Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session E7: Turbulent Boundary Layers III: Roughness |
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Chair: Karen Flack, U.S. Naval Academy Room: 310 |
Sunday, November 20, 2011 4:40PM - 4:53PM |
E7.00001: A Study of the Mean Force Structure of Rough-Wall Turbulent Boundary Layers Faraz Mehdi, Joseph Klewicki Analysis of existing data by Mehdi, Klewicki \& White [Physica D 239(2010)] provides evidence that the traditional classifications do not fully account for the combined effects of roughness and Reynolds number. We continue to explore this further, and in the present talk report on experiments that used 24-grit sandpaper and pea gravel for roughness over an 8m fetch. Two-component LDV measurements are used to acquire well-resolved mean velocity and Reynolds stress profiles over a modest range of Reynolds numbers. These data are used to estimate the terms in the appropriate mean statement of dynamics, which directly reveals the operative time-averaged balance of forces. The present results further reinforce the previous observation that the mean viscous force retains dominant order above (and often well-above) the roughness elements. Force balance data are shown to be usefully organized relative to the length scale that defines the region from the wall to where the leading order mean dynamics are described by a balance between mean advection and the mean effect of turbulent inertia. In the smooth-wall flow, this length scale is only a function of Reynolds number. In rough-wall flows, the data indicate it to be a function of roughness and Reynolds number. [Preview Abstract] |
Sunday, November 20, 2011 4:53PM - 5:06PM |
E7.00002: Scale-separation phenomena in rough-wall turbulent boundary layers Rachel Ebner, Caleb Morrill-Winter, Joseph Klewicki Analysis of data from smooth-wall flows indicates that scale- separation and scale-selection phenomena occur between the velocity and vorticity fields as a function of distance from the wall and Reynolds number. These phenomena are significant since correlations between specific velocity and vorticity components underlie the momentum and energy transport mechanisms operative in boundary layer turbulence. The current effort explores these phenomena in rough-wall flows by examining the behaviors of velocity and vorticity component spectra and co-spectra as derived from multi-sensor hot-wire measurements. The data were acquired at multiple Reynolds numbers and for two different sizes of homogeneously distributed roughness. A four-wire probe configuration allowed for well-resolved measurements of the streamwise and wall-normal velocity components, as well as the spanwise component of vorticity. Pre-multiplied power spectra and the associated co-spectra are interpreted relative to the wall- normal variations of the Reynolds stress and kinetic energy gradient. These results are compared to the smooth-wall analysis, and the recent rough-wall findings of Hong et al. (\textit{J. Fluid Mech.} \textbf{667}, 2011) indicating that roughness effectively sets the scale of the vortical motions throughout the boundary layer. [Preview Abstract] |
Sunday, November 20, 2011 5:06PM - 5:19PM |
E7.00003: Spatial Characteristics of Large-Scale Motions in Smooth and Rough Turbulent Boundary Layers J.M. Barros, K.T. Christensen Wide field of view stereo PIV measurements were conducted in the wall-normal--spanwise plane of smooth and rough zero-pressure- gradient turbulent boundary layers. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials and contained a broad range of scales arranged in an irregular manner. Inspection of smooth- and rough-wall instantaneous velocity fields in this cross plane uncovered the spatial signatures of low- and high-momentum regions, the former of which have been previously linked to hairpin vortex packets in the outer layer of wall turbulence. Proper orthogonal decomposition was employed to study the spatial characteristics of the large-scale motions in smooth- wall flow and revealed a significant spanwise coherence in the form of alternating low- and high-momentum regions well beyond that reflected in two-point velocity correlations. In contrast, the rough-wall results revealed weaker coherence of these large- scale motions. [Preview Abstract] |
Sunday, November 20, 2011 5:19PM - 5:32PM |
