16:10
Reduction of turbulent skin friction
Flow topology and potential for passive spanwise forcing of undulating streamwise grooves for turbulent drag reduction
Bas Van Oudheusden, Max Knoop, Luuk Pelkmans, Ferry Schrijer
Abstract: The present investigation addresses an alternative passive means of spanwise flow forcing, by a geometry referred to as sinusoidal undulations (SU). It constitutes grooves that meander along the streamwise direction, with the proposed working principle that this will induce an oscillatory spanwise flow, in direct analogy to a spatial (standing-wave) actuation. The scope of the present communication is to investigate the capability of this geometry to achieve such a flow pattern, as a first step in assessing its potential for turbulent drag reduction.
|
Towards drag reduction through sinusoidal riblets in adverse pressure gradient
Edoardo Fracchia, Gabriele Simone Pipino, Jacopo Serpieri, Gioacchino Cafiero
Abstract: We experimentally investigated the effects of different riblets patterns on the development of a turbulent boundary layer in adverse pressure gradient conditions. Streamwise aligned riblets as well as wavy riblets, characterized by a sinusoidal pattern in the mean flow direction, are considered in this study.
Hot wire anemometry measurements are performed to characterize the flow statistics at different distances from the wall, in addition to direct drag force measurements using a floating-element balance.
|
Design and characterization of the transient response of oscillating DBD plasma actuators for skin-friction drag control in turbulent pipe flows
Lorenzo Magnani, Gabriele Neretti, Jacopo Serpieri, Arturo Popoli, Andrea Cristofolini, Alessandro Talamelli, Gabriele Bellani
Abstract: Reducing the aerodynamic drag of next-generation aircraft is a central goal in sustainable development strategies at national, European, and international levels. To decrease friction drag, which accounts for roughly 50% of total drag during cruise [1], advanced smart surfaces are being designed and tested. Lowering the friction drag of large commercial aircraft requires effective control of turbulent fluctuations, particularly those associated with the near-wall cycle [6].
Active flow control
Flow control systems are devices which try to manipulate the flow over a surface, so to produce a desired effect, such as drag reduction, lift enhancement or noise abatement. Usually, they are classified into active or passive methods: the former require energy to be actuated, while the latter donβt. Due to their adaptability to changing conditions, active flow control are usually preferred. In particular, several studies have demonstrated that oscillating a wall in the spanwise direction can lead to a significant reduction in friction drag [2]. The technological challenge is to mimic the Stokes layer induced by wall-oscillations with actuators that do not require any moving surface. To this end, arrays of Dielectric Barrier Discharge Plasma Actuators (DBD- PAs) have been developed and have shown promising performance [3, 4].
To achieve the best result in terms of drag reduction, flow control schemes that are implemented at the wall often prescribe a forcing of similar time and/or length scales pertaining to the so-called near-wall streaks [5, 6]: this is the idea behind Inner Scale Actuation (ISA) systems. On the other end, Outer Scale Actuation (OSA) systems target Large Scale Motion (LSM) and Very Large Scale Motion (VLSM), associated to larger characteristic time scales[8]. Despite both ISA and OSA systems produce positive drag-reduction values, the high actuation frequencies required by ISA make this kind of actuation energy-inefficient. Furthermore, in contrast to ISA, the performance of OSA improves with increasing Reynolds number. This increases the interest in developing flow-control strategies in high-Reynolds number flow conditions.
Long Pipe: a high-Reynolds number facility
To evaluate and optimize the performance of the array of plasma actuators under realistic flow conditions, the geometry, actuation timing, and electrical parameters of this device must be tailored to match the temporal and spatial scales of high-Reynolds-number turbulent flows. The Long Pipe facility at the Centre for International Cooperation in Long Pipe Experiments (CICLoPE) [7] offers an ideal setting for such investigations: with a diameter of 0.9 m and a length of 111.5 m, it enables precise measurements of near-wall flow fluctuations, serving as a reliable reference for assessing smart surface technologies. To generate an oscillating wall-parallel flow, multiple pairs of actuators must be alternately activated and deactivated. Therefore, it is essential to thoroughly characterize the actuatorsβ transient response, rather than relying solely on their steady-state behaviour.
Plasma Actuators layout
The layout of the array of plasma actuators is depicted in Fig.1. Each single plasma actuator is composed of a 500ΞΌm-thick layer of Kapton interposed, as a dielectric barrier, between two High Voltage electrodes (HV1 and HV2) and a ground electrode, both aligned in the streamwise direction. The high voltage electrodes are exposed to the flow, and the ground electrode is electrically insulated with an epoxy resin to avoid undesired plasma discharges.
Results and discussion
In this work, we present a full characterization of the transient response of an array of plasma actuators specifically designed to operate in the high-Reynolds number regime of the Long Pipe facility (6000<π
ππ<43000). Experiments were conducted using the Schlieren technique, which visualizes flow density variations induced by the actuators. Exploiting a Phantom Miro M340 high-speed camera, up to 50,000 images per second are acquired and processed to track the propagation front of the tangential flow induced at the wall by plasma actuators (Fig.2).
Tangential velocity profiles (Fig.3) of the plasma-induced vortex front generated by both HV1 and HV2 are retrieved based on two different image post-processing techniques for several actuation parameters, including modulation period (5β80 ππ ) and peak-to-peak input voltage (3.5β5 ππ). The similarity between the tangential velocity profiles of the vortex front generated by the two high voltage electrodes is presented in Fig.4.
Future work
Based on these results, we discuss potential and limitations of this approach deployed on a large-scale, high-Reynolds number application. Results will also be compared in light of a newly developed mathematical model for plasma generation and its interaction with the surrounding flow.
Key Words: Plasma actuators, Active flow control, Drag reduction
|
|