## About

207

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September 1992 - August 2018

August 1989 - January 1992

## Publications

Publications (207)

We report on an experimental study of turbulent Rayleigh–Bénard convection with asymmetric top and bottom plates. The plates are covered with pyramid-shaped roughness elements whose aspect ratios are $\lambda =1$ or $\lambda =4$ . In the low-Rayleigh-number regime ( $Ra<1.9\times 10^9$ ), the heat transport efficiencies in the asymmetric cells, cha...

An experimental investigation is conducted to study the flow patterns, spectral properties and energy fluxes in thin-layer turbulence with varying system sizes and damping rates. It is found that although a system-size vortex (an indicator of spectral condensation) occurs for small system sizes and does not for large ones, the spectra for different...

We present a systematic study on the effects of small aspect ratios $\varGamma$ on heat transport in liquid metal convection with a Prandtl number of $Pr=0.029$ . The study covers $1/20\le \varGamma \le 1$ experimentally and $1/50\le \varGamma \le 1$ numerically, and a Rayleigh number $Ra$ range of $4\times 10^3 \le Ra \le 7\times 10^{9}$ . It is f...

We report an experimental study of heat transport in a three-layer turbulent Rayleigh-Bénard convection. The experiments were conducted in a cylindrical cell (with diameter D) filled with a FC77 layer with height H=D. A very thin layer of water and a very thin layer of mercury were introduced to the top and bottom of the FC77 layer to provide slipp...

We report a home-built velocity-gradient-tensor-resolved particle image velocimetry (VGTR-PIV) system which spatio-temporally resolves all components of the velocity gradient tensor. This technique is applied to the paradigmatic turbulent Rayleigh–Bénard convection system in a cylindrical cell at three representative positions, i.e. centre, side an...

We present a physically motivated low-dimensional model for the dynamics of two interacting large-scale circulations (LSC) in two-layer turbulent convection. Inspired by our experimental results of the flow dynamics and coupling in two-layer turbulent convection [J. Fluid Mech. 728, R1 (2013)], the model extends previous studies of single-LSC dynam...

Using thermal convection in liquid metal, we show that strong spatial confinement not only delays the onset Rayleigh number $Ra_c$ of Rayleigh–Bénard instability but also postpones the various flow-state transitions. The $Ra_c$ and the transition to fully developed turbulence Rayleigh number $Ra_f$ depend on the aspect ratio $\varGamma$ with $Ra_c\...

Using direct numerical simulations, we investigate the heat transport in bulk and boundary flows separately in rotating Rayleigh–Bénard convection in cylindrical cells. In the bulk we observe a steep scaling relationship between the Nusselt number ( $Nu$ ) and the Rayleigh number ( $Ra$ ), which is consistent with the results from simulations using...

We investigate the spatial distribution and dynamics of the vortices in rotating Rayleigh–Bénard convection in a reduced Rayleigh number range $1.3\le Ra/Ra_{c}\le 83.1$ . Under slow rotations ( $Ra\approx 80\,Ra_{c}$ ), the vortices are distributed randomly, which is manifested by the size distribution of the Voronoi cells of the vortex centres be...

We experimentally studied the effect of cell tilting on the temperature oscillation in turbulent Rayleigh–Bénard convection. We simultaneously measured the temperature using both in-fluid and in-wall thermistors for Ra=1.7×109 and 5.0×109 at Prandtl number Pr = 5.3. The tilt angles relative to gravity are set to 0°, 0.5°, 1°, 2°, and 7°. It is foun...

We present an experimental study of Rayleigh-B\'enard convection using liquid metal alloy gallium-indium-tin as the working fluid with a Prandtl number of $Pr=0.029$. The flow state and the heat transport were measured in a Rayleigh number range of $1.2\times10^{4} \le Ra \le 1.3\times10^{7}$. The temperature fluctuation at the cell centre is used...

We report an experimental and numerical study on Rayleigh-Bénard convection in a slender rectangular geometry with the aspect ratio Γ varying from 0.05 to 0.3 and a Rayleigh number range of 10 5 Ra 3 × 10 9. The Prandtl number is fixed at Pr = 4.38. It is found that the onset of convection is postponed when the convection domain approaches the quas...

