Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session M14: Vortex Dynamics: General |
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Chair: Nicholas Kevlahan, McMaster University Room: 202 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M14.00001: The role of Reynolds number in the fluid-elastic instability of tube arrays Nicholas Kevlahan, Ali Ghasemi The onset of fluid-elastic instability in tube arrays is thought to depend primarily on the mean flow velocity, the Scruton number and the natural frequencies of the tubes. However, there is evidence from experiments and numerical simulations that the Reynolds number is also an important parameter, although the available data are not sufficient to understand or quantify this effect. We use a high resolution pseudo-spectral scheme to solve two-dimensional penalized Navier--Stokes equations in order to accurately model turbulent flow through tightly packed tube arrays. To investigate the Reynolds number effect we perform simulations that vary Reynolds number between about 100 and 13,600 independent of flow velocity at fixed Scruton numbers, and then analyze the tube responses. Increasing Reynolds number has a strong de-stabilizing effect for rotated arrays. For in-line arrays, although Reynolds number still affects the instability threshold, the effect is not monotonic with increasing Reynolds number. The main de-stabilizing effect of increasing Reynolds number appears to be broadening of the vortex shedding frequency spectrum. This study increases reduces uncertainties in the experimental data, which usually do not account for the effect of Reynolds number. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M14.00002: Defining and Computing Vortices Objectively from the Vorticity George Haller, Alireza Hadjighasem, Mohammad Farazmand, Florian Huhn We introduce the notion of rotationally coherent Lagrangian vortices as tubular material surfaces in which fluid elements complete equal bulk material rotation relative to the mean rotation of the fluid. We find that initial positions of such tubes coincide with tubular level surfaces of the Lagrangian-Averaged Vorticity Deviation (LAVD), the trajectory integral of the normed difference of the vorticity from its spatial mean. LAVD-based vortices turn out to be objective, i.e., invariant under time-dependent rotations and translations of the reference frame. In the limit of vanishing Rossby numbers in geostrophic flows, cyclonic LAVD vortex centers can be proven to coincide with the observed attractors for light particles. A similar result holds for heavy particles in anticyclonic LAVD vortices. We also discuss a relationship between rotationally coherent Lagrangian vortices and their instantaneous Eulerian counterparts. The latter are formed by tubular surfaces of equal material rotation rate, objectively measured by the Instantaneous Vorticity Deviation (IVD). We show how the LAVD and the IVD detect rotationally coherent Lagrangian and Eulerian vortices objectively in analytic flow models and numerical flow data. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M14.00003: Vortex force generation of an impulsively started wing at high angle of attack Xiang Fu, Fuxin Wang, Hong Liu, Suyang Qin, Yang Xiang A wing at high angle of attack (AoA) impulsively started from rest is a fundamental motion employed by insects during flight. Previous studies have almost solely focused on the lift enhancement by the leading-edge vortex (LEV). However, the influences of the starting vortex and secondary vortex on both the lift and drag generation have been less studied. In this paper, the vorticity fields for three AoAs of 45°, 58.5° and 72° are obtained numerically. The roles of the LEV, starting vortex and secondary vortex in generating the lift and drag are quantitatively studied using the vorticity moment theory. It is revealed that the LEV provides positive lift whereas the starting vortex and secondary vortex provide negative lift during the whole motion. The negative lift produced by the starting vortex or secondary vortex is not trivial and cannot be ignored. Regarding the drag, the LEV reduces the total drag whereas the starting vortex, the secondary vortex increases the total drag. As the AoA increases, the drag resulting from the starting vortex increases quickly and comprises almost all the total drag for the AoA of 72°. The relations between the motion of the vortical structures and the forces are also investigated. