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The Physics, Clinical Measurement and Equipment of Anaesthetic Practice for the FRCA$
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Patrick Magee and Mark Tooley

Print publication date: 2011

Print ISBN-13: 9780199595150

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199595150.001.0001

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Behaviour of Fluids (Liquids and Gases, Flow and Pressure)

Behaviour of Fluids (Liquids and Gases, Flow and Pressure)

chapter 7 Behaviour of Fluids (Liquids and Gases, Flow and Pressure)
The Physics, Clinical Measurement and Equipment of Anaesthetic Practice for the FRCA

Patrick Magee

Mark Tooley

Oxford University Press

A fluid can be either a liquid or a gas. Fluids exhibit different flow behaviours depending on their physical properties, in particular viscosity and density. Flow characteristics also depend on the geometry of the pipes or channels through which they flow, and on the driving pressure regimes. These principles can be applied to any fluid, and the complexity of the analysis depends on the flow regimes described in this section [Massey 1970]. Fluid flow is generally described as laminar or turbulent. Laminar flow, demonstrated by Osborne Reynolds in 1867, is flow in which laminae or layers of fluid run parallel to each other. In a circular pipe, such as a blood vessel or a bronchus, velocity within the layers nearest the wall of the pipe is least; in the layer immediately adjacent to the wall it is probably actually zero. In fully developed laminar flow, the velocity profile across the pipe is parabolic, as shown in Figure 7.1, and as discussed in Chapter 1. Peak velocity of the fluid occurs in the mid line of the pipe, and is twice the average velocity across the pipe at equilibrium, and layers equidistant from the wall have equal velocity. The importance of laminar flow is that there is minimum energy loss in the flow, i.e. it is an efficient transport mode. This is in contrast to turbulent flow, where eddies and vortices (flow in directions other than the predominant one) mean that energy in fluid transport is wasted in production of heat, additional friction and noise. The result is that the pressure drop required to drive a given flow from one end of the pipe to the other is greater in turbulent than in laminar flow. The shear stress τ, which is the mechanical stress between layers of fluid and between the fluid and the tube wall, is proportional to the velocity gradient across the tube (dv/dr) of the fluid layers. The constant of proportionality between these two variables is the dynamic viscosity, η.

Keywords:   Newtonian fluids, Rotameter, Torricellian vacuum, atmospheric pressure, diffusion, flow measurement, fluid flow, hyperbaric pressure, hypobaric pressure, isotonic fluids, laminar flow, manometers, oncotic pressure, osmolality, osmolarity, osmometers, osmosis, osmotic pressure, partial pressure, plasma osmolarity, turbulence, venturi flowmeter

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