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 FR: Turbulent Boundary Layers: Roughness Effects |
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Chair: Ivan Marusic, University of Minnesota Room: Hilton Chicago Stevens 3 |
Monday, November 21, 2005 8:00AM - 8:13AM |
FR.00001: Effects of surface roughness on turbulent pipe flow Michael Shockling, Alexander Smits, James Allen Mean flow measurements are presented for fully developed turbulent pipe flow over a Reynolds number range $57 \times 10^3$ to $21 \times 10^6$ where the flow exhibits hydraulically smooth, transitionally rough, and fully rough behavior. The surface of the pipe was prepared with a honing tool, typical of many engineering applications. A unique aspect of the present experiment is the very small ratio of characteristic roughness height to pipe diameter, $1:17000$. Results for the friction factor show that in the transitionally rough regime this surface follows a Nikuradse type inflectional relationship rather than the monotonic Colebrook relationship used in the Moody diagram. This result supports previous suggestions that the Moody diagram in the transitional regime must be used with caution. Outer scaling of the mean velocity data shows excellent collapse and strong evidence for Townsend's outer layer similarity hypothesis for rough walled flows. Finally, the pipe exhibited smooth behavior for $k_s^+ \le 3.5$, which supports the suggestion that the original Superpipe was hydraulically smooth for $Re_D \leq 24 \times 10^6$. [Preview Abstract] |
Monday, November 21, 2005 8:13AM - 8:26AM |
FR.00002: Evaluation of a universal transitional resistance diagram for pipes with honed surfaces James Allen, Michael Shockling, Alexander Smits A method for evaluating a universal transitional resistance diagram for pipes that relates the pressure drop in the pipe to Reynolds number, as a function of relative surface roughness, is presented. The method assumes a universal wake function coupled with a logarithmic overlap region and a power fit in the viscous and buffer layer. Estimates can be made of the friction factor-Reynolds number relationship for arbitrary relative roughness. The size of the non-dimensional velocity shift as a function of roughness Reynolds number comes from the honed pipe data of Shockling, Allen {\&} Smits$^{ }$[1]. Honed roughness demonstrates an inflectional behavior in the transitionally rough regime, much like sandgrain roughness, but the method proposed here applies to any given roughness behavior. \newline \newline [1] Shockling, M.S., Allen, J.J. and Smits, A.J. ``Roughness effects in turbulent pipe flow,'' Submitted to\textit{ Journal of Fluid Mechanics}, (2005). [Preview Abstract] |
Monday, November 21, 2005 8:26AM - 8:39AM |
FR.00003: Theoretical and numerical calculation of the Eddy viscosity in smooth and rough turbulent boundary layers Jorge Bailon-Cuba, Luciano Castillo A numerical implementation of the theory of George and Castillo
(GC-97) for smooth and Seo (2003) for rough, is being done for
the computation of the Reynolds shear stresses, $- |
Monday, November 21, 2005 8:39AM - 8:52AM |
FR.00004: Roughness Effects on the Third Moments in a Turbulent Boundary Layer Donald Bergstrom, Olajide Akinlade Profiles of the third moments of the fluctuating velocity field ($<$u$^{/3}>$, v$^{/3}$, $<$u$^{/2}$v$^{/}>$, and $<$u$^{/}$v$^{/2}>)$ and distributions of the skewness of the longitudinal and wall-normal velocity fluctuations over smooth and three different rough surfaces were measured in a zero pressure-gradient turbulent boundary layer using cross hot-film anemometry. The freestream velocity and physical geometry of the three rough surfaces (i.e. perforated sheet, sand grain, and woven wire mesh) were chosen to create fully rough flow regimes. The Reynolds numbers based on momentum thickness were approximately the same for all three rough-wall flows. Two different scaling parameters, i.e. the friction velocity, U$_{\tau }$ , and a mixed scale, U$_{e}$U$_{\tau }^{2}$, proposed by George and Castillo (1997), were used to assess the effect of roughness on the triple velocity correlations. The experimental results indicate that surface roughness significantly alters some components of the third moment in the inner region, and this effect also extends into the outer region of the boundary layer. This observation is at variance with the wall similarity hypothesis. On the other hand, the distributions of the skewness for both the longitudinal and wall-normal velocity fluctuations are largely unaffected by surface roughness. [Preview Abstract] |
