#Airfoil reviews series
#Airfoil reviews free

#Airfoil reviews series

To understand the mechanical factors that affect the transformation of the structure of the LSB, a series of experimental studies have been carried out.

#Airfoil reviews free

(2) The ratio of the laminar to the transition region in the LSB is 1.6–3.įor the LSB, in addition to the separation of the boundary layer caused by the strong adverse pressure gradient, the following known conditions (Reynolds number and turbulent intensity of free flow) can affect the formation of the separation bubble. 24 Marxen and Henningson 31 improved the criterion used to distinguish between short and long bubbles and summarized the following characteristics of short bubbles: (1) The influence of the bubble on potential flow is limited and local. In general, “short” LSBs affect only the local pressure distribution, and “long” LSBs can affect the pressure distribution over the entire chord. 21 “Short” bubbles have lengths of only a few percent points of the chord, about 10 2 δ s * to 10 3 δ s *⁠, where δ s * is the displacement thickness at the separation, while long bubbles may cover 20%–30% of the entire airfoil, about 10 4 δ s *⁠, and form from the trailing edge of the airfoil. The results shown in Table I represent the relationship between different types of bubbles and the thickness of the displacement. 29 The LSB can be classified into several types (Owen and Klanfer 30), and “short” and “long” LSBs depend on the length of the bubble in comparison with the thickness of the displacement in it during separation. The classical semi-empirical model of the LSB is not perfect for example, there is not simply one type of structure of the LSB at a low Reynolds number. This region is usually called the “dead air region.” In the latter part of the bubble, the stronger circulation velocity and an abrupt reattachment of the shear layer lead to a rapid increase in surface pressure. As a result, the gradient of streamwise pressure is nearly zero and results in the plateau pressure being distributed.

24–26 Near the separation point, the recirculation region is characterized by slow-moving reverse flow. This pressure distribution is clearly different from that of inviscid flow. 3), the distribution of plateau pressure from the separation point to the transition point can be observed, and the pressure rapidly recovers from the transition point to the reattachment point. Combined with the pressure distribution (see Fig. The region of recirculation flow can be divided into two sub-regions relative to the average position of transition. 23 The transition in the separated shear layer is reduced to a point rather than a region. It is described by the mean dividing streamline, which intersects the surface at the separation and the reattachment points. The classic two-dimensional (2D) schematic diagram of the laminar separation bubble is shown in Fig. However, because such flows involve many problems, such as the mechanism of the triggering effect of laminar flow separation, the transition mechanism, and complex nonlinear aerodynamic characteristics, few systematic reviews of the issue are available.

Some excellent reviews 18,19 of flows over an airfoil at low Reynolds numbers have been published. Mueller 17 noted in a review that laminar separation bubbles have a significant influence on the aerodynamic performance of airfoils at low Reynolds numbers and it is necessary to accurately predict the occurrence and evolution of the separation bubbles when designing highly efficient low-speed airfoils. Lissaman 5 subsequently summarized the problems of an airfoil at a low Reynolds number based on mechanical theory, experimental research, and theoretical design and concluded that flow over the airfoil in this case is closely related to separation and transition. 16 It describes the separation, transition, reattachment, and other physical phenomena in the laminar separation bubble (LSB). The theoretical basis for the aerodynamics at a low Reynolds number is the laminar separation bubble (LSB) theory proposed by Horton et al. Flow at a low Reynolds number has been widely researched as a branch of aerodynamics.