Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session BE: Turbulence: Boundary Layers II |
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Chair: Beverley McKeon, California Institute of Technology Room: Salt Palace Convention Center 151 D-F |
Sunday, November 18, 2007 10:34AM - 10:47AM |
BE.00001: Turbulent boundary layers over a systematically varied rough wall Michael Schultz, Karen Flack Results of an experimental investigation of the flow over a model roughness are presented. The series of roughness consists of close-packed pyramids in which both the height and the slope are systematically varied. The aim of this work is to gain insight into the physical roughness scales which contribute to drag. The mean velocity profiles for all nine rough surfaces collapse with smooth-wall results when presented in velocity-defect form. The Reynolds stresses also show good agreement with smooth-wall results outside the roughness sublayer when presented in outer variables. The results for the six steepest surfaces indicate that the roughness function, $\Delta U^{+}$, scales almost entirely on the roughness height with only a weak dependence on the slope of the pyramids. However, $\Delta U^{+ }$for the three surfaces with the smallest slope does not scale on the roughness height, indicating that these surfaces might not be thought of as surface `roughness' in a traditional sense but instead surface `waviness'. [Preview Abstract] |
Sunday, November 18, 2007 10:47AM - 11:00AM |
BE.00002: Development of roughness scaling parameters in the fully rough regime. Karen Flack, Michael Schultz The most important unresolved question regarding surface roughness is to identify suitable roughness length scales that can be used to predict the frictional drag of a body covered with any generic roughness. The focus of this research is to identify length scales of the surface topography, obtained from profilometry, that correlate with the roughness functions, obtained from detailed boundary layer velocity profiles and towing tank tests. Results for three-dimensional roughness in the fully rough regime indicate that the two most important scales are a roughness height, such as the rms roughness height, and the skewness of the probability distribution function of the roughness height. Correlations will be presented for a variety of three-dimensional roughness types. [Preview Abstract] |
Sunday, November 18, 2007 11:00AM - 11:13AM |
BE.00003: Applicability of Townsend's similarity hypothesis to very-rough-wall channel flow David M. Birch, Jonathan F. Morrison There has been considerable recent interest in the scaling of turbulent flows over rough walls, and especially in the applicability of Townsend's similarity hypothesis to flows over walls with values of $k/\delta$ exceeding 2.5 \% (where $k$ is the mean wall-normal roughness height and $\delta$ is the outer length scale). In order to investigate outer-layer similarity under these very-rough wall conditions, an experimental study of the turbulent flow through a channel has been carried out using both a broadly distributed grit-type roughness with $k/h$ = 0.035 and $ku_{\tau}/\nu$ $\sim$ 200 (where $h$ is the channel half-height, $u_{\tau}$ is the wall friction velocity and $\nu$ is the kinematic viscosity) and a mesh-type roughness with $L_ {x}/L_{z}$ = 2.6, $k/h$ = 0.031 and $ku_{\tau}/\nu$ $\sim$ 190 (where $L_{x}$ and $L_{z}$ are the axial and spanwise mesh spacings, respectively). The channel flow in both cases is shown to be fully developed and statistically stationary, and the turbulence kinetic energy budgets are examined. Scaling of the Reynolds stresses is also examined and the behaviour of the higher-order statistics is compared with previous investigations. Single- and two-point velocity correlations are used to deduce the behaviour of the structures with special attention being given to the scaling of the spectra of the streamwise and wall-normal velocity components. [Preview Abstract] |
Sunday, November 18, 2007 11:13AM - 11:26AM |
