DNS of Instantaneous Behavior in Turbulent Forced and Mixed Convection of Liquid Metal Past a Backward-Facing Step
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DNS of Instantaneous Behavior in Turbulent Forced and Mixed Convection of Liquid Metal Past a Backward-Facing Step Chaozheng Wang2 · Pinghui Zhao1 · Mingzhun Lei1 · Kun Lu1 · Zhihao Ge2 · Jiaming Liu2 · Yuanjie Li2 · Gang Pei3 Received: 3 February 2020 / Revised: 23 August 2020 / Accepted: 25 August 2020 © Springer Nature B.V. 2020
Abstract A direct numerical simulation has been performed to study instantaneous behavior in leadbismuth eutectic flows past a vertical, backward-facing step. A turbulent forced convection case and two cases of mixed convection, the first buoyancy-aided flow at a Richardson number Ri of 0.1 and the second buoyancy-opposed flow at Ri = 0.02 , are simulated and discussed. The Reynolds number based on the bulk velocity and step height is 4805. A uniform heat flux is imposed on the expansion wall behind the step. In the forced convection case, the numerical results reveal two characteristic unsteady flow phenomena. The first is vortex-shedding motion along the separating shear layer, while the second is wall-normal flapping of the shear layer. These unsteady motions have significant influences on the thermal field. The vortex-shedding motion induces some streak-like low-temperature structures on the heated wall, while the flapping motion induces oscillation of the maximum temperature on the wall. In the mixed convection cases, buoyancy alters the flow field substantially. The two unsteady flow phenomena noted above constitute motions inherent to backwardfacing step flow. Buoyancy plays a material role in vortex development, affecting vortex ranges and time-scales. While the vortex shedding frequency is insensitive to buoyancy, the frequency of the flapping motion increases with the buoyancy. These results contribute to an improved understanding of separating and reattaching flows, especially in association with buoyancy and temperature fluctuations. The data serve to aid future development and validation of improved heat-flux modeling of low-Prandtl-number fluids. Keywords DNS · Liquid metal · Backward-facing step · Instantaneous behavior · Buoyancy
* Pinghui Zhao [email protected] 1
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
2
School of Physical Sciences, University of Science and Technology of China, Hefei 230026, China
3
School of Engineering Sciences, University of Science and Technology of China, Hefei 230026, China
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Flow, Turbulence and Combustion
List of Symbols AR Aspect ratio, W∕h 𝜏w Cf Skin friction coefficient, 0.5𝜌u 2 b
Cp Specific heat capacity ER Expansion ratio, H/(H − h) f Physical frequency g Gravitational acceleration 3 Gr Grashof number, g𝛽ΔTh 𝜈2 H Channel height behind the step h Step height ⟨ ⟩ k Turbulent kinetic energy, 12 u′i u′i L Length of the computational domain n Dimensionless frequency, fU⋅h b
N Number of numerical grid points for backflow region Pr Molecular Prandtl number p Pressure divided by density q̇ Heat flux density U h Re Reynolds number, 𝜈b f ⋅h St Strouh
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