Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A33: Convection and Buoyancy-driven Flows: General I |
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Chair: Joerg Schumacher, Technical University of Ilmenau Room: Georgia World Congress Center B405 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A33.00001: Regime transitions in turbulent horizontal convection at low Prandtl numbers. Pierre-Yves Passaggia, Alberto Scotti, Brian White Transitions between flow regimes in natural horizontal convection are analyzed using Direct Numerical Simulations starting from laminar and up to turbulent dynamics. This particular flow is driven by a gradient of buoyancy along a horizontal boundary and it has attracted increasing attention over the last ten years. Horizontal convection is thought to play an important role in driving large-scale overturning circulations as found in the oceans and the atmosphere. Recently, a new picture of the different types of theoretical regimes has been proposed by Shishkina et al. (2016) but this theory has yet to be validated. The scaling laws describing the transport of momentum and heat inside the domain are verified against numerical experiments at low Prandtl numbers. Our numerical experiments show clear evidence of the regimes predicted by Shishkina et al. (2016) and include other regimes previously observed by Gayen et al. (2014). In particular, we report the first observations of two new limiting turbulent regimes in natural horizontal convection at low Prandtl numbers. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A33.00002: Experiments on turbulent horizontal convection at high Schmidt numbers. Nadia F Cohen, Pierre-Yves Passaggia, Alberto Scotti, Brian White We explore regimes of turbulent horizontal convection at high Schmidt numbers in a laboratory experiment. This study unveils flow regimes, previously observed at intermediate Prandtl numbers, which are diagnosed using the relationship between fresh and salt-water fluxes at the surface and the resulting overturning flow. We set up the experiment using a transparent acrylic tank. Along the top of the tank are two wells made up of two semipermeable membranes. One has fresh water continuously pumped in and out, while the other has salt water. The diffusion of the salt ions across the membranes act as a horizontally driving force. To measure the overturning circulation, we perform particle image velocimetry together with conductivity measurements to determine the properties of the horizontally-driven convective flow. Results are analyzed using the scaling laws theoretically predicted by Shishkina et al. (2016) and Griffith et al. (2005). These results allow for drawing new conclusions on the regime diagram of horizontal convection and complement our understanding on the role of large-scale horizontal buoyancy gradients in driving large-scale overturning circulations. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A33.00003: Horizontal convection driven by temperature and salinity gradients over the top surface Junyi Li, Yantao Yang Horizontal convection is studied numerically as a model problem to investigate the convection flow driven by surface density distribution, which is relevant to the ocean environment. Unlike most existing works of thermally driven horizontal convection, we consider the gradients of both temperature and salinity over the top surface. Much more complex flow morphology is obtained in our simulations. Within certain region below the top surface, both mean temperature and salinity show strong dependences on the depth. At one end of the domain they both increase as the depth becomes larger, and multiple layers of horizontal mean flows emerge. At the opposite end the mean temperature and salinity drop simultaneously versus the depth, which favors the fingering double diffusive convection. And indeed we observe salt-finger structures. The vertical fluxes at the surface and the horizontal fluxes within the domain are also discussed for both temperature and salinity fields. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A33.00004: Exchange flows in rotating pipes Seyed Mohammad Taghavi, Shan Lyu In this work, the effects of a pipe axial rotating motion on buoyant miscible flows in inclined pipes are studied via experimental methods. The experiments are carried out in a long transparent pipe with closed ends and an automated gate valve making a lock-exchange flow configuration. A system of a stepper motor is employed in order to generate a stable and precise axial rotation speed. The experiments are recorded using a high-speed digital camera to visualize flow patterns in the pipe. The analysis of our experimental results reveals how the interpenetration of the light and heavy fluids are affected by changing the experimental parameters, such as the pipe inclination angle, density difference, viscosity and pipe rotation speed. In particular, the results show in detail how the interpenetration front velocities vary with the pipe rotation speed. The results are quantitatively analyzed using the flow dimensionless groups, such as the Rossby number, and the characteristic velocities that describe the flow dynamics. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A33.00005: The stability of stratified, gravity driven exchange flow Chris Pringle Exchange flows are found in a wide variety of scenarios, both industrial and natural, with a wide range of parameter regimes – for instance from low viscosity oceanographic flows to high viscosity oils to the extreme viscosity of lava. The possible arrangements of such flows and their respective stabilities are the subject of much ongoing interest. Here we analyse the stability of stratified, gravity driven exchange flow. We consider developed flow profiles, rather than an initialisation problem, in order to establish what long-time arrangements may be possible.