BE.00004: Influence of wall roughness on turbulent boundary layer Lukas Vesely, Christian Haigermoser, Michele Lavecchia, Michele Onorato Measurements were taken with time resolved PIV System in streamwise wall normal plane and plane parallel to the wall for different distances over smooth and 3 types of rough surfaces ($\delta $/k$_{S} \quad <$ 40) For smooth case Re$_{\theta }$ = 935 and for rough cases Re$_{\theta }$ = 1418$\div $1922. Instant flow images for all roughned walls show features similar to this as expected in a smooth wall turbulent boundary layer. The main differences occur in relation to scaling and level of organization. Large irregularly shaped zones of flow, having relatively retarded uniform values of the streamwise momentum, separated by thin regions of large $\partial $u/$\partial $y are shown. Along the boundaries trains of negative swirling motions are irregularly aligned, representing the signature on the measurement plane of packets of vortical structures: hairpin vortices and skewed quasi-longitudinal vortices. In wall parallel planes dominant structures typical for smooth wall as low and high momentum zones elongated in the streamwise direction remain. Roughness effects show a similar behavior to that seen in the smooth wall case, where a shift towards the wall of the organized structures can be noted. Detailed analysis was performed to enlighten difference between the different flow fields. [Preview Abstract] |
Sunday, November 18, 2007 11:26AM - 11:39AM |
BE.00005: Structural Characteristics of Flow Over a Highly-Irregular Surface Topology Y. Wu, K.T. Christensen The structural character of a zero-pressure-gradient turbulent boundary layer over highly-irregular surface roughness is investigated. Two-dimensional PIV measurements are made in the streamwise--wall-normal plane of this flow and stereoscopic PIV measurements are performed in wall-parallel planes both within and at the outer edge of the roughness sublayer. The roughness under consideration is replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topological scales. Previous analysis of the streamwise--wall-normal data reveals that the single-point turbulence statistics collapse with those of smooth-wall flow outside the roughness sublayer, consistent with the notion of outer-layer similarity. This similarity also appears to extend to the spatial structure of the outer-layer turbulence. The wall-parallel measurements within the roughness sublayer reveal strong, yet local, heterogeneities in the turbulence statistics that occur spatially-coincident with the most intense surface defects. Two-point correlations and conditional averaging are employed to contrast the spatial footprints of the dominant vortical structures within and outside the roughness sublayer to those of smooth-wall turbulence. [Preview Abstract] |
Sunday, November 18, 2007 11:39AM - 11:52AM |
BE.00006: The response of turbulent boundary layers to dynamic roughness perturbations Jacob George, Jeffrey LeHew, Beverley McKeon We describe a fundamental experimental study into the effects of spatially sudden, temporally-varying wall roughness on a canonical turbulent boundary layer at moderate Reynolds number. Results for dynamic disturbances are compared with boundary layer structure, development and return to equilibrium in the presence of constant amplitude ``static'' roughness. The ``dynamic roughness'' can be described as a perturbation to the wall morphology with a variable ratio of peak amplitude, $k$, to boundary layer thickness, $\delta$, and frequencies from d.c. to the order of the burst frequency (limited here by the mechanical drive mechanism). The input wall boundary condition is characterized with the aid of image velocimetry techniques and the response of the zero pressure gradient turbulent boundary layer is obtained using Pitot probes and hot-wire anemometry. [Preview Abstract] |
Sunday, November 18, 2007 11:52AM - 12:05PM |
BE.00007: The Effect of Freestream Turbulence on a Rough Surface Turbulent Boundary Layer Brian Brzek, Sheilla Torres-Nieves, Jose Lebron-Bosques, Raul Cal, Charles Meneveau, Luciano Castillo Measurements are performed in the Stanley Corrsin Wind Tunnel Facility at The Johns Hopkins University to study the effects of freestream turbulence on a rough surface turbulent boundary layer. The upstream turbulence was generated with the use of an active grid, resulting in freestream turbulence levels of 6.2 {\%} and 5.2 {\%}. The effects of the freestream turbulence on this rough surface, alter the mean velocity deficit profiles. The effects of freestream turbulence are identifiable in the streamwise Reynolds stress profiles throughout the entire boundary layer, but not necessarily for the wall-normal and shear stress profiles. The reduced velocity gradient reduced the production term in the streamwise Reynolds stress despite the increase in the $<$u$^{2}>$ profiles. Thus, an important mechanism becomes the turbulent diffusion from the freestream turbulence as it relates to the Reynolds stress equations. [Preview Abstract] |
