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Development and Test of Fluidic Actuators for Active Flow Control Applications

Final Report Summary - DT-FA-AFC (Development and Test of Fluidic Actuators for Active Flow Control Applications)

Executive Summary:

Active flow control by means of pulsed blowing has proven to be effective and efficient to delay or avoid flow separation on aerodynamic bodies and therefore increases their operating range. Such active flow control systems, however, require effective, efficient, and reliable flow control actuators, the design and validation of which was the aim of the project DT-FA-AFC. Different flow control actuator concepts have been the focus of research in the past decades. The variety ranges from piezo-driven Zero-Net-Mass-Flux (ZNMF) actuators, plasma based actuators, and mechanical valves to pyrotechnic actuator and moving pistons. The control authority and frequency range of those actuators vary greatly, all have their pros and cons, and all concepts were tested more or less extensively in lab scale experiments. Only very few concepts reached the maturity level to be flight tested and no concept was ever considered for commercial (civil) application.

Within the frame of DT-FA-AFC it was attempted to develop a core flow control actuator system that has the potential to be considered for commercial, civil application. In the course of the project an aircraft scale fluidic actuator system was devised that produces high amplitude pulsed air jets without incorporating any moving or electrical components. The system performance was evaluated (in close cooperation with SFWA project FloCoSys) in bench-top experiments. Its design was provided to SFWA project AFCIN to study the integrability into a trailing edge flap modified for AFC application. The combined findings of DT-FA-AFC, FloCoSys, and AFCIN show that it is possible to design an actuator system for active flow control that is capable of providing the necessary control authority in a relevant frequency range, while being sufficiently robust und compact to be considered for integration in a civil airliner.

Project Context and Objectives:

As an aircraft spends most of its flight time in cruise its wings are optimized for that part of the mission. Nevertheless, during take-off and landing, the wing needs to provide sufficient lift at low speed and therefore requires a high-lift system, which is usually a complex combination of leading and trailing edge flaps. These components increase complexity and weight of the overall system and therefore increase the direct operating costs and fuel consumption. Active flow control on those devices enables the design of systems of less weight, less complexity, or simply smaller elements. Such active flow control systems, however, require effective, efficient, and reliable flow control actuators. The design and validation of which was the aim of SFWA project DT-FA-AFC.

First a single element fluidic actuator was designed in close cooperation with Airbus and the SFWA partner projects FloCoSys and AFCIN. An actuator shape was found (using CFD tools) which fulfils the AFC parameter requirements specified by Airbus and fits into the relevant civil aircraft trailing edge flap. In addition, steps were taken to derive a driving system for the output stage actuator elements and to implement the single elements into a larger array of actuators.

After having arrived at a satisfying shaping for an individual outlet stage actuator element, the integration of those elements into an overall actuator system was worked on. The efforts comprised the manufacturing and testing of prototypes, the design of a driving stage, and the downscaling of the aircraft scale actuators to wind tunnel model scale. The testing and evaluation process consisted of two stages. First the performance of ONE single element of the outlet stage was evaluated. It was found that at a frequency of 150Hz the actuator is fully modulated at the design point. There is also sufficient margin to low performance especially with respect to undersupply of control mass flow.

The second stage focused on the testing of the flueric actuator array. This AFC system consists of a plenum, a driving stage, and an outlet stage. Only driving stage and outlet stage are considered to be the core actuator system. Both stages are supplied with the same pressure level through the plenum. The design point for the flueric actuator array is at an inlet pressure level of 500mbar above ambient. As a result of employing only one pressure level for both stages, the frequency is a function of the pressure in the plenum - and therefore the mass flow rate.

For the flueric actuator array the total pressure of the air jets was measured at the outlets relative to ambient pressure. The velocity distribution along the actuators' span of 600mm is sufficiently even and the actuators output is fully modulated for all tested pressure levels applied at the plenum.

In addition to design of a flow control system at aircraft scale, this design was downscaled to wind tunnel model level. Here two different pressure levels were applied to driving and outlet stage to allow for setting the a frequency independent of the actuation amplitude (within limits). Extensive pressure measurements proved the functionality of the downscaled actuator. Analogous to the large scale actuator, modulation was considered to be the parameter to qualify the actuators' operability. For main flow rates greater than approximately 2000NL/min/meter, the actuator shows a modulation of more than 90% for all tested parameter combinations. The achievable frequency ranges from 170Hz to 240Hz.

Therefore, in the course of the project, significant progress was made with respect to flow control hardware development. Starting from industry requirements and a technology concept a robust and efficient flow control actuator system was designed, optimized, and tested. It was proven in bench top experiments that it is possible to design an AFC system that has sufficient control authority to be implemented at aircraft scale. In addition, the project provided input to partner projects FloCoSys, AFCIN, and the ongoing SFWA project robustAFC.

Project Results:

Please see attached pdf file of part 4.1 for a comprehensive presentation of the main results.

Potential Impact:

Concerning the potential impact of DT-FA-AFC it is to note that active separation control by means of pulsed blowing from the flap shoulder is a most promising aspect of the active wing concept and might be a major step towards reaching the ACARE 2020 goals.

Specific impact of flow control may be:

- Reduction of mechanical complexity
- Reduction of needed space for TE flap inside the main element
- Reduction of DOC
- Reduction of weight
- Reduction of fuel consumption

Within this project a (core) fluidic actuator system was developed. The design has high relevance for industry application, as the requirements for the flow control system were specified by SFWA partner Airbus. DT-FA-AFC therefore contributed to furthering the understanding on a rather under researched aspect of flow control - the system aspect (Most research focuses on the understanding of the aerodynamic effect of flow control and the identification of an optimal parameter set.). This impact on the development of flow control hardware was further increased through the close cooperation of TUB with SFWA partner EADS-IW, whose industry perspective on the overall flow control system allowed a detailed study of not only actuator performance, but also of the viability of integrating such a system in the infrastructure of a civil airliner.

The immediate contribution of DT-FA-AFC on flow control hardware design was the establishment of a novel two-stage fluidic actuator concept. The advantages of such a system are (among others):

- High efficiency in terms of total pressure to dynamic pressure conversion, as several diverters are driven by only one much more lossy oscillator.
- Independent (within limits) setting of actuation amplitude and actuation frequency when using two different pressure supplies for diverter and driving stage.
- Reduction of required installation space: Diverters are more compact than oscillators if low frequencies are to be realized, as the operating frequency of an oscillator mainly depends on the length and volume of its feedback lines.

The dissemination of the projects results comprises mostly the provision of input for other SFWA CfP projects, for which the detailed actuator designs were necessary boundary conditions to further their respective goals. Those projects were:

- FloCoSys
- robustAFC

The final outcome is a highly reliable and durable actuator system, which provides the required flow control authority, is in line with integration requirements and functions without incorporating any moving or electrical components.

List of Websites:

Prof. Dr.-Ing. Wolfgang Nitsche
Fachgebiet Aerodynamik, Sekr. F2, Technische Universität Berlin
Marchstrasse 12-14
10587 B e r l i n

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