Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session E06: Free and Rayleigh-Benard Convection I |
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Chair: Joerg Schumacher, TU Ilmenau Room: North 122 AB |
Sunday, November 21, 2021 2:45PM - 2:58PM |
E06.00001: Enhanced Heat Transfer in Rayleigh-Bénard Flows at Transcritical Conditions Jack Guo, Matthias Ihme Rayleigh-Bénard (RB) flows arise from a horizontal fluid layer subject to a downwards gravitational force coupled with heating from the underside and cooling from above. Despite extensive computational, theoretical, and experimental research of this canonical configuration, the behavior at supercritical pressures is poorly understood. At these conditions, strong gradients and fluctuations in thermodynamic properties lead to significant deviations from classical theory in the Oberbeck-Boussinesq (OB) regime. We present a series of highly-resolved RB simulations at transcritical and supercritical conditions, at Rayleigh numbers of up to 10^{9}. Heat transfer values, as characterized by the Nusselt number, are enhanced by up to O(100%) compared to OB values at the same Rayleigh number. In characterizing this intensified heat transfer behavior, we present statistics and flow structures, highlighting differences not only among each region of the domain but also with OB simulations. |
Sunday, November 21, 2021 2:58PM - 3:11PM |
E06.00002: Simulation and validation of the effects of thermally buoyant flow on subsea laser transmission Stefan Mrozewski, Michael Krieg Point-to-point underwater laser communication systems are being developed to address increasing subsea data needs and bandwidth requirements. Many use cases - such as transmitting data from distributed sensors installed on subsea oilfield infrastructure - feature laser beams passing above heat sources such as heated pipelines. These thermal sources induce natural convection in the surrounding seawater, changing its refractive index. Maintaining point-to-point laser telemetry thus requires characterizing and mitigating the effects of these thermally buoyant flows on beam propagation. A direct numerical simulation was used to model heat and fluid flow for simple two-dimensional geometries representative of conditions where laser communication beams pass over subsea pipelines. Lineouts from the temperature fields and computed refractive index are used to calculate beam deflection for various beam locations and flowline temperatures. These results are used to develop practical guidelines on laser transceiver placement. The simulation output for a subset of test cases is validated experimentally in a laboratory water tank using scaled thermal sources. Laser beam deviation is measured directly and, as intermediate verification, fluid flow and temperature are measured using digital particle image velocimetry, showing good agreement with simulated results. |
Sunday, November 21, 2021 3:11PM - 3:24PM |
E06.00003: Natural convection enhanced highly efficient and salt rejecting solar evaporator Xiangyu Li, Lenan Zhang, Yang Zhong, Arny Leroy, Lin Zhao, Zhenyuan Xu, Evelyn N Wang Thermally localized solar evaporation provides a sustainable solution to address the water-energy nexus, which is promising for the applications in vapor generation, seawater desalination, wastewater treatment, and medical sterilization. However, salt accumulation has been identified as one of main practical challenges, which induces undesired fouling and reduces device lifetime. Here, we develop a novel confined water layer structure enabling a simultaneously highly efficient and salt rejecting solar evaporation. With high-fidelity modeling and experimental characterization of salt transport, we optimize the fluidic flow in the confined water layer. With the optimized flow field, our design is capable of salt transport without sacrificing energy efficiency. In addition, we further applied our design to the contactless solar evaporator and report a record-high efficiency. The fundamental understanding of salt transport shown in this work paves a new avenue toward the high-performance and reliable solar evaporation with low-cost and high material flexibility. |
Sunday, November 21, 2021 3:24PM - 3:37PM |
E06.00004: Rayleigh-Bénard-like convection with an asymmetric vertical cooling Soohyeon Kang, Liu Hong, Shyuan Cheng, Leonardo P Chamorro Natural convection is a fundamental phenomenon of high relevance in nature and engineering systems. Here, we experimentally studied Rayleigh-Bénard-like convection conducted in a rectangular tank heated from below and cooled from the top with a cooled sidewall at a Rayleigh number Ra ≈ 1.6 × 10^{10}. Instantaneous flow fields were characterized at various vertical planes using particle image velocimetry (PIV). We will discuss transient features of the flow motions and characterization of coherent structures using proper orthogonal decomposition POD. In particular, the inspection of the POD coefficients reveals a distinct, coherent convective roll structure along the wider direction of the tank. Complementary temperature distribution, measured using thermochromic particles, illustrates the modulation of the sidewall effects on the temperature fluctuations. |
Sunday, November 21, 2021 3:37PM - 3:50PM |