A novel experiment was performed in rotating Rayleigh–Bénard convection (RRBC), wherein the convection cell with radius $R$ was shifted away from the rotation axis by a distance $d$ . In this case, the centrifugal force felt by a fluid parcel (characterized by the Froude number $Fr$ ) can be decomposed into an axisymmetrical part $Fr_R$ and a direc...

We investigate the spatial distribution and dynamics of the vortices in rotating Rayleigh-B\'enard convection in a reduced Rayleigh-number range $1.3{\le}Ra/Ra_{c}{\le}166$. Under slow rotations ($Ra{\gtrsim}10Ra_{c}$), the vortices are randomly distributed. The size-distribution of the Voronoi cells of the vortex centers is well described by the s...

We present an experimental study of the velocity circulation in a quasi-two-dimensional turbulent flow. We show that the area rule of circulation around simple loops holds in both the forward cascade enstrophy inertial range (ΩIR) and the inverse cascade energy inertial range (EIR): When the side lengths of a loop are all within the same inertial r...

We report an experimental study on the enhancement of heat transfer efficiency in turbulent thermal convection via manipulation of the bulk flow, which is achieved by inserting artificial structures into the convection domain. These structures are designed to alter the bulk flow without perturbing the boundary layer. Two types of convective domains...

Tuning transport properties through the manipulation of elementary structures has received a great success in many areas, such as condensed matter physics. However, the ability to manipulate coherent structures in turbulent flows is much less explored. This article reviews a recently discovered mechanism of tuning turbulent heat transport via coher...

We present an experimental study of Rayleigh–Bénard convection using liquid metal alloy gallium-indium-tin as the working fluid with a Prandtl number of $Pr=0.029$ . The flow state and the heat transport were measured in a Rayleigh number range of $1.2\times 10^{4} \le Ra \le 1.3\times 10^{7}$ . The temperature fluctuation at the cell centre is use...

We report an experimental study of the viscous boundary layer (BL) properties of turbulent Rayleigh–Bénard convection in a cylindrical cell. The velocity profile with all three components was measured from the centre of the bottom plate by an integrated home-made particle image velocimetry system. The Rayleigh number $Ra$ varied in the range $1.82...

We report a laboratory study on the scattering, energy dissipation and mean flow induced by internal gravity waves incident upon slopes with varying surface roughness. The experiment was performed in a rectangular box filled with thermally stratified water. The roughness of the slope surface, $\lambda$ , defined as the height of a roughness element...

We present an experimental and numerical study of turbulent thermal convection in the presence of an effective horizontal buoyancy that generates extra shear at the boundary. Geometrical confinements are also applied by varying the streamwise and spanwise aspect ratios of the convection cell to condense the plumes. With these, we systematically exp...

The effect of centrifugal force in turbulent rotating Rayleigh–Bénard convection (RRBC) is studied experimentally in an aspect-ratio $\varGamma =1$ cylindrical convection cell and in the ranges of the Froude number $0.004\leq Fr \leq 0.363$ and the Rayleigh number $2.8\times 10^8 \leq Ra \leq 9.5\times 10^9$ , and with the Prandtl number fixed at $...

We present a numerical study on how tidal force and topography influence flow dynamics, transport and mixing in horizontal convection. Our results show that local energy dissipation near topography will be enhanced when the tide is sufficiently strong. Such enhancement is related to the height of the topography and increases as the tidal frequency...

An important and unresolved issue in rotating thermal turbulence is when the flow starts to feel the centrifugal effect. This onset problem is studied here by a novel experiment in which the centrifugal force can be varied over a wide range at fixed Rossby numbers by offsetting the apparatus from the rotation axis. Our experiment clearly shows that...

We present a numerical study of horizontal convection using both step and linear forcing profiles as the imposed surface thermal driving, the former of which is commonly used in many studies but the latter is closer to the real ocean. Our investigation focuses on how forcing profiles can affect the theoretical and experimental results previously ob...

When a fluid system is subject to strong rotation, centrifugal fluid motion is expected, i.e., denser (lighter) fluid moves outward (inward) from (toward) the axis of rotation. Here we demonstrate, both experimentally and numerically, the existence of an unexpected outward motion of warm and lighter vortices in rotating thermal convection. This ano...

