Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T37: Separated Flows: Experiments |
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Chair: Karen Mulleners, École polytechnique fédérale de Lausanne (EPFL) Room: 203AB |
Monday, November 20, 2023 4:25PM - 4:38PM |
T37.00001: Dynamics of laminar separation bubbles under accelerating inflow conditions Wolfgang Dierl, Serhiy Yarusevych, Rainer Hain, Christian J Kähler Laminar separation bubbles mainly occur at low Reynolds numbers, where they alter the flow significantly. Investigations performed in the past preferably focused on steady inflow conditions. The work presented here examines the influence of acceleration on the formation of a laminar separation bubble. The experiments were performed in a towing tank with an SD7003 airfoil model accelerated from rest to a constant chord Reynolds number and then decelerated. Quantitative flow field measurements were performed using two-component time-resolved Particle Image Velocimetry over a range of conditions. The aim is to provide insight into the mechanism of LSB formation by a detailed analysis of the spatio-temporal flow development focusing on the LSB formation and dynamics. The associated transient flow development is shown to persist over several convective time scales after steady state free stream velocity is reached, with no significant effect of acceleration on the overall transient duration. However, the acceleration rate has a substantial effect on flow development during the acceleration, with a delayed development of shear layer perturbations observed at lower accelerations. |
Monday, November 20, 2023 4:38PM - 4:51PM |
T37.00002: Flow Topology of Laminar Separation and Reattachment on a Wall-Bounded Wing Charles Klewicki, Geoffrey R Spedding, Gustaaf B Jacobs, Bjoern F Klose Airfoil performance at transitional Reynolds numbers depends strongly on the dynamics of boundary layer separation and reattachment. Though it is convenient to assume two-dimensional flow, recent evidence suggests wing-wall junctions affect transition dynamics, particularly for low aspect ratio wings. Here, a NACA 65(1)412 airfoil with aspect ratio $AR=3$, is set between two end walls, and at $alpha = {0, 2, 4, 6, 8, 10}^{circ}$, with Reynolds number, Re$_c = {2, 4, 6, 8} imes 10^4$. Two-dimensional PIV is used to quantify the flow topology at midspan, the end walls, and within the boundary layer of the airfoil suction surface. Both spanwise and chord-normal directions are investigated. A curved laser sheet is employed to capture the flow at the suction surface in the spanwise-streamwise directions. The line of laminar separation depends only weakly on Re, and inside the separation region, spanwise velocities up to $0.2U_{infty}$ are found. The flow is never two-dimensional, and implications of this for control strategies are considered. |
Monday, November 20, 2023 4:51PM - 5:04PM |
T37.00003: Dynamic stall landmark events and time scales on a vertical-axis wind turbine blade Sebastien Le Fouest, Daniel Fernex, Karen Mulleners Vertical-axis wind turbines (VAWT) enable urban and high-density offshore wind power applications. The occurrence of dynamic stall on the blades of VAWT has hampered their industrial deployment. To improve the efficiency and lifespan of these turbines, we aim to develop data-driven models and control strategies that take into account the timing and duration of subsequent events in the unsteady flow development. In this talk, we present the chain of events that leads to dynamic stall on a VAWT blade and quantify the influence of the turbine operation conditions on the duration of the individual flow development stages. These timescales are characterised using time-resolved flow and unsteady load measurements of a wind turbine model undergoing dynamic stall for a wide range of tip-speed ratios. We use proper orthogonal decomposition to identify dominant flow structures and distinguish six characteristic stall stages: the attached flow, shear-layer growth, vortex formation, upwind stall, downwind stall and flow reattachment stage. Dynamic stall stages are also identified based on aerodynamic force measurements. These findings can help improve low-order modelling of dynamic stall on vertical-axis wind turbines and inform flow control strategies. |
Monday, November 20, 2023 5:04PM - 5:17PM |
T37.00004: Unsteadiness in turbulent separated flow over a three-dimensional Gaussian hill Kevin H Manohar, Hariprasad Annamalai, Owen Williams, Robert J Martinuzzi, Chris Morton The turbulent separated flow over the three-dimensional Gaussian `Boeing’ speed-bump benchmark geometry exhibits phenomena across a wide range of frequencies. This talk aims to characterize the unsteady dynamics in the separated zone, and to explore the streamwise and spanwise three-dimensional separation topology. High-rate surface-pressure measurements are used to identify the dominant frequencies, while planar two-component particle image velocimetry is employed to elucidate the wake structure along multiple planes. |
Monday, November 20, 2023 5:17PM - 5:30PM Author not Attending |
