10:20   Passive Wake flows Chair: Anne-Marie Schreyer Experimental Characterization of a high Reynolds number turbulent flow separating from a 2D smooth body Parisa Ghanoni Bostanabad, Woutijn Baars, Marios kotsonis Abstract: Flow separation with a turbulent incoming boundary layer is common in many wall-bounded flows. Understanding turbulent separated flows is critical, as they exhibit a broad range of unsteadiness. Among the various forms of unsteadiness explored, the large-scale (low-frequency) motion is the most dominant and most energetic feature, yet a consistent physical explanation in the literature is still lacking. To address this gap, the objective of the present experiment is to replicate the key physics of a pressure- induced separating flow, allowing for further investigate the spatial mechanisms linking upstream boundary-layer structures to the large-scale downstream separated region. To achieve this objective, a unique experimental setup was designed to provide initial Turbulent Boundary Layer (TBL) growth over a flat plate, followed by a curvature. Combined with variations in angle of attack, this allows different extents of separated flow without reattachment. A two-dimensional particle image velocimetry (PIV) system with three side-by-side cameras was used to acquire instantaneous velocity fields that simultaneously captured the incoming boundary layer and the separated shear layer. Wake Characteristics of a Circular Cylinder with a Flexible Splitter Plate Fan Duan, Jin-Jun Wang Abstract: The splitter plate is one of the most effective devices for controlling the flow over a bluff body. Recently, the flexible splitter plate has garnered significant attention due to its self-adaptive behavior and flutter-induced perturbations, which hold promise for a more efficient control effect. In this experimental investigation, we explore the influences of different splitter plates on the wake characteristics of a circular cylinder, with a particular focus on the wake spatial scale and the wake fluctuation. Two characteristic streamwise positions in the wake are identified, that is, x/L_r = 1.0 and x/L_r = 1.8 (L_r is the recirculation region length). The wake flow field is categorized into three stages based on the two positions. The wake overall fluctuation undergoes a linear increase initially, then attains a saturation stage, and finally decays as a power law. For all the cases, the wake overall fluctuations scale with their saturation-stage magnitudes, resulting in a collapse of all the data onto a universal scaling curve. Visualization and control of a turbulent boundary layer Fulvio Scarano, Kushal Kempaiah Abstract: The organisation of wall turbulence is altered when actuators produce spanwise forcing, which reduces skin friction drag. A turbulent boundary layer with mechanically oscillated wall is investigated with high-resolution planar PIV and tomographic PIV. The direct measurement of wall shear yields a 15% reduction. Tomographic PIV visualizes the three-dimensional organisation of low- and high-speed streaks, along with ejection events and hairpin packets. A conceptual model of the drag reduction mechanism is proposed, whereby hairpin auto-generation is inhibited by the tilting action at the tail of low-speed streaks. An array of AC-DBD plasma actuators is intended to surrogate the mechanical oscillation. The preliminary characterisation in quiescent flow returns high peaks of induced spanwise velocity. However, discrete wall jets erupting from the wall produce starting vortices, deemed detrimental for drag reduction.

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