We present high-precision experimental and numerical studies of the Nusselt number Nu as functions of the Rayleigh number Ra in geostrophic rotating convection with the domain aspect ratio Γ varying from 0.4 to 3.8 and the Ekman number Ek varying from 2.7×10−5 to 2.0×10−7. With decreasing Ra our heat-transport data Nu(Ra) reveal a gradual transitio...

We report an experimental study of Lagrangian velocity and acceleration in turbulent Rayleigh-Bénard convection using particle tracking velocimetry, with the Rayleigh number Ra spanning from 5.4×108 to 1.3×1010. The measurements were made in two representative regions of a cylindrical convection cell with aspect ratio unity, where an abundant amoun...

We consider the situation of a misalignment between the global temperature gradient and gravity in thermal convection. In such a case an effective horizontal buoyancy arises that will significantly influence the transport properties of heat, mass and momentum. It may also change the flow morphology in turbulent convection. In this paper, we present...

When a fluid system is subject to strong rotation, centrifugal fluid motion is expected, i.e., denser (lighter) fluid moves outward (inward) from (toward) the axis of rotation. Here we demonstrate, both experimentally and numerically, the existence of an unexpected outward motion of warm and lighter vortices in rotating turbulent convection. This a...

Brownian motion of particles in fluid is the most common form of collective behavior in physical and biological systems. Here, we demonstrate through both experiment and numerical simulation that the movement of vortices in a rotating turbulent convective flow resembles that of inertial Brownian particles, i.e., they initially move ballistically an...

We present high-precision experimental and numerical studies of the Nusselt number $Nu$ as functions of the Rayleigh number $Ra$ in geostrophic rotating convection with domain aspect ratio ${\Gamma}$ varying from 0.4 to 3.8 and the Ekman number Ek from $2.0{\times}10^{-7}$ to $2.7{\times}10^{-5}$. The heat-transport data $Nu(Ra)$ reveal a gradual t...

We present an investigation of the root-mean-square (r.m.s.) temperature $\unicode[STIX]{x1D70E}_{T}$ and the r.m.s. velocity $\unicode[STIX]{x1D70E}_{w}$ in the bulk of Rayleigh–Bénard turbulence, using new experimental data from the current study and experimental and numerical data from previous studies. We find that, once scaled by the convectiv...

We present an experimental study of the reversal of the large-scale circulation (LSC) in quasi-two-dimensional turbulent Rayleigh–Bénard convection. It is found that there exists a transition in the Rayleigh number ( $Ra$ ) dependence of the reversal rate $f$ with two distinct scalings: for $Ra$ less than a transitional value $Ra_{t}$ , the non-dim...

A primary objective in turbulent thermal convection research is to understand and control the heat transport scaling behaviour. Previous studies have shown that the heat transport can be tuned by manipulating the boundary layer topographies with monoscale roughness elements. Now, Zhu et al. ( J. Fluid Mech. , vol. 869, 2019, R4) have demonstrated t...

We present an experimental and numerical study of natural convection with moist air as convecting fluid. By simplifying the system as two-component convection, an experimental method is proposed for indirectly measuring the moisture transfer rates in buoyancy-driven flows. We verify the results using direct numerical simulations. It is found that t...

We present an investigation of the root-mean-square (rms) temperature $\sigma_T$ and the rms velocity $\sigma_w$ in the bulk of Rayleigh-B\'enard turbulence, using new experimental data from the current study and experimental and numerical data from previous studies. We find that, once scaled by the convective temperature $\theta_*$, the value of $...

We present a numerical study of quasistatic magnetoconvection in a cubic Rayleigh–Bénard (RB) convection cell subjected to a vertical external magnetic field. For moderate values of the Hartmann number $Ha$ (characterising the strength of the stabilising Lorentz force), we find an enhancement of heat transport (as characterised by the Nusselt numbe...

In rotating Rayleigh-B\'enard convection, columnar vortices advect horizontally in a stochastic manner. When the centrifugal buoyancy is present the vortices exhibit radial motions that can be explained through a Langevin-type stochastic model. In a centrifugation-dominant flow regime anomalous outward motion of cyclones is observed which is contra...