T37.00005: Experimental Real-Time observation of very low-frequency phenomena downstream of a Backward-Facing Step Juan S Pimienta, Jean-Luc Aider The Backward-Facing Step (BFS) flow is considered and used as a relevant benchmark case for shear flows and separated flows. Its relevance lies in its simple geometry, a downward step of height h, with a sharp edge, producing very complex fluid structures such as a shear layer, vortex rolling/paring/shedding, and formation of coherent structures, superimposed to a large recirculation bubble. In this study, Real-Time velocity field data are recorded for several hours and various Reynolds numbers by means of a 2D2C Real-Time Optical Flow Particle Image Velocimetry (RT-OF PIV) system. The signal computed and recorded by the RT-OF system is the time-evolution of the instantaneous recirculation area A_{R}(t) of the separated flow, together with multiple local velocity probes distributed downstream of the BFS. The observation and recording of this information over such a long time (hours of recording) have allowed us to explore high energy low-frequency phenomena (of the order of 10^{-3} Hz) downstream of the BFS. To our knowledge, this is the first experimental evidence of very low-frequency of large-scale structures in separated flows. |
Monday, November 20, 2023 5:30PM - 5:43PM |
T37.00006: Three-Dimensionality in Swept Wing-Gust Encounters Oliver D Wild, Antonios Gementzopoulos, Anya R Jones Flow disturbances pose a significant risk to aerial vehicles, particularly smaller ones operating in urban areas, as they can lead to flight instability and potentially fatal accidents. Currently, the understanding of the physics behind three-dimensional gust encounters is limited since previous studies have mainly focused on transverse gusts encountered by unswept flat plates. Therefore, the objective of this study is to bridge this knowledge gap by examining encounters with transverse gusts at non-zero sideslip angles. The present experimental investigations conducted in a water towing tank facility, utilizing a transverse gust jet nozzle, revealed a lift response dependent on the square of the cosine of the sideslip angle and the gust ratio. The lift force curves collapse when employing the proposed normalization approach for sideslip angles surpassing 30 degrees. Three-dimensional particle tracking velocimetry using the Shake-The-Box approach gave more insight into the three-dimensionality of the leading edge vortex. These findings will contribute to the development of simplified low-order models and controllers essential for ensuring safe autonomous flight in the future. |
Monday, November 20, 2023 5:43PM - 5:56PM |
T37.00007: Spectral proper orthogonal decomposition of Ahmed-body wake using time-resolved three-dimensional flow measurements Chin Wang Chen, Sen Wang, Sina Ghaemi The turbulent wake behind a flat-back Ahmed body was investigated using a methodology that combines time-resolved tomographic particle image velocimetry (tomo-PIV) measurements and spectral proper orthogonal decomposition (SPOD). The SPOD analysis shows four types of flow motions in the wake of the Ahmed body, each occurred at a specific frequency range. The bi-stability mode was asymmetric and consisted of a tilted toroidal vortex and two counter-rotating streamwise vortices. This motion resulted in significant variations in the separation bubble volume. A newly identified category of motions, referred to as swirling motions, was observed. The swirling motions consisted of rotational movements of the wake barycenter in the crossflow plane in addition to streamwise motions of the barycenter. The swiriling motions resulted in expansion and contraction of the separation volume similar to those associated with bubble pumping motions, as well as oscillations between asymmetric and symmetric states previously described as switching attempts. The vortex-shedding motions consisted of quasi-streamwise vortices that advected in the downstream direction and resulted in oblique movements of the barycenter. Furthermore, SPOD showed shear layer instabilities that generate small vertical and spanwise velocity fluctuations along the shear layers. |
Monday, November 20, 2023 5:56PM - 6:09PM |
T37.00008: Vortex-Induced Vibrations on a curved path Yahya Modarres-Sadeghi, Adrian Carleton We present the results of a series of experiments on vortex-induced vibrations of a cylinder that is free to oscillate on a curved path perpendicular to the direction of the incoming flow. We conducted these experiments in a re-circulating water tunnel and measured the oscillation amplitudes and frequencies. We also visualized the cylinder's wake using bubble image velocimetry. We conducted these experiments for different values of the radius of curvature and over a range of reduced velocities. We observed that at low radii of curvature, oscillations are suppressed, and at a critical radius of curvature, maximum amplitude of oscillations is obtained. We also show that the lock-in range, the amplitude of oscillations and the wake of the cylinder depend on whether the cylinder oscillates on a concave or a convex path. |
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