Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G13: Geophysical: General IV 
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Chair: Alberto Scotti, University of North Carolina Room: 27A 
Monday, November 19, 2012 8:00AM  8:13AM 
G13.00001: Erosion sculptures Leif Ristroph, M.N.J. Moore, Stephen Childress, Michael Shelley, Jun Zhang Erosion by flowing fluids carves the striking landscapes imprinted on the Earth and on the surfaces of our neighboring worlds. In these processes, solid boundaries both influence and are shaped by the surrounding fluid, but the emergence of morphology as a result of this interaction is not well understood. We study the coevolution of shape and flow in the context of clay bodies immersed in fast flowing water. Although commonly viewed as a smoothing process, we discover that erosion sculpts surprisingly sharp points and corners that persist as the body shrinks. These features result from a natural tendency to form surfaces that erode uniformly, and we argue that this principle may also apply to the more complex scenarios that occur in nature. [Preview Abstract] 
Monday, November 19, 2012 8:13AM  8:26AM 
G13.00002: Simulation of Sediment Wave Generation and Maintenance Gary Hoffmann, Eckart Meiburg, Mohamad NasrAzadani Deepsea sediment waves are a common feature throughout the world, forming under the influence of turbidity currents, thermohaline currents, and/or deformational processes. Past efforts for modeling turbiditycurrent generated sediment waves have focused on 1D (depthaveraged) NavierStrokes equations, whereas here we use the 2D equations, as implemented in TURBINS 2D. We employed two experimental setups: 1) Repeated flows in a lockexchange configuration, in which deposition and erosion are observed to lead to waveforms developing on an initially linear ramp, and 2) single continuous inflow currents that are allowed to flow over a preexisting sinusoidal geometry. Using these two setups, we examine both the initial generation of sediment waves over the course of many episodic events, as well as maintenance of sediment waves under the influence of a quasisteady flow. In both setups, we examined a range of flow parameters, such as ramp length, sediment settling velocity, etc. [Preview Abstract] 
Monday, November 19, 2012 8:26AM  8:39AM 
G13.00003: Selfsimilarity of an eroding body M. Nick J. Moore, Leif Ristroph, Stephen Childress, Michael Shelley, Jun Zhang Motivated by the erosion of natural landforms, we study the interaction between an eroding body and surrounding fluidflow using experiments and simulations. Both reveal the emergence of a unique shape that forms early and then shrinks in a selfsimilar fashion. Here, I focus on simulations in which erosion rate is dictated by local shear stress. In this highReynoldsnumber context, we determine shear stress by combining an outer, inviscid flow with a boundary layer flow. We discover that a broad range of initial shapes morph into a terminal shape characterized by nearly uniform shear. [Preview Abstract] 
Monday, November 19, 2012 8:39AM  8:52AM 
G13.00004: Numerical simulation of turbulence and sandbed morphodynamics in natural waterways under live bed conditions Ali Khosronejad, Fotis Sotiropoulos We develop and validate a 3D numerical model for coupled simulations of turbulence and sandbed morphodynamics in natural waterways under live bed conditions. We employ the FluidStructure Interaction Curvilinear Immersed Boundary (FSICURVIB) method of Khosronejad et al. (Adv. in Water Res., 2011). The mobile channel bed is discretized with an unstructured triangular grid and treated as the sharpinterface immersed boundary embedded in a background curvilinear mesh. Transport of bed load and suspended load sediments are combined in the nonequilibrium from of the ExnerPoyla for the bed surface elevation, which evolves due to the spatiotemporally varying bed shear stress and velocity vector induced by the turbulent flow field. Both URANS and LES models are implemented to simulate the effects of turbulence. Simulations are carried out for a wide range of waterways, from small scale streams to largescale rivers, and the simulated sandwaves are quantitatively compared to available measurements. It is shown that the model can accurately capture sandwave formation, growth, and migration processes observed in nature. The simulated bedforms are found to have amplitude and wave length scales ranging from the order of centimeters up to several meters. [Preview Abstract] 
Monday, November 19, 2012 8:52AM  9:05AM 
G13.00005: Spectral description of migrating bedforms and sediment transport Michele Guala, Nicholas BadHeartBull, Arvind Singh, Efi FoufoulaGeorgiou The spatiotemporal evolution of migrating bedforms in an experimental straight flume with erodible bottom and fixed sidebanks is presented through a joint spectral analysis of surface elevations in the frequency and wave number domains. In this framework any generic bedform can be described as a combination of Fourier modes propagating with well defined, scaledependent, convection velocities. Quasisimultaneous measurement of bed elevations z=f(x,t) in time (t) and space (x, along the flow direction) are used to estimate the propagation velocity of each Fourier mode, and to propose a purely statistical, spectral approach to quantify sediment transport rates in the presence of bedforms. A functional relationship between the length scale and time scale of migrating bedforms is developed into a dimensionless expression for scaledependent convection velocities. The latter is further used to provide an estimate of sediment transport rate in the presence of generic migrating bedforms. Experiments were conducted in the Tilting Bed Flume at SAFL under varying flow and bed material conditions. [Preview Abstract] 
