Session Z28: Convection and Buoyancy-Driven Flows: Environmental

Experimental study on the flow behavior of a finite dense fluid release at upstream from cubic building face for different Richardson numbers

The dispersion of dense polluted gas has become of great interest as an ever-increasing number of individuals live in big cities. Hence, it is important to understand dense gas dispersion in urban areas better. To this end, this study illustrates the characteristics behavior of the dense gas dispersion at the wake of a cubic building for Richardson number (Ri) ranging from 1 to 33. A series of small-scale experiments were carried out in a water channel following the same geometry of the Thorney Island Phase II Trials 26-29. Salt water was used as the dense gas and it was released instantaneously at different locations upstream from the building face. Light-Induced Fluorescence (LIF) technique was used to visualize the flow in the wake and Acoustic Doppler velocimetry (ADV) was used to measure the velocity profile. This study measured the time taken for the heavy gas to reach the building wake, the cloud height formed by the release on the leeward face of the building, and the time taken to flush the gas out of the building wake for the different Ri and release distances. It is observed that a building block significantly impacts the behavior of the dense gas movement in its wake. Initially, the dense fluid did not enter the building wake and flowed over and around the block. The dense fluid, however, later entered the wake; it was drawn upstream to the leeward face of the block and pushed up towards the top of the building.

    

3D Simulations of the Interior of an Ice-Covered Lake subjected to Spatially Heterogeneous Solar Radiation Intensity

In this talk, I present the results of 3D high resolution, non-hydrostatic simulations subjected to spatially heterogeneous solar radiation intensity in the cold-water regime (< 4 °C). Spatial variations in optical properties of ice or snow cover leads to these conditions. In these simulations, solar radiation is shadowed by 90% in a circular region at the corner of the computational domain. We find that lateral intrusions develop in the horizontal away from the shadowed region along the surface. These intrusions interact with Rayleigh-Taylor instabilities that develop away from the shadowed region, modifying their properties and enhancing local turbulence production.

    

How Fuel Moisture Influences Near-Field Plume Development in Prescribed Fires: A Large-Eddy Simulation Study

Prescribed fire is a universally applicable technique of fuel treatment used for forest restoration and wildfire hazard mitigation. The smoke plumes from prescribed fires, however, may be troublesome for the neighboring communities and may substantially impact the air quality and increase the risk of road accidents. The near-field plume development and transport heavily depend on the fuel moisture content. By understanding the spatiotemporal variation of fuel moisture and its effect on turbulent plume dynamics, it may be possible to better guide the prescribed fire plan. This study investigates the effect of fuel moisture on the near-field plume dynamics. Through a series of computations involving a detailed dynamic fuel moisture model coupled with large-eddy simulation, this study compares the plume dynamics and fire propagation for a heading fire over a flat grassland for cases with dynamic and constant fuel moisture content.   

Plume merger from area sources

Rising temperatures pose a threat to industrial projects requiring cooling. At large-scale, cooling is often provided by wet cooling towers. Water vapor is then discharged to the atmosphere where, depending on ambient conditions, it may condense to form a visible plume. Because such plumes are generally undesirable, major investments have been made into abatement technologies. However, any successful deployment of these technologies requires an understanding of plume merger and its impact on entrainment. Previous theoretical work on plume merger has used velocity potential contours to mark the boundary between the plume interior vs. exterior. Analyses are performed in the point source limit where the plume source is small. We propose a modification that allows plumes to originate from sources of arbitrary size. We study merger in three contexts: (i) two plumes placed side-by-side, (ii) two parallel rows of plumes in a quiescent environment, (iii) a single row of plumes in a crosswind. In the former case, comparisons are drawn with a classical and purely-geometric description of plume merger. In the latter case, comparisons are drawn against existing tow tank experimental data. Results of these comparisons and implications for industrial-scale plume abatement are discussed.   

Influence of movement kinematics and buoyancy on the fluid dynamics of transmission in a waiting line.

Waiting in a line is a critical social interaction that occurs frequently in many public spaces. It represents a situation that could potentially increase the risk of transmission of respiratory viruses. Here we use a combination of laboratory experiments and direct numerical simulations (DNS) to assess the flow patterns created by periodic movements mimicking the kinematics of a waiting line. We matched the relevant non-dimensional numbers, including the Reynolds number of the line movement, and the Grashof number representing the buoyancy-driven fluid motion. UV-induced fluorescence, combined with particle imaging velocimetry, reveals the presence of competing counter-currents due to line kinematics and thermal gradients. We systematically explore the competing roles of the two using the DNS, which can either increase or decrease the risk of transmission depending on the pattern of movements and the temperature difference with the ambient.   

