wind tunnel in Braunschweig, Germany. The considered geometry is a nine-bladed propeller installed in front of a high-lift wing (employing a Coanda flap). The results illustrate the influence of propeller rotational speed, blade pitch angle, wind tunnel velocity, and angle of attack variations on the sound radiation. Furthermore, with a source localisation technique insight is gained in the dominant sound sources, and reveals the importance of periodic as well as broadband noise for the considered geometry.

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...

Overset DNS with Application to Homogeneous Decaying Turbulence

In this contribution an application of a computational aeroacoustics code as a hybrid Zonal DNS tool is presented. The derivation of the Non-Linear Perturbation Equations (NLPE) extended with viscous terms is shown as well as information related to the numerical method is given. The application of the simulation tool to a generic three-dimensional test case, i.e., the Taylor-Green Vortex (TGV), is presented. This TGV, initially consists of only one length scale, develops into homogeneous decaying turbulence. Results of energy spectrum and grid convergence study are given. It is shown that the observed accuracy of the numerical code matches well with the expected theoretical order of four.

Overset DNS with Application to Sound Source Prediction

In this contribution, we present an application of a computational aeroacoustics code as a hybrid Zonal DNS tool. The extension of the Non-Linear Perturbation Equations (NLPE) with viscous terms is presented as well as information related to the numerical method. The applicability of the simulation tool is illustrated with two testcases, i.e., a 2D circular cylinder in a uniform flow at moderate Reynolds numbers and a 3D decaying flow initialised with Taylor-Green vortices. Both testcases provide results which match well with data reported in literature. The cylinder testcase verifies that the viscous terms are indeed correctly implemented (at least in 2D) and the Taylor-Green vortex case illustrates that the numerical scheme introduced minimal numerical dissipation.

Validation of Aerodynamic and Aeroacoustic Simulations of Contra-Rotating Open Rotors at Low-Speed Flight Conditions

Contra Rotating Open Rotor (CROR) propulsion systems have been the focus of a number of research projects in recent years, aiming to establish this propulsion system as an economic and environmentally friendly powerplant for future transport aircraft. In the frame of the EU 7th Framework Joint Technology Initiative Smart Fixed Wing Aircraft project, the DLR Institute of Aerodynamics and Flow Technology is participating as an associated partner in the Airbus-led studies of the Contra-Rotating Open Rotor. Due to significant technical challenges in terms of noise emissions, installation effects and certification that still need to be addressed, the numerical activities require the use of sophisticated multidisciplinary analysis tools and approaches covering both aerodynamics and aeroacoustics. Having been widely applied to the simulations of single as well as contra-rotation propellers, the DLR CFD code TAU and the aeroacoustic analysis tool APSIM+ allow for a detailed analysis and an improved understanding of the complex aerodynamics and aeroacoustics of this type of propulsion system. Drawing benefit from a set of high- quality results from aeroacoustic wind tunnel tests of a generic Airbus designed CROR configuration, this paper will present an in-depth analysis and validation of the DLR numerical approach to coupled CFD-CAA-simulations to enable reliable predictions of the aerodynamic and aeroacoustic performance of CROR propulsion systems.

Assessment of Front-Rotor Trailing-Edge-Blowing for the Reduction of Open Rotor Noise Emissions

A possible way to decrease the noise contribution of the interaction tones of a contrarotating open rotor (CROR) is through application of trailing edge blowing (TEB), by reducing the momentum deficit of the front rotor wake and therefore its interaction with the aft rotor. In this contribution, we present a novel assessment of front rotor TEB for the reduction of CROR noise. For this purpose, the Airbus Clean Sky generic Open Rotor configuration AI-PX7 configuration has been modified to include TEB at the front rotor blades. With the DLR CFD-code TAU, uRANS simulations have been made of the baseline and the TEB configuration. Subsequently, an aeroacoustic analysis has been performed with the Ffowcs-Williams/Hawkings tool APSIM+ for both configurations. The results show a negligible degradation of the aerodynamic performance, with slightly higher unsteady loading of the aft rotor. This is resulting from the front rotor tip vortices which impinge on the aft rotor due to insufficient aft rotor cropping. On the aeroacoustics side, interaction tones are most affected showing a slight decrease in maximum overall sound pressure level. Rotor-alone tones show approximately similar sound pressure levels near the rotor plane. It is demonstrated that indeed it is possible to influence CROR noise emission from interaction tones with TEB. However, the considered CROR geometry decreases the noise reduction potential due to the aforementioned small aft rotor cropping. New simulations are underway, addressing the current shortcomings of the CROR geometry (aft rotor cropping) and practical applicability (massflow reduction of TEB).

Overset-LES with Stochastic Forcing for Sound Source Simulation

An application of a hybrid RANS/LES method to trailing-edge noise, is presented in this contribution. The numerical framework of the CAA-code PIANO offers a basis for extending the Non-Linear Perturbation equations with viscous terms. As a result, a full Navier-Stokes equations perturbation analysis, denoted with “Overset”, can be performed on top of a steady background flow. With such a scale-resolving simulation tool, consisting of optimized higher-order numerical schemes for aeroacoustics, the investigation of sound source mechanism on first principals become feasible. An issue of such hybrid zonal approaches, namely the seeding of proper inflow turbulence, is solved with the Fast Random Particle-Mesh method in combination with the Eddy-Relaxation source term. The overall process chain of the intended Overset-LES usage is illustrated by means of NACA0012 trailing-edge noise computations at moderate Reynolds number and zero angle-of-attack.