Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session GL: Turbulent Boundary Layers II* |
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Chair: Thomas Lundgren, University of Minnesota Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 8 |
Monday, November 20, 2006 10:30AM - 10:43AM |
GL.00001: Instability of a Local Downflow in a Turbulent Boundary Layer Thomas Lundgren In the laminar Hiemenz stagnation point flow a downflow produces a straining flow along the boundary. This flow is unstable (Hammerlin,1955) with neutral eigensolution consisting of alternating sign vortex cells of uniform strength along the flow direction. Low-speed and high-speed streaks at the cell boundaries increase in amplitude in the flow direction. In the turbulent problem a local downflow is envisioned to be caused by large scale structures in the outer part of the boundary layer. The Reynolds-averaged equations were employed with an eddy viscosity which depens only on the distance from the wall. The resulting equations are unstable to longitudinal vortices with a structure similar to the Hiemenz neutral egensolutin, with a continuous spectrum of the spanwise wavenumber. It is found that the wavenumber must be smaller than a critical value which depends on the local strainrate. In general when the downflow is weaker the critical wavenuber is smaller (streak spacing larger). [Preview Abstract] |
Monday, November 20, 2006 10:43AM - 10:56AM |
GL.00002: On the outskirts of a turbulent spot in plane Couette flow Maher Lagha, Paul Manneville Transition to turbulence in wall flows is an important problem that we approach here by simulating a partial differential Galerkin model of plane Couette flow [1] and focussing on the growth of turbulent spots. Outside a spot, numerical results show the development of large scale flow, spanning the whole gap, characterized by streamwise inflow and spanwise outflow, giving it a quadrupolar shape. The spot is associated to a region where the streamwise velocity correction is dominantly opposed to the base flow, as seen by filtering out small-scale fluctuations. The origin of the quadrupole-like flow is traced back to the shearing of this negative correction by the base flow, whereas the correction itself is generated by the appropriate component of the local average of the small-scale Reynolds stresses (streaks and streamwise vortices). The role of this large scale flow on the spot spreading mechanism is discussed. \newline \newline [1] M.Lagha, P.Manneville, ``Modeling plane Couette flow,'' Eur. Phys. J. B submitted. [Preview Abstract] |
Monday, November 20, 2006 10:56AM - 11:09AM |
GL.00003: The Turbulence Structure of Compressible Boundary Layers using Pattern Recognition and Tracking Algorithms in DNS data G.C. Richdale, M. Ringuette, M.P. Martin, D. Silver We use a direct numerical simulation database (Martin, JFM 2006) of compressible turbulent boundary layers to develop tracking algorithms for the turbulence structures. We use the pattern recognition and tracking algorithm of Wang and Silver (IEEE Transactions on Visualization and Computer Graphics, 1997) and incorporate the criteria used by Ringuette et al. (AIAA 2006-3539) to identify and characterize turbulence structures individually and in packets. In turn, the evolution of small and large coherent turbulence structures in space and time is visually monitored and the effects of Mach number and local compressibility are studied. [Preview Abstract] |
Monday, November 20, 2006 11:09AM - 11:22AM |
GL.00004: Variation in the Turbulence Structure of Supersonic Boundary Layers with Mach Number using DNS Data Matthew Ringuette, M. Pino Martin, Alexander Smits, Minwei Wu We utilize a direct numerical simulation database to characterize the properties of coherent structures in supersonic turbulent boundary layers at Mach numbers from 3 to 5. Tools to calculate the average structure angle, convection velocity, and length scale are developed, and the results show good agreement with the available experimental data. We find that the structure angle and convection velocity increase with higher Mach number, while the streamwise integral length scale decreases. The structures are taller with increasing Mach number, consistent with the trend in structure angle. The spacing of the wall streaks is slightly narrower and more uniform with increasing Mach number. We observe hairpin vortices clustered into streamwise packets at all Mach numbers, and create an algorithm that identifies and characterizes these hairpin packets. The average packet convection velocity, length, and number of hairpins increase with higher Mach number, while the packet height and angle decrease. [Preview Abstract] |
