Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session E21: Flow Instability: Boundary LayersBoundary Layers Instabilities
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Chair: Jae Sung Park, University of Nebraska-Lincoln Room: 706 |
Sunday, November 19, 2017 4:55PM - 5:08PM |
E21.00001: Transient disturbance growth in flows over convex surfaces Michael Karp, M. J. Philipp Hack Flows over curved surfaces occur in a wide range of applications including airfoils, compressor and turbine vanes as well as aerial, naval and ground vehicles. In most of these applications the surface has convex curvature, while concave surfaces are less common. Since monotonic boundary-layer flows over convex surfaces are exponentially stable, they have received considerably less attention than flows over concave walls which are destabilized by centrifugal forces. Non-modal mechanisms may nonetheless enable significant disturbance growth which can make the flow susceptible to secondary instabilities. A parametric investigation of the transient growth and secondary instability of flows over convex surfaces is performed. The specific conditions yielding the maximal transient growth and strongest instability are identified. The effect of wall-normal and spanwise inflection points on the instability process is discussed. Finally, the role and significance of additional parameters, such as the geometry and pressure gradient, is analyzed. [Preview Abstract] |
Sunday, November 19, 2017 5:08PM - 5:21PM |
E21.00002: Stochastic modeling of mode interactions via linear parabolized stability equations Wei Ran, Armin Zare, M. J. Philipp Hack, Mihailo Jovanovic Low-complexity approximations of the Navier-Stokes equations have been widely used in the analysis of wall-bounded shear flows. In particular, the parabolized stability equations (PSE) and Floquet theory have been employed to capture the evolution of primary and secondary instabilities in spatially-evolving flows. We augment linear PSE with Floquet analysis to formally treat modal interactions and the evolution of secondary instabilities in the transitional boundary layer via a linear progression. To this end, we leverage Floquet theory by incorporating the primary instability into the base flow and accounting for different harmonics in the flow state. A stochastic forcing is introduced into the resulting linear dynamics to model the effect of nonlinear interactions on the evolution of modes. We examine the H-type transition scenario to demonstrate how our approach can be used to model nonlinear effects and capture the growth of the fundamental and subharmonic modes observed in direct numerical simulations and experiments. [Preview Abstract] |
Sunday, November 19, 2017 5:21PM - 5:34PM |
E21.00003: Instability properties in the bottom boundary layer under a model mode-1 internal tide. John Segreto, Peter Diamessis The instability properties of the bottom boundary layer (BBL) under a model mode-1 internal tide in linearly stratified finite-depth water are studied, using 2-D fully nonlinear and non-hydrostatic direct numerical simulations (DNS) based on a spectral multidomain penalty method model. Low-mode internal tides are known to transport large amounts of energy throughout the oceans. One possible mechanism, among others, through which the energy of the particular tidal waves can be directly dissipated, without transfer to higher modes, is through wave-BBL interactions, where strong near-bottom shear layers develop, leading to localized instabilities and ultimately mixing. In the model problem, the stability response of the time-dependent wave-induced BBL is examined by introducing low-amplitude perturbations near the bed. For the linear stage of instability evolution, the time-dependent perturbation energy growth rates are computed by tracking the largest perturbation energy density in the domain through the wave-modulated shear and stratification, ultimately the formation of distinct localized near-bed Kelvin-Helmholtz billows are observed. The average growth rate, $\sigma $, is then compared to the time, T, that a parcel of fluid is subject to a local Richardson number less than 1/4, resulting in a nondimensional criterion for instability, $\sigma $T. A stability boundary is then constructed as a function of perturbation amplitude, wave steepness, aspect ratio and Reynolds number. [Preview Abstract] |
Sunday, November 19, 2017 5:34PM - 5:47PM |
E21.00004: Experimental and numerical investigation of low-drag intervals in turbulent boundary layer Jae Sung Park, Sangjin Ryu, Jin Lee It has been widely investigated that there is a substantial intermittency between high and low drag states in wall-bounded shear flows. Recent experimental and computational studies in a turbulent channel flow have identified low-drag time intervals based on wall shear stress measurements. These intervals are a weak turbulence state characterized by low-speed streaks and weak streamwise vortices. In this study, the spatiotemporal dynamics of low-drag intervals in a turbulent boundary layer is investigated using experiments and simulations. The low-drag intervals are monitored based on the wall shear stress measurement. We show that near the wall conditionally-sampled mean velocity profiles during low-drag intervals closely approach that of a low-drag nonlinear traveling wave solution as well as that of the so-called maximum drag reduction asymptote. This observation is consistent with the channel flow studies. Interestingly, the large spatial stretching of the streak is very evident in the wall-normal direction during low-drag intervals. Lastly, a possible connection between the mean velocity profile during the low-drag intervals and the Blasius profile will be discussed. [Preview Abstract] |
Sunday, November 19, 2017 5:47PM - 6:00PM |
E21.00005: Receptivity to Kinetic Fluctuations: A Multiple Scales Approach Luke Edwards, Anatoli Tumin The receptivity of high-speed compressible boundary layers 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. Fedorov and Tumin (AIAA J., 2017) solved the receptivity problem using an asymptotic matching approach which utilized a resonant inner solution in the vicinity of the generation point of the second Mack mode. Here we take a slightly more general approach based on a multiple scales WKB ansatz which requires fewer assumptions about the behavior of the stability spectrum. The approach is modeled after the one taken by Luchini (AIAA J., 2016) to study low speed incompressible boundary layers over a swept wing. The new framework is used to study examples of high-enthalpy, flat plate boundary layers (see Edwards Tumin, AIAA 2017) whose spectra exhibit nuanced behavior near the generation point, such as first mode instabilities and near-neutral evolution over moderate length scales. The configurations considered exhibit supersonic unstable second Mack modes despite the temperature ratio $T_w/T_e > 1$, contrary to prior expectations. [Preview Abstract] |
Sunday, November 19, 2017 6:00PM - 6:13PM |
E21.00006: Stability characteristics of compressible boundary layers over thermo-mechanically compliant walls Fabian Dettenrieder, Daniel Bodony Transition prediction at hypersonic flight conditions continues to be a challenge and results in conservative safety factors that increase vehicle weight. The weight and thus cost reduction of the outer skin panels promises significant impact; however, fluid-structure interaction due to unsteady perturbations in the laminar boundary layer regime has not been systematically studied at conditions relevant for reusable, hypersonic flight. In this talk, we develop and apply convective and global stability analyses for compressible boundary layers over thermo-mechanically compliant panels. This compliance is shown to change the convective stability of the boundary layer modes, with both stabilization and destabilization observed. Finite panel lengths are shown to affect the global stability properties of the boundary layer. [Preview Abstract] |
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