Structure of turbulence

Abstract

Studies of turbulent flow structure at Southampton over the past ten years have been primarily concerned with the quantitative description of those components concerned with aerodynamic noise generation and those concerned with the unsteady loading of structures. The aims of, and experimental approach to, individual experiments has changed over this period with the growth of knowledge and development of facilities. For example, excellent analogue facilities for the measurement of the statistical properties of turbulent flow structure have existed throughout the ten years while fast digital data processing equipment has recently provided what amounts to a new environment for our research. Initially the studies concerned statistical descriptions of the structure of turbulent jets and boundary layers based on time delayed cross-correlation measurements. These provide useful descriptions of practical existing flows but offer no obvious hints how these observed flows may be controlled or modified. More recently the aim has been to describe in more detail the physical structure of the individual eddies that constitute the turbulence and to clearly identify those features of the flow that led to the observed state. Three lines of active development are described, with some promising initial results. The first concerns time domain analysis of the velocity, displacement and pressure records from turbulent flows to identify the details of the characteristic eddy patterns in the flow. The second concerns potential flow modelling of impulsively started flows where arrays of elementary vortices represent vortex sheets and regions of concentrated vorticity. The third concerns modified Orr-Sommerfeld stability analysis of shear flow turbulence. These approaches are clearly complementary and so provide the basis of a comprehensive attack on the problem.