Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session G21: Separated Flows |
Hide Abstracts |
Chair: Kenneth Christensen, Notre Dame University Room: 2010 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G21.00001: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 8:13AM - 8:26AM |
G21.00002: Investigation of Bio-Inspired High Lift Devices for Stall Mitigation Esteban Hufstedler, Beverley J. McKeon A passive upper-surface flap has been shown to increase the lift on a wing after stall and reduce the severity of stall at a wide range of Reynolds numbers. Experiments at Re=20,000 have been conducted that examined the forces and flow fields around an airfoil with passively moving and static upper-surface flaps. Force measurements confirm the reported post-stall lift-enhancing effect. Particle image velocimetry measurements display the interaction of a significant region of reversed flow with the flap in the lift-enhancing regime. Application of proper orthogonal decomposition techniques to the velocity field data leads to identification of relevant timescales in the separated region and a quantification of the intermittency of vortex shedding that occurs after stall. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G21.00003: Formation of Three-Dimensional Stall Cells on Two-Dimensional Airfoils Victor Sivaneri, Burak Tuna, Edward DeMauro, Michael Amitay Stall cells are a pattern of three-dimensional mushroom-shaped structures that form within the separated region of stalled, thick airfoils within a certain range of Reynolds numbers. The occurrence and number of stall cells are dependent on the wing camber, aspect ratio, angle of attack, and Reynolds number. While much work within the literature has been conducted to visualize and measure this phenomenon, to date a comprehensive explanation for their existence remains elusive. The present work aims to identify these structures, quantify them, and understand the mechanisms by which they are formed. This was conducted using oil flow visualization and stereoscopic particle image velocimetry (SPIV) on a two-dimensional NACA 0015 airfoil, pitched to 18$^{\circ}$ angle of attack, at Reynolds numbers ranging from 160,000 to 400,000. Oil flow visualization was used to qualitatively identify the signature of the stall cells on the airfoil surface and resolve the associated skin friction vector fields. In addition, SPIV measurements were taken in order to quantify the flow field in the presence and absence of stall cells within the region of separated flow above the surface of the airfoil. Results showed that the stall cells are highly sensitive to Reynolds number, with evidence of an apparent bi-stable state existing at a Reynolds number of 320,000. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G21.00004: Effect of rib length on characteristics of separation and reattachment Jacques W. Van der Kindere, Bharathram Ganapathisubramani Ribs reproduce key elements in engineering. Their aerodynamics can be detrimental to vehicles, or harnessed favorably in motors, and heat exchangers. The flow around such obstacle includes separation upstream of the obstacle, separation and reattachment on the top surface, and separation downstream. The interaction between these different recirculation regions is affected by the obstacle's length. This study examines experimentally how the interaction between different recirculation regions evolves with rib length. The rib is submerged in a fully turbulent boundary layer ($\delta/H = 1.37$, where $\delta$ and $H$ are respectively incoming boundary layer thickness and rib height), and the Reynolds number based on rib height is $Re_H = 20,000$. Particle Image Velocimetry synchronized with pressure measurements was carried out on the flow past ribs of different lengths. The length of the rib (distance between the two vertical faces) varied between $L=0.1H$ and $L=8H$. Results from this experiment will be used to compare the mean recirculation lengths of the different separation regions. Pressure distribution within the separation regions will also be examined and compared. Finally, the interaction between the different shear layers will be examined and contrasted across all cases. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G21.00005: Near-wake characteristics of a rotating dimpled sphere Jooha Kim, Haecheon Choi In this study, we investigate the characteristics of flow around a rotating dimpled sphere in the subcritical, critical and supercritical Reynolds number (\textit{Re}) regimes. The experiment is performed in a wind tunnel at \textit{Re} $=$ $0.3\times 10^{5}$ -- $2.4\times 10^{5}$ and the spin ratio ($\alpha $; ratio of surface velocity to the free-stream velocity) of 0 (no spin) -- 2.6. We directly measure the drag and lift forces and the velocity field in the near wake using PIV and smoke visualization. In the subcritical \textit{Re} regime, the wake of a stationary dimpled sphere shows large-scale wavy structures and the hairpin-shaped vortices are shed changing its azimuthal orientation quasi-randomly in time. As \textit{Re} increases from subcritical to critical regime, the recirculation bubble length decreases significantly and the drag coefficient reduces rapidly to about 0.23. The wavelength of wake also decreases and the shedding orientation of the hairpin-shaped vortices becomes fixed in time. In the supercritical regime, both the recirculation bubble length and drag coefficient remain almost constant, whereas the wavelength of wake decreases further. With rotation, the recirculation bubbles disappear at very small $\alpha $ in the critical and supercritical regimes, resulting in a faster increase in the lift coefficient with $\alpha $ than that in the subcritical regime. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G21.00006: Flight trajectory of a rotating golf ball with grooves Moonheum Baek, Jooha Kim, Haecheon Choi Dimples are known to reduce drag on a sphere by the amount of 50{\%} as compared to a smooth surface. Despite the advantage of reducing drag, dimples deteriorate the putting accuracy owing to their sharp edges. To minimize this putting error but maintain the same flight distance, we have devised a grooved golf ball (called G ball hereafter) for several years. In this study, we modify the shape and pattern of grooves, and investigate the flow characteristics of the G ball by performing wind-tunnel experiments at the Reynolds numbers of $0.5\times 10^{5}-2.5\times 10^{5}$ and the spin ratios (ratio of surface velocity to the free-stream velocity) of 0 -- 0.6 that include the real golf-ball velocity and rotational speed. We measure the drag and lift forces on the rotating G ball and compare them with those of a smooth ball and two well-known dimpled balls. The lift-to-drag ratio of the G ball is much higher than that of a smooth ball and is in between those of the two dimpled balls. The trajectories of flying golf balls are computed. The flight distance of G ball is almost the same as that of one dimpled ball but slightly shorter than that of the other dimpled ball. The fluid-dynamic aspects of these differences will be discussed at the talk. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G21.00007: Prediction of flow separation from aircraft tails using a RSM turbulence model Andrea Masi, Jeremy Benton, Paul G. Tucker Enhancing engineers' capability to predict flow separation would generate important benefits in aircraft design. In this study the attention is focused on the vertical tail plane (VTP), which consists of a fixed part (the fin) and a moveable control surface (the rudder). For standard two-engine aircraft configurations, the size of the VTP is driven by the condition of loss of an engine during takeoff and low speed climb: in this condition the fin and the rudder have to be sufficient in size to balance the aircraft. Due to uncertainties in prediction of VTP effectiveness, aircraft designers keep to a conservative approach, risking specifying a larger size for the VTP than it is probably necessary. Uncertainties come from difficulties in predicting the separation of the flow from the surfaces of the aircraft using current CFD techniques, which are based on the use of RANS equations with eddy viscosity turbulence models. The CFD simulations presented in this study investigate the use of a RSM turbulence model with RANS and URANS. The introduction of a time-dependency gives benefits in the accuracy of the flow solution in presence of massive flow separation. This leads to the investigation of hybrid RANS/LES techniques with the aim of improving the solution of the detached flow. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G21.00008: Numerical Dissipation and Subgrid Scale Modeling for Separated Flows at Moderate Reynolds Numbers Francois Cadieux, Julian Andrzej Domaradzki Flows in rotating machinery, for unmanned and micro aerial vehicles, wind turbines, and propellers consist of different flow regimes. First, a laminar boundary layer is followed by a laminar separation bubble with a shear layer on top of it that experiences transition to turbulence. The separated turbulent flow then reattaches and evolves downstream from a nonequilibrium turbulent boundary layer to an equilibrium one. In previous work, the capability of LES to reduce the resolution requirements down to $1\%$ of DNS resolution for such flows was demonstrated (Cadieux et al, JFE 136-6). However, under-resolved DNS agreed better with the benchmark DNS than simulations with explicit SGS modeling because numerical dissipation and filtering alone acted as a surrogate SGS dissipation. In the present work numerical viscosity is quantified using a new method proposed recently by Schranner et al. and its effects are analyzed and compared to turbulent eddy viscosities of explicit SGS models. The effect of different SGS models on a simulation of the same flow using a non-dissipative code is also explored. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G21.00009: Mechanisms Of Pressure Distributions Within Laminar Separation Bubble At Different Reynolds Numbers DongHwi Lee, Soshi Kawai, Taku Nonomura, Akira Oyama, Kozo Fujii Large-eddy simulation around $5\%$ thickness flat plate at $Re = 5,000, 6,100, 11,000$ and $20,000$ are performed and the physical mechanisms of the pressure distributions ($C_p$) in laminar separation bubbles are analyzed. Depending on the Reynolds number, a gradual pressure recovery and plateau pressure distribution are observed as experiments by Anyoji et al. [AIAA paper 2011-0852]. The causes of the pressure distributions are quantitatively shown by deriving the pressure gradient (momentum budget) equation from the steady momentum equation. From the results, we identify that the viscous diffusion term near the surface has a major contribution to the pressure gradients, and a different growth of the separated shear layer relying on the Reynolds numbers affects the viscous stress near the surface. The gradual pressure recovery at the lower Reynolds numbers is caused by the progressive development of separated shear layer due to the viscous stress which makes a non-negligible viscous stress. On the other hand, a thin laminar separated shear layer is created at the higher Reynolds numbers because of the relatively small viscous diffusion effects, which results in a negligible shear stress distribution. It makes $dp/dx \approx 0$ and the plateau pressure distribution is generated. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G21.00010: Flow-induced instabilities of a flexibly-mounted rigid plate placed in water Pariya Pourazarm, Yahya Modarres-Sadeghi, Matthew Lackner Flow-induced instabilities of a flat rigid plate placed in water with either one degree of freedom (torsional) or two degrees of freedom (transverse and torsional) are studied. The onset of dynamic instability for each configuration was pinpointed and the post-critical behavior of the system in both 1 DoF and 2 DoF cases was investigated. For the 1DoF case, for all flow velocities higher than the critical, a periodic motion was observed. The amplitude of oscillations increased with increasing flow. For the 2DoF system, a period-doubling route to chaos was observed. The post-critical periodic oscillations were followed by period-2 and later on period-4 oscillations, which led to chaotic oscillations at higher flow velocities. Flow visualizations showed that for periodic oscillations, one vortex was shed in each cycle, and for period-2 oscillations, two vortices were shed in each cycle. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700