Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L35: Geophysical Fluid Dynamics: General |
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Chair: Alfredo Pinelli, City University London Room: Georgia World Congress Center B407 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L35.00001: Large-eddy simulations of flows over rigid canopies in different canopy-flow regimes Alfredo Pinelli, Alessandro Monti, Mohammad Omidyeganeh We have performed fully resolved large-eddy simulations of turbulent flows over rigid canopies in an open channel of depth H at low Reynolds number (Reb=Ub H/ν = 6000, b stands for bulk) in sparse, transitional and dense canopy-flow regimes (Nepf ARFM 44, 2011).The different regimes are obtained by varying the height h of the rigid filaments that constitute the canopy while maintaining unchanged the filaments thickness and number per wall unit surface. In particular, we have considered variations from H/h=2.5 to H/h=20 using an immersed boundary method to model each rigid filament of the canopy. The variations of the canopy height determines the locations of the mean velocity internal inflection point in the canopy and of the virtual origin seen by the external flow. The latter has a strong impact in determining the outer flow regime (i.e. the skin friction Reynolds number of the outer flow) and the eventual appearance, size and intensity of the Kelvin-Helmholtz rollers induced by the inflection points.More details on the effect of the canopy height (i.e., the λ parameter proposed by Nepf et al. to characterise canopy flows) in establishing the mechanisms of the interaction between the flow inside and outside the canopy layer. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L35.00002: Sculpting stone forests Leif Ristroph, Jinzi Mac Huang, Joshua Tong, Michael John Shelley Landforms are readily recognized by their stereotyped shapes, but their formation processes are often hidden in geological history and involve complex interactions between flowing fluids and solid but shapeable boundaries. Spectacular examples are so-called stone forests, which are found worldwide and consist of thousands of tall, upright pillars of dissolvable mineral rock. Using candy as a stand-in for mineral in lab experiments on dissolution, we show that forests can be 'grown' under surprisingly simple conditions. We also focus on a single tree in the forest, and boundary layer theory modeling shows how dissolution-generated flows 'lick' and reshape the pillar into a singularly sharp spike. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L35.00003: Suspended Sediment Size and Concentration Characteristics of Particle-driven Gravity Currents Firat Yener Testik, Jin Ikeda This study investigated the vertical concentration and particle size profiles of non-cohesive particle-driven gravity currents with significant settling. Laboratory experiments were conducted using a 2D lock-exchange type tank 3.7m (length) x 0.2m (depth) x 0.3m (width). Gravity currents with non-cohesive silicon carbide particles of three different median particle diameters (17, 34 and 45 μm) were considered. Using siphon rakes, suspended sediment samples from different vertical distances were collected at three different stages (early, middle, and late stages) of the current propagation. Concentrations and sediment sizes of the collected suspended sediment samples were obtained using the total suspended solid test and the laser diffraction technique, respectively. In this presentation, our findings on the vertical concentration and sediment size profiles, and their evolution as the gravity current propagates will be discussed. Our findings are of importance in understanding the propagation characteristics and numerical modeling of particle-driven gravity currents with significant settling. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L35.00004: Gravity current in a channel of general cross-section with open top surface Marius Ungarish Consider the idealized steady-state gravity current of height h and density ρ_{1} that propagates into an ambient motionless fluid of height H and density ρ_{2} (< ρ_{1}) in a channel of general cross-section (rectangle, triangle, semi-circle, trapezoid, etc.) with an upper surface open to the atmosphere (open channel) at high Reynolds number. The current propagates with speed U and causes a depth decrease χ of the top surface. This is a significant extension of the solution for the fixed-top channel χ = 0. Here the determination of χ is a part of the problem. For a given cross-section geometry, the dimensionless parameters of the problem are a=h/H and r = ρ_{2}/ρ_{1}. We show that a control-volume analysis determines χ/H and Fr = U/(g' h)^{½} as functions of a and r, where g' = (1/r -1)g is the reduced gravity. The system satisfies balance of volume and momentum (explicitly), and vorticity (implicitly). We present solutions and insights for various cross-section geometries. For a Boussinesq system with r ≈ 1, we obtain χ/H ≈ 0 , and the present Fr and dissipation results differ only slightly from the fixed-top predictions, as expected. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L35.00005: Time-resolved and statistical PIV measurements in a refractive index matched flume to elucidate the turbulent flow structure over 3D bedforms Nathaniel Bristow, Gianluca Blois, Jim Best, Kenneth Thor Christensen Barchan dunes are three-dimensional, crescent-shaped bedforms, and while most commonly associated with aeolian environments, recent observations have shown them to form in subaqueous and extraterrestrial environments as well. As barchans migrate in the direction of the flow, they interact with their neighbors, typically by way of a collision. The morphodynamics of such collision processes are complex, where the role of the turbulent flow structure is strongly coupled to that of the sediment transport and morphological change. Here we study the flow structure in a decoupled manner through measurements of the turbulent flow over fixed-bed models of barchan dunes in various configurations involved in a barchan collision process. Particle image velocimetry is used to measure the flow in a refractive-index matched flume environment that enables uninhibited access to the whole flow field around these geometrically complex bedforms. Presented herein are results from temporally resolved stereo PIV measurements showing the dynamics of turbulent flow structure in the cross-plane, as well as turbulent statistics from low frame-rate 2D PIV measurements in streamwise–wall-parallel planes. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L35.00006: Gravity currents propagating over fixed beds of spherical particles Thomas Köllner, Alex Meredith, Claudia Cenedese, Roger Nokes, Eckart Meiburg We investigate full-depth, saline lock-exchange gravity currents propagating over densely packed layers of spherical particles by laboratory experiments and direct numerical simulations. Our objective is to explore how the speed and composition of the current is influenced by this macro-rough bed. Particle Tracking Velocimetry was employed to measure flow fields along the centerline of the flume while light attenuation was used to measure the spanwise averaged density field. Numerically, we solve the three-dimensional Navier-Stokes equations in the Boussinesq approximation, with an immersed boundary method to fully resolve the flow around each particle. The no-flux condition for the salinity at the particle surface is implemented by a volume of fluid approach. We vary the number of particle layers in the bed as well as the current height. For an increasing ratio of particle to current size, the current mixes more strongly with the bed fluid, so that it becomes progressively more diluted and its propagation speed is reduced. Simulations furthermore show that the dense fluid in the gravity current tail triggers a Rayleigh-Taylor instability within the porous bed. |
Monday, November 19, 2018 5:23PM - 5:36PM |
L35.00007: Thermo-viscous fingering of magma flow in volcanic fissures Dipin Pillai, Jason R Picardo, Ranganathan Narayanan The stability of hot magma flow through a thin, long volcanic fissure is investigated. Magma flow through the fissure is modeled as a fluid with temperature-dependent viscosity flowing through a narrow slot. The side-walls of the slot are held at a lower temperature, mimicking the colder rock walls surrounding the fissure, thus cooling the magma as it flows. Flow and heat advection equations are averaged across the slot thickness where the averaged model is developed using a weighted-residual integral boundary layer technique. The model accounts for the non-Poiseuille velocity profile in the slot due to temperature-dependent viscosity unlike the Poiseuille flow-based Darcy models used in earlier works. Linear stability analysis reveals that for sufficiently strong temperature-dependence of viscosity, one-dimensional flow of magma undergoes a thermo-viscous fingering type of instability. This usually occurs for parameter values exhibiting a multi-valued relationship between base-state flow rate and pressure-drop. The uniform flow then gives rise to alternate interior bands of low-viscosity fast-moving and high viscosity slow-moving magma. The parametric regions of instability are further investigated and compared with Darcy models. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L35.00008: Dispersion in hyper-porous fractured systems and the impact of matrix permeability on fracture transmissivity Bowen Ling, Alexandre Tartakovsky, Mart Oostrom, Ilenia Battiato Current studies of fractures generally assume purely diffusive transport in the matrix. Yet, this assumption is invalid for fractures embedded in hyperporous matrices that can be highly permeable to flow. By means of perturbation theory and asymptotic analysis, we derive a set of upscaled equations describing mass transport in a coupled fracture-matrix system and an analytical expression relating macro-scale dispersion coefficient and matrix permeability. Our analysis shows that its impact on dispersion coefficient strongly depends on the magnitude of the Peclet number, i.e. on the interplay between diffusive and advective mass transport. Additionally, we demonstrate different scaling behaviors of the dispersion coefficient for thin or thick porous matrices. Our analysis shows the possibility of controlling the dispersion coefficient, i.e. transversal mixing, by either active (i.e. changing the operating conditions) or passive mechanisms (i.e. controlling matrix effective properties) for a given Peclet number. We compare the upscaled model against experiments conducted on microchannels with surfaces patterned with different topologies. The experimental data are in agreement with the developed theory at different Peclet numbers. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L35.00009: On the vertical variation of turbulence statistics in open channel flow Erika Johnson, Edwin A Cowen Nezu (1977) established a set of universal exponential expressions for the vertical variation of the turbulence intensities following a series of open channel flow laboratory experiments. Each of these relationships contains an empirical coefficient that differs with the component direction (D_{u} = 2.3, D_{v} = 1.27, D_{w} = 1.63). Recent laboratory experiments have demonstrated that for open channel flows these constants are not universal but vary with the Reynolds number and bed composition. A method for the remote prediction of D_{u} and hence, the vertical variation of the streamwise turbulence intensity is developed here. The method uses the free surface turbulence anisotropy as a proxy for the redistribution of vertical velocity fluctuations into surface parallel components enabling a correlation between the free surface turbulence intensity and D_{u}. Implications for the remote prediction of bed composition, sediment transport and bed shear stress are discussed. |
Monday, November 19, 2018 6:02PM - 6:15PM |
L35.00010: Entrainment models for turbulent axisymmetric jets and forced plumes Himanshu Mishra, Jimmy Philip Entrainment models have been used for several decades to make growth predictions for turbulent jets and plumes. Although useful, these models are based on ad-hoc assumptions, and say very little about the underlying physics. Recently, van Reeuwijk & Craske (2015) derived an energy consistent unified entrainment relation, providing contributions from mean, turbulence, buoyancy and pressure via profile coefficients. Here, we present the results of simultaneous time resolved measurement of velocity and density in an axisymmetric turbulent plume using 2D-2C particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF), respectively; using refractive index matched fluids to remove uncertainties due to optical distortions. We test the existing models, and compare them to our measurements and data available in the literature. We also discuss the effects of variability in the value of entrainment coefficient and scalar to velocity spread rate. Finally, employing our experimental data we develop an entrainment model based on the relation developed by van Reeuwijk & Craske (2015), parameterizing the profile coefficients based on local Richardson number. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L35.00011: Measurements of air drag on two-dimensional soap-film flow Yuna Hattori, Rory T Cerbus, Pinaki Chakraborty The gravity-driven soap film is a well-established experimental setup for investigating two-dimensional (2D) flows. As with any 2D geophysical flow, the soap film interacts with the 3D environment in which it is embedded. Even in numerical simulations of 2D turbulence, this interaction is modeled by adding a drag term to the governing equations, which also serves as an energy sink at large scales. In a soap-film flow, the interaction with the surrounding air engenders drag, which has never before been directly measured. We measure the movement of the air around the film by seeding the air with atomized droplets, whose motion we track using Particle Tracking Velocimetry (PTV). This yields the velocity profile in the air, with which we compute the drag on the soap film. Besides addressing a fundamental question in laboratory 2D flows, our work also has implications for understanding the interaction between the atmosphere and the ocean surface currents. |
Monday, November 19, 2018 6:28PM - 6:41PM |
L35.00012: Local Ensemble Transform Kalman Filtering Implemented on 2D and 3D Dynamo Flows Sarah C Burnett, Nathanaël Schaeffer, Kayo Ide, Daniel Perry Lathrop The behavior of the Earth's magnetic field and the influence of the core dynamics has been investigated in recent years through experiments and numerical models. At UMD, the geodynamo is replicated by experimental studies of the three-meter spherical Couette device filled with liquid sodium driven by two independently rotating concentric shells and an applied dipole magnetic field. These experiments incorporate high velocity flows to recreate the turbulence of convection-driven flows in the Earth. Collaborators at ISTerre have created the numerical code XSHELLS which features finite difference methods in the radial direction and pseudospectral spherical harmonic transforms for the angular directions. Highly turbulent flows are unfeasible to resolve in numerical models and in experiments full measurements can be intrusive. Our goal is to synchronize the outputs from the numerical code with the experimental magnetic boundary data to get an idea of the unknown velocity field. We present our preliminary studies of observation system simulation experiments using a kinematic 2D dynamo model and the full 3D model. This research provides an avenue for making predictive models of the Earth's magnetic field. |
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