Monday, November 21, 2005 8:52AM - 9:05AM |
FR.00005: Outer Layer Turbulence Similarity for Rough Wall Boundary Layers Karen Flack, Michael Schultz, Jonathan Connelly The outer layer similarity hypothesis of Townsend states that the turbulence beyond a few roughness heights from the wall is independent of the surface condition. In order to explore a limiting roughness height for boundary layer similarity, an experimental investigation was carried out on six rough surfaces representing two types of three dimensional roughness (sandpaper and woven mesh) in which the boundary layer thickness to roughness height varied from 16 to 110. The measurements were conducted in a closed return water tunnel, over a momentum thickness Reynolds number range of 6,100 to 13,000, using a two-component, laser Doppler velocimeter (LDV). The Reynolds stresses, and higher-order moments, as well as quadrant analysis of the rough surfaces show collapse with smooth wall results outside of a roughness sublayer. These results indicate that turbulence similarity in the outer layer may be more robust than previously thought and changes to the turbulent structure appear to be confined to a roughness sublayer, even for very large roughness. [Preview Abstract] |
Monday, November 21, 2005 9:05AM - 9:18AM |
FR.00006: Effects of Roughness on Turbulence Spectra and Correlation Measurements Ralph J. Volino, Michael P. Schultz, Karen A. Flack Zero pressure gradient, fully-developed, turbulent boundary layers have been documented on a smooth flat plate and a fully rough surface in a water tunnel facility. The roughness is produced using a wire mesh affixed to the surface. Profiles of the streamwise and wall-normal velocity components were acquired using laser Doppler velocimetry (LDV). Additional LDV data were acquired with high sampling rates at two locations in the profiles, corresponding to $y/\delta_{99}=0.1$ and $y/\delta_{99} =0.4$. From these data, turbulence spectra of the Reynolds normal stresses and shear stress are computed. Flow field measurements were also made using digital particle image velocimetry (PIV) in streamwise planes both normal to the surfaces and parallel to the surfaces. From the PIV data, two point correlations of the turbulence quantities are computed. The presentation will focus on the similarities and differences between the spectra and correlations on the rough and smooth walls, adding to comparisons of mean flow and turbulence statistics between rough and smooth surfaces presented previously. [Preview Abstract] |
Monday, November 21, 2005 9:18AM - 9:31AM |
FR.00007: Velocity-defect scaling for turbulent boundary layers with a range of relative roughness Michael Schultz, Karen Flack, Jonathan Connelly Velocity profile measurements in zero pressure gradient, turbulent boundary layer flow were made on a smooth wall and on two types of rough walls with a wide range of roughness heights. The ratio of the boundary layer thickness ($\delta$) to the roughness height ($k$) was 16 $< \delta/k <$ 110 in the present study, while the ratio of $\delta$ to the equivalent sand roughness height ($k_s$) ranged from 6 $< \delta/k_s <$ 91. The results show that the mean velocity profiles for all the test surfaces collapse in velocity-defect form in the overlap and outer layer when normalized by the friction velocity obtained using two different methods. The velocity-defect profiles also agree within experimental uncertainty when normalized with the velocity scale proposed by Zagarola \& Smits (1998). The results provide evidence that roughness effects on the mean flow are confined to the inner layer, and outer layer similarity of the mean velocity profile applies even for relatively large roughness. [Preview Abstract] |
Monday, November 21, 2005 9:31AM - 9:44AM |
