Session A10: Convection and Buoyancy I

An Anomaly in Natural Convection for Binary Mixtures subject to Flux Boundary Conditions

The problem of natural convection in a binary mixture subject to realistic boundary conditions of imposed zero mass flux on the solid walls shows solutions that might lead to unrealistic negative values of the mass fraction / solute ocncentration. This effect was investigated and presented in this paper and a possible way of addressing it is suggested via a mass-fraction-dependent thermodiffusion coefficient that can have negative values in regions of low mass fractions. The latter suggests the posibilty of particles moving locally up the mass fraction / solute concentration gradient  rather than the usual Fick's law direction.   

Anomalous natural convective sculpting of melting ice

We study the shape dynamics of ice melting in cold, initially quiescent fresh water, subject to the natural convective flows generated during melting. Experiments reveal three shape motifs associated with increasing far-field water temperature: sharp pinnacles pointed downward, scalloped waves, and sharp pinnacles pointed upward. Phase-field simulations reproduce these morphologies, which are closely tied to the anomalous density-temperature profile of liquid water. Analysis shows pinnacles sharpen with accelerating growth of tip curvature, while scallops emerge from a Kelvin-Helmholtz-like instability caused by counter currents that roll up to form arrays of wall-bound vortices.   

Turbulent Entrainment in Buoyancy Driven Flows in a Stratified Environment using WRF-LES.

Entrainment is an important phenomenon in a wide variety of geophysical fluid flows (density currents, pyroclastic flows, wildland fire plumes).  Ordinarily it is difficult to quantify and measure this.  We use WRF-LES to simulate two-way interactions between the plume and the atmospheric boundary layer (ABL), which has been used to simulate denser and lighter gas plumes into the convectively stratified atmospheric boundary layer by varying initial heat-flux conditions for different gases. In this presentation we examine the unsteady nature of entrainment and its relation to initial source conditions such as reduced gravity terms (g'_T and g'_ρ ) and their relationship to entrainment and further plume development for characteristics such as the plume-(height/width). We further show good agreement with powerlaw relation in the literature. Finally, we present dimensionless parameters to represent this unsteady component of entrainment with the Reynolds and Froude's numbers.   

Impact of canopy-induced instability on buoyant plumes

Characterizing the interaction between buoyancy-driven plumes and ambient cross winds is crucial to understanding and modeling wildfire spread in forest canopies. However, current classifications of buoyancy or wind-dominated wildfire systems are based only on scalings using ambient crossflow velocities rather than canopy flow properties. Analogous to mixing-layers, flow through forest canopies is characterized by the presence of Kelvin-Helmholtz instabilities at the canopy top that affect momentum, turbulence, and scalar fluxes into and out of the canopies. Therefore, in this study we explore the impact that these canopy-induced instabilities have on buoyant plumes, over a wide parametric range, in order to develop a scaling that better encapsulates the behavior of convective plumes in canopy flow conditions. The study is conducted in a recirculating flume with submerged model vegetation and thermally buoyant plumes. In varying how wind or buoyancy-dominated the system is, we vary the convective Froude number (Fr_c) of the plumes. Additionally we control the momentum thickness of the canopy mixing layer to alter the strength of the canopy signal. Our results indicate that while the Fr_c may set the mean behavior of the plume, the canopy signal has a strong impact on the unsteady beavhior of plume and its trajectory. Plume visualization using Synthetic-Schlieren methods and thermocouple readings are used to analyze the behavior and trajectory of the plume under different conditions.   

The dynamics of a line plume confined by a single inclined boundary

A buoyant turbulent line plume developing from a source on a horizontal plane boundary in an unconfined environment will rise vertically, spreading linearly with height as it entrains fluid from the surroundings. If the plume instead develops from a source on an inclined plane, it might be anticipated that the flow behaviour will remain largely unaffected unless the inclination of the plane is sufficiently steep that the plume perimeter impinges on the boundary. We will show, however, that the presence of such a boundary dramatically alters the flow dynamics at much shallower inclinations. Indeed, in this experimental study two novel flow configurations with non-vertical plume trajectories have been identified, which thereby enable the parameter space of boundary inclination to be divided into three distinct regimes of plume behaviour. Furthermore, we will put forward insights into how the presence of the boundary modifies the entrainment flow into the plume. Such insights have allowed us to begin clarifying the physical mechanisms underlying the observed flow behaviours. The findings of this work have numerous potential applications across the engineered, and natural, world - for example in designing a natural ventilation scheme for a steeply-raked lecture theatre.   

