Unsteady inlet flow distortion can influence the stability and performance of any propulsion system, in particular for more novel, short and slim intakes of future aero-engine configurations. As such, the requirement for measurement methods able to provide high spatial resolution data is important to aid the understanding of these flow fields. This work presents flow field characterisations at a crossflow plane within a short aeroengine intake using stereoscopic particle image velocimetry (SPIV). A series of tests were conducted across a range of crosswind and high angle of attack conditions for a representative short and slim aspirated intake configuration at two operating points in terms of mass flow rate. The velocity maps were measured at a crossflow plane within the intake at an axial position L/D = 0.058 from where a fan is expected to be installed. The diameter of the measurement plane was 250 mm, and the final spatial resolution of the velocity fields had a vector pitch of 1.5 mm which is at least two orders of magnitude richer than conventional pressure-based distortion measurements. The work demonstrates the ability to perform robust non-intrusive flow measurements within modern intake systems in an industrial wind tunnel environment across a wide range of operating conditions; hence, it is suggested that SPIV can potentially become part of standard industrial testing. The results provide rich datasets that can notably improve our understanding of unsteady distortions and influence the design of novel, closely coupled engine-intake systems.

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A flying aircraft consumes fuel to overcome air resistance during its motion. A significant part of this consumption (55%) is due to turbulent skin friction arising at the interface of the aircraft surface and the air. The current work aims to develop and investigate relevant turbulent skin-friction reduction techniques and assist in advancing technologies that will contribute to achieving a reduction in turbulent skin-friction drag and, consequently, fuel consumption and the associated emissions. The thesis examines active flow control techniques derived from the spanwise wall oscillation concept. The latter involves introducing a time-dependent spanwise motion to the wall over which a turbulent boundary layer is present. The current work relies on the experimental investigation using particle image velocimetry to quantify the effect of the active control techniques.....@en

In the present work, planar and tomographic PIV are used to investigate how the organisation of wall turbulence is altered when actuators are operated with the objective of reducing the skin friction drag. When the wall is mechanically oscillated in the spanwise direction, high-resolution planar PIV enable the direct measurement of wall shear and a 15% reduction of skin friction is observed. The use of tomographic PIV enables access to the three dimensional organisation of low- and high-speed streaks, along with ejection events and associated hairpin vortices. These observations help forming a conceptual model of the salient drag reduction mechanism, whereby hairpin auto-generation is inhibited through a tilting action at the tail of low-speed streaks. The second part of the study documents an effort to surrogate the mechanical oscillation by means of a densely distributed array of AC-DBD plasma actuators. The latter are first characterised in quiescent flow, where the induced velocity distribution is obtained and compared to the solution of the classical second Stokes problem that models the mechanical oscillation. The induced peak of spanwise velocity is found to surpass the velocity of the oscillating wall. However, the wall jet height develops further away from the wall. More importantly, the spatially inhomogeneous distribution of the unsteady body force produces an unwanted lattice of starting streamwise vortices. The latter are deemed to be detrimental for the purpose of drag reduction, compared to the orderly and homogeneous sideways motion induced by mechanical wall motion. The application of the AC-DBD actuator currently leads to a pronounced momentum deficit in the logarithmic region and increases skin friction. Finally, further research directions are anticipated, that potentially circumvent the formation of streamwise vortices by means of AC-DBD actuators operating in steady regime. @en