Monday, November 20, 2006 11:22AM - 11:35AM |
GL.00005: Analysis of turbulent structures in supersonic wall-bounded flows Sergio Pirozzoli, Francesco Grasso The statistical properties of compressible wall-bounded flows are analyzed by means of direct numerical simulations of a spatially evolving boundary layer on an adiabatic flat plate at $M_{\infty}=2.25$, with the objective to characterize the turbulent structures in terms of flow visualizations as well as by quantitative analysis of the joint PDF of the velocity gradient tensor invariants. Several vortex identification criteria have been considered, including the $\Delta$ criterion, the Q-criterion, and the $\lambda_2$ criterion. Preliminary results indicate that hairpin-like structures, universally observed in low-speed wall bounded flows, also dominate in the (low) supersonic regime, and shocklets are virtually absent under the selected test conditions. Such scenario is consistent with the commonly accepted notion that compressibility effects in wall bounded flows are negligible up to very large main flow Mach numbers. The analysis of the joint PDF of the velocity gradient tensor invariants at several stations in the wall-normal direction (viscous sublayer, buffer layer, log region, outer region) seems to confirm the findings of Chong et al. (JFM 98) and Pirozzoli \& Grasso (PF 04): the joint PDF in the Q-R plane exhibits (in large part of the boundary layer) a universal, tear-drop shape, and the vorticity is preferentially aligned with the intermediate eigenvector of the (traceless part of the) strain- rate tensor, which accounts for the vortex stretching mechanism. [Preview Abstract] |
Monday, November 20, 2006 11:35AM - 11:48AM |
GL.00006: Turbulent structure above wind-sheared air-water interface Kamran Siddiqui, Nasir Uddin The flow dynamics immediately above and below the air-water interface in the presence of wind and waves is very challenging. The understanding of flow dynamics in near-interfacial regions is vital to improve our knowledge of the physical mechanisms responsible for the heat, mass and momentum transport across the interface. The measurement of airside velocity field close to the fluctuating water surface in the presence of waves is very challenging. The difficulty arises in the region between the wave crest and trough, where any particular spatial location lies sometimes in water and sometimes in air, which is the main obstacle in using point measurement techniques in this region. We report on a series of laboratory experiments conducted to investigate the airflow structure above the wavy water surface using PIV. The results have shown that the flow characteristics in the crest-trough region above the waves are significantly different from that at greater heights. The results showing the turbulent structure in this region will be presented and discussed. [Preview Abstract] |
Monday, November 20, 2006 11:48AM - 12:01PM |
GL.00007: Streamwise Reynolds Stress Partitioned into Active and Inactive Correlations. Ronald Panton The streamwise Reynolds stress \textit{$<$uu$>$ } in wall layers has a peak near the wall that is an order of magnitude higher that the shear stress <$u$v>. Townsend attributed this to an ``inactive'' swirling motion $u-w$ that does not contribute to the shear stress. The peak continues to increase with Reynolds number. Degraaff and Eaton ( JFM, \textbf{422, }p 319 ) and Metzger and Klewicki ( P of F, \textbf{13}, p 692 ) have essentially shown that the peak increases as \textit{$<$uu$>$}$_{MAX}/ u_{\ast }^{2}$\textit{ $\sim $ U}$_{0}/u_{\ast . }$ With these facts in mind, it is proposed that the inactive motion $u_{I}$ scales with ($U_{0}u_{\ast })^{1/2}$ , and the active motion $u_{A}$ scales with $u_{\ast }$. With these ideas, an asymptotic expansion for the streamwise stress consists of three terms with gauge functions 1, $(u_{\ast }/U_{0}$\textit{(Re*))}$^{1/2},$ and$ u_{\ast }/U_{0}$\textit{(Re*)}: \textit{$<$uu$>$/(u}$_{\ast }U_{0}$\textit{) $\sim $ f}$_{0}(y)+ f_{1}(y)(u_{\ast }/U_{0})^{1/2}+ f_{2}(y)(u_{\ast }/U_{0}). ^{ }$ The terms,$ f_{0}$\textit{(y) {\&} f}$_{2}(y),$represent the autocorrelations of the inactive and active motions respectively, while the$ f_{2}(y)$ term represents the cross-correlation of those motions. This form appears to be valid for both inner and outer regions. Data for channel flow was analyzed by making crude approximations for $ f_{1}$\textit{(y) {\&} f}$_{2}(y) $and solving for $f_{0}(y). $Equations fitted to $f_{0}(y)$ allow a composite expansion to predict the streamwise Reynolds stress as a function of $y$ and \textit{Re}$_{\ast }$. [Preview Abstract] |
