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This book presents experimental and numerical findings on reducing shock-induced separation by applying transition upstream the shock wave. The purpose is to find out how close to the shock wave the transition should be located in order to obtain favorable turbulent boundary layer interaction.

The book shares findings obtained using advanced flow measurement methods and concerning e.g. the transition location, boundary layer characteristics, and the detection of shock wave configurations. It includes a number of experimental case studies and CFD simulations that offer valuable insights into the flow structure. It covers RANS/URANS methods for the experimental test section design, as well as more advanced techniques, such as LES, hybrid methods and DNS for studying the transition and shock wave interaction in detail. The experimental and numerical investigations presented here were conducted by sixteen different partners in the context of the TFAST Project.

The general focus is on determining if and how it is possible to improve flow performance in comparison to laminar interaction. The book mainly addresses academics and professionals whose work involves the aerodynamics of internal and external flows, as well as experimentalists working with compressible flows. It will also be of benefit for CFD developers and users, and for students of aviation and propulsion systems alike.



Introduction—TFAST Overview

In recent decades huge effort has been placed on maintaining laminar boundary layers with respect to a number of applications. To a large extent this research concerned laminar wings, especially focusing on maintaining laminar boundary layer on the longest possible distance. The research of laminar boundary layer is inherently coupled with the investigations of instabilities leading to the laminar-turbulent transition. These research directions are still in progress. The existing knowledge, at its present stage of development, has been fully utilised in the project for the analysis of laminar boundary layer development and the following transition. It must be emphasized that TFAST objective was not to improve knowledge of laminar boundary layers and of transition. The general aim of TFAST was to avoid that the laminar boundary layer is penetrated by the shock wave. The benefits of having laminar boundary layer are so important that the transition should occur as late as possible. The sensitivity of the solution is calling for basic studies before the configurations closer to application may be investigated. It is necessary to carry out advanced experiments and accurate CFD work with the most advanced methods as LES and DNS. The topic of laminar/transitional/turbulent interaction with a shock wave is the most challenging problem in aeronautics, even more when unsteady interaction effects are to be treated. For these challenging tasks a consortium of the high quality was 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. Due to the experience obtained from the consortium that was forming the UFAST project, and because the proposed research was based on the experience gained in UFAST, the core of the TFAST consortium consisted mainly of UFAST partners. Due to the industrial requirements and needs those partners being “only” observers in UFAST were employed as full partners in TFAST. Another “new” partner was DLR which carried out the experimental compressor and turbine cascade investigations. In summary one may conclude that the consortium is not only of the high European level, but that it has already a long tradition of successful collaboration. The main objective of the project was to study the effect of transition location on the structure of interaction between a shock wave and a boundary layer. Main question was how close the induced transition may be to the shock wave while still maintaining a typical turbulent character of the interaction. In other words, how far the laminar boundary layer may extend without changing the turbulent character of interaction. This question have been answered in WP-2 (i.e. in the basic type of the test section), and also in the flow cases characterising different applications in WP-3, WP-4 and WP-5.
Piotr Doerffer

Basic Flow Cases


WP-1 Reference Cases of Laminar and Turbulent Interactions

In order to be able to judge the effectiveness of transition induction in WP-2, reference flow cases were planned in WP-1. There are two obvious reference cases—a fully laminar interaction and a fully turbulent interaction. Here it should be explained that the terms “laminar” and “turbulent” interaction refer to the boundary layer state at the beginning of interaction only. 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. 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. As mentioned earlier, shock reflection at M = 1.6 may be related to incipient separation. Therefore two additional test cases were planned with different Mach numbers. ITAM conducted an M = 1.5 test case, and TUD an M = 1.7 test case. These partners have also previously made very specialized and successful contributions to the UFAST project.
Jean-Paul Dussauge, Reynald Bur, Todd Davidson, Holger Babinsky, Matteo Bernardini, Sergio Pirozzoli, Pierre Dupont, Sébastien Piponniau, Lionel Larchevêque, Rogier Giepman, Ferry Schrijer, Bas van Oudheusden, Pavel Polivanov, Andrey Sidorenko, Damien Szubert, Marianna Braza, Ioannis Asproulias, Nikos Simiriotis, Jean-Baptiste Tô, Yannick Hoarau, Andrea Sansica, Neil Sandham

WP-2 Basic Investigation of Transition Effect

An important goal of the TFAST project was to study the effect of the location of transition in relation to the shock wave on the separation size, shock structure and unsteadiness of the interaction area. Boundary layer tripping (by wire or roughness) and flow control devices (Vortex Generators and cold plasma) were used for boundary layer transition induction. As flow control devices were used here in the laminar boundary layer for the first time, their effectiveness in transition induction was an important outcome. It was intended to determine in what way the application of these techniques induces transition. These methods should have a significantly different effect on boundary layer receptivity, i.e. the transition location. Apart from an improved understanding of operation control methods, the main objective was to localize the transition as far downstream as possible while ensuring a turbulent character of interaction. The final objective, involving all the partners, was to build a physical model of transition control devices. Establishing of such model would simplify the numerical approach to flow cases using such devices. This undertaking has strong support from the industry, which wants to include these control devices in the design process. Unfortunately only one method of streamwise vortices was developed and investigated in the presented study.
Holger Babinsky, Pierre Dupont, Pavel Polivanov, Andrey Sidorenko, Reynald Bur, Rogier Giepman, Ferry Schrijer, Bas van Oudheusden, Andrea Sansica, Neil Sandham, Matteo Bernardini, Sergio Pirozzoli, Tomasz Kwiatkowski, Janusz Sznajder

