Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session A26: Surface Roughness: ExperimentsBoundary Layers Turbulence
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Chair: Jimmy Philip, University of Melbourne Room: 707 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A26.00001: Effect of truncated cone roughness element density on hydrodynamic drag Kristofer Womack, Michael Schultz, Charles Meneveau An experimental study was conducted on rough-wall, turbulent boundary layer flow with roughness elements whose idealized shape model barnacles that cause hydrodynamic drag in many applications. Varying planform densities of truncated cone roughness elements were investigated. Element densities studied ranged from 10\% to 79\%. Detailed turbulent boundary layer velocity statistics were recorded with a two-component LDV system on a three-axis traverse. Hydrodynamic roughness length ($z_0$) and skin-friction coefficient ($C_f$) were determined and compared with the estimates from existing roughness element drag prediction models including Macdonald et al. (1998) and other recent models. 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 19, 2017 8:13AM - 8:26AM |
A26.00002: Experimental investigation of turbulent flow-roughness interaction over surfaces of rigid and flexible roughness Mostafa Toloui, Jiarong Hong The influence of flexible surface roughness on wall-bounded turbulent flows is examined experimentally via simultaneous 3D fluid velocity and roughness deformation measurements using Digital inline holographic PTV (i.e. DIH-PTV, Toloui et al. Meas. Sci. {\&} Tech 2017). The experiments are conducted in a refractive-index-matched turbulent channel over two rough surface panels of similar geometry but with an order of magnitude difference in elastic modulus (1.8 Mpa vs. 0.2 Mpa). The roughness elements (i.e. tapered cylinders of 0.35 mm in base diameter, 3 mm in height, 4 mm spacing) are designed such that the rough surface with higher modulus shows no deformation (namely rigid roughness) while the one with lower elasticity deforms appreciably under the same flow conditions (\textit{Re}$_{\mathrm{h}} \quad \approx $ 32500, based on centerline velocity and channel width). The concurrent fluid velocity and roughness deformation measurements are acquired with 160 $\mu $s temporal, 1.1 mm/vector velocity, and \textless 20 $\mu $m deformation resolutions from a 10 \texttimes 50 \texttimes 10 mm$^{\mathrm{3}}$ sampling volume. Despite minimal influence on mean velocity fields, the effect of roughness compliance on turbulence kinetic energy is observed and linked to roughness deformation. The fingerprint of this energy exchange on shortening the instantaneous flow structures, reduction of Reynolds stresses as well as flow features in energy spectra are examined and will be presented in detail. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A26.00003: A spatial picture of the synthetic large-scale motion from dynamic roughness David Huynh, Beverley McKeon Jacobi and McKeon (2011) set up a dynamic roughness apparatus to excite a synthetic, travelling wave-like disturbance in a wind tunnel, boundary layer study. In the present work, this dynamic roughness has been adapted for a flat-plate, turbulent boundary layer experiment in a water tunnel. A key advantage of operating in water as opposed to air is the longer flow timescales. This makes accessible higher non-dimensional actuation frequencies and correspondingly shorter synthetic length scales, and is thus more amenable to particle image velocimetry. As a result, this experiment provides a novel spatial picture of the synthetic mode, the coupled small scales, and their streamwise development. It is demonstrated that varying the roughness actuation frequency allows for significant tuning of the streamwise wavelength of the synthetic mode, with a range of 3$\delta$-13$\delta$ being achieved. Employing a phase-locked decomposition, spatial snapshots are constructed of the synthetic large scale and used to analyze its streamwise behavior. Direct spatial filtering is used to separate the synthetic large scale and the related small scales, and the results are compared to those obtained by temporal filtering that invokes Taylor's hypothesis. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A26.00004: The flow field around a pair of cubic roughness elements with different spacings immersed in turbulent boundary layer Karuna Agarwal, Jian Gao, Joseph Katz The shape, size, and spacing between roughness elements in turbulent boundary layers affect the associated drag and noise. Understanding them require data on the flow structure around these elements. Dual-view tomographic holography is used to study the 3D 3-component velocity field around a pair of cubic roughness elements immersed in a turbulent boundary layer at Re$_{\mathrm{\tau \thinspace }}=$ 2500. These $a=$1 mm high cubes correspond to 4{\%} of the half channel height and 90 wall units ($\delta _{\mathrm{\nu }}=$11 $\mu $m). Tests are performed for spanwise spacings of $a$, 1.5$a$ and 2.5$a$. The sample volume is 385$\delta_{\mathrm{\nu }}\times $250$\delta_{\mathrm{\nu }}\times $190$\delta_{\mathrm{\nu }}$ and the vector spacing is 5.4$\delta_{\mathrm{\nu }}$. Conversed statistics is obtained by recording 1500 realizations in volumes centered upstream, downstream and around a cube. The boundary layer separating upstream of the cube does not reattach until the wake region, resulting in formation of a vortical ``canopy'' that engulfs each cube. It is dominated by spanwise vorticity above the cube and separated region, bounded by vertical vorticity on the sides. Flow channeling in the space between cubes causes asymmetry in the vorticity distributions along the inner and outer walls. The legs of horseshoe vortices remain near the wall between cubes, but grow and expand in the wake region. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A26.00005: Spatio-Temporal Signatures of One- And Two-Mode Rough Walls in Turbulent Boundary Layers Jonathan Morgan, Beverley McKeon A rough wall acts on a turbulent boundary layer flow by altering the wall boundary condition and creating persistent spatial inhomogeneity in the velocity field compared to a smooth-wall flow. A single Fourier mode of roughness, with height which varies in the streamwise and spanwise directions, will create a single static Fourier mode in the velocity field. Two or more Fourier modes of roughness will interact to create more complicated inhomogeneity. A finite number of roughness modes representing the largest scales of a realistic roughness geometry will begin to approximate the flow physics of the full roughness as the number of modes increases (Mejia-Alvarez and Christensen 2010). This study investigates the effect of simple roughnesses, consisting of one or two static height Fourier modes which vary in the streamwise and spanwise directions. The direct effect of the altered boundary condition is apparent in the inhomogeneous mean velocity field, while the indirect effect on the turbulent fluctuations is observed through the spatial inhomogeneity of the turbulent power spectrum and associated with specific nonlinear (triadic) interactions. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A26.00006: Skin friction measurements of systematically-varied roughness: Probing the role of roughness amplitude and skewness Julio Barros, Karen Flack, Michael Schultz Real-world engineering systems which feature either external or internal wall-bounded turbulent flow are routinely affected by surface roughness. This gives rise to performance degradation in the form of increased drag or head loss. However, at present there is no reliable means to predict these performance losses based upon the roughness topography alone. This work takes a systematic approach by generating random surface roughness in which the surface statistics are closely controlled. Skin friction and roughness function results will be presented for two groups of these rough surfaces. The first group is Gaussian (i.e. zero skewness) in which the root-mean-square roughness height ($k_{rms})$ is varied. The second group has a fixed $k_{rms}$, and the skewness is varied from approximately -1 to $+$1. The effect of the roughness amplitude and skewness on the skin friction will be discussed. Particular attention will be paid to the effect of these parameters on the roughness function in the transitionally-rough flow regime. For example, the role these parameters play in the monotonic or inflectional nature of the roughness function will be addressed. Future research into the details of the turbulence structure over these rough surfaces will also be outlined. [Preview Abstract] |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A26.00007: Downstream development of a turbulent boundary layer following a step change in roughness height: influence of virtual origin M. Li, C. M. de Silva, R. Baidya, D. Chung, I. Marusic, N. Hutchins In this study we examine a streamwise heterogeneous roughness where a step change in wall condition occurs from a rough to a smooth wall along the flow direction. Our work focuses on the impact of the height difference between the virtual origins of the rough- and smooth-walled surfaces. Accordingly, a set of particle tracking velocimetry experiments are conducted where the smooth wall is located (1) above the roughness peak, (2) below the roughness valley and (3) in between the roughness canopy. These data are used to provide direct measurements of the wall shear stress from the viscous sublayer over the smooth wall, and are compared against estimates from oil film interferometry and through empirical fits to hotwire databases, as well as large-eddy simulation results at matching Reynolds number obtained at California Institute of Technology. Differences in virtual origin between the two surfaces of less than 5\% of the boundary layer thickness lead to significant changes in the evolution of the shear stress after a change in surface conditions. These observations may explain the large scatter reported in past studies on the evolution of a turbulent boundary layer and associated internal layer after a sudden change in surface conditions. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A26.00008: Wall roughness induces asymptotic ultimate turbulence Ruben A. Verschoof, Xiaojue Zhu, Dennis Bakhuis, Sander G. Huisman, Roberto Verzicco, Chao Sun, Detlef Lohse For real-world applications of wall-bounded turbulence, the underlying surfaces are virtually always rough; yet understanding the effects of wall roughness for turbulence remains a challenge. By combining experiments and numerical simulations, here, taking as example the paradigmatic Taylor-Couette system (the closed flow between two independently rotating coaxial cylinders), we uncover the mechanism that causes the considerable enhancement of the overall transport properties by wall roughness. If both walls are rough, the viscosity dependence is thoroughly eliminated and we thus achieve what we call {\it asymptotic ultimate turbulence}, i.e. the upper limit of transport, whose existence had been predicted by Robert\ Kraichnan in 1962 (Phys. Fluids {\bf 5}, 1374 (1962)). [Preview Abstract] |
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