Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session X17: Convection and Buoyancy-Driven Flows: General III |
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Chair: Andrew Wynn, Imperial College London Room: 145B |
Tuesday, November 21, 2023 8:00AM - 8:13AM |
X17.00001: Floating bodies propelled by convective dissolution. Michael Berhanu, Martin Chaigne, Arshad Kudrolli Self-propulsion can be achieved by modifying anisotropically the properties of the surrounding fluid. It has been shown that an asymmetrical submerged object, one side of which consists of a heating plate, can be propelled by the convection flow it generates [1]. In this work, we demonstrate an original propulsion mechanism in which convection is no longer of thermal origin but solutal, via the dissolution of an immersed soluble object [2]. To do this, we use boats just a few centimetres long, consisting of a plastic buoy and a sloping candy plate. Once placed in the water, these boats start to move forward, at speeds up to 5 mm/s and in a straight line, under the effect of the gravitational flow fed by their dissolution. We show that propulsion is mainly due to the flow beneath the boat, where the concentration boundary layer destabilizes to form plumes. We characterize the flow generated to determine the origin of the thrust. We propose also a model predicting the body speed depending on geometry and material properties and show that it captures the observed trends reasonably. The dynamics of dissolving solids demonstrated here applies equally well to solids undergoing phase change and may thus contribute to the drift of melting icebergs. |
Tuesday, November 21, 2023 8:13AM - 8:26AM |
X17.00002: Fast MILES solver: validation of a new scalable tool to study and optimize indoor ventilation Niklaus Leuenberger, R. Yang, L. Münzel, R. Verzicco, D. Lohse, L. Bourouiba, P. Jenny Indoor displacement ventilation systems have several important functions for occupants such as thermal comfort, supply of fresh air and reduction of contaminants exposure, and they have been favored in parts of the world for their energy-efficiency. Indeed, the flow rate of such a ventilation system is driven by the buoyancy induced by the heat output of the occupants. In the context of respiratory infectious diseases, improvement of indoor ventilation is key for the reduction of transmission, as clearly demonstrated yet again during the COVID-19 pandemic. |
Tuesday, November 21, 2023 8:26AM - 8:39AM |
X17.00003: Geometry of Spatially Varying Solar Radiation Intensity Affects Buoyancy Driven Flow in Ice Covered Lakes Donovan J Allum, Marek Stastna In this talk, I present the results of three, three-dimensional high resolution, non-hydrostatic simulations subjected to spatially heterogeneous solar radiation intensity in the cold-water regime (< 4 °C). Each simulation has a shadowed region (intensity reduced by 90%) at the edge of the domain with a different shape. The three shapes are a quarter circle, a rectangle and a quarter ellipse. Lateral intrusions develop in the horizontal away from the shadowed region along the surface and interact with Rayleigh-Taylor (RT) instabilities that develop away from the shadowed region. Initially, each case exhibits strong symmetry in the azimuthal (circle and ellipse) or transverse (rectangle) direction that eventually breaks down at the intrusion front as the RT instabilities grow. The depth and timescales of the intrusion change depending on the geometry of the shadowed region. |
Tuesday, November 21, 2023 8:39AM - 8:52AM |
X17.00004: Mixed convection in an idealized coastal urban environment with momentum and thermal surface heterogeneities. Yuanfeng Cui, Shuolin Xiao, Leiqiu Hu, Yongling Zhao, Qi Li Coastal marine heatwaves (MHWs) modulate coastal climate through ocean-land-atmosphere interactions, but little is known about how coastal MHWs interact with coastal cities and modify urban thermal environment. In this study, a representative urban coastal environment under MHWs is simplified to a mixed convection problem. Fourteen large-eddy simulations (LESs) are conducted to investigate how coastal cities interact with MHWs. We consider the simulations by simple urban roughness setup (Set A) as well as explicit urban roughness representation (Set B). Besides, different MHW intensities, synoptic wind speeds, surface fluxes of urban and sea patches are considered. Results suggest that increasing MHW intensity alters streamwise air temperature gradient and vertical velocity direction. The magnitude of vertical velocity and urban heat island (UHI) intensity decrease with increasing synoptic wind speed. Changing urban or sea surface heat flux also leads to important differences in flow and temperature fields. Comparison between Set A and B reveals a significant increase of vertical velocity magnitude and UHI intensity. To further understand this phenomenon, a canopy layer UHI model is proposed to explain different mechanisms that urban canopy, thermal heterogeneity and mean advection contribute to UHI intensity. The effect of urban canopy is considered in terms of an additional vertical velocity scale that facilitates heat transport from the heated surface and therefore increases UHI intensity. The model can well explain the trend of the simulated results and implies that overlooking the effect of urban canopy may severely misunderstand the overall flow and temperature fields in urban coastal environment. |
