General Introduction

At a time when the dramatic progress in computer performance and numerical methods enable the resolution of the Navier–Stokes equations giving a remarkably accurate prediction of air motion (at least in the usual conditions of aerodynamics), the question may be raised of the utility of the concepts and theories developed in the present book. Is it not sufficient to present the general motion equations (established in Chapter 4) and adequate developments on numerical techniques? Indeed, there exist codes to faithfully compute complex flows, as in a separated propulsion nozzle, past a transonic airfoil, and even around a complete aircraft. Sometimes, we call this approach numerical aerodynamics to distinguish between experimental and theoretical aerodynamics. However, we can put forward some arguments to justify our approach.

The solution of the Navier–Stokes equations is still far from being a panacea. Determination of a solution, including accurately taking into account turbulent motion, is still out of reach because of the vast amount of calculation resources that would require the prediction of the flow, even past simple shapes. This ultimate weapon, known as Direct Numerical Simulation (DNS), is still at a developmental stage and only applied to basic flow cases. Thus, we have to rely on some turbulence modeling that is frequently uncertain and not always reliable in complex flows. Even with the use of a turbulence model (we speak of the RANS approach: ...

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