Author not Attending

Turbulent thermally driven buoyancy plumes have important applications in the atmosphere such as in wildfire plumes, volcanic plumes, and chemical plumes from anthropogenic sources. The purpose of the study is to propose a new model for entrainment of turbulent buoyant plumes within the convective atmospheric boundary layer. Previous entrainment models do not consider the background effects of a convective environment. Here we present a model that incorporates the factors of sensible heat flux and the atmospheric stability parameters into the entrainment model. An existing in-house WRF-LES- bPlume model with a two-way coupling between the ABL and the plume which was implemented within the Weather Research and Forecasting (WRF) Model was used. WRFLES-bPlume was used in two steps: I. Ambient Quasi-steady convective ABL was simulated, II. The plume was released within the ambient convective ABL and data collected for 2 hours. The cases of 1. Thermal plumes, 2. Heated buoyant plumes released from a large circular source (diameter -400m). in various unstably-stratified environments at geophysical scales have been investigated. The results are presented to quantity entrainment in terms of plume Reynolds number, Froude's number, non-dimensional surface heat flux and scaled Atmospheric stability parameters ( Richardson number and Monin-obhukov length scale).   

Interplay between advective, diffusive, and active barriers in Rayleigh-Bénard flow

Recent developments in the theory of objective coherent structures have provided a wealth of approaches to identifying transport barriers in three-dimensional (3D) turbulent flows. Specifically, theoretical advances have been incorporated into numerical algorithms that extract the most influential advective, diffusive and active barriers to transport from data sets in a frame-indifferent fashion. To date, however, there has been very limited investigation into these objectively defined transport barriers in 3D unsteady flows with complicated spatiotemporal dynamics. Similarly, no systematic comparison of advective, diffusive and active barriers has been carried out in a 3D flow with both thermal and convective features. In our study, we utilize data from turbulent Rayleigh-Bénard convection simulations to uncover the interplay between advective transport barriers (Lagrangian coherent structures), material barriers to diffusive heat transport and objective Eulerian barriers to momentum transport. For a range of Rossby numbers, we visualize each type of barrier and assess their evolving physical role under changes in the relative influence of mechanical and thermal forces.   

Investigation on Ice Melting by Simultaneous Thermometry and Velocimetry Method in Oil

Crude oil is accidentally released into open water each year. Since the released oil is harmful to the environment, the trajectory needs to be predicted in order to remove the oil minimizing the polluted area by oil. The prediction of the spilled oil trajectory becomes complicated when the ice is present like in Arctic Ocean. When the oil is adjacent to ice, the oil causes the ice melting. In order to simulate this ice melting, the heat transfer between oil and ice needs to be understood. Spatial- and time-resolved temperature and velocity change in oil are required. Several optical measurement techniques can provide spatial- and time-resolved information in fluids, however, none of those techniques are available to measure the temperature and velocity distribution in oil simultaneously. In this research, the simultaneous thermometry and velocimetry in oil is accomplished by combining Dual Luminescence Imaging (DLI) for thermometry and Particle Image Velocimetry (PIV). Pyranine-induced particles are used for PIV. By selecting proper luminophores for DLI, temperature and velocity distributions in oil can be captured by a color camera in the developed method. With this technique, the ice melting is studied and will be discussed in the presentation.   

Pair dispersion in Rayleigh-Bénard convection: effets of roughness at the boundaries

The Rayleigh-Bénard cell is a useful model system to understand the physics of turbulent thermal convection, because the

boundary conditions are well defined. However, boundaries in nature or in the industry are seldom hydrodynamically smooth. It is therefore

tempting to add roughness at the boundaries of the Rayleigh-Bénard cell, to investigate how the heat transfer and the flow

features are modified.

Several rough Rayleigh-Bénard cells have been operated in the last decade, in particular in the group of K.-Q. Xia,

and in our group in Lyon. These experiments evidence a roughness triggered regime with enhanced heat transfer

efficiency when the thickness of the thermal boundary layer matches the roughness size. The enhancement appears to

saturate at larger Rayleigh numbers, when the boundary layers are much smaller.

If the effect of roughness is clear on non dimensionnal number such as the Nusselt, the effect on flow structures and

statistics of turbulence is less known. We performed lagrangian tracking in a thin square Rayleigh Bénard cell, with rough

hot plate and smooth cold place, at different Ra number from Ra = 2.1 × 10¹⁰ to Ra = 6.3 × 10¹⁰. From the tracks,

we were able to compute the pair dispersion in different region of the flow to compare the case with or without

rough boundaries.