Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session R29: Separated Flows: General |
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Chair: Xinyi Huang, California Institute of Technology Room: 255 A |
Monday, November 25, 2024 1:50PM - 2:03PM |
R29.00001: Pressure Measurments of Turbulent Boundary Layer Behavior over a Bump Nissrine J Aziz, Theresa Ann Saxton-Fox Experiments will be conducted in a subsonic turbulent boundary layer within a 15'' x 15'' x 12' test section. A 3D bump, inspired by the "Boeing bump" is selected for its tendency to induce flow separation and subsequent reattachement. The modified bump design features an aspect ratio (h/δ) between 1 and 1.4 at Reynolds numbers (Reh) of 1.4x104 to 1x105. Time-resolved pressure measurements on the bump will be conducted at lower Re number and lower (h/δ) values compared to previous literature (Owens, 2021). Time-resolved measurements of surface pressure will be performed at two streamwise positions and four spanwise positions on the bump surface. Skin friction will be measured with IC2 sensors at two streamwise positions and four spanwise positions. Statistics and correlations between the pressure measurements and shear stress measurements at various positions will be calculated and presented. Furthermore, oil flow visualization will be implemented to detect the 3D topology to gain deeper understanding of the surface flow over the bump. This study aims to provide detailed measurements on the bump to analyze the impact of strong pressure gradients on turbulent boundary layer development across various Reynolds numbers and bump heights. |
Monday, November 25, 2024 2:03PM - 2:16PM |
R29.00002: Decay of laminar separation bubbles under decelerating inflow conditions Wolfgang Dierl, Serhiy Yarusevych, Rainer Hain, Christian J Kaehler The formation of a laminar separation bubble on the suction side of a lifting surface is common when airfoils are operated at low Re. The investigation of unsteady inflow conditions, however, is relatively rare, although these conditions typically occur in practical applications. The work presented here explores the effects of deceleration on laminar separation bubbles. Experiments were conducted in a water towing tank using an SD7003 airfoil model decelerated from a constant chord Reynolds number to rest. Quantitative flow field measurements were performed using time-resolved two-component Particle Image Velocimetry over a range of decelerations. The aim is to gain insight into the time-dependent topology of laminar separation bubbles. The results show that deceleration of the model leads to a gradual shift of the separation point and vortex roll-up towards the leading edge, which is in contrast to the trends expected for a quasi-steady decrease in Reynolds number. At lower deceleration values in the range considered here, the vortex shedding frequency decreases. The effect of deceleration on the pressure gradient indicates that the deviation of the results from quasi-steady trends in the cases studied is due to significant inertial effects. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R29.00003: Triglobal resolvent analysis of separated flow around a NACA0012 finite wing: Reynolds number effects Laura Victoria Rolandi, Luke Smith, Kunihiko Taira Resolvent analysis offers valuable insights into flow unsteadiness and the design of flow control techniques. In this study, we employ triglobal resolvent analysis to investigate the effects of Reynolds numbers Re∈[600;5000] on separated flow over a NACA0012 finite wing of semi-aspect-ratio sAR=2 and at an angle of attack α=14°. Depending on the Reynolds number, the time-averaged base flow is obtained by direct numerical simulations or wall-resolved large eddy simulations. Our focus is on understanding the influence of Reynolds number on the response and forcing modes, as well as the energy gain associated with different frequencies. For all the Reynolds numbers investigated, we observe a shift of the modal structures toward the outboard region as the forcing frequency increases, consistent with observations at lower Reynolds numbers. Furthermore, we find that sufficiently strong tip vortices support modal structures at the highest frequencies. We also show that varying the forcing frequency causes the modal structures to change their inclination relative to the wing span. In particular, the maximum energy gain is achieved for structures that are parallel to the wing and at forcing frequencies that increase with the Reynolds number. In this regard, we propose a Reynolds number-based normalization for the mechanisms driving the most energetic dynamics, providing a more comprehensive understanding of the flow behavior across different Reynolds numbers. |
Monday, November 25, 2024 2:29PM - 2:42PM |
R29.00004: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 2:42PM - 2:55PM |
