Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session GB: Turbulent Boundary Layers IV |
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Chair: Hassan Nagib, Illinois Institute of Technology Room: Long Beach Convention Center 101B |
Monday, November 22, 2010 8:00AM - 8:13AM |
GB.00001: Modeling roughness effects in turbulent boundary layers using elliptic relaxation Jacob George, Alejandro De Simone, Gianluca Iaccarino, Javier Jimenez We present results from the efforts towards modeling roughness in turbulent boundary layers using elliptic relaxation. This scheme, included in the $v^{2}-f$ model and first formulated by Durbin (1993, JFM, vol. 249, p.465) for smooth-walls, uses an elliptic partial differential equation to incorporate near-wall turbulence anisotropy and non-local pressure-strain effects. The use of the elliptic PDE is extended to model roughness effects in various transitionally-rough and fully-rough boundary layers consisting of a uniform and sparse distribution of cylinders for which experimental data is available. The roughness effects are incorporated through the elliptic PDE by including the length and time scales that the roughness imposes upon the flow, which the experiment has shown to be constant within the rough-walls. Further modeling of roughness effects is considered by altering the source terms in the elliptic PDE. [Preview Abstract] |
Monday, November 22, 2010 8:13AM - 8:26AM |
GB.00002: DNS of a Turbulent Boundary Layer with Surface Roughness Yi Chen, James Cardillo, Guillermo Araya, Luciano Castillo, Kenneth Jansen A Direct numerical simulation (DNS) of a high Reynolds number, zero pressure gradient, turbulent boundary layer (Re$_{\theta }$= 2400) subjected to sandpaper surface roughness is performed. The surface roughness is modeled with a roughness parameter k$^{+} \sim$ 25 to match the experiments at similar Reynolds number and roughness distribution. The employed computational method involves a synergy of the multi-scale dynamic approach devised by Araya et al. (2010) and a new method for mapping high-resolution topographical data onto a computational domain. When dealing with rough surfaces, where calculation of the wall shear stress is very challenging the multi-scale dynamic method provides a major advantage. Contrary to traditional thought, it has been shown that the different types of surface roughness yield different types of flow fields. In light of these challenges the current roughness methodology aims to provide the community with the tools to use real topographical data to simulate surface roughness. The present simulations may shed light on our understanding of the interaction of the outer and inner layers at various scales. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GB.00003: Inner and Outer Flow Interactions on Rough, Turbulent Boundary Layers Sheilla Torres-Nieves, Hyung-Suk Kang, Charles Meneveau, Luciano Castillo Laser Doppler and hotwire anemometry measurements are performed to study the effects of surface roughness on the different length scales of the turbulent boundary layer. Measurements are carried out downstream of an active grid, with free-stream turbulence and Reynolds number, based on momentum thickness, of up to 6\% and 4,300, respectively. Second-order structure functions and energy spectra distributions are used to identify and examine how surface roughness affects the inner and outer regions of the boundary layer. Second order structure function analysis suggests that, for favorable pressure gradient flows, surface roughness directly interacts with, not only the small length scales of the flow, but also with intermediate and even large scales. These observations are even more evident when additional levels of turbulence are present in the free-stream. Power spectral density plots are analyzed in order to understand the mechanism by which larger length scales interact with the surface roughness at the wall. [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GB.00004: Combined Roughness and Favorable Pressure Gradient Effects in a Turbulent Boundary Layer D. Min, K.T. Christensen The combined impact of irregular surface roughness and moderate favorable-pressure-gradient (FPG) conditions ($K \approx 2.5 \times 10^{-7}$) on the structure of a turbulent boundary layer is assessed using two-dimensional particle image velocimetry (PIV) measurements in the streamwise--wall-normal plane. The roughness under consideration is replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topographical scales. These measurements are compared to measurements of smooth-wall flow under both identical FPG conditions as well as zero-pressure-gradient (ZPG) conditions in order to reveal the synergistic impact of roughness and FPG conditions on the underlying structure of the flow. While vortex organization is found to persist under both smooth- and rough-wall FPG conditions, its characteristics are altered compared to smooth-wall ZPG flow. Inspection of instantaneous velocity fields reveals this organization to be focused closer to the wall in the smooth- wall FPG case, with a shallower inclination angle noted as well as an elongated streamwise extent. In contrast, the rough-wall FPG results reveal packet structures more consistent with the smooth-wall ZPG case, indicating that roughness mitigates the FPG-induced focusing of these structural attributes toward the wall. Two-point correlations of streamwise velocity support these instantaneous observations. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GB.00005: Studies of the Combined Effects of Roughness and Reynolds Number in Turbulent Boundary Layers Faraz Mehdi, Joseph Klewicki Mehdi, Klewicki \& White [Physica D 239(2010)] provide evidence from existing studies that the prevalent scheme for classifying roughness regimes is likely to be incomplete. To further pursue these findings, more data are required, and for this purpose, additional rough-wall experiments are being performed. We report on our studies of the combined roughness-Reynolds number problem conducted in a 8m long wind-tunnel. The roughness considered is the randomly distributed type and introduced in the form of 24-grit sandpaper and pea gravel. The primary measurement tool is two-component LDV. The basis of the analysis is the mean equation of dynamics. In this regard, the length scale defining where the mean dynamics become dominated by inertia is of central importance. