Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H26: Rough Wall Boundary Layers II |
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Chair: Michael Schultz, United States Naval Academy Room: 2007 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H26.00001: A study of transient channel flow in a transitionally rough regime Mehdi Seddighi, Shuisheng He, Tom O'Donoghue, Dubravka Pokrajac, Alan Vardy DNS has been used to investigate the transient behaviour of turbulence following a rapid flow acceleration from an initially turbulent flow in a channel with a smooth top wall and a roughened bottom wall made of close-packed pyramids. Simulations have been performed at various flow conditions in the transitionally rough regime with equivalent roughness heights ($k_{s}^{+}$) ranging from 12 to 42. It is shown that the transient responses of the flow over the smooth and rough walls are practically independent of each other. Also, the nature of the process over the rough wall varies strongly as the influence of the roughness increases during the early stages of the acceleration. Whereas the transient flow over the smooth-wall undergoes a process strikingly similar to laminar-turbulent bypass transition, the corresponding behaviour over the rough wall depends on the wall condition. When the equivalent roughness height of the final flow condition is below $\sim$30, bypass-like transition dominates, although the roughness induces early transition. When $k_{s}^{+} >30$, however, the rough-wall flow undergoes a highly transient process resembling roughness induced transition. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H26.00002: Properties of the advective transport contribution to the inertial mean dynamics of rough-wall boundary layers Rachel Ebner, Joseph Klewicki Measurements and scaling analyses are conducted to clarify the combined effects of roughness and Reynolds number on momentum transport in rough-wall turbulent boundary layers. Experiments employing a four element (``Foss style'') spanwise vorticity probe cover nearly a decade in Reynolds number, and nearly three decades in sand grain roughness, $k_s^+$. Here we leverage the expression that decomposes the Reynolds stress gradient into the difference of two velocity-vorticity correlations, i.e., $-\partial \overline{uv}/{\partial y} = \overline{v\omega_z}-\overline{w\omega_y}$. The present analyses focus on the first term on the left hand side, $v\omega_z$, in the logarithmic layer and outer regions, as it is known from smooth-wall studies that this advective transport mechanism is the largest contributor to $-\partial \overline{uv}/{\partial y}$ in the domain where the mean dynamics are inertially dominated. Streamwise correlation maps and length scales associated with the spectra and correlations of $v$ and $\omega_z$ are used to clarify the scaling behaviors of the motions underlying $-\partial \overline{uv}/{\partial y}$. The results are shown to further support the combined roughness Reynolds number description of Mehdi et al. 2013, \textit{J. Fluid Mech.} \textbf{731}, 682 [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H26.00003: Characteristics of Large-Scale Motions in a Turbulent Boundary Layer Overlying Complex Roughness J.M. Barros, K.T. Christensen The characteristics of large-scale motions in a turbulent boundary layer overlying complex roughness are explored with high-frame-rate stereo PIV measurements in the wall-normal--spanwise plane. It was previously reported that the single-point turbulence statistics of this flow display strong spanwise heterogeneity, particularly spanwise-alternating low- and high-momentum flow pathways in the mean flow bounded by large-scale streamwise-oriented roll cells and marked by enhanced Reynolds stresses and turbulent kinetic energy. These patterns were interpreted as the imprints of roughness-induced turbulent secondary flows owing to the streamwise elongation and spanwise heterogeneity of the topography. Frequency spectra of all three velocity components at fixed wall-normal location also display strong dependence on spanwise position, principally that of the streamwise velocity. In particular, the roughness promotes enhanced turbulent kinetic energy content of the large-scale motions and smaller-scale motions, coupled with strong spanwise dependence in the energy content of the very-large-scale motions when compared to smooth-wall flow. Modifications of Reynolds shear stress content as a function of scale are also explored from the three-component velocity measurements. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H26.00004: Comparison of the coherent structure of rough and smooth wall turbulent boundary layers at high Reynolds number Dougal Squire, Charitha de Silva, Michael Schultz, Nicholas Hutchins, Ivan Marusic A comparison of structural aspects of the log- and wake-regions of smooth and rough wall zero pressure gradient turbulent boundary layers is presented at a friction Reynolds number of approximately 12,000. The roughness consists of P36 sandpaper, installed over the 54 m$^2$ working section in a continuous sheet. The results from four measurements are discussed, consisting of two eight-camera PIV arrangements above each surface. The field of view of both arrangements captures the full wall-normal extent of the boundary layer, but differs in the streamwise direction; one camera array captures a streamwise domain that spans approximately twice the boundary layer thickness; the other has a narrower streamwise extent in order to obtain an enhanced spatial resolution in the order of the Kolmogorov microscale. Combined, the two arrangements enable investigation of structural features with reasonably large streamwise dimension---using the large field of view data---and provide well resolved information on the wall-normal structure of the boundary layer---using the narrow field of view data. Generally, the data in the inertial dominated region confirm that the studied smooth and rough wall bounded flows are structurally similar, providing support for the outer-layer similarity hypothesis. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H26.00005: Perturbation of roughness-induced secondary flow in a turbulent boundary layer overlying complex roughness G. Pathikonda, K.T. Christensen Recent experiments investigating flow over more heterogeneous and organized roughness have revealed spanwise inhomogeneity in turbulence statistics interpreted as roughness-induced secondary flow induced by streamwise elongation and spanwise heterogeneity of the topography itself. In particular, spanwise alternating regions of low- and high-momentum pathways in mean streamwise velocity have been observed, each flanked by streamwise oriented counter-rotating roll cells, for flow over both the complex roughness investigated herein and organized roughness reported in the literature. We explore perturbation of this roughness-induced secondary flow as a means of studying its origin and persistence. Spanwise--wall-normal stereo PIV measurements of flow over complex roughness are made, first with an incident smooth-wall turbulent boundary layer upstream of the roughness followed by perturbation of this incident smooth-wall turbulent boundary layer with organized hemispherical roughness elements prior to transition to the complex roughness (with the hemisphere scale being distinct from that of the complex roughness). Hot-wire measurements are also made to capture the energy distribution/re-distribution at various flow scales in both flow conditions. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H26.00006: Turbulent secondary flows in high Reynolds number boundary layers induced by streamwise-elongated complex roughness William Anderson, Julio Barros, Kenneth Christensen It has been reported that complex roughness with a predominant streamwise elongation induces secondary mean flow heterogeneities in the above turbulent boundary layer (Mejia-Alvarez and Christensen, 2013: \textit{Phys. Fluids} \textbf{25}:115109, MAC; Nugroho et al., 2013: \textit{Int. J. Heat Fluid Flow} \textbf{41}:90-102). These mean secondary flows exist as transverse variations of mean streamwise velocity (so-called low- and high-momentum pathways, MAC) and are flanked by mean counter-rotating, boundary layer-scale circulations (Christensen and Barros, 2014: \textit{J. Fluid Mech.} \textbf{748}:R1). In related work, we have used large-eddy simulation to model turbulent boundary layer flow over a suite of topographies composed of ``strips'' of high and low roughness length (drag imposed with the equilibrium logarithmic law); in all cases, we observe the formation of high- and low-momentum pathways (Willingham et al., 2013: \textit{Phys. Fluids} \textbf{26}:025111.). Here, we investigate turbulence statistics from large-eddy simulation such as magnitudes and spatial gradients of Reynolds stresses and turbulence kinetic energy, to discern underlying physical processes responsible for the secondary flows. We demonstrate that elevated production of turbulence above ``high'' roughness necessitates the mean circulations by virtue of turbulent kinetic energy production-dissipation non-equilibrium. We propose that the mean flow is Prandtl's secondary flow of the second kind. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H26.00007: Boundary-layer structure of shallow free-surface flows with high relative roughness Olivier Eiff, Emma Florens, Fr\'ed\'eric Moulin The boundary-layer structure of shallow free-surface flows over very rough walls is investigated with particle image velocimetry (PIV) both within the canopy and above, without disturbing the flow, by gaining complete optical access. This enabled reliable estimates of the double-averaged mean and turbulence profiles to be obtained by minimizing and quantifying the usual errors introduced by limited temporal and spatial sampling. It is shown that poor spatial sampling can lead to erroneous vertical profiles in the roughness sublayer. In order to better define and determine the roughness-sublayer height, a methodology based on the measured spatial dispersion is proposed which takes into account temporal sampling errors. The results reveal values well below the usual more ad hoc estimates. Then, the double-averaged statistics were used to investigate the effect of low relative submergence of the roughness elements on the friction velocity and the logarithmic law. The measurements show that the dispersive stresses are necessary to estimate correctly the total shear stress above the canopy top. The logarithmic law is shown to persist for submergence ratios at least as high as 0.33, even though the roughness sublayer largely extends into it. A dependence of the roughness length on submergence is observed, but not for the displacement height. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H26.00008: Investigation of Wall Shear Stress Behavior for Rough Surfaces with Blowing Jacob Helvey, Colby Borchetta, Mark Miller, Alexandre Martin, Sean Bailey We present an experimental study conducted in a turbulent channel flow wind tunnel to determine the modifications made to the turbulent flow over rough surfaces with flow injection through the surfaces. Hot-wire profile results from a quasi-two-dimensional, sinusoidally-rough surface indicate that the effects of roughness are enhanced by momentum injection through the surface. In particular, the wall shear stress was found to show behavior consistent with increased roughness height when surface blowing was increased. This observed behavior contradicts previously reported results for regular three-dimensional roughness which show a decrease in wall shear stress with additional blowing. It is unclear whether this discrepancy is due to differences in the roughness geometry under consideration or the use of the Clauser fit to estimate wall shear stress. Additional PIV experiments are being conducted for a three-dimensional fibrous surface to obtain Reynolds shear stress profiles. These results provide an additional method for estimation of wall-shear stress and thus allow verification of the use of the Clauser chart approach for flows with momentum injection through the surface. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H26.00009: The structure of turbulence overlying impermeable and permeable rough walls T. Kim, G. Blois, J. Best, K.T. Christensen Turbulent flow overlying complex topographies, both impermeable and permeable, occur across a broad range of scales in both natural and engineering environments. Permeability of the wall introduces a higher degree of both structural and conceptual complexity, with previous studies suggesting that interactions between the turbulent free flow and pore flow occur along the permeable interface and play a defining role in momentum exchange across the interface. Here we employ a Refractive-Index-Matching (RIM) technique in order to access the flow across the permeable interface with the particle image velocimetry (PIV) method, resulting in unimpeded optical access to the fluid flow at and within a permeable bed. Cubic-packed hemispheres are studied in both impermeable and permeable configurations, with models cast by an acrylic resin whose refractive index matched that of the working fluid (aqueous sodium iodide). The statistical and structural features of the flow in the near-wall region of the impermeable case and the interfacial region of the permeable case are compared to understand the role of permeability in driving momentum exchange processes as a function of Reynolds number. Comparisons to recent numerical simulations are also made. [Preview Abstract] |
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