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Passive Wake flows II
Experimental investigation of the flowfield around propellers operating at low Reynolds numbers
Gabriele Salomone, Gerardo Paolillo, Tommaso Astarita, Gennaro Cardone, Carlo Salvatore Greco
Abstract: Characterizing the flow field around propellers operating at low Reynolds numbers is important for design optimization, since the presence of phenomena, such as laminar separation bubbles (LSBs), can negatively impact propeller performance, leading to noise emissions, stall and drag penalties. The present work uses Infrared Thermography (IRT) to examine the relevant features of the flow field surrounding a 610 mm-diameter propeller that mounts a NACA 4412 airfoil throughout the span.
The propeller is operated at rotational velocity equal to 130 rad/s, corresponding to tip chord-based Reynolds number equal to 4.13 × 10^4.
Preliminary results highlight the presence of a laminar separation bubble in the tip region and larger separation regions towards the root.
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Kirigami sheets for passive flow control
Adrian Carleton, Yahya Modarres-Sadeghi
Abstract: Kirigami designs consist of a pattern of slits, or a combination of slits and creases, which makes them different from origami designs that are solely based on creases. The ease of production and deployment (which is often done simply by applying external tension on a 2D sheet) has caused several recent applications for kirigami sheets in ``structural mechanics, materials, optics, electronics, robotics, and bioengineering". Despite this extensive use in other fields, kirigami designs have not been used for fluid mechanic applications. Here, we discuss the potentials of kirigami sheets as passive flow control devices by considering the behavior of kirigami sheets placed perpendicular to an incoming flow and observing their response to the flow forces and their corresponding wakes.
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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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