Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F25: Instability: Boundary Layers |
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Chair: Jovan Nedic, McGill University Room: Georgia World Congress Center B313 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F25.00001: Receptivity analysis of boundary layer flows subject to stochastic excitation Wei Ran, Armin Zare, M. J. Philipp Hack, Mihailo R Jovanovic We utilize the externally forced linearized Navier-Stokes equations to study the receptivity of pre-transitional boundary layers to persistent stochastic excitation sources. In contrast to the widely used resolvent analysis that quantifies amplification mechanisms associated with input-output pairs of identical temporal frequency, our approach determines the steady-state response to white-in-time excitation. Stochastic forcing is used to model the effect of free-stream turbulence that enters at various wall-normal locations and the fluctuation dynamics are studied from locally parallel and global perspectives. In addition to forcing with trivial (identity) covariance, we utilize the spatial spectrum of homogeneous isotropic turbulence to model the effect of free-stream turbulence. Global flow analysis predicts the amplification of a cascade of streamwise scales throughout the streamwise domain. Even though parallel flow analysis does not account for the effect of the spatially evolving base flow, we demonstrate that it captures important trends and prevailing length-scales. Our study offers a systematic, computationally efficient framework for quantifying the influence of free-stream turbulence on the dynamics of velocity fluctuations in weakly non-parallel flows. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F25.00002: Non-modal stability analysis of low-Re separated flow around a NACA 4415 airfoil in ground effect Wei He, Jóse Miguel Pérez, Peng Yu, Larry K.B. Li In this numerical-theoretical study, we perform a linear non-modal stability analysis of the separated flow around a NACA 4415 airfoil over a no-slip ground at low Reynolds numbers (300≤Re≤500) and high angles of attack (15° ≤α≤20°). We find that: (i) the strength of the recirculation zone behind the airfoil is a key parameter controlling the absolute/convective nature of the instability in the boundary layer downstream; (ii) when Re, α or the ground clearance increases, the energy gain also increases, with the optimal perturbations switching from being three dimensional to two dimensional; and (iii) classical hairpin vortices, or Klebanoff modes, can be produced by three-dimensional optimal perturbations on a two-dimensional steady base flow containing a laminar separation bubble. Knowledge of the spatiotemporal properties of the optimal mode could aid the design of advanced strategies for flow control. This study offers new insight into the transient growth behavior of airfoil-ground flow systems at low Re and high α, contributing to a better understanding of the ground-effect aerodynamics of small insects and micro aerial vehicles. |
Monday, November 19, 2018 8:26AM - 8:39AM |
F25.00003: Nonlinear solutions for wall-bounded transition in the frequency domain Georgios Rigas, Denis Sipp, Tim Colonius In a linear 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. Two approaches will be compared: a global approach considering the full Jacobian of the problem and an efficient spatial marching technique based on the One-Way Navier-Stokes (OWNS) equations, which substantially reduces the computational cost. We demonstrate the framework on a Blasius boundary layer by considering three-dimensional spanwise-periodic perturbations triggered by a few optimal forcing modes of finite amplitude. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F25.00004: Growth mechanisms of perturbations in boundary layers over a compliant wall Roland Bouffanais, M. Malik, Martin Skote The temporal modal and nonmodal growth of 3D perturbations in the boundary layer flow over an infinite compliant flat wall is considered. Using a wall-normal velocity/wall-normal vorticity formalism, the dynamic boundary condition at the compliant wall admits a linear dependence on the eigenvalue parameter, as compared to a quadratic one in the canonical formulation of the problem. As a consequence, the continuous spectrum is accurately obtained. This enables us to effectively filter the pseudospectra, which is a prerequisite to the transient growth analysis. An energy-budget analysis for the least-decaying hydroelastic and Tollmien--Schlichting modes in the parameter space reveals the primary routes of energy flow. Moreover, the maximum transient growth rate increases more slowly with the Reynolds number than for the solid wall case. Unlike the solid-wall case, viscosity plays a pivotal role in the transient growth. The initial and optimal perturbations are compared with the boundary layer flow over a solid wall; differences and similarities are discussed. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F25.00005: Boundary-layer control using “Deterministic Turbulence” Kwing-So Choi, Yaxing Wang, Mike Gaster, Chris Atkin, Yury Kachanov, Vladimir Borodulin The aim of this experimental study is to develop and optimise the control strategy to reduce the skin-friction drag of turbulent boundary layers. This will be tested on a pseudo-randomly excited boundary-layer during the late stage of transition, where the velocity fluctuations (therefore the flow structures) are repeatable even when the turbulence statistics becomes very similar to that of a fully developed turbulent flow. Sensors are not required for the boundary-layer control with “deterministic turbulence” since the large turbulence structures are repeated each time the experiment is run using the same excitation signal. Experiments were carried out in a very low-turbulence (less than 0.01%) wind tunnel, where the laminar boundary layer over a flat plate was excited by broadband noise. Therefore, the receptivity of the boundary layer to the background noise is much weaker than that to the initial excitation with broadband noise. The turbulence structures such as the Λ-vortices and the turbulent spots were repeatably realised in the “deterministic turbulence”. Control will be through suction/blowing and plasma actuators. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F25.00006: The dynamics of a rotating cubic cavity flow under libration Ke Wu, Bruno D Welfert, Juan Lopez In this study, we numerically explore the flow dynamics of a fluid inside a cube which is rotating at a large rate which is subject to modulation. For small modulation amplitudes the flow is synchronous, dominated by inertial wave beams and intrinsic eigenmodes of the cube, and preserves all symmetries of the configuration. Larger modulation amplitudes generate time-dependent boundary layers which penetrate into the interior of the cube. In this regime the flow is susceptible to instabilities that tend to trigger symmetry breaking or period doubling bifurcations. |