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M14.00004: Transient Crossings of a NACA0012 Wing into a Streamwise-Oriented Vortex Daniel Garmann, Miguel Visbal High-fidelity numerical simulations are conducted to examine the unsteady interactions of a finite NACA0012 wing maneuvering into a streamwise-oriented vortex as a representative problem of wake encounters. Three crossing speeds are examined at a Reynolds number of $Re=2.0\times10^{5}$, and a mostly quasi-stationary response is revealed with vortex position as each encounter traverses a similar range of flow regimes, including instances of vortex pairing of the tip and incident vortices with mutual induction/attenuation, tip vortex suppression as the impingement passes inboard of the wingtip, and induced separation that precipitates an abrupt transition at the leading edge. However, an advanced upstream development of a spiraling mode instability is observed in the incident vortex with slower encounters, which is attributed to prolonged exposure of the vortex to the adverse pressure gradient on the underside of the wing that decelerates the core axial flow below known stability bounds of the vortex, and precipitates a more pronounced spiraling mode. A dynamic loading effect is also identified prior to the vortex crossing inboard of the wingtip, whereby the incident structure's relative position at peak loading shifts outboard with higher speeds due to a strengthened tip vortex. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M14.00005: Pressure Evolution in the Shear Layer of Vortex Rings as an Indicator of Pinch-Off Kristy Schlueter, John Dabiri Vortex development and shedding occurs in a variety of biological settings, but the physical mechanisms driving this process are poorly understood. In a seminal 1998 paper, Gharib et al. linked the shedding of vortex rings to the integrated velocity of a piston pushed through a hollow cylinder, a metric referred to as formation number. However, the absence of a piston/cylinder configuration in most biological settings makes formation number a confusing metric by which to develop generalized principles of vortex ring pinch-off. In this study, a recently developed algorithm was used to generate pressure fields from PIV data of vortex ring development and pinch-off. The pressure evolution in the shear layer feeding the vortex ring was examined in detail. The formation time at the occurrence of a local maximum in the pressure in the shear layer was found to be a strong indicator of vortex ring pinch-off. It is hypothesized that a pressure maximum separates fluid that becomes a part of the leading vortex ring from fluid that ends up in the trailing jet. By focusing on the pressure evolution in the shear layer, instead of the formation number, which can be difficult to measure or ambiguous to define for biological flows, generalizations to other vortex shedding flows are possible. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M14.00006: Objective detection of vortices in massively-separated flow Yangzi Huang, Alireza Hadjighasem, Melissa Green, George Haller We study the formation and shedding of vortices in two vortex-dominated flows around a pitching panel in order to detect coherent structures objectively (i.e., in a frame invariant fashion) in massively-separated flow. We employ a recently developed objective definition and extraction technique for rotationally coherent Lagrangian vortices. This methods renders material vortex boundaries as outermost convex level surfaces of the Lagrangian-Averaged Vorticity Deviation (LAVD), i.e., the trajectory integral of the normed deviation of the vorticity from its spatial mean. We also employ the derivative of the LAVD, the Instantaneous Vorticity Deviation (IVD), to uncover instantaneous Eulerian vortex boundaries in an objective fashion. These Eulerian vortex boundaries, therefore, remain the same in all possible rotating and translating unsteady frames. The multiple methods we use identify and track both leading edge and trailing edge vortices as they form and shed. This helps in describing the relationship between the vortex dynamics and the loss of lift during dynamic stall on a 2D flat plate undergoing a 45 degree pitch-up maneuver. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M14.00007: On the growth of enstrophy in axisymmetric 3D Euler flows with swirl Diego Ayala, Charles Doering By numerically solving suitable PDE-constrained optimization problems, we study a family of axisymmetric vector fields, having the structure of a pair of vortex rings with swirl, that maximize the instantaneous production of enstrophy in the context of 3-dimensional (3D) incompressible Euler flows. The axisymmetric fields are parametrized by their energy $\mathcal{K}$, enstrophy $\mathcal{E}$ and helicity $\mathcal{H}$. The imposed symmetry is justified by the results from the seminal work of Doering \& Lu (2008), recently confirmed independently by Ayala \& Protas (2015), where highly localized pairs of colliding vortex rings were found to be instantaneously optimal for enstrophy production in 3D Navier-Stokes flows. The axial symmetry allows for an exhaustive exploration of the parameter space $(\mathcal{K},\mathcal{E},\mathcal{H})$, as the 3D problem is effectively reduced to a 2-dimensional system of partial differential equations for the modified azimuthal vorticity and the azimuthal circulation density, with the corresponding reduction in computational complexity. Possible connections between these optimal axisymmetric fields with swirl and the ``blow-up'' problem are discussed. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M14.00008: Darwinian drift: Effects of Wake Vortices and Multiple Obstacles Sergei Melkoumian, Bartosz Protas When a body passes through an unbounded fluid, it induces a net displacement of fluid particles. The difference between the initial and final positions of a fluid particle is defined as the Darwinian drift and plays an important role in the characterization of the stirring occurring in multiphase flows and in the context of biogenic mixing. Traditional studies of drift have mainly focused on single obstacles moving in a potential flow. In the present investigation we consider the effect of wake vorticity, represented by a pair of F\"oppl point vortices, and the combined effect of multiple obstacles. The drift in various configurations is determined using methods of complex analysis and careful numerical computations. It is demonstrated that, while the total drift increases with the size of the wake for large vortex strengths, it is actually decreased for small circulation values. We also discuss how the interaction of two obstacles affects the drift in comparison to the case of two isolated obstacles. In particular, we identify the lower and upper bound on the drift due to two identical cylinders. In certain cases our results are supported by asymptotic analysis. A physical explanation of the observed affects is offered in terms of the trajectories of individual particles. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M14.00009: Momentum transport in the wake of a finite-length thin flat plate Arman Hemmati, David H. Wood, Robert J. Martinuzzi A comparison of the wakes of thin flat plates with aspect ratios (AR) 1.0, 1.6, 2.0 and 3.2, normal to a uniform stream, are conducted based on Direct Numerical Simulations (DNS) at Re=1200. Typical anti-symmetric Karman shedding of high AR plates, AR>2.0, is initiated by detachments at the plate corners. Shear layer detachment on the longer edges triggers shedding from the shorter edges. Thus, there is only a single shedding frequency detected in the wake. At lower AR, however, an interaction between adjacent shear layers occurs prior to detachment, which elongates the base vortex, i.e. from 1.56H for AR=3.2 to 2.69H for AR=1.6. This change of shedding mechanism has significant impact on wake structures and instantaneous pressure loads. The dominant shear layers on the longer sides appear to maintain the Karman shedding at higher AR. Karman shedding is intermittently interrupted for lower AR plates due to shear layer interactions, which increases the turbulence kinetic energy, production and dissipation rates and Reynolds stresses. To better understand dependence of the wake topology on AR, mean and fluctuating flow variables are evaluated at various locations along the chord. Moreover, comparisons to wakes of finite-height cylinders and circular plates are considered. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M14.00010: Rolling up of Large-scale Laminar Vortex Ring from Synthetic Jet Impinging onto a Wall Yang Xu, Chong Pan, Jinjun Wang Vortex ring impinging onto a wall exhibits a wide range of interesting behaviors. The present work devotes to an experimental investigation of a series of small-scale vortex rings impinging onto a wall. These laminar vortex rings were generated by a piston-cylinder driven synthetic jet in a water tank. Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) were used for flow visualization/quantification. A special scenario of vortical dynamic was found for the first time: a large-scale laminar vortex ring is formed above the wall, on the outboard side of the jet. This large-scale structure is stable in topology pattern, and continuously grows in strength and size along time, thus dominating dynamics of near wall flow. To quantify its spatial/temporal characteristics, Finite-Time Lyapunov Exponent (FTLE) fields were calculated from PIV velocity fields. It is shown that the flow pattern revealed by FTLE fields is similar to the visualization. The size of this large-scale vortex ring can be up to one-order larger than the jet vortices, and its rolling-up speed and entrainment strength was correlated to constant vorticity flux issued from the jet. [Preview Abstract] |
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