Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H15: Turbulent Boundary Layers: Roughness and Atmospheric Turbulence |
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Chair: Inanc Senocak, University of Pittsburgh Room: 310 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H15.00001: Altered large-scale structures in turbulent boundary layers formed over drag-increasing riblets Ryan Newton, Daniel Chung, Nicholas Hutchins Riblet drag reduction only occurs within a small, viscous-scaled riblet spacing envelope, outside of which drag is significantly increased. In this study, non-optimal, drag-increasing trapezoidal riblet surfaces, with a $30{^\circ}$ tip angle and height of $0.5s$ are experimentally investigated in the regime where the viscous-scaled riblet spacing is large ($40 < s^+ < 250$). The motivation behind this is to understand the cause of the drag increase in this regime. Results indicate that riblets with a large $s^+$ provide a considerable drag penalty, which appears to asymptote to fully rough behavior as $s^+$ increases. An analysis of the turbulence close to the surface reveals that there is little evidence of either lodgment of near-wall turbulence in the riblet grooves, or of a Kelvin--Helmholtz instability above the riblet crests. This suggests that the two previously hypothesized mechanisms for the breakdown in drag reduction at $s^+\approx20$, do not play a role at large $s^+$. In addition, energy spectra show that the riblets greatly diminish the footprint of large-scale turbulent motions in the near-wall and logarithmic regions, questioning the presence of a typical log layer and the validity of outer-layer similarity for these cases. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H15.00002: Scale interactions in velocity and pressure over urban-like roughness Manuel Ferreira, Bharathram Ganapthisubramani An integral description of a boundary layer developing over a large-cube array is obtained using velocity data to reconstruct the underlying pressure field via 2D-TH. Coupled-statistics of the pressure forces acting on a target roughness element provide insight into the relevant mechanisms responsible for surface drag. This is complemented by conditional analysis and extended POD of the pressure field based on velocity modes to further understand the velocity-pressure interrelations. Coherent motions, at different scales, leave a strong imprint on the pressure field. Larger turbulent features dominate pressure variance, but their direct contribution to surface drag appears to be mitigated by the relative size of the roughness obstacles that are considerably smaller. Pressure waves induced by the passage of alternating high and low-momentum regions evenly affect the flow field over a wide region, coupling the forces on the windward and leeward sides of the cube which, in turn, partially cancel each other out. This suggests that uncorrelated, intermediate and small-scale pressure fluctuations are more important to the drag force variance. Large-scale structures are nonetheless significant for the role they play in modulating the small-scale pressure events in the near wall region. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H15.00003: Post fully rough regime in turbulent pipe flow Daniel Cruz, Hamidreza Anbarlooei, Fabio Ramos, Cecilia Mageski, Atila Freire The phenomenological model of Gioia and Chakraborty (2006) tries to relate the friction and energy spectrum in canonical flows. In this model, the momentum transfer by eddies at a specified surface above the wall balances the wall shear stress. At low Reynolds numbers, the dominant eddies are in the size of the Kolmogorov length scale (Blasius regime), while at high Reynolds numbers the dominant eddies are in the size of the roughness elements (fully rough regime). The present authors recently showed that instead of a simple transition between these two extremes, there is a third regime where the effects of the wall (through attached eddies) dominate. This results in a new power-law friction regime between the Blasius and fully rough regimes. The effects of the roughness geometry on turbulence are studied experimentally in the present work. Beside the common sand-roughed surfaces, some well geometrically defined rough walls have also been examined. As expected, by increasing the Reynolds number, the fully rough regime appears for the latter type of roughness. However, a further increase in the Reynolds reveals an unexpected behavior, where friction starts to decrease again in a power-law manner (with the same trend as observed for the third regime). [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H15.00004: The influence of regular and random roughness on outer-layer similarity Michael Schultz, Kristofer Womack, Ralph Volino, Charles Meneveau An experimental investigation was carried out on rough-wall, turbulent boundary layers with regular and random roughness element arrangements. Varying planform densities of truncated cone roughness elements in square staggered patterns and random arrangements were investigated. Velocity statistics were measured via two-component laser Doppler velocimetry and stereo particle image velocimetry. Differences in the mean streamwise velocity profiles recorded over the regular arrangements were confined to within one roughness height above the roughness crests whereas the random cases exhibited significant variation across the span that persisted into the outer layer. Evidence is shown that the differences result from low momentum pathways (LMPs) and high momentum pathways (HMPs) over the random surfaces which are not observed over the regular surfaces. The LMPs and HMPs were marked by elevated and depressed Reynolds shear stress as seen by previous investigators. However, previous studies had systematic or regular surface topography which they linked to the secondary flows. The present results indicate that secondary flows may develop even in the absence of these surface characteristics. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H15.00005: Roughness effects on scalar transport Zvi Hantsis, Ugo Piomelli The momentum transport in wall-bounded flows has been studied extensively, for both smooth and rough surface; passive-scalar transport has also been the focus of experimental and numerical investigations. The mechanisms that govern scalar transport in the presence of roughness, however, are less well-known. For smooth walls, the similarity between their respective transport equations results in an analogy between the velocity and scalar (Reynolds analogy), that relates the velocity and scalar statistics. For rough walls, however, such analogy fails. We perform Direct Numerical Simulations of passive-scalar transport with $Pr=0.7-1.4$ in a fully-developed turbulent channel with smooth and rough walls at $Re_\tau \approx 1,650$. The downward shift of the mean scalar log-layer is smaller than that of the velocity, and depends on the Prandtl number. While the $\overline{u'u'}$ fluctuations (in wall units) decrease significantly, the scalar ones are less affected. For the budget of $\overline{\theta'\theta'}$, scalar dissipation is the only significant sink and is roughly equal to the sum of the two significant sinks of $\overline{u'u'}$: dissipation and pressure work. The form-induced velocity affect the budget of $\overline{\theta'\theta'}$ more than that of $\overline{u'u'}$. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H15.00006: Turbulence structure over idealized and natural barchan dune fields Chao Wang, William Anderson LES simulation has been used to study inertia-dominated turbulent flows responding to aeolian sand dunes. The former is recovered from simulations initialized with a Reynolds-averaged flow, absent any small-scale features, which highlights the emergence of salient structures within the dune field roughness sublayer. The latter is based upon computation of integral lengths. In the interest of generality, these exercises are based upon flow over canonical dune geometries -- which serve as a comparative benchmark -- and flow over a section of the White Sands National Monument aeolian dune field. In both applications, we report the emergence of mixing layer-like processes, although the distinct geometric nature of the dunes shows the prevalence of a persistent interdune roller, which is aligned most closely with the streamwise direction. In order to demonstrate underlying similarities in processes aloft idealized and natural dune fields, we normalize~the integral lengths by characteristic length scales -- vorticity thickness, attached eddy hypothesis mixing~length, and dissipation length. This exercise reveals a distinct growth and collapse pattern that is robust across all considered dune arrangements. Herein, ``growth'' refers to the stage of downflow thickening of~vortices produced via vortex shedding off the upflow dune; growth is regulated by the lesser of distance~to the wall or distance to the upflow dune, where the latter marks the beginning of the ``collapse'' stage. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H15.00007: On the unsteady dynamics of fully submerged, rigid and flexible canopies Liu Hong, Robert C Houseago, Daniel Parsons, James L Best, Lai Wing, Leonardo P Chamorro The unsteady dynamics of a fully submerged ßexible canopy, and the near-field turbulence were explored experimentally under various ßow conditions. The flow surrounding the flexible canopy was compared with that from a similar case with a rigid canopy. The flexible and rigid canopies consisted of thin prismatic elements of large-aspect ratio, which were placed in a staggered pattern in a developed boundary layer. Planar and volumetric particle image velocimetry (PIV) and particle tracking velocimetry (PTV) were used to characterize the turbulence and the motions of the elements within the center of the canopy along the span of the array. Results show that the flow and structures reached equilibrium state at different downstream locations. Structure dynamics resulted reconfigured the turbulent exchange between the inner and outer flows within a range of scales. Analysis of the tip motions of the canopy elements allowed exploring the local flow dynamics across the canopy. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H15.00008: Scaling and dynamics of turbulent flows over sparse canopies Akshath Sharma, Ricardo Garcia-Mayoral We study turbulent flows over sparse canopies using direct numerical simulations. We find that these canopies affect the surrounding flow via two mechanisms. The first is through the flow induced by the presence of the canopy elements. The second, which can dominate, is through a change in scale for the background turbulence. When the element-induced flow is filtered out, the remaining background turbulence exhibits a balance of the viscous and the Reynolds shear stresses within the canopy layer similar to that over smooth walls. We propose a scaling for the background turbulence based on the sum at each height of the viscous and the Reynolds shear stresses, $\tau_f$, as in Tuerke \& Jimenez (2013). Using this scaling, the background-turbulence fluctuations within the canopies also show similarities to those over smooth walls. This suggests that the background turbulence scales with $\tau_f$ at each height, rather than the conventional scaling based on the total stress. We show that this effect can be captured by substituting the canopy by a drag that acts on the mean flow alone, aiming to produce the correct $\tau_f$ without modifying the fluctuations directly. This forcing produces better estimates for the turbulent fluctuations than a conventional, homogeneous-drag model. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H15.00009: LiDAR measurements of the Turbulent/non-turbulent Interface in the Atmospheric Surface Layer. Giacomo Valerio Iungo, Behzad Najafi, Matteo Puccioni, Sebastian Hoch, Marc Calaf, Eric Pardyiak Wind velocity measurements have been performed in the atmospheric surface layer (ASL) with scanning Doppler wind LiDARs in order to characterize the turbulent/non-turbulent interface (TNTI), which is a shear-layer in the ASL separating the turbulent region from the non-turbulent region aloft. The TNTI plays a crucial role for the engulfment process, which consists in drawing large packets of non-turbulent fluid into the turbulent region leading to redistribution of TKE. This field campaign was performed at the SLTEST facility of the US Army Dugway Proving Ground center in Utah. The LiDAR velocity data are post-processed in order to retrieve the horizontal streamwise velocity component acquired simultaneously at different heights throughout the ASL height with a vertical resolution smaller than 1 m. The analysis of the velocity variance is used to detect the local height of the TNTI. The geometric characteristics of the TNTI, such as height and thickness, are analyzed together with its velocity gradient and variance. The conditional averaging of the velocity field as a function of the TNTI height shows velocity increases (reductions) that are consistent with Q4 (Q2) events and relative decrease (increase) of TNTI height. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H15.00010: Quality and reliability of general purpose finite volume solvers for the simulation of atmospheric boundary layer flow Beatrice Giacomini, Marco Giometto In the present work, the quality and reliability of a colocated, unstructured, finite-volume solver (OpenFOAM framework) is analyzed for the simulation of a pressure-driven atmospheric boundary layer flow. First and second order statistics, as well as velocity spectra and two-point velocity correlations, are compared to predictions from a "battle-tested" pseudo-spectral solver. The solution is found to be particularly sensitive to the grid aspect ratio and to the chosen numerical scheme. First and second order statistics obtained using a non-dissipative setup compare well between the solvers, with the finite volume one featuring an overdissipative behavior that leads to enhanced sub-grid scale stress contributions. When considering velocity spectra, the finite volume solver features a rapid decay of energy density within the inertial subrange, irrespectively of the discretization scheme that is adopted. In addition, a spurious pile up of energy density at high wave numbers is observed across all of the considered cases. The cause of this behavior will be discussed and mitigation strategies proposed. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H15.00011: Moving horizon estimation of turbulent velocity fields in the atmospheric boundary layer using lidar measurements and large eddy simulations Pieter Bauweraerts, Johan Meyers Pulsed lidar sensors measure the line-of-sight (LOS) projected wind velocity in the atmospheric boundary layer at a range of locations along the LOS spanning several kilometres. Despite the vast amount of measurement information, the data remains sparse and unidirectional. To reconstruct a full 3D velocity field, we employ a non-linear moving horizon estimation (or 4D-Var) approach, using large-eddy simulations (LES) as a state space model. The cost function, which represents the relative probability of the state space trajectories, is comprised of two terms, the first term regularizes the optimization based on prior statistical knowledge of the velocity field fluctuations embedded in the 2-point covariance tensor of the atmospheric boundary layer. To this end, the covariance tensor is averaged offline over a sufficiently long time window, to ensure statistical convergence. The second part penalizes the difference between the LES and real observations over the time horizon. Instead of using experimental data, we use a fine-grid LES simulation as a virtual reality, creating a controlled and flexible testing environment. The methodology is demonstrated on two measurement setups, a standard plan-position indicator scanning mode and a 3D trajectory based on a Lissajous curve. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H15.00012: Resolving near-wall particle dynamics for turbulence-induced saltation and dispersion Taehoon Kim, Samuel Hamermesh, Rui Ni The Stanley-Corrsin wind tunnel at Johns Hopkins University has recently been repurposed for studying sediment transport over an erodible surface. To resolve the near-wall particle dynamics, a hybrid technique that uses both the back illumination and scattering light illumination was adopted to measure the particle dynamics over a wide range of scales. In particular, it allows us to resolve particle motion across the near-wall saltation layer using our in-house particle tracking code. From this experiment, the entire time history of particle behaviors can be acquired. In this presentation, we will discuss the statistics of near-wall particle motion and its relationship with the near-wall turbulence. [Preview Abstract] |
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