Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session E33: Convection and Buoyancy Driven Flows: EnvironmentalConvection
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Chair: Geno Pawlak, University of California San Diego Room: 106 |
Sunday, November 19, 2017 4:55PM - 5:08PM |
E33.00001: Residual streaming flows in buoyancy driven cross-shore exchange Geno Pawlak, Ricardo Felez, Kristen Davis, Antonio Sanchez Cross-shore exchange processes are critical for coastal ecosystems such as coral reefs with implications for transport of nutrients, larvae and heat. We present an analytical study of two-dimensional flow in a wedge driven by a time-dependent surface heat flux as a model problem to understand buoyancy-induced cross-shore flow. Besides the turbulent Prandtl number and the relevant Rayleigh number, both assumed to be of order unity, the solution is seen to depend on the geometry through a small parameter $\beta$ measuring the bottom slope. Following previous efforts (e.g., Farrow and Patterson, JFM 1993) an analytic solution is sought in the asymptotic limit $\beta \ll 1$ for a water layer bounded by an adiabatic bottom surface subject to a harmonic heat flux on the upper surface. The analysis reveals that the motion at leading order can be expressed as the sum of a harmonic component and a steady component, the latter driven by the nonlinear advection terms. This steady-streaming motion includes a near-shore vortex with associated counterclockwise recirculating motion that could have a significant effect on the near-short transport dynamics. The analytical solution is compared with numerical integrations of the complete conservation equations for small values of $\beta$. [Preview Abstract] |
Sunday, November 19, 2017 5:08PM - 5:21PM |
E33.00002: Plume Splitting in a Two-layer Stratified Ambient Fluid Yongxing Ma, Morris Flynn, Bruce Sutherland A line-source plume descending into a two-layer stratified ambient fluid in a finite sized tank is studied experimentally. Although the total volume of ambient fluid is fixed, lower- and upper-layer fluids are respectively removed and added at a constant rate mimicking marine outfall through diffusers and natural and hybrid ventilated buildings. The influence of the plume on the ambient depends on the value of $\lambda$, defined as the ratio of the plume buoyancy to the buoyancy loss of the plume as it crosses the ambient interface. Similar to classical filling-box experiments, the plume can always reach the bottom of the tank if $\lambda>1$. By contrast, if $\lambda<1$, an intermediate layer eventually forms as a result of plume splitting. Eventually all of the plume fluid spreads within the intermediate layer. The starting time, $t_v$, and the ending time, $t_t$, of the transition process measured from experiments correlate with the value of $\lambda$. A three-layer ambient fluid is observed after transition, and the mean value of the measured densities of the intermediate layer fluid is well predicted using plume theory. [Preview Abstract] |
Sunday, November 19, 2017 5:21PM - 5:34PM |
E33.00003: The role of double diffusion on the entrainment in turbulent plumes M. Dadonau, J. L. Partridge, P. F. Linden Turbulent plumes are common features in environmental fluids, which occur whenever buoyant fluid is persistently released from a localised source. Such flows play a fundamental role in a large number of geophysical and industrial applications. The evolution and ultimate fate of the plume fluid is largely determined by the process of turbulent entrainment. It is not uncommon for the buoyancy to arise from two components with different molecular diffusivities, forming a double-diffusive configuration, e.g. a plume of cold and fresh meltwater rising along the sidewall of a melting iceberg in a relatively warm and salty ambient ocean. We study the effect of molecular diffusion on turbulent entrainment through experimental investigation of doubly-diffusive plumes. The experiments used the ‘filling-box’ technique developed by Baines (1983), with water as the working fluid. To ensure mass and energy conservation, instead of the conventional temperature-salinity combination, double-diffusive configurations were obtained by releasing sucrose solution into a salty ambient. Our findings suggest that the entrainment rate is decreased when the source of buoyancy is double-diffusive. Thus, it seems that turbulent entrainment can depend on mixing processes and diffusion at the molecular level. [Preview Abstract] |
Sunday, November 19, 2017 5:34PM - 5:47PM |
E33.00004: Initial Descent of a Thermal and the Application to Downdraughts Emily Kruger, Henry Burridge, Gabriel Rooney, Paul Linden Downward moving cold air within thunderstorms, known as downdraughts, can be used to determine the severity of a storm. Therefore an understanding of them is useful for weather forecasting. Typically in weather forecasting these downdrafts are modelled using the theory of a plume from from Morton, Taylor and Turner (1956), which inherently assumes that the plume is long and thin. Downdrafts are generally wider than they are high and hence deviate from the Morton, Taylor and Turner theory. We perform experiments using finite releases of dense fluid from a cylinder of varying lengths. By tracking the edges of the release we can find the velocity and radius of the descent and gravity current after impact. In doing so we find that the descent has two regimes, an initial phase and then a self similar thermal, and the axisymmetric gravity current varies depending on which regime the descent impacts the ground in. A theoretical model is proposed for this initial phase and compared to the experimental data. We hope that these results and future work will allow us to better inform forecasting of weather arising from such downdraughts. [Preview Abstract] |
Sunday, November 19, 2017 5:47PM - 6:00PM |
E33.00005: ABSTRACT WITHDRAWN |
Sunday, November 19, 2017 6:00PM - 6:13PM |
E33.00006: Influences of source condition and dissolution on bubble plume in a stratified environment Shigan Chu, Andrea Prosperetti A cross-sectionally averaged model is used to study a bubble plume rising in a stratified quiescent liquid. Scaling analyses for the peel height, at which the plume momentum vanishes, and the neutral height, at which its average density equals the ambient density, are presented. Contrary to a widespread practice in the literature, it is argued that the neutral height cannot be identified with the experimentally reported intrusion height. Recognizing this difference provides an explanation of the reason why the intrusion height is found so frequently to lie so much above predictions, and brings the theoretical results in line with observations. The mathematical model depends on three dimensionless parameters, some of which are related to the inlet conditions at the plume source. Their influence on the peel and neutral heights is illustrated by means of numerical results. Aside from the source parameters, we incorporate dissolution of bubbles and the corresponding density change of plume into the model. Contrary to what's documented in literature, density change of plume due to dissolution plays an important role in keeping the total buoyancy of plume, thus alleviating the rapid decrease of peel height because of dissolution. [Preview Abstract] |
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