A further significant reduction of aircraft noise per operation is only achievable with major design changes and disruptive aircraft concepts and noise reduction technologies beyond the proven noise reduction means of today TurboNoiseBB builds in particular on the results of previous European projects as well as UK national research programmes. EC funded projects PROBAND (FP6) and FLOCON (FP7) respectively explored advanced fan broadband noise prediction methods and adaptive / passive flow control techniques for fan broadband noise reduction. TurboNoiseBB will apply to the full scale design of an UHBR fan stage the most promising passive noise reduction technologies taken from FLOCON small scale experiments, such as serrated wavy OGV leading edges and various innovative design effects of the OGV grids. Broadband noise prediction methods developed and evaluated in PROBAND will be used and improved in order to assist the design process of such low noise OGV grids with industrial methods. All the methods developed in the previous European programmes will be brought to a higher TRL and validated for a real engine configuration and high flow Mach number conditions.
In the UK, the Aerospace Technology Institute together with the Technology Strategy Board (ATI / TSB), have funded HARMONY and SILOET II, two research programmes with the aim of respectively; down selecting a suitable method to predict the broadband noise and developing the chosen method for aerospace applications. A physically based prediction methods for fan broadband noise has been developed and low TRL validation performed using small scale low speed wind tunnel. The experimental database and the subsequent research activity in TurboNoiseBB will validate the prediction methods at higher TRL. This will allow the aero-mechanical design of fan OGVs with low noise technology to be done in TurboNoiseBB.
The accurate prediction of turbomachinery broadband noise, which results from the turbulence in the flow (boundary layers, wakes, fan tip-gap), is still very challenging due to the complexity of the flow (confined flow, strong swirl behind the fan) and also due to the strong aero-acoustic interactions between the fan, the OGV, their proximity and the environment within a confined duct. TurboNoiseBB is an approach to combine the multiple skills and experience of European aeronautical industries, research institutes and universities to work together on the specification and data analysis of a large scale fan stage geometry and validate the results against industry-exploitable and complex numerical methods to understand fan broadband noise on future aircraft platforms. The collaboration between all partners required to achieve such a progress will be ensured by the use of a common fan-OGV geometry, shared between all partners, as well as a full disclosure of experimental data.
Advanced aero-acoustic instrumentation will allow precise aerodynamic measurements (aerodynamic performance of the fan stage, mean flow, highly resolved unsteady-turbulence flow measurements, stress monitoring on the blades), near field (in-duct beamforming, modal decomposition) and far field noise in order to establish a comprehensive aero-acoustic database. This data will be used to understand, predict and reduce broadband fan stage noise of UHBR engines.
Based on aero-acoustic measurements of the baseline fan-stage tests, TurboNoiseBB will also design several low noise OGV grids using state-of-the-art multi-disciplinary methods (aerodynamics, acoustics, mechanics) to predict and reduce broadband noise as well as cutting edge high fidelity CFD / CAA methods (LES, ZDES, RBC, LEE). The aero-acoustic performance of such low noise OGV grids will be assessed by numerical methods. While costly direct noise simulation will be attempted using CFD LES/DES type methods targeting very high fidelity results, prediction methods with lower computational costs will use a two-step process in which the turbulence in the rotor wake flow is characterized and then the acoustics associated with the response of the OGV to the turbulence is computed. In these methods, the vane response computation is based on the solution to the linearized Euler equations for a simplified geometry. The model input is derived from the turbulence characteristics in the wake. However, the methods differ in how the wake information is utilized. For instance, some methods require knowledge on the distribution of the turbulence intensity in wake passages, while others only utilize the average passage value of turbulence intensity. TurboNoiseBB will gather the data required to validate and feed the different models. Experimental data from hotwire measurements with simultaneous noise measurements in a representative case will be used to specify the OGVs inflow and validate the models. The aero-acoustic performance of such low noise OGV grids will therefore be assessed with each numerical method and the best ones will be used for the low noise OGV design. The state-of-the-art industrial design methods will be tested against the experimental database as well as complex CFD/CAA methods in order to establish their applicability for the design of low noise fan stages and, if necessary, identify the required improvements to be applied to fast industrial broadband noise prediction methods to be taken into account in the design process. In turn, TurboNoiseBB is a first step to integrate high fidelity prediction methods into the design process to reduce broadband noise through pure design effects as well as passive noise reduction technologies based on the results of former European FP7 projects PROBAND and FLOCON. While OPENAIR achieved the broadband noise reduction at the source by a simple reduction of the number of OGVs (or on propagated noise with liner concepts) and FLOCON exploited a 2D acoustic design for a low speed flow; in TurboNoiseBB for the first time, the noise reduction will be achieved by a fully 3D design of the OGV with a compete aero/noise/mechanical optimisation and integration into the real engine architecture. The fan stage noise reduction obtained through a better design of the low noise OGV grids will finally be assessed on Long Range and Short-Medium Range aircraft platforms defined in other projects (e.g. Cleansky 2), which integrate all noise sources on various aircraft fitted with UHBR engines.
The noise benefits of each of the low noise OGV designs will be assessed for each of the ICAO Annex 16 certification points (sideline, flyover, and approach, compare the figure above). The typical aircraft noise prediction methodology to be used is based on the transposition into flight of engine noise sources (jet, forward and aft fan, turbine, core) measured or predicted in static conditions and tabulated in engine static noise source databases. The transposition includes cycle effects (thermodynamic effect on noise sources between ground static conditions and in-flight conditions), flight effects on jet noise (impact on the jet noise generation of the aircraft speed), installation effects (impact of the installation of the engine / powerplant on the aircraft vs the engine alone in static conditions), and propagation effects (distance, atmospheric absorption, ground reflections, lateral attenuation and aircraft speed effects: Doppler and convective effects). Airframe noise sources (landing gears, high lift devices…) will also be included in order to obtain a proper evaluation of the OGV low noise benefits.
TurboNoiseBB will also evaluate the relevance of fan stage broadband noise prediction methods to turbine applications. Indeed, although the physics of the flow are very different, it is believed that the broadband noise mechanisms are very similar and semi-analytical methods should apply well also to predict and evaluate trends of turbine stage designs.
Besides, the uncertainty that will be demonstrated in the project for those methods will be propagated up to the aircraft EPNL predictions with the same process outlined above for the low noise OGV sets in order to assess whether the tested methods reach sufficient accuracy to support the development process of future aircraft and contribute to targeted noise reductions.