Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session M24: Convection and Buoyancy Driven Flows II |
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Chair: Ken Kiger, University of Maryland Room: 327 |
Tuesday, November 22, 2011 8:00AM - 8:13AM |
M24.00001: Is horizontal convection really ``non-turbulent''? Alberto Scotti, Brian White The oceanic Meridional Overturning Circulation (MOC) is the slow residual motion that turns over the world ocean over a millennial time scale. What drives the MOC has been the subject of intense speculation. Equatorial heating and polar cooling, i.e. horizontal convection (HC), would seem to be a natural candidate to drive (at least partially) the MOC, but within the oceanographic community HC has been considered irrelevant or nearly so, based primarily on an inference based on a century old experiment by Sandstr\"om (1908), though all modern experiments contradict it (Hughes and Griffiths, 2008), and on a theoretical argument that would prevent HC to sustain a true turbulent flow (Paparella and Young, 2002), the latter deemed necessary to achieve mixing. We revisit Paparella and Young's argument with the aid of DNS of HC at Rayleigh number up to $10^{10}$. We argue that the latter argument is overly restrictive. On the contrary, analysis of invariants (such as Q-R plots) show that HC possesses the characteristic of turbulent flows. The surprising result is that HC can transport very large quantities of heat and sustain large amount of diapycnal mixing with a surprisingly small amount of dissipation. Hence, HC alone could be an important component behind the MOC, even though it cannot explain the levels of dissipation observed in the ocean, which have to be accounted from other sources. [Preview Abstract] |
Tuesday, November 22, 2011 8:13AM - 8:26AM |
M24.00002: Horizontal convection PIV study Stefano Discetti, Tommaso Astarita Horizontal convection has received limited attention, despite of its well recognized implication in geophysical flows of relevant interest. E.g., oceans are often modeled as being cooled and heated in a thin surface layer, which is nearly horizontal, due to solar irradiance forcing differentially in latitude and determining strong stable density stratification. The phenomenology is simulated in a plexiglass box, with a piecewise thermal boundary condition on its bottom surface while all the other boundaries are thermally insulated. A Particle Image Velocimetry (PIV) investigation of the natural convection generated by a differential thermal boundary condition along the same horizontal boundary is proposed. A preliminary investigation of the structure of the circulation cell, generated by the hot water rising up in correspondence of the heat flux input zone, and sinking down nearby the cooled zone, is carried out. A strong convection cell, consisting of a rising intense plume (filling the whole box height) and a slow downward average flow, are observed in all the tested configurations. [Preview Abstract] |
Tuesday, November 22, 2011 8:26AM - 8:39AM |
M24.00003: Horizontal Convection in the Presence of a Surface Stress Katarzyna Matusik Horizontal convection, the flow driven by buoyancy fluxes imposed along a horizontal boundary of a fluid, is a model of the meridional overturning circulation of the oceans. We explore features of horizontal convection in the presence of a surface stress. The buoyancy flux is achieved by injecting a plume of dense water into a fresh-water tank, while continuously imposing a fresh-water surface boundary condition. The magnitude of the stress is varied by adjusting the flow rate of fresh water traversing along the surface. We measure the steady-state density field using the Synthetic Schlieren method, and qualitatively observe the flow via discrete dye injections. The vertical and horizontal velocities are determined by PIV techniques. Our goal is to examine the effects of a surface stress of varying magnitude and direction on the general horizontal convection. We compare laboratory data to scaling analyses for the boundary- layer thickness and the strength of the overturning circulation. The addition of a surface stress to horizontal convection may offer insight into the effects of wind on the ocean surface, namely the implications of a kinetic energy source on the overall energetics of the circulation. [Preview Abstract] |
Tuesday, November 22, 2011 8:39AM - 8:52AM |
M24.00004: DNS of unstable stratification in turbulent channel flow Alfredo Soldati, Francesco Zonta Turbulence subject to unstable stratification (bottom-up heating) is of great interest due to its dynamical importance in engineering and geophysical flows. Unstably-stratified flows often occur in the presence of high temperature gradients. In this situation, it is important to account for variation of the fluid properties with temperature. According to this idea, we use direct numerical simulation to analyze the physics of unstably-stratified turbulence with temperature-dependent fluid properties: specifically, viscosity and thermal expansion coefficient. Due to unstable stratification, wall-normal transport of momentum and heat increases with respect to the neutrally-buoyant case. Results show that flow modifications due to temperature-dependent properties may become important. In particular, the effect of temperature-dependent thermal expansion coefficient is to increase the wall normal transport of momentum and heat. By contrast, the effect of temperature-dependent viscosity appears negligible. [Preview Abstract] |
