OUR RESEARCH

Nonlinear Physics and Turbulence

Nonlinear physics is concerned with the study of instabilities, chaos, pattern formation, turbulence, and other complex dynamics in open systems driven away from thermal equilibrium. Nonlinear phenomena can be found in many fluid, solid, chemical, and biological systems. Our research is concerned mainly with non-equilibrium dynamics in flow systems. Our interests in this area concern with the statistical properties of fully developed turbulence, high Rayleigh number thermal convection, transport phenomena in turbulent flows, and pattern formation in rotating suspensions of non-Brownian particles. Dynamic light scattering, laser Doppler velocimetry (LDV), thermometry, and various imaging techniques are used in the experiments.

In recent years, we have studied temperature and velocity fluctuations in turbulent thermal convection. When a horizontal layer of fluid is heated from below, Rayleigh–Bénard convection will occur if the temperature difference DT across the layer exceeds a critical value DTc. The control parameter in thermal convection is the Rayleigh number Ra, which is proportional to DT. In the past decades attention has been focused on Rayleigh–Bénard convection both in the chaotic regime, where Ra slightly exceeds the critical value Rac (~ DTc), and in the turbulent regime where Ra >> Rac. In the latter case, the temperature gradient across the fluid layer is concentrated in the upper and lower thermal boundary layers, and the flow in the central region is turbulent. Our experiments focus on the statistical properties of high-Ra turbulent convection in water.

The recent discovery of scaling laws in the heat flux and temperature statistics in turbulent convection has stimulated considerable theoretical and experimental efforts, aimed at explaining the observed scaling laws in the temperature field. The theoretical calculations arrive at similar conclusions for the temperature field, but have different assumptions and predictions for the velocity field in turbulent bulk regions and near viscous and thermal boundary layers. Direct measurements of the velocity field, therefore, become important to verify assumptions and test predictions of various theoretical models. In contrast to the great number of temperature measurements, experimental information about velocity fluctuations and their statistics in turbulent convection is limited. The lack of the velocity information is partially due to the fact that the conventional methods for measuring velocity, such as hot-wire anemometry, are not suitable for thermal turbulence.

To overcome the experimental difficulties, we have developed new laser light scattering techniques to probe the local velocity and its fluctuations in turbulent convection. With these new techniques, we measure local-velocity fluctuations in the central region and the mean velocity profile near the boundary at various Ra ranging from 107 to 1011. In an attempt to further understand the structure and dynamics of thermal plumes, we have recently carried out a systematic study of the temperature and velocity fields in turbulent convection. Using the techniques of laser Doppler velocimetry (LDV), thermometry, and flow visualization, we map out the temperature and velocity structures in the plane of the large-scale circulation. These temperature and velocity measurements are conducted in convection cells with different aspect ratios and over varying Rayleigh numbers and spatial positions across the entire cell. Water is used as the convecting fluid, in which the temperature and velocity measurements can be made simultaneously with high accuracy. These local measurements provide a body of reliable velocity and temperature data and complement the global measurements of heat transport in turbulent convection.

We have also carried out a novel convection experiment in a cell with rough upper and lower surfaces. The experiment reveals that the main effect of the surface roughness is to increase the emission of thermal plumes, which enhances the heat transport across the cell. These measurements are of fundamental interest for understanding the nature of convective turbulence and are also relevant to many engineering applications for efficient heat transfer. Understanding the heat transport phenomena in turbulent convection under different boundary conditions will certainly shed new light on technological improvements in various industrial applications ranging from heat exchangers to reentry vehicles in the space flight.