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 B34: Flow Instability: Boundary Layers Transition |
Hide Abstracts |
Chair: Tim Colonius, Caltech Room: 616 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B34.00001: Volume-Area and Area-Perimeter Dimensions of Turbulent-Spots Interfaces in Transitional Boundary-Layer Flow Zhao Wu, Tamer Zaki, Charles Meneveau The nature of turbulent spots in transitional boundary layers, and whether their internal structure shares characteristics of equilibrium turbulence, remain open questions of considerable interest. Here we study scaling properties of the interface separating the spots from the outside flow. For high-Reynolds-number turbulence, such interfaces are known to display fractal scaling with a fractal dimension near D=2+1/3, where the 1/3 can be related to the Kolmogorov scaling of velocity fluctuations (e.g. de Silva et al. PRL 2013). We measure the volume-area fractal scaling of the naturally triggered turbulent spots. The data are from the DNS of a transitional boundary layer available at the JHTDB (http://turbulence.pha.jhu.edu). The spot boundaries (interfaces) are determined without arbitrary threshold selection using an unsupervised machine learning method, namely the self-organizing map (Zhao et al. PRF 2019). Results from the volume-area fractal dimension confirm D=7/3, i.e. trends consistent with fully developed turbulence. Applying an alternative area-perimeter analysis on planar cuts at various heights shows D decreasing then increasing. It is argued that these trends could be associated to changes in the thickness of the interface at different heights from the wall. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B34.00002: On the evolution of the velocity gradient in a minimal simulation unit of transitional boundary layers Ahmed Elnahhas, Perry Johnson, Adrian Lozano-Duran, Parviz Moin The transition of a boundary layer from laminar to turbulent state is associated with a rapid increase in friction and heat transfer coefficients and is accompanied by the rapid growth of velocity gradients throughout the boundary layer. This can be appreciated by visualizing isosurfaces of Q-criterion or other vortex identifiers and is particularly evident in late-stage transition when growing structures abruptly break down into more chaotic flow, generating turbulent spots. We consider the evolution of several velocity gradient invariants integrated in cross-stream planes using direct numerical simulations of canonical transition scenarios with minimal spanwise extent. The spanwise domain is restricted to fit only one wavelength of the oblique wave, leading to a single $\Lambda $-vortex being present at any streamwise location. The minimal unit simulation displays the main features of transition in larger domains, such as a realistic skin friction coefficient profile. The budget of the squared Frobenius norm of the velocity gradient tensor shows that the pressure term is orders of magnitude smaller compared to the source and sink terms. Furthermore, the profile of the Q-criterion squared exhibits a distinct plateau after the initial emergence of the hairpin vortex, and appears to be a good indicator of the onset of transition. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B34.00003: Development for a Theoretical Model of Crossflow-induced Boundary-layer Transition Makoto Hirota, Yuki Ide, Takahisa Hayashida, Yuji Hattori On widely-used swept wings of aircrafts, laminar-turbulent transition of three-dimensional boundary layer mainly occurs through the process in which (i) a crossflow instability (referred to as the primary) first grows spatially and generates a vortex street and, then, (ii) the vortex street further induces low-speed streaks in the mainstream distribution that becomes unstable to a high-frequency secondary instability. Direct numerical simulation (DNS) can reproduce this process accurately, but the transition location sensitively depends on how the two kind of disturbance sources are fed to the primary and secondary instabilities, respectively. It is moreover difficult to understand the dependency on various flow parameters. In this study, we develop a theoretical model to estimate the growth rates of both the primary and secondary instabilities according to linear stability analyses. By noting a scale similarity to the Kelvin-Helmholtz instability, the inflection point and the shear profile of the flow enable us to estimate the growth rates. Our method is expected to be not only efficient to predict the transition location, but also useful for finding less unstable flow profiles that control devices should attain. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B34.00004: Emergence of streaks and turbulent spots in an unsteady boundary layer beneath a solitary wave Asim Onder, Philip Li-Fan Liu Bypass route to transition is studied in a bottom boundary layer developing under solitary wave. First, the conditions for streak growth and breakdown are analyzed using a linear input-output framework and secondary stability analysis. Vortical perturbations whose intensity is about 1{\%} of the maximum free-stream velocity are found to be sufficient to induce unstable streaks in moderate to high Reynolds numbers. In the second step, a natural bypass transition scenario is realized using direct numerical simulations, where a weak turbulent current is introduced to initiate the transition. The breakdown of streaks to turbulent spots is shown. Depending on their nucleation phase, the turbulent spots can grow to occupy the whole domain leading to a premature transition bypassing the emergence of two-dimensional modal instabilities. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B34.00005: Linear and nonlinear dynamics of second-mode instability in hypersonic boundary layers (HBL) Unnikrishnan Sasidharan Nair, Datta Gaitonde Hypersonic transition is often dominated by the second-mode instability. We perform a direct numerical simulation (DNS) informed by linear stability theory, to understand the eventual three-dimensional breakdown of this instability. Linear amplification and nonlinear saturation of this two-dimensional wave eventually culminate in the appearance of oblique waves, which break down the HBL into a turbulent state. A modal analysis of the transitional region identifies lambda-shaped vortices belonging to both fundamental and subharmonic categories. While the former appears relatively downstream, the subharmonic waves are observed immediately following nonlinear saturation. This nonlinear stage of transition is further analyzed through a novel unsteady flow perturbation (UFP) technique. UFP essentially tracks the linear evolution of perturbations on a nonlinearly saturated background flow, using body force constraints, thus approximating a Floquet analysis for general configurations. UFP is shown to identify the most receptive superharmonic/subharmonic components in the periodically distorted flow. In addition to providing insights into the dynamics, relative to DNS, it provides an accurate low cost approximation of the breakdown spectrum in the early transitional stages. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B34.00006: Nonlinear input-output analysis of laminar-turbulent transition for wall-bounded flows Georgios Rigas, Denis Sipp, Tim Colonius In a linear input-output analysis framework, the most amplified instabilities are typically described by considering singular vectors of the resolvent operator of the linearized Navier-Stokes equations. In this study, we extend the methodology to take into account nonlinear triadic interactions by considering a finite number of harmonics in the frequency domain using the Harmonic Balance Method. Optimal nonlinear forcing mechanisms that lead to transition and maximize the skin-friction coefficient are identified using direct-adjoint looping. We demonstrate the framework on a zero-pressure flat-plate boundary layer by considering three-dimensional perturbations triggered by a few optimal forcing modes of finite amplitude. Depending on the frequency, spanwise wavenumber, amplitude and symmetries of the perturbation, we recover all the transition stages associated with K-type and H-type transition mechanisms, oblique waves, streaks, and their breakdown. The proposed frequency-domain framework identifies the worst-case frequency disturbances for wall-bounded laminar-turbulent transition. [Preview Abstract] |
Saturday, November 23, 2019 5:58PM - 6:11PM |
B34.00007: The Role of Fluctuating Dissipative Fluxes in the Receptivity of High-Speed Chemically Reacting Boundary Layers in Binary Mixtures to Kinetic Fluctuations Kevin Luna, Anatoli Tumin In this talk we present progress toward understanding the role that the numerous fluctuating dissipative fluxes that occur in chemically reacting mixtures play in the kinetic fluctuations boundary layer receptivity problem. These fluctuations are modeled using fluctuating hydrodynamics where the molecular nature of fluids is expressed through stochastic white noise contributions to the dissipative fluxes. While the problem of boundary layer receptivity to kinetic fluctuations has been studied for some time now and its relevance for flight conditions has been established, there are few works that provide the full description of all fluctuating dissipative fluxes that must be modeled when working with non-perfect gasses such as multi-species air models under flight conditions. To make progress toward understanding this wide spectrum of effects, we work with binary mixtures of Oxygen and Nitrogen as a limiting case for modeling air flows. In this talk we present a new model that allows for a quantitative description of these effects in regions where previous results of Tumin and Luna (2018) could not provide a precise description. Using this model, a detailed description of the roles of these fluctuating dissipative fluxes is established. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700