Tuesday, November 21, 2023 8:52AM - 9:05AM |
X17.00005: Experimental study of parameters affecting Firebrand Transport Hayoon Chung, Laura K Clark, Erika MacDonald, Nicholas Ouellette, Jeffrey R Koseff Increasing attention within the wildfire community has been on the role of spotfire spread in disastrous fire events through firebrand transport. Spotfires arise when burning firebrands are lofted into the tree canopy crossflow and transported much further downwind of the source fire to start secondary fires. Despite their significant role in rapidly increasing the rates of spread of wildfires, there is a general lack of understanding of what parameters control the transport of these firebrands. In this experimental flume study, we model the interaction of four key physical parameters that can affect firebrand transport. These include the shape of the firebrands (modeled using particles), the nature of the buoyant thermal plumes that loft the firebrands, gaps in the tree canopy, and the strength of the crosswind and ambient canopy turbulence. Our experimental study tracks the characteristics of the particle transport (velocity, spatial dispersion of settling position) through the use of PTV (Particle Tracking Velocimetry). We also simultaneously characterize the shape and position of the buoyant plumes that loft the particles. Results suggest in addition to their role in the initial lofting of the particles, the nature of the buoyant plumes at the time of lofting have a strong influence on where the particles settle. We also find that the ambient turbulence, due to the presence of large-scale rollers at the canopy/crossflow interface, also strongly affects the distribution of the particle settling distances. |
Tuesday, November 21, 2023 9:05AM - 9:18AM |
X17.00006: The Cooling Box Problem: Experimental Parameterization of the Dynamic Surface Temperature Jason Olsthoorn Rayleigh-Bénard convection is often modelled with a constant surface temperature. However, the surface temperature of many geophysical systems, such as lakes, is coupled to the atmospheric forcing. In this presentation, we present both two- and three-dimensional numerical simulations that account for this dynamic surface temperature using an additional coupling parameter $eta$. Results from these simulations show good agreement with laboratory experiments. We hope that this work will simplify the application of Rayleigh-Bénard theory in geophysical contexts, such as lakes. |
Tuesday, November 21, 2023 9:18AM - 9:31AM |
X17.00007: On the state space structure of spiral-defect chaos in Rayleigh-Bénard convection Chi Hin Chan, Mohammad Zakir Hossain, Spencer J Sherwin, Yongyun Hwang The existence of bistability between ideal straight rolls (ISRs) and spiral-defect chaos (SDC) in Rayleigh-Bénard convection is well established. Extensive SDC have been observed in large domains, characterised by localised structures of a specific length scale. This study aims to isolate these structures by systemically reducing the domain size, starting from a large domain. In a moderate domain, a transient chaotic state is observed before transitioning into multiple stationary, travelling-wave and time-periodic states. We identify and label 14 of these states as elementary states of SDC. Subsequently, we investigate the state space structure of the elementary states and explore their connections to SDC and ISRs. In particular, we examine the linear instabilities outside of the boundaries of the Busse balloon and determine their asymptotic behaviours. Near the Busse balloon, the asymptotic states lead to ISRs. As we move further away from the Busse balloon, we identify linear pathways leading to transient chaotic state, ultimately setlling into a travelling-wave state. The state space structure of these linear pathways will be presented in this presentation. |
Tuesday, November 21, 2023 9:31AM - 9:44AM |
X17.00008: Wall mode transition and dynamics in quasistatic magnetoconvection Matthew McCormack, Andrei Teimurazov, Olga Shishkina, Moritz Linkmann In quasistatic magnetoconvection, subjected to a strong vertical magnetic field with no-slip sidewalls, the onset of convection is determined by a linear instability at the sidewalls leading to the so-called wall mode regime. However, later stages of the transition to turbulence past this primary instability are not currently well understood. In this talk, we present new 3D DNS results in an aspect ratio one box which track the development of the equilibrium solutions produced by this primary instability through a series of bifurcations leading to a variety of more dynamically complex invariant solutions, and further to states exhibiting chaotic dynamics. In particular, we will discuss how different transition mechanisms occur at different magnetic field strengths. At low magnetic field strengths, weakly chaotic solutions feature a coexistence between wall modes and a large-scale roll in the centre of the domain which persists to higher Rayleigh numbers (Ra), but at high magnetic field strengths, the large-scale roll exists only for a small range of Ra, and chaotic dynamics primarily arise due to the unsteady dynamics of the wall modes themselves. |
