Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M16: Vortex Wakes and Propulsion |
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Chair: Bartosz Protas, McMaster University Room: Georgia World Congress Center B303 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M16.00001: Wake structure of an oscillating cylinder in a flowing soap film at low Reynolds number Wenchao Yang, Emad Masroor, Mark A. Stremler When a circular cylinder oscillates with respect to a uniform background flow, a variety of vortex wake patterns are observed as a function of the oscillation amplitude and frequency. Williamson and collaborators have determined pattern bifurcations in parameter space for 3D water flows with Re>300; Leontini and collaborators have studied the problem using a 2D computational method, which is physically relevant for Re<200. There are several differences between these prior computational and experimental results. We will present results of an ongoing experimental study of the wake structure produced by a circular cylinder undergoing forced oscillations transverse to the background flow in an inclined soap film system with Re<200. Wake patterns are observed directly and clearly via the interference fringes caused by thickness variations in the soap film. Comparisons will be made with the previous studies, and features of the two-dimensional experimental wake behavior will be explored. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M16.00002: Seeking insight from the performance of machine learning classifiers in determining the generating mechanism of vortex patterns Jonathan H. Tu Machine learning algorithms have been applied with impressive success in Silicon Valley, helping solve problems that are complex and often without a governing equation, and doing so using enormous datasets. As such, there has been a rapidly growing interest in whether or not these tools can be applied with similar success in fluid mechanics, where nonlinearities, complex dynamics, and large datasets are common. In this work, we consider the vortex patterns generated by the motion of rigid flat plates at low Reynolds numbers, a common model flow for bio-inspired propulsors. We numerically simulate prescribed pitching and plunging motions, respectively, choosing parameters such that 2S vortex streets are observed. Using machine learning methods such as linear discriminant analysis, we are able to use downstream measurements to accurately identify which upstream motion has generated the observed vortex pattern. In addition, we evaluate the performance of our machine learning classifiers as various hyperparameters are changed; these include the number of sensors, the measured flow variables, and the sensor locations. Observed changes in classifier performance provide insight into the underlying flow physics and suggest best practices for sensor design. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M16.00003: Computational analysis of physical mechanisms at the onset of three-dimensionality in the wake of a square cylinder Gilou Agbaglah, Catherine Mavriplis A high-order spectral element method is used to perform direct numerical simulations of flow past a square cylinder at the transition from the two-dimensional von Kármán vortex street in the wake of the cylinder to three-dimensionality. A Reynolds number range between 100 and 300 has been considered and good agreement with previous numerical and experimental results is obtained. At transition, the spanwise perturbation observed in the cylinder wake occurs before the onset of the streamwise vortex separation, while the former mechanism is commonly shown, in the literature, to originate from the latter by an instability of the cores or interactions of shed vortices in the vicinity of the cylinder. It is shown that the first three-dimensional unstable mode, the mode A, originates from an axial stretching of the upstream perturbed vorticity, existing on the braids, due to the strain field created by the spanwise vortices which evolve under a shear instability of the wake when viscous effects are small. On the other hand, the mode B is observed to arise only after spanwise vortices are shed downstream of the cylinder. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M16.00004: Comparative Analysis of End Conditions on Vortex Shedding from a Circular Cylinder in Sub-Critical Flow Vahid Nasr Esfahani, Ronald E Hanson, Eric Khoury, Alis Ekmekci In this experimental study, we examine the effect of several end conditions on a circular cylinder in subcritical cross flow. Measurements are performed in a water tunnel with removable top section. The end conditions considered in this study include the free-surface, direct wall mount, end plates of various geometries and fetch, as well as an upstream control device. A range of Reynolds numbers is considered from 3000 to 30000. The variation in the mean flow structure is examined in terms of the ensemble averaged streamwise velocity along the cylinder axis. This analysis demonstrates that the vortex confluence point is highly sensitive to end condition, and varies about the span. Further insight into the effect of end conditions is drawn from a time-resolved analysis of the vortex filament, which is captured from PIV data obtained along the axial and streamwise flow direction. Particular focus is given to the analysis of the axial variation in what is denoted as the vortex filament. Identification methods are used to show that this parameter exhibits sensitivity to both end conditions and Reynolds number. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M16.00005: Vortex wakes of a flapping foil in a viscous uniform flow Min Je Kim, Jae Hwa Lee We perform numerical simulations for two types of rigid and flexible thin foils in a viscous uniform flow to explore the effects of the flexibility on wakes. The thin foils are prescribed by the heaving motions and the relevant non-dimensional parameters are the chord length based Strouhal number and heaving amplitude. When the dynamical features of the wakes are varied with respect to the two parameters, it is possible to make a direct comparison between a both foils. We find three transition boundaries in the wake maps of the thin foils: the transition from a von Kármán to a reverse von Kármán wake, transition from reverse von Kármán to asymmetric reverse von Kármán wake, and a further transition from asymmetric reverse von Kármán to complex wake. The wake transition boundaries of the rigid thin foil are predicted by constant amplitude based Strouhal number(StA) lines, consistent with previous studies. However, contrary to the observation from the rigid thin foil, the wake transition boundaries of the flexible thin foil are not predictable by constant StA lines. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M16.00006: Wake characterization of the forced rotational oscillation of a circular cylinder with an attached splitter plate for consistent periodic vortex shedding Matthew Rockwood, Albert Medina An experimental study of a circular cylinder with an attached splitter plate of length equal to two cylinder diameters is presented. The cylinder-splitter plate assembly is forced to oscillate at frequencies near the natural von Karman shedding frequency at various angular amplitudes to remove variations in the vortex shedding frequency normally present in cylinder experiments. The size, spacing, and strength of vortices downstream of the oscillating cylinder assembly are compared with the static cylinder case. |
Tuesday, November 20, 2018 9:18AM - 9:31AM |
M16.00007: A data-driven model for predicting velocity field around a circular cylinder based on pressure on the cylinder Xiaowei Jin, Peng Cheng, Wen-Li Chen, Hui Li The flow field in bluff-body fluid dynamics involves high-dimensional spatial-temporal evolution laws. Though such problems have caused numerous concerns and researches, it is far from getting an analytical solution. However, numerous data from experiments and high-fidelity numerical simulations make it possible to employ deep learning to establish a data-driven quantitative relation model for bluff-body fluid dynamics. Among deep learning algorithms, convolutional neural networks (CNNs) are suitable for extracting abstract features from grid-like data. In this work, a deep CNN architecture consists of paths with and without a pooling layer is employed to establish the model for predicting velocity field around the circular cylinder based on pressure on the cylinder surface. The input of the CNN is the pressure fluctuations on the cylinder surface, which are transformed into a grid-like topology. The output of the CNN is the velocity field around the circular cylinder to be predicted. The CNN is trained by Adam algorithm. The predicted results over various Reynolds numbers indicate that the intrinsic relationship between the wake and the pressure on the cylinder surface is learned by the data-driven model. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M16.00008: Steady outer-flow boundary conditions for incompressible unsteady two-dimensional flows past bluff bodies on an unbounded domain Jiaqi Mai, Paul F. Fischer, Arne J. Pearlstein A new "far-field" computational boundary condition for use in unsteady two-dimensional bluff-body flows is presented. At a given Reynolds number, we impose the steady-flow solution on the upstream and lateral boundaries, providing higher accuracy than conventional far-field boundary conditions imposed on the same domain. Alternatively, for a solution of specified accuracy, this approach allows use of a considerably smaller computational domain. The approach, which reduces the effects of "blockage" for any finite domain, is illustrated for vortex shedding by a circular and square cylinders. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M16.00009: Characterizing differences between surface-mounted square and circular cylinder wakes protruding thin laminar boundary layers. Robert J. Martinuzzi, Maryam Shahroodi, Matthew G. Kindree A comparative experimental study is conducted for the flow around a cantilevered circular and a square cylinder protruding thin laminar boundary layers at a Reynolds number of 10500. Comparison to cases with thin turbulent boundary layers shows differences in the turbulent wake structure and dynamics. Unlike for turbulent boundary layers, spectra of velocity and surface pressure fluctuations show the existence of a low-frequency instability in addition to the periodicity associated with Karman-like shedding. The low-frequency fluctuations on opposing obstacle sides are in-phase and contribute to symmetric POD modes. The instability is only observed on and near the obstacles. The low-frequency spectral signature differ for the two geometries. Interactions between the low-frequency instability and shedding process only occur for the square cylinder. Auxiliary measurements suggest that the instability’s existence depends on the boundary layer state, but is not directly related to the instability of the horseshoe vortex system. In addition to the dynamics, the topologies of the velocity field and surface flow patterns also indicate that the circular and square geometries give rise to fundamentally distinct wakes. |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M16.00010: Evolution of impulse and thrust in the single-pulsed jet Lei Gao, Xin Wang, Cheng-lei Wang It has been previously suggested that the unsteady jet propulsion and the associated vortex formation can increase propulsive efficiency relative to an equivalent steady jet. To reveal the physical connection between the propulsive characteristics and the vortex ring dynamics, the transient changes in impulse and thrust of single-pulsed jets are investigated numerically for cases with different velocity programs. The simulation quantitatively demonstrates that the unsteady thrust generation is highly sensitive to the jet velocity program. By exploiting the over-pressure effect at the jet initiation and mitigating the strength of the stopping vortex at the jet termination, the cases with fast acceleration and slow deceleration (FASD) velocity program have the maximum thrust augmentation over its steady counterparts. The pressure term of the jet impulse is found to be determined by two factors, i.e., the relative magnitude of jet acceleration to deceleration and the axial position of the leading vortex ring during the jet termination stage. Finally, a qualitative explanation on the unsteady propulsive characteristics is proposed according to the effects of formation and evolution of the leading vortex ring and the stopping vortex on the impulse provided by the jet. |
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