Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session QG: Stratified Flows I |
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Chair: Sutanu Sarkar, University of California, San Diego Room: Long Beach Convention Center 103B |
Tuesday, November 23, 2010 12:50PM - 1:03PM |
QG.00001: Interleaving intrusions between adjacent layered stratifications Benjamin Maurer, Paul Linden Interfacial gravity currents occur when horizontal density gradients result in the intrusion of a fluid along the interface between two layers of ambient fluid. This system has traditionally been studied in the case where the ambient fluid consists of only two layers, however, many oceanographic and atmospheric flows involve the interleaving adjacent stratifications consisting of multiple layers. We present an experimental and numerical study of the interleaving of multiple interfacial intrusions propagating in both directions. For the simple case of two adjacent layered stratifications where the average density of both sides is equivalent, the adjacent layers interleave at uniform speeds. However, if the average density of one side is increased, the individual current speeds show a marked departure from the speeds predicted from local initial conditions. We present a model to account for this departure. [Preview Abstract] |
Tuesday, November 23, 2010 1:03PM - 1:16PM |
QG.00002: Intrusive gravity currents and the solitary wave lifecycle in a cylindrical geometry Justine McMillan, Bruce Sutherland An ``intrusive gravity current'' or ``intrusion'' arises when a fluid of one density propagates at an intermediate depth within a stratified ambient. Numerous experimental and theoretical studies have examined the propagation of these currents in a rectilinear geometry, however, the dynamics of radially spreading axisymmetric intrusions is less well established. By way of full-depth lock release experiments and numerical simulations, we examine the propagation of vertically symmetric intrusions in a two-layer ambient in a cylindrical geometry. We show that the strong stratification at the interface supports the formation of a mode-2 solitary wave that surrounds the intrusion head and carries it outwards at a constant speed beyond 6 lock radii. The wave and intrusion propagate faster than a linear long wave; therefore, there is strong evidence to support that the wave is indeed nonlinear. By extending rectilinear KdV theory to allow the wave amplitude to decay as $r^{-p}$ with $p\approx\frac{1}{2}$, we show that from a single measurement of wave amplitude, the theory can be used to accurately predict the amplitude, speed and spread of the wave during its nonlinear evolution phase. [Preview Abstract] |
Tuesday, November 23, 2010 1:16PM - 1:29PM |
QG.00003: Horizontal Convection Katarzyna Matusik, Stefan Llewellyn Smith Horizontal convection, caused by differential heating along the horizontal boundary of a fluid, is a model of the meridional overturning circulation of the oceans. We explore aspects of horizontal convection using laboratory experiments. We use salt rather than heat, with sinks and sources of dense and fresh water on the upper boundary of the tank so that the net flux of salt into the tank is statistically zero. We measure the density using the Synthetic Schlieren method and a conductivity probe, and dye the incoming fluid and measure its concentration using optical methods. We also use particle tracking to visualize the velocity field within the tank. Our goal is to examine the role of the aspect ratio and governing dimensionless parameters of the system, as well as the effect of the location of sources and sinks, and relate these to features of the flow such as boundary-layer thickness and net overturning circulation. The use of salt rather than temperature results in a high Schmidt number, with implications for the understanding of the experiments and their relation to the ocean. [Preview Abstract] |
Tuesday, November 23, 2010 1:29PM - 1:42PM |
QG.00004: Lattice Boltzmann simulation of buoyancy-driven flow of two immiscible fluids in an inclined channel S.P. Vanka, K.C. Sahu Buoyancy-driven flow due to unstable density stratification of two immiscible fluids in a confined inclined channel is studied by Lattice Boltzmann method using the color segregation approach followed by Wu et al. (Int. J. Numer. Mech. Fluids, 2008, 57, 793-810) and Lishchuk et al. (Phys. Rev.E, 2008, 77, 036702). Initially, the upper and lower halves of the channel are filled with heavier and lighter fluids, respectively. In this system Rayleigh-Taylor instability would occur for any perturbation along the ``interface'' of the two fluids. The fingers of the heavier and lighter fluids then propagate in the downward and upward directions, respectively. The results are compared with earlier experimental and theoretical studies. The spatio-temporal evolution of flow structures and propagation of the finger tips for different angles of inclination and different channel aspect ratio are investigated. [Preview Abstract] |
Tuesday, November 23, 2010 1:42PM - 1:55PM |
QG.00005: ABSTRACT WITHDRAWN |
Tuesday, November 23, 2010 1:55PM - 2:08PM |
QG.00006: Turbulence-induced secondary motion in a buoyancy-driven flow in a circular pipe Jacques Maganudet, Yannick Hallez We analyze the results of a direct numerical simulation of the turbulent buoyancy-driven flow that sets in after two miscible fluids of slightly different densities have been initially superimposed in an unstable configuration in an inclined circular pipe closed at both ends. In the central region located midway between the endwalls, where the turbulent flow is fully developed, the resulting mean flow is found to exhibit nonzero secondary velocity components in the tube cross section, resulting in a four-cell pattern in the tube cross-section. We analyze the generating mechanism of this secondary flow which turns out to be due to the combined effect of the lateral wall and the shear-induced anisotropy between the transverse components of the turbulent velocity fluctuations. Although of small magnitude compared to the main flow, this secondary mean motion is found to be of primary importance in the mixing process between the light and heavy streams. [Preview Abstract] |
