Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session KN: Geophysical Flows III |
Hide Abstracts |
Chair: Bruce Sutherland, University of Alberta Room: Hilton Chicago PDR 2 |
Monday, November 21, 2005 4:10PM - 4:23PM |
KN.00001: Bumps, witches and bouncing beams: lab investigations of internal waves Thomas Peacock, Paula Echeverri, Neil Balmforth, Ali Tabaei There is a great, ongoing effort to better understand the processes surrounding internal wave generation, propagation and dissipation in the oceans. To contribute to this effort, we are in the process of establishing a state-of-the-art experimental facility. The facility, based around the digital schlieren method, is designed to investigate both linear and non-linear phenomena in a laboratory setting. We here report the latest experimental results concerning tidal conversion by typical topographic features, such as a Gaussian bump and a knife-edge. The quantitative results compare very well with theoretical predictions developed from the classic work of Bell and Hurley, and more recent analysis of subcritical topography by Balmforth et al. In addition, we present more details on recently published results concerning the nonlinear generation of second-harmonic wavebeams at reflecting boundaries. [Preview Abstract] |
Monday, November 21, 2005 4:23PM - 4:36PM |
KN.00002: 2D PIV Measurements of flow between a pair of model buildings with varying geometries Bhagirath Addepalli, Eric Pardyjak The study of flow within a pair of three dimensional model buildings assumes paramount importance in developing an understanding of the mechanisms involved in the transport of pollutants within urban areas. For this work, 2D PIV measurements have been performed to add insight into urban flow physics. The experiments were performed in a 7.9 m long boundary layer wind tunnel facility with a cross section of 0.61 m x 0.91 m and at a free stream velocity of $\sim $7 m s-1. Two sets of experiments have been considered. In the first set of experiments, the spacing between two cubes was varied. For these highly idealized experiments, the results correspond well with the results in the literature for the wake interference and skimming flow regimes, but discrepancies have been found corresponding to the isolated roughness flow regime. The second set of experiments considers flow between two buildings with variable upwind and downwind building heights and separation distances. These experiments indicate that the mean and turbulence flow characteristics associated with even these very simple types of building configurations can vary considerably and thus significantly modify pollutant transport. [Preview Abstract] |
Monday, November 21, 2005 4:36PM - 4:49PM |
KN.00003: Atmospheric Flow through Urban Street Canyons H.J.S. Fernando, Dragan Zajic, Ronald Calhoun Flow and turbulence through a network of urban street canyons (streets located within large buildings) were studied during two large-scale field experiments: the Mock Urban Setting Test (MUST-2000) at the US Army Dugway Proving Grounds and the Joint-Urban 2003 field experiment in Oklahoma City. Instrumented towers and tethersondes deployed by the authors and several other groups were analyzed in the framework of flow regimes corresponding to each of the sites (``isolated roughness'' at Dugway and ``skimming flow'' at OKC). The results show that the flow patterns are highly sensitive to the approach angle for angles greater than about 5 deg, and that when the flow is normal to the building cluster the canyons are dominated by recirculating flow. The production of turbulence is highest near the ground and near the top of the buildings, and the variations of turbulent shear stresses could be scaled using local similarity variables. The mean flow in the roughness and inertial layers were compared with available theoretical formulations, and the flow in MUST was also studied using numerical simulations. [Preview Abstract] |
Monday, November 21, 2005 4:49PM - 5:02PM |
KN.00004: Stratified Flow over Rough Topography Bruce Sutherland, Dawn Aguilar Linear theory and Long's model for stratified flow over topography both assume free-slip lower boundary conditions and so neglect the possibility of boundary-layer separation either between successive hill crests or in the lee of a range of hills. Here we report upon laboratory experiments that focus upon periodic finite-amplitude hills which are representative of the Earth's major mountain ranges as well as the fractured crevasses of the ocean floor. The topographic shapes are selected to encompass varying degrees of roughness, from smoothly-varying sinusoidal hills to steeper triangular and rectangular hills. For low flow speeds, U (and hence low values of the excitation frequency), the internal wave frequencies are consistent with those predicted by linear theory. However, when the excitation frequency exceeds the buoyancy frequency, N, internal waves are still excited and their frequencies are found to be an approximately constant fraction of N. In all experiments the wave amplitudes are much smaller than predicted because, through boundary-layer separation, fluid is trapped in the valleys between hills effectively reducing the peak-to-peak hill height, H. This is true even if NH/U is moderately less than 1. For rough topographies, turbulent structures emerge even at low towing speeds and waves are generated by the nonlinear interactions between the flow, lee waves and turbulence far in the lee. [Preview Abstract] |
