Improved Analytical Prediction of Boundary Layer Induced Rotor Noise using Circumferential Modes

2.5 Noise Prediction by Analytical or Numerical Models

Modern aircrafts have to meet strict regulations concerning the emission of pollutants and noise to limit the environmental impact and ensure the acceptance of growing air travel. This demand will not be satisfied by the classical tube-wing aircraft architecture, as it has already come close to its maximum of efficiency. The introduction of revolutionary technologies, such as boundary layer ingesting engines, has the potential to bridge the gap towards a transformation of aircraft shape to highly fuel efficient blended or hybrid wing bodies with embedded engines. Beside the potential benefits of a boundary layer ingesting engine concept new concerns arise related to the noise emission of such configurations resulting from the highly distorted engine inflow.
The DLR department of engine acoustics is developing an analytical tool that allows the modular prediction of several tonal and broadband noise sources. In an earlier publication an analytical theory, based on a modal approach in the frequency domain, was presented for the prediction of broadband noise caused by the ingestion of a turbulent boundary layer. This theory is able to predict peaks in the broadband noise spectrum at the blade passing frequency and its higher harmonics for various operating conditions of the rotor
The current work aims on the improvement of this theory. The prior used one-dimensional isotropic von Kármán spectrum is replaced by a one-dimensional Kerschen-Gliebe spectrum to account for the anisotropy of the inflow. The estimation of the transversal coherence length is substituted by a recently published model that was specifically developed for a turbulent boundary layer. This model replaces an ad-hoc assumption made for the axial coherence length, which determines the intensity of blade-to-blade correlation and is therefore a steering parameter.
To verify the adapted theory, noise predictions are compared with acoustic measurements of the "Fundamental Test Case 3 (FC3)" published in the framework of the "Fan Broadband Noise Prediction Workshop 2015-17".