Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - Rotary Wing CLFC (Closed-Loop Flow Control to Enhance Aerodynamic and Aeroacoustic Performance of Wind-Turbine Blades)
Teaser
Aerodynamic loads and noise emitted by rotating blades are of interest in many industrial applications, such as wind-turbines, aircraft propellers, and helicopter blades. The focus of the Rotary-Wing CLFC project is harvesting of wind-energy while reducing negative...
Summary
Aerodynamic loads and noise emitted by rotating blades are of interest in many industrial applications, such as wind-turbines, aircraft propellers, and helicopter blades. The focus of the Rotary-Wing CLFC project is harvesting of wind-energy while reducing negative environmental impact. These double aims are important given that it is crucial to increase energy harvesting from the environment without producing more green-house gases. To achieve the latter, we have to improve our understanding of the contributing processes. Wind-turbines should be designed such that they have a low impact on the environment and on society. Additionally, to reduce the impact on inhabited areas, their noise emission should be low. Similar arguments hold for aircraft propellers and helicopter blades as well. Wind-turbine blades operate under a highly turbulent atmospheric boundary layer where they encounter gusts and variation in wind direction and speed. These decidedly changeable harsh conditions affect both aerodynamic and aeroacoustic performance. The aerodynamic noise is, in fact, a result of interaction between the turbulent flow and the blade; accordingly, flow unsteadiness and noise interact strongly. For this reason, the Rotary-Wing CLFC project aimed to achieve a combined aerodynamic and aeroacoustic optimization of the rotary wing by experiments conducted in a controlled environment of gusting flow and rotating motion. The first objective was to develop an aero-acoustic model that could be incorporated in a control strategy. The second objective was directly aimed at the development of strategies for the reduction of blade noise. The third key technological objective was to develop a control system. The last objective was to investigate the effect of rotation. The investigations carried under the Rotary-Wing CLFC project focused on wind-turbine applications. To impact a large flow region, so-called active flow control technology in the form of wall normal blowing was chosen. Using this technique, dynamic processes in the flow are excited by boundary actuation, exploiting amplification effects due to flow instability so that results can be obtained with a minimal amount of input energy. Moreover, passive flow control technologies were developed to enhance the amount of energy harvested from the wind, while improving the aero-acoustic properties. Passive methods benefit from a simple and robust design. Leading- and trailing-edge serration were proven to be effective in reducing the noise emitted from wind-turbine blades. The leading-edge region of the wind turbine blade was modified to incorporate leading-edge serrations and active flow control in the form of wall normal blowing.
Work performed
The project commenced with knowledge transfer from the lead scientist and his research team to the researcher. The researcher acquired knowledge in advanced techniques in system identification and control theory. The researcher summarized this knowledge transfer in the book chapter, “Closed-loop Active Flow Control,†published by Wiley in “Advance UAV Aerodynamics, Flight Stability and Control: Novel Concepts, Theory and Applicationsâ€. In the next stage, the researcher worked on development of sensors, actuators, standalone data acquisition and control system to be embedded into rotating blades. To incorporate complex control systems, the internal structure of the model was manufactured with advanced rapid prototyping methods. Active flow control in the form of wall normal blowing was incorporated inside a blade. One main challenge was presented by the limited size of an airfoil model that could fit into the unique gust wind tunnel facility at TU Berlin. The facility consists of a main wind tunnel that generates the baseline flow and a second wind tunnel capable of generating unsteady gusts. In this respect, a comprehensive CAD design study was performed to find an airfoil shape, actuator geometry, and sensor locations together with an appropriate choice of sensors. The aerodynamic experiment was complimented by measurements of the small rotor acoustic signature. The objective of these experiments was to understand the source of aerodynamic noise in a rotating frame. Aeroacoustic measurements were recorded simultaneously with free-field microphones in an anechoic room. The extraneous rotor noise comprised the tonal peak at blade passing frequency and broadband noise components. The effect of spanwise tripping (passive technique) was investigated. The results showed that it is possible to reduce the level of low frequency tones by 2-6 dB while shifting the acoustic energy to high frequencies. The ability to reduce low frequency tones is of great importance given that under atmospheric conditions, low frequencies travel longer distances since atmospheric absorption is relatively low. This work was presented at the 57th IACAS (Israel Annual Conference on Aerospace Sciences) and the researcher was awarded the Neev-Ya Hadas Durban Prize. The experimental work was supported by mathematical models development to describe blade aerodynamic and aero-acoustic behaviour in unsteady inflow conditions. The aerodynamic model is based on classical unsteady thin airfoil theory and the aero-acoustic model on the Amiet theory. Although fluid flow systems, as described by the Navier-Stokes equation, are inherently non-linear, much can be achieved in closed-loop-exploiting linear concepts. Thus, the mathematical model was oriented toward development of a closed-loop reliable physics-based low order model that is based on physical principals.
Final results
Today, it is widely recognized that noise pollution is a major environmental concern for quality of life in urban environments across the EU-28 and in OECD countries. The public perceives noise as a major environmental issue, particularly in urban areas. Further spread of onshore wind-farms in inhabited areas is limited due to noise concerns. As a result of the Fellowship, new tools and methods were developed. The progress beyond the current state-of-the-art includes development of passive and active strategies, mathematical models, and standalone data acquisition. It should be emphasized that the data acquisition system is a key component in further understanding of noise emitted by rotating blades since it allows measurement of noise at source. Beyond the energy sector, the tools and methods developed during the Rotary-Wing CLFC project will have a tangible impact on other fields such as transportation, where the noise emitted by rapidly growing rotary-wing UAV applications is a concern. In this field, the level of technological readiness of noise mitigation strategies and their modelling lags behind the level of expertise reached by the aeronautical and energy industries. The presence of these non-aeronautical important societal actors within the Users Committee will greatly enhance the dissemination of mature tools and best practices developed within the project towards a wide industrial community, which affects the lives of many European citizens. The findings will hopefully pave the way for development of novel engineering concepts that will incorporate both passive and active technologies to enhance performance. All techniques and methods developed during the Fellowship now form the basis for future scientific work by the researcher in her new position as an Assistant Professor at the Technion—Israel Institute of Technology.