Tuesday, November 22, 2011 8:52AM - 9:05AM |
M24.00005: Stable stratification in turbulent channel flow Francesco Zonta, Miguel Onorato, Alfredo Soldati Stable stratification (bottom-up cooling) is of great importance, since it is encountered in industrial applications, environmental processes and geophysical flows. Turbulent entrainment and mixing across density interfaces in terrestrial water bodies (oceans, lakes and rivers) or at the boundaries between water bodies and the atmosphere, are just some important examples of stably-stratified flows. In this work we use direct numerical simulation to investigate the fundamental physics of stably-stratified turbulent channel flow of water with temperature dependent viscosity and thermal expansion coefficient. Compared to the neutrally-buoyant case, stable stratification induces a general suppression of turbulence levels, momentum and buoyancy fluxes. We observe that the effect of temperature-dependent fluid properties may be important. The most striking feature produced by temperature-dependent fluid properties is local flow laminarization in the cold side of the domain (for temperature-dependent viscosity) or in the hot side of the domain (for temperature-dependent thermal expansion coefficient). [Preview Abstract] |
Tuesday, November 22, 2011 9:05AM - 9:18AM |
M24.00006: ABSTRACT WITHDRAWN |
Tuesday, November 22, 2011 9:18AM - 9:31AM |
M24.00007: Unsteady Heat Transfer in Channel Flow using Small-Scale Vorticity Concentrations Effected by a Vibrating Reed Pablo Hidalgo, Ari Glezer Heat transfer enhancement by small-scale vorticity concentrations that are induced within the core flow of a mm-scale heated channel are investigated experimentally. These small-scale motions are engendered by the cross stream vibrations of a streamwise cantilevered reed that spans most of the channel's width. The interactions between the reed the core flow over a range of flow rates lead to the formation, shedding, and advection of time-periodic vorticity concentrations that interact with the wall boundary layers, and increase cross stream mixing of the core flow. Heating of the channel walls is controlled using microfabricated serpentine resistive heaters embedded with streamwise arrays of temperature sensors. It is shown that the actuation disrupts the thermal boundary layers and result in significant enhancement of the local and global heat transfer along the channel compared to the baseline flow in the absence of the reed. The effect of the reed on the cross flow is measured using high resolution particle image velocimetry (PIV), and the reed motion is characterized using a laser-based position sensor. The blockage induced by the presence of the reed and its cross stream motion is characterized using detailed streamwise pressure distributions. Supported by DARPA and UTRC. [Preview Abstract] |
Tuesday, November 22, 2011 9:31AM - 9:44AM |
M24.00008: Mixing from diffusion and natural convection in binary non-equilibrium fluid phases Laurence Rongy, Kjetil Haugen, Abbas Firoozabadi The mixing of two non-equilibrium fluid phases is relevant to a large number of problems in industry and in nature. Important applications are improved oil recovery and carbon sequestration. Our work provides a realistic description of the mixing in non-ideal fluids, which is needed to assess the efficiency of oil extraction and the storage capacity in geological formations. We also show how to determine diffusion coefficients accurately in non-ideal fluids at reservoir conditions from measurements such as pressure evolution and gas-liquid level data whereas most techniques are restricted to measurements at atmospheric pressure only. [Preview Abstract] |
Tuesday, November 22, 2011 9:44AM - 9:57AM |
M24.00009: Buoyancy-driven instabilities of acid-base fronts: the case of a color indicator L.A. Riolfo, S. Kuster, P.M.J. Trevelyan, C. El Hasi, A. Zalts, C. Almarcha, A. D'Onofrio, A. De Wit Buoyancy-driven hydrodynamic instabilities of acid-base fronts are studied both experimentally and theoretically in the case where an aqueous solution of a strong acid is put above a denser aqueous solution of a color indicator in the gravity field. The neutralization reaction between the acid and the color indicator as well as their differential diffusion modifies the initially stable density profile in the system and can trigger convective motion both above and below the initial contact line. The type of patterns observed as well as their wavelength and the speed of the reaction front are shown to depend on the value of the initial concentrations of the acid and of the color indicator and on their ratio. A reaction-diffusion model explains how the hydrodynamic instability scenarios change when the concentration of the reactants are varied. [Preview Abstract] |
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