Application Flow Cases


WP-3 Internal Flows—Compressors

In the case of a civil turbofan engine operating at particularly high altitudes the Reynolds number can drop by a factor of 4, when compared to the sea 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 forms. 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. Forcing transition within the boundary layer can be achieved through the application of surface roughness patches. Although such passive control methods are already in use, the mechanism of the shock wave-laminar boundary layer interaction, and in particular the source of the strong shock unsteadiness are still not well understood. Furthermore, the benefits of boundary layer control obtained for low Reynolds numbers can turn into loss increase at the higher levels of Reynolds numbers. Another possibility of transition control is to use Vortex Generators driven by Air Jet (AJVG). In the compressor application the jets may be driven by the pressure difference between pressure and suction side of the blade. There are two effects which are present. The main effect is resulting from streamwise vortices generated on the blade suction side. The second effect is the suction of the boundary layer on the pressure side. The goal of Work Package 3 was to improve the understanding of the shock wave–laminar boundary layer interaction on the transonic compressor blade. This can potentially lead to successful new design solutions.
Patrick Grothe, Pawel Flaszynski, Ryszard Szwaba, Michal Piotrowicz, Piotr Kaczynski, Benoit Tartinville, Charles Hirsch, Alexander Hergt

WP-4 Internal Flows—Turbine

Modern HP turbine stages consist of highly loaded aerofoils, including transonic and even supersonic flow regions. In terms of stators a normal shock wave in the passage throat chokes the flow, stabilizing the flow conditions at the operating point. In addition to these flow phenomena, the strong acceleration along the early suction side leads to a relaminarisation of the flow, which in turn has a strong impact on the size of the shock induced separation bubble. Finally, the injection of film coolant via rows of cooling holes further influences the boundary layer state. As heat transfer and film cooling effectiveness are of crucial importance in high pressure turbines, an in-depth understanding of transition mechanisms is needed for a competitive design. The main objective was to study transition location effects (from natural transition to fully turbulent) on separation size, shock structure and unsteadiness. The transition interaction with cooling flow was a real challenge for the TFAST project. This research delivered new knowledge, which is crucial for further improvement of HP turbine stages. As transition control devices AJVG and disturbance strip were used.
Anna Petersen, Piotr Doerffer, Pawel Flaszynski, Ryszard Szwaba, Michal Piotrowicz, Piotr Kaczynski, Benoît Tartinville, Charles Hirsch

WP-5 External Flows—Wing

Study of transition location effect (from natural transition to fully turbulent) on separation size, shock structure and unsteadiness was the focus of this WP. Boundary layer tripping (by wire or roughness) and flow control devices (VG) were used for boundary layer transition induction. Although this type of flow field had been studied widely in the past, there remains considerable uncertainty on the effects of transition on transonic aerofoil performance. In particular it is not known how close to the shock location transition has to occur to avoid detrimental effects associated with laminar shock-induced separation. Furthermore, it was unclear how best to provoke transition on an airfoil featuring significant laminar flow and how close to the shock this needs to be performed. Finally, current CFD methods are particularly challenged by such transitional flows. In this work package some of the findings from the basic research performed in other WPs was applied. Specialized large-scale transonic wind tunnels running cost is very high therefore using such facilities is not appropriate for upstream research programs such as TFAST. Therefore we have used existing wind tunnels within our consortium. One of these is a transonic test section at UCAM where laminar and transitional profiles were studied previously at Reynolds numbers up to 2 million (based on chord length). This wind tunnel allowed basic investigations of the transition location effects on a shock induced separation and unsteadiness for a relatively large number of parameters. A larger wind tunnel at Institute of Aviation in Warsaw was used, which enabled the investigation of a much larger aspect ratio profile. In this facility it was possible to measure a whole force polar up to and including the buffet boundary. The research was carried out for the natural b/l transition location as well as different methods of tripping.
Flavien Billard, Todd Davidson, Holger Babinsky, Robert Placek, Marek Miller, Paweł Ruchała, Wit Stryczniewicz, Tomasz Kwiatkowski, Wieńczysław Stalewski, Janusz Sznajder, Sara Kuprianowicz, Matteo Bernardini, Sergio Pirozzoli, George Barakos, George Zografakis, Benoit Tartinville, Charles Hirsch, Damien Szubert, Marianna Braza, Ioannis Asproulias, Nikos Simiriotis, Jean-Baptiste Tô, Yannick Hoarau

Summary, Conclusions and Lessons Learned


Closing Remarks

The TFAST project covered very wide range of knowledge starting from the basic research to more applied flow configurations. New knowledge generated in this project casts new light on the possibilities of flow control methods in reducing negative effects of shock wave interaction with laminar boundary layer. On the other hand application of flow control devices in the laminar boundary layer, just to inspire transition, is a valuable result. As usually in such complex flow structure the results are strongly case dependent, but the complexity of the presented approach shows great potential for improvements and provides a good basis for future research.
Pawel Flaszynski, Piotr Doerffer, Sergio Pirozzoli, Jean-Paul Dussauge, Pierre Dupont, Lionel Larchevêque, Holger Babinsky, Patrick Grothe, Anna Petersen, Flavien Billard
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