Double-Diffusive Brine Rejection

It is common knowledge that hot air rises, so we might expect that hot water will rise above cold water. However, the exact opposite happens in cold lakes, where cold water rises! Through surface cooling and mixing, most seasonally ice-covered lakes transition between the intuitive hot-over-cold temperature stratification in the summer, and the reverse cold-over-warm stratification in the winter. Simultaneously, as ice forms, salts in the water are rejected to the ice-water interface. This surface salt-flux destabilizes the stable reverse temperature stratification. Thus, there is a competition between the unstable salt stratification and the stable temperature stratification.

We will show that this configuration will lead to double-diffusive finger-like instabilities under the ice. We will discuss the key parameters that control the evolution of this double-diffusive instability, taking into account the nonlinear equation of state of water. We will analyze both experimental and numerical observations of this phenomenon.    

Not Participating

Early experiments of rotating magnetoconvection in a liquid mercury layer and linear stability analysis suggested that at a critical field strength, the horizontal length scale of the convection cells is significantly enlarged (Nakagawa, Proc. Roy. Soc. A, 1959; Chandrasekhar, 1961). We have numerically reproduced those experiments as close as possible, using a cylinder filled with a fluid with Prandtl number Pr = 0.025, a rotation rate corresponding to an Ekman number of Ek = 1.2 × 10^-4, and a wide diameter-to-height aspect ratio of Γ = 8. We varied both the magnetic field strength and the thermal forcing in the Chandrasekhar and Rayleigh number ranges 9.5 × 10¹ ≤ Ch ≤ 5.5 × 10⁴ and 10⁵ ≤ Ra ≤ 10⁷, respectively. Employing polar spectral analysis, we find that the pronounced multimodality of liquid metal flows leads to a rich variety of flow regimes with strongly varying distinct length scales. However, contrary to the early laboratory experiments, none of these regimes is characterised by a single large-scale mode. Moreover, we detect a strong boundary zonal flow which does not originate from a nonlinear wallmode.   

Investigation of a buoyancy-driven instability during horizontal miscible displacement

Hydrodynamic instabilities in miscible reactive systems have significant impact on applications such as: CO₂ sequestration, soil reparation, or particle formation in precipitation systems. A buoyancy-induced instability has been observed when a fluid horizontally displaces another miscible fluid of different density, resulting in a complex flow field. With this study, we intended to characterize the flow patterns in such systems and to investigate their dependence on parameters such as the injection flow rate and the fluids density difference. We performed reactive and non-reactive experiments in a microscale geometry and explored the intricate 3D flow conditions of the system, using micro-Particle Image Velocimetry. The results evidence the existence of flow structures present in equivalent numerical studies and reveal the dependency of the instability on the direction of the displacement.   

Experimental investigation of double-diffusive convection in a quasi-two-dimensional chamber during the unidirectional solidification of binary solutions

Importance of double diffusive convection during unidirectional solidification of binary solutions has been realized in the literature. The present experimental work, conducted using a combination of non-intrusive measurement techniques, pertains to the study of the onset and further development of double diffusive layers (DDLs) during unidirectional solidification of Succinonitrile-ethanol binary solution. The associated flow (PIV) and transported parameters (temperature and species concentration through dual-wavelength interferometry) have been mapped in real-time. In addition, rainbow-schlieren deflectometry has been used for recording the real-time convection transients, plumes and DDLs, which, in turn, lead to the creation of stepped structure of composition and temperature in the fluid domain. The experimental observations from different techniques revealed the continuous evolution of convective rolls inside DDLs along with several key characteristics e.g., size, shape, inclination, sense of rotation, downward movement etc. A combination of solutal and thermal Rayleigh numbers have been quantified to characterize the double-diffusive convection. The experimental results are additionally supported with analytical scales of length, time, velocities etc.