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Leading Edge Box Design for Swept Flow Control Wing

Final Report Summary - LEBOX (Leading edge box design for swept flow control wing)

Executive summary:

The objective of the LEBOX project was the design of an integrated leading edge box with pneumatic flow control devices for mid-scale wind tunnel testing on a suitable swept constant chord (2.5D) model.

Dynamic vortex generator jets (VGJs) have been chosen as the most suitable active device to control turbulent flow separation at the leading edge. The important step beyond the state of the art was the design, integration and testing of the actuator geometries for swept (twisted) boundary layers.

The geometry of these devices has been evaluated by means of comprehensive 2.5D numerical simulations to highlight the sensitivity of important parameters such as the skew angle, pitch angle, spacing and size, e.g. relative to the boundary layer. A number of comparative simulations have demonstrated the influence of these parameters on the effectiveness of the flow control system. Furthermore, for selected configurations, the phenomenology of the stall of the actuated case has been analysed. Also, the results have been extrapolated to higher Reynolds numbers using the numerical approach. TU-BS participated in the concept evaluation for the swept mid-scale wind tunnel model. It has been concluded that the best setup is a so-called swept constant chord wing. A wingtip has been designed for this setup, also a number of comparative numerical simulations to check the setup against wind tunnel influence.

An existing leading edge box piece has then been modified and instrumented with the actuation system, also consisting of the VGJ geometry that has been developed in the above described work packages (WPs). The model with the leading edge active flow control (AFC) system was then tested in the mid-scale facility DNW-NWB. The results have been analysed with respect to effectiveness and efficiency.

Project context and objectives:

The core objective of the smart fixed wing aircraft call for proposals (SFWA CfP) project 'Leading edge box design for swept flow control wing' (LEBOX) was to demonstrate the potential of AFC at the leading-edge of a swept airfoil, i.e. to suppress a turbulent leading-edge flow separation at high angles of attack and thus increase maximum lift of the configuration. Experimental studies in a mid-scale, low speed wind tunnel - including the respective model design and instrumentation - as well as numerical studies have been performed to achieve this objective.

Context

As a response to the SFWA call for proposal, CfP participant TU Braunschweig, institute of fluid mechanics (TU-BS), proposed to use the so-called DLR-F15 airfoil model, which is owned by the SFWA partner DLR. This model was available via DLR to the SFWA community and TU-BS has extensive background knowledge about the application of AFC to this airfoil, which emanates from several national research programs (the German 'Luftfahrtforschungsprogramme', LuFos), also together with DLR. At the beginning of the project, the model was physically available in form of a two-dimensional (2D) wing (non-swept model) that can be integrated into various wind tunnels. Up to then, all wind tunnel integrations have been done in a wall-to-wall-setup, e.g. the model is mounted on both sides to a rotating table, as it is a very typical setup for 2D airfoil studies. Also available was a raw-piece of a leading edge module (called the 'leading-edge-box') that has been used in German national projects to apply AFC to the leading edge of this airfoil, though this module was designed for AFC at non-swept configurations. Not only the physical hardware was available, but also data from several wind tunnel entries with this model, without and with the application of AFC to the leading-edge. The data available to the participants (TU-BS as well as DLR) ranges from small / medium-scale, low-speed results in laboratory-type tunnels, over mid-scale, low-speed results in industrial facilities up to mid-scale, large-Re cryogenic testing results, all without and with AFC, but all in a 2D setup.

Beside experimental hardware and knowledge (e.g. existing databases) also, at the beginning of the project LEBOX, some initial work was existent on the numerical simulation of such problems. For example, it was known that such flows can be solved with a Reynolds-averaged Navier-Stokes (RANS) approach and do not necessarily require large-eddy resolving techniques such as large-eddy simulation (LES) or detached eddy simulation (DES), if the cell size in the surrounding of the AFC device is not too coarse. Also it was known that typically high-quality two-equation models (such as the so-called Menter shear-stress transport model) must be utilised to model the turbulent terms in the RANS equation. Certainly, because of these two requirements, significant computational power is needed.

Regarding the AFC technique, the use of a row of vortex generating jets (VGJs) at the leading-edge was proposed, as used by TU-BS in several preceding studies. Such VGJs basically consist of small orifices slightly upstream of the airfoil leading edge. The orifices are typically circular holes, pitched and skewed with respect to the oncoming flow, to create a local streamwise vortex structure. The vortex structure enhances the turbulent mixing inside the boundary layer and is therefore able to suppress a turbulent separation. Hence, the flow control is enabled by embedding a longitudinal (streamwise) vortex into the boundary layer. The use and sensitivities of this technique were well known for nominally 2D boundary layers or airfoils, respectively. However, it is known that swept wings feature significantly twisted boundary layers in the leading edge region. Such twist will always favour one sense of rotation of embedded streamwise vortices. It was suspected that the local boundary layer twist can be utilised in a positive manner to enhance the flow control effect. However, the general sensitivities have been more-or-less unknown, but it was clear that the geometry of the VGJ orifices (in terms of, e.g. pitch and skew, orientation co- or counter-rotating, size, distances) needed to be adjusted to this specific, swept flow case.

Coming back to the airfoil, TU-BS proposed to use the DLR-F15 airfoil in a so-called 'slatless' two-element configuration, consisting of an airfoil main element and a gapped trailing edge fowler flap. Regarding the choice of the high-lift flap configuration the relevant partners (which are: TU-BS within this CfP project, TU Berlin, who will do AFC at the flap within their CfP project 'AFC-TEFL-HLC' in the same wind tunnel entry and DLR who is the main point of contact for these tests and serves as an interface between the wind tunnel DNW-NWB and the two CfP partners) pre-selected one configuration, which was known from background knowledge to serve as a good reference for leading edge as well as for T/E flow control studies.

