Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session L27: Boundary-Layer Instability I |
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Chair: Mujeeb Malik, NASA Langley Research Center Room: 31C |
Monday, November 19, 2012 3:35PM - 3:48PM |
L27.00001: Evolution of wavepacket over short compliant panels in a Blasius boundary layer Ige Bori, K.S. Yeo, Hua-Shu Dou, Xijing Zhao Compliant surface has been proved in various theoretical studies as a promising tool in delaying transition. This study concerns our recent work carried on the evolution of pulse-initiated disturbance wavepackets over finite-length compliant panels in a Blasius boundary layer by direct numerical simulation (DNS) method. A finite section of the wall was replaced by a tensioned membrane on a damped foundation. By comparing with the rigid wall case, the upstream intervention by a finite compliant panel was found to effectively delay the onset of the incipient turbulent spot -- an increase of about 51{\%} in the transition distance with respect to the initiation point was obtained. Transition distance to the occurrence of the incipient turbulent spot was increased further to about 84{\%} relative to a rigid wall when a second compliant panel was introduced. Spectral analysis shows the important role of the fundamental 2D modes in wavepacket evolution and the roles played by compliant panels in transition delay. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L27.00002: Nonlinear intrinsic streaks in the flat plate boundary layer Carlos Martel, Juan A. Martin Luchini [JFM, vol.327, 1996] analyzed the flow near the leading edge of a flat plate boundary layer using the linearized problem around the Blasius solution. He found that there is just one single streaky mode (periodic in the spanwise direction) that grows downstream from the leading edge. The existence of this growing mode indicates that there is a one parameter family of 3D steady streak solutions that emerge from the leading edge of the boundary layer. In this presentation, we will numerically continuate downstream this family of intrinsic streaks (intrinsic because they appear in complete abscense of any free stream perturbations) using the Reduced Navier Stokes formulation, and we will show and comment the characteristics of the resulting Reynolds-number independent, fully nonlinear streaks. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L27.00003: Dynamic mode decomposition of H-type transition to turbulence Taraneh Sayadi, Joseph Nichols, Peter Schmid, Parviz Moin Dynamic mode decomposition (DMD) [1] is applied to a direct numerical simulation database of H-type transition to turbulence of a compressible, nominally-zero-pressure-gradient, spatially developing flat-plate boundary layer. The objective of this work is to identify the structures of dynamical importance throughout the transition region. DMD, viewed as an optimal phase averaging process in the context of the triple decomposition [2], is employed to assess the contribution of each coherent structure to the total Reynolds shear stress. In this region, it is observed that the total Reynolds shear stress gradient can be estimated accurately from only a few low-frequency DMD modes. These low-frequency modes are observed to correspond to the legs of hairpin vortices. Furthermore, DMD is applied to a large-eddy simulation (LES) database of the same configuration, generated using the dynamic Smagorinsky subgrid-scale model. The low-frequency DMD modes extracted from the LES, are, however, of lower amplitude than in the DNS, resulting in an underprediction of the Reynolds shear stress gradient and corresponding skin-friction coefficients.\\[4pt] [1] Schmid, P. J. JFM, 1-24, 2010.\\[0pt] [2] Reynolds, W. C. and Hussain, A. K. M. F. JFM, \textbf{54}, 236-288, 1972. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L27.00004: Self-sustained localized structures in a boundary-layer flow identified by edge tracking Dan Henningson, Yohann Duguet, Philipp Schlatter, Bruno Eckhardt When a boundary layer starts to develop spatially over a flat plate, only disturbances of sufficiently large amplitude survive and trigger turbulence subcritically. Direct numerical simulation of the Blasius boundary-layer flow in a long and wide domain is carried out to track the dynamics in the region of phase space separating transitional from relaminarizing trajectories. In this intermediate regime, the corresponding disturbance is localized both in streamwise and spanwise directions, and spreads slowly in space. This structure is dominated by a robust pair of low-speed streaks, whose convective instabilities spawn hairpin vortices evolving downstream into transient disturbances. In contrast to previous work we find that the hairpin vortices are dynamically insignificant. A quasicyclic mechanism for the generation of offspring is unfolded using dynamical rescaling with the local boundary-layer thickness. The obtained quasi-cyclic character may be interpreted as an approach to an edge state in a spatially developing boundary layer. [PRL 108, 044501, 2012] [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L27.00005: Global stability and receptivity of swept attachment line boundary layer Gianluca Meneghello, Peter Schmid, Patrick Huerre The global stability and receptivity of the incompressible, viscous flow in the leading edge region of a swept wing is examined by solving the eigenvalues/eigenmodes problem associated with the Navier-Stokes operator linearized around a steady state base flow. A branch of eigenvalues is identified, which is associated with eigenvectors displaying a connection between attachment line and crossflow structures. The wavemaker region for these eigenvectors is shown to be close to the attachment line by computing the corresponding solution to the adjoint eigenvalue problem. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L27.00006: Effect of low freestream turbulence on crossflow instabiltiy Mohammad Hosseini, Ardeshir Hanifi, Dan Henningson The effect of freestream turbulence on the generation of crossflow disturbances in swept wings is investigated through direct numerical simulations. The set up follows the experimental set up provided by Hunt \textit{et al}.\footnote{Hunt, L. E. 2011 Boundary-layer receptivity to three-dimensional roughness arrays on a swept-wing. PhD thesis, Texas A\&M University.} in their TAMU experiment. In this experiment the authors use ASU(67)-0315 wing geometry which promotes growth of crossflow disturbances. In this study, we fully reproduce the freestream isotropic homogenous turbulence through a DNS code using detailed freestream spectrum data provided by the experiment. The generated freestream fields are then applied as the inflow boundary condition for direct numerical simulation of the wing. The geometrical set up follows the experiment along with application of distributed roughness elements near the leading edge to precipitate stationary crossflow disturbances. Two different levels of freestream turbulence intensities are produced in order to study their effects on the initial amplitudes of the boundary layer perturbations. Additionally their influence on the transition location is examined. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L27.00007: Linear Stability Analysis of Fully Three-Dimensional Boundary-Layers Wei Liao, Mujeeb Malik, Fei Li, Meelan Choudhari, Chau-Lyan Chang Stability and transition of three-dimensional (3D) finite-span swept-wing boundary layers is of interest here. It is common practice to use quasi-3D boundary-layer codes for generating mean flows for stability analysis of swept-wing flows because of their efficiency and simplicity. However, the use of infinite span approximation or the spanwise conical flow assumption in these codes becomes questionable for fully three-dimensional boundary layers. In this work, the results of stability analysis based on mean flows generated by a quasi-3D boundary layer solver and an unstructured-grid Navier-Stokes solver (FUN3D\footnote{http://fun3d.larc.nasa.gov/}) are compared for a swept wing-glove assembly.\footnote{Belisle et al., AIAA-2012-2667.}$^,$\footnote{Liao et al., AIAA-2012-2690.} The N-factor evolution based on the full-Navier-Stokes computation is shown to differ significantly from that based on the quasi-3D boundary layer codes owing to the un-sweep of the isobars caused by the limited glove span. This points to the need for stability analysis based on Navier-Stokes solutions or possibly fully 3D boundary layer codes when the underlying flow develops strong three-dimensionality. The substantial reduction in maximum N factor (from about 20 to 12 for the case studied here) also indicates the possibility of stabilizing crossflow instability by using fully 3D design methods. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L27.00008: Effect of Surface Imperfections and Excrescences on the Crossflow Instability Matthew Tufts, Glen Duncan, Jr., Brian Crawford, Helen Reed, William Saric Presented is analysis of the planned SWIFTER experiment to be flown on Texas A\&M University's O-2A aircraft. Simultaneous control of the crossflow and streamwise boundary-layer instabilities is a challenge for laminar flow control on swept wings. Solving this problem is an active area of research, with a specific need to quantify the effect of surface imperfections and outer mold line excrescences on crossflow instabilities. The SWIFTER test article is a modification of a prior-tested flight model, with the additional capability of creating controlled excrescences in flight. Using a finite-element Navier-Stokes solution and a spectrally accurate boundary-layer solver, coupled with linear and nonlinear stability analyses, we show that the flow field over the test article is well suited to this study. Results are compared with flight data. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L27.00009: Computations of Crossflow Transition in Supersonic Swept Wing Boundary Layers Lian Duan, Meelan Choudhari, Fei Li, Minwei Wu A common cause for transition over swept wing configurations of a supersonic aircraft is the crossflow instability of the three-dimensional boundary layer flow. This study seeks to analyze the nonlinear stages of transition due to crossflow instability, with the eventual goals of enabling efficient yet accurate predictive models and, potentially, obtaining clues for better control of the transition process. To achieve these goals, direct numerical simulations are performed to examine the laminar breakdown process in a supersonic swept airfoil boundary layer. Different mechanisms of transition are studied with an emphasis on the breakdown initiated by the high-frequency secondary instability of stationary crossflow modes. The secondary instability is introduced via inflow forcing derived from a two-dimensional, partial-differential-equation based eigenvalue computation. The simulation tracks the linear and nonlinear growth of the secondary instability wave, the resulting onset of laminar-turbulent transition, and the fully turbulent flow downstream. As the secondary instability grows, small rib-like structures develop on top of the tubular structure. These structures are aligned at an oblique angle to the axis of the crossflow vortex, and are similar to the rib-like structures observed in previously reported computations of secondary instability in incompressible flows. Farther downstream, even smaller structures emerge and the laminar breakdown process ensues. [Preview Abstract] |
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