Vortices play an unique role in heat and momentum transports in astro- and geo-physics, and it is also the origin of the Earth's dynamo. A question existing for a long time is whether the movement of vortices can be predicted or understood based on their historical data. Here we use both the experiments and numerical simulations to demonstrate some...

We present a numerical study of quasistatic magnetoconvection in a cubic Rayleigh-B\'enard (RB) convection cell subjected to a vertical external magnetic field. For moderate values of the Hartmann number Ha, we find an enhancement of heat transport. Furthermore, a maximum heat transport enhancement is observed at certain optimal $Ha_{opt}$. The enh...

We report an experimental measurement of the rms temperature (σT) profiles in two regions inside a large aspect ratio (Γ=4.2) rectangular convection cell. It is found that, in the region where the boundary layer is sheared by a large-scale wind, σT has a power-law dependence on the vertical distance (z) from the plate, whereas in the region where p...

In scalar turbulence it is sometimes the case that the scalar diffusivity is smaller than the viscous diffusivity. The thermally-driven turbulent convection in water is a typical example. In such a case the smallest scale in the problem is the Batchelor scale lb, rather than the Kolmogorov scale lk, as lb=lk/Sc1/2, where Sc is the Schmidt number (o...

We report an experimental observation of a flow topology transition via global bifurcation in a turbulent Rayleigh-Bénard convection. This transition corresponds to a spontaneous symmetry breaking with the flow becomes more turbulent. Simultaneous measurements of the large-scale flow (LSF) structure and the heat transport show that the LSF bifurcat...

In scalar turbulence it is sometimes the case that the scalar diffusivity is smaller than the viscous diffusivity. The thermally-driven turbulent convection in water is a typical example. In such a case the smallest scale in the problem is the Batchelor scale $l_b$, rather than the Kolmogorov scale $l_k$, as $l_b = l_k/Sc^{1/2}$, where Sc is the Sc...

A critical ingredient of a meridional overturning circulation (MOC) is vertical mixing, which causes dense waters in the deep sea to rise throughout the stratified interior to the upper ocean. Here, we report a laboratory study aimed at understanding the contributions from surface thermal forcing (STF) to this mixing process. Our study reveals that...

We study, using direct numerical simulations, the effect of geometrical confinement on heat transport and flow structure in Rayleigh-B\'enard convection in fluids with different Prandtl numbers. Our simulations span over two decades of Prandtl number $Pr$, $0.1 \leq Pr \leq 40$, with the Rayleigh number $Ra$ fixed at $10^8$. The width-to-height asp...

We present systematic investigation of the effects of roughness geometry on turbulent Rayleigh-B\'enard convection (RBC) over rough plates with pyramid-shaped and periodically distributed roughness elements. Using a parameter $\lambda$ defined as the height $h$ of roughness element over its base width, heat transport, flow dynamics and local temper...

The effects of insulating lids on the convection beneath were investigated experimentally using rectangular convection cells in the flux Rayleigh number range 2.3 × 10 9 Ra F 1.8 × 10 11 and cylindrical cells in the range 1.4 × 10 10 Ra F 1.2 × 10 12 with the Prandtl number Pr fixed at 4.3. It is found that the presence of the insulating lids leads...

Many natural and engineering systems are simultaneously subjected to a driving force and a stabilizing force. The interplay between the two forces, especially for highly nonlinear systems such as fluid flow, often results in surprising features. Here we reveal such features in three different types of Rayleigh-B\'enard (RB) convection, i.e. buoyanc...

Motivated by a desire to understand the geothermal heating effects on ocean circulation, a large-scale circulation generated and sustained by thermal forcing at the surface subject to a small amount of heating from the bottom boundary is investigated through laboratory experiments. Despite its idealization, our experiments demonstrate that the lead...

We present experimental studies of higher-order modes of the flow in turbulent thermal convection in cells of aspect ratio ( $\unicode[STIX]{x1D6E4}$ ) 1 and 0.5. The working fluid is water with the Prandtl number ( $Pr$ ) kept at around 5.0. The Rayleigh number ( $Ra$ ) ranges from $9\times 10^{8}$ to $6\times 10^{9}$ for $\unicode[STIX]{x1D6E4}=1...