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GB.00006: Structural Aspects of Wall Turbulence Over Low-Order Representations of Irregular Roughness R. Mejia-Alvarez, K.T. Christensen Low-order representations of roughness replicated from a turbine blade damaged by deposition of foreign materials are generated using singular value decomposition (SVD) to decompose the surface into a set of topographical basis functions (383 total) of decreasing importance to the original (``full") surface character. The low-order surface models are then formed by truncating the full set of basis functions at the first 5 and 16 modes (containing approximately 71\% and 95\% of the full surface content, respectively), so that only the most dominant, and large-scale, topographical features are included in the models. Physical replications of the full surface and the two models are created by rapid prototyping and PIV is used to measure velocity fields for all cases in both wall-normal and wall-parallel planes from which the structural aspects of these flows are explored. While the three rough-wall flows are found to be similar to smooth-wall flow outside the roughness sublayer, differences are noted within the roughness sublayer. Persistent low- (LMR) and high-momentum (HMR) regions often bounded by regions of enhanced Reynolds stresses are observed at preferential spanwise positions that vary depending upon the details of the topography. These observations suggest the possibility of enhanced production and dissipation at preferential locations within the roughness sublayer that depend upon the details of the topography. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GB.00007: Roughness effects on fully developed pipe flow at high Reynolds numbers Marcus Hultmark, Margit Vallikivi, Alexander Smits Well resolved turbulence measurments were conducted in the Princeton/ONR Superpipe throughout the smooth and transitional regime and into the fully rough regime. The pipe tested was a commercial steel pipe with $k_{rms}=5\mu$m. This pipe is smooth up to about $Re_D=800\times10^3$, and fully rough above about $Re_D=5\times10^6$. In order to resolve the turbulence at these high Reynolds numbers a nano-scale thermal anemometry probe (NSTAP) was used to obtain the data. This sensor has a sensing volume of $60 \times 2 \times 0.1 \mu$m which is one order of magnitude smaller than conventional techniques, allowing to study the effects of rougness independently of any spatial filtering effects at high Reynolds numbers. This new data reveals information about the behaviour of roughness both in the inner layer and the outer layer. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GB.00008: An analytical framework for the study of rough-wall turbulent boundary layer Kiran Bhaganagar, Richard Leighton To study the dynamics of rough-wall turbulent boundary layer, an alternate set of transport equations that contain an implicit roughness drag and roughness production have been developed. The Canonical Reynolds averaged Navier-Stokes equations and transport equations are not well suited for this purpose, as they do not contain any roughness information. In this talk we present an analytical framework suitable for a rough-wall based on three-level decomposition of velocity. Direct numerical simulations have been used to simulate flow in a channel with rough-walls. We present the results for the transport equation of the mean momentum equation and discuss the significance of explicit roughness drag term that arises due to this formulation. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GB.00009: Frictional drag measurements for pitted surface roughness Karen Flack, Michael Schultz An important unanswered question in fluid mechanics is the prediction of the frictional drag for a generic surface roughness. Based on previous research by the authors and others, a new correlation was proposed to estimate the frictional drag for a surface covered with three-dimensional, irregular roughness in the fully rough regime. The correlation relies solely on a measurement of the surface roughness profile. A relationship is given for the equivalent sandgrain roughness height as a function of the root-mean-square roughness height and the skewness of the roughness probability density function. The correlation was developed using results from boundary layer measurements. Towing tank tests were performed to obtain the overall increase in drag due to surface roughness in order to validate the correlation for a wider range of surfaces. Results from rough surfaces with pitting (negatively skewed \textit{pdf's}) will be presented. [Preview Abstract] |
Monday, November 22, 2010 9:57AM - 10:10AM |
GB.00010: Turbulent boundary layer structure over sparsely-distributed roughness Michael Schultz, Bharathram Ganapathisubramani Experiments were performed on a surface consisting of sparsely-distributed rigid circular cylinder roughness elements to examine the effects of solidity on turbulence structure. The solidity ($\lambda$) of the roughness (defined as frontal area of the roughness elements per unit wall-parallel area) is 0.08. This value was chosen to lie in the ``sparse'' regime ($\lambda \leq$ 0.1) while maintaining a large equivalent sandgrain roughness height ($k_s$). Measurements were made using both two-component LDV and wide-field planar PIV techniques at two Reynolds numbers, $Re_\tau$ = 1800 and 4000. The results indicate that the relative roughness height of these elements ($k/\delta$) is approximately 0.09. The value of ${k_s}^+$ is greater than 350 in both cases indicating that the flow is in the fully-rough regime. The mean velocity profile in defect form at both Reynolds numbers conforms to outer-layer similarity. The Reynolds stresses appears to exhibit outer-layer similarity for $y > 3k_s$. However, differences between the two Reynolds numbers are observed closer to the wall. Specifically, a broad peak in the streamwise Reynolds normal stress is observed around $y/\delta$ = 0.12 for $Re_\tau$ = 4000. Further analysis using both the PIV and LDV data will be performed and presented. [Preview Abstract] |
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