Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M25: Turbulence: Curvature and Flow Instability |
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Chair: Leonardo Chamorro, University of Illinois at Urbana-Champaign Room: 2005 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M25.00001: ABSTRACT WITHDRAWN |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M25.00002: Development of second mode instability in a Mach 6 flat-plate boundarylayer with two-dimensional roughness Qing Tang, Chuanhong Zhang, Cunbiao Lee The PCB pressure sensors and particle image velocimetry (PIV) are used to study the development of the second mode instability in a Mach 6 flow over a flat plate with two-dimensional roughness. A two-dimensional transverse wall blowing is used to enhance the second mode instability in the boundary layer and seeding tracer particles for PIV measurement. Three roughness elements with different heights are mounted at 125mm downstream the leading edge of the flat plate. It is proved that two-dimensional roughness could enhance the second mode fluctuation upstream the roughness. The second mode instability waves in flat-plate boundary layer are clearly shown by PIV and the boundary layer separation zone upstream the roughness is carefully measured. The boundary layer then reattaches the wall and the second mode instability waves are found damping downstream the roughness. It is also proved that the amplification and damping effect of the second mode instability waves depend on the height of the roughness. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M25.00003: Tollmien-Schlichting Wave Cancellation by Feedback Control SH Sankarasarma Vemuri, Jonathan Morrison, Eric Kerrigan Tollmien-Schlichting (TS) waves are primary instabilities in the boundary layer and by actively interfering with these naturally occurring waves, the transition could be delayed. The present active cancellation scheme involves a feedback control loop between an array of sensors and actuators to generate the desired actuation to attenuate the growth of these spatially evolving unstable waves. Experimental results of growing TS-waves from a point source on a flat-plate model will be presented. Numerical calculations based on linear stability theory have been carried out to predict the evolution of TS waves downstream to model the control system. We will present the spatial transfer functions between the excitation source and sensors and those between the actuators and sensors for various sensor-actuator configurations. A H$_{\mathrm{\infty }}$ optimal controller is designed for each of these configurations to obtain an optimal sensor-actuator configuration and the controller will be implemented to attempt real-time cancellation of TS waves on a flat-plate model using these optimal configurations. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M25.00004: Edge states and the spatio-temporal transition dynamics in a boundary layer driven by free stream turbulence Bruno Eckhardt, Tobias Kreilos, Taras Khapko, Philipp Schlatter, Yohann Duguet, Dan S. Henningson We present a cellular automaton model for the transition to turbulence in a boundary layer exposed to free stream turbulence. The model is based on the presence of an invariant flow structure (aka ``edge state'') intermediate between laminar and turbulent (Phys. Rev. Lett. 108, 044501 (2012)) and replaces the complex initiation of turbulence by a stochastic process with parameters that are related to the properties of the free stream turbulence and the edge state. The model uses a discretization of space and time and includes spanwise and streamwise spreading of a turbulent nucleus. It reproduces the downstream variation of the nucleation rate, the intermittency factor, and the number and widths of turbulent spots, including their variation with the free stream turbulence intensity. The model thus connects the observed characteristics of boundary layer transition with the transition scenario that has been developed for parallel shear flows, such as pipe flow or plane Couette flow. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M25.00005: Stability analysis of the experimental and the simulated flow past miniature vortex generators in a Blasius boundary layer Simone Camarri, Lorenzo Siconolfi, Jens H.M. Fransson It is shown in the literature that Tollmien-Schlichting (TS) waves can be damped and transition delayed if properly shaped modulations of the streamwise velocity (streaks) are generated inside a Blasius boundary layer. In [1] velocity streaks are generated experimentally by installing miniature vortex generators (MVGs) on the plate wall so as to obtain a significant streak amplitude where the uncontrolled flow would be convectively unstable. When TS waves are excited upstream with respect to the MVGs, they undergo an amplification in the near wake past the MVGs and, if the streaks amplitude is sufficiently high, they decay further downstream, delaying transition. In order to investigate this behavior, representative experimental cases among those documented in [1] are selected and simulated by DNS, and local bi-global stability analysis is applied both to the experimental and to the DNS flow fields. As a result, stability curves for the BL with MVGs are computed and compared to that of an uncontrolled Blasius BL. It is shown that available experimental results agree with the computed stability curves and results from the stability analysis are used to investigate the involved stabilization mechanisms. \\[4pt] [1] Shahinfar et al., Phys. Rev. Lett. 109, 074501, (2012) [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M25.00006: Accelerated Transonic Flow past a curvature discontinuity Thomas de Cointet, Anatoly Ruban The aim of this talk is to investigate High Reynolds number Transonic flow past a discontinuity in body curvature. Starting with the inviscid flow outside the boundary layer, our analysis will focus on the flow in a vicinity of the point of discontinuity, where a solution of the Euler equations will be sought in self-similar form. This reduces the Euler equations to an ordinary differential equation. The analysis of this equation shows that the pressure gradient on the airfoil surface develops a strong singularity, which is proportional to $(x_0-x)^{-1/3}$ as the discontinuity point $x_0$ is approached. We then study the response of the boundary layer to this extremely favourable pressure gradient. We show that the boundary layer splits into two parts, the main body of the boundary layer that becomes inviscid on approach to the singularity, and a thin viscous sublayer situated near the wall. The analysis of the behaviour of the solution in the viscous sublayer shows that Prandtl's hierarchical concept breaks down in a small region surrounding the singular point, where the viscous-inviscid interaction model should be used. In the final part of this talk we present a full formulation of the viscous-inviscid interaction problem and discuss numerical results. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M25.00007: Effect of curvature modulation on Gortler vortices in boundary layers Hui Xu, Philip Hall, Spencer Sherwin The stability of a high-Reynolds-number flow over a curved surface with varying curvature is studied. The investigation is concentrated on spanwise-periodic vortices of wavelength comparable with the boundary layer thickness. Motivated by the amendment of Rayleigh's criterion (Hall,2013), the effect of wavy-wall modulation on Gortler vortices is addressed. Both linear and nonlinear investigations are performed to understand the destabilization and stabilization mechanisms of the vortices. Furthermore, due to the wavy-wall curvature modulation, the growth or decay rate of the vortices is discussed. Finally, a control strategy of the vortices is proposed based on distributing the curvature. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M25.00008: Flow past a hump in subsonic and transonic regimes: Comparisons between triple deck theory and DNS Gianmarco Mengaldo, Marina Kravtsova, Anatoly Ruban, Spencer Sherwin The prediction of laminar-turbulent transition is a key factor for reducing the drag and for improving the aerodynamic performance of an aircraft. In the past few years several studies, theoretical, numerical and experimental, have been conducted on roughness elements and isolated humps in order to investigate their role in the transition process. Many comparisons already exist between numerical and experimental data while little work has been carried out in comparing theoretical and numerical results. In this work we present a comparative study between triple deck theory and DNS. Specifically we consider a flat plate with a hump of various heights in a compressible regime at a relatively high Reynolds number. Different Mach numbers are taken into account, ranging from subsonic to transonic regimes and various temperatures are applied to the wall for each Mach number considered. The main questions we aim to answer in this work are the following: \begin{itemize} \item Are triple deck and DNS data comparable and how can we perform this comparison? \item Which are the limits and the advantages of the first (triple deck) and the second (DNS) approach for the simple test case under investigation? \end{itemize} [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M25.00009: Flow-field characterization over 1D and 2D periodic wavy walls using PIV in a refractive-index-matched channel Leonardo P. Chamorro, A.M. Hamed, Carlo Zuniga Zamalloa Flow over two wavy walls was experimentally investigated using high- and low-frame-rate particle image velocimetry (PIV). The first wall has 1D streamwise waves with amplitude-to-wavelength ratio a/$\lambda = $ 0.05. The second wall has streamwise and spanwise waves with a/$\lambda = $ 0.05 and 0.025, respectively. A refractive-index matching approach was used to minimize image distortion and reflections. It grants unobstructed optical access and allows for very near-wall velocity measurements. Flow-field measurements were acquired at multiple streamwise-wall-normal and wall-parallel planes. The low-frame-rate measurements were used to obtain high-resolution ensemble-averaged flow fields and turbulence statistics, while the high-frame-rate measurements were used to map the structure of the turbulence at various wall-normal locations and to determine scale-dependent correlations across the topological features of the walls. The results were studied to understand the link between the turbulence structure and wall undulations. Linking the turbulence to wall topology has many implications in environmental flows, sediment transport and advection diffusion of scalars. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M25.00010: Turbulence structure over 1D and 2D periodic wavy walls: Coupling between coherent motions and large-scale undulations Nicolas Tobin, Pranav Suresh, Pratap Vanka, Leonardo P. Chamorro Understanding the turbulence dynamics over topographies with mild perturbations is of great relevance at geophysical scale and on a number of other wall-bounded flow phenomena. Turbulence statistics and its spectral distribution in the boundary layer are heavily modulated by the topological features of walls in a very complex fashion. In this study we aim to understand some of the basic processes modulating the interaction of near-wall turbulence and large-scale mild perturbations superimposed to smooth walls. Large Eddy Simulations (LES) of channel flows with 1D and 2D periodic wavy surfaces are performed at a Reynolds number of 104 based on the channel depth. The computational domain spans 10$\lambda $ (where $\lambda $ is the wavelength) in the streamwise direction and 4$\lambda $ and 1$\lambda $ in the spanwise and vertical directions respectively. The sinusoidal waves have amplitude of 0.1$\lambda $. Turbulence structures and high-order statistics as well as features of energetic coherent motions are discussed in terms of the wall topology. The effect of the wall shape on the flow is examined through the pre-multiplied spectral features of the turbulence at key locations. [Preview Abstract] |
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