Monday, November 19, 2018 9:18AM - 9:31AM |
F25.00007: Receptivity of supersonic boundary layers over smooth and wavy surfaces to impinging slow acoustic waves Carlos G. Hernández, Xuesong Wu In this talk, we investigate the receptivity of a supersonic boundary layer to impinging acoustic waves. Unlike previous studies of acoustic receptivity, where the sound waves have phase speeds comparable with or larger than the free-stream velocity U_\infty, the acoustic waves here have much slower (O(R^{-1/8}U_\infty)) phase velocity, and their wavelength and frequency are of O(R^{-3/8} L) and O(R^{1/4} U_\infty /L) respectively, compatible with the triple-deck structure, where L is the distance to the leading edge and R the Reynolds number based on L and U_\infty. The first receptivity mechanism is completely new, involving the interaction of two waves and operating in a boundary layer over a smooth wall. The second involves the interaction between an acoustic wave and the steady perturbation induced by a wavy wall. The sound-sound, or sound-roughness, interactions generate a forcing in resonance with a neutral T-S wave. The latter is thus excited near the lower branch of the neutral curve, and subsequently undergoes exponential amplification. The two receptivity processes are much more effective compared with those involving usual sound waves, with the coupling coefficient being greater by a factor of O(R^{1/4}) and O(R^{1/8}) in the S-S and S-R interactions, respectively. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F25.00008: Receptivity of high-speed boundary layers in binary mixture of gases to kinetic fluctuations Anatoli Tumin, Kevin Luna The receptivity of high-speed boundary layers in Binary Mixture of gases (oxygen and nitrogen) to kinetic fluctuations (KF) is considered within the framework of fluctuating hydrodynamics. The formulation is based on the idea that KF-induced dissipative fluxes may lead to the generation of unstable modes in the boundary layer. The binary mixtures may serve as limiting cases for real gas effects in an airflow. It is assumed that KF manifest themselves in this context through “stochastic forcing” by a random stress tensor, a random heat flux, a random species diffusion, and a random chemical source term. The approach taken here follows that of Fedorov and Tumin (2017), but includes previously unconsidered effects such as fluctuating species diffusion and fluctuating species production. The results are in agreement with Fedorov and Tumin (2017) where it was shown that in the case of a calorically perfect gas, that boundary layer flow is most susceptible to kinetic fluctuations in the critical layer and that the fluctuations of the shear stress in the streamwise momentum equation are the dominant fluctuations. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F25.00009: Nonmodal traveling disturbances within the entropy layer of hypersonic boundary layers Pedro Paredes, Meelan Choudhari, Fei Li Laminar-turbulent transition in boundary layers is a critical factor in the design of hypersonic vehicles, because of its impact on overall aerodynamic and heat transfer characteristics. Numerical and experimental studies have demonstrated that the modal growth of planar Mack modes is responsible for laminar-turbulent transition on sharp cones. However, the physical mechanisms that lead to transition onset within the swallowing distance of the entropy layer on sufficiently blunt cones are not well understood as yet. Because modal amplification is too weak to initiate transition at large bluntness values, transient growth has been investigated as the potential basis for a physics-based model of the transition reversal phenomenon. The present computations examine disturbance growth over both a variable bluntness, 7-degree cone that was tested in the AFRL Mach-6 high-Reynolds-number facility and a cone-eogive-cylinder configuration that was employed during experiments in the Boeing/AFOSR Mach-6 Quiet Tunnel at Purdue University. Nonmodal instability results are compared with the Purdue data to address the nature of the traveling disturbances measured outside the boundary layer. The implications of these disturbances for transition onset on these configurations are also discussed. |
Monday, November 19, 2018 9:57AM - 10:10AM |
F25.00010: Balancing stability and sensitivity in hypersonic boundary layers using input-output analysis David Cook, John Thome, Joseph Brock, Joseph William Nichols, Graham Candler We apply Input/Output (I/O) analysis to study instabilities of hypersonic boundary layers over sharp and blunt cones. Compared to conventional stability analysis, I/O analysis represents a paradigm shift from a local view to a global view of instabilities. Based on standard techniques from linear systems theory, I/O analysis uses the resolvent operator to find the total system frequency response to external excitation. We first verify that the global view provided by I/O analysis is correct by comparing its results to well-known results corresponding to the development of second mode instability on a sharp cone. We then show that I/O analysis seamlessly handles the complex flow created by the tip of a blunt cone that otherwise violates the assumptions underpinning traditional stability analysis. Because of its global scope, I/O not only captures second mode instability, but also identifies additional instability mechanisms on blunt cones that traditional stability analyses have been unable to predict. We also demonstrate how I/O analysis incorporates sensitivity information to the disturbance input location, which provides a physical measure of the minimum amplitude of disturbances that can lead to flow transition. |
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