Spanwise wall oscillations alter the organization of low-speed streaks and ejections in turbulent boundary layers, eventually leading to skin friction drag reduction. Such flow regimes are represented by pointwise statistics or spatial correlation. This work attempts to quantify the systematic distortions of the dominant turbulent structures by feature-analysis, intended to overcome the dispersion observed in pointwise statistics and correlation functions. Furthermore, data from tomographic particle image velocimetry are employed to clarify the mechanism that inhibits hairpin auto-generation, as described in Kempaiah et al. ["3-dimensional particle image velocimetry based evaluation of turbulent skin-friction reduction by spanwise wall oscillation,"Phys. Fluids 32(8), 085111 (2020)]. Based on the instantaneous distribution of Reynolds stresses, a specific spatial template is defined for low-speed streaks and flow ejections. Events corresponding to this template are collected and parametrized with their occurrence, geometrical properties (length and orientation), and dynamics (intensity). The approach is compared with most practiced statistical analysis to explain the significance of the features extracted by the detection algorithm in relation to the drag reduction mechanism. Data comparing stationary and oscillating wall in a drag-reducing regime (A+osc = 100, T+osc = 100) are investigated in the near-wall region (y+ < 100). Ejections and low-speed streaks systematically exhibit a positive pitch, supporting the hypothesis that only the rear region, close to the wall, is affected by the wall motion. A side-tilt of elongated ejection events is observed past the phase of maximum oscillation velocity, which is hypothesized to inhibit hairpin auto-generation. The latter indicates a phase dependence of the side-tilt in the oscillating regime. The results also indicate that low-speed streaks and ejection events are reduced by approximately 10% and 15%, respectively, compared with the stationary wall, further consolidating the mechanism of rapid lateral distortion being responsible for the different organizations of the turbulent structures in the near-wall region.

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Rotating discs, flush-mounted within the wall beneath a turbulent boundary layer, affect the large-scale flow dynamics. An organized array of rotating discs is a surrogate for the transverse-oscillating wall concept that is known to reduce turbulent friction drag, since the convecting flow encounters a travelling wave, particularly in the spanwise centre of the discs. This work experimentally assesses the flow manipulation of this wall-based actuation method using planar and stereoscopic particle image velocimetry (PIV). Experiments were conducted in a developing turbulent boundary layer, at Re_τ ≈ 910, and with an optimized viscous-scaled sizing and layout of the discs following the direct numerical simulation (DNS) study of Ricco & Hahn (2013). Planar PIV in the streamwise-wall-normal plane over the spanwise centre of the discs revealed a reduction of the in-plane Reynolds stresses, suggesting a suppression of the near-wall turbulence auto-generation process. Wall-parallel planes of velocity data at a height of 70 viscous units above the wall revealed two distinct types of streamwise-oriented regions, comprising low- and high-momentum pathways. These spanwise alternating regions were also captured using the stereo-PIV measurements downstream of the disc-array. It was observed that the mean boundary layer flow is pulled closer to the wall in the disc center, resulting in a higher mean velocity and a less intense streamwise Reynolds stress for a given wall-normal height. With this effect being maximum in the disc center, while being absent between the discs, this type of flow manipulation could be optimized in terms of turbulence suppression (and potentially in terms of friction drag reduction at high Reynolds numbers), by considering larger discs.

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The reduction of turbulent skin-friction drag and the response of vortical structures in a zero-pressure gradient, turbulent boundary layer subjected to spanwise wall oscillation is investigated using planar and tomographic particle image velocimetry (PIV). The experiments are conducted at a momentum based Reynolds number of 1000, while the range of spanwise oscillation amplitude and frequency is chosen around the optimum reported in previous studies. A high-resolution planar PIV measurement is employed to determine the drag reduction directly from wall shear measurements and to analyze the accompanying modifications in the turbulent vortical structures. Drag reduction of up to 15% is quantified, with variations following the trends reported in the literature. The analysis of the turbulence structure of the flow is made in terms of Reynolds shear stresses, turbulence production, and vortex visualization. A pronounced drop of turbulence production is observed up to a height of 100 wall units from the wall. The vorticity analysis, both in the streamwise wall-normal plane and in the volumetric results, indicates a reduction of vorticity fluctuations in the near-wall domain. A distortion of the hairpin-packet arrangement is hypothesized, suggesting that the drag-reduction mechanism lies in the inhibition of the hairpin auto-generation by the spanwise wall oscillations.

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