Monday, November 21, 2005 5:02PM - 5:15PM |
KN.00005: Intrusions on a density interface Paul Linden, Hyong-Bin Cheong, Jeroen Keunen, Stuart Dalziel An accurate theoretical prediction of the speed of an intrusion propagating along the interface between two uniform layers has defied analysis for the past 25 years. Theories by Holyer \& Huppert (1980) and, more recently, Sutherland, Kyba \& Flynn (2004) give only approximate agreement with experiments. We describe an experimental and numerical study of an intrusion and show that, except when the density of the intrusion is the depth-weighted mean of the layer densities, the interface ahead of the intrusion is displaced vertically. We predict this vertical displacement, which takes the form of an upstream-propagating long wave, and use the predicted value to determine the intrusion speed. For the case when the interface is undisturbed the intrusion propagation speed is a minimum. We develop an energy argument that describes the observed variation of the intrusion speed from this minimum speed as a function of the intrusion and layer densities and the ratio of the layer depths. We also show that if, and only if, the layer depths are equal, the speed of the intrusion is \emph {independent} of the density of the intrusion. [Preview Abstract] |
Monday, November 21, 2005 5:15PM - 5:28PM |
KN.00006: Internal Wave Transmission Across the Equatorial Undercurrent Joshua Nault, Bruce R. Sutherland We examine the propagation of internal waves from the surface mixed region of the equatorial Pacific Ocean through the equatorial undercurrent. Thus we are able to assess the spectrum of waves capable of penetrating to the deep ocean and energising the deep equatorial countercurrents. We show that heuristics based on ray theory are not sufficient to make this assessment. A numerical code is developed to solve the Taylor-Goldstein equation for arbitrary buoyancy frequency and background flow profiles. In addition to numerical integration of the governing equation, the code also uses a initial condition driver that finds a unique causal wave-like solution and thus determines the transmitted and reflected wave amplitudes. From these we determine the transmission coefficient defined to be the ratio of transmitted to incident pseudoenergy. Using equatorial ocean density and background flow speed observations, we develop characteristic analytic basic state profiles. Applying the code, the transmission coefficient is calculated for given incident initial internal wave frequencies $\omega$ and horizontal wavenumbers $k$. For a range of $\omega$ and $k$ for which waves do not encounter a critical level and the Doppler-shifted frequency is less than the local buoyancy frequency at all depths (and hence for waves that one might expect on the basis of ray theory to transmit perfectly) we find wave transmission is largest for incident waves that are close to being harmonic with vertical modes in the duct. [Preview Abstract] |
Monday, November 21, 2005 5:28PM - 5:41PM |
KN.00007: Internal Gravity Waves in a Dipolar Wind. Jean-Marc Chomaz, Ramiro Godoy-Diana, Claire Donnadieu An experimental study on the interaction of the internal wave field generated by oscillating cylinders in a stratified fluid with a pancake dipole is presented. The experiments are carried out in a salt-stratified water tank with constant Brunt-V\"{a}is\"{a}l\"{a} frequency. When the wave and the dipole propagate horizontally in opposite directions (counterpropagating case), the phase line of the gravity wave beam steepens towards the vertical as it enters the dipolar field and it may even reach a turning point where the wave is reflected. When the dipole and the wave propagate in the same direction (copropagating case), the wave beam is bent towards the horizontal and may be absorbed by the dipole. In that case, the waves encounter a critical layer, and momentum is transferred to the dipole. Three-dimensional effects of the dipolar velocity field on the propagating internal waves induce focusing and refraction of a wave beam, that in ocean flows may lead to wave breaking. [Preview Abstract] |
Monday, November 21, 2005 5:41PM - 5:54PM |
KN.00008: Effects of particle inertia on gravity current flows Mariano Cantero, S. Balachandar, Marcelo Garcia Gravity currents are buoyancy-driven flows well known to be one of the main sediment transport mechanism into deep sea. A new mathematical model for particulate gravity currents based on the well-accepted formalism of two-phase flow is introduced. The model includes settling and particle inertia effects to $O(\tau V_s + \tau^2 + V_s^2$) (tau represents the dimensionless particle inertia and $V_s$ the dimensionless settling rate). By means of highly resolved simulations the effect of particle inertia on front velocity, ambient fluid entrainment, flow structure and deposition patterns is addressed. The simulations are performed with a de-aliased pseudo-spectral code which allows accurate representation of length and time scales present in the flow. The results show a strong variation of the flow structure caused by particles migration from regions with strong vorticity and accumulation into regions of high shear. These effects contribute to the variation of the current front velocity with varying particle size, even in the case of same net buoyancy. The analysis of the effect of particle inertia on deposition patterns and ambient fluid entrainment is under way. [Preview Abstract] |
