1 - Introduction

Abstract

This chapter introduces the concept of turbulent flow, and explains its various features and measurement techniques. Turbulence is an irregular motion that generally makes its appearance in fluids, gases or liquids, when they flow past solid surfaces or even when neighboring streams of the same fluid flow past or over one another. Turbulent fluid motion is an irregular condition of flow in which various quantities show a random variation with time and space coordinates, so that statistically distinct average values can be discerned. Turbulent motion can occur at all speeds and under all sorts of conditions: in water, in hypersonic flow, in channels, in rocket nozzles, or on or near external surfaces such as airfoils. The Navier-Stokes equations and their reduced forms leading to Euler and boundary-layer equations are derived by considering flow and forces about an element of infinitesimal size, with the flow being treated as a continuum. It is observed that the oscilloscope trace of a hot wire placed in a stream flowing at 100mph would definitely show a far more gradual fluctuation if the average eddy were 3 ft in diameter than it would have had the average eddy been half-inch diameter.