Objective and approach

As mentioned above, the key objective of LEBOX was to demonstrate the effectiveness and efficiency of leading-edge flow control on an experimental as well as on a numerical basis. To fulfil this objective LEBOX was split in three WPs. WP1 deals with numerical evaluation of flow separation control at the airfoil leading edge by means of a state-of-the-art RANS solver. WP2 was about the concept design, manufaction of components and preparation of the mid-scale wind tunnel tests in DNW-NWB. Finally in WP3 the wind tunnel test was accomplished and the data has been analysed and put into contest. The most important and vital connection between the numerical and the experimental WP was the layout / design of the VGJ orifices, which was developed by numerical solutions and then used in the wind tunnel tests. Similarly important was the feedback of the respective experimental results in order to validate the numerical approach.

Broken down to WP-level, the following objectives have been formulated:

WP1: Numerical evaluation
- setup of a framework to perform 2.5D computations including VGJs at the leading edge;
- evaluation of VGJ orifice geometry and position;
- sensitivity studies for steady operation; amplitude and relative size;
- extrapolation of results to higher Re-number.

WP2: Experimental concept, design and manufaction
- development of concept for '2.5D' testing using the DLR-F15 model;
- design of leading edge box with integrated AFC;
- manufaction of new or modification of existing leading edge box;
- implementation of AFC and peripheral systems.

WP3: Testing, analysis and reporting
- swept constant-chord mid-scale W/T-testing (with partners);
- analysis of the data, assessment of efficiency.

Potential impact:

Impact

AFC and - more specific - suppression of (turbulent) separation is a key enabler to meet the challenges of the 'Vision 2020'. Future aircraft with innovative high-lift systems are expected to be lighter, more efficient, less noisy, easier in maintenance and will probably utilise synergies on aircraft level, e.g. between a system for AFC as a high-lift device and a system for hybrid laminar flow. Extensive research on future technologies such as AFC is thus crucial to keep the European aircraft industry up to date with global challengers.

The CfP-project LEBOX created extensive knowledge about operation of VGJs as a high-lift device. The results are a valuable re-validation of previous mid-scale tests and an essential input for future work (see chapter 'exploitation'). The results of LEBOX provide a reliable and high-quality data base for further scientific development of fluidic vortex generators as a means to increase maximum lift and maximum angle of attack of transport aircraft. Within the numerical WP an industrial approach for the design of such VGJs has been developed.

Dissemination

Dissemination of the project results is based on: SFWA WP meetings, conferences and paper publications.

Within the WP1.1.4 regular meetings were held and the coordinator of LEBOX attended all of these meetings. A comprehensive presentation about the status of the project has been given and also the opportunity for intensive discussion of technical details was taken. These WP meetings disseminate the results to the relevant SFWA partners.

2-3 September 2010 - WP1.1.4 meeting, Delft
3-4 May 20 - WP1.1.4 meeting, Bukarest
21-22 November 2011 - WP1.1.4 meeting , Braunschweig
22 November 2012 - WP1.1.4 meeting, Southhampton

Key results of LEBOX have been disseminated by presentations on conferences. Up to this point in time two conferences have been attended (24 October 2011, Venice and 6 November 2012, Stuttgart), furthermore, a proposal for presentation at the 2013 AIAA summer conference series has been accepted.

For all conferences written proceeding papers are available and also contribute to dissemination:

MAHMOOD, S.S. SCHOLZ, P., RADESPIEL, R., Numerical Design of Leading Edge Flow Control over Swept High-Lift Airfoil, , 3rd CEAS Air&Space Conference / 21st AIDAA Congress, Venice, Italy, 24-28 October 2011

HÜHNE, C.-P. SCHOLZ, P., RADESPIEL, R.: Aerodynamic Effects of Tip Tanks on a Swept Wing Wind Tunnel Model, 18. DGLR-Fach-Symposium der STAB, 6-7 November 2012, Stuttgart, 2012

HÜHNE, C.-P. MAHMOOD, S.S. SCHOLZ, P.: AFC at a Swept Wing: A Comparison of Numerical and Experimental Results, 31st AIAA Applied Aerodynamics Conference, 24-27 June 2013, San Diego, USA, 2013 (upcoming)

Apart from the proceeding the Venice paper was also edited into a peer-reviewed journal paper that has been accepted for publication (though publication date is pending: MAHMOOD, S.S. SCHOLZ, P., RADESPIEL, R., Numerical Design of Leading Edge Flow Control over Swept High-Lift Airfoil, Aerotecnica Missili & Spazio (accepted for publication). Results of LEBOX are also expected to be part of dissertation / doctor's thesis of two Doctor of Philosophy (PhD) students, namely Mr Phillip Hühne and Mr Saquib S. Mahmood, who worked in the project. Publication dates of these theses are however pending.

Exploitation

Since LEBOX is a CfP project in SFWA, exploitation is ensured due to the close connection to and participation of the European aircraft industry.

The exploitation of foreground results of LEBOX is mainly the usage of the experimental data for comparison and validation of similar setups. Furthermore the numerical design approach can be easily transferred to other setups and problems.

On European level, the numerical design approach as well as some of the experimental reference data is exploited in another SFWA WP1.1.4 CfP-project 'ATTESI', where the VGJs are applied more at the trailing edge of the same airfoil in a drooped spoiler configuration. On national (German) level, again the numerical design approach as well as some of the experimental reference data is exploited in the recent national research programme 'Luftfahrtorschungsprogramm IV (LuFo IV)' as a part of the project MOVE.ON. Here, VGJs will be applied to a slightly swept, finite span wing similar to the setup of LEBOX. This new wing features a winglet at the wingtip and AFC by means of VGJs will be applied at the very outboard part of the wing, where a slat cannot be mounted.