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 C12: Vortex Dynamics and Vortex Flow: Wakes |
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Chair: Diana Sher, University of Toronto Room: 303 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C12.00001: Drag forces on a bluff body shedding a 2P wake Emad Masroor, Mark A. Stremler We develop analytical expressions for the drag experienced in a uniform flow by a bluff body shedding a 2P wake. Following von Karman's well-known method for the drag experienced by a fixed cylinder with a typical `von Karman street'-type wake, we set up a potential flow past a general bluff body and model its wake as an infinite periodic collection of point vortices with 4 vortices per period. The relative positions of the 4 vortices are calculated by determining the relative equilibrium configurations of 2P wakes. By considering the momentum flux into and out of a rectangular region enclosing the body and cutting through the wake, we determine the drag forces exerted on the cylinder over one period of vortex shedding. The drag coefficients predicted by this method will be compared to those predicted for objects shedding a standard von Karman street, giving insight into the effect of wake type on the drag. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C12.00002: 2P or not 2P? Jason Dahl, Erdem Aktosun The characterization of modes in the wake of an oscillating cylinder in a free stream has become a common method for describing the spatial variation of vorticity and coherent vortices in these wakes. Arguably, the most commonly observed wake modes come from the classification of Williamson and Roshko (1988) for a cylinder oscillating in the crossflow direction with definitions such as `2S' to describe two single vortices shed per cycle of motion and `2P' to describe two pairs of opposite rotating vortices shed per cycle. When a cylinder is allowed to move in both the in-line and cross-flow direction, however, these simplified classifications of the wake become muddled, as the phasing and pinch-off of vortices can result in intermediate wakes that might fall in between typically observed wake modes, form new complex combinations of vortices, or form strongly three-dimensional wakes with little coherent vortex structure in a 2-D plane. Through a series of forced motion experiments of a circular cylinder undergoing forced in-line and cross-flow motion in a free stream, this complex wake variability is demonstrated, with focus particularly on formation of the `2P' mode, leading to the question: 2P or not 2P? [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C12.00003: Optimization of mangrove-root inspired arrangements for minimizing wake disturbances Aishwarya S. Nair, Amirkhosro Kazemi, Oscar M. Curet, Siddhartha Verma Coastal protection measures such as seawalls are vital for reducing the impact of storms and risk of flooding. The root systems of red mangroves, which are a part of certain coastal ecosystems, provide a natural barrier that dissipates wave energy effectively. The replication of such systems could benefit the design of coastal protection infrastructure substantially. In this work, we present simulations of flow across arrays of circular cylinders, as a simplified model of the root network. The objective is to determine the optimal root arrangements that minimize the disturbances in their wake. We couple these simulations with a genetic optimization algorithm, NSGA - II, to discover the optimal configurations that generate the least enstrophy in their wake. The configurations that produce the highest and lowest enstrophies were studied experimentally using Particle Imaging Velocimetry, and a soap film setup. The resulting flow patterns are analyzed to relate the increase or decrease in wake-disturbance to interactions among vortices shed at the individual cylinder level within the arrangements. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C12.00004: Vortex Shedding Dynamics for Nonparallel Tandem Cylinders Diana Sher, Sean Blaney, Philippe Lavoie Nonparallel tandem cylinder geometries are found in many industrial applications. This type of geometry can lead to significant flow induced vibration and noise emissions. However, the flow dynamics is not well understood since this geometry has not received as much attention as isolated or parallel tandem cylinders. This work considers an upstream circular cylinder perpendicular to the flow and a yawed downstream cylinder. Measurements were conducted in an open jet anechoic wind tunnel (AWT) at UTIAS. Yaw angles up to 45$^\circ$ over a range of Re = 40,000 –120,000 were tested. The vortex shedding behavior along the span of the cylinders was investigated using surface pressure measurements. Peak patterns observed in the spectra indicate the presence of spanwise-dependent flow cells. A single dominant peak in the frequency spectrum exists where the spacing between the cylinders is narrow, two prominent peaks appear at intermediate spacings, and at locations where the spacing is wide, a single peak is present. There are two distinct shedding frequencies at which these peaks occur for each yaw angle, suggesting the existence of two flow cells. The shedding frequency increases for larger yaw angles independently of Re, with the lower frequency peak exhibiting a larger shift. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C12.00005: Wake mode variability for a circular cylinder in a free stream with force sinusoidal in-line and cross-flow motion Erdem Aktosun, Jason Dahl Quantitative flow visualization and force measurements were systematically obtained for a circular cylinder undergoing forced sinusoidal in-line and cross-flow motion in a free stream. Variation of the in-line amplitude, cross-flow amplitude, reduced velocity, and phase between in-line and cross-flow motions were made for a fixed Reynolds number of 7620, with a total of 819 experiments for flow visualization and 9555 for force measurements. Visual categorization of the wake shows a wide variability in pattern of vortices shed in the wake, resulting in a wide variability in the resultant forces acting on the cylinder. Although all motions are symmetric in the cross-flow direction, some parameter combinations are observed to produce an asymmetric wake and strong mean lift forces. For a simple demonstration of the complex variation of the wake, one example baseline motion is shown, where small perturbations from the baseline motion are given to demonstrate wake changes as a function of the motion parameters. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C12.00006: Vortex-Induced Vibrations of a One-Degree-of-Freedom Cylinder Transitioning from the Inline to the Transverse Degree of Freedom Bridget Benner, Yahya Modarres-Sadeghi Flow-induced oscillations of a cylinder with one degree of freedom in the purely inline direction, and at angles that deviate from the inline direction, are studied experimentally. Experiments are conducted in a recirculating water tunnel using a setup with a low mass-damping coefficient. Force and displacement measurements together with flow visualization of the wake are used to characterize the response of the cylinder over a range of reduced velocities as the single degree of freedom incrementally deviates from the inline direction. It is shown that the two non-zero amplitude regions that are observed at reduced velocities of 1.8 and 2.5 in a purely inline vortex-induced vibration (VIV) response of the cylinder slowly disappear as the angle is increased and the lock-in region, usually observed in a pure crossflow VIV response, appears at higher angles. It is also shown how the observed vortex shedding modes change as the degree of freedom deviates from a pure inline direction. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C12.00007: Pulsating Flow Past a Square Cylinder at Low Reynolds Number: Analysis of Vortex Structures Thomas Fowler, IV, Freddie Witherden, Sharath Girimaji Flow past a fixed square cylinder is a canonical problem for investigating vortex-induced vibration and various wake flow physics of interest to several engineering fields. A variant of this problem is that of a pulsating inflow condition. In this work, direct numerical simulations were performed for the case of pulsating flow at Re = 200 over a range of forcing frequencies. As in literature, three regimes are identified: (i) Pre-Lock-in; (ii) Lock-in; and (iii) Post-Lock-in. In Pre-Lock-in, vortex shedding is asymmetric and aperiodic, with the shedding frequency matching that of the uniform case. During Lock-in, vortex shedding remains asymmetric, but becomes distinctly periodic owing to the synchronization of the detachment of the primary and secondary vortices. Here the vortex shedding frequency is determined by the forcing frequency, leading to an increase in the forces experienced by the body. Transitioning to Post-Lock-in, the increasingly strong pulsations lead to symmetric detachment of the primary vortices, disrupting the asymmetric shedding of secondary vortices, and returning to aperiodicity. Spectral analysis then provides further insight regarding the sharp transition into the lock-in regime as opposed to the gradual transition beyond. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C12.00008: Effect of Eliminating Trailing Edge Vortices on Thrust Coefficient in a Plunging Flat Plate Aevelina Rahman, Danesh Tafti Plunging motion, characterized by frequency and amplitude is a key component in the kinematics of many flying and swimming organisms. We studied plunging of a flat plate with a broad range of reduced frequencies 0.25$\le k\le $16 and plunge amplitudes 0.03125$\le h\le $8 giving plunge velocities of 0.25$\le $\textit{kh}$\le $4 at \textit{Re}$=$100. This study observed that, unlike previous investigations for small plunge amplitudes, thrust does not increase monotonically with \textit{kh} but reaches a maximum and then decreases. It is shown that Leading Edge Vortices (\textit{LEV}s) are responsible for thrust production whereas Trailing Edge Vortices (\textit{TEV}s) induce drag on the plate. At higher \textit{kh}, vortex induced velocities dominate the flow with strong nonlinear vortex-vortex interactions (\textit{VVI}). Three main \textit{VVI} mechanisms are identified; in two of them \textit{TEV}s adversely affect thrust production. It is shown that by introducing a splitter plate that eliminates the formation of \textit{TEV}s, the thrust coefficient ($C_{T})$ increases monotonically \quad with \textit{kh. }A parametrization of thrust coefficient is done with frequency ($k)$ and amplitude ($h)$ [$C_{T\thinspace }=$ $A$.$k^{\mathrm{1.4}}h -B$ where $A$ and $B$ are constants, with a $R^{2}=$0.96 for the proposed equation]. Additionally, a scaling analysis is done between $C_{T}$ and circulation to see the effect of eliminating \textit{TEV} on \textit{LEV} dynamics. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C12.00009: Sparse sensor placement for machine learning classification of pitching and plunging plates Jonathan Tu We consider the task of using downstream measurements to characterize the upstream motion of a rigid flat plate, which can be thought of as a simplified model of a swimming fish. Specifically, we numerically simulate pitching and plunging plates at a Reynolds number of 100, then apply Linear Discriminant Analysis (LDA) to distinguish between the two upstream motions. To reduce the dimension of the feature space, we apply LDA to projection coefficients of the flow field obtained using proper orthogonal decomposition (POD). However, at inference time this still requires collecting full flow field data, in order to perform the POD projection. To avoid having to collect full flow fields at all, we also use the Sparse Sensor Placement Optimization for Classification (SSPOC) algorithm to find a small number of point sensors that provide the same information as a POD projection. We implement both the original algorithm and a new extension of SSPOC for vector-valued measurements. Whereas the standard SSPOC algorithm might choose certain sensor locations for the $u$ velocity and others for the $v$ velocity, our vector SSPOC algorithm places sensors for $u$ and $v$ at the same locations. Classification using either SSPOC variant achieves similar accuracy to that using full flow field data. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C12.00010: Dynamics of wing tip vortices in the near and far wake Marie Couliou, Kevin Azim, Jean-Claude Monnier, Vincent Brion Wing tip vortices are a major challenge for civil aviation, both regarding air transport management (efficiency, safety) and environmental impact (induced drag, condensation trails). In this work, an experimental investigation on the development of cooperative instabilities of vortex wakes in the near and far field of a finite wing is carried out. The experiment takes place in a 22 m long hydrodynamic towing tank capable of towing speeds up to 5 $m/s$ and enabling PIV measurements up to 200 spans of wake development. PIV data provides an extensive characterization of the vortex evolution. The effect of a perturbation placed along the span of the wing, following recent theoretical results on the control of vortex wake dynamics, is particularly discussed. [Preview Abstract] |
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