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Experimental characterization of turbulent pressure fluctuations on realistic Contra-Rotating Open Rotor (CROR) 2D airfoil in representative high subsonic Mach number

Periodic Reporting for period 1 - CRORTET (Experimental characterization of turbulent pressure fluctuations on realistic Contra-Rotating Open Rotor (CROR) 2D airfoil in representative high subsonic Mach number)

Reporting period: 2016-08-01 to 2018-01-31

Open rotors are a type of aircraft engine designed reduce fuel consumption by as much as 20% compared to currently used engines. This is not only more cost efficient but also better for the environment. Unfortunately the lack of a nacelle prevents the reduction of aircraft engine noise significantly, compared to conventional engines. Advances in open rotor engine development have resulted in the reduction of tonal noise from their spectrum, leaving broadband noise to constitute a significant part of their emitted noise. However, being able to model and predict open rotor broadband noise is still in its infancy, with dedicated research efforts necessary to bring predictive tools to the same level as for tonal noise. A major problem is the lack of data for the high subsonic Mach conditions encountered by rotor blades during aircraft approach (0.5) take-off (0.7) or cruise (0.9). To address this issue the CRORTET project successfully wind-tunnel tested 2 relevant airfoils to build an accurate industry database of such conditions. The main goal of this project was to deliver a high quality database with boundary layer turbulence statistics at high Mach numbers that can be used for semi-analytic broadband trailing edge noise prediction models, but also constitute a high quality database for future high fidelity numerical computation.
An inverse aero design method is used by NLR to obtain a pressure distribution representative of the pressure distribution of a CROR front blade section at 87.5% of blade span in take-off condition, which is sufficiently thick to install the requested pressure sensors in the near trailing edge region and also strong enough to be used as a 2D airfoil section in DNW-TWG wind tunnel.

NLR has performed the structural design (engineering) and manufacture of two 2D wind tunnel models:
1. The CROR like 2D airfoil based on the inverse design
2. The baseline airfoil, i.e. the ValeoCD as requested by the topic manager at T0.

The structural design and manufacturing included the integration of unsteady sensors selected, the static pressure orifices (50 by profile) and the installation of an additional accelerometer sensor in the airfoils. The most optimal type and layout of the unsteady pressure sensors was determined for the unsteady pressure measurements. The use of interchangeable instrumentation pads on both models was considered in order to reduce sensor procurement costs for the two instrumented 2D models, but proved infeasible. Flush mounted sensors were selected.

The selected DNW-TWG wind tunnel allowed profiles with a chord length of up to 0.4 m to be tested in the requested alpha, Mach and Reynolds range, without significant side wall effects. This larger chord length of 0.40 m (compared to the target chord length of 0.3 m) was preferred, because it facilitates the installation of closely packed unsteady pressure sensors in the trailing edge region.

Details on mounting of the model in DNW-TWG, strength requirements (safety factors) and instrumentation cable routing from the model to the data acquisition systems and possibility to install an inflow-microphone near the profile trailing edge were discussed with the WT operator. Efficient solutions were sought to mount and dismount the models, in order to allow an easy model exchange. Solutions for optical access to the wind tunnel with the PIV camera’s, the infrared camera and laser light sheet as well as the PIV flow seeding were found.

The WT operator DNW-TWG executed the tests in accordance with the test matrix of the Test Plan in five productive testing days. During the week before the actual test preparations were performed, e.g. the wind tunnel model was shipped to the wind tunnel, the PIV, IR and pressure measurements were setup and the PIV/IR foil and tripping were applied to the models. This resulted in optimal use of wind tunnel time during the tests themselves.

At the end of the project, NLR will collect and store all aerodynamic and aero-acoustic test data together. Two copies of the database were delivered to the Topic Manager, i.e. one at Airbus Lyon and one at Airbus Bremen. NLR will also preserve it to enable further analysis when needed. Both original and processed data will be stored to allow re-processing of original data if needed.
The Contra Rotating Open Rotor is a promising innovative engine concept that is targeting a fuel burn reduction in de order of 15% to 20% versus the performance of year 2014 state of the art propulsion system. The potential increase in fuel efficiency is significantly higher than the one obtained from other fuel reducing engine designs, including the Ultra High Bypass Ratio engine (UHBR), another innovative engine concept investigated in the Clean Sky (2) program. The absence of casing and nacelle on a CROR engine, enables the extreme increase of the bypass ratio, the engine performance and a reduction of weight and size, but also limits the blade-off protection and noise shielding possibilities.

Europe’s Clean Sky program was not the first time the open rotor concept was investigated. During the fuel crisis in the late 80s General Electric already demonstrated the GE36 Unducted Fan in flight, however when the fuel crisis ended the concept was shelved. Earlier this decade NASA funded GE’s wind tunnel research on better blade designs to increase performance and reduce noise. When NASA stopped the project, Snecma - partner of GE in the joint venture CFM International - and the Clean Sky program took the lead in advancing the maturity of the open rotor concept. Maturing the CROR-research within Clean Sky (2) will result in lighter and smaller engines with substantial increased fuel efficiency, reduce time to market including certification and reduce life cycle cost. This combination of effects will establish Europe as an industrial leader in terms of future engine technology.

Reducing engine noise and CO2 emissions contributes to enhanced mobility due to noise containment and altitude flexibility in dense traffic environments and due to accessibility of secondary airports that are usually restricted because of noise regulation. Furthermore the increased fuel efficiency will lead to more cost effective transport.

All the deliverables and results were shared with not only the consortium, the wind tunnel and the topic manager, but also with the closely linked SCONE project and leading experts on the controlled diffusion airfoil. Furthermore the high quality database with boundary layer turbulence statistics at high Mach numbers that can and will be used at NLR for semi-analytic broadband trailing edge noise prediction models and for comparison with future high fidelity numerical computation, both in other European projects (e.g. in combination with DREAM data) and for its other customers. Individual technology advances, e.g. the innovative solutions for installing closely packed unsteady flow sensors, the method of inverse design or the sensor layout design, can be exploited by NLR for its future wind tunnel model customers.
Model clamped in 2D test-section (outside of tunnel).
DNW- TWG, 2D test section
Application of boundary layer tripping
Model in tunnel, with PIV camera