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 H24: Convection and Buoyancy Driven Flows I |
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Chair: Colm-cille Caulfield, University of Cambridge Room: 327 |
Monday, November 21, 2011 10:30AM - 10:43AM |
H24.00001: Buoyancy driven acceleration in a hospital operating room indoor environment James McNeill, Jean Hertzberg, John Zhai In hospital operating rooms, centrally located non-isothermal ceiling jets provide sterile air for protecting the surgical site from infectious particles in the room air as well as room cooling. Modern operating rooms are requiring larger temperature differences to accommodate increasing cooling loads for heat gains from medical equipment. This trend may lead to significant changes in the room air distribution patterns that may sacrifice the sterile air field across the surgical table. Quantitative flow visualization experiments using laser sheet illumination and RANS modeling of the indoor environment were conducted to demonstrate the impact of the indoor environment thermal conditions on the room air distribution. The angle of the jet shear layer was studied as function of the area of the vena contracta of the jet, which is in turn dependent upon the Archimedes number of the jet. Increases in the buoyancy forces cause greater air velocities in the vicinity of the surgical site increasing the likelihood of deposition of contaminants in the flow field. The outcome of this study shows the Archimedes number should be used as the design parameter for hospital operating room air distribution in order to maintain a proper supply air jet for covering the sterile region. [Preview Abstract] |
Monday, November 21, 2011 10:43AM - 10:56AM |
H24.00002: Particle-Laden Viscous Gravity Currents Sandeep Saha, Laurent Talon, Dominique Salin The extension of a gravity current in lock-exchange problems, proceeds as square root of time in the viscous regime. In the presence of particles, however, this scenario is drastically altered due to sedimentation in a manner similar to the well- known Boycott effect. The spreading of particle-laden gravity currents is investigated with numerical simulations based on a Lattice-Boltzmann method. The settling of particles is modelled using a flux function for capturing sudden discontinuities in particle concentration travelling as kinematic shock waves. Contrary to conventional gravity currents, sedimentation leads to the formation of two additional fronts: a horizontal front descending vertically and a sediment layer that ascends as the particles accumulate. Two regimes emerge in the spreading process: the latter corresponding to the lateral advance of the sediment deposit and the former characterised by the vertical motion of the two fronts. An increase in the initial concentration hastens the time at which the regime change occurs and impedes the overall spreading process. The sedimentation velocity of the particles either slows down or speeds up the edges of the gravity current. A model based on lubrication theory is derived to explain the results and identify scaling laws. [Preview Abstract] |
Monday, November 21, 2011 10:56AM - 11:09AM |
H24.00003: Self-consistent definition for the variable depth of unsteady, turbulent gravity currents Hafiz Junaid Anjum, Jim McElwaine, C.P. Caulfield We used the data from two-dimensional direct numerical simulations of Boussinesq gravity currents to define a self-consistent depth, $h$, and reduced gravity, $g'=\frac{g}{\rho_0}(\rho-\rho_0)$, for the current in terms of moments of the current density field. We demonstrate that using these definitions to calculate the Froude number, Fr=$u/\sqrt{g'h}$, gives a constant Froude number in constant-velocity and self-similar regime. At sufficiently high Reynolds number, our results are consistent with previous experimental and theoretical models (Shin \emph{et al}. 2004, Nokes \emph{et al}. 2008). We also develop a simple model to quantify the loss of mass from the gravity current head due to shear-induced vortices which propagate away from and behind the head. \\[4pt] Shin, J. O., Dalziel, S. B., Linden, P. F. (2004). Gravity currents produced by lock exchange. \emph{J. Fluid Mech.} \textbf{521},1-34. \\[0pt] Nokes, R.I., Davidson, M.J., Stepien, C.A., Veale, W. B., Oliver, R.L. (2008). The front condition for intrusive gravity currents. \emph{J. Hydraul. Res.} \textbf{46} (6) 788-801. [Preview Abstract] |
Monday, November 21, 2011 11:09AM - 11:22AM |
