Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A21: Turbulence: Environmental Flows |
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Chair: Lian-Ping Wang, University of Delaware Room: 209 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A21.00001: Simulations of turbulence and dispersion in idealized urban canopies using a new kinetic scheme Lian-Ping Wang, Pablo Huq, Zhaoli Guo In this talk, we will demonstrate the capabilities of a new kinetic scheme, known as the Discrete Unified Gas Kinetic Scheme (DUGKS), by simulating turbulent flow and scalar dispersion in an idealized urban canopy. DUGKS is a finite-volume formulation of the Boltzmann equation which can incorporate a non-uniform grid. It could be used as an direct numerical simulation tool or as a large-eddy simulation tool for turbulent flow in a complex geometry. We will describe this mesoscopic CFD method, and details in setting up a non-uniform grid, no-slip boundary condition on solid surfaces, and far-field boundary conditions. The model urban canopy contains an array of buildings with a prescribed building-height-to-width aspect ratio. Several aspect ratios will be considered in the simulations. A passive scalar is continuously released from a near-ground point source. Profiles of mean velocity, turbulence statistics, and scalar concentration obtained from the simulations will be compared to data from water-tunnel measurements. Grid refinement will be performed to study the convergence of the simulated results on the grid resolution. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A21.00002: Flow over and within large-scale porous topography: Impact of surface heterogeneity on turbulence structure Ali M. Hamed, Prateek Ranjan, Matthew J. Sadowski, Heidi M. Nepf, Leonardo P. Chamorro An experimental investigation of the flow within and above model canopies was carried out to determine the effect of canopy height heterogeneity on the structure and spatial distribution of the turbulence. Two 800 mm long models with 20{\%} blockage were placed in a 2.5 m long refractive-index-matching channel. The first model (base case) is constituted of equal height (h) square bar elements arranged in a staggered configuration. The other model bars had two heights (h$+$1/3h and h-1/3h) alternated every two rows. Particle image velocimetry was used to map the flow field at three locations spanning the length of the canopy under three confinement ratios H/h$=$2, 3, and 4, where H is the free surface height. The experiments were performed at Reynolds number Re$_{\mathrm{H}} \quad =$ 6800, 10200, and 13600. Refractive index matching renders the canopy invisible and grants full optical access allowing the flow field within the canopy to be measured by PIV. Turbulence statistics complemented with POD, quadrant analysis, and LES decomposition reveal the distinctive effect of the height heterogeneity on the shear layer that forms on top of the canopy, and on the free flow over the canopies. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A21.00003: A large-eddy simulation study on statistical attributes of urban-like geometries relevant to parameterizing bulk aerodynamic characteristics Xiaowei Zhu, William Anderson The inherent spatial heterogeneity exhibited by real urban environments complicates a priori estimation of the roughness height needed to parameterize the inertial layer mean streamwise velocity. A large-eddy simulation study of turbulent flow over 3-D random urban-like topographies is conducted to explore the effects of surface geometry on bulk aerodynamic characterization. In a mean sense, we find that statistical attributes including surface height root mean square and skewness can adequately capture the spatial heterogeneities and randomness of real urban geometries. We find, however, that higher-order statistical moments have a negligible affect on aerodynamic drag (i.e. kurtosis may be omitted). The results enable exploration of applicability of some recently-proposed roughness parameterizations that are relevant to complex, urban-like roughness (including the model proposed by Flack and Schultz, 2010: J. Fluids Eng. 132, 041203-1). We evaluate empirical parameters needed in these models for the present urban-like cases. We find that two empirical parameters (relevant to height rms and skewness) can characterize the bulk aerodynamic roughness of topographies with statistical attributes comparable to dense urban environments. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A21.00004: LES for wind turbulence in canopy layer at large urban area Tetsuro Tamura, Hidenori Kawai, Rahul Bale, Keiji Onishi, Makoto Tsubokura, Koji Kondo, Tsuyoshi Nozu In order to accurately predict the wind flow in canopy layer of large urban area, we introduce LES based on BCM, Building Cube Method which is formulated on the very fine Cartesian mesh system. Houses and buildings were not aerodynamically modelled but directly reproduced their shapes in the numerical model, because the wind profile parameterization in cities requires the correct estimation of local flow field in the canopy layer close to the ground. Recent high-performance computing, HPC technique has developed distinctly, so high-resolution computation can apply to flows around a complicated configuration. In this case we have to deal with buildings, vegetation and street etc. as a part of numerical model. Actually LES using the Cartesian coordinate encounters the non-correspondence of directions between the street lines and the discretized mesh lines. Very fine mesh system by BCM can solve this problem, supported by Immersed Boundary Method. Also, in this numerical scheme, the computational process is so simple that the parallel algorithm and the memory access obtain the perfect efficiency. In this study, we have applied LES by BCM to the wind flow estimation over the real complicated urban surface. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A21.00005: Entrainment Across a Sheared Density Interface in High Richardson Number Cavity Flow Nicholas Williamson, Michael Kirkpartick, Steve Armfield The turbulent entrainment of fluid across a sharp density interface has been examined experimentally in a purging cavity flow. In the experiments, a long straight cavity with sloped entry and exit boundaries is located in the base of a straight open channel. Saline fluid is entrained from the cavity into the overflow. The cavity geometry has been designed to ensure there is no separation of the overflow in the cavity region with the goal of obtaining a single mode of entrainment, related only to the interface properties rather than to cavity specific mechanisms. The bulk entrainment rate has been measured and correlated with bulk Richardson number over $Ri= 1.0-20$ at Reynolds number $Re=7100-15100$. The entrainment rate is shown to scale with the local bulk Richardson number $E\simeq C Ri^{-1.38}$, very close to the established result for entrainment across a sharp two layer density interface in a recirculating water channel (Strang and Fernando, J Fluid Mech., 428, 2001) but with an order of magnitude lower coefficient C. Experiments instrumented with PIV/LIF were used to relate the bulk Ri to the local gradient Richardson number of the interface. In the cavity setting the interface appears to remain sharper, resulting in larger $Ri_g$ and reduced entrainment. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A21.00006: Entrainment of Vertical Jets in Turbulent Cross Flow Graham Freedland, Karen Roberts, Larry Mastin, Stephen Solovitz, Raul Cal Volcanic eruptions produce high concentrations of ash that produce clouds in the atmosphere that are hazardous for private and commercial aviation. Without accurate models to predict ash concentrations, air traffic is unable to safely navigate ash clouds downwind of an eruption as critical concentrations are difficult to identify visually. Current models rely on inputs such as plume height, eruptive dissipation and cross-flow wind speeds as well as empirical parameters such as the entrainment ratio between the cross-flow and the plume velocity. A wind tunnel experiment has been designed to investigate these models by injecting air orthogonally into a cross-flow. The ratio of the cross-flow and jet velocities is varied to simulate a weak plume and flow response is measured using particle image velocimetry. Grids upstream of the plume create different turbulence intensities, which, combined with different jet geometries, allow us to study the flow field, mean and second order moments and thereby obtain information to accurately model volcanic ash concentrations in the atmosphere. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A21.00007: Development of Turbulence Downstream of a Submerged Aquatic Canopy in Unidirectional and Combined Wave-Current Flows Francisco Zarama, Robert Zeller, Jeffrey Koseff Seagrasses and corals form the essential building blocks of many coastal ecosystems, and the turbulence generated from these canopies have been investigated heavily. However, the effect of these canopies on the downstream flow is poorly understood, particularly for combined wave-current flows. Furthermore, the development of flow characteristcs may have a profound impact on propagule transport and sediment dynamics downstream of the canopy. The present study focuses on the adjustment of turbulence and flow characteristics downstream of a model canopy. These experiments comprise three different canopy heights, three different wave conditions, and three different flowrates. Measurements are taken using an acoustic velocimeter and 2D particle image velocimetry. This work proposes the existence of four distinct regions downstream of a model canopy: the mixing layer, the transition region, the turbulence decay region, and the boundary layer. Each of these regions has distinct characteristics regarding the mean flow, bed stress, TKE, and Reynolds shear stress. The delineation and description of these four regions will allow ecosystem managers and sediment modelers to better understand coastal dynamics. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A21.00008: Dynamic Mode Decomposition of Flow Around Interacting Barchan Dunes Nathaniel Bristow, Gianluca Blois, Taehoon Kim, Peter Schmid, Jim Best, Kenneth Christensen Barchan dunes are crescentic bedforms located in environments with unidirectional flow and limited sediment supply, including deserts, river beds and the craters of Mars. The evolution of, and interactions between, barchans are highly dynamic, involving feedback mechanisms between the fluid flow, morphological change and sediment transport. A series of experiments were undertaken to discretely simulate the collision of a smaller barchan with a larger, downstream one using fixed bedform models, each experiment representing a successive snapshot in the dune collision process. These experiments thus capture the turbulent flow over fixed-bed morphologies that correlate with rapid morphological change and high rates of sediment transport using time-resolved PIV in the wall-parallel plane. The use of a Refractive Index Matching (RIM) flow facility allows for the light to pass through the model, capturing areas which are otherwise obscured, such as around the horns of the dune and the sheltered region behind the crest. Dynamic Mode Decomposition is used to identify the most dominant modes contributing to flow dynamics in each collision stage. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A21.00009: Experimental investigation of drifting snow in a wind tunnel Philip Crivelli, Enrico Paterna, Stefan Horender, Michael Lehning Drifting snow has a significant impact on snow distribution in mountains, prairies as well as on glaciers and polar regions. In all these environments, the local mass balance is highly influenced by drifting snow. Despite most of the model approaches still rely on the assumption of steady-state and equilibrium saltation, recent advances have proven the mass-transport of drifting snow events to be highly intermittent. A clear understanding of such high intermittency has not yet been achieved. Therefore in our contribution we investigate mass- and momentum fluxes during drifting snow events, in order to better understand that the link between snow cover erosion and deposition. Experiments were conducted in a cold wind tunnel, employing sensors for the momentum flux measurements, the mass flux measurement and for the snow depth estimation over a certain area upstream of the other devices. Preliminary results show that the mass flux is highly intermittent at scales ranging from eddy turnover time to much larger scales. The former scales are those that contribute the most to the overall intermittency and we observe a link between the turbulent flow structures and the mass flux of drifting snow at those scales. The role of varying snow properties in inducing drifting snow intermittency goes beyond such link and is expected to occur at much larger scales, caused by the physical snow properties such as density and cohesiveness. [Preview Abstract] |
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