Flow control for oblique shock wave reflections

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. Aerodynamic

On Transition Delay with Plasma Actuators: PIV Diagnostics, Reduced Order Modeling and Adaptive Control

With oil prices regularly topping one hundred dollars per barrel it becomes ever more important to reduce the fuel consumption of airplanes. Besides economical motives, the environmental impact of airplanes also forms a driving factor for reducing fuel consumption. Drag reduction forms an important component in the complete package of fuel reduction measures. On average 50% of total drag can be attributed to skin friction drag, and on average a turbulent boundary layer results in a wall shear stress that is ten times higher than that of a laminar boundary layer [1]. These numbers show the potential of drag reduction by postponing the laminar to turbulent transition.Aerospace Engineerin

A parametric study of laminar and transitional oblique shock wave reflections

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 (Rexsh 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.greenAerodynamic

Flow control of an oblique shock wave reflection with micro-ramp vortex generators: Effects of location and size

The effects of micro-ramp height and location on a shock induced separation bubble were quantified using planar particle image velocimetry measurements. Conditional averaging was used to show that the amount of separation is related to the momentum flux in the near-wall region (< 0.5?) of the incoming boundary layer. The momentum flux added to this region scales linearly with micro-ramp height and larger microramps are shown to be more effective in stabilizing the interaction. Full boundary layer mixing is attained 5.? downstream of the micro-ramp and this forms a lower limit on the required distance between microramp and reflected shock foot.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin

High-resolution PIV measurements of a transitional shock wave–boundary layer interaction

This study investigates the effects of boundary layer transition on an oblique shock wave reflection. The Mach number was 1.7, the unit Reynolds number was 35 × 106 m?1, and the pressure ratio over the interaction was 1.35. Particle image velocimetry is used as the main flow diagnostics tool, supported by oil-flow and Schlieren visualizations. At these conditions, the thickness of the laminar boundary layer is only 0.2 mm, and seeding proved to be problematic as practically no seeding was recorded in the lower 40 % of the boundary layer. The top 60 % could, however, still be resolved with good accuracy and is found to be in good agreement with the compressible Blasius solution. Due to the effects of turbulent mixing, the near-wall seeding deficiency disappears when the boundary layer transitions to a turbulent state. This allowed the seeding distribution to be used as an indicator for the state of the boundary layer, permitting to obtain an approximate intermittency distribution for the boundary layer transition region. This knowledge was then used for positioning the oblique shock wave in the laminar, transitional (50 % intermittency) or turbulent region of the boundary layer. Separation is only recorded for the laminar and transitional interactions. For the laminar interaction, a large separation bubble is found, with a streamwise length of 96 ??i,0 . The incoming boundary layer is lifted over the separation bubble and remains in a laminar state up to the impingement point of the shock wave. After the shock, transition starts and a turbulent profile is reached approximately 80–90 ??i,0 downstream of the shock. Under the same shock conditions, the transitional interaction displays a smaller separation bubble (43 ??i,0 ), and transition is found to be accelerated over the separation bubble.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin

Experimental Study on the Body Force Field of Dielectric Barrier Discharge Actuators

An experimental investigation on thrust and body force of Dielectric Barrier Discharge (DBD) /plasma actuators aimed at low power flow control applications is presented. A parametric study on thrust is conducted for a wide range of geometrical configurations as well as several electrical operational conditions. Direct measurements of the induced thrust are taken using a highly sensitive load cell. Simultaneous readings of current and voltage are also performed, providing the power consumption. Furthermore a novel technique for determination of the spatial distribution of the body-force is proposed, developed and tested. The technique involves the use of a high-speed PIV system to resolve all terms of the Navier-Stokes equation representation of the flow field including body force. Results reveal the existence of an explicit relation between voltage, thrust and consumed power. Furthermore the influence of the geometrical configuration of the actuator on the thrust is shown. The body force obtained with the proposed technique agrees well with the thrust measurements.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin

Tomographic-PIV investigation of the 3D Separation behaviour of a micro-ramp controlled SWBLI

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.Structural Integrity & CompositesAerodynamic