Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F11: Vortex Dynamics & Vortex Flows: Structure InteractionVortexes
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Chair: Gr/'egoire Winckelmans, Universit/'e Catholique de Louvain (UCL) Room: 504 |
Monday, November 20, 2017 8:00AM - 8:13AM |
F11.00001: LES of an Advancing Helicopter Rotor, and Near to Far Wake Assessment Denis-Gabriel Caprace, Matthieu Duponcheel, Philippe Chatelain, Grégoire Winckelmans Helicopter wake physics involve complex, unsteady vortical flows which have been only scarcely addressed in past studies. The present work focuses on LES of the wake flow behind an advancing rotor, to support the investigation of rotorcraft wake physics and decay mechanisms. A hybrid Vortex Particle-Mesh (VPM) method is employed to simulate the wake of an articulated four-bladed rotor in trimmed conditions, at an advance ratio of 0.41. The simulation domain extends to 30 rotor diameters downstream. The coarse scale aerodynamics of the blades are accounted for through enhanced immersed lifting lines. The vorticity generation mechanisms, the roll-up of the near wake and the resulting established far wake are described (i) qualitatively in terms of vortex dynamics using rotor polar plots and 3D visualizations; (ii) quantitatively using classical integral diagnostics. The power spectra measured by velocity probes in the wake are also presented. The analysis shows that the wake reaches a fully turbulent equilibrium state at a distance of about 30 diameters downstream. [Preview Abstract] |
Monday, November 20, 2017 8:13AM - 8:26AM |
F11.00002: Experimental framework to study tip vortex interactions in multirotor wakes Rongnan Yao, Daniel Araya We present an experimental study to compare the dynamic characteristics of tip vortices shed from a propeller in a crossflow to similar characteristics of an isolated vortex column generated in a closed system. Our aim is to evaluate the feasibility of using this simple isolated system to study the more complicated three-dimensional vortex interactions inherent to multirotor wakes, where the local unsteadiness generated by one rotor can strongly impact the performance of nearby rotors. Time-resolved particle image velocimetry is used to measure the velocity field of the propeller wake flow in a wind tunnel and the vortex column in a water tank. Specific attention is placed on analyzing the observed vortex core precession in the isolated system and comparing this to characteristic tip-vortex wandering phenomenon. [Preview Abstract] |
Monday, November 20, 2017 8:26AM - 8:39AM |
F11.00003: Dynamics of helical vortices behind a wind turbine in a stratified atmosphere Xuerui Mao, Fazle Hussain The wind turbine wake features helical vortices, which are shed from the tips of blades and inflict undesirable fatigue loading on downstream turbines. Prior studies of helical vortices focused on their hydrodynamic instabilities and the following breakup in the neutrally stable, isothermal atmospheres in which the buoyancy force is balanced by gravity. However, the atmosphere is typically mostly unstable during the day and mostly stable at night, but is seldom neutral. The present numerical work addresses the development of helical vortices in a thermally stratified atmosphere and also concentrates on the stable condition which is typical for offshore applications. The Boussinesq approximation is invoked to account for the thermal stratification effect, and an actuator line model is adopted for the turbine blades. In our direct numerical simulations, the helical vortices are found to be increasingly elliptic downstream and subsequently interact with the hub vortex to produce a new mode of breakup into turbulence. Such elliptic structures increase the width of the wake and subsequently the interaction between aligned turbines in large-scale offshore wind farms. [Preview Abstract] |
Monday, November 20, 2017 8:39AM - 8:52AM |
F11.00004: Upstream Influence of Axisymmetric Bodies on Trailing Vortices in Formation Flight Tanya Johnson, Justin Jaworski In formation flight, a leader wing generates a trailing vortex that travels downstream and interacts with a secondary, follower wing. The pressure field of the follower wing can affect the trajectory and character of the trailing vortex prior to impingement, and this upstream influence is investigated by analytical and numerical means. The linearized boundary-layer form of the Navier-Stokes equations models the finite-distance evolution of a Batchelor vortex in an imposed pressure gradient that is axisymmetric about the vortex axis. The analysis results in a heat conduction problem that can be solved using Green's functions and the effects of the vortex boundary condition on the finite domain are included. Results are presented for constant and linear pressure gradients, as well as pressure fields representative of canonical axisymmetric follower bodies. This work may be extended and applied to the stability analysis of streamwise finite-core vortices arising in formation flight. [Preview Abstract] |
