Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session L28: Vortex Dynamics: Simulation/Turbulence |
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Chair: Jie Yao, Texas Tech University; Samik Bhattacharya, University of Central Florida Room: 236 |
Monday, November 21, 2022 8:00AM - 8:13AM |
L28.00001: Creation and control of an isolated turbulent blob fed by vortex rings Takumi Matsuzawa, Noah P Mitchell, Stephane Perrard, William T Irvine We experimentally study a stationary, isolated blob of turbulence, initiated and sustained by the collisions of multiple vortex rings. Our PIV and 3D PTV measurements reveal that the blob consists of a turbulent core (Taylor Reynolds number: 50-300) surrounded by comparatively quiescent fluid. By examining the mass and enstrophy flux, distribution of energy and enstrophy, and turbulent statistics, we assemble a complete picture of its three-dimensional structure, onset, energy budget, and tunability. Crucially, the injected vortex rings can be endowed with conserved quantities such as helicity. We demonstrate that helicity can be controllably transferred to a turbulent state. This system provides an ideal playground to investigate the generation and decay of turbulence with controlled inputs of energy, enstrophy, and helicity. |
Monday, November 21, 2022 8:13AM - 8:26AM |
L28.00002: Establishing a physical mechanism behind the inverse cascade in two-dimensional turbulence: vortex mergers in the presence of background shear Joshua L. Pughe-Sanford, Roman O Grigoriev The inverse energy cascade, which causes energy to accumulate at large scales, is a unique and characteristic feature of two-dimensional turbulence. Despite a long history of systematic study, the underlying physical mechanism behind the cascade is still not well understood. One ubiquitous physical process that mediates the transfer of energy from small to large scales is vortex mergers. Previous work focused mainly on the interaction of pairs of isolated co-rotating vortices immersed in an irrotational background flow. In this talk, we extend this work to a configuration more relevant to turbulence, where small-scale vortices are immersed in a large-scale background flow with shear. We find that whether the vortices merge or scatter is determined by a pair of nondimensional parameters, analogous to the ratio of small- and large-scale vorticity and the ratio of the separation distance to vortex size. |
Monday, November 21, 2022 8:26AM - 8:39AM |
L28.00003: Investigate the dynamics of vortex ring using holographic vorticimetry Wenkai Zhu, Ben Sorge, Jiaqi Li, Lei Feng, Jiarong Hong Characterizing vortex structures and associated vorticity dynamics is vital to enhance our fundamental understanding of turbulence beyond conventional statistical averaging and stochastic dynamics frameworks. We have recently developed a method for direct vorticity measurement in fluid flows based on digital inline holography[1]. This approach was shown to be able to simultaneously measure the Lagrangian rotation and translation of multiple tracers and obtain the vorticity in regions less than 100 µm. Following up this study, here we use this approach to examine the dynamics of a vortex ring in a tank filled with refractive-index-matched fluid. The vortex ring is generated using a piston arrangement through a circular nozzle and seeded with specially made tracers with internal markers under a variety of conditions (i.e., piston velocity and stroke ratio). Using our approach, we can reconstruct the rotation of tracers trapped in the vortex core and determine the spatial distribution of vorticity and the change of vorticity of vortex ring as it evolves from its generation to its final stage of dissipation. This study paves the way for implementing our approach to characterize the dynamics of small-scale vortex structures in turbulent flows. |
Monday, November 21, 2022 8:39AM - 8:52AM |
L28.00004: A complex variable boundary element method for principal value integrals Adam C DeVoria The trapezoid rule is often used to evaluate the singular integral equation for the induced velocity of vortex sheets. This quadrature, or Gaussian quadrature more generally, results in an induced velocity field that is meromorphic, i.e. analytic everywhere except at a discrete subset of isolated points. More plainly, such quadrature rules are equivalent to a set of point singularities. On the other hand, the velocity field induced by a continuous vortex sheet is sectionally holomorphic, meaning a discontinuity (in tangential velocity) exists at any point comprising the sheet. We take an alternative approach that aims to preserve this topological feature of the sheet. The velocity discontinuity is effected by the branch cuts of complex logarithms that appear in the quadrature expression. Using complex variables allows a proper treatment of the local contribution to the principal value integral. |
Monday, November 21, 2022 8:52AM - 9:05AM |
L28.00005: Interaction of a Spanwise Non-Uniform Gust with a Flexible Wing: A Numerical Study Alex S Ruiz, Samik Bhattacharya In recent years, there has been an increasing interest in the investigation of effects of gusts on aircraft wings using both experiments and computations. Most of these studies used rigid wings and a spanwise uniform gust to characterize the effect of different gust parameters. In the present study, the effect of a spanwise nonuniform gust on a flexible wing is investigated numerically. Three different methods were compared and contrasted with each other and with published literature: (a) where both the gust generator and wing were held fixed; (b) where the gust generator moved while the wing stayed fixed; (c) where the gust generator was fixed and the wing moved. The upper surface of the gust generator is prescribed as a velocity distribution in the form of ramp function, which maps to the surface. The velocity is turned on and off using a rectangle function that smoothly initiates and terminates the prescribed velocity distribution. By comparing the evolution of the unsteady coefficient of lift, the various gust implementation techniques are compared. Differences were found in the way the lift forces developed and in the evolution of the three-dimensional vorticity field as well. A major difference was observed in the formation and the pinching off of the leading edge vortex which formed in all the three cases considered. |
Monday, November 21, 2022 9:05AM - 9:18AM |