Sunday, November 18, 2007 12:05PM - 12:18PM |
BE.00008: High Reynolds number rough wall turbulent boundary layer experiments using Braille surfaces Michael Harris, Jason Monty, Todd Nova, James Allen, Min Chong This paper details smooth, transitional and fully rough turbulent boundary layer experiments in the New Mexico State high Reynolds number rough wall wind tunnel. The initial surface tested was generated with a Braille printer and consisted of an uniform array of Braille points. The average point height being 0.5mm, the spacing between the points in the span was 0.5mm and the surface consisted of span wise rows separated by 4mm. The wavelength to peak ratio was 8:1. The boundary layer thickness at the measurement location was 190mm giving a large separation of roughness height to layer thickness. The maximum friction velocity was $u_\tau=1.5m/s$ at $Re_x=3.8 \times 10^7$. Results for the skin friction co-efficient show that this surface follows a Nikuradse type inflectional curve and that Townsends outer layer similarity hypothesis is valid for rough wall flows with a large separation of scales. Mean flow and turbulence statistics will be presented. [Preview Abstract] |
Sunday, November 18, 2007 12:18PM - 12:31PM |
BE.00009: On Scaling Rough-Wall Turbulent Boundary Layers Faraz Mehdi, Joseph Klewicki The problem of scaling rough-wall turbulent boundary layers is explored relative to the prevalent inner/outer/overalap layer model of turbulent wall flows and relative to the four layer structure associated with the mean momentum balance. Of particular interest is the characterization of the combined roughness/Reynolds number problem using the characteristic scales inherent to each model. Within this context, available rough turbulent boundary layer normal and shear stress data are examined. The locations of the peaks in these profiles are determined and sorted according to $\delta^+$ and $k_s^+$. For all of the data examined, these analyses indicate that the peak positions occur within the roughness sublayer. The implications of these are discussed relative to the position of the Reynolds shear stress peak with increasing Reynolds number and relative to the effects of roughness on the structure of the mean momentum field. The present survey also reveals the lack of data sets sufficient to characterize the rough-wall problem, even under the utilization of general parameters such as equivalent sand grain roughness. Experiments addressing some of the open questions are suggested. [Preview Abstract] |
Sunday, November 18, 2007 12:31PM - 12:44PM |
BE.00010: A Numerical Study of a Scaling Parameter for Turbulent Boundary Layer with Large Roughness Erika Johnson, Chelakara Subramanian Several investigations have shown that when a turbulent boundary layer is subjected to large surface roughness (on the order of the inner layer thickness), the friction velocity, u$_{\tau}$, is not a suitable scaling parameter for the mean and turbulent velocities. Experimental studies concerning large surface roughness have consistently shown a dramatic effect on the mean and turbulent velocities, with the effect being particularly strong on the wall normal velocity fluctuation, $\overline{\mbox{v'}^2} $. Additionally, Lebrun (2004) showed a concomitant pressure gradient normal to the rough surface, even when no external pressure gradient exists. In this investigation, we perform a numerical simulation of a boundary layer over a surface with large roughness (k/$\delta $ = 0.07 - 0.3) to gain more insight on the correlation between the wall normal pressure gradient and the wall normal velocity fluctuation, $\overline{\mbox{v'}^2} $. Four different size regular roughness elements are considered and the pressure gradient velocity scale, u$_{P}$, is applied to each case. It is believed that the pressure gradient velocity scale, u$_{P}$, is a better alternative for capturing the effects of the roughness induced pressure gradient. While a perfect similarity is not achieved, the results scaled by u$_{P}$ are encouraging. [Preview Abstract] |
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