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Basics of Fluid Mechanics, 2014a

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CHAPTER 8<br />

Differential Analysis<br />

8.1 Introduction<br />

The integral analysis has a limited accuracy, which leads to a different approach <strong>of</strong> differential<br />

analysis. The differential analysis allows the flow field investigation in greater<br />

detail. In differential analysis, the emphasis is on infinitesimal scale and thus the analysis<br />

provides better accuracy 1 . This analysis leads to partial differential equations which are<br />

referred to as the Navier-Stokes equations. These equations are named after Claude–<br />

Louis Navier–Marie and George Gabriel Stokes. Like many equations they were independently<br />

derived by several people. First these equations were derived by Claude–Louis–<br />

Marie Navier as it is known in 1827. As usual Simon-Denis Poisson independently, as<br />

he done to many other equations or conditions, derived these equations in 1831 for<br />

the same arguments as Navier. The foundations for their arguments or motivations are<br />

based on a molecular view <strong>of</strong> how stresses are exerted between fluid layers. Barré de<br />

Saint Venant (1843) and George Gabriel Stokes (1845) derived these equation based<br />

on the relationship between stress and rate–<strong>of</strong>–strain (this approach is presented in this<br />

book).<br />

Navier-Stokes equations are non–linear and there are more than one possible<br />

solution in many cases (if not most cases) e.g. the solution is not unique. A discussion<br />

about the “regular” solution is present and a brief discussion about limitations when<br />

the solution is applicable. Later in the Chapters on Real <strong>Fluid</strong> and Turbulence, with<br />

a presentation <strong>of</strong> the “non–regular” solutions will be presented with the associated<br />

issues <strong>of</strong> stability. However even for the “regular” solution the mathematics is very<br />

complex. One <strong>of</strong> the approaches is to reduce the equations by eliminating the viscosity<br />

effects. The equations without the viscosity effects are referred to as the ideal flow<br />

equations (Euler Equations) which will be discussed in the next chapter. The concepts<br />

1 Which can be view as complementary analysis to the integral analysis.<br />

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