L28.00006: Exploring finite time singularity via collision of inviscid vortex rings Jie Yao, Fazle Hussain Motivated by the recent model on the possible formation of finite-time singularity (FTS) by Moffatt & Kimura [J. Fluid Mech. 861, 930 (2019); J. Fluid Mech. 870, R1 (2019)], we conducted a numerical study of two colliding slender vortex rings with the same condition as those used in Yao & Hussain (J. Fluid Mech., vol. 888, 2020a, R2) for three-dimensional incompressible Euler equations. While most metrics – such as the separation s and curvature κ at the two tipping points – exhibit similar trends as predicted by the Moffatt-Kimura model, significant differences are indeed found between them. Specifically, computed κ and the maximum vorticity ∥ω(x, t)∥∞ grow much slower than the model’s prediction. Several phenomena that may affect the formation of a finite-time singularity are observed. First, when the two vortices come close (i.e., the separation distance is comparable to the core size), significant core flattening occurs – deforming the initially circular vortex cross-section into elliptical and then into the typical head-tail structures. In addition, near the end of the simulation, the curvature κ at the tipping points eventually saturates. Finally, the strong core dynamics along the vortex centerline can further prevent core size from decreasing and the maximum vorticity from increasing considerably. Whether a finite-time singularity can develop for this configuration still remains an unresolved issue; if it does at all, the singularity time should be much later than the predicted value (i.e., tc ≈ 0.243). |
Monday, November 21, 2022 9:18AM - 9:31AM |
L28.00007: Turbulent transition of helical vortices destabilized by short-wave instability Yuji Hattori Helical vortices appear in devices which possess rotating wings. It is important to understand the dynamics of helical vortices because they affect the performance of the devices. Helical vortices are subject to several types of instability. Among them the elliptic and curvature instabilities, both of which are short-wave instabilities, are of particular interest since they can lead to turbulent transition of the helical vortices. We study the time evolution of a helical vortex disturbed by an unstable mode of the elliptic instability by direct numerical simulation. The incompressible Navier-Stokes equations in the cylindrical coordinate system are solved numerically by a high-precision method. For low Reynolds numbers, the instability saturates as the vortex core grows by viscous diffusion to lose resonance condition for the elliptic instability; then another mode of the elliptic instability grows and saturates. For large Reynolds numbers, the first instability causes turbulent transition of the vortex core; then the turbulence decays gradually. In both cases, the helical vortex is eventually destroyed by interaction between the adjacent spirals. |
Monday, November 21, 2022 9:31AM - 9:44AM |
L28.00008: Large-Eddy Simulation of Tip-Region Vortex Interactions for a Ducted Marine Propeller Thomas Kroll, Krishnan Mahesh The experiments by Oweis et al. [Journal of Fluids Engineering (2006)] studied the three-bladed, ducted propellers David Taylor Model Basin (DTMB) 5407 and 5206 operating in uniform flow at Re=0.7x10^6 to 9.2x10^6. Analysis of 3-D LDV and planar PIV in the tip region unveiled interactions of the primary tip-leakage vortex and secondary vortices related to the trailing edge vortex as they evolve downstream. Large-eddy simulation (LES) is used to study this propeller contrasting to available experimental data and previous computational work. Details of the flow field and the mechanisms behind the vortex interactions are discussed. The simulations use a novel, unstructured overset grid methodology, developed by Horne and Mahesh [J. Comput. Phys (2019) 376:585-596]. |
Monday, November 21, 2022 9:44AM - 9:57AM |
L28.00009: High resolution vortex layer computations in 2D Euler flows using a characteristic mapping method Julius Bergmann, Thibault OUJIA, Xi-Yuan (Bruce) Yin, Jean-Christophe Nave, Kai Schneider The goal of this study is to get insight into singular solutions of the 2D Euler equations for non-smooth initial data, in particular for vortex sheets. To this end high resolution computations of vortex layers in 2D incompressible Euler flows are performed using the characteristic mapping method. This semi-Lagrangian flow method evolves the flow map using the gradient-augmented level set method (GALSM). The semi-group structure of the flow map allows its decomposition into submaps (each over a finite time interval), and thus the error can be controlled by choosing appropriate remapping times. It yields exponential resolution in linear time and fine scale flow structures are resolved which can be analyzed in detail. Here the roll-up process of vortex layers is studied varying the thickness of the layer showing its impact on the growth of palinstrophy. The self-similar structure of the vortex core is investigated in the vanishing thickness limit. Conclusions on the non-uniqueness of weak solutions of 2D Euler for non-smooth initial data will be drawn and the presence of flow singularities is revealed. |
Monday, November 21, 2022 9:57AM - 10:10AM Author not Attending |
L28.00010: Vortex dynamics in the wake of a finite span wing in stable stratification. Mohd. Suhail Naim, Navrose Navrose A numerical investigation is carried out to study the evolution of wing-tip vortices in a stably stratified environment. Earlier studies have mostly modeled the wake behind a wing using a pair of counter rotating vortices (Spalart 1996, Ortiz et al. 2015). In the present work we simulate the full wake behind a finite span wing upto a downstream distance of 60 chord lengths for various levels of stratification. In the unstratified case, nearwake consists of the vortex rollup and far wake consists of fully developed wing-tip vortices (Navrose et al. 2019). The calculations are carried out at Re (Reynolds number)=1000 and 1≦ Fr (Froude number) ≦ 10. The results show that as strength of stratification increases, the rollup process of wing-tip vortex formation is inhibited. The stratification suppresses the vertical motion and baroclinic vorticity of opposite sense as that of wing-tip vortices is generated that persists along with vortex sheet. At relatively large stratification elongated flat 'V'-shaped structures are observed in the near wake. The far wake in this case comprises of a mixture of pancake like strutures (Pal et al. 2017) and streamwise elongated vortical structures. The vortical structures appears to have a well defined wavelength in the streamwise direction. |
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