Effects Of Rotational Hypergravity And Asymmetric Wall Roughness On Thermal Turbulence

Thermal convection driven by temperature difference is a common physical phe-nomenon in nature and in industrial processes,which plays crucial roles in astrophysical flows,in the ocean and atmosphere,in aerospace engineering,and in energy and chemical engineering.Studying the scaling dependence of Nusselt number(quantifying the con-vective heat transport)on the Rayleigh number(measuring the thermal driving strength)is the main attention of high-Rayleigh number thermal turbulence,which is also the key to testify whether Kraichnan ultimate scaling exists.However,it is extremely challenging to push the Rayleigh number to an ultra-high value in both experiments and numerical simulations.In addition,as the Rayleigh number increases,the boundary layer of the convection system gradually becomes thinner,and the influence of the wall roughness is prominent.But up to now,the vast majority of related studies focus on ordered and symmetrical structures.In view of the above,this thesis combines experiments and nu-merical simulations to explore the ultimate regime in thermal turbulence by means of a hypergravitational turbulent thermal convection system in which the Rayleigh number can be boosted through rapid rotation.Meanwhile,the effects of asymmetrical Feynman ratchet on the global heat transport and flow dynamics in thermal turbulence are studied experimentally and numerically.This thesis proposes a novel approach to boost Rayleigh number by exploiting cen-trifugal acceleration induced by rapid rotation.A hypergravitational thermal convection system with an effective gravity up to 100 times the Earth's gravity is designed and built,which is also provided with high-precision heat transfer and flow measurement.Before the studying of high-Rayleigh number thermal turbulence,effects of rapid rotation on heat transport and flow structures of thermal convection are explored,and we show that,Coriolis force tends to suppress the axial flow,leading to a two-dimensional flow field and a reduction of global heat transport.Next,the convective rolls move around the cen-ter azimuthally,signifying the emergence of zonal flow,which is associated with Coriolis force and the difference of curvature between the outer and inner cylinders.Finally,based on the hypergravitational turbulent thermal convection setup,the existence of Kraichnan ultimate scaling is convincingly demonstrated in nearly two decades of Rayleigh number range,which is backed up by the appearance of a logarithmic layer in velocity profile,the enhanced strength of the shear Reynolds number,and the new statistical properties of local temperature fluctuations.By exploiting the Feynman ratchet,we have studied the influences of asymmetric roughness on thermal turbulence experimentally and numerically in different convective systems,including Rayleigh-Bénard convection,vertical natural convection,and slant-wise convection.Firstly,in Rayleigh-Bénard convection,due to the symmetry breaking caused by the presence of the ratchet structures,the orientation of the large scale circula-tion roll is locked to a preferred direction.By introducing a small tilt to the system,the orientation of the flow can be controlled,and two distinct states exist.When the large scale wind faces towards the steeper slope side of the ratchets(case B),the heat transport is larger than the case in which the large scale wind flows along the ratchets in the di-rection of their smaller slopes(case A).Flow structures and quantitative analysis indicate that these findings are connected to the dynamics of thermal plume emissions.Remark-ably different from Rayleigh-Bénard convection,in vertical natural convection,case A has a more efficient heat transport than that of case B.Velocity fields,temperature fields and quantitative analysis demonstrate that the heat transfer in vertical natural convection is governed by the boundary layer flow and the horizontal intrusion flow instead of dis-persed thermal plumes.The sharp corners of the ratchets in case B hinder the development of the boundary layer flow.Finally,it is found that the arrangement of ratchets in the way of case A can delay the transition where the heat transfer decreases sharply with the tilting angle,which may be useful to enhance the robustness of the heat exchangers.