Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgments
- 1 A Preview
- 2 Basic Principles
- 3 Unidirectional and One-Dimensional Flow and Heat Transfer Problems
- 4 An Introduction to Asymptotic Approximations
- 5 The Thin-Gap Approximation – Lubrication Problems
- 6 The Thin-Gap Approximation – Films with a Free Surface
- 7 Creeping Flow – Two-Dimensional and Axisymmetric Problems
- 8 Creeping Flow – Three-Dimensional Problems
- 9 Convection Effects in Low-Reynolds-Number Flows
- 10 Laminar Boundary-Layer Theory
- 11 Heat and Mass Transfer at Large Reynolds Number
- 12 Hydrodynamic Stability
- Appendix A Governing Equations and Vector Operations in Cartesian, Cylindrical, and Spherical Coordinate Systems
- Appendix B Cartesian Component Notation
- Index
4 - An Introduction to Asymptotic Approximations
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- Preface
- Acknowledgments
- 1 A Preview
- 2 Basic Principles
- 3 Unidirectional and One-Dimensional Flow and Heat Transfer Problems
- 4 An Introduction to Asymptotic Approximations
- 5 The Thin-Gap Approximation – Lubrication Problems
- 6 The Thin-Gap Approximation – Films with a Free Surface
- 7 Creeping Flow – Two-Dimensional and Axisymmetric Problems
- 8 Creeping Flow – Three-Dimensional Problems
- 9 Convection Effects in Low-Reynolds-Number Flows
- 10 Laminar Boundary-Layer Theory
- 11 Heat and Mass Transfer at Large Reynolds Number
- 12 Hydrodynamic Stability
- Appendix A Governing Equations and Vector Operations in Cartesian, Cylindrical, and Spherical Coordinate Systems
- Appendix B Cartesian Component Notation
- Index
Summary
Although the full Navier–Stokes equations are nonlinear, we have studied a number of problems in Chap. 3 in which the flow was either unidirectional so that the nonlinear terms u · ∇u were identically equal to zero or else appeared only in an equation for the cross-stream pressure gradient, which was decoupled from the primary linear flow equation, as in the 1D analog of circular Couette flow. This class of flow problems is unusual in the sense that exact solutions could be obtained by use of standard methods of analysis for linear PDEs. In virtually all circumstances besides the special class of flows described in Chap. 3, we must utilize the original, nonlinear Navier–Stokes equations. In such cases, the analytic methods of the preceding chapter do not apply because they rely explicitly on the so-called superposition principle, according to which a sum of solutions of linear equations is still a solution. In fact, no generally applicable analytic method exists for the exact solution of nonlinear PDEs.
The question then is whether methods exist to achieve approximate solutions for such problems. In fluid mechanics and in convective transport problems there are three possible approaches to obtaining approximate results from the nonlinear Navier–Stokes equations and boundary conditions.
First, we may discretize the DEs and boundary conditions, using such formalisms as finite-difference, finite-element, or related approximations, and thus convert them to a large but finite set of nonlinear algebraic equations that is suitable for attack by means of numerical (or computational) methods.
- Type
- Chapter
- Information
- Advanced Transport PhenomenaFluid Mechanics and Convective Transport Processes, pp. 204 - 293Publisher: Cambridge University PressPrint publication year: 2007