R29.00005: Investigation of the three-dimensional dynamics of a turbulent separation bubble by means of volumetric velocity field measurements Ben Steinfurth, Mogeng Li, Fulvio Scarano, Julien Weiss A turbulent separation bubble (TSB) is formed when a turbulent boundary layer detaches from the surface and the resulting separated shear-layer reattaches further downstream. Planar velocity field measurements inside the TSB symmetry plane have shown in past studies that the dynamics of such flows are characterized by low-frequency fluctuations of the streamwise TSB extent. The objective in this study is to advance our knowledge regarding the time-dependent behaviour by identifying the three-dimensional representation of the low-frequency motion. To this end, volumetric particle tracking velocimetry measurements are conducted in an open-return blowdown wind tunnel (Reθ ≈ 2000) where flow separation is produced in a backward-facing ramp test section with 600 mm span. Three-dimensional velocity field measurements are performed by supplying helium-filled soap bubbles to the flow and tracking them through space-time. The measurement data are assimilated using a physics-informed network that is constrained by the Navier-Stokes equations, and the dynamical analysis is based on spectral proper orthogonal decomposition. The latter returns multiple large-scale regions along the spanwise direction that capture anti-correlated streamwise velocity fluctuations at low frequency. This suggests that the instantaneous state of TSB expansion observed in the symmetry plane is accompanied by an increase in velocity near the side walls of the test section, and (vice versa) by a reduction for the contracted TSB. The findings of this study support the hypothesis that the low-frequency motion, now observed in several experimental configurations, is a fully three-dimensional phenomenon. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R29.00006: Comparison of Techniques for Measuring Flow Residence Time in Flow Separation over a Backward-Facing Step Sucheta Tamragouri, Maduka Maduka, Andy DiFut, Zhenglun Alan Wei Flow separation is a common phenomenon in physiological systems. It often leads to increased flow residence time (FRT) has been linked to various physiological adverse events, such as energy loss, blood damage, and thrombus potential. Numerous computational and experimental techniques have been developed to measure FRT. However, direct comparisons between these methods to demonstrate performance differences are lacking. This study aims to provide such a comparison using a canonical configuration for flow separation: flow over a backward-facing step (BFS). We compared popular techniques for measuring FRT, including computational fluid dynamics (CFD), particle image velocimetry (PIV), and dye injection across various BFS configurations. These configurations varied in dimensions (5 mm – 20 mm) and flow conditions to achieve laminar (Re = 654) and transitional-to-turbulent (Re = 3923) flows, which are commonly observed in physiological systems. All techniques showed general agreement with each other. Notably, the dye injection parameters (such as injection speed and position) were fine-tuned as they are critical to obtaining reasonable results. This study directly compared computational and experimental methods for quantifying FRT in a simple, canonical configuration, providing new insights into these techniques. Future studies should include similar direct comparisons of FRT techniques under more physiological conditions. |
Monday, November 25, 2024 3:08PM - 3:21PM |
R29.00007: Abstract Withdrawn
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Monday, November 25, 2024 3:21PM - 3:34PM |
R29.00008: Thrust Generation by Shark Denticles Wen Wu, Benjamin Sam Savino Increasing interest has emerged in studying the potential of shark dermal scales - known as denticles - to reduce pressure drag by controlling flow separation. Direct numerical simulations of turbulent channel flow over a bump on the bottom wall were performed, comparing a smooth bump with one where the lee side featured an array of shark denticle replicas. Complete shark denticles, including the neck and crown, were employed. As the flow over the bump was subjected to an adverse pressure gradient (APG), the flow over the denticle array was injected through the slits between the crowns, forming a reverse pore flow (RPF) in the cavity region created by the denticle necks underneath. The staggered necks in this region accelerate the RPF in the gaps between them, forming arrays of wall-parallel jets that produce thrust through the augmented shear and pressure forces. This resulted in more than a twofold increase in thrust on the lee side and a net 4\% total drag reduction compared with the smooth bump. The DNS provided detailed pore flow statistics and revealed the geometrical features responsible for the generation and maintenance of the RPF and thrust generation. In particular, the RPF is activated by the APG before the onset of massive flow separation and does not appear when the APG is absent. These mechanisms indicate a flow-activated, on-demand passive drag-reduction mechanism that leverages and transforms an APG into a favorable outcome. |
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