Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M33: Free and Rayleigh-Benard Convection III |
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Chair: Shidi Huang, Chinese University of Hong Kong Room: Georgia World Congress Center B405 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M33.00001: Direct numerical simulation of heat transport in two-dimensional Rayleigh-Bénard convection Shidi Huang, Zhenyuan Gao, Yun Bao We investigate the heat transport efficiency in two-dimensional Rayleigh-Bénard convection with high resolution direct numerical simulation. The study is carried out in a box with unit aspect ratio in the Rayleigh number Ra range 107 ~ 1012 for Prandtl numbers Pr = 0.7 and 4.3. It is found that while the heat transport efficiency, i.e. Nusselt number Nu, scales as Ra0.3 approximately over the whole Ra range, there exist obvious changes in the local scaling exponent of Nu-Ra relation in different Ra ranges, which could be understood with the changes in the flow structures and their dynamics. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M33.00002: Numerical and experimental study of the influence of a cylindrical obstacle on the heat transfer in a differentially heated cavity Romain Gers, Juan Pablo Robinson, Paul Chorin, Florian Moreau, Olivier Skurtys, Didier Saury Heat transfer enhancement in free convection systems is a challenge for many industrial processes, such as nuclear reactors cooling, electronic circuits or building's ventilation. The rectangular differentially heated cavities (DHC) are the best model to study it. The key factor to succeed in is to destabilize the laminar thermal boundary layers in order to enhance the temperature gradient at the walls. In this work, we numerically study the influence of a conductor or not conductor cylinder located at the bottom of the hot plate, perpendicular to it, in a DHC of aspect ratio 4. Various Rayleigh numbers (or temperature difference) are considered. When the cylinder is not conductor we perturb the velocity fields first and in a second step, the temperature fields. When it is conductor we perturb both fields by both mechanisms, thermal and mechanical. The cavity, filled with air (Prandtl number = 0.71). Direct numerical simulations of the three-dimensional incompressible Navier-Stokes-Boussinesq equations, are performed using the spectral element method with a Pn-Pn-2 formulation, with polynomial order 7, written in the open source code Nek5000. The results are contrasted with experimental data and we show the effect of a cylinder on the heat transfer. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M33.00003: Heat transfer analysis in a tilted cavity with rotation Diana Perez-Espejel, Ruben Avila A heat transfer analysis of the natural convection in an inclined rotating cavity is presented. The top and bottom surfaces of the cavity are rigid and have fixed temperatures. The rotation axis passes orthogonally through the hot and cold surfaces. The heat transfer analysis is performed for different Taylor numbers and inclination angles (δ) of the cavity for longitudinal, transverse and oblique convective rolls solutions. The Nusselt number is obtained for the critical parameters of the onset of convection (Rayleigh number, wave number and roll orientation of oblique rolls) previously obtained from a linear stability analysis. Both unstable and stable thermal conditions are treated, with inclination angles of δ <90° (heated from below) and 90° < δ <= 120° (heated from above) respectively. Three dimensional non-linear numerical simulations are also presented for a Boussinesq fluid. The simulations are obtained using a spectral element method, considering periodic lateral boundaries and no-slip boundary conditions at the top and bottom surfaces. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M33.00004: On the Lagrangian Acceleration in Rayleigh-Be ́nard Convection: effect of aspect ratio Jin-Tae Kim, Shikun Shen, Yaqing Jin, Leonardo P. Chamorro A laboratory investigation using 3D particle tracking velocimetry is performed to uncover distinctive Lagrangian dynamics of turbulent Rayleigh-Be ́nard convection at aspect ratios Γ=1.2 and 2 with emphasis on Lagrangian statistics of thermal plumes. A laterally extended investigation volume allows us to capture a part of high vertical motion. Approximately 3.5×107flow particles were measured at a Rayleigh number of Ra ≈1.1×1010 for Γ =1.2 and Ra ≈2.8×109 for Γ =2. The PDF of the vertical Lagrangian acceleration exhibited heavier tails than the lateral counterpart away from the near-wall boundary layer, evidencing higher Lagrangian intermittency in the vertical direction. Inspection of the 2nd-order acceleration PDF filtered by vertical velocity magnitude thresholds indicates that intense vortical motion induced by the thermal plumes promotes stronger vertical acceleration respect to the lateral. A comparison with previous works reveals the influential role of roll dynamics induced by Γ on the distribution of the vertical acceleration. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M33.00005: Synchronization of natural convection in thermostatically controlled adjacent cavities: effect of common wall properties Rafael Chavez-Martinez, Mario Sanchez-Lopez, Francisco Javier Solorio, Mihir Sen Synchronization is a phenomenon that was first reported by Christian Huygens in the 17th century. Since then it has been found in several science and engineering applications such as electrical, mechanical, optical and thermal, among others. In thermal systems, analytical studies of two similar cavities separated by a common wall, with similar heat sources and independent thermostatic temperature control shown the existence of temperature synchronization inside the cavities, recently it was confirmed experimentally. In the present work, the experimental model consists of two cuboid cavities separated by a common wall, inside each one a heat source was installed, and the temperature was controlled by a computer-based thermostat. Different common walls were used in the experiments. In all experiments the cavities maintained their inner dimensions. The experiments showed the effect of the thickness and material (thermal resistance) of the wall on the phase difference Δ∅. The results demonstrate that the thickness affects Δ∅, but the frequency of the temperature is barely affected. In general, thermal resistance has a significant effect on Δ∅, changing his value and its sign. