Julian Winkler

Large-Eddy Simulation and Trailing-Edge Noise Prediction of an Airfoil with Boundary-Layer Tripping

This paper deals with hybrid methods for trailing edge noise prediction of a single NACA 6512-63 airfoil at zero angle-of-attack. The procedure is based on two steps. First, an incompressible large-eddy simulaton (LES) of the airfoil trailing-edge flow is performed. Then, in a second step, the far-field acoustic pressure is predicted from the LES source terms using three different methods based on the acoustic analogy. These are Amiet’s and Ffowcs Williams & Hall’s trailing-edge noise theories and Curle’s compact dipole solution. Aerodynamic and acoustic results are then compared to experimental measurements performed in the aeroacoustic wind tunnel of the University of Siegen. For comparison purposes with experimental measurements, the large-eddy simulation includes wind tunnel installation effects by a choice of suitable inflow boundary conditions. The experimental chord-based Reynolds number is 1.9× 10 5 , which results in long laminar flow regions along the airfoil that lead to Tollmien-Schlichting instabilty waves with an associated additional tonal and broadband noise radiation. To avoid this additional noise source, the boundary layer of the airfoil had been tripped on both sides in the experiment. This tripping has been included in the numerical grid of the LES in its full complex configuration (forward facing serrations) and also as a simplified geometry (simple stair-step). Eventually three different LES computations are compared that differ in boundary condition and boundarylayer tripping modeling. The modeling effects are assessed with regard to the aerodynamic and aeroacoustic prediction capability of the respective LES approaches. The comparison stresses once more the necessity of accurate boundary conditions in the LES in order to arrive at comparable results with wind-tunnel measurements.

Airfoil Trailing-Edge Blowing: Broadband Noise Prediction from Large-Eddy Simulation

Trailing-edge blowing has in recent studies found to be a potential technique for broadband noise reduction in turbomachinery applications with rotor-stator arrangement. The key-idea is to inject fluid into the wake of the rotor in such a manner that the wake will become momentumless and the turbulence structure will be modified by the enhanced mixing process. Ultimately, this wake manipulation should lead to a reduced aeroacoustic response of the downstream stator-vane with the modified turbulent wake of the rotor. The study presented within this paper investigates the trailing edge blowing mechanism by numerical means in a simplified configuration. A large-eddy simulation on a NACA 6512-63 airfoil without and with trailing-edge blowing is undertaken to investigate the effect of blowing on wall-pressure statistics of the blowing airfoil and also its wake turbulence structure and characteristics. The broadband self-noise of the airfoil is predicted by Amiet’s trailing edge noise theory. It appears that two competing mechanisms exist: a potentially increased airfoil self-noise due to the blowing jet interacting with the jet slot-lip and the trailing edge, and a reduction in interaction noise through manipulation of the wake turbulence and early wake-turbulence decay. Overall this study provides a first insight into some relevant parameters concerning the potential for broadband noise reduction through trailing-edge blowing.

Airfoil Trailing Edge Noise Prediction from Large-Eddy Simulation: Influence of Grid Resolution and Noise Model Formulation

This study is concerned with the effect of a grid refinement in large-eddy simulation (LES) for airfoil trailing edge noise prediction using different model approaches. Three different LES studies are carried out with a difference either in the spanwise grid resolution or the computational spanwise domain size. The airfoil simulated is equipped with a forward-facing serration trip on both sides to trigger the transition from a laminar to turbulent boundary layer. Differences in the evolution of the wall-pressure traces are discussed and several different analytical trailing edge noise models are tested. In addition, the airfoil noise is predicted with a finite element solution to Lighthill’s equation.

Fan Broadband Interaction Noise Modeling

Results from a detailed investigation into the eect of modeling assumptions used with the RSI method to compute broadband interaction noise downstream of a turbofan engine’s fan stage are presented. The modeling assumptions that are considered include the use of a Green’s function to obtain the exhaust noise from the unsteady vane surface pressure, the implementation of a 2D vs. 3D vane model, and the form of the turbulence velocity correlation function. Calculation of the duct acoustics via the Green’s function is shown to be robust when one selects the frequencies used for the calculation such that they do not coincide with a duct cut-on/cut-o edge frequency. The unsteady vane response calculated by strip theory is found to be dierent than that predicted with a three-dimensional vane model. However, it is not clear yet how these dierences specically impact the predicted exhaust noise. Inclusion of the inhomogeneity of the turbulence across the passage is not so important because the average passage value provides good results. The form of the correlation function used to model the inow turbulence is shown to have a strong impact on the overall sound power level. Within the RSI framework, it is shown that using a common 3D spectrum (e. g. Liepmann and Gaussian spectra) but disregarding the k3 contribution gives results 20 dB lower than when the nontraditional RSI spectrum is used. The inclusion of the k3 eect with the common 3D spectrum within RSI leads to a dierence of 10 dB

Direct Numerical Simulation of the Self-Noise Radiated by an Airfoil in a Narrow Stream

A direct numerical simulation (DNS) is conducted for the first time of an airfoil that is embedded in a wind-tunnel flow at a realistically high Reynolds number of Rec = 150, 000, based on the chord length. The nominal wind-tunnel speed corresponds to a Mach number of M = 0.25. The simulation domain comprises only the near field around the airfoil; the aerodynamic effect of the wind tunnel is included by using an appropriate set of inflow boundary profiles—a technique that has been used successfully in previous numerical airfoil studies by the authors of this paper. The boundary layer on the airfoil suction side is tripped using a purposely developed immersed-boundary method (IMBM). The results of this simulation are compared to an incompressible large-eddy simulation (LES) and experimental data. Both simulation approaches yield very good predictions of the steady and unsteady flow field and the acoustic far field.

Turbofan Inlet Distortion Noise Prediction with a Hybrid CFD-CAA Approach

Modern turbofan engines have drooped inlets which cause the mean flow into the fan to be circumferentially non-uniform at practically all flight conditions. This effect leads to the appearance of additional fan noise sources which propagate even at subsonic rotor tip speeds. In this paper we demonstrate a hybrid CFD-CAA methodology for predicting fan inlet noise in the presence of such distorted mean flows. A compressible URANS simulation is performed to first capture the unsteady acoustic source terms, which are then propagated using a linear potential-based wave equation solver. Coupling between the two simulations relies on Mohring’s acoustic analogy which is applicable to non-uniform, irrotational flow and uses the stagnation enthalpy as the acoustic variable. The CFD-CAA coupling is successfully verified and shown to accurately reproduce the acoustic field directly simulated by highly resolved CFD. It is shown that the current methodology offers significant improvement in accuracy over the commonly used approach of specifying sources in terms of uniform-flow duct modes. The methodology demonstrated here enables, for the first time, high fidelity simulation of the acoustic field within an inlet using computationally affordable finite-element based methods. The methodology therefore allows rapid parametric studies of inlet liner impedance effects on far-field sound in the presence of realistic acoustic sources.