Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session GT: Boundary Layer Instabilities II |
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Chair: Joseph Liu, Brown University Room: Hilton Chicago Stevens 5 |
Monday, November 21, 2005 10:34AM - 10:47AM |
GT.00001: Flight experiments on laminar flow control in swept-wing boundary layers William Saric, Andrew Carpenter, Celine Kluzek, Lauren Hunt, Christopher McKnight, Shane Schouten Recent experiments have shown that the interaction of freestream turbulence and surface roughness not only strongly influences both the location and 3-D aspects of boundary-layer transition, but has a sensitivity to spanwise distribution and roughness Reynolds number heretofore unknown. This work then concentrates on conducting boundary-layer transition control within a low-disturbance flight environment. A swept-wing model is hung from the wing of a Cessna O-2 and flown at chord Reynolds numbers up to 7.5 million. The laminarization scheme of spanwise-periodic discrete roughness elements (DRE) elements is investigated. We use a combination of hotfilm anemometry and infra-red thermography to document the transition delay due to the roughness. The hotfilm measurements give the important passband and spanwise scales while the thermography gives transition location. It is shown that DRE can effectively delay transition from the smooth-surface reference case. [Preview Abstract] |
Monday, November 21, 2005 10:47AM - 11:00AM |
GT.00002: Transition delay by means of a passive mechanism Jens H.M. Fransson, Alessandro Talamelli, Luca Brandt, Carlo Cossu Reducing the skin friction is important in nature and in many technological applications when larger speeds or lower energy consumptions are sought for. This reduction may be achieved by reducing stresses in turbulent boundary layers for instance tailoring biomimetic rough skins. Here we take a second approach consisting in keeping the boundary layer laminar as long as possible$\footnote{Joslin, R. D. 1998 {\it{Annu. Rev. Fluid Mech.}} {\bf{30}}, 1--29}$. We report the results of wind-tunnel experiments, motivated by previous theoretical analyses$\footnote{Cossu \& Brandt 2002 {\it{Phys. Fluids}} {\bf{14}}, L57--L60.}$$^{,}$$\footnote{Cossu \& Brandt 2004 {\it{Eur. J. Mech./B Fluids}} {\bf{23}}, 815--833.}$ in which a well controlled spanwise periodic modulation of the boundary layer thickness is induced by using suitably designed$\footnote{Fransson, Brandt, Talamelli \& Cossu 2004 {\it{Phys. Fluids}} {\bf{16}}, 3627--3638.}$$^{,}$$\footnote{Fransson, Brandt, Talamelli \& Cossu 2005 {\it{Phys. Fluids}} {\bf{17}}, 054110.}$ roughness elements placed on the skin. We show, both with smoke visualization and measurements, that using this passive control technique it is possible to sensibly delay transition to turbulence. [Preview Abstract] |
Monday, November 21, 2005 11:00AM - 11:13AM |
GT.00003: Receptivity of Tollmien--Schlichting Waves to Acoustic Disturbances and Combined Two- and Three-Dimensional Surface Roughness James E. Kless, Edward B. White Recent computational studies have demonstrated that transient disturbances such as those generated by three-dimensional (3D) roughness elements can suppress the growth of unsteady Tollmien--Schlichting-like (TS) disturbances. This finding suggests that deliberately introduced 3D roughness might be used for passive transition control. Laboratory studies on the effectiveness of this technique have included TS waves produced by a two-dimensional (2D) source near the waves' first neutral point and transient disturbances produced by spanwise arrays of 3D roughness elements. To date, these studies have been inconclusive because the introduction of 3D roughness into situations where TS waves exist changes the TS waves' initial amplitudes. The present experimental study explores the origin of the changed initial amplitudes using TS waves produced by freestream acoustic waves and a 2D roughness strip. The key objective is to determine whether the modified initial amplitudes that are observed when 3D roughness is introduced result from superposed TS-like waves that originate at the 2D roughness strip and also the 3D roughness array or whether the spanwise-varying steady flow generated by the 3D roughness modifies the receptivity characteristics of the 2D source in a more fundamental manner. [Preview Abstract] |
