Optimised Aerodynamic Design of an OGV With Reduced Blade Count for Low Noise Aircraft Engines
Authors: Kröger Georg , Schnell Rainer , Humphreys Nicholas D. 
Published: July 2013

In order to meet the ACARE environmental goals further noise reduction from aircraft operation is required relative to that achieved to date. To support this aim the European Commission Seventh Framework Programme promotes the “Optimisation for low Environmental Noise impact AIRcraft (OPENAIR)” programme. Within the OPENAIR programme Rolls-Royce leads the Integrated Propulsion System Design sub platform containing a task to produce multi disciplinary optimized Fan System outlet guide vane (OGV) designs. As a partner in this task, DLR have undertaken an aerodynamic optimized design for a novel OGV concept as the first part within this task.

As a starting point, Rolls-Royce specified a high bypass ratio turbo fan engine style and a novel OGV number and space envelope within the engine. Within the EU FP6 VITAL programme a lower number of OGVs compared to current practice was investigated [1] and Rolls-Royce had previously undertaken rig tests of a range of fan / OGV blade ratios and showed potential noise penalties but also benefits, particularly for the higher tone harmonics and broadband noise for very low numbers of OGVs [2]. The advent of multidisciplinary optimization techniques for noise, aerodynamics and mechanical constraints led Rolls-Royce to specify a very low number of OGVs to investigate the benefits and the mitigation of the penalties using these optimization techniques. The multidisciplinary design optimization was planned to be undertaken in two parts. The aim of the first part, to be reported in this paper, was to achieve a good aerodynamic design for the novel OGV concept. Controlling the aerodynamic secondary losses was expected to be a challenge for the novel concept. The second part of the optimisation will also include noise.

As part of the initial design optimization study within OPENAIR, DLR present a multi objective aerodynamic design optimization for the novel very low OGV number and space envelope specified. The blade number of the OGV was reduced from 42 to 14. The first design phase covered the two dimensional airfoil section design for the reduced blade count OGV. In order to achieve an enhanced aerodynamic performance with a reduced blade number an automated process chain was used to couple the DLR in house optimiser AutoOpti [3, 4] with the two dimensional flow solver MISES [5]. Three operating points were considered in the aerodynamic design covering a wide range of the compressor operating map.

It has been found that the design of a low blade count OGV is feasible by means of good aerodynamic performance. Additionally the aerodynamic losses could be reduced in all operating points further and small flow separation areas on the suction surface closed to the hub and tip endwalls implied a further improvement potential. Therefore, the second design phase was focused on the three dimensional endwall optimisation including a variable flow duct and fillet geometry. An automated process chain was established for this purpose. As a result the flow separation was prevented in all operating points. In the last step of the DLR work a high fidelity URANS simulation of the configuration with the newly designed OGV was conducted to assess its aero-acoustical behavior at this stage of the aerodynamically optimized OGV design and the emitted sound power level downstream the OGV for the optimised low blade number configuration is presented in this work.