Monday, November 20, 2017 8:52AM - 9:05AM |
F11.00005: Scattering and trapping of vortex pairs by a flat plate Monika Nitsche The interaction of a counter-rotating vortex pair with a flat plate in its path is studied numerically. The vortices are initially separated by a distance $D$ and placed far upstream of a plate of length $L$. The plate is stationary, inclined relative to the incoming vortex trajectory, at an incident angle $\beta_i$. Generally, the vortices surround the plate and then leave as a dipole with unchanged velocity, but with a large change in the transmitted travel direction. This transmitted angle depends sensitively on changes in the incident angle, with increasing sensitivity as $D/L$ decreases. In fact, for sufficiently small $D/L$, the dependence on $\beta_i$ is highly singular. We show that there are intervals of incident angles in which the vortex trajectory undergoes repeated topological discontinuities, characterized by jumps in the vortex winding number and in the time they take to leave the plate. The discontinuities occur in a fractal self-similar fashion within the whole interval. These intervals furthermore contain incident angles that trap the vortices, which never leave the plate. The number of such trapping intervals increases as the parameter $D/L$ decreases, and the dependence of the motion on $\beta_i$ becomes increasing complex. [Preview Abstract] |
Monday, November 20, 2017 9:05AM - 9:18AM |
F11.00006: Viscous flow past wedge Ling Xu Direct numerical simulations are applied to study viscous flow past an infinite wedge with the angle ranging from 60 to 150 degrees. The background flow is potential and increases at a fixed acceleration rate $p\in[0,1]$. The work is focused on the dynamics of the starting vortex near the wedge tip. Results show the time evolution of the vorticity, streamlines, streaklines at varying wedge angles and acceleration rates. Since the wedge length is infinite, At early times, it is shown that solutions of different viscosity are identical upon a scale; the trajectory of the vortex core follow the inviscid similarity theory. At later times, hierarchical recirculating regions corresponding to alternating signs of vorticity appear at wedge tip. We also compare the results with experiments, and consistency and discrepancy are discussed. [Preview Abstract] |
Monday, November 20, 2017 9:18AM - 9:31AM |
F11.00007: Numerical investigation of a vortex ring impinging on a coaxial aperture JiaCheng Hu, Sean D Peterson Recent advancements in smart materials have sparked an interest in the development of small scale fluidic energy harvesters for powering distributed applications in aquatic environments, where coherent vortex structures are prevalent. Thus, it is crucial to investigate the interaction of viscous vortices in the proximity of a thin plate (a common harvester configuration). Hence, the present study systematically examines the interaction of a vortex ring impinging on an infinitesimally thin wall with a coaxially aligned annular aperture. The rigid aperture serves as an axisymmetric counterpart of the thin plate, and the vortex ring represents a typical coherent vortex structure. The results indicate that the vortex dynamics can be categorized into two regimes based on the aperture to ring radius ratio ($\mathrm{Rr}$). The rebound regime ($\mathrm{Rr} < 0.9$) exhibits the classical unsteady boundary layer interaction in a vortex ring-wall collision. The vortex ring is able to slip past the aperture when $\mathrm{Rr} \geq 0.9$, and an increase in the vortex ring impulse is observed for $1.0 \leq\mathrm{Rr} \leq 1.3$ due to fluid entrainment. Furthermore, pressure loadings are also compared to elucidate an optimal energy harvesting strategy in vortex impact configurations. [Preview Abstract] |
Monday, November 20, 2017 9:31AM - 9:44AM |
