Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D32: Rough Wall Turbulent Boundary Layers  II 
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Chair: Corey Markfort, University of Iowa Room: Oregon Ballroom 201 
Sunday, November 20, 2016 2:57PM  3:10PM 
D32.00001: Surface fluxes in atmospheric boundary layer flows over complex terrain Wei Zhang, Corey Markfort, Fernando PortÃ©Agel Interactions between the atmosphere and the land/water surface can be described by fluxes of momentum, heat and other scalars. While predicting the atmospheric boundarylayer (ABL) flows and modeling regional/global weather and climate, these surface fluxes need to be specified as boundary conditions. It is a common practice to use formulations based on the MoninObukhov similarity theory even for flows over a wide range of complex terrain, which maybe deviate significantly from the conditions of steady, fullydeveloped ABL flow, due to the knowledge gap for turbulent transport of fluxes across the interface. This work aims to provide insights for spatial distribution of the surface fluxes in ABL flows involving typical complex terrain cases, including surface roughness transition, steep topography and canopy patches. Results from windtunnel experiments will be presented to characterize the surface momentum and heat fluxes for different flow regimes and their correlation to the turbulent flow properties in thermallystratified boundary layers. Application of the similarity theory to such cases is evaluated by comparing to the measurements. Ultimately, new knowledge of surface fluxes will help to improve parameterization of the surfaceatmosphere interaction in numerical models. [Preview Abstract] 
Sunday, November 20, 2016 3:10PM  3:23PM 
D32.00002: Measurements of turbulent boundary layer flow and surface fluxes over roughness and temperature transitions Corey Markfort, Wei Zhang, Fernando PorteAgel Often natural and engineered surfaces have spatially heterogeneous properties at a variety of scales that affect the structure of the turbulent boundary layer, which is no longer in equilibrium with the local surface. Predicting the spatial distributions of surface momentum and scalar fluxes over heterogeneous surfaces remains a challenge. We present measurements made in a thermally stratified boundary layer wind tunnel to characterize the turbulent flow and surface fluxes for abrupt transitions in surface temperature and roughness. We compare the development of internal boundary layers for momentum and heat, and associated mean surface flux for two cases. The first is a smooth boundary layer with an abrupt change in surface temperature and the second also involves a change from a fully rough to a smooth wall.~ The effects of roughness change on surface heat flux and implications for prediction are examined. The data will be compared to typical models that utilize MoninObukhov similarity theory. [Preview Abstract] 
Sunday, November 20, 2016 3:23PM  3:36PM 
D32.00003: Contrasts Between Momentum and Scalar Exchanges Over Very Rough Surfaces Elie BouZeid, Qi Li Understanding of the physical processes modulating transport of momentum and scalars over very rough walls is essential in a large range of engineering and environmental applications. Since passive scalars are advected with the flow, broad similarity is expected between momentum and scalar transport. However, unlike momentum, which is dominated by form drag over very rough walls, scalar transport must occur through the viscous exchanges at the solidfluid interface, which might result in transport dissimilarity. To examine these similarities and differences of momentum and passive scalar exchanges over large threedimensional roughness elements, a suite of largeeddy simulations is conducted. The turbulent components of the transport of momentum and scalars within the canopy and roughness sublayers are found to be similar. However, strong dissimilarity is noted between the dispersive fluxes. The dispersive components are also found to be a significant fraction of the total fluxes within and below the roughness sublayer. Increasing frontal density induces a general transition in the flow from a rough boundary layer type to a mixedlayerlike type, which is found to have contrasting effects on momentum and scalar transport. [Preview Abstract] 
Sunday, November 20, 2016 3:36PM  3:49PM 
D32.00004: On the turbulent boundary layer over geophysicallike topographies. Leonardo P. Chamorro, Ali M. Hamed, Luciano Castillo The developing and developed flows over 2D and 3D largescale wavy walls were experimentally studied with highresolution planar PIV in a refractiveindexmatching channel. The 2D wall is described by a sinusoidal wave in the streamwise direction with amplitude to wavelength ratio a/$\lambda $x $=$ 0.05, while the 3D wall has an additional wave in the spanwise direction with a/$\lambda $y $=$ 0.1. The flow was characterized at Re$=$ 4000 and 40000, based on the bulk velocity and the channel half height. The walls have amplitude to boundary layer thickness ratio $a/\delta_{99} \quad \approx $ 0.1 and resemble largescale and geophysicallike roughnesses found in rivers and natural terrain. Instantaneous velocity fields and timeaveraged turbulence quantities reveal strong coupling between largescale topography and the turbulence dynamics near the wall, and the presence of a wellstructured shear layer that enhances the turbulence for both walls. However, the 3D wall exhibits spanwise flow that is thought to be responsible for distinctive flow features, including comparatively reduced spanwise vorticity and decreased turbulence levels. Further insight is drawn in the developed and developing regions through proper orthogonal decomposition and quadrant analysis. [Preview Abstract] 
