Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session AE: Instability: Boundary Layers I |
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Chair: Mujeeb Malik, NASA Langley Room: Long Beach Convention Center 102C |
Sunday, November 21, 2010 8:00AM - 8:13AM |
AE.00001: A Comparative Study of Subgrid Scale Models, for Prediction of Transition in Turbulent Boundary Layers Taraneh Sayadi, Parviz Moin Large eddy simulation of subharmonic transition of a spatially developing zero pressure gradient boundary layer at $Ma = 0.2$ is investigated using three different subgrid scale (SGS) models: Dynamic Smagorinsky [1], dynamic model involving the SGS kinetic energy [2] and dynamic scale similarity model. The interest lies in assessing the capability of each model in predicting the location of transition and the overshoot in the skin friction coefficient which is specific to this transition scenario. In the case of dynamic Smagorinsky model results were obtained for four different grid resolutions and it is observed that the location of transition is largely unaffected, indicating robust performance of the dynamic model in this respect. However, after breakdown and in the turbulent region the simulations with coarsest grids produce insufficient eddy viscosity to sustain the correct value of skin friction along the plate. As a result the coarsest resolution is employed to compare the performance of these three models. The point of transition is estimated correctly in each case, but the value of the overshoot and the turbulent statistics are affected by the model. [1] Moin P. \textit{et. al.} Phys Fluids A, \textbf{3}(11), 2746-2757, 1991. [2] Ghosal. S. \textit{et. al.} JFM, \textbf{286}, 229- 255, 1995. [Preview Abstract] |
Sunday, November 21, 2010 8:13AM - 8:26AM |
AE.00002: Sensitivity of non-modal instabilities to base-flow modifications Luca Brandt, Jan Pralits, Denis Sipp, Olivier Marquet Non-modal analysis determines the potential for energy amplification in stable flows. This is quantified in the frequency domain by the singular values of the resolvant operator and in the time domain by the singular values of the evolution operator. The present work extends previous analysis on the effect of base flow variation on flow stability by considering the sensitivity of the flow non-modal behavior. Using a variational technique, we derive an analytical expression for the gradient of the resolvent norm of the system with respect to a base flow modification and show how it depends on the optimal forcing and optimal response. The potential of such an approach is illustrated for zero-pressure-gradient boundary layers where the different instability mechanisms of wall-bounded shear flows are all at work. Results confirm previous findings and clearly indicate that base flow modifications can stabilize Tollmien-Schlichting waves whereas the amplification of streamwise streaks is more difficult to hamper. This result is now explained simply examining the expression for the gradient of the resolvant norm. [Preview Abstract] |
Sunday, November 21, 2010 8:26AM - 8:39AM |
AE.00003: Formation of roll/streaks structures in boundary layers as an instability of turbulence/mean-flow interaction Petros Ioannou Stochastic Structural Stability Theory (SSST) provides a deterministic nonlinear dynamical system for evolving the statistical mean state of a turbulent system. In this presentation SSST is applied to the problem of understanding the origin of the roll/streak structures that arise from free stream turbulence and are associated with bypass transition in boundary layers. Roll structures in the cross-stream/spanwise plane and associated streamwise streaks are shown to arise as a linear instability of interaction between the free stream turbulence and the mean flow. In this interaction Reynolds stresses arising from free stream turbulence are organized by perturbation streamwise streaks to force perturbation rolls giving rise to an amplification of the streamwise streak that in turn further organize the free stream turbulence to produce through this feedback interaction an instability of the roll/streak/turbulence complex. The dominant turbulent perturbation structures involved in supporting the roll/streak/turbulence complex instability are the non-normal oblique optimal perturbations. The emergence of the roll/streak structure arises at a bifurcation in the parameter of free stream turbulence intensity. The instability eventually equilibrates nonlinearly producing perturbation stable streaks and vortex circulations in agreement with the observed structures in transitional boundary layers. [Preview Abstract] |
Sunday, November 21, 2010 8:39AM - 8:52AM |
AE.00004: Receptivity, Growth and Breakdown of G\"{o}rtler Vortices Tamer Zaki, Lars-Uve Schrader, Luca Brandt The flow over a concave plate with constant radius of curvature is considered using direct numerical simulations. The boundary layer forming on the plate is exposed to wall roughness and to free-stream vortical disturbances. These sources trigger steady or traveling G\"{o}rtler modes composed of counter-rotating streamwise rolls and streamwise streaks. We present a parametric study of receptivity to localized roughness elements with various length scales, shapes and downstream positions and to vortical modes with different wavenumbers and frequencies. The associated receptivity mechanisms are characterized in terms of receptivity coefficients. We discuss linear and nonlinear receptivity to free-stream vorticity and determine the relevance of these mechanisms in boundary layers exposed to turbulent free streams with different frequency spectra. [Preview Abstract] |
Sunday, November 21, 2010 8:52AM - 9:05AM |
