Temporal modulation on mixed convection in turbulent channels

Abstract

We studied flow organization and heat transfer properties in mixed turbulent convection within Poiseuille-Rayleigh-Bénard channels subjected to temporally modulated sinusoidal wall temperatures. Three-dimensional direct numerical simulations were performed for Rayleigh numbers in the range $10^6\le Ra\le 10^8$, a Prandtl number $Pr=0.71$ and a bulk Reynolds number $R{e}_b\approx 5623$. We found that high-frequency wall temperature oscillations had minimal impact on flow structures, while low-frequency oscillations induced adaptive changes, forming stable stratified layers during cooling. Proper orthogonal decomposition (POD) analysis revealed a dominant streamwise unidirectional shear flow mode. Large-scale rolls oriented in the streamwise direction appeared as higher POD modes and were significantly influenced by lower-frequency wall temperature variations. Long-time-averaged statistics showed that the Nusselt number increased with decreasing frequency by up to 96\%, while the friction coefficient varied by less than 15\%. High-frequency modulation predominantly influenced near-wall regions, enhancing convective effects, whereas low frequencies reduced these effects via stable stratified layer formation. Phase-averaged statistics showed that high-frequency modulation resulted in phase-stable streamwise velocity and temperature profiles, while low-frequency modulation caused significant variations due to weakened turbulence. Turbulent kinetic energy (TKE) profiles remained high near the wall during both heating and cooling at high frequency, but decreased during cooling at low frequencies. A TKE budget analysis revealed that during heating, TKE production was dominated by shear near the wall and by buoyancy in the bulk region; while during cooling, the production, distribution and dissipation of TKE were all nearly zero.

32 pages, 22 figures

References in corpus (20)

• Large scale dynamics in turbulent Rayleigh-Benard convection
• The physics of wind-blown sand and dust
• Boundary layer structure in turbulent thermal convection and its consequences for the required numerical resolution
• Lattice Boltzmann simulations of three-dimensional thermal convective flows at high Rayleigh number
• Mixed convection in turbulent channels with unstable stratification
• Comparison of computational codes for direct numerical simulations of turbulent Rayleigh-Bénard convection
• Lagrangian temperature, velocity and local heat flux measurement in Rayleigh-Benard convection
• Jump Rope Vortex in Liquid Metal Convection
• Changes in turbulent dissipation in a channel flow with oscillating walls
• Periodically modulated thermal convection
• Flow organization and heat transfer in turbulent wall sheared thermal convection
• Multi-GPU thermal lattice Boltzmann simulations using OpenACC and MPI
• Supergravitational turbulent thermal convection
• Counter-gradient heat transport in two-dimensional turbulent Rayleigh-Bénard convection
• The effect of Prandtl number on turbulent sheared thermal convection
• Tristable flow states and reversal of the large-scale circulation in two-dimensional circular convection cells
• Law of the wall in an unstably stratified turbulent channel flow
• Thermal Waves and Heat Transfer Efficiency Enhancement in Harmonically Modulated Turbulent Thermal Convection
• Oscillatory large-scale circulation in liquid-metal thermal convection and its structural unit
• Turbulent mixed convection in vertical and horizontal channels