Monday, November 20, 2006 12:01PM - 12:14PM |
GL.00008: The velocity field in the cavity between two square two-dimensional obstacles at the wall of a turbulent boundary layer. S. Simoens, J. Wallace Measurements have been made of the scalar dispersion of smoke released from a two-dimensional slot in the wall perpendicular to a boundary layer flow with $R_{\theta} \approx 980$ and located parallel to and midway between two square obstacles placed on the wall. Two optical systems with CCD cameras facing each other have been used to measure simultaneously the velocity and scalar concentration fields, respectively, with PIV and Mie scattering diffusion to ultimately provide detailed information about the scalar fluxes for this environmentally relevant geometry. Here the results of the velocity field measurements in the streamwise plane will be reported for spacings between the obstacles of 1-8 obstacle heights. The mean flow measurements reveal the increased complexity of the canyon flow with increasing obstacle spacing. A primary vortex, with negative spanwise vorticity, occurs for all spacings, and its circulation region extends above the level of the tops of the obstacles. For spacings of $2h$ and greater, a secondary vortex with positive vorticity appears in the upstream corner of the canyon, and a tertiary vortex with negative spanwise vorticity begins to appear in the downstream corner for a $6h$ spacing. Spatial distributions of the turbulent kinetic energy and Reynolds shear stress fields will also be presented. [Preview Abstract] |
Monday, November 20, 2006 12:14PM - 12:27PM |
GL.00009: Velocity Measurements of a Cylindrical Turbulent Boundary Layer in a Submarine Wake Deborah Furey, Paisan Atsavapranee, Kimberly Cipolla, Damien Bretall High resolution stereo-PIV measurements were made on a long, small diameter cylinder towed from the control surface of a 1/18$^{th}$ scale submarine model. The experiments were performed in the tow tank at NSWCCD at 5 kts. 3-D velocity fields over ten body lengths downstream were obtained. The cylinders were approximately neutrally buoyant and towed through a stationary laser sheet oriented perpendicular to the tow direction. The objective of the study was to quantify the effect of the flow behind the submarine control surfaces and propeller on the boundary layer development and dynamics of a model towed array where $\delta >>$ the cylinder radius, a. Analysis required stepwise tracking of the array through the dissipating wake field and isolation of the contribution from the cylindrical boundary layer. Approximately 40 instantaneous vector fields were obtained for each location. Mean and fluctuating streamwise and cross-stream velocities will be presented. Initial results indicate that the contribution of the wake to the total flow field and its effect on the boundary layer development varies in the streamwise direction. [Preview Abstract] |
Monday, November 20, 2006 12:27PM - 12:40PM |
GL.00010: Thoughts on bypass ``transition'' Donald M. McEligot One may consider the situation when a plate is immersed in a fully-turbulent jet. If there is no angle-of-attack, one has effectively a turbulent wall jet with the leading edge region in the near field. This situation describes many experiments examining so-called ``bypass transition'' -- which perhaps could be treated and interpreted in this light. At the surface, turbulent fluctuations become zero thanks to the no-slip condition while the free stream is completely turbulent -- in contrast to the steady laminar free stream of ``natural'' transition. In the limit of ``high'' free stream turbulence levels, the situation described is probably not one of conventional laminar-to-turbulent transition but rather one of suppression of turbulent flow approaching a surface. Is there really a laminar boundary layer in the near field or just the viscous layer of a turbulent wall flow? In some ways it may be comparable to ``laminarization'' of a turbulent boundary layer or duct flow. The study reported explores similarities between observations of bypass transition and those of the viscous layer of a turbulent wall flow. [Preview Abstract] |
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