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Sunday, November 18, 2018 9:05AM - 9:18AM |
A33.00006: Rotating salt-fingering convection in the limit of small diffusivity ratio and large density ratio Jin-Han Xie, Keith A. Julien, Edgar Knobloch This work theoretically and numerically studies the rotating salt-fingering convection using an asymptotically derived reduced model valid in the limit of small diffusivity and large density ratios, where the strength of convection and rotation are captured by Ra, the ratio of Rayleigh numbers of the destabilizing and stabilizing buoyancy forces, and the Taylor number Ta, respectively. Their relative importance is captured by a single parameter G=(Ra-1)1/2/Ta with critical value Gc=1 separating strongly (G<Gc) and weakly (G>Gc) rotating regimes, which are best distinguished when the rotation vector is at an angle to the local vertical gravity with broken left-right symmetry. In the strongly rotating regime the saturated states are characterized by layered structures and the magnitudes of domain-averaged salinity flux, kinetic and salinity potential energies increase linearly as Ta increases, which differs from that in the weak rotation regime and is explained based on the rotation-confined region of linearly unstable modes and an assumption of self-similar energy spectra. Two-peak shapes are observed for both kinetic and potential energy spectra, and the power-law spectra between the two peaks are explained with exponents calculated. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A33.00007: Control of Rayleigh-Taylor instability properties by differential diffusion effects Anne De Wit, Shyam Gopalakrishnan, Jorge Carballido-Landeira, Bernard Knaepen Fingering instabilities of a miscible interface between two fluids in a gravitational field can develop due to adverse density gradients as in the well-known Rayleigh-Taylor (RT) and double-diffusive (DD) instabilities. In the absence of differential diffusion, the mixing rate and the onset time of the RT instability developing when a denser solution of a given solute A overlies a less dense solution of a solute B are respectively proportional and inversely proportional to the initial density difference ∠ρ0 between the two superposed layers. We show both experimentally and theoretically that when the mechanisms of RT and DD instabilities are combined, the properties of the convective growth of the fingers are controlled by the dynamic density jump ∠ρm of the non-monotonic density profile induced by the differential diffusion effects. In particular, the onset time and mixing rate can be controlled by varying the ratio of the diffusion coefficients of the solutes. Ref: Gopalakrishnan et al. Phys. Rev. E, 98, 011101(R) (2018). |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A33.00008: High Schmidt number "washout" of a viscosifying solute downstream of a backward-facing step Dahhea Min, Paul F. Fischer, Arne J. Pearlstein We consider high Schmidt number, two-dimensional, unsteady laminar convective mass transfer downstream of a backward-facing step, behind which a nonreactive viscosifying solute is initially confined to a layer whose thickness is equal to the height of the step. The solute is "washed away" by a flow of "clean" fluid over the step. The relationship between solute concentration, viscosity, and molecular diffusivity is approximated using the Stokes-Einstein relation. We focus on the distribution and removal of the solute for several combinations of Reynolds number (based on step height and infinite-dilution viscosity; primarily 10 and 100) and Schmidt number (the ratio of kinematic viscosity to the infinite-dilution molecular diffusivity; primarily 500 and 2650). Comparison is made to the case in which the viscosity and diffusivity are independent of solute concentration. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A33.00009: Transport and mixing of miscible fluids inside a capillary tube Ziyang Huang, Ivan C. Christov, Arezoo M Ardekani The dynamics of mixing of two miscible fluids has been widely modeled by Fick’s Law, while interfacial and surface tension effects were neglected. However, these effects are not negligible for the dynamics of mixing of miscible fluids when the interface persists for a long period before the two fluids are fully mixed. In addition, Fick’s Law is not suitable for the case of a large concentration gradient, which has been confirmed to fail to reproduce the experimental results of the dynamics of mixing of water and glycerol in a capillary tube. We propose a Phase-Field model, where the diffusion flux is modeled by the gradient of a chemical potential that consists of a single-well potential, to model the mixing, and we incorporate the gradient of the Phase-Field parameter to capture interfacial effects. When coupling this Phase-Field model to the Navier-Stokes equations, we reproduce similar behavior as in experiments, where the interface of two miscible fluids is more inclined when the capillary tube is wider, or the diffusivity is smaller. The long-term behavior of the displacement of the interface scales as t2/3 at intermediate times and then as t1/3 at later times, which is consistent with the experiments. |
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