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 H40: Turbulence: Wakes |
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Chair: Ronald Adrian, Arizona State University Room: 6b |
Monday, November 25, 2019 8:00AM - 8:13AM |
H40.00001: The Age of a Wake David Lewis, Timour Radko This study attempts to quantify decay rates of stratified wakes in active oceanic environments, characterized by the presence of intermittent turbulence and double-diffusive convection. The investigation is based on a series of direct numerical simulations of wakes produced by a sphere uniformly propagating in stratified two-component fluids. We examine and compare the evolution of wakes in fluid systems that are initially quiescent, double-diffusively unstable, or contain preexisting turbulence. Model diagnostics are focused primarily on dissipation of turbulent kinetic energy ($\varepsilon )$ and thermal variance ($\chi )$. Analysis of decay patterns of $\varepsilon $ and $\chi $ indicates that microstructure generated by an object of D $=$ 0.6 m in diameter moving at the speed of U $=$ 0.02 m/s could be detected, using modern high resolution profiling instruments, for 0.5--0.7 h. Convective overturns are shown to be particularly effective in terms of dispersion of microscale wake signatures. Extrapolation of model results to objects of $\sim $10 m in diameter propagating with speeds of $\sim $10 m/s suggests that this form of detection is feasible for at least 4 h after the object's passage through the monitored areas. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H40.00002: Characterization of the Far-Wake of an Inclined 6:1 Prolate Spheroid Jonathan Peck, Ethan Lust The study of flow around prolate spheroids has informed our understanding of fundamental hydrodynamics. For example, they have been used to study boundary layer development, turbulent flow transition, and vortex shedding. Because of the simplicity of the model and the rich hydrodynamic result, prolate spheroid data are often used to validate computational fluid dynamics codes. There are many examples from the literature featuring spheroids in various configurations: aspect ratio (L:D), Reynolds number, and inclination angle. However, there is little consideration of the far-wake, at downstream diameters greater than 6D, particularly at Reynolds numbers in excess of 4 X 10$^{\mathrm{6}}$. The focus of the present study was on this region. The experiment was conducted in the large towing tank at the U.S. Naval Academy. The 6:1 prolate spheroid, measuring 54 in. in length and 9 in. in diameter, was sting-mounted at a 20$^{\mathrm{o}}$ angle of inclination. An underwater, stereo particle image velocimetry system was attached to a structure fixed at a point approximately half the length of the towing tank. The spheroid was towed through the field of view in a number of configurations. The field of view was oriented laterally, capturing the strength, size, and evolution of the trailing vortex. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H40.00003: Spreading and decay of axisymmetric drag wakes: theory Scott Wunsch, Theodore D. Drivas, D. Curtis Saunders The spreading and decay of drag wakes is of interest to a wide variety of applications in geophysics and engineering, such as the wakes of mountains, seamounts, windfarms, and buildings. For axisymmetric bodies, the classical self-similar scaling law formulated by Swain (1929) has long been widely accepted (Tennekes and Lumley 72) and used to describe the wakes of spheres (Bevilaqua and Lykoudis 78) and slender bodies (Pao {\&} Lin 73). However, recent experiments have cast doubt on the classical decay law (Bonnier and Eiff 02; Nedic. Vassilicos, and Ganapathisubramani 13), indicating more rapid wake spreading and decay than the classical result. Here, new laboratory data from a dimpled sphere (Re $=$ 50,000) are compared with various theoretical explanations for non-classical self-similar wake decay. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H40.00004: Spreading and decay of axisymmetric drag wakes: experiment D. Curtis Saunders, Scott Wunsch The spreading and decay of drag wakes is of interest to a wide variety of applications in geophysics and engineering, such as the wakes of mountains, seamounts, windfarms, and buildings. For axisymmetric bodies, the classical self-similar scaling law formulated by Swain (1929) has long been widely accepted (Tennekes and Lumley 72) and used to describe the wakes of spheres (Bevilaqua and Lykoudis 78) and slender bodies (Pao and Lin 73). However, results from recent experiments have cast doubt on the classical decay law (Bonnier and Eiff 02; Nedic. Vassilicos, and Ganapathisubramani 13), indicating more rapid wake spreading and decay than the classical result. Here, new laboratory data from a dimpled sphere are presented. The Reynolds number (50,000) exceeds previous experiments, and the dimples alleviate issues with Strouhal vortex shedding. Particle Imaging Velocimetry (PIV) data in both along-track and cross-track configurations are used to illustrate various aspects of the wake decay. The observed wake decays at a greater rate than suggested by the classical result, and extends the self-similar decay to further downstream distances than previously reported. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H40.00005: Investigation of Large-Scale Coherent Structures in Flow past a Sphere for Scale-Resolving Simulations (SRS) Chetna Kamble, Freddie Witherden, Sharath Girimaji Success of Scale-Resolving Simulations (SRS) depends on accurately reproducing the large-scale structures in complex turbulent flows. Yet, no clear procedure exists for a quantitative assessment of the fidelity of these flow structures. The objective of this work is to develop such an assessment framework in the context of wake flow past a sphere which is chosen due to its inherent complexity and three-dimensional features. Numerical computations at different degrees of physical resolution are performed using the Partially-averaged Navier-Stokes (PANS) bridging-SRS method. Proper Orthogonal Decomposition (POD) is employed for a quantitative analysis of the computed large-scale structures in the wake. Most energetic structures resolved at different physical resolution are extracted and compared. Furthermore, the dominant structures are also qualitatively examined using the iso-surfaces of Q-criterion, i.e., second invariant of velocity gradient tensor. These results are compared to Direct Numerical Simulations (DNS) studies to establish the efficacy of the PANS-SRS simulations in capturing the large-scale vortical structures. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H40.00006: Direct measurements of the mode B instability in the wake of a two-dimensional blunt trailing edge Bradley Gibeau, Sina Ghaemi Recent work has revealed contradictory results regarding the secondary instabilities that appear in the wake of a two-dimensional body with a blunt trailing edge (BTE). A linear stability analysis at Reynolds numbers near transition predicted that the aspect ratio (AR) of the body, i.e. the ratio of chord length to body thickness, dictates the secondary instabilities that exist in the wake. These authors predicted that a mode B' instability will appear in the wake for AR \textgreater 7.5, in contrast to the mode B that exists in the wakes of cylindrical bodies (AR $=$ 1). There are experimental results in the literature to support the existence of mode B'. However, the indirect analysis techniques used to show its existence have recently been found to be inadequate. We match the AR of 12.5 used in the studies that reported mode B' and also extend to a large AR of 46.5 and conduct particle image velocimetry measurements to unambiguously characterize the secondary instability in the wake. Mode B is found to be present in the wake for all Reynolds numbers investigated (Re(h) $=$ 2600 to 26000 where h is the thickness of the BTE), suggesting that AR does not play an important role in the formation of the secondary instability. No evidence of mode B' was found. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H40.00007: Stratified near wake of a slender body Jose Luis Ortiz-Tarin, Sheel Nidhan, Sutanu Sarkar Motivated by applications in submersible hydrodynamics, the effect of density stratification on the wake of a slender body is numerically investigated. Large-eddy simulations with an immersed boundary method are employed to simulate the flow past a 6:1 prolate spheroid at zero angle of attack. The Reynolds number based on the minor-axis diameter is fixed to $Re_D=UD/\nu=10^5$, and different levels of stratification measured by $Fr = U/ND$ are compared: $Fr =2$ which is close the critical value of Froude number ($Fr_c = 6/\pi$) where the turbulence suppression in the wake is maximal; a moderately stratified case ($Fr= 4$) and the unstratified case ($Fr=\infty$). A trip placed near the nose of the body is used to force laminar-turbulent transition and the wake is simulated to $25D$. Preliminary results that include the evolution of the boundary layer, mean velocity and turbulence statistics will be presented. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H40.00008: High Reynolds Number Stratified Turbulent Sphere Wake Measurements Kenneth Kalumuck, Alan Brandt There are virtually no data on wake turbulence for Reynolds numbers (Re) of one million or larger, above the drag crisis, even though aircraft and ships operate at Re \textgreater \textgreater 10$^{\mathrm{6}}$. This study seeks to characterize the near-field of a stratified wake at large Re \textasciitilde 2 x 10$^{\mathrm{5}}$ -- 1.4 x 10$^{\mathrm{6}}$, by towing a large diameter (\textasciitilde 0.5 m) sphere through a thermally stratified lake and a thermally stratified large salt water tow tank. The stratification produced BV frequencies, N, up to 0.07 s$^{\mathrm{-1}}$ with Froude numbers F $=$ U/ND $\ge $ 15. Three component turbulent velocity and temperature measurements were obtained using Acoustic Doppler Velocimeters (ADVs) and an array of fast response thermistors at various downstream distances. Wake turbulence characteristics including rms velocity and temperature fluctuation spectra, dissipation, integral scales, and Reynolds stresses are presented. Velocity and temperature power spectra exhibit clear -5/3 slopes while velocity co-spectra and transverse spectra exhibit -7/3 slopes over order-of-magnitude ranges in wavenumber, which are generally not clearly evident for lower Re laboratory experiments. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H40.00009: Spectral Proper Orthogonal Decomposition of Flow Around a Disk in Homogeneous and Stratified Fluids Sheel Nidhan, Karu Chongsiripinyo, Sutanu Sarkar, Oliver Schmidt The eduction of coherent structures and quantification of their role is a cornerstone of research in turbulent flows. Proper orthogonal decomposition (POD) introduced by Lumley to turbulent flows is a technique which can be used to identify the coherent structures that optimally capture the fluctuation kinetic energy of the flow. In this study, a space-time variant of POD, namely spectral POD (SPOD) is used to study the dynamics of the wake behind a disk at $Re=50,000$. Two cases, a stratified flow with $Fr=U/ND=2$ that evolves into a regime of strongly stratified turbulence (SST) and an unstratified case with $ Fr=\infty$, are analyzed. Preliminary results for $Fr=\infty$ show that the wake fluctuation energy is dominated by azimuthal modes, $m=0, 1$ and $2$. As the wake evolves downstream beyond $x/D \approx 60$, $m=2$ (at Strouhal number, $St=0$) becomes dominant while the $m=1$ mode (at $St=0.136$) dominates in the near wake. At $Fr=2$, the energy captured in the most energetic POD mode increases downstream, indicating the increasing importance of coherent modes in the stratified wake as it evolves downstream. In further analysis of modal dynamics, the Reynolds stresses carried by the dominant modes and the intermodal energy transfer among these modes will be quantified. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H40.00010: Direct Numerical Simulation of a Turbulent Wake behind a Body of Revolution at $Re_D=5000$ Fengrui Zhang, Yulia Peet, Ronald Adrian This study is concerned with the numerical investigation of a three-dimensional wake behind a body of revolution via Direct Numerical Simulations. Direct Numerical Simulations with the Reynolds number $Re_D=5000$ based on the bluff body diameter are performed using a high-order spectral-element solver Nek5000. The focus of the study is on characterizing the wake asymmetries and low-frequency behavior observed in previous experimental studies with similar bluff body models. Modal analysis will be presented to show the wake dynamics in both the near-wake and the far-wake region. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H40.00011: Vertical Flux of Mean Flow Kinetic Energy in the Near-Wake of a Marine Propeller in the Presence of External Turbulence Luksa Luznik, Bennitt Hermsen This experimental study examines the spatial evolution of the three-bladed marine propeller model (D$=$13cm) near wake in the early stages from the immediate wake behind the propeller up to 7 propeller diameters downstream for two inflow conditions: one with imposed external turbulence with 7{\%} intensity and integral scale compared to propeller blade geometry, and the second one with clean inflow conditions as a reference. Resulting Reynods number is Re$_{\mathrm{0.7R}}=$4.7x10$^{\mathrm{5}}$. All components of the vertical fluxes of mean flow kinetic energy are calculated from available three-dimensional PIV data and compared for the two inflow conditions. Influence of external turbulence on the wake instability process of mutual induction is examined and it was found that external turbulence enhances tip-vortex roll up mechanism resulting in earlier breakdown of individual vortices into small scale turbulence. Conditional sampling is performed to analyze the mechanisms of mean flow kinetic energy transport and it was found that outward interactions and sweep events contribute the most to the vertical transfer of mean flow kinetic energy from the inner wake to the free stream. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H40.00012: Do ribs help succulents to cope with aerodynamic loads in their natural environment? Oleksandr Zhdanov, Angela Busse Ribs on cacti stems play an important role for their survival not only in terms of water storage and evaporation control but also in attenuating high wind loads. Studies of flow past circular cylinders with many ribs inspired by the Saguaro cactus, which is native to the North American deserts, showed a reduction in drag and amplitude of unsteady force fluctuations compared to the smooth circular cylinder. In the Eastern Hemisphere some succulents have independently developed a similar plant structure as cacti but with a low number of ribs. If aerodynamics is one of the driving factors behind this convergent evolution, shapes with a low number of ribs should show similar benefits. In the present work, we experimentally investigated the aerodynamics of a cylinder with four ribs inspired by the succulent \textit{Euphorbia Abyssinica}. As its outer shape is approximately square, i.e. non-circular, the aerodynamic coefficients show a strong dependence on its orientation with respect to the mean flow. The experimental results are compared to the square cylinder at the same angular orientations and to results obtained from large-eddy simulations. The optimal orientation of this shape where aerodynamic loads are minimised is also discussed. [Preview Abstract] |
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