F11.00008: Interactions of Vortex Pairs with Flat and Wavy Walls Sarah Morris, C.H.K. Williamson In this research we examine the interaction of a vortex pair with flat and wavy walls. When a vortex pair approaches a ground plane, the boundary layer that forms on the surface between the vortices and the wall separates, generating secondary vorticity and causing the primary pair to `rebound' from the wall. When a vortex pair with the long-wave ``Crow'' (1970) instability interacts with a flat boundary, rebounding vortical structures are produced, the form of which is distinctly different to that seen in long-wave instability out of ground effect (Asselin {\&} Williamson, 2017). Similarly, when the ``complementary'' experiment of a straight vortex pair encounters a wavy wall, the topology of the pair changes significantly. By using PIV and LIF to study these structures, certain characteristic features can be seen. The 2D vortex pair first interacts with the ``hills'' of the boundary, resulting in earlier localized secondary vorticity generation. This leads to the formation of secondary vortex loops in the ``valleys'', wherein these loops pinch off into rising vortex rings, two per fundamental wavelength. We also observe an increased rate of circulation decay compared to a 2D pair impinging on a flat wall. Research has also been undertaken on delta wings in ground effect. Some features of the spatially developing vortex pair in the wake of a delta wing are seen to relate to our previous results with a temporally developing vortex pair impinging on a wall. [Preview Abstract] |
Monday, November 20, 2017 9:44AM - 9:57AM |
F11.00009: Vortex forcing model for turbulent flow over spanwise-heterogeneous topogrpahies: scaling arguments and similarity solution William Anderson, Jianzhi Yang Spanwise surface heterogeneity beneath high-Reynolds number, fully-rough wall turbulence is known to induce mean secondary flows in the form of counter-rotating streamwise vortices. The secondary flows are a manifestation of Prandtl's secondary flow of the second kind -- driven and sustained by spatial heterogeneity of components of the turbulent (Reynolds averaged) stress tensor. The spacing between adjacent surface heterogeneities serves as a control on the spatial extent of the counter-rotating cells, while their intensity is controlled by the spanwise gradient in imposed drag (where larger gradients associated with more dramatic transitions in roughness induce stronger cells). In this work, we have performed an order of magnitude analysis of the mean (Reynolds averaged) streamwise vorticity transport equation, revealing the scaling dependence of circulation upon spanwise spacing. The scaling arguments are supported by simulation data. Then, we demonstrate that mean streamwise velocity can be predicted a priori via a similarity solution to the mean streamwise vorticity transport equation. A vortex forcing term was used to represent the affects of spanwise topographic heterogeneity within the flow. Efficacy of the vortex forcing term was established with large-eddy simulation cases, wherein vortex forcing model parameters were altered to capture different values of spanwise spacing. [Preview Abstract] |
Monday, November 20, 2017 9:57AM - 10:10AM |
F11.00010: A model of the trajectory of an inclined jet in incompressible crossflow Robert Breidenthal, Yanyan Feng, Yanping Song A simple model of the flow field induced by an inclined jet into an incompressible cross-flow is proposed. In general, such a jet generates an asymmetric pair of vortices, with a larger one farther from the wall. The model accounts for the influence of the jet pitch angle (with respect to the wall), skew angle (with respect to the freestream direction), and velocity ratio (the jet velocity to the freestream velocity) on the jet trajectory in the transverse plane in the far field. From the model, the increase of the jet penetration (the jet edge distance from the nozzle exit plane) component $s$ in the transverse plane to the downstream distance $x$ obeys to the same one-third law in the case of normal transverse jet. The circulation of the large vortex in the transverse plane decreases with a power of one-third of $x$. The effect of velocity ratio on the jet trajectory is also similar with that in the case of normal transverse jet. Penetration and circulation in the transverse plane both increase as the jet pitch angle increases, and they reach a maximum at a skew angle of 90 degrees. Data from the literature are normalized and compared to the model. While there is considerable scatter, the normalized data are generally in accord with the predictions of the model. However, for low velocity ratios $\le $ 1.0 when skew angle is near 90 degrees, the effects of jet entrainment as well as horseshoe and wake vortices may create a low-pressure region on the wall, and hence alter the jet trajectory and vortex circulation. [Preview Abstract] |
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