Tuesday, November 21, 2023 9:44AM - 9:57AM |
X17.00009: Flow instability and cellular vortex formation in natural convection from a heated horizontal cylinder under an adiabatic wall Stavros Tavoularis, Marc-Etienne Lamarche-Gagnon Laminar flow instability between a heated horizontal cylinder and an adiabatic ceiling was investigated computationally and experimentally for relatively small cylinder-to-ceiling distances h (h ≤ D; D is the cylinder diameter) and for Rayleigh numbers Ra_{D} in the range from 1.4×10^{4} to 3.5×10^{7}. Computations for a uniformly heated cylinder wall and for h=0.2D show that, in the range 3×10^{6} ≤ Ra_D ≤ 5×10^{6}, two pairs of steady roller-type vortices appear in the gap region. For Ra_{D }>1×10^{7}, however, the rollers break down to well defined, toroidal convection cells, which are arranged quasi-periodically along the cylinder axis direction. These cells are relatively stable in time, although they occasionally break down and, following some time, they get reconnected. A physical model describing the formation of these cells is proposed. The presence of quasi-periodic convection cells is confirmed experimentally for a larger range of conditions, i.e., for 0.1D ≤ h ≤ 0.5D. The h = 0.1D, 0.3D and 0.5D configurations were found to be stable for all Rayleigh numbers investigated, for both uniform temperature and uniform heat flux conditions on the cylinder wall. |
Tuesday, November 21, 2023 9:57AM - 10:10AM |
X17.00010: Role of buoyancy work in forming superstructures in rapidly rotating Rayleigh-B{'e}nard convection Veeraraghavan Kannan, Xiaojue Zhu Turbulent superstructures in the form of large-scale vortices are an intrinsic feature in the rotating convection framework often used to study physics in geophysical and astrophysical systems. It is presumed that work done by buoyancy plays a minor role compared to non-linear energy transfer in forming large-scale vortices and only influences the dynamics of small-scale motions. Here we performed a direct numerical simulation in rapidly rotating Rayleigh-B{'e}nard convection paradigm at $Ra = 5 imes 10^{8}$, $E = 5 imes 10^{-6}$, $Pr=1$ and for an aspect ratio of unity to simulate a domain-filling large-scale coexisting cyclone and anticyclone superstructure. The role of buoyancy work in forming the superstructure is understood by decomposing the flow into 3D (eddies) and depth-averaged 2D (vortex) motions to understand their energy evolution from a quiescent initial state. The time evolution of energy budget equations is studied separately for 3D and 2D flow during the growth phase of the superstructure. It is observed that convective eddies organise at domain size corresponding to horizontal wavenumber $k_h=1$ during the start of the growth phase of the superstructure and acts as a dominant energy source through the buoyancy term to non-linear 3D to 2D energy transfer term, which in turn helps in the energy growth of 2D motions at $k_h=1$ resulting in the large-scale superstructure. Thus, we show that the buoyancy work has a direct influence on the formation of the large-scale superstructure. |
Tuesday, November 21, 2023 10:10AM - 10:23AM |
X17.00011: Prandtl Number Influence on Heat Transfer and Flow Dynamics in Narrow and Unbounded Differentially Heated Cavities Tyler R Kennelly, Sadegh Dabiri In this study, we investigate the influence of the Prandtl number on turbulent natural convection thermal transport in a Differentially Heated Cavity (DHC) at high Rayleigh numbers. The range of Rayleigh numbers explored is from 10^5 to 10^9, and Prandtl numbers vary from 1/2 to 10. |
Tuesday, November 21, 2023 10:23AM - 10:36AM |
X17.00012: Thermal boundary layer slip at a stable liquid-liquid interface Penger Tong, Hailong Huang, Wei Xu, Yin Wang, Xiaoping Wang, Xiaozhou He We report a systematic experimental study of the mean temperature profile θ (δ z) and temperature variance profile η(δ z) across a stable and immiscible liquid-liquid (water-FC770) interface formed in two-layer turbulent Rayleigh-Bénard convection. The measured θ (δ z) and η(δ z) as a function of distance δ z away from the interface for different Rayleigh numbers are found to have the scaling forms θ (δ z/λ ) and η(δ z/λ ), respectively, with varying thermal boundary layer (BL) thickness λ . By a careful comparison with the simultaneously measured BL profiles near the solid conducting surface, we find that the measured θ (δ z) and η(δ z) near the liquid interface can be well described by the BL equations for a solid wall, so long as a thermal slip length ℓ_T is introduced to account for the convective heat flux passing through the liquid interface. Direct numerical simulation results further confirm that the turbulent thermal diffusivity κ_t near a stable liquid interface has a complete cubic form, κ_t(ξ )/κ ∼ (ξ +ξ0)^3, where κ is the molecular thermal diffusivity of the convecting fluid, ξ = δ z/λ is the normalized distance away from the liquid interface, and ξ0 is the normalized slip length associated with ℓ_T . |
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