| The Project |
| Introduction |
| Objectives |
| Concept |
| Participants |
| Consortium |
| Impact |
| Partners |
| Members area |
| Consortium |
| Concept and project objective |
| Background |
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Drag reduction and separation control are directly related to more efficient air transportation and less emission of harmful gases into the environment. While the aerospace industry is striving to have more and more optimised designs, it is still some way away from the targets set out in the ACARE 2020 vision for 50% reduction in aircraft emissions. Separation control and drag reduction contribute directly towards this target and active flow control could plays an important role in achieving it. Active flow control provides an additional dimension for further improving aircraft performance, in particular, for performance at different operational points, such as at cruise and take-off and landing. After many decades of development, the highly optimised aircraft designs make further large improvements difficult without a ‘game-changing’ technology such as active flow control.
The proposed project is based on the mutual understanding of the EU and Chinese partners, mostly from the AeroChina and AeroChinaII consortia funded by the EU. In AeroChinaII, a Working Group on Flow Control was established due to a strong interest in the area from both sides. A series of EU-China Flow Control workshops were held, which hugely improved our understanding of the capabilities in both experiments and simulations and pacing problems in industrial applications. While the MARS consortium is based on the key members of this working group, some new EU and Chinese partners were invited in the current project in order to further strengthen the current consortium. The importance of reducing skin friction on meeting the global fuel burn targets is obvious. At cruise approximately one half of the total drag of a modern commercial transport aircraft is attributed to skin friction. The importance of improved separation control is less obvious unless we acknowledge the influence of aircraft mass on fuel burn. In fact the sensitivity of fuel burn to mass is greater than that to skin friction. Therefore if the structural mass of the aircraft can be reduced by more efficient low speed configurations or improved load alleviation off-design then this carries a significant direct benefit on cruise fuel burn. |
| Concept |
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The turbulence Reynolds stress is the most important dynamic quantity affecting the mean flow as it is responsible for a major part of the momentum transfer in the wall bounded turbulent flow. It has a direct relevance to both skin friction (for a turbulent boundary layer) and flow separation (occurs when skin friction drops to zero). The near wall region for a turbulent boundary layer can be divided into the viscous sublayer, where the mean viscous stress is important and the approximately constant Reynolds stress region, where the viscous stress drops to zero and the Reynolds stress peaks. As the Reynolds number increases, the peak Reynolds stress approaches the value of the viscous stress at the wall. Therefore active manipulation of the Reynolds stress can directly lead to changes in the viscous stress at the wall so as to effectively control the flow for effective flow control.
However, there is a lack of current understanding of the inter-relationship between the various flow control devices and the Reynolds stresses in the flow field they produced. An improved understanding can potentially significantly improve the effectiveness of flow control as the Reynolds stresses are closely related to the flow behaviour at the surface for effective separation control or drag reduction. A variety of control devices are available and new ones are invented but which one for what purpose is an open question yet to be fully answered. The majority of prior work has focused on the introduction of changes to the mean flow that resulted in changes to the Reynolds stress. In the present proposal we propose to reverse that process and consider the long term goal of controlling dynamic structures that then influence the Reynolds stress that in turn changes the mean flow. This radical approach recognises that we are still some way away from hardware to implement the concept at flight scales but if successful, would establish a first important step towards our ultimate ambition. The focus of the present project will be on the effects of a number of active flow control devices on the discrete dynamic components of the turbulent shear layers and the Reynolds stress. From the application point of view, the current proposal provides a positive and necessary step in the right direction wherein it will demonstrate the capability to control individual structures that are larger in scale and lower in frequency compared to the richness of the time and spatial scales in a turbulent boundary layer. In order to focus the current project with the limited duration and budget, the current project excludes laminar flow control for transition delay, another area of extensive previous and current research. Furthermore, the project will investigate active flow control means rather than passive controls. To explain our basic strategy in manipulating Reynolds stresses through the dynamic components of the turbulent shear layers, it is helpful to start with the triple decomposition proposed by Reynolds and Hussain (1972) for an instantaneous velocity, U 
The first term on the RHS is the time averaged mean velocity. If we attempt to control this via flow control then most devices offer little gain in efficiency on a global energy basis i.e. change in energy out equals energy in. The second term on the RHS is the periodic/dynamic component of the flow and for some specific flow scenarios this can be shown to be dominant in determining the flow state and characteristics. The stresses produced from this term are referred as the periodic stresses or “apparent stresses”. It offers some interesting opportunities for demonstrating the way in which to deploy flow control technologies for dynamic environments (responsive environments, smart inputs and sensible control). This also implies that, for statistically steady flows, where the second term disappears, artificial introduction of the periodic term may be necessary for effective control. The final term on the RHS represents the broadband ‘random’ turbulent fluctuations, from which the Reynolds stresses are defined. While direct control of the “random” components is the ultimate goal, the current project aims to investigate the control of the periodic stresses, the dynamic components of the flow, in order to manipulate the Reynolds stress for the benefit of flow control. |
| Test Cases |
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Following this strategy, two fundamental and distinct flow cases have been chosen for the study of the effects of various flow control devices. The methodology will be based on both wind tunnel experimental investigation and numerical simulation, complementing each other for extracting flow details regarding the dynamic components and the Reynolds stresses in the shear layers, in addition to the surface properties.
The backward facing step presents a separated turbulent shear layer from a fixed sharp edge. Depending on the step height in relation to the incoming flow boundary layer thickness, various separated flow characteristics can be generated. At certain conditions, periodic unsteady flows can be achieved and the effects of the periodic component on the turbulence Reynolds stresses can be investigated in detail. For the step flow, the reattachment zone exhibits a characteristic frequency as a function of the external fluid velocity, the flow state and the step geometry. Flow control actuators will be investigated actuators regarding their influence on the characteristic frequency, amplitude and coherence of the periodicity. The capability of the flow control on increasing and decreasing the Reynolds stresses downstream of the step will be studied. The project will also investigate whether a more responsive environment can be created for flow control. The second basic test case is closer to a realistic wing regarding the pressure gradient but the separation from a smooth surface is more difficult to handle for unsteady flows. For the NACA0015 wing, extensive studies have been carried out with different flow control devices, including the control of training edge separation and stall characteristics. Previously, fluidic vortex generators and synthetic jets were studied for suppressing trailing edge separation and wing stall. Unsteady flow separation was observed and the responsiveness of the flow to the control input was investigated. At some flow conditions, periodicity of the dynamic components was observed. However, the detailed behaviour of the Reynolds stresses near and downstream of the control devices is not very clear so far. The project will focus on the control of the periodic component to manipulate the Reynolds stresses. The interaction of the underlying flow periodicity and the control device periodicity will be identified. For the wing case, the effects of the flow control on the Reynolds stresses can be more directly related to separation control and drag reduction. |
| Global Aims of MARS |
With the basic strategy for MARS established the global aims of the project can be defined. They are:
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