H24.00004: Laminar-turbulent cycles in strongly-confined lock-exchange flows Yukie Tanino, Frederic Moisy, Jean-Pierre Hulin It is well established that certain shear flows display a transitional regime in which they alternate regularly between laminar and turbulent states. Here, we consider laminar- turbulent cycles in strongly-confined lock-exchange flows in an inclined tube. Simultaneous measurements of density and velocity fields using laser-induced fluorescence and particle image velocimetry demonstrate that the axial velocity evolves in a distinctive ramp-cliff pattern during each laminar-turbulent cycle. This pattern indicates that the flow accelerates as it relaminarizes, then decelerates rapidly with the breakdown of Kelvin-Helmholtz billows. In contrast to conventional interpretation, a single value of the Richardson number does not distinguish flows that subsequently exhibit turbulence from flows that do not. While the density contrast drives the shear flow macroscopically, local stratification does not directly control the onset of instability within experimental conditions. Instead, the measurements suggest that a necessary criterion for a ramp-cliff laminar-turbulent cycle in this flow configuration is for the local Reynolds number to exceed 2200-2300. [Preview Abstract] |
Monday, November 21, 2011 11:22AM - 11:35AM |
H24.00005: Heat transport in a tilted channel Xavier Riedinger, Jean-Christophe Tisserand, Francesca Chilla, Bernard Castaing We study heat convection in a channel between two chambers, the hot one being lower than the cold one. The channel is tilted with an angle varying up to $50$ degrees and the power applied, to heat the hot chamber, varying up to $80$ Watts. We perform both thermal and velocity measurements, through a PIV technique. We define a Nusselt number and a Rayleigh number according to the measured thermal gradient inside the channel. We show how increases the Nusselt number with the angle of the channel. Otherwise, the evolution of the thermal gradient shows four different regimes depending of the tilt angle and the power. When increasing power, the fluid flow changes from laminar to turbulent. For the turbulent case, we give a scaling law for the mean axial velocity. For intermediate parameters we differentiate another soft turbulent regime for which the velocity scaling law still apply but where the thermal gradient evolves much slowly with power. PIV measurements allow us to illustrate the transition from the laminar flow to the turbulent one and to distinguish a fourth regime charaterized by the destabilisation of laminar lateral jets. [Preview Abstract] |
Monday, November 21, 2011 11:35AM - 11:48AM |
H24.00006: Effect of rate of change of external conditions on the transition from abnormal to normal steady-state convection Albert Sharifulin, Anatoly Poludnitsin Steady-state thermal convection regimes in a closed cavity undergo bifurcation on smooth variation of the external parameters. Bifurcations originating from changes in the Rayleigh number and angle of cavity inclination are investigated in the present work. We experimentally investigate the influence of the crossing rate through bifurcation curve on the transition process. The experiments were conducted with tilted cubic cavity filled with air, as described in [1], for fixed values of the Rayleigh number and different speeds of the slope of the cavity. It was found that at low velocities the slope of the rate of inclination does not affect the critical angle, beyond which is about the spontaneous transition from the anomalous convection mode to normal one. With no low-speed tilting the region of existence of the anomalous flow is sufficiently increased, and the transition process, as well as at low speeds, the slope is in rapid rotation around the vertical axis of the mass of air that fills the cavity. Possibility of applications to atmospheric behavior explanation and to Earth's mantle one is discussed. \\[4pt] [1] A.N. Sharifulin, A.N. Poludnitsin A.N., A.S. Kravchuk Laboratory Scale Simulation of Nonlocal Generation of a Tropical Cyclone. Journal of Experimental and Theoretical Physics, 2008, Vol.107, No.6, pp.1090-1093. [Preview Abstract] |
Monday, November 21, 2011 11:48AM - 12:01PM |
H24.00007: Convection, thermal stratification and the energy penalty of localised heating with and without thermal mass Andrea Kuesters, Andrew W. Woods When a large enclosed space is heated from a point source, the space becomes vertically stratified in temperature. We develop a model for this stratification in the case of constant mixing ventilation, in which there is high level supply and extract of air, testing our model predictions with some new laboratory experiments. We demonstrate that with large ventilation flow or a small heat flux a strong two layer stratification develops in the space, whereas with low ventilation flux or large heat supply the stratification is weak. We explore how thermal mass affects this stratification and generalise the results to allow for a combination of a space with a distributed and a point source of heating. One important consequence of the stratification is that in order to achieve comfortable temperatures in the occupied zone near the floor of the space, temperatures become elevated higher in the space; with mixing ventilation this increases the heat loss associated with the ventilation in comparison to that in a well-mixed space, in which the temperature is maintained at the comfort temperature throughout. Using our model, we compare the cost of such stratification in terms of elevated heating load, accounting for the role of the thermal mass in controlling the strength of the stratification. [Preview Abstract] |
