Bulletin Spring‧Summer 2001

Two Key Issues in Turbulence Research I n t u r b u l e n t f l o w s , the p h y s i c a quantities of interest, such as velocity, pressure, and temperature , display irregular and complex temporal and spatial fluctuations. A key issue i n f u n d ame n t a l studies o f turbulence is to make sense of these complex fluctuations. H o w e v e r , i t is h i g h l y d i f f i c u l t to derive quantitative results for probability distribution of a turbulent quantity from the equations of fluid motion. One feature o f turbulence is that it is composed o f eddies or vortices. Larg e vortices continually break up into small ones, wh i ch in turn break up into even smaller ones, u n t il the effec t of f l u i d v i s cos i ty dissipates the kinetic energy of the smallest v o r t i c es i n to heat. L ew i s R i c h a r d s on described this process in a well-circulated verse: Big whorls have little whorls, Which feed on their velocity, And little whorls have lesser whorls, And so on to viscosity. In 1941, Andrei Kolmogorov translated, using mathematical language, this picture of energy transfer from large-scale motion to small-scale motion int o a theory. Although his predictions have not been completely borne out by experiments, Kolmogorov's ideas have dominated turbulence research for more than 50 years. The deviations ar e believed to be associated w i th the uneven distribution of turbulent activity. Therefore, another major focus in turbulence research is to try to understand this intermittent nature of turbulence, that is, to solve the so-called intermittency problem. Research Results P r o f . E m i l y S . C . C h i n g o f t he Department o f Physics has been do i ng theoretical research work on turbulenc e since 1990. She was awarded a grant f r om the Research Grants Council (RGC) in 1995 for her f i r s t project at the University. The project, an extension of her doctoral work 1 , gave rise to a f r amewo r k for s t udy i ng turbulence using conditional statistics 2 . U s i n g this app r oach , the p r o b a b i l i t y d i s t r i bu t i on of an y physical quantit y o f interest (such as velocity, pressure, and temperature) is obtained exactly in terms of two conditional averages, which are taken upon satisfaction o f specific conditions. This framework has attracted a lot of interest, and has been extensively applied in the analyses of turbulent experiments. Interesting general features o f different turbulent flows have since been discovered, and the deviation o f the probability distribution of temperature fluctuations in thermal convection from a Gaussian has been understood. Recently, this framework has also found applications i n other systems such as in the analysis of the Hang Seng Index. Sc i en t i s t s g e n e r a l l y b e l i e v e that understanding of the intermittent natur e of Prof. Emily S.C . Ching obtained her B. Sc. from the University of Hong Kong in 1986 and her M.Phil from The Chinese University of Hong Kong in 1988. She then went abroad to the United States where she received her Ph.D. from the University of Chicago in 1992. After doing post-doctoral work in the Instit Physics at the University of California, Santa Barbara, she joined The Chinese University of Hong Kong as lecturer in 1995. In recognition of her contributions to the understanding of the complex fluctuations in turbulence. Prof. Ching was awarded the 1999 Achievement in Asia Award by the Overseas Physics A s s o c i a t i o n . Prof. Ching is a theoretical physicist and her research interests are non-equilibrium nonlinear systems, in particular, fluid turbulence and fracture dynamics. Turbulence: A 19th Century Problem with a Challenge for the 21st 35