Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session PR: Convection III |
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Chair: Thomas Peacock, Massachusetts Institute of Technology Room: 200F |
Tuesday, November 24, 2009 11:40AM - 11:53AM |
PR.00001: A novel way to measure molecular diffusivity Thomas Peacock, Michael Allshouse We present a new technique for measuring molecular diffusivity. The method exploits the phenomenon of diffusion-driven flow, in which buoyancy-driven convection occurs on sloping surfaces due to the diffusive-flux boundary condition affecting the local fluid density. By measuring the volume flux drawn into the boundary layer flow on a sloping surface, and exploiting the properties of an analytical solution, the molecular diffusivity is determined. The results are compared with predictions from numerical simulations. This approach allows one to readily investigate such issues as the effect of concentration on molecular diffusivity. [Preview Abstract] |
Tuesday, November 24, 2009 11:53AM - 12:06PM |
PR.00002: Sailing On Diffusion Michael Allshouse, Mike Barad, Thomas Peacock When a density-stratified fluid encounters a sloping boundary, diffusion alters the fluid density adjacent to the boundary, producing spontaneous along-slope flow. Since stratified fluids are ubiquitous in nature, this phenomenon plays a vital role in environmental transport processes, including salt transport in rock fissures and ocean-boundary mixing. Here we show that diffusion-driven flow can be harnessed as a remarkable means of propulsion, acting as a diffusion-engine that extracts energy from microscale diffusive processes to propel macroscale objects. Like a sailboat tacking into the wind, forward motion results from fluid flow around an object, creating a region of low pressure at the front relative to the rear. In this case, however, the flow is driven by molecular diffusion and the pressure variations arise due to the resulting small changes in the fluid density. This mechanism has implications for a number of important systems, including environmental and biological transport processes at locations of strong stratification, such as pycnoclines in oceans and lakes. There is also a strong connection with other prevalent buoyancy-driven flows, such as valley and glacier winds, significantly broadening the scope of these results and opening up a new avenue for propulsion research. [Preview Abstract] |
Tuesday, November 24, 2009 12:06PM - 12:19PM |
PR.00003: Diffusive or convective regime: role of vibrations in Space experiments Valentina Shevtsova Despite the dramatic decrease of gravity level in spacecrafts, diffusive transport in liquid phase can be significantly perturbed by convection. Although it is recognized that on-board g-jitters may have a major impact on diffusion and thermal diffusion measurements, very few experiments have been carried out in the past. There is a lack of experimental data to validate numerous numerical studies. In the frame of ESA program the experiment SODI / IVIDIL (Influence of Vibration on Diffusion in Liquids) will be performed in September-October 2009 on ISS. Purpose of the project is to measure thermal and isothermal diffusion coefficients in binary systems subjected to controlled vibrations under different values of amplitude and frequency. The measurements will be repeated at wide range of amplitudes and frequency in order to validate the results. The IVIDIL experiment will be performed in two cell arrays each composed of two cells: the primary cell that is probed by MZI and the companion cell that is filled with tracer particles and is probed by digital holography for PIV. Different cell arrays are filled with various liquids. Over the experiment run, the cells are temperature controlled. IVIDIL science team foresees to present first experimental results. [Preview Abstract] |
Tuesday, November 24, 2009 12:19PM - 12:32PM |
PR.00004: Optimal Prandtl number for heat transfer in rotating Rayleigh-B\'enard convection Richard Stevens, Herman Clercx, Detlef Lohse The heat transfer in Rayleigh-B\'enard convection (RBC) is determined by the Rayleigh number $Ra$ and the Prandtl number $Pr$ \footnote{Ahlers et al. {\emph{Rev. Mod. Phys.}} {\bf{81}}, 503 (2009)}. In case of rotation about the vertical axis the third dimensionless control parameter is the Rossby number $Ro$. Here we present numerical data for the heat transfer in rotating RBC for $Ra = 10^8$ as a function of $Pr$ and $Ro$. When $Ro$ is fixed the heat transfer enhancement with respect to the non- rotating value as function of $Pr$ shows a maximum. This maximum is due to the reduced efficiency of Ekman pumping when $Pr$ becomes too small or too large. When $Pr$ becomes too small the heat that is carried by the vertical vortices spreads out in the middle of the cell, i.e. it makes Ekman pumping less efficient, due to the larger thermal diffusivity \footnote{Zhong et al. {\emph{Phys. Rev. Lett.}} {\bf{102}}, 044502 (2009); Stevens et al. {\emph{Phys. Rev. Lett.}} {\bf{103}}, 024503 (2009)}. For higher $Pr$ the thermal boundary layers (BLs) are much thinner than the kinetic BLs and therefore the Ekman vortices do not reach the thermal BL. This means that the fluid that is sucked into the vertical vortices is colder than for lower $Pr$ and this limits the efficiency of Ekman pumping at high $Pr$. [Preview Abstract] |
Tuesday, November 24, 2009 12:32PM - 12:45PM |
PR.00005: Convection-driven pattern formation in lawn grasses Sally Thompson, Karen Daniels Spatial patterns of 'dead' lawn grass have often been ascribed to Turing-type reaction-diffusion processes related to water scarcity. We present an alternative hypothesis: that the air within the grass canopy is unstable to a convective instability, such that chill damage caused by falling cold air is responsible for the creation of brown and green bands of grass. This hypothesis is consistent with several features of small-scale vegetation patterns, including their length scale, rapid onset and transient nature. We find that the predictions of a porous medium convection model based are consistent with measurements made for a particular instance of lawn-patterning in North Carolina. [Preview Abstract] |