E7.00004: The Impact of Favorable-Pressure-Gradient Conditions on Large-Scale Motions in Smooth- and Rough-Wall Turbulence D. Min, K.T. Christensen The combined impact of irregular surface roughness and moderate favorable-pressure-gradient (FPG) conditions ($K \approx 4.0-4.4 (10{^{-7}})$) on large-scale motions in a turbulent boundary layer was assessed using stereo PIV measurements in the wall- normal--spanwise 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. These measurements were compared to measurements of smooth-wall flow under identical FPG conditions to reveal the combined impact of roughness and FPG conditions on the larger-scale motions. Instantaneous smooth- and rough- wall velocity fields embodied spanwise-alternating patterns of low- and high-momentum regions (LMRs and HMRs). While these motions in the smooth-wall case were found to penetrate less deep into the boundary layer compared to similar motions in zero- pressure-gradient flow, they often extended much further away from the wall in the presence of roughness. Two-point correlations of low-pass- filtered velocity fields embodying only the larger-scale motions of smooth-wall flow revealed a remarkable spanwise- alternating nature of low- and high-momentum regions well beyond that reflected in correlations of unfiltered velocity. Roughness was found to reduce this spanwise coherence. [Preview Abstract] |
Sunday, November 20, 2011 5:32PM - 5:45PM |
E7.00005: Large eddy simulation of accelerating turbulent boundary layers over rough surfaces Junlin Yuan, Ugo Piomelli Rough-wall turbulent boundary layers subject to favourable pressure gradients (FPGs) and freestream acceleration are found in many engineering applications. On a smooth wall with a strong FPG the flow may revert to a quasi-laminar state. Roughness, on the other hand, enhances turbulent fluctuations near the wall. We investigate strongly accelerating turbulent boundary layers over rough surfaces by large eddy simulation. We use an immersed-boundary method to model random sandgrain-type roughness. Flow statistics show that the extent of relaminarization depends on the degree of acceleration, on the roughness height, and on the flow Reynolds number. The stabilizing effect of the FPG (laminar-like mean velocity, decreased Reynolds stresses, increased anisotropy) is only found outside the roughness sublayer: close to the wall, roughness results in an increase of all components of the Reynolds stress and decreases Reynolds-stress anisotropy. An increase in the Reynolds number intensifies the roughness effect. Small eddies are generated in the wake of the roughness elements; they tend to disrupt the streak stabilization observed in FPG boundary layers, and increase bursting. This study confirms previous findings which indicate that roughness competes with the FPG stabilizing effect near the wall, and might deactivate acceleration effect in the outer layer. [Preview Abstract] |
Sunday, November 20, 2011 5:45PM - 5:58PM |
E7.00006: HPIV based volumetric 3D flow description in the roughness sublayer of a turbulent channel flow Siddharth Talapatra, Joseph Katz Microscopic HPIV is utilized to resolve the 3D flow in the roughness
sublayer of a boundary layer over a rough wall at Re$_{\tau}$=3400,
consisting of pyramidal elements with height of $k$=0.45mm and 3.3mm
wavelength. Typically, $\sim $7000 particles are tracked in a
3.2$\times
$2.1$\times $1.8mm$^{3}$ volume, whose wall-normal extent is
-0.2$ |
Sunday, November 20, 2011 5:58PM - 6:11PM |
E7.00007: Coherent structures and associated sub-grid scale energy transfer in a rough-wall turbulent channel flow Jiarong Hong, Joseph Katz, Charles Meneveau, Michael Schultz Our earlier study has revealed spectral bumps and enhanced dissipation of turbulent kinetic energy (TKE) by roughness scale eddies in the outer parts of a rough wall channel flows at Re$_{\tau }$=3520-5360. Here, conditional averaging of PIV data, based on high subgrid-scale (SGS) energy flux, shows an inclined large structure with negative spanwise vorticity in streamwise-wall-normal (\textit{xy}) planes, and two pairs of counter-rotating vortices in streamwise-spanwise (\textit{xz}) planes, both near the wall and in the outer-layer. In instantaneous snapshots, this structure consists of periodic bundles of roughness-scale eddies spaced by a roughness wavelength, resulting from lifting and re-orientation of near-wall, quasi-streamwise vortices. Near the wall, the peak of SGS energy flux occur at the intersection of the quasi-streamwise eddies, while in the outer-layer, elevated flux occurs within the large structure owing to interaction among eddies in a bundle. At the conditioning point, the SGS kinetic energy peaks, but the resolved TKE has a minimum. The location of the resolved TKE maximum varies with elevation due to the increasing impact of ejections with distance from the wall. [Preview Abstract] |