E06.00005: Boundary layers in turbulent vertical convection at high Prandtl number Christopher J Howland, Chong Shen Ng, Roberto Verzicco, Detlef Lohse Many environmental flows arise due to natural convection at a vertical surface, from flows in buildings to dissolving ice faces at marine-terminating glaciers. We use three-dimensional direct numerical simulations of a vertical channel with differentially heated walls to investigate such convective, turbulent boundary layers. Through the implementation of a multiple-resolution technique, we are able to perform simulations at a wide range of Prandtl numbers Pr. This allows us to distinguish the parameter dependences of the horizontal heat flux and the boundary layer widths in terms of the Rayleigh number Ra and Prandtl number Pr. For the considered parameter range 1 ≤ Pr ≤ 100, 10^{6} ≤ Ra ≤ 10^{9}, we find the flow to be consistent with a ‘buoyancy-controlled' regime where the heat flux is independent of the wall separation. For given Pr, the heat flux is found to scale linearly with the friction velocity V_{∗}. Finally, we discuss the implications of our results for the parameterisation of heat and salt fluxes at ice-ocean interfaces. |
Sunday, November 21, 2021 3:50PM - 4:03PM |
E06.00006: Data Assimilation of Rayleigh-Bénard Convection using Thermal Measurements Lokahith Narendra Agasthya, Patricio Clark Di Leoni, Luca Biferale Nudging^{1} is a continuous data assimilation technique which introduces a penalty term in the equations of motion to keep the evolution of the flow close to a given set of measurements. We employ a nudging^{2} scheme to reconstruct the temperature and velocity fields in Rayleigh-Bénard convection using only thermal data. |
Sunday, November 21, 2021 4:03PM - 4:16PM |
E06.00007: Bounding Heat Transfer in Internally Heated Convection Ali Arslan, Giovanni Fantuzzi, Andrew Wynn, John Craske Rigorously bounding emergent properties of turbulent flows provides a means for improving our mathematical understanding of turbulence, while yielding results with real world applications. Using the background method formulated in terms of quadratic auxiliary functions we prove bounds on the mean convective heat transport < wT > in internally heated (IH) convection. Boundary driven Rayleigh-Bénard convection has seen extensive analysis with the background method, yet an extension of the same analysis to IH convection is incomplete. The change in mechanism driving convection presents a problem of importance in geophysical and astrophysical phenomena such as convection in the mantle or the Venusian atmosphere. This talk will demonstrate that Rayleigh dependent bounds on < wT > can be obtained, the scaling of which can be verified with numerical optimisation and shown to be optimal within our bounding framework. |
Sunday, November 21, 2021 4:16PM - 4:29PM |
E06.00008: Free Convection with a Viscosity that is Exponentially Dependent Upon Temperature Kelsey A Everard, Megan S Davies Wykes, Sam Pegler Using numerical and asymptotic analyses, we explore the situation where viscosity is an exponential function of temperature in free convection in the large Prandtl number limit. We identify two distinct asymptotic limits. For the case of a heated wall, all of the variation in velocity is contained in a thin lubrication layer near the wall. Whereas, for the case of a cooled wall, a conductive plug in which there is no velocity forms near the wall and all velocity variation is confined to a narrow transition region beyond the plug. This work has important applications to diapir dynamics, and to other flows of fluids, like honey, whose viscosity is strongly affected by changes in temperature. |
Sunday, November 21, 2021 4:29PM - 4:42PM |
E06.00009: Heat transport enhancement in confined Rayleigh-Bénard convection feels the shape of the container Robert Hartmann, KAI LEONG CHONG, Richard Stevens, Roberto Verzicco, Detlef Lohse Moderate spatial confinement enhances the heat transfer in turbulent Rayleigh-Bénard (RB) convection [Chong et al., PRL 115, 264503 (2015)]. Here, by performing direct numerical simulations, we answer the question how the shape of the RB cell affects this enhancement. We compare three different geometries: a box with rectangular base (i.e., stronger confined in one horizontal direction), a box with square base (i.e., equally confined in both horizontal directions), and a cylinder (i.e., symmetrically confined in the radial direction). In all cases the confinement can be described by the same confinement parameter Γ^{-1}, given as height-over-width aspect ratio. The explored parameter range is 1≤Γ^{-1}≤64, 10^{7}≤Ra≤10^{10} for the Rayleigh number, and a Prandtl number of Pr=4.38. We find that both the optimal confinement parameter Γ^{-1}_{opt} for maximal heat transfer and the actual heat transfer enhancement strongly depend on the cell geometry. The differences can be explained by the formation of different vertically-coherent flow structures within the specific geometries. The enhancement is largest in the cylindrical cell, owing to the formation of a domain-spanning flow structure at the optimal confinement parameter Γ^{-1}_{opt}. |
Sunday, November 21, 2021 4:42PM - 4:55PM |
E06.00010: Two-Dimensional Convective Boundary Layer: Numerical Analysis and Echo State Network model Florian Heyder, Juan Pedro Mellado, Joerg Schumacher The numerical study of global atmospheric circulation processes requires the parametrization of turbulent buoyancy fluxes in the lower convective boundary layer which typically cannot be resolved by the coarse-scale computational grids. We investigate a two-dimensional model of a shallow convective boundary layer in the Boussinesq limit by direct numerical simulations. A series of simulation runs evaluates the turbulent transport properties as a function of the flux ratio at the top and bottom. Furthermore, the resulting simulation data records are used to train and test a recurrent neural network realized in this work by an echo state network. We show that an echo state network, which was trained with simulation data at two different top fluxes, can approximate an unseen third data record with a different top flux. We find, amongst others, that the statistical properties of the buoyancy flux across the layer are reproduced by our model without solving the underlying highly nonlinear equations of motion. |
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