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 L23: Turbulent Boundary Layers IX: Rough / Wavy / Bent Walls |
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Chair: Jonathan Naughton, University of Wyoming Room: 30D |
Monday, November 19, 2012 3:35PM - 3:48PM |
L23.00001: Index-Matched PIV Measurements of Turbulence inside a Fractal-Tree Canopy Kunlun Bai, Joseph Katz, Charles Meneveau Turbulence inside vegetation canopies has a significant impact on various physical and biological processes such as forest microclimate, rainfall evaporation distribution and climate change. In most scaled laboratory experimental studies, the canopy element models, for example vertical strips or rods, typically have only one or a few characteristic length scales. However, natural canopies usually contain multiple scales with branches and sub-branches. In this study, a model canopy is constructed by twelve fractal-like trees. Each tree contains five generations with three branches and a scale reduction factor 1/2 at each generation and fractal similarity dimension of Df $\sim$ 1.58. In order to capture the flow fields inside the trees and between the branches, an index-matching technique is applied. Two trees are made by clear urethane plastic with refractive index about 1.49. The solution running in the facility is carefully prepared by mixing Sodium-Iodine in distilled water to match the refractive index of the urethane-plastic trees. In this talk, experiments will be discussed in detail and measured velocity and turbulence statistics inside and above the canopy will be presented and discussed. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L23.00002: On the routes to inertial mean dynamics in smooth- and rough-wall turbulent boundary layers Joseph Klewicki, Faraz Mehdi Connections between the structure of smooth- and rough-wall turbulent boundary layers are established within the context of the order of magnitude properties exhibited by the terms in the mean momentum equation. These properties are shown to be associated with the processes by which inertial mean dynamics emerge with distance from the wall. A key element is the process by which the vorticity field becomes three-dimensional. In the smooth-wall case, vorticity stretching leads to the three-dimensionalization of the vorticity field in the region where the mean viscous force retains leading order. This underlies the well-established Reynolds number scaling behaviors exhibited by smooth-wall flows. Roughness modifies (generally augments) the process by which the vorticity field becomes three dimensional, rendering scalings for the route to inertial mean dynamics that depend on the relative scale separations between the inner, roughness, and outer scales. Evidence (from existing and recent experiments) of these combined scaling regimes is presented. The present analyses provide a basis for predicting where and physically why Townsend's similarity hypothesis should hold, as well as under what conditions outer similarity loses validity. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L23.00003: Investigation of Turbulence Modification by Momentum Injection Into Turbulent Flow Over a Rough Surface Mark Miller, Alexandre Martin, Sean Bailey We present an experimental study conducted in a turbulent channel flow wind tunnel to determine the modifications made to the turbulent velocity spectrum by a sinusoidally rough, micro-perforated surface, both with and without flow injection through the surface. Preliminary results at moderate Reynolds numbers demonstrate that Townsend's hypotheseis is approximately valid at low momentum injection ratios. Whereas the magnitude and location of a near-wall peak in turbulence intensity remains largely unaffected by increasing flow injection, turbulence intensity in the logarithmic and outer layers increases and Townsend's hypothesis becomes invalid. Spectral analysis indicates that this increase in turbulence intensity reflects significant modifications made to the turbulence structure within these layers, even for very small injected momentum ratios. At high blowing rates, although the signature of very long wavelength motions persisted, the largest proportion of turbulent kinetic energy in the outer layer was found to be increasingly contained within turbulent scales corresponding to the thickness of the wall layer. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L23.00004: Coherent structures and associated sub-grid scale energy transfer in a rough-wall turbulent channel flow Jiarong Hong, Joseph Katz, Charles Meneveau, Michael Schultz The turbulence structure in a rough-wall channel flow and its role in subgrid scale (SGS) energy transfer are studied utilizing PIV data obtained in an optically index-matched facility. In streamwise-wall-normal ($x-y)$ planes, the averaged flow structure conditioned on high SGS flux exhibits a large inclined shear layer containing negative vorticity, bounded by an ejection below and a sweep above. The peaks of SGS flux and kinetic energy are spatially displaced from region of high-resolved TKE. In wall-parallel $x-z $planes, the conditional flow exhibits two pairs of counter-rotating vortices that induce a contracting flow at the SGS flux peak. Instantaneous realizations in the roughness sublayer confirm the presence of these vortex pairs at the intersection of two vortex trains. In the outer layer, the SGS flux peaks within isolated vortex trains that retain the roughness signature. To explain the planar signatures, we propose a flow consisting of U-shaped quasi-streamwise vortices that develop as spanwise vorticity is stretched in regions of high streamwise velocity between roughness elements. Flow induced by legs of adjacent U-shaped structures causes powerful ejections, which lift these vortices away from the wall. As a sweep is transported downstream, its interaction with the roughness generates a series of such events, leading to the formation of inclined vortex trains. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L23.00005: Effect of Immersed Wall-Bounded Cylinders on Turbulent Boundary Layer Structure Shaokai Zheng, Ellen Longmire, Michael Hallberg, Mitchell Ryan Single spanwise arrays of wall-mounted cylinders with H/$\delta \quad \le $ 0.2, where H is the cylinder height and $\delta $ is the boundary layer thickness, were used to modify turbulent boundary layers (Re$_{\tau }$=2500) in an attempt to affect the organization of the coherent structures in the logarithmic and outer regions. Flow downstream of several array spacings was investigated and compared against an unperturbed case. Instantaneous and averaged velocity fields in streamwise-spanwise planes were obtained by stereo PIV. The PIV cameras and laser sheet optics could be traversed at the local mean flow speed in order to track the evolution of larger structures in the flow. The results are analyzed to determine the streamwise evolution of dominant spanwise modes. Different array spacings are shown to either inhibit or reinforce the organization of vortex packet structures over streamwise distances up to 8$\delta $. The flying stereo PIV measurements suggest also that dominant structures upstream of the arrays can strongly affect the organization and location of structures downstream. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L23.00006: Experimental test of the spectral analogue of the law of the wall in rough-pipe flows Carlo Zuniga Zamalloa, Gustavo Gioia, Pinaki Chakraborty We extend the recently proposed spectral analogue of Prandtl's law of the wall to obtain a scaling relation for the turbulent energy spectra in rough-pipe flows. To test this scaling relation we measure the streamwise component of the turbulent energy spectrum on numerous locations along the radii of three rough-walled pipes, for flows spanning a decade in Reynolds number. Our results are in excellent accord with the scaling relation. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L23.00007: Measurement of turbulent flow upstream and downstream of a circular pipe bend Jun Sakakibara, Nobuteru Machida We measured velocity distribution in cross sections of a fully developed turbulent pipe flow upstream and downstream of a 90-degree bend by synchronizing two sets of a particle image velocimetry (PIV) system. Unsteady undulation of Dean vortices formed downstream from the bend was characterized by the azimuthal position of the stagnation point found on the inner and outer sides of the bend. Linear stochastic estimation (LSE) was applied to capture the upstream flow field conditioned by the azimuthal location of the stagnation point downstream from the bend. When the inner-side stagnation point stayed below (above) the symmetry plane, the conditional streamwise velocity upstream from the bend exhibited high-speed streaks extended in a quasi-streamwise direction on the outer side of the curvature above (below) the symmetry plane. Similarity of the estimated structure and the very large scale motion (VLSM) will be presented. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L23.00008: Evolution of turbulence characteristics from straight to curved pipes George K. El Khoury, Azad Noorani, Philipp Schlatter, Paul F. Fischer Large-scale direct numerical simulations are performed to study turbulent flow in straight and bent pipes at four different Reynolds numbers: $Re_b=5300$, 11700 (bent and straight) and 19000 and 37700 (only straight). We consider a pipe of radius $R$ and axial length $25R$ with curvature parameter $\kappa$ taken to be $0, 0.01$ and $0.1$ for zero, mild and strong curvatures, respectively. The code used is Nek5000 based on the spectral element method. In the straight configuration, the obtained DNS data is carefully checked against other recent simulations, highlighting minute differences between the available data. Owing to a centrifugal instability mechanism, the flow in bent pipe $(\kappa \neq 0)$ develops counter-rotating vortices, so-called Dean vortices. The presence of the secondary motion thus induces substantial asymmetries both in the mean flow and turbulence characteristics for the bent pipe. These asymmetries tend to damp turbulence along the inner side and correspondingly enhance it along the upper side. The results are validated with recent experiments, and we could confirm the peculiar behaviour of the friction factor for specific curvatures and $Re$, leading to a lower friction in curved pipes than in straight pipes for the same mass flux. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L23.00009: Secondary motions induced by a $90^{\circ}$ bend in turbulent pipe flow Leo Hellstr\"om, Alexander Smits Continuous stereoscopic particle image velocimetry (SPIV) was used to investigate the temporal behavior of curvature induced motions downstream a $90^{\circ}$ bend in fully developed turbulent pipe flow. The velocity field was fully resolved in time for Reynolds numbers ranging from $1.3\times 10^4$ to $3.6\times 10^4$. Snapshot Proper Orthogonal Decomposition was performed on the data to extract the most energetic modes in the flow, which are believed to correctly identify the curvature induced secondary motions. These motions appear to be governed by a small number of highly energetic modes, active at different times. These modes may be used to reconstruct the flow, filtering the smaller structures, showing a ``swirl switching'' behavior, first noted by Tunstall \& Harvey (1968). [Preview Abstract] |
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