Sunday, November 20, 2011 6:11PM - 6:24PM |
E7.00008: POD Analysis of a ZPG Turbulent Boundary Layer with and without Surface Roughness Jensen Newman, Pat Russo, Lucianno Castillo, Guillermo Araya, Don Drew Effects of surface roughness on coherent structures of a zero pressure gradient turbulent boundary layer are investigated via the POD method. In this study we seek to gain better understanding of how the small scales of turbulence are able to influence the large scales; particularly in rough surface flows. Comparison with coherent structures in a smooth case by Baltzer et al (2010) will be shown to assess the role of surface roughness in a transitional rough regime. The POD analysis is performed using the method of snapshots on two turbulent boundary layer DNS databases created using the dynamic multi scale recycling plane method developed by Araya et al. (2011) at Reynolds numbers of 2000-2300. Snapshots are separated by a time of $150\theta _0 /U_\infty $ Resulting POD modes are qualitatively compared to assess the effects of roughness on the coherent structures. Baltzer, R.J.; Adrian, J.; Wu, X., ``Turbulent Boundary Layer Structure Identification via POD,'' in \textit{Stanford Center for Turbulence Research Summer Program}, 2010. G. Araya\textit{, et al.}, ``A dynamic multi-scale approach for turbulent inflow boundary conditions in spatially developing flows,'' \textit{Journal of Fluid Mechanics, }vol. 670, pp. 581-605, 2011. [Preview Abstract] |
Sunday, November 20, 2011 6:24PM - 6:37PM |
E7.00009: Scaling of transitionally-rough flow structures in DNSes of channels with riblets up to $\mbox{Re}_\tau \approx$ 550 Garc\'Ia-Mayoral Ricardo, Javier Jim\'enez The linear, drag-reducing effect of vanishingly small riblets breaks down once their size is in the transitionally-rough regime. We have previously reported that this breakdown is caused by the additional Reynolds stresses produced by the appearance of elongated spanwise rollers just above the riblet surface. These rollers are related with the Kelvin--Helmholtz instability of free shear layers, and to similar structures appearing over other rough and porous surfaces. However, because of the limited $\mbox{Re}_\tau \approx 180$ in our previous DNSes, it could not be determined whether those structures scaled in inner or outer units. Furthermore, it is questionable if results in the transitionally-rough regime at $\mbox{Re}_\tau \approx 180$ can be extrapolated to configurations of practical interest. At such small Reynolds numbers, roughness of transitional size can perturb a large portion of the boundary layer, which is not the case in most industrial and atmospheric applications. To clarify these issues we have conducted a set of DNSes at $\mbox{Re}_\tau \approx 550$. Our results indicate that the spanwise rollers scale in wall units, and support the validity of the extrapolation to configurations of practical interest. [Preview Abstract] |
Sunday, November 20, 2011 6:37PM - 6:50PM |
E7.00010: Optimization of drag-reducing surface geometries in turbulent channel flow Asghar Yarahmadi, Meredith Metzger Turbulent channel flow over surfaces comprised of spanwise-wavy arrays of longitudinally-oriented riblets was simulated computationally to determine the optimal riblet configuration yielding maximum drag reduction. Surface riblet patterns were characterized by geometric parameters including riblet height, spacing, and cross-sectional shape (i.e., side-wall curvature/slope) as well as the spanwise amplitude and wavelength of the sinusoidal undulation. This parameter space allowed investigation of the following types of riblet surfaces: rectangular, triangular, trapezoidal, notched top, scalloped semicircular, and U-shaped. The surface configuration affording the highest cost savings in terms of skin friction drag reduction was obtained using a multi-objective evolutionary optimization method driven by numerical simulations that were performed using LES with a localized dynamic SGS model, RANS (RSTM and two equations methods) and Detached Eddy Simulation model. The computational efficiency and accuracy of the turbulence models are also discussed. [Preview Abstract] |
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