Tuesday, November 24, 2009 12:45PM - 12:58PM |
PR.00006: An Experimental Study of Flow Reversals in Quasi-2D Rayleigh-Benard Convection Rui Ni, Tak-Shing Chan, Sheng-Qi Zhou, Heng-Dong Xi, Ke-Qing Xia, Kazu Sugiyama, Enrico Calzavarini, Chao Sun, Detlef Lohse, Siegfried Grossmann We present an experimental study of flow reversals of the large-scale circulation in turbulent Rayleigh-Benard convection in a quasi-2D rectangular convection cell with aspect ratio 0.84. Using water as the convecting fluid the range of Rayleigh number varied from around 10$^{8}$ to 10$^{10}$ and the Prandtl number is around 5.8 in the experiment. From local temperature measurements it is found that, contrary to the 3D case, the rate of flow reversals has a strong dependence on the Rayleigh number, i.e. it decreases with increasing Ra. By using thermal plumes as tracer particles, we use shadowgraphs to obtain the velocity field of plume clusters. The results suggest the role played by the plumes in the reversal process. [Preview Abstract] |
Tuesday, November 24, 2009 12:58PM - 1:11PM |
PR.00007: Thermal and PIV measurements of heat convection in a tilted channel Jean-Christophe Tisserand, Mathieu Creyssels, Bernard Castaing, Francesca Chilla The Rayleigh-Benard system, in which a fluid is cooled from above and heated from below, is one of the most studied systems in thermal convection. Nevertheless, in this configuration, the neighborhood of the plates controls the heat transfer. Subsequent, we have built a system (a vertical channel) where the flow forgets the cold and the hot plate. Moreover, we have built a structure which allows to tilt this channel from an angle of 0 to 90 degrees. The main goal of this paper is to highlight how the flow in the bulk of the inclined channel is. In a first part, we present new measurements obtained thanks to particle image velocimetry (PIV) technique and a model which interprets our results. The second part of the paper will be focused on thermal measurements and on the influence of the difference of temperature between the two plates. [Preview Abstract] |
Tuesday, November 24, 2009 1:11PM - 1:24PM |
PR.00008: Buoyancy-Induced Columnar Vortices Mark Simpson, Ari Glezer Large-scale inherent instability of a thermally stratified air layer is exploited for deliberate formation of intense vertical column vortices. In hot-climate regions, buoyancy-driven vortices (``dust devils'') occur spontaneously, with core diameters of 1-50 m at the surface, heights up to one kilometer, with induced air flow of considerable angular and linear momentum. Meter-scale laboratory experiments have demonstrated the nucleation and sustainment of strong buoyancy-driven vortices over a plane heated surface driven by a controllable power source. Optical diagnostics includes high-speed video imaging and particle image velocimetry. It is shown that vortices having a nominal 10 cm diameter core with nearly-uniform vorticity distribution can be triggered by and ``anchored'' to small ground protrusions, and their circulation and angular momentum can be controlled by geometrical modifications of these surface protrusion and by simple flow vanes. [Preview Abstract] |
Tuesday, November 24, 2009 1:24PM - 1:37PM |
PR.00009: Convective instability of a fluid layer induced by an inclined temperature gradient Alejandro Sebastian Ortiz-Perez, Luis Antonio Davalos-Orozco We present new results of the convective stability of a shallow layer of Newtonian fluid under the presence of an inclined temperature gradient. The horizontal component produces the so called Hadley circulation. The vertical component induces the well known Rayleigh convection with vertical Rayleigh number Ra$_{v}$. Here, the numerical results of Nield (1994) are corrected and extended for larger horizontal Rayleigh numbers, Ra$_{H}$. Two modes, the longitudinal and the transversal, compete as the most unstable in different regions of the parameters space which also includes the Prandtl number Pr. The calculations of these corrections were motivated by the paper by Kaloni and Qiao (1996) where they present only particular cases of the nonlinear energy stability of the problem. The horizontal temperature gradient stabilizes until certain magnitude after which the curves of criticality start to decrease because the magnitude of Ra$_{H}$ is very near to its critical value. At this critical value the vertical Rayleigh number is zero. Our results agree very well with those of Kuo and Korpela (1986) (presented also in Wang and Korpela (1989)) who made numerical analysis of the linear instability of the Hadley circulation, presenting corrections to the classical paper by Hart (1972). [Preview Abstract] |
Tuesday, November 24, 2009 1:37PM - 1:50PM |
PR.00010: New results in bioconvective linear stability of gravitactic microorganisms Ildebrando Perez-Reyes, Luis Antonio Davalos-Orozco New results on the linear bioconvective instability of a suspension of gravitactic microorganisms have been calculated. Use is made of the model equations presented by Ghorai (2000,2007) some years ago but that have not been used to determine the linear instability in an infinite horizontal fluid layer. The hydrodynamic stability is characterized by dimensionless parameters such as the bioconvection Rayleigh number Ra, the gyrotaxis number G, the motility of microorganisms d and the wavenumber k of the perturbation. Analytical and numerical solutions are calculated. Two numerical methods are used for the sake of comparison. They are a shooting method and a Galerkin method. Marginal curves of Ra against k for fixed values of d and G are presented along with curves corresponding to the variation of the critical values of Rac and kc. The important result of this work is that those critical values are compared with the experimental data presented in Table I of Bees and Hill (1997) and Table II of Bees and Hill (1998), where the gyrotactic algae Chlamydomonas nivalis is used, and it is found that the agreement is very good. A discussion of the results will be given. [Preview Abstract] |
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