Monday, November 21, 2005 5:54PM - 6:07PM |
KN.00009: Mixing efficiency in lock release gravity currents C.P. Caulfield, M.D. Patterson, J.N. McElwaine, S.B. Dalziel We consider numerically and experimentally mixing in Boussinesq lock release gravity currents. We show quantitatively that mixing is strongly dependent on the lower boundary condition. As the fluid in the current slumps and flows under gravity, Kelvin-Helmholtz billows grow at the current head, entrain ambient fluid as they are swept backwards, and then fall down over the current tail, interacting strongly with a thin layer of dense fluid which remains at the lower boundary. For flows with free-slip lower boundaries, the billows remain largely two- dimensional. Conversely, for flows with no-slip lower boundaries, the current front develops lobes and clefts as ambient fluid is engulfed horizontally and overrun vertically, thus leading inevitably both to convection, and also to strong three-dimensionality. Using the APE framework of Winters et al. (1995), we quantify the time-dependent mixing associated with the engulfed fluid, the overrun fluid, and the billows as they develop streamwise secondary instabilities which trigger turbulence. We also compare the cumulative mixing efficiency with laboratory experiments. [Preview Abstract] |
Monday, November 21, 2005 6:07PM - 6:20PM |
KN.00010: Interaction of finite volume gravity currents with a two-layer stratified interface Periandros Samothrakis, Aline Cotel An experimental study of two-dimensional gravity currents impinging on a stratified interface in a two-layer stratified environment has been conducted. The gravity currents are created by the release of a finite volume of dense fluid along a 6$^{o}$ inclined boundary. The effect of the stratified interface on the entrainment and mixing processes is quantified by the use of Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV). The instantaneous velocity and vorticity fields are quantified and averages are computed over 0.2 seconds. For both experimental techniques, the laser sheet is positioned at mid-span and extends in the streamwise direction. This allows for the measurements to be centered on the impact region between the gravity current and the stratified interface. We have previously determined the entrainment rate and studied the internal structure of a gravity current created by a continuous source with similar experimental conditions (for the slope and the ambient stratification) as in the current study. A thorough comparison of the two cases is provided. [Preview Abstract] |
Monday, November 21, 2005 6:20PM - 6:33PM |
KN.00011: Front velocity of lock exchange gravity currents on a slope Vineet Birman, Eckart Meiburg, James Rottman, Paul Linden We present an investigation of lock exchange gravity currents in sloping channels. Two-dimensional Navier-Stokes simulations show the existence of two phases for the flow. During the initial phase, the front velocity is seen to be constant. Its magnitude depends on the slope angle and reaches a maximum near 40 degrees. This quasisteady initial phase gives way to a second phase of higher, unsteady front velocity. This second phase is dominated by the accelerating motion of the two stratified fluid layers past each other. We develop a simple model that predicts the time of the transition between the two phases. Experimental observations are presented that support the numerical findings. [Preview Abstract] |
Monday, November 21, 2005 6:33PM - 6:46PM |
KN.00012: Mixing induced in a dense current flowing down a sloping bottom in a rotating fluid Claudia Cenedese, Claudia Adduce A density driven current was generated in the laboratory by releasing dense fluid over a sloping bottom in a rotating freshwater system. Over a wide range of parameter values, the following four flow types were found: laminar, wave, turbulent and eddy regime. The amount of mixing between the dense and the ambient fluids was measured and its dependence on the Froude number and on the distance downslope was determined for increasing values of the Reynolds number. Mixing increased significantly when passing from the laminar to the wave regime; i.e. with increasing Froude number. We believe that mixing between the dense salty water and the lighter fresh water was caused by breaking waves. We quantified the amount of mixing observed and estimated the value of the entrainment velocity at the interface between the dense fluid and the fresh overlying fluid. The results have been compared with previous laboratory experiments which presented the classic turbulent entrainment behavior and observational estimates of the Mediterranean and Denmark Strait overflow. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700