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.

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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.

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High-resolution particle image velocimetry measurements were performed on laminar and transitional oblique shock wave reflections for a range of Mach numbers (M D 1:6-2:3), Reynolds numbers (Re_xsh D 1.4×10⁶-3.5×10⁶) and flow deflection angles (θ 1°-5° or p3=p1 D 1.11-1.64). The laminar interactions revealed a long, flat and triangular shaped separation bubble. For relatively strong interactions (p3=p1 > 1.2), the bubble grows linearly in the upstream direction with increasing shock strength. Under these conditions, the boundary layer keeps an on average laminar velocity profile up to the shock impingement location, followed by a quick transition and subsequent reattachment of the boundary layer. For weaker interactions (p3=p1 < 1.2), the boundary layer is able to remain laminar further downstream of the bubble, which consequently results in a later reattachment of the boundary layer. The pressure distribution at the interaction onset for all laminar cases shows excellent agreement with the free-interaction theory, therefore supporting its validity even for incipiently separated laminar oblique shock wave reflections.

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Micro-ramp vortex generators (or micro-ramps) have gained popularity as promising alternatives to boundary layer bleed for high speed flow-control applications. Micro-ramps generate a counter-rotating vortex pair which leads to a more separation resistant boundary layer. Furthermore, they modulate the size of the shock induced separation bubble along the span such that individual cells of three-dimensional separation are formed. In the current study, the highly three-dimensional nature of such a micro-ramp controlled SWBLI is visualized by using tomographic-PIV. The 3D mean flow field of the interaction is presented, while the 3D separation behaviour is further characterized by the separation probability of the interaction Psep at different wall-parallel planes. The largest reduction in Psep occurs along the micro-ramp centreline where separation is basically eliminated. Additionally, it is found that the total volume of separation is decreased by 70% when compared to an uncontrolled SWBLI. @en

Shock wave-boundary layer interactions are prevalent in many aerospace applications that involve transonic or supersonic flows. Such interactions may lead to boundary layer separation, flow unsteadiness and substantial losses in the total pressure. Flow control techniques can help to mitigate these adverse effects and stabilize the interaction. This thesis focuses on passive flow control techniques for oblique shock wave reflections on flat plates and presents experimental results for both laminar (part I) and turbulent interactions (part II). Particle image velocimetry (PIV) measurements were used as the main flow diagnostics tool throughout this thesis, where especially the laminar case proved to be challenging due to its very small boundary layer thickness of ∼0.2 mm. Laminar boundary layers are extremely prone to separation and long separation bubbles (∼50δ99) were recorded even for relatively weak shock waves (p3/p1∼1.2). The bubble has a flat / triangular shape and extends mostly upstreamof the incident shockwave. The incoming boundary layer is lifted over the bubble and remains in an apparent quasilaminar state up to the incident shockwave, afterwhich the boundary layer quickly transition into a turbulent state (30-40δ99). Only for very weak shock waves it was found that the boundary layer can remain laminar up to reattachment. The separation bubble for laminar interactions can be removed by enforcing boundary layer transition a short distance upstream of the interaction. Transition strips that introduce three-dimensional features in the flow were found to be more effective at this task than purely two-dimensional trips (e.g. a step) and could therefore be placed closer to the interaction while still maintaining their effectiveness. Forced boundary layer transition, however, comes at the price of having a substantially thicker (∼50%) turbulent boundary layer downstream of the interaction, which is the result of losses at the trip, a larger portion of turbulent flow and higher shock-induced losses. It therefore appears that there is no added value to tripping the boundary layer for laminar flat plate interactions, especially given the fact that the untripped laminar interaction shows no signs of any large-scale type of unsteadiness. For the turbulent interactions, micro-ramp vortex generators were studied as flow control devices. Micro-ramps transport high-momentum fluid towards the near-wall region of the flow by the action of streamwise vortices, thus creating a fuller boundary layer profile that is less prone to separation. A net transport of streamwise momentum has been observed up to 5-7δ99 downstream of the micro-ramp, after which a plateau level is reached in which, on average, no momentumis added or removed fromthe nearwall region of the flow. Consequently, a similar distance between the trailing edge of the micro-ramp and the onset of the interaction is required to ensure a maximumreduction in separation bubble size and shock unsteadiness. The application of micro-ramps leads to a spanwise modulation of the separation bubble, with the micro-ramp being most effective along its centreline. The control effectiveness of the micro-ramp is virtually independent of the Reynolds number and is slightly reduced for higherMach numbers. @en