FR.00008: Large Eddy Simulation of a boundary layer flow over urban-like roughness Kojiro Nozawa, Tetsuro Tamura In this study, Large Eddy Simulation (LES) of a boundary layer flow over large-scale roughness was performed targeting the experiments conducted by Cheng and Castro [Boundary-Layer Meteorology 104, 229-259 (2002)]. In order to duplicate the experimental conditions, the pseudo periodic boundary method for rough-wall boundary flows [Nozawa and Tamura (2000)] was applied to the inlet boundary conditions and the computational area was reduce to less than 50\% in the streamwise direction. The roughness blocks were modeled using immersed boundary approach which would reduce the grid points around the roughness blocks. The characteristics of the turbulent flow over large-scale roughness, whose roughness area density was 25\% and the ratio $\delta/h$ ($\delta$: boundary layer thickness, $h$ roughness height) was almost $7\sim10$, were different from those whose ratio $\delta/h$ is more than 40[Jimenez, Annu. Rev. Fluid Mech., 36, 173-196 (2003)]. The spatial variation of vertical profiles of mean and fluctuation velocities are studied in detail and compared to the experimental data. We focus on the influence to the spatial variations of turbulence structure deduced by large-scale roughness. This study was conducted using the Earth Simulator in Yokohama, Japan. [Preview Abstract] |
Monday, November 21, 2005 9:44AM - 9:57AM |
FR.00009: Unveiling the Ties Between Nikuradse and Kolmogorov: How to Derive the Diagram From the Spectrum Gustavo Gioia, Pinaki Chakraborty We model the relation, first evinced by Nikuradse, among the Reynolds number, the relative roughness, and the friction coefficient ($f$) of turbulent flows in rough pipes. To that end, we identify the eddies that effect most of the momentum transfer between the viscous layer and the turbulent flow, and derive an expression for $f$ in terms of the characteristic velocity of those eddies, $u_s$. Then we use Kolmog\'orov's spectrum for the inertial range to determine $u_s$, and show that the resulting expression for $f$ gives a gradual transition between the scalings of Blasius and Strickler, but fails to give the hump or the bellies of Nikuradse's diagram. To obtain an expression for $f$ that also gives the bellies, we include an exponential spectrum for the dissipation range. Last, to obtain an expression for $f$ that also gives the hump, we include von K\'arm\'an's spectrum for the energy-containing range. This final expression for $f$ is in minute qualitative agreement with Nikuradse's diagram; it affords a way of interpreting successive portions of the diagram as manifestations of the varying habits of momentum transfer; and it reveals the existence of close ties between two milestones of experimental and theoretical turbulence. [Preview Abstract] |
Monday, November 21, 2005 9:57AM - 10:10AM |
FR.00010: A-priori Wind Tunnel Study of Subgrid-scale Models for LES of a Boundary Layer Over a Rough-to-Smooth Transistion Matt Carper, Fernando Port\'{e}-Agel Improving our understanding of subgrid-scale (SGS) physics is of key importance to large-eddy simulation (LES) becoming a reliable tool to study atmospheric boundary layer fluxes over heterogeneous land surfaces. The largest errors in LES of the atmospheric boundary layer are associated with parameterization of the subgrid-scale fluxes required to account for the effect of the unresolved (subgrid) scales on the resolved scales. In particular, the SGS stresses need to be specified through the entire boundary layer, using a SGS model, and at the surface, as a boundary condition (or wall model). Many of these SGS models have limited accuracy due to their assumptions of horizontal spatial homogeneity. In this wind tunnel experiment we evaluate, {\it a priori}, the SGS stresses and energy transfers from measurements using stereoscopic particle image velocimetry (PIV) in a boundary layer ($Re_{\theta} = 10,000$) over a rough- to-smooth transition. The high-resolution, planar 3-D velocity fields, obtained at various positions downstream of the roughness transition, are spatially filtered in 2-D and used to calculate SGS stresses and filtered strain rates. These in turn are used to compute local SGS transfers of resolved energy. The results of this analysis are used to characterize the SGS stresses and energy transfers as a function of position in the flow, and to evaluate ({\it a priori}) several SGS models. [Preview Abstract] |
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