Bulletin Spring‧Summer 2001

Turbulence A 19th Century Problem with a Challenge for the 21st What is Turbulence? L i q u i d s and gases flow, hence they are known as f l u i d s . C o m m o n examples o f f l u i ds are water and air. I t is often noticed that when the water tap is turned on slightly, the stream that comes out is smooth and clear, but when it is turned on further, the stream becomes sinuous and there is no steady flow p a t t e r n. These are two kinds of flows. When the fluid speed is slow, the flow is steady and l am i n a r . When it is fast, the flow becomes irregular and erratic. The latter is known as turbulence. Impor tant Physical Question Turbulence is the r u le rathe r than exception. Most fluid flows in nature and in engineering application s are turbulent . Hence turbulence is a problem o f practical interest. A solid grasp o f this u b i q u i t o u s p h e n ome n on w i l l , for example, a l l ow engineers to reduce the drag on automobiles or airplanes, and, as a result, save fuel or enhance the mixin g of fuel and oxidizer i n combustio n e n g i n e s , s u b s e q u e n t l y producing cleaner and more efficient combustion. Turbulence also poses challenging and profound questions for physics. How can such complex and chaotic behaviour occur whe n the physical laws that govern fluid motion are simple and well defined? Ho w can we completely and accurately describe in quantitative terms the properties of turbulent flows? In fact fluid motion equations were derived in the 19th century, and scientists have been studying turbulence since the last century. Yet today, in the 21st century, it still remains a great challenge i n physics. Laminar flow Turbulent flow