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 D7: Turbulent Boundary Layers II: Roughness |
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Chair: Michael Schultz, U.S. Naval Academy Room: 310 |
Sunday, November 20, 2011 2:10PM - 2:23PM |
D7.00001: Effect of surface finish on the onset of roughness effects in turbulent channel flow Michael Schultz, Karen Flack The ability to reliably predict the onset of roughness effects in wall-bounded turbulent flows is an important yet elusive aim. This is especially relevant for the test engineer or experimental researcher who must specify the surface finish for a wind or water tunnel model. In this study, the effect of the surface finish resulting from sanding was investigated systematically using a turbulent channel flow facility. The experiments were conducted over a Reynolds number range of 6,000 -- 64,000 based on the channel height and the bulk mean velocity. Painted surfaces sanded with a range of grit sizes were tested. The results indicate that all of these surfaces depart from hydraulically smooth behavior at roughness Reynolds numbers ($k_{rms}^+ )$ of between 0.5 and 0.7. The implications of these results with regard to model surface finish required in order to avoid roughness effects will be discussed. [Preview Abstract] |
Sunday, November 20, 2011 2:23PM - 2:36PM |
D7.00002: Important scales for predicting the onset of roughness effects in the transitionally rough regime Karen Flack, Michael Schultz Accurately predicting the departure from hydraulically smooth behavior for a given surface has significant engineering applications including determining the frictional drag of vehicles or the pressure drop in piping systems. A series of experiments have been performed for a range of three-dimensional rough surfaces to determine the appropriate roughness scales for the prediction of the onset of roughness effects. The experiments were performed in a small water channel with a Reynolds number based on channel height range of 6,000-64,000. The wall shear stress was determined from the pressure drop in the in the channel. Scaling parameters based on the statistics of the surface topography in addition to the roughness density will be presented. [Preview Abstract] |
Sunday, November 20, 2011 2:36PM - 2:49PM |
D7.00003: Direct numerical simulation of a high-entrainment turbulent boundary layer Guillem Borrell, Ayse G. Gungor, Javier Jimenez It has been reported that certain rough surfaces modify the outer region of turbulent boundary layers, but not those of channels or pipes. Besides their surface geometries, all those experiments share relatively large spreading and entrainment rates, which is known to modify the outer intermittent layers of external turbulent flows, but is absent from channels. To separate the effect of surface geometry from that of entrainment, we present a direct simulation of a zero-pressure-gradient turbulent boundary layer, at $Re_\theta = 1400-4500$, in which the friction coefficient is augmented by a smooth volumetric force, restricted to the viscous layer below $y^+=25$, and proportional to the streamwise component of the velocity. The spreading rate increases by 70\%, equivalent to a sand roughnes $k_s^+\approx 60$. The resulting changes in the velocity and pressure fluctuations, and in the velocity correlation lengths, are compared with those of rough-wall experiments. [Preview Abstract] |
Sunday, November 20, 2011 2:49PM - 3:02PM |
D7.00004: Investigation of wall-bounded turbulence over sparsely distributed roughness Marco Placidi, Bharath Ganapathisubramani The effects of sparsely distributed roughness elements on the structure of a turbulent boundary layer are examined by performing a series of Particle Image Velocimetry (PIV) experiments in a wind tunnel. From the literature, the best way to characterise a rough wall, especially one where the density of roughness elements is sparse, is unclear. In this study, rough surfaces consisting of sparsely and uniformly distributed LEGO$^{\textregistered}$ blocks are used. Five different patterns are adopted in order to examine the effects of frontal solidity ($\lambda_f$, frontal area of the roughness elements per unit wall-parallel area), plan solidity ($\lambda_p$, plan area of roughness elements per unit wall-parallel area) and the geometry of the roughness element (square and cylindrical elements), on the turbulence structure. The Karman number, $Re_{\tau}$, has been matched, at the value of approximately 2300, in order to compare across the different cases. In the talk, we will present detailed analysis of mean and rms velocity profiles, Reynolds stresses and quadrant decomposition. [Preview Abstract] |
Sunday, November 20, 2011 3:02PM - 3:15PM |
D7.00005: Non-equilibrium layers in rough-wall channel flow Jonathan Morrison, David Birch Streamwise and wall-normal velocity statistics in a fully- developed turbulent channel flow over large relative roughness are examined, for $k/h=3.9\%$, and $Re_{\tau}=5130$. The surface is fully rough, $k^+=200$, and the appearance of a log region is demonstrated by an overlap region with inner and outer scaling, where the procedure defines a roughness lengthscale and a zero-plane displacement, $d^+=130$ for $\kappa=0.41$. Although the zero-plane displacement is somewhat smaller than the roughness height, it is large relative to the thickness of the log region, $205 < (y-d)^+ < 308$. The zero-plane displacement obtained by the matching to the log law is somewhat smaller than that required for the measured shear-stress gradient to match the linear variation required by the momentum equation, and the latter does not provide a convincing log law. The reasons for this are discussed and the behaviour of the turbulence in the log region is examined, with particular reference to the maximum in the Reynolds shear stress which appears above the outermost extent of the log region. The effect of the roughness offset on the energy balance is examined: despite the appearance of the log law, it does not conform to the local-equilibrium approximation. Scaling of the velocity spectra and second- and third-order structure functions is examined: the energy balances in the log region and at the centre-line where estimates of the dissipation rate can be made are examined. [Preview Abstract] |