Tuesday, November 23, 2010 2:08PM - 2:21PM |
QG.00007: Negatively buoyant fluid projectiles Ole Joergen Myrtroeen, Gary R. Hunt We describe the rise-height behaviour of a finite volume saline release dispensed vertically upwards into a still fresh-water environment. The dynamics of the non-continuous release, or projectile, differ significantly from the continuous version that produces a turbulent fountain. The projectile can be characterised in terms of the release aspect ratio $L/D$ (the length $L$ of the dispensed column to the nozzle diameter $D$) and the source Froude number $Fr_0$, expressing the ratio of inertia and buoyancy. In a continuous high Reynolds number fountain $L/D \rightarrow \infty$ and the behaviour is characterised solely by $Fr_0$. We dispensed, over a time $t_d$, each release and recorded the extent of its maximum vertical propagation as a function of $L/D$ and $Fr_0 = (L/t_d)/ (g' D/2)^{1/2}$, where $g'$ denotes the reduced gravity of the fluid released. For $Fr_0 \rightarrow \infty$, the release propagates as a vortex ring with a trailing jet for $L/D > 4$. As $Fr_0$ decreases, gravitational effects limit the vertical propagation and a maximum rise height $z_m/D$ is reached. We find that the releases are sensitively dependent upon $Fr_0$ and $L/D$ and three rise height regimes, `the weak fountain regime', `the vorticity development regime' and `the forced release regime', are identified by considering rise heights and morphologies. Finally, we discuss some aspects of the transition from a non-continuous release to the continuous fountain as achieved on increasing $L/D$. [Preview Abstract] |
Tuesday, November 23, 2010 2:21PM - 2:34PM |
QG.00008: Mixing induced by exchange flows in a confined volume of fluid A. Kuesters, Andrew W. Woods We investigate the transient mixing and stratification associated with the buoyancy-driven exchange flow of a confined volume of fluid, connected to the exterior through two openings in the roof. The inflowing flux of dense exterior fluid develops a turbulent buoyant plume which mixes with the interior fluid as it cascades to the base of the volume. We show that the resulting transient stratification of the interior fluid asymptotes to a profile of constant shape with decaying amplitude. We show how contaminant, released locally, rises through the space to form a front of high concentration, with relatively little mixing by the plume. New laboratory experiments of the process are shown to be consistent with our predictions of the interior stratification and evolution of the flow. In enclosures with multiple stacks, the efficiency of mixing increases, although there are now multiple flow regimes which can develop. We show that the process is key for modelling the release of smoke or other contaminant through a roof stack from an enclosed space. [Preview Abstract] |
Tuesday, November 23, 2010 2:34PM - 2:47PM |
QG.00009: Buoyancy-induced mixing caused by a dense fluid in a narrow vertical channel D.D.J.A. van Sommeren, C.P. Caulfield, Andrew W. Woods We consider the turbulent mixing caused by a constant flux of dense saline fluid at the top of a long, relatively thin vertical tank with square cross-section, filled initially with either a homogeneous or salt-stratified fluid, such that the incoming fluid is more dense than the fluid at the base of the tank. The dynamics of buoyancy-induced turbulent mixing is strongly influenced by the confined geometry of a vertical tank. Dalziel et al ({\it Phys. Fluids \bf 20}, 065106 2008) found that the turbulent diffusion coefficient is not constant but rather decreases as the strength of the unstable density gradient driving the flow decreases. The vertical propagation of the first front of the turbulent dense fluid is therefore characterized by a $t^{2/5}$ power law. In our work, we apply image analysis to retrieve the propagation speed of the first front, and also to obtain density profiles along the length of the tank for the early- and late-times. We analyze both the propagation speed and the density profiles for a homogeneous and stratified environment. We investigate hereby the influence of the density of the fluid flux relative to the fluid density in the tank, and the length scale of the stratification. We briefly compare and contrast the dynamics of a single- (saline) and double-phase (particle-laden) dense fluid flux. [Preview Abstract] |
Tuesday, November 23, 2010 2:47PM - 3:00PM |
QG.00010: Inertia versus gravity Valentina Shevtsova Einstein postulated the equivalence of gravitational and inertial mass. As such it deserves the most extensive testing possible. In the absence of buoyancy inertia exists, as the tendency of a body to resist acceleration. A good approach is to test action of inertia on board the International Space Station where gravitational effects are absent. When a container filled with liquid is subjected to high frequency vibrations and density gradients are present, inertia will not be uniform, resulting in convective motion. This convective motion is similar to the gravity-induced convection. In the frame of ESA program the experiment SODI / IVIDIL (Influence of Vibration on Diffusion in Liquids) has been performed in October 2009-January 2010 on ISS. Two liquid mixtures with positive and negative Soret effect were studied. The closed cells filled with different binary mixtures were vibrated with different frequency and amplitude. The liquids were density stratified due to temperature and concentration gradient. The applied experimental technique (digital optical interferometry) allowed to IVIDIL science team to obtain a clear evidence of different convective patterns created by vibrations. [Preview Abstract] |
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