Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session E26: Turbulent Boundary Layers III |
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Chair: Ivan Marusic, University of Melbourne Room: 2007 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E26.00001: Prediction of Frictional Drag over Rough Walls using Surface Statistics Karen Flack, Michael Schultz Although the frictional drag of rough-wall-bounded flows has been studied extensively, several practical questions remain largely unresolved. First, the relationship between the shape of the roughness function in transitionally-rough regime and the surface topography which gives rise to it are not well understood. Second, it is not completely clear which textural parameters best describe a rough surface in a hydraulic sense. Furthermore, the range of roughness wavelengths that influence the skin-friction is not well established. The focus of the present work is to attempt to address these questions with a systematic study of the skin-friction of fifteen rough surfaces that were generated by grit blasting. The hydrodynamic tests were carried out over a large Reynolds number range. Five surfaces were prepared by grit blasting with a single scale blast media. These underwent hydrodynamic testing and were subsequently blasted with secondary and tertiary scale media in order to investigate the role that the incorporation of additional roughness length scales plays in determining the shape of the roughness function and the resulting hydraulic length scale. The presentation will focus on the appropriate statistical scales for prediction of the roughness function. Spatial filtering prior to the calculation of surface statistics will also be discussed. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E26.00002: Effects of Sudden Change in Surface Roughness on Turbulent Boundary Layers Ronald Hanson, Bharathram Ganapathisubramani In almost all practical applications the Reynolds number of the turbulent boundary layer is high and the separation between the inner and outer layer scales become appreciable. Interaction between these scales has profound consequences for the control of turbulent wall flows. In this experimental study we consider the response of the turbulent boundary layer occurring over a surface which transitions from a rough to smooth boundary condition. The transition in surface condition leads to the formation of an internal layer. Above this layer the flow is characteristic of the upstream condition. Within the internal layer the near-wall turbulence establishes itself under the influence of the outer region that remains from the incoming rough-wall. We examine the interaction between the newly established near-wall region and the outer region that persists from upstream conditions. Single and two-component hot-wire measurements are performed simultaneously; the single wire is used to improve the spatial resolution of the measurement of $u$ and to acquire near-wall data. Wide-field Particle Image Velocimetry measurements capture the entire development of the boundary layer over the smooth wall downstream of the roughness. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E26.00003: Reorganisation of the large-scale structures in turbulent boundary layers using highly ordered and directional surface roughness - Kevin, Bagus Nugroho, Gokul Pathikonda, Julio Barros, Kenneth Christensen, Jason Monty, Nicholas Hutchins The potential of riblet-type surface roughness with converging-diverging (herring-bone type) arrangements to reorganise the large-scale coherent structures that populate the logarithmic region of turbulent boundary layers is investigated at moderate Reynolds number. The ability of this transitionally rough surface to generate large-scale counter rotating roll-modes suggests that a preferential arrangement of the naturally occurring large-scale structures may have been introduced. Prior analysis of the pre-multiplied energy spectra of streamwise velocity fluctuation indicates an increase (or decrease) in the large-scale streamwise turbulence energy over the converging region (or diverging) of the riblets. In this study we examine this possible spanwise redistribution of the coherent structures using instantaneous planar Particle Image Velocimetry (PIV) in the wall-parallel plane (within the logarithmic region) as well as cross-plane Stereoscopic PIV. The characteristics of the large-scale structure over the converging-diverging surface are compared with those of the corresponding smooth-wall case, revealing pronounced modification of the size, strength and alignment of these features over the directional surface. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E26.00004: Wind Tunnel Simulation of the Atmospheric Boundary Layer Tristen Hohman, Tyler Van Buren, Alexander Smits, Luigi Martinelli We aim to generate an artificially thickened boundary layer in the wind tunnel with properties similar to the neutral atmospheric boundary layer (ABL). We implement a variant of Counihan's technique which uses a combination of a castellated barrier, elliptical vortex generators, and floor roughness elements to create a thick boundary layer in a relatively short wind tunnel. We demonstrate an improved spanwise uniformity than in Counihan's original design by using a tighter vortex generator spacing with a smaller wedge angle to keep frontal area approximately constant. This is achieved while keeping the turbulence intensity and power spectral density unchanged. It was found possible to generate a boundary layer at $Re_{\theta} \sim 10^{6}$, displaying logarithmic mean velocity behavior, a constant stress region, and turbulence intensities that compare favorably with full scale ABL measurements and laboratory rough-wall boundary layers. In addition, the longitudinal power spectral density agrees well with von K\'arm\'an's model spectrum and the integral length scale agrees well with data from ABL measurements. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E26.00005: Statistical structure of spanwise vorticity in high Reynolds number rough-wall turbulent boundary layers Caleb Morrill-Winter, Joseph Klewicki, Ivan Marusic A defining characteristic of boundary layers is the presence of vorticity. Within the 2-D turbulent boundary layer the only component of vorticity to have a non-negligible mean value is the spanwise component, $\omega_z$. In the present experiments, a compact four element (``Foss-style'') hotwire probe was used to acquire well-resolved $\omega_z$ fluctuations over the range, $3,000 \le \delta^+ = \delta u_\tau/\nu \le 20,000$ for 36 grit sandpaper roughness. Over the entire Reynolds number range good spatial resolution was maintained by utilizing the low speed, large scale attributes of the HRNBLWT at the University of Melbourne. The present talk addresses the statistical structure of $\omega_z$ above a rough wall including comparisons with its smooth wall counterpart. The observed low Reynolds number smooth wall self-similarity between the mean and the rms profiles of $\omega_z$ is clarified for the rough-wall case. The rough wall $\omega_z$ behavior is described in a context consistent with the mean momentum equation. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E26.00006: Cylinder array height effects on evolution of tracked vortex packets within a turbulent boundary layer Yan Ming Tan, Ellen Longmire A zero pressure gradient turbulent boundary layer with $Re_{\tau } =2480$ was perturbed by a spanwise array of cylinders. When a narrowly spaced array extended to the top of the log region, perturbed packets appeared to reorganize via a top-down mechanism, suggesting that packet organization can originate from above. We test this hypothesis by extending the array height to the edge of the boundary layer to completely disrupt the packet organization. On the other hand, previous measurements showed that the downstream packet organization was reinforced by an array spacing matching the dominant spanwise spacing of unperturbed packets. A shorter array with reduced blockage was tested to see whether the same effect is achievable. To compare the flow organization in the different cases, fixed and flying PIV measurements were obtained in streamwise-spanwise planes at multiple wall normal locations. The flying PIV system allows tracking and quantification of packet evolution through the array and over a distance of $7\delta $ downstream. [Preview Abstract] |
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