Monday, November 19, 2012 9:05AM  9:18AM 
G13.00006: Entraining gravity currents Chris Johnson, Andrew Hogg Largescale gravity currents, such as those formed when industrial effluent is discharged at sea, are greatly affected by the entrainment and mixing of ambient fluid into the current, which both dilutes the flow and causes an effective drag between the current and ambient. We study these currents theoretically by combining a shallowwater model for gravity currents flowing under a deep ambient with an empirical model for entrainment, and seek longtime similarity solutions of this model. We find that the dependence of entrainment on the bulk Richardson number plays a crucial role in the current dynamics, and results in entrainment occurring mainly in a region close to the flow front, reminiscent of the entraining current `head' observed in natural flows. The longtime solution of an entraining lockrelease current is a similarity solution of the second kind, in which the current grows as a power of time that is dependent on the form of the entrainment model, approximately as $t^{0.44}$. The structure of a current driven by a constant buoyancy flux is quite different, with the current length growing as $t^{4/5}$. Scaling arguments suggest that these solutions are reached only at very long times, and so may be attained in large natural flows, but not in smallscale experiments. [Preview Abstract] 
Monday, November 19, 2012 9:18AM  9:31AM 
G13.00007: Numerical simulation of reversing buoyancy gravity currents Senthil Radhakrishnan, Erik Lenk, Michael Boekels, Eckart Meiburg Sediment laden fluid propagates as an underflow when its bulk density is higher than the density of the ambient fluid. If the density of the interstitial fluid in gravity current is smaller than the density of the ambient fluid, the gravity current can become positively buoyant after sufficient particles have settled. The current then lifts off from the bottom surface and travels as a surface gravity current over the heavier ambient fluid. These types of currents, where the buoyancy reverses its direction, have been observed when sediment laden fresh water enters the sea or during volcanic eruption that creates a pyroclastic flow. We use a lockexchange configuration with monodisperse and bidisperse particles to study the lofting characteristics of reversing buoyancy currents. This talk will focus on results obtained from Largeeddy Simulation of high Reynolds number currents. In particular, the deposit profiles show a sharp decay at the liftoff point unlike a ground hugging turbidity current whose deposit profile has a slow monotonic decay from the lock region. [Preview Abstract] 
Monday, November 19, 2012 9:31AM  9:44AM 
G13.00008: Powerlaw for gravity currents produced from instantaneous sources propagating on inclined boundaries in the deceleration phase Albert Dai The powerlaw for gravity currents on slopes is essentially an equivalent form of the solution of thermal theory, when the gravity current is sufficiently far into the deceleration phase. However, the hypothesis that gravity current is sufficiently far into the deceleration phase is hardly satisfied in experiments. In this paper, we rederived the powerlaw, considering the influence of bottom friction, and corrected an error in an early version of powerlaw given by Maxworthy (2010). When the gravity current is not sufficiently far into the deceleration phase, we showed that the powerlaw still robustly describes the front location versus time relationship, but the amount of heavy fluid in the head can be easily underestimated. The underestimation of heavy fluid in the head also depends on where the gravity current is in the deceleration phase. Therefore, a correction factor is suggested according to the location of gravity current. [Preview Abstract] 
Monday, November 19, 2012 9:44AM  9:57AM 
G13.00009: Laminar flow of constantflux released gravity currents: Friction factorReynolds number relationship Firat Testik, Nazli Yilmaz, Mijanur Chowdhury This study aims to provide a relationship for the friction factor, $C_{f}$, in terms of the Reynolds number, \textit{Re}, for twodimensional constantflux release gravity currents during viscousbuoyancy propagation phase. Motivation of this study was related to the pipeline disposal of highconcentration dredged fluidmud. Such disposal operations form nonNewtonian gravity currents that propagate over the coastal seafloor. Our theoretical and experimental analysis resulted in $C_{f}$\textit{Re} relationships for both Newtonian (e.g. saline solution) and powerlaw (e.g. nonNewtonian fluid mud) fluids. A large number of experiments were conducted with different concentrations of both fluid mud mixtures (Kaolinite clay mixed with tap water) and saline solutions in a laboratory tank [dimensions: 4.3$m$ x 0.25$m$ x 0.5$m$]. In the experiments, different depths of ambient fluid (tap water) were considered. To determine the experimental $C_{f}$ values for the viscousbuoyancy propagation phase, theoretical analysis was conducted to relate $C_{f}$ to the experimental measurables. Based upon experimental observations, $C_{f}$ is shown to relate to \textit{Re} of the gravity currents inversely for both Newtonian and powerlaw fluids. While Newtonian gravity currents revealed a single value of the constant of proportionality for the $C_{f}$\textit{Re} relationship, powerlaw gravity currents revealed multiple values of the constant of proportionality that depends on the fluidmud concentration. [Preview Abstract] 

G13.00010: ABSTRACT WITHDRAWN 
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