Sunday, November 20, 2011 3:15PM - 3:28PM |
D7.00006: DNS of Rough Surface Turbulent Boundary Layer James Cardillo, Guillermo Araya, Yi Chen, Kenneth Jansen, Onkar Sahni, Luciano Castillo A dynamic method for prescribing realistic inflow boundary conditions is presented for simulations of spatially developing turbulent boundary layers subject to surface roughness. Direct Numerical Simulation (DNS) of a moderate Reynolds number, zero pressure gradient (ZPG) turbulent boundary layer was performed. The boundary layer was subjected to transitional, $24-$grit sand grain surface roughness, modeled with a roughness parameter of $k^+ \sim $12 and a Reynolds number of $R_\theta = 2400$. The computational method involves a synergy of the multi-scale dynamic approach and a new methodology for mapping high-resolution topographical surface data into a computational fluid dynamics environment. It is shown here that the multi-scale dynamic approach can be successfully extended to simulations, which incorporate surface roughness. In terms of the mean velocity and Reynolds stresses, the DNS results are encouraging as they demonstrate good agreement with the LDA measurements performed under similar conditions. [Preview Abstract] |
Sunday, November 20, 2011 3:28PM - 3:41PM |
D7.00007: Direct Numerical Simulation of a turbulent channel flow with 3D wedges randomly placed on a wall Stefano Leonardi, Luis Martinez Tossas, Edgardo Garcia Cartagena Direct numerical simulations of a turbulent channel flow with 3D wedges of random height have been performed. In addition, two other simulations have been carried out to assess the effect of the geometry on the overlying flow. In the first simulation, the elements in the wake of higher wedges were removed while in the other, a uniform distribution of wedges with the same area was used. A wedge is considered in the wake of another element when the line joining the crests is steeper than 45 degrees. The bulk Reynolds numbers is 7000 which correspond in case of smooth walls to $Re_\tau=300$. The comparison of the 3 surfaces has shown that near the wall, the uniformly distributed roughness represents only a poor approximation of the surface with wedges of random height. The surface obtained removing the wedges in the wake of previous wedges present Reynolds stresses, pressure distribution and spectra in good agreement with the original surface. Therefore, it is possible to reduce the geometrical complexity of a rough surface while retaining the same flow physics. [Preview Abstract] |
Sunday, November 20, 2011 3:41PM - 3:54PM |
D7.00008: Direct numerical simulation study in turbulent boundary layers with the cube-roughened walls Junsun Ahn, Jae Hwa Lee, Hyung Jin Sung Direct numerical simulations of turbulent boundary layers over the cube-roughened walls were performed to investigate the effects of the streamwise ($p_{x}/k)$ and spanwise ($p_{z}/k)$ extents between the cubes on the properties in the outer-layer statistics. The streamwise extent of the cubes was varied from $p_{x}/k$=2, 3, 4, 6, 8, and 10 at fixed $p_{z}/k$=2 and, the spanwise extent of the cubes was varied from $p_{z}/k$=2, 3, 4 6 including aligned configuration at fixed $p_{x}/k$=3, where $k$ is the roughness height. The results showed that the outer peak values of the Reynolds stresses are increased with increasing streamwise spacing, indicating the strong inner and outer-layer interaction at large $p_{x}/k$ and the profiles of the Reynolds stresses with varying spanwise spacing show the similar trends in the outer region except $p_{z}/k$=4. In addition, we found that there is no difference between staggered and aligned cube geometries. [Preview Abstract] |
Sunday, November 20, 2011 3:54PM - 4:07PM |
D7.00009: Direct Numerical Simulations of the flow over a dimpled flat plate Nikolaos Beratlis, Kyle Squires, Elias Balaras Golf balls use dimples to reduce the drag by as much as 50 percent when compared to a smooth sphere in the subcritical regime. Recent experiments and computations indicate that dimples introduce perturbations that drive high momentum fluid towards the wall, thus delaying separation and reducing drag. The nature of these perturbations and how they are affected by the Reynolds number, boundary layer thickness, and dimple shape is not well understood. In the present study we will report a series of DNS of the flow past a dimpled flat plate. A stability map with the important parameters that control the onset of the perturbations (Reynolds number, ratio of dimple depth to boundary layer thickness, etc) will be presented. We will show that dimples induce perturbations that lead to turbulent-like boundary layers at relatively low Reynolds numbers, where the flow would otherwise remain laminar. The origin of these perturbations is a thin shear layer forming over a dimple, which becomes unstable and generates vortices. These undergo complex three-dimensional instabilities transforming themselves into structures that resemble hairpin-like vortices typically found in turbulent wall bounded flows. They are very effective in mixing the flow and already within one dimple diameter downstream of the dimple the flow attains characteristics of low Reynolds number turbulence. [Preview Abstract] |
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