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 L20: Turbulent Boundary Layers VII: Rough Walls I |
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Chair: Kenneth Christensen, University of Illinois at Urbana-Champaign Room: 30A |
Monday, November 19, 2012 3:35PM - 3:48PM |
L20.00001: Structural Aspects of Flow Over Highly Irregular Roughness Revealed from Wall-Normal--Spanwise Plane Stereo PIV Measurements J.M. Barros, K.T. Christensen The structural attributes of turbulent flow over a complex roughness topography were explored with both low-frame-rate and time-resolved stereo particle-image velocimetry in a wall-normal--spanwise ($y-z$) measurement plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topographical scales arranged in a highly irregular manner. Instantaneous velocity fields in the cross-flow measurement plane revealed structural attributes qualitatively consistent with smooth-wall flow structure, particularly patterns of spanwise-alternating, large-scale regions of low and high streamwise momentum. However, single-point turbulence statistics revealed significant statistical heterogeneity in the form of low- and high-momentum flow pathways marked by enhanced Reynolds stresses and turbulent kinetic energy. The low-momentum flow pathways were also marked by intense vortical activity along their spanwise boundaries, indicating that these pathways could represent preferential ``channeling'' of large-scale motions due to the roughness below or the generation of ``trains'' of vortical structures shed from the roughness that advect along a common path downstream. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L20.00002: Roughness effects in turbulent channel flow for the transitionally rough regime Karen Flack, Michael Schultz Skin friction coefficients and roughness functions are presented for sandpaper roughness (220, 320 and 500 grit) throughout the transitionally rough regime from hydraulically smooth to fully rough. The experiments were performed in a 8:1 aspect ratio water channel with a Reynolds number based on channel height range of 12,000 - 280,000. The wall shear stress was determined from the pressure drop in the channel. Mean flow and turbulence statistics were also obtained for the 320 grit sandpaper using a two-component LDV at Karman numbers ranging from \textit{Re}$_{\tau }$=1000 - 6000. The mean flow and turbulence data are compared to smooth wall channel results from experiments and DNS at similar \textit{Re}$_{\tau }$. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L20.00003: Effects of increased entrainment in turbulent boundary layers Guillem Borrell, Javier Jimenez It has been reported that certain rough surfaces modify the outer region of turbulent boundary layers. One of the effects of surface roughness is additional friction that causes an accelerated entrainment rate, which is also known to modify the outer intermittent layers of external turbulent flows. One and two-points statistics are presented from a direct numerical simulation of a zero-pressure-gradient turbulent boundary layer in the range $Re_\theta = 1400-4500$, in which the spreading rate is increased by 70\% by a smooth volumetric force restricted to the layer below $y^+=25$, and equivalent to a sand roughness of $k_s\sim 60$. The goal of this simulation is to separate the effects of surface geometry from those of entrainment. The velocity fluctuations, Reynolds stresses and spatial correlations $C_{\xi \xi}(x;x^\prime,y;y^\prime,k_z)$, that are consistently different from those in smooth-wall boundary layers at similar Reynolds numbers, will be compared with experimental and numerical data sets available in the literature. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L20.00004: Time Resolved Tomographic PIV Measurements of Rough-Wall Turbulent Channel Flow Rinaldo Miorini, Cao Zhang, Patrick Luckett, Dev Patel, Joseph Katz Turbulent channel flow is investigated via time-resolved tomographic particle image velocimetry. The optical refractive index of the transparent channel rough wall is matched with that of the fluid, allowing measurements very close to its surface. A thick, high-speed laser sheet illuminates tracers whose scattered light is recorded by four high-speed cameras. The roughness consists of staggered pyramidal elements whose aspect ratio satisfies the ``well-characterized'' flow conditions, with h/k close to 50 and k+=60-100 (h and k are the channel half-height and roughness height, respectively). The measurements are performed at Re(h) in the 40000-60000 range. Following the work of Hong et al. (JFM, 2011, 2012), data analysis is aimed at understanding the interaction between outer layer large-scale structures and the roughness elements. We examine the processes involved with vortex generation at the pyramid front ridge, vortex evolution in the non-uniform flow in the roughness sub-layer, its rise as neighboring structures interact, and its subsequent development under the influence of outer layer structures. Associated trends of Reynolds stresses and TKE are also explored, taking advantage of the available three-dimensional velocity gradients. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L20.00005: Large-Scale Secondary Flows in a Turbulent Boundary Layer Caused by Highly Ordered and Directional Surface Roughness Bagus Nugroho, Nick Hutchins, Jason Monty The effects of highly-ordered and directional surface roughness on zero-pressure-gradient turbulent boundary layers has been investigated experimentally. The surface roughness geometry is a converging-diverging riblet-type, which is shown to induce large-scale secondary flows within the boundary layer. Detailed studies using a single-normal hot-wire indicate that the surface roughness promotes large-scale spanwise modifications of the boundary layer characteristics on the wavelength of the converging-diverging pattern. The data reveals that the local mean velocity above the diverging region increases, while the turbulent intensity and boundary layer thickness decreases. The opposite phenomena occurs over the converging region. Parametric studies reveal that the magnitude of the induced spanwise modifications is determined by the viscous scaled riblet height and spacing ($h^+$ and $s^+$), the converging / diverging angle ($\alpha$), and the streamwise fetch of the boundary layer flow over the surface roughness. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L20.00006: Investigation of wall-bounded turbulence over regularly distributed roughness Marco Placidi, Bharathram Ganapathisubramani The effects of regularly distributed roughness elements on the structure of a turbulent boundary layer are examined by performing a series of Planar (high resolution $l^{+}\approx30$) and Stereoscopic Particle Image Velocimetry (PIV) experiments in a wind tunnel. An adequate description of how to best characterise a rough wall, especially one where the density of roughness elements is sparse, is yet to be developed. In this study, rough surfaces consisting of regularly and uniformly distributed LEGO$^{\textregistered}$ blocks are used. Twelve different patterns are adopted in order to systematically examine the effects of frontal solidity ($\lambda_f$, frontal area of the roughness elements per unit wall-parallel area) and plan solidity ($\lambda_p$, plan area of roughness elements per unit wall-parallel area), on the turbulence structure. The Karman number, $Re_{\tau}$, is approximately 4000 across the different cases. Spanwise 3D vector fields at two different wall-normal locations (top of the canopy and within the log-region) are also compared to examine the spanwise homogeneity of the flow across different surfaces. In the talk, a detailed analysis of mean and rms velocity profiles, Reynolds stresses, and quadrant decomposition for the different patterns will be presented. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L20.00007: Drag and Turbulence Production by Random Roughness Richard Leighton, Kenneth Christensen, Kiran bhaganagar The effects of roughness in an incompressible turbulent boundary layer include the increased production of turbulence kinetic energy (TKE) and altered the nature and distribution of the skin drag. By formulating the exact Reynolds-averaged Navier-Stokes turbulence kinetic energy equations in a manner that includes an arbitrary roughness, the averaged terms representing the roughness production of TKE and the roughness drag can be written explicitly. Similar transport equations for TKE can be formulated wherein the roughness geometry is represented using the immersed boundary methodology. These terms are calculated from a collection of direct numerical simulations (DNS). The roughness geometry employed is based on both real turbine blade roughness and a set of spectrally defined random roughness with varying amounts of skew. The primary results include an examination of thr distribution of the roughness drag and the partitioning of the production of TKE into canonical shear production and into production by roughness, and the partitioning of drag into form drag and viscous shear drag. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L20.00008: Holographic particle tracking elucidates coherent structures in the roughness sublayer of a channel flow Siddharth Talapatra, Joseph Katz The 3D, volumetric flow in the inner part of a turbulent rough channel, at \textit{Re}$_{\tau }$=3520, is resolved using microscopic holographic particle tracking in an optically index-matched facility. The channel walls consist of uniformly distributed pyramids, with height $k$=0.46mm, $k^{+}$=65 and $h/k$=54 ($h$ is the channel half height). Mean velocity and Reynolds stress profiles agree with 2D PIV results except for very close to the wall ($< 0.7 k$), where discrepancies are attributed to the higher resolution of the holographic data. All the Reynolds stress components increase rapidly within the roughness sublayer as the wall is approached. Instantaneous realizations indicate that the roughness sublayer is flooded by low lying spanwise, groove parallel and quasi-streamwise vortices. Linear stochastic estimation and conditional sampling reveal that the prevalent sublayer structure consists of interacting U-shaped vortices with spanwise base located in the low speed region above the pyramid ridgeline, and quasi streamwise legs extending between ridgelines, where the velocity is higher. Interactions among legs of vortices generated above neighboring pyramids induces ejection, lifting the quasi-streamwise legs and aligning them preferentially at angles of 54\r{ }-63\r{ } to the streamwise direction. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L20.00009: Extraction of very-large scale structures in turbulent boundary layer St\'ephane Roux, Franck Kerherv\'e, Michel Stanislas, Jean Marc Foucaut, Joel Delville The examined flow is a zero-pressure gradient turbulent boundary layer. The data used are taken from the joined experimental campaign conducted during the european WALLTURB program in the large wind tunnel at Laboratoire de M\'ecanique de Lille (LML). The free-stream velocity is 10 m/s. At the investigated position, the boundary layer thickness is 30 cm and the Reynolds number based on the momentum thickness is 19100. A methodology for eduction of \emph{super-structures} is presented. These structures are characterised by a large degree of persistance and are thought to participate actively to the turbulence regeneration in the near-wall region~(Marusic et al. 2010. A time-resolved estimate of the three-dimensionnal structures is obtained by combining low-speed two-dimensional stereo-PIV at 4Hz and a two-dimensionnal rake of 143 single hot-wire probes at 30 kHz. The very large scale structures are clearly reconstructed which exhibit a streamwise extent an order of magnitude larger than the boundary layer thickness. Interest is particulary focused on the low-speed species of these structures. Associated coounter-rotating vortices are also evidenced in good agreement with the litterature. [Preview Abstract] |
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