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Periodic Report Summary 3 - TFAST (Transition Location Effect on Shock Wave Boundary Layer Interaction)

Project Context and Objectives:
The greening of air transport systems means a reduction of drag and losses, which can be obtained by keeping laminar boundary layers on external and internal airplane parts. Increased loads make supersonic flow velocities more prevalent and are inherently connected to the appearance of shock waves, which in turn may interact with a laminar boundary layer. Such an interaction can quickly cause flow separation, which is highly detrimental to aircraft performance, and poses a threat to safety. In order to diminish the shock induced separation, the boundary layer at the point of interaction should be turbulent.
The main objective of the TFAST project is to study the effect of transition location on the structure of interaction. The main question is how close the induced transition may be to the shock wave while still maintaining a typical turbulent character of interaction.
The main study cases - shock waves on wings/profiles, turbine and compressor blades and supersonic intake flows - help to answer open questions posed by the aeronautics industry and to tackle more complex applications. In addition to basic flow configurations, transition control methods (stream-wise vortex generators and electro-hydrodynamic actuators) will be investigated for controlling transition location, interaction induced separation and inherent flow unsteadiness. TFAST for the first time provides a characterization and selection of appropriate flow control methods for transition induction as well as physical models of these devices.
Emphasis is placed on closely coupled experiments and numerical investigations to overcome weaknesses in both approaches.
For these challenging tasks a consortium of the highest quality is employed. It is not only the high scientific level and skills of the partners which is important but also the quality of partner's cooperation and integration in the consortium. Because of the good experience obtained from the consortium that was forming the UFAST project, and because the proposed research is based on the experience gained in UFAST, the core of the TFAST consortium consists mainly of UFAST partners.

Project Results:
In order to be able to judge the effectiveness of transition induction in WP-2, reference flow cases are planned in WP-1. There are two obvious reference cases – a fully laminar interaction and a fully turbulent interaction.
There are two basic configurations of shock wave boundary layer interaction and these are a part of the TFAST project. One is the normal shock wave, which typically appears at the transonic wing and on the turbine cascade. The characteristic incipient separation Mach number range is about M=1.2 in the case of a laminar boundary layer and about M=1.32 in the case of turbulent boundary layer.
Two test sections for the investigation of normal shock wave interaction are used in WP-1 and WP-2. One at ONERA DAFE provides measurements for M=1.25. The second test section at University of Cambridge concerns M=1.3. This is a very important flow case which is directly related to the laminar wing, where the shock is shifted downstream in order to extend the length of the laminar boundary layer. Both Partners have experienced serious experimental worries. None was able to obtain stable laminar normal shock wave interactions for such low Mach number, with upstream laminar flow with a plate sharp leading edge . UCAM succeeded to obtain a stable configuration using an elliptical leading edge.
The second typical flow case is the oblique shock wave reflection. The most characteristic case in European research is connected to the 6th FP IP HISAC project concerning a supersonic business jet. The design speed of this airplane is M=1.6. Therefore the TFAST consortium decided to use this Mach number as the basic case. Pressure disturbance at this Mach number is not very high and can be compared to the disturbance of the normal shock at the incipient separation Mach number mentioned earlier. Therefore two additional test cases are carried out with different Mach numbers. ITAM conducts an M=1.45 test case, and TUD an M=1.7 test case.
All experimental partners succeeded in developing operational set-ups for the oblique shock reflections. It should be noted that highly stable and repeatable experimental conditions have been obtained, allowing cross comparisons between several metrologies and experiments. Laminar to turbulent upstream conditions have been obtained, documented and used for numerical simulations. In parallel, a wide panel of numerical simulations has been used: 2D URANS, DDESs, 3D LES and DNS. LES and DNS results confirm the experimental observations.
A comparison of tripped and un-tripped configurations revealed that in many cases it is not detrimental to allow the shock wave ‘to do the work’ and promote transition in the interaction zone. These findings are valid for interactions on flat surfaces (WP-2).
For more industrial applications new test sections have been built. In the case of an civil turbofan engine operating at particularly high altitudes the Reynolds number can drop by a factor of 4, when compared to the see level values. The laminar boundary layer on the transonic compressor rotor blades interacts with shock waves and as a result a strong boundary layer separation will form. This can seriously affect the aero-engine performance and operation. One way to avoid strong separation is to ensure that the boundary layer upstream of the shock wave is turbulent. In order to have a better insight into the operation of flow control devices besides typical cascade experiments a “close up” (single passage) test section has been introduced.
IMP PAN and DLR experiments show that pressure losses can be reduced when tripping the boundary layer upstream the shock, however over tripping easily appears when incoming boundary layer is increased too much and causes massive increase in pressure losses.
The same approach concerns also the turbine application. It was shown that boundary layer separation downstream of the shock can be reduced, if upstream boundary layer tripped by strip (IMP PAN) or AJVG (DLR) .
Classical wing design through shape optimisation enables separation to be controlled to a certain extent but only offers limited potential for further improvements at given design conditions. Active flow control technologies, when carefully applied and developed, may open the door to significant progress. These technologies are clearly multi-disciplinary and imply not only a good knowledge of the aerodynamic improvement, but also a strict analysis of their impact on the wing structure and systems in order to globally identify the related benefits and penalties.
A considerable amount of experimental and numerical data were produced to characterize tripping location influence on shock wave boundary layer interaction prior, at, and beyond buffeting onset.

Potential Impact:
The research topic of TFAST project finds the application in the Flight Physics as well as in the Propulsion. Therefore the impact of TFAST on Air Transport Greening will be manifold. Regarding the strategic development in drag reduction the laminarisation of boundary layers on lifting parts of the airplane has a potential to reduce fuel consumption by 10%. In propulsion systems enhanced laminarity of boundary layers may reduce fuel consumption by 1 or 2 % but it affects also the maximum mass flow rate output of compressor and reduces the heat flux to turbine blades, improving durability of equipment and improving safety. These aspects can not be expressed in terms of fuel consumption.
It is foreseen that the TFAST project very well complies with the structure of the AAT Thematic Priority concerning research for strengthening the competitiveness of the European aircraft industry in the global market but also by providing a new fundamental knowledge base for shock wave/boundary layer interaction. This scope of work may be realised only on the wide international platform, - a European level.
Different groups in TFAST, ensuring a sound critical mass, are working together on a particular set of flows (unsteady transonic and supersonic) and are fostering work by cross-fertilisation and close knowledge dissemination.
The TFAST project also plays a major role in providing reliable methods that are applicable to unsteady and highly compressible flows, with the clear goal of becoming recommended for industrially relevant application challenges. It is the technical achievement of the TFAST project which is – supported by the new data base – served as the basis for a European knowledge of all aspects in the area of transition location effect on unsteady shock wave/boundary layer interaction.
Moreover, it is the formation of trans-national teams - meeting the challenging goals of TFAST - which will contribute to the establishment of a more common (at least in the aeronautics sector) environment in Europe that is favourable to innovation. The latter is even more supported by the number of universities in the TFAST project, that will – on the basis of the knowledge gained – educate students by using knowledge base provided by TFAST.

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