Sunday, November 20, 2016 3:49PM  4:02PM 
D32.00005: Physical modeling of the atmospheric boundary layer in the UNH Flow Physics Facility Gregory TaylorPower, Stephanie Gilooly, Martin Wosnik, Joe Klewicki, John Turner The Flow Physics Facility (FPF) at UNH has test section dimensions W$=$6.0m, H$=$2.7m, L$=$72m. It can achieve high Reynolds number boundary layers, enabling turbulent boundary layer, wind energy and wind engineering research with exceptional spatial and temporal instrument resolution. We examined the FPF's ability to experimentally simulate different types of the atmospheric boundary layer (ABL) using upstream roughness arrays. The American Society for Civil Engineers defines standards for simulating ABLs for different terrain types, from open sea to dense city areas (ASCE 4912). The standards require the boundary layer to match a power law shape, roughness height, and power spectral density criteria. Each boundary layer type has a corresponding power law exponent and roughness height. The exponent and roughness height both increase with increasing roughness. A suburban boundary layer was chosen for simulation and a roughness element fetch was created. Several fetch lengths were experimented with and the resulting boundary layers were measured and compared to standards in ASCE 4912: Wind Tunnel Testing for Buildings and Other Structures. Pitot tube and hot wire anemometers were used to measure average and fluctuating flow characteristics. Velocity profiles, turbulence intensity and velocity spectra were found to compare favorably. [Preview Abstract] 
Sunday, November 20, 2016 4:02PM  4:15PM 
D32.00006: The flow field around a pair of cubes immersed in the inner part of a turbulent boundary layer measured using holographic microscopy Jian Gao, Joseph Katz The objective of this study is to characterize the interaction of a turbulent boundary layer with roughness elements. Digital holographic microscopy is applied to measure the flow structure and turbulence around a pair of cubic roughness elements aligned in the spanwise direction with height $a=90\delta_{\nu } $ and embedded in the inner layer of a turbulent channel flow at $Re_{\tau } =2500$. The ratio of halfchannel height to cube height is 25, and the cubes are separated by 1.5$a$. The fieldofview size is 385$\delta_{\mathrm{\nu }}\times $250$\delta_{\mathrm{\nu }}\times $190$\delta_{\mathrm{\nu }}$ and the vector spacing of the 3D 3component velocity fields is $5.4\delta_{\nu } $. Ensemble statistics, which provide distributions of mean velocities and Reynolds stresses, demonstrate the boundary layer separation upstream of the obstacles and resulting formation of vortical semirings around each cube, dominated by vertical vorticity on the sides and spanwise vorticity on top of the cubes. The vortical ``canopy'' persists and hovers around the separated region behind the cubes, but then becomes scrambled in the turbulent wake further downstream. The necklace vortices form upstream of the cubes and remain concentrated near the wall as streamwise vortices between the cubes, but expand substantially in the wake region. Effects of the neighboring cube include accelerated flow channeling in the space between the obstacles and differences between the spatial distributions of vorticity at the inner and outer sides. [Preview Abstract] 
Sunday, November 20, 2016 4:15PM  4:28PM 
D32.00007: Effect of truncated cone roughness element density on hydrodynamic drag Kristofer Womack, Michael Schultz, Charles Meneveau An experimental study was conducted on roughwall, turbulent boundary layer flow. Varying planform densities of truncated cone roughness elements were investigated. Element densities studied ranged from 10\% to 57\%. Detailed turbulent boundary layer velocity statistics were recorded with a twocomponent LDV system on a threeaxis traverse. Hydrodynamic roughness length ($z_0$) and skinfriction coefficient ($C_f$) were determined and compared with the estimates from existing roughness element drag prediction models including Macdonald et al. (1998) and Yang et al. (2015). The roughness elements used in this work model idealized barnacles, so implications of this data set for ship powering are considered. [Preview Abstract] 
Sunday, November 20, 2016 4:28PM  4:41PM 
D32.00008: Skin friction measurements of mathematically generated roughness in the transitionally to fullyrough regimes Julio Barros, Michael Schultz, Karen Flack Engineering systems are affected by surface roughness which cause an increase in drag leading to significant performance penalties. One important question is how to predict frictional drag purely based upon surface topography. Although significant progress has been made in recent years, this has proven to be challenging. The present work takes a systematic approach by generating surface roughness in which surfaces parameters, such as $rms$, skewness, can be controlled. Surfaces were produced using the random Fourier modes method with enforced powerlaw spectral slopes. The surfaces were manufactured using high resolution 3Dprinting. In this study three surfaces with constant amplitude and varying slope, $P$, were investigated ($P = 0.5, 1.0, 1.5$). Skinfriction measurements were conducted in a high Reynolds number turbulent channel flow facility, covering a wide range of Reynolds numbers, from hydraulicsmooth to fullyrough regimes. Results show that some long wavelength roughness scales do not contribute significantly to the frictional drag, thus highlighting the need for filtering in the calculation of surface statistics. Upon highpass filtering, it was found that $k_{rms}$ is highly correlated with the measured $k_s$. [Preview Abstract] 

D32.00009: ABSTRACT WITHDRAWN 

D32.00010: ABSTRACT WITHDRAWN 
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