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Introduction of the research works (Last Updated 2019-12-31)

 

The main research works I did in the past focuses on heat transport and dynamics of coherent structures in thermally-driven turbulence. They can be categrized into the following main research topics:

 

(a) Heat transport by thermally-driven turbulence


1. Manupulating heat transport scaling in turbulent convection using wall roughness

 One of the key issues in the study of turbulent thermal convection is to understand the heat transport efficiency by the convective flow. This is usually expressed in terms of a scaling relation between the dimensionless heat transprot efficiency, namely the Nusselt number Nu, and the dimensionless drivingh force of the system, i.e., the Rayleigh number Ra.

 

In this work, we demonstrate that the heat transport can be manupulated using wall roughness (see fig.1). We found three regimes of the heat transport scaling. The boundaries between different regimes depends on the typical length scales of the system, i.e., the roughness height, the thermal boundary layer thickness and the viscous boundary layer thickness.  In the heat transprot enhaced regime, we showed that the heat transport scaling could be manupulated using a roughness parameter $lambda$, which is defined as the inverse of the aspect ratio of the roughness element (see fig. 2).

 

Refrences: Xie and Xia, Journal of Fluid Mechanics, 825,573-599, 2017.

 

Fig.1 Photograph of the rough plate with roughness parameter $lambda$=0.5. (a) Top view; (b) Side view.

 

Fig.2 The exponent of the heat transport scaling with incresing the roughness parameter $lambda$. 

 

2. Heat transport enhancement by polymer additives

The effect of polymer induced drag reduction has been well-know since the pineer work by Toms in 1948. This technique has been utilized to enhace the oil transport in pipelines. The effects of polymer on turbulent convective flows is less studied. We showed, through combied velocity and temperature measurement, that polymer additives can significantly enhance the heat transport in the bulk of turbulent convection (see fig. 3). We also illustrate that the possible mechanism for such enahcement is that on one hand, polymer enhances the coherency of thermal plumes which are the fundenmenal heat carriers, and on the other hand, polymer reduces the random fluctuations that does not contribute to heat transport.

 

Reference:Xie, Huang, Funfschilling,  Li, Ni and Xia, Journal of Fluid Mechanics,784,R3, 2015.

Fig.3 The normalized time averaged heat transport in the bulk Jz(c)/Jz(0) as a function of polymer concentration c.

 

 

(b) Dynamics of coherent structures in turbulent flows

 

One facinating feature of fluid turbulence is the emergence of cohernt structures. These structures are mainly responsible for the momentum and heat transport of the systems. In turbulent thermal convection, the coherent sturcutres are in the form of a system-sized large scale circulation (LSC). This LSC is formed by the self-organization of another fundmental coherent sturucure, namely thermal plumes. 

 

1. Flow topology transition via global bifurcation

In this work, we present experimental evidences of flow topology transition from a high-symmetry state to a low- symmetry state via global bifurcation with the increase of turbulence level in thermally driven turbulence (see fig.4). The two states are characterized by different flow structures and distinctive heat transport efficiencies. This transition cor- responds to a spontaneous symmetry breaking in a system far away from equilibrium. In the transition zone, the system exhibits stochastic switching between two long- lived metastable states. As spontaneous symmetry breaking is one of the unifying themes in modern physics, such as in particle and condensed matter physics; the present finding could thus be of interest to a wider range of fields beyond fluid dynamics.

 

Reference: Xie, Ding and Xia, Physical Review Lettes, 120, 214501, 2018

Fig.4 Mean flow structure for (a) Ra=1.2e8 and (b) Ra=1.1e9.

 

2. Dynamics and flow coupling in two-layered turbulent thermal convection

Reference: Xie and Xia, Journal of Fluid Mechanics, 728, R1, 2013

Fig.5 Schematics of the themal-couling mode and the viscous coupling mode in two-layered turbulent thermal convection

 

3. Dynamics of the large-scale circulation in high-Prandtl-number turbulent thermal convection

 Reference: Xie, Wei and Xia, Journal of Fluid Mechanics, 717, 322-346, 2013