We study the effect of severe geometrical confinement in Rayleigh–Bénard convection with a wide range of width-to-height aspect ratio Γ , 1/128 Γ 1, and Rayleigh number Ra, 3 × 10 4 Ra 1 × 10 11 , at a fixed Prandtl number of Pr = 4.38 by means of direct numerical simulations in Cartesian geometry with no-slip walls. For convection under geometrica...

We report an experimental study on the statistical properties of the thermal plumes in turbulent thermal convection. A method has been proposed to extract the basic characteristics of thermal plumes from temporal temperature measurement inside the convection cell. It has been found that both plume amplitude A and cap width w, in a time domain, are...

Motivated by a desire to understand the geothermal heating effects on ocean circulation, a large-scale circulation generated and sustained by thermal forcing at the surface subject to a small amount of heating from the bottom boundary is investigated through laboratory experiments, motivated by understanding the geothermal heating effects on ocean...

We report an experimental study of confinement effects in quasi-2-D turbulent Rayleigh–Bénard convection. The experiments were conducted in five rectangular cells with their height
$H$
and length
$L$
being the same and fixed, while the width
$W$
was different for each cell to produce lateral aspect ratios (
${\it\Gamma}=W/H$
) of 0.6, 0.3,...

Coherent structures are ubiquitous in turbulent flows and play a key role in transport. The most important coherent structures in thermal turbulence are plumes. Despite being the primary heat carriers, the potential of manipulating thermal plumes to transport more heat has been overlooked so far. Unlike some other forms of energy transport, such as...

We present experimental evidence that a minute amount of polymer additives can significantly enhance heat transport in the bulk region of turbulent thermal convection. The effects of polymer additives are found to be the enhancement of coherent heat fluxes and suppression of incoherent heat fluxes. The enhanced heat transport is associated with the...

We report the first experimental study of the influences of the thermal boundary condition on turbulent thermal convection. Two configurations were examined: one had a constant heat flux at the bottom boundary and a constant temperature at the top (CFCT cell); the other had constant temperatures at both boundaries (CTCT cell). In addition to produc...

Comparing to laminar flows, the most important effects of fluid turbulence are the greatly enhanced transport of mass, momentum and heat. With increased frictional drag from the container walls, one would ordinarily expect decreased transport of these quantities. Therefore, it comes as a surprise when heat transfer enhancement was observed as the c...

We report an experimental study of the large-scale circulation (LSC) reversal in quasi-2D turbulent thermal convection, in which the aspect ratio Γ (= height/length of a rectangular box) is used as a parameter to perturb the stability of the LSC. It is found that the mean time interval τ between two successive reversals increases strongly with incr...

We present an experimental investigation of turbulent thermal convection in a cylindrical cell with two superposed immiscible fluid layers, namely water layer above fluorinert FC-77 electronic liquid (FC77) layer. The flow dynamics and coupling are studied using a multi-thermal-probe method. It is found that while one large-scale circulation (LSC)...

Using the classical Rayleigh-Bénard convection, we show that a simple geometrical confinement can greatly increase the convective heat transfer efficiency, the Nusselt number Nu. The recent experimental and numerical studies by Huang et al. [7] showed that when the aspect ratio (lateral dimension over height) of the system is decreased from 0.6 to...

Geometrical confinement of turbulent Rayleigh-Bénard convection (RBC) in Cartesian geometries is found to reduce the local Bolgiano length scale in the centre of the cell LB,centre and can therefore be used to study cascade processes in the bulk of RBC. The dependence of L B,centre versus Γ suggests a cut-off to the local L B, which depends on the...

We present an experimental study of turbulent thermal convection with smooth and rough surface plates in various combinations. A total of five cells were used in the experiments. Both the global Nu and the Nu for each plate (or the associated boundary layer) are measured. The results reveal that the smooth plates are insensitive to the surface (rou...

The system of turbulent thermal convection is introduced. Progresses in recent decades in the four major areas of research in turbulent convection are briefly reviewed. Some of the recent trends of the field are then discussed, which also serve to point out that the future directions in this important field of fluid mechanics lie in the extension t...