Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H18: Flow Instability: Vortex and Wakes |
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Chair: Adam Edstrand, Florida State University Room: D135 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H18.00001: DNS of two-phase flow in an inclined pipe Fangfang Xie, Xiaoning Zheng, Michael Triantafyllou, Yiannis Constantinides, George Karniadakis We study the de-stabilization mechanisms of two-phase flow in an inclined pipe subject to gravity with a phase-field approach. At the inlet, a stratified flow is imposed with a parabolic velocity profile. We found that due to gravity, the stratified flow will become unstable, causing a complex transitional flow inside the pipe. Firstly, a 2D channel geometry is considered. When the heavy fluid is injected in the top layer, inverted vortex shedding emerges, interacting periodically with the bottom wall as it develops further downstream. The accumulation of heavy fluid in the bottom wall causes a backflow, which interacts with the previous jet. On the other hand, when the heavy fluid is placed in the bottom layer, a big slug is formed and subsequently breaks into small pieces, some of which will be shed along the pipe. To describe the generation of vorticity from the two-phase interface and pipe walls, we analyze the circulation dynamics and connect it to the two-phase flow pattern. Moreover, we analyze the two-phase flow induced forces along the pipe, which is capable of producing unwanted and destructive vibrations. Finally, we conduct 3D simulations in the circular pipe and compared the differences of flow dynamics against the 2D simulation results. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H18.00002: A parabolized stability analysis of a trailing vortex wake Adam Edstrand, Peter Schmid, Kunihiko Taira, Louis Cattafesta To aid in understanding how best to control a trailing vortex, we perform a parabolized stability analysis on a flow past a wing at a chord-based Reynolds number of 1000. At the upstream position, the wake instability branch dominates, with only a single vortex instability present in the spectrum. With downstream progression, the growth rate of the wake instability decays, but remains unstable 10 chords downstream. With the wake mode being unstable so far downstream, these results imply that the excitation of the wake instability, despite the varying base flow, will continue to see growth and potentially disrupt the trailing vortex. Conversely, the vortex instability in its formative region rapidly decays to the stable half-plane, then at 11 chords downstream becomes unstable again. We hypothesized the renewed instability growth far downstream is developing as a result of vortex instabilities, however the excitation of these instabilities proves to be challenging in the vortex far field. From these results, control near the two-dimensional wake behind the airfoil may better interfere with the trailing vortex formation; however, to determine the optimal disturbances, an adjoint analysis is required and is included in the future work of the project. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H18.00003: Flow Control Behind Bluff Bodies through the Interaction of an Attached Resonant Flexible Tail Samuel Shelley, John Smith, Alastair Hibbins, Roy Sambles, Simon Horsley Steady uniform flow, incident upon a bluff body can separate downstream causing a wake to form, this leads to the periodic shedding of vortices behind the body. By adding a thin flexible tail to the rear of the body one may reduce the drag as well as change the vortex shedding frequency (VSF). In this work we model the flow past a cylinder, in the Laminar flow regime, with an attached tail, varying the length and stiffness of the tail to couple the resonant frequencies of the tail to the natural VSF of the structure. We use this to explore how the drag and VSF of the system change as we couple to different vibrational modes of the tail. On increasing tail length, or decreasing tail stiffness progressively on passing where the natural VSF of the cylinder and tail resonances couple we see sharp increases in both the drag and VSF, which both gradually decrease again. The effect of changing the shape of the end of the tail is also investigated by exploring tails with square, rounded and triangular trailing edges. Experiments are being conducted in water at a higher Reynolds number using a tail made out of Neoprene to confirm these modelling results. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H18.00004: Sensitivity analysis of small circular cylinders as wake control Julio Meneghini, Gustavo Patino, Rafael Gioria We apply a sensitivity analysis to a steady external force regarding control vortex shedding from a circular cylinder using active and passive small control cylinders. We evaluate the changes on the flow produced by the device on the flow near the primary instability, transition to wake. We numerically predict by means of sensitivity analysis the effective regions to place the control devices. The quantitative effect of the hydrodynamic forces produced by the control devices is also obtained by a sensitivity analysis supporting the prediction of minimum rotation rate. These results are extrapolated for higher Reynolds. Also, the analysis provided the positions of combined passive control cylinders that suppress the wake. The latter shows that these particular positions for the devices are adequate to suppress the wake unsteadiness. In both cases the results agree very well with experimental cases of control devices previously published. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H18.00005: The correlation between 2D-3D wake transition and propulsive efficiency of a flapping foil Liping Sun, Jian Deng, Xueming Shao We study numerically the propulsive wakes produced by a flapping foil. As a major contribution of this report, we find an interesting coincidence that the efficiency maximum agrees well with the 2D-3D transition boundary. Although lack of direct 3D simulations, it is reasonable to conjecture that the propulsive efficiency increases with Strouhal number until the wake transits from a 2D state to a 3D state. By comparing between the pure pitching motion and the pure heaving motion, we find that the 2D-3D transition occurs earlier for the pure heaving foil than that of the pure pitching foil. Consequently, the efficiency for the pure heaving foil peaks more closely to the wake deflection boundary than that of the pure pitching foil. Furthermore, since we have drawn the maps on the same parametric space with the same Reynolds number, it is possible to make a direct comparison in the propulsive efficiency between a pure pitching foil and a pure heaving foil. We note that the maximum efficiency for a pure pitching foil is $15.6\%$, and that of a pure heaving foil is $17\%$, indicating that the pure heaving foil has a slightly better propulsive performance than that of the pure pitching foil for the currently studied Reynolds number of $Re=1700$. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H18.00006: Mechanism of Secondary Instability of Flow around a Circular Cylinder Hua-Shu Dou, An-Qing Ben Flow around a circular cylinder in infinite domain is simulated with large eddy simulation at Re$=$200, and the mechanism of the origin of secondary vortex street is analyzed. The simulation results show that the vortex street generated in the cylinder near wake disappears as the flow moving downstream. Secondary instability occurs in far wake of the cylinder after the primary vortex street dying away. The processes of first instability and secondary instability in the cylinder wake are recorded in the simulation. The instability of the entire flow field is studied with the energy gradient theory. It is found that it is the high value of the energy gradient function generated by the zero velocity gradients that leads to the instability. As the vortex developing at rear of the cylinder, the value of the energy gradient function becomes low downstream, which leads to the vortex dying away. At further downstream, the value of the energy gradient function is enlarged again because of the role of perturbation, which leads to the secondary instability. It can be concluded that the interaction of the variation of the value of the energy gradient function and the perturbation leads to the occurrence of secondary instability. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H18.00007: Unstable shear flows in two dimensional strongly correlated liquids -- a hydrodynamic and molecular dynamics study Akanksha Gupta, Rajaraman Ganesh, Ashwin Joy In Navier-Stokes fluids, shear flows are known to become unstable leading to instability and eventually to turbulence. A class of flow namely, Kolmogorov Flows ( K-Flows) exhibit such transition at low Reynolds number. Using fluid and molecular dynamics, we address the physics of transition from laminar to turbulent regime in strongly correlated-liquids such as in multi-species plasmas and also in naturally occurring plasmas with K-Flows as initial condition. A 2D phenomenological generalized hydrodynamic model is invoked wherein the effect of strong correlations is incorporated via a viscoelastic memory. To study the stability of K-Flows or in general any shear flow, a generalized eigenvalue solver has been developed along with a spectral solver for the full nonlinear set of fluid equations. A study of the linear and nonlinear features of K-Flow in incompressible and compressible limit exhibits cyclicity and nonlinear pattern formation in vorticity. A first principles based molecular dynamics simulation of particles interacting via Yukawa potential is performed with features such as configurational and kinetic thermostats for K-Flows. This work reveals several interesting similarities and differences between hydrodynamics and molecular dynamics studies. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H18.00008: Spiral vortex formation in cross-slot flow Simon Haward, Noa Burshtein, Robert Poole, Paulo Oliveira, Manuel Alves, Amy Shen Fluid flow through bisecting channels (cross-slots) results in the formation of a steady spiral vortex as the Reynolds number (Re) is increased above a modest critical value (Re$_{c})$. The value of Re$_{c}$ is strongly dependent on the channel aspect ratio, $\alpha =d/w$, where $d$ and $w$ are the depth and width of the channel, respectively. Quasistatic experiments and numerical simulations over a range of Re show that for low $\alpha $ this symmetry-breaking bifurcation is supercritical, however subcritical behavior develops as $\alpha $ is increased. The system can be described by a Landau-type 6$^{\mathrm{th}}$-order polynomial potential and we identify a value of $\alpha \approx 0.55$ for which a tricritical point can be found. Dynamic experiments and simulations conducted across the transition indicate a plausible mechanism for the onset of the instability. Our analysis suggests that the transition results from the growth of center-point vorticity induced by random imbalances between two pairs of Dean vortices that form in the channel cross-section. Vorticity growth is governed by two distinct time scales. At short times, viscous diffusion dominates and vorticity grows slowly. Once the vorticity is sufficiently high, vortex stretching dominates and the vorticity grows rapidly until steady state is reached. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H18.00009: Instability of a vortex sheet leaving a right-angled wedge Stefan Llewellyn Smith, Anthony Davis We examine the dynamics of a semi-infinite vortex sheet attached not to a semi-infinite plate but instead to a rigid right-angled wedge, with the sheet aligned along one of its edges. The resulting linearised unsteady potential flow is forced by an oscillatory dipole in the uniform stream passing along the top of the wedge, while there is stagnant fluid in the remaining quadrant. The essentially quadrant-type geometry replaces the usual Wiener--Hopf technique by the Mellin transform. The core difficulty is that a first-order difference equation of period $4$ requires a solution of period unity. As a result the complex fourth roots $(\pm 1\pm i)$ of $-4$ appear in the complementary function. The Helmholtz instability wave is excited and requires careful handling to obtain explicit results for the amplitude of the instability wave. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H18.00010: Unstable flow of worm-like micelles in rectangular microfluidic channels Paul Salipante, Steven Hudson We investigate a jetting instability of shear banding worm-like micelle (WLM) solutions in microfluidic channels with rectangular cross-sections. The flow is tracked using both 3-D and 2-D particle tracking methods in channels of different aspect ratio, size, and wall materials. We observe that the instability forms in high aspect ratio channels within an intermediate range of volumetric flows. The location of the high velocity jet in the channel appears to be sensitive to stress localizations induced by channel defects and wall roughness. A lower concentration WLM solution, with a monotonic stress curve, does not show the banding instability but displays non-negligible velocity gradients across the channel width. The transient development of the instability at the entrance of the microfluidic channel is observed in various geometries. The experimental measurements are compared to finite volume simulations using the Johnson-Segalman viscoelastic model. The simulations show a qualitatively similar behavior to our experimental observations and indicate that normal stresses in the cross stream directions lead to the development of the jetting flow. [Preview Abstract] |
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