Physics and physiology
© Cambridge University Press 2005. Physics of airflow Gas flowing through tubes can be characterized by possessing either laminar or turbulent flow, or more usually, a mixture of both. Before we explore these, we need to familiarize ourselves with a fundamental thermodynamic concept detailed below. In laminar flow, although gas molecules in different parts of the ‘stream’ have different velocities, the vectors of these velocities are parallel, as shown in Figure 3.1. For a viscous Newtonian fluid such as air, molecules can be thought of as being arranged in slippery ‘sheets’ or ‘layers’ and these have been schematically labelled 1, 2, 3, etc. in Figure 3.1. The sheet of molecules closest to the wall of the tube (sheet 1) is stationary and bound to the wall of the tube. Each sheet of molecules can exert a force on its neighbour (‘shear’ force) so that a slow moving sheet will tend to retard a quicker moving neighbour, and quicker moving sheets will tend to drag slower neighbours along. For example, the shear forces exerted on sheet 2 by the stationary sheet 1 would tend to retard it, but the shear force exerted by its other neighbour, the faster moving sheet 3, would tend to increase its velocity. The sheets of molecules in the middle of the stream (e.g. sheet 3) are the least influenced by the static layers at the edges, and so these have the highest velocity. The amount of grip which one sheet has on another (the shear force) is a property of the viscosity of the fluid. For low viscosity (i.e. slippery) fluids, the stationary boundary layer has little ‘grip’ on its neighbour and so the velocity of subsequent layers rises very quickly as one moves towards the centre of the stream.