Separation: Some Relevant Boundary-Layer Properties, Interaction Issues, and Drag

Abstract

This chapter is devoted to an introduction to some of the concepts used when dealing with separated and vortical flow.We note first that the flow problems we are dealing with in general are Galilean invariant [1]. Therefore we can consider a flight vehicle in our mathematical models and in ground simulation (computational simulation, ground-facility simulation) in a fixed frame with the air-stream flowing past it. In the reality the vehicle moves through the-quasi-steady and quasi-uniform-atmosphere.In passing we also note that usually the Eulerian description of the flow is employed, which in contrast to the Lagrangian description considers the flow at every fixed point in space as steady or unsteady flow. For the different possible formulations of the governing equations see, e.g., [2,3].The flow field in our perceived reality (Model 1 in Sect. 1.5) is a one-domain entity. In aerodynamics and fluid mechanics, however, that entity is differentiated into several domains: the uniform far field, the inviscid flow field past the vehicle, the boundary layer flow at the vehicle's surface, the separated flow, the wake.The classical one-domain models to describe inviscid flow fields are the modeled potential flow (Model 4) and the discrete modeled Euler flow (Model 8). General one-domain approaches to describe viscous flow fields are the Navier-Stokes (NS) methods (Model 9), the Reynolds-Averaged Navier-Stokes (RANS) methods (Model 10), and the scale-resolving methods (Model 11).Applying a boundary-layer method (modeled viscous flow, Model 2) means working with a two-domain model, because always an inviscid flow model-first-has to be employed. The recently evolved hybrid RANS/LES methods are two-domain models, too. Strong interaction models are three-domain approaches, see, e.g., [4,5].Considering the two-domain model "boundary layer flow and inviscid flow" leads to the topic of interaction. In this chapter we discuss the most important types of interaction between boundary-layer flow and inviscid flow.