AE.00005: Secondary Instability of Roughness-Induced Transient Growth Nicholas Denissen, Edward White Optimal perturbation methods have provided the primary means of studying transient growth in theoretical and computational frameworks. Many interesting aspects of transient growth have been analyzed with these approaches, including the onset of secondary instability leading to transition-to-turbulence. However, while optimal perturbations are those that experience the most transient energy growth, this is not synonymous with the perturbations most likely to cause transition. The present work performs stability analysis of the flow field behind an array of periodic roughness elements. The incompressible flow over spanwise-periodic circular cylinders was previously computed via Direct Numerical Simulation, and the present work shows the resulting sub-optimal transient growth is more susceptible to secondary instabilities than optimal disturbances. The results agree qualitatively with experimental work and the implications of this are discussed for future work on more realistic rough surfaces. [Preview Abstract] |
Sunday, November 21, 2010 9:05AM - 9:18AM |
AE.00006: Boundary-layer receptivity of three-dimensional roughness arrays on a swept-wing Lauren Hunt, William Saric This experimental study extends the knowledge base of swept-wing receptivity mechanisms to three-dimensional surface roughness arrays, quantifying the relationship between surface roughness height and initial disturbance amplitudes within a boundary layer that is dominated by a crossflow instability. The experimental configuration includes the ASU(67)-0315 swept-wing installed in the low-turbulence Klebanoff-Saric Wind Tunnel at Texas A{\&}M University. It has a 45-degree sweep, 1.83m chord and a pressure minimum at 71{\%} chord. Three types of spanwise-periodic discrete roughness elements are used. Appliqu\'{e}, pneumatic, and plasma-actuated roughness are placed near the leading edge of the swept wing to investigate the effectiveness of each shape in creating the initial amplitudes of unstable stationary crossflow waves over a chord-Reynolds-number range of 2.0 million to 2.8 million. Results of naphthalene flow visualization and detailed boundary-layer scans using hotwire anemometry are provided. [Preview Abstract] |
Sunday, November 21, 2010 9:18AM - 9:31AM |
AE.00007: Bypass transition delay via oscillating Stokes layers Philipp Hack, Luca Burini, Tamer Zaki The breakdown of laminar boundary layers to turbulence is accompanied by a large increase in skin friction drag. Therefore, flow modification strategies are sought in order to delay transition and reduce drag. Our work addresses the influence of wall oscillation on the proceedings of bypass transition, and in particular on the amplification of boundary layer streaks. Direct numerical simulations demonstrate that appropriate choice of oscillation amplitude and frequency can yield a considerable reduction in local skin friction, and thereby drag. Linear, transient analyses are performed in order to explain the physical mechanism and the optimal parameters of wall oscillation. Finally, it is shown that an overall energetic advantage is possible, where the reduction in theoretical propulsion power outweighs the required input into the wall movement. [Preview Abstract] |
Sunday, November 21, 2010 9:31AM - 9:44AM |
AE.00008: Linear stability analysis of an evaporating binary liquid layer with fully transient reference profiles Hatim Machrafi, Alexey Rednikov, Pierre Colinet, Pierre Dauby This study deals with an evaporating horizontal binary-liquid layer (aqueous solution of ethanol; mass fraction 0.1) in contact with air with an imposed transfer distance. Fully transient solutions for the reference state are first calculated by means of a finite difference method. Then, the linear stability problem is solved using the frozen-time approach. After decomposition into normal modes, we obtain a problem for the eigenvalues, depending on the time as a parameter, which is numerically solved using the Chebyshev decomposition. Solutal and thermal Rayleigh-Benard-Marangoni instabilities are taken into account together with the Soret effect. The critical times needed for instability to occur and corresponding liquid thicknesses, are calculated, showing that a critical liquid thickness can be found under which no instability can occur. The latter point coincides approximately with the diffusive boundary layer reaching the bottom of the liquid layer. For instance, for a gas/liquid thickness layer ratio fixed at 10, the critical liquid thickness appears to be rather small, about 18.5 $\mu $m, which is illustrative of the general tendency. [Preview Abstract] |
Sunday, November 21, 2010 9:44AM - 9:57AM |
AE.00009: Inviscid Stability Analysis of Chemically Reacting Boundary Layers in Binary Gas Mixtures Jill Klentzman, Erman Ulker, Anatoli Tumin The inviscid stability of boundary layers in binary mixtures of nitrogen and oxygen in thermal equilibrium is investigated. Both temporal and spatial frameworks are considered. With the assumption of inviscid flow, the simplicity of the governing equations allows for a clear, direct comparison between the structure of the eigenvalue spectrums of real and perfect gases. Our results indicate that qualitatively the eigenvalue spectrum of a real gas should be similar to that of a perfect gas. When we examine the impact of real gas effects on the first and second modes, we find that real gas effects act to stabilize the first and destabilize the second. This can be attributed to the decrease in temperature at the solid surface due to variations in the specific heat. If we isolate the real gas effect of changes in species concentrations due to chemical reactions, we find that this effect alone stabilizes the second mode, which agrees with studies of flows at finite Reynolds number. This could be the result of energy being absorbed and used for dissociation rather than for the growth of disturbances. [Preview Abstract] |
Sunday, November 21, 2010 9:57AM - 10:10AM |
AE.00010: Stabilty of corner boundary layers Jim Denier, Nathaniel Jewell We reconsider the problem of the stability of the flow in an internal corner, focusing attention on determining both the most unstable mode and the critical Reynolds number for instability. Our results predict that the zeroth, the so called, inviscid mode becomes unstable first with a critical Reynolds number of 43,500. Comparison with previous work on this problem will also be provided. [Preview Abstract] |
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