58747 - Structural Integrity of Serrated Leading Edge Guide Vane Blades for Noise Reduction 

The constant challenge of obtaining more efficient and environmentally friendly aero engines led to a current trend of replacing the purely aerodynamically designed bypass outlet guide vanes with structurally loaded vanes, integrated within the load-carrying case structure. In this new arrangement, the bypass outlet guide vanes must fulfil both aerodynamic and structural requirements, withstanding the engine loads in all operating conditions. On the other hand, one of the major noise sources of high bypass ratios modern turbofan engines arises from the interaction between the turbulent rotor wake and the leading edge of the downstream outlet guide vanes. Recent research has confirmed that leading edge serrations are an effective passive control for reducing noise. Transferring the serrated leading edge concept to bypass outlet guide vanes for noise reduction purposes, therefore, requires an assessment of the structural behaviour changes introduced by the presence of the serrations.

The current study characterizes the structural behavior when adding serrations to the leading edge, as a function of serration amplitude and wavelength. The key criteria for comparing the structural response of a serrated leading edge OGV blade compared with a classical, straight leading edge one, have been identified as: buckling, peak stress, global stiffness and vibrations. The loads applied were generic, unit loads.

First, an analysis on three different methods of generating the leading edge serrations was conducted on a simplified bypass outlet guide vane geometry. The three methods for creating the serrations are characterised by: adding 10% of the chord to five equally distributed span sections while maintaining the chord length in the sections positioned at half of the wavelength, by subtracting 10% of the chord at the same sections and by adding 5% of the chord for the five sections and subtracting 5% for the half wavelength positions. The results comparison showed that the mean serrated leading edge geometry, created by both adding and subtracting from the chord, has the overall best structural behaviour from all the serrated leading edge geometries analysed. With an added mass of only 0.2% as compared to the datum straight leading edge geometry, it has the smallest buckling loading factor, the lowest relative difference to the datum for maximum displacement of the centre point and the best stress overall behaviour. All of the first 20 vibration modes also fall under 1% of the datum.

This method was then further used to create the geometries for the test matrix in a parametric study characterizing the structural behaviour of an outlet guide vane blade of a more engine relevant geometry for different serration amplitudes and wavelengths. It was found that the structural performances deteriorate with increasing number of serrations, directly related to the serrations wavelength, and with their amplitude. The buckling loading factor decreases and the total deformation increases constantly with the increase of both parameters. The presence of the troughs of the serrations introduce stress concentrators. For large enough values of either the wave length or amplitude of the serrations, the maximum von Mises stress increases significantly, and can as much as double in value. At the same time, the location of the maximum stress on the blade moves from its original position, on the leading edge of the casing fillet, to the troughs of the serrations. Overall, this work can provide reference for the designing of outlet guide vanes leading edge serrations to minimize the impact on the structural behaviour.