Session F15: Convection and Buoyancy-Driven Flows: Environmental (3:55pm - 4:40pm CST)

How the growth of lake ice depends on the fluid dynamics underneath

Convective flows coupled with solidification or melting in water bodies play a major role in shaping geophysical landscapes. Particularly in relation to the global climate warming scenario, it is essential to be able to accurately quantify how water-body environments dynamically interplay with ice formation or melting process. By combining experiments, numerical simulations and theoretical model, we investigate solidification of fresh water, properly considering phase transition, water density anomaly, and real physical properties of ice and water phases, which we show to be essential for correctly predicting the different qualitative and quantitative behaviors. We identify, with increasing thermal driving, four distinct flow-dynamics regimes, where different levels of coupling among ice front, stably and unstably stratified water layers occur. Despite the complex interaction between the ice front and fluid motions, remarkably, the average ice thickness and growth rate can be well captured with the theoretical model. It is revealed that the thermal driving has major effects on the temporal evolution of the global icing process, which can vary from a few days to a few hours in the current parameter regime.   

Mixing of a Finite Dense Fluid Release Around a Downstream Cube.

Results are presented from experiments examining the flow of an instantaneous release of dense fluid upstream of a cubic obstacle. This is the same geometry as the Thorney Island Phase II Trials 26-29. Experiments were run in a water channel with salt water used to create the density difference. The flow of dense fluid into the building wake was visualized using Light Induced Fluorescence (LIF). The volume and buoyancy of the release were varied as well as the ambient velocity and distance from the building at which the dense fluid was released. A range of flow phenomena were observed and will be discussed. We observe that the presence of an obstacle has a significant impact on the dense gas dispersion. In some cases, the fluid initially flowed around and over the obstacle and the building wake remained initially free of the dense fluid. However, later the dense fluid was drawn into the wake from the downstream end, struck the downstream face of the obstacle and rose up it. Results are presented including the time taken for the dense fluid to reach the downstream building face and the height to which it rises up the face as a function of the release Richardson number, non-dimensional release volume, and non-dimensional release distance.   

Plume Outflow and Deflection Time for a Line Plume in a Filling Box

We examined the behavior of a line plume that spans the full width of a rectangular filling box. For the case of a centrally located plume the flow s symmetric, and the plume falls vertically throughout the filling process. However, if the plume is off-center then the symmetry is broken and after a finite time, the plume is significantly deflected toward the nearest end wall. The plume deflects when the plume outflow returns to the plume having reflected off the end wall. Experimental results for the outflow thickness, outflow velocity and plume deflection time were compared to a theoretical model and showed good agreement. The experimental and model results show that the line plume outflow is substantially thicker than that of a round plume. Further, the line plume outflow has a constant velocity which again differs from a round plume that has an outflow that slows with distance from the plume. Finally, the deflection time is shown to scale on the filling box time for the portion of the box between the plume and near wall.   

Modeling of plume merger with applications to cooling towers

We report on the irrotational flow analysis of plume merger under different plume configurations, including two adjacent area-source plumes and two long rows of plumes. A key assumption underlying this analysis is that the boundaries of $n$ ($n\geq2$) merging plumes can be approximated by the velocity potential contours for $n$ line sinks. For two adjacent area-source plumes, we propose a modification to the equation describing the velocity potential contours for two point-source line sinks; the modified contours can therefore originate from the actual area source. On this basis, we propose a novel entrainment assumption that relates the entrainment coefficient to the plume boundary curvature. The theory in question agrees well with previous experimental and theoretical results of the plume volume flux and the full merger height. For two long rows of plumes, we reveal an intermediate line plume behavior between the near- and far-field similarity limits when the spacing between two rows is moderate or large. An application of this model to back-to-back cooling towers indicates that the entrainment of plumes at the center is greatly curtailed, which significantly increases the visible plume length.   

Comparing CFD and Regulatory Modeling of Pollutant Dispersion Under Different Thermal Stabilities

Modeling pollutant dispersion in the atmospheric boundary layer is a complex task. Computationally efficient Gaussian plume-based dispersion models, such as AERMOD, are empirically formulated and validated for regulatory modeling in a wide range of meteorological conditions. On the other hand, CFD modeling -- although more detailed and can potentially provide more accurate and predictive analysis -- is not very common for atmospheric pollutant dispersion due to its computational cost. Large variations of meteorological conditions, high Reynolds numbers, large length scales and heat fluxes, and thermal plumes can make a rigorous CFD modeling of atmospheric dispersion computationally intractable. Also, limited detailed and high-resolution field measurements make high-fidelity validation of CFD modeling difficult. In this work, we present a comparison of CFD modeling with AERMOD calculations for pollutant dispersion in an idealized urban setting under various atmospheric stabilities as an indirect means for preliminary validation. In addition to pollutant concentration, derived atmospheric parameters, e.g., Monin-Obukhov length, are also compared by post-processing CFD results.   

Unbalanced Exchange Flow and Its Implications for the Night Cooling of Buildings

The passive ventilation of buildings at night, when the outside air is cooler, is integral to many natural ventilation schemes, purging the building of heat accumulated during the day. These schemes often use displacement flow, where warmer air exhausts through a high-level opening and cooler air enters through a low-level opening. In order to design a natural ventilation system, it is necessary to be able to predict flow rates and the time to complete a purge. Current models assume that displacement flow is maintained throughout the purge, however we show that this is not possible. Instead, we show that the flow must transition to an ‘unbalanced exchange flow’ at a critical flow rate, below which there will be both warm outflow and cool inflow simultaneously through the high-level opening, but still cool inflow through the low-level opening. The redistribution of buoyancy this causes in the room changes the predicted flow rates and time to complete a purge. We develop and present a theoretical model that captures this behaviour and predicts the unbalanced exchange flow rates and resulting purge times.