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

Abstract

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 Möhring'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.

Turbomachinery Noise Predictions: Present and Future

“…Holewa et al studied the effect of the bifurcations on fan noise [38]. Winkler et al considered the effect of an inlet distortion on short engine inlets [39]. But only recently Daroukh et al considered all the effects together by computing the 360 • of a typical UHBR turbofan including a short nacelle, a strong OGV heterogeneity and the IGV of the primary engine stream, both at approach [11] and cutback conditions [40].…”

Tone Noise Predictions of a Full-Scale UHBR Engine at Approach Condition with Inflow Distortion Effects

“…This heterogeneity of the entering flow in short intakes is amplified at OP (Operating Point) conditions with non-zero AoA (Angle of Attack), typically for AoA = 10-15° for which the inflow direction is not aligned with the axis of the nacelle. As shown by recent studies [1] [2] [3][4] [5], such inflow distortions can highly modify the tone noise characteristics known from conventional turbofans. This paper presents aeroacoustic investigations performed by ONERA on a full-scale UHBR engine in the framework of the CleanSky2 European project, ASPIRE, in collaboration with NLR, DLR, and Airbus.…”

Shockwave Generation and Radiation from an UHBR Engine with Flow Distortion Using a CFD/CAA Chaining Strategy

“…This is typically the case during landing or take-off operations (angle of attack around 10°-15°) where airport neighbours are the most exposed to noise nuisance. Several recent studies have therefore focused on the impact of distortion on the noise emitted by turbofan engines [1][2][3][4][5][6][7][8]. They have shown that the tone noise characteristics can be significantly modified by: (a) introducing new acoustic sources caused by the distortion/fan blades interaction [3][4][5], (b) modifying the conventional fan-blade wakes/Outlet Guide Vanes (OGVs) interaction [3][4][5]8], and (c) modifying the generation and propagation of fan-blade shocks at transonic speeds [1][2][3][5][6][7].…”

“…However, the associated computational cost is very high and the radiation of acoustic waves outside of the nacelle cannot be correctly captured over an important distance. Most of the studies [1,3,7] are therefore based on a two-step methodology where the CFD solution is chained to a Computational AeroAcoustics (CAA) solver to propagate and radiate the shocks. The latter methodology is chosen here using the CFD/CAA chaining strategy developed by Thisse et al [9,10].…”