Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G10: Separated Flows: Wakes / Boundary Layer |
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Chair: Michael Amitay, Rensselaer Polytechnic University Room: 3A |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G10.00001: Effect of sweep on the laminar separated flows over finite-aspect-ratio wings Kai Zhang, Kunihiko Taira Separated flows over finite-aspect-ratio wings at low Reynolds numbers can exhibit rich wake dynamics across a range of aspect ratio, angles of attack, and sweep angles. In this talk, we focus on the effect of sweep on the three-dimensional flow physics behind finite NACA0015 wings. The post-stall wake of an unswept finite wing features a prominent tip vortex at the free end and unsteady wake vortices near the midspan. For a swept wing, a steady wake region forms from the midspan and the unsteady shedding region repositions toward the tip. The sweep-induced spanwise flow counteracts the roll-up of the flow over the wing tip, hindering the formation of the tip vortex. Corresponding to the spanwise variation of the vortical structures, the sectional lift coefficient is largest at the midspan, and decreases towards the wing tip. At large sweep angles, the wake of finite wings exhibits streaks of steady streamwise vortices that are embedded in the spanwise-undulated vortex sheets. In this talk, we discuss findings from a large parametric study and offer comparisons with wake behind unswept wings. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G10.00002: Effect of boundary conditions on 3-D separation over an airfoil Shelby Hayostek, Michael Amitay Boundary conditions greatly affect the wake topology around a finite-span, low aspect ratio airfoil. Two sets of boundary conditions were imposed on the airfoil and their effect was explored in water tunnel experiments. First, the model was cantilevered from a wall where the thickness of the boundary layer, at the junction between the airfoil and the wall, was about half the airfoil’s span. The second model was with two free ends, where the model was held by a thin rod attached to the wall. In both cases, the airfoils had a NACA 0015 profile with an aspect ratio of 2, and were tested at a range of angles of attack and chord Reynolds numbers of 600 and 1,000. In addition, the airfoils were either unswept or 30 degrees swept back. Stereo particle image velocimetry and dye flow visualization were used to capture the flow field. It was found that for the unswept case, the boundary layer greatly affected the flow structures in the wake, with a dual vortex system forming at the tip. However, for the sweep case, the spanwise velocity component pushed the wake towards the tip and the effect of the boundary condition was smaller compared to the unswept cases. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G10.00003: Three-dimensional separation over a finite span NACA 0015 airfoil Jacob Neal, Michael Amitay Three-dimensional separation over a finite span, cantilevered NACA 0015 airfoil is influenced by several parameters. The effects of aspect ratio, Reynolds number, and angle of attack were explored in a series of wind tunnel experiments. Oil flow visualization (OFV) was performed on wings of aspect ratio four, two, and one. For all aspect ratios, it was seen that the Reynolds number did not affect the structures in the oil. For angles of attack above stall, two distinct foci were formed, one near the root and another counter-rotating near the wing tip. In addition, for a couple of cases, the flow field over the airfoil and the structures in its wake were explored using stereo particle image velocimetry at Reynolds number 330,000 for aspect ratio of two. At the higher angle, a clear three-dimensional separation bubble was present at the middle of the wing, and a surface normal vortex was seen to emerge from the focus points seen in the OFV and bend downstream into the flow. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G10.00004: Dynamic stall and its post-stall lift decay Sabrina Henne, Karen Mulleners Dynamic stall occurs when an airfoil undergoes dynamic changes in the angle of attack beyond its static stall angle. It can be observed for example on helicopter or wind turbine blades. Dynamic stall is associated with an increase in maximum lift, but this initial peak is followed by strong load fluctuations. These fluctuations can cause vibrations, structural damage, and failure. To identify the origin of the decay of post-stall lift fluctuations, the flow around a flat plate undergoing a ramp-up motion from an angle of attack of zero to an angle beyond the static stall limit was investigated using time-resolved flow field and load measurements. Immediately following the ramp-up motion, multiple subsequently shed leading edge vortices are observed and their separation coincides with local maxima in lift. The magnitude of these local lift maxima decays in time. A detailed analysis of the size and trajectory of the dominant leading edge vortices using Eulerian and Lagrangian methods helped to characterise the decay of post-stall lift peaks and quantify the importance of variations in the development of the leading edge vortices and changes in the wake topology on the airfoil's post-stall performance. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G10.00005: Time-Resolved PIV Airwake Measurements of a Frigate Ship Model Zheng Zhang, Dhuree Seth, Ebenezer Gnanamanickam, Gordon Leishman To better understand the complex, unsteady airwake structure produced on the rear deck of a ship used for helicopter operations, a ship model with standard Navy SFS2 geometry was tested in a 6 ft (W) by 4 ft (H) by 12 ft (L), closed circuit, low-speed wind tunnel. A time-resolved planar PIV system was used to measure the temporal flow field at the centerline of the deck behind the funnel and hangar structures. The measurements were conducted at Reynolds numbers up to 8 million based on the length of the ship model. The flow field was noted to be dominated by two major structures, namely the wake of the funnel and the shear layer emanating from the superstructure of the ship and top of the hangar. The recirculation behind the hangar structure induced the shear layer reattach to the surface at approximately the middle of deck, resulting in a highly energetic turbulent flow close to the surface. Dynamic model decomposition of the time-resolved flow field revealed existence of multiple dominant frequencies, indicating complex vortex flows in the vicinity of the deck. The ship model was also tested in a simulated atmospheric boundary layer, the results suggesting that the associated velocity gradient and higher turbulence weakens the shear layers produced over and near the deck. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G10.00006: Effects of spanwise inhomogeneity on wake dynamics of a cylinder Chao Jiang, Yefei Yang, Shujin Laima, Hui Li The flow over a twisted cylinder at low Reynolds numbers of Re$=$30\textasciitilde 300 is investigated using direct numerical simulation based on the finite volume method. The elliptic cross-section of the cylinder is rotated along its axial direction, thus resulting in extrinsic inhomogeneity of the geometry. For comparison, the flow past a smooth and a wavy cylinder is also calculated. The twisted cylinder achieves reductions of approximately 90{\%} both in mean drag and lift fluctuation compared with smooth and wavy cylinders. When Re\textgreater 160, the time trace of drag and lift for the twisted cylinder reveals the presence of multi-frequency oscillations, resulting in harmonic behavior of the power spectra, which is different from the other two. The plot of Strouhal number vs. Reynolds number for the twisted cylinder strikingly exhibits four discontinuities, while only two for the smooth cylinder. The first discontinuity corresponding to the start point for transition of the flow, moves towards a lower critical Reynolds number. The extrinsic geometry inhomogeneity always induces the three-dimensional separation and vortical structures, even at very low Reynolds numbers. Effects of the angle of rotation are further discussed. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G10.00007: Boundary Layer Separation from Sports Balls with Seams Andrew Smith, Barton Smith Results of a study on the behavior of boundary layer separation of various sports balls with seams using particle image velocimetry (PIV) is presented. Our primary focus is the boundary layer state at separation, the location of separation, and the angle formed by the separated shear layer. The study uses moving balls in free space rather than a wind tunnel, which may alter critical pressure gradients and affect separation behavior. The bulk of the results presented will be for non-rotating major league baseballs. The seams on baseballs play two distinct roles: 1) the may cause the laminar boundary layer to become turbulent (when located on the front of the ball) and 2) they often form the separation location (when on the back of the ball). The effect of surface roughness (scuffs) on boundary layer separation was also studied. To better understand the unique effects of baseball seams, other balls are examined, including a cricket ball, a smooth ball, a sliotar (hurling ball), an artificially roughened leather baseball, and a printed baseball with exaggerated seams. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G10.00008: Separating and reattaching turbulent boundary layer due to an unsteady adverse pressure gradient Junshin Park, Donghyun You Turbulent boundary layer subject to an unsteady adverse pressure gradient (APG) is studied using direct numerical simulation (DNS). The unsteady APG is imposed with a suction/blowing velocity profile at the top numerical boundary to evoke boundary layer separation and reattachment. In particular, the suction/blowing velocity profile is varied sinusoidally in time at several different reduced frequencies: $k=3.75, \,0.75$ and $0.25$. Results have shown that for reduced frequencies $k=0.75$ and $0.25$, flow features differ greatly from those of separated flow with a steady APG. The separated shear layer lifts up at the first half-period of the APG oscillation. Flow near the reattachment region is entrained back inside the separation bubble. Large vortices are subsequently generated and convect downstream while the separated shear layer leans down to the wall at the latter half-period of the APG oscillation. Differences in time-averaged turbulent kinetic energy are also observed compared with the steady counterpart. To investigate the origin of such differences between flows with steady and unsteady APG, dynamic mode decomposition is also applied. Identified dynamic modes show features related to the aforementioned unsteady behavior. [Preview Abstract] |
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