Monday, November 21, 2005 11:13AM - 11:26AM |
GT.00004: Vortical waves incident on junctions between different wave-bearing media Peter Carpenter, Pradeep Sen, Shriram Hegde, Christopher Davies We present a solution to a receptivity-type problem whereby a TS wave or other vortical wave propagating along a plane Poiseuille flow is incident on a junction between a rigid and compliant wall. Depending on whether the upstream wall is rigid or compliant there can be both transmitted and reflected waves. We show that the incident wave creates a virtual wave-driver. The problem then reduces to determining the strength of the driver in terms of the incident wave amplitude from which the amplitudes of the reflected and transmitted waves can then be determined. A high-Reynolds-number asymptotic theory combined with the use of adjoint eigenmodes is developed to solve this problem. We believe that the main features of this theory are generic for all vortical waves. [Preview Abstract] |
Monday, November 21, 2005 11:26AM - 11:39AM |
GT.00005: DNS and the theory of receptivity in a hypersonic boundary layer Anatoli Tumin, Xiaowen Wang, Xiaolin Zhong Direct numerical simulation of receptivity in a boundary layer over a sharp wedge of half-angle of 5.3 degrees was carried out with perturbations introduced into the flow by periodic-in-time blowing-suction through a slot. The free stream Mach number is equal to 8. The perturbation flow field at a small distance from the slot was decomposed into normal modes with the help of the multimode decomposition technique based on the spatial biorthogonal eigenfunction system. Filtered-out amplitudes of two discrete normal modes of interest are compared with the linear receptivity problem solution. The results illustrate how the multimode decomposition technique may serve as an efficient tool for validation of DNS results and for gaining insight into the flow dynamics. [Preview Abstract] |
Monday, November 21, 2005 11:39AM - 11:52AM |
GT.00006: Disturbance growth in boundary layer subjected to weak anisotropic free stream turbulence. Toshiaki Kenchi, Masaharu Matsubara The present experiment focused on revealing effect of scales and directional components of free stream turbulence on flat plate boundary layer transition, especially the growth and structure of the disturbances energy in the streamwise direction and transition process. Free stream turbulence generated by a turbulence grid mounted upstream of a contraction, is weak axisymmetric turbulence with strong anisotropy. The experimental results with the anisotropic free stream turbulence show existence of the non-modal growth disturbance even at 0.7 {\%} turbulence intensity. Downstream variation of distribution of the streamwise fluctuation and flow visualization suggest new transition scenario which is mingled non-modal growth process and emergence of wave packets. [Preview Abstract] |
Monday, November 21, 2005 11:52AM - 12:05PM |
GT.00007: Generation of boundary-layer disturbances by freestream forcing Karen Kudar, Peter Carpenter, Christopher Davies We present an investigation based on simplified DNS of the generation of boundary layer disturbances by free-stream vorticity in Falkner-Skan boundary layers. The free-stream vorticity is generated by a source of the form $\delta (x)\delta (z-z_{f})$exp(\textit{i$\beta $y}) derived from a body force, where ($x$,$y$,$z)$ are the streamwise, spanwise and wall-normal co-ordinates, $z_{f }$is located above the boundary layer and near its edge, and \textit{$\beta $} is the spanwise wave-number. Both streamwise and spanwise vorticity and stationary and oscillating sources are used. A steady streamwise vorticity source creates a diffused and corrugated sheet of streamwise vorticity that drives the boundary layer with a wall-normal velocity, thereby generating streak-like structures within the boundary layer. Oscillating the source acts to generates two different structures; one similar to a Klebanoff mode and the other, depending on the frequency of oscillation, either grows exponentially or decays and cannot be seen. [Preview Abstract] |
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