Monday, November 21, 2011 12:01PM - 12:14PM |
H24.00008: Turbulent buoyant convection from a maintained source of buoyancy in a narrow vertical tank Daan D.J.A. van Sommeren, C.P. Caulfield, Andrew W. Woods We describe new experiments to examine the buoyancy-induced mixing which results from the injection of a small constant volume flux of fluid of density $\rho_{s}$ at the top of a long narrow vertical tank with square cross-section that is filled with fluid of density $\rho_{0}<\rho_{s}$. The injected fluid vigorously mixes with the less dense fluid that initially occupies the tank, such that a dense mixed region of turbulent fluid propagates downwards. The density at any height within this mixed region increases with time. For an ideal point source of constant buoyancy flux $B_{s}$, we show that the height of the mixed region grows as $h \sim B_s^{1/6} d^{1/3} t^{1/2}$ and that the reduced gravity $g'=g(\rho-\rho_{0})/\rho_0$ at the top of the tank increases as $g'(0) \sim B_{s}^{5/6}d^{-7/3} t^{1/2}$, with $d$ the width of the tank. Once the mixed region reaches the bottom of the tank, the turbulent mixing continues, and we demonstrate that the reduced gravity at each height increases approximately linearly with time. Our results are consistent with Prandtl's mixing length theory, which suggest that the local turbulent flux is given by $J=\lambda d^{2}(\partial \overline{g'}/\partial z)^{3/2}$, with $\lambda$ an $O(1)$ constant. We solve the corresponding nonlinear turbulent diffusion equation, and show a good agreement with experimental profiles obtained with a dye attenuation technique. [Preview Abstract] |
Monday, November 21, 2011 12:14PM - 12:27PM |
H24.00009: Flushing of a dense fluid from an urban canyon part 1: Steady state measurements Nigel Kaye, Zahra Baratian We consider the role of buoyancy on the vertical transport of a dense gas due to a horizontal wind flow above a street canyon. The density of the pollutant suppresses vertical mixing as the turbulent shear flow at the top of the canyon must do work to raise the dense gas up above the canyon top. We present results of a series of experiments to measure the rate of removal of a dense miscible fluid from a two dimensional square canyon open at the top. The cavity is formed by square blocks up- and down-stream. Dense fluid is introduced at a constant rate at the base of the cavity and is removed by mixing with the flow passing over the top of the cavity. Two different steady flows are observed. For higher Richardson numbers, a two layer stratification develops in which there is a relatively sharp interface. In this case the mixing is parameterized in terms of an entrainment velocity across the interface that is a function of the Richardson number and the fractional depth of the interface below the cavity top. For lower Richardson numbers no interface is observed and the buoyancy increases linearly with height above the cavity base. We also found a range of Richardson numbers for which both steady stratifications are possible and for which the steady flow depends on the initial conditions. [Preview Abstract] |
Monday, November 21, 2011 12:27PM - 12:40PM |
H24.00010: Flushing of a dense fluid from an urban canyon part 2: Transient flows Zahra Baratian, Nigel Kaye We consider the wind driven flushing of a finite volume of dense fluid from a canyon. Unlike the case of a continuous release of dense fluid into the canyon, no steady state flow is established. Instead the total buoyancy in the canyon decreases monotonically with time until it is completely flushed. The time taken to flush an initially full canyon depends on the initial flow Richardson number. Two flow phenomena were observed. For high Richardson numbers a two layer stratification is maintained for much of the flushing process. Dense fluid is skimmed off the top of the dense layer. However, the buoyancy of the layer also slowly decreases over time indicating that fresh ambient fluid is mixed down into the dense layer. The two layer transient flushing is modeled using an entrainment coefficient for mixing across the interface and a skimming coefficient to describe change in interface height over time. Both these coefficients were derived from steady-state constant buoyancy flux experiments. For lower Richardson numbers the fluid in the cavity is relatively well mixed and the buoyancy decays approximately exponentially over time consistent with previous observations of flushing of a non-buoyant fluid from a canyon. The exponential decay rate decreases with increasing Richardson number. [Preview Abstract] |
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