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M33.00006: Experimental Study of Heat Transfer and Fluid Flow Inside a Differentially Heated Closed Rectangular Cavity Using Two Non-Invasive Techniques Vimal Kishor, Atul Srivastava, Suneet Singh Buoyancy-induced flow in closed cavities is subject of interest since last few decades because of its complexity and a wide range of applications in different areas. To understand such types of phenomena, experiments are performed in the differentially heated vertical closed rectangular cavity for air (Pr=0.71). Temperature potential has been applied across the two vertical walls of the cavity while other two walls have been kept insulated. The aspect ratio of the cavity (height to width ratio) is kept as 3. Experiments are done using two non-invasive techniques, namely, Mach Zehnder Interferometer (MZI) and smoke visualization. MZI and smoke visualization techniques are used to get two-dimensional full field temperature profiles and streaklines of the flow inside the cavity. Experiments are performed for different Rayleigh numbers. Results obtained through two experiments techniques also have been compared with numerical results (performed using COMSOL). Thermal and hydraulic boundary layer formation near the vertical walls has been observed from the results of MZI and smoke visualization respectively. The dominance of corner flow has been observed where the maximum heat transfer takes place while in the core of the cavity, the weak convective heat transfer has been seen. |
Tuesday, November 20, 2018 9:18AM - 9:31AM |
M33.00007: POD-ROM model for analyzing the onset of natural convection and stability in a differentially heated top open cavity Suneet Singh, Ashish Saxena, Abhinav Gairola, Hitesh Bindra The deformation of the conduction layers in a differentially heated cavity leads to the onset of convection currents. The top open cavity investigated here showed lower critical Rayleigh number as compared to the classical Rayleigh-Benard convection, which can be due to the free interaction of cavity air with the external atmosphere due to the non-negligible vertical velocity component at the open surface. These conditions lead to the corner flow patterns in the cavity even at Rayleigh numbers below the critical limit. The corner flow leads to non-equilibrium condition before the onset of natural convection which complicates the stability analysis for this problem. One of the complication is due to non-equilibrium pre-convection flow conditions and Proper Orthogonal Decomposition (POD) of the numerical solutions is used to model the initial corner flow conditions. POD based Reduced Order Model (ROM) is then devised by projecting the Navier-Stokes-Boussinesq equations on empirical eigenvectors. The resulting ROM is used to predict the critical Rayleigh number and further bifurcation analysis. Some of the experimentally and numerically observed physics such as symmetry breakdown, mirror image solution have been analyzed using reduced order model. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M33.00008: Effect of wall roughness on the heat transfer in rotating Rayleigh-Bénard convection Pranav Joshi, Hadi Rajaei, Rudie Kunnen, Herman Clercx This study focuses on the heat transfer in rotating Rayleigh‒Bénard convection in the presence of horizontal walls with pyramid-shaped roughness elements. Measurements are performed at a Rayleigh number of 2.2×109 and a Prandtl number of 6.2. The effect of the roughness elements on the heat transfer depends on their size, k, relative to the thickness of the Ekman boundary layer, δE. The Ekman pumping mechanism, which is responsible for the heat transfer enhancement under rotation in case of smooth walls, is unaffected by the roughness when δE>>k. As the rotation rate increases (and hence δE decreases), the roughness elements penetrate the radially inward boundary layer flow into the columnar Ekman vortices, generating more thermal structures and further enhancing the heat transfer. This enhancement continues to increase with decreasing δE until δE ≈ k. As δE decreases further below k, the heat transfer starts decreasing as the strong perturbation of the Ekman boundary layer suppresses the Ekman pumping mechanism. The mechanism is perhaps re-established when δE becomes sufficiently smaller than k, i.e., when the surfaces of the roughness elements act locally like sloping walls on which Ekman layers can be formed. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M33.00009: New cryogenic nitrogen facility for Rayleigh-Bénard convection at high Rayleigh number - Part 1: Facility Design and Capabilities Jessica Imgrund, Stephen R Johnston, Enrico Fonda, Jacob A McFarland, Katepalli Raju Sreenivasan, Devesh Ranjan Multiple theories predict the existence of an ultimate regime of heat transfer for Rayleigh-Bénard convection (RBC), yet the experimental evidence supporting the transition to this regime has been contradictory. The study of RBC under rotation, specifically at underexplored geophysical-scale Rayleigh numbers, is of particular interest. We present a newly developed, multi-functional facility for studying both RBC at high Rayleigh numbers, and rotating RBC. This novel facility uses cryogenic nitrogen near the gas-liquid coexistence curve from atmospheric pressure up to its critical point as the working fluid. Interchangeable sidewalls permit studying RBC in test cells of various aspect ratios, and optical access allows for application of optical diagnostic techniques such as PIV. The facility is designed to exceed Rayleigh numbers of 1015 for a test cell of aspect ratio 1, and 1017 for a test cell of aspect ratio 0.2. We present its salient features in this talk. |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M33.00010: New cryogenic nitrogen facility for Rayleigh-Bénard convection at high Rayleigh number - Part 2: Validation and Initial Data Stephen R Johnston, Jessica Imgrund, Enrico Fonda, Katepalli Raju Sreenivasan, Devesh Ranjan We present initial results from our newly constructed cryogenic nitrogen facility for studying Rayleigh Bénard convection at high Rayleigh numbers. Our validation experiments cover the Rayleigh number range 108 ≤ Ra ≤ 1013, and order-unity Prandtl number in a test cell of unity aspect ratio (diameter-to-height ratio). The Nusselt number scaling for this range is compared with other experimental data. Additionally, we outfit the test cell with fast response thermistors in the bulk of the flow, and show initial fluctuation data over an extended range of Rayleigh numbers. |
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