Low Reynolds Number Terminology

The concepts of modern airfoil design and the attendant jargon are famil­iar mostly to specialists. In addition, some of the terms commonly used even by aerodynamicists have modified or expanded meanings when applied to low Reynolds number airfoils. For example, the concept of a bubble ramp is derived from a transition ramp, but they are not synonymous. To assist the general reader and to avoid confusion we here define those terms that are specific to low Reynolds number aerodynamics in order to supplement what can be found in textbooks15’16. We also have gone into greater detail in the earlier airfoil discus­sions in Section 5.1 so that the concepts will be familiar when they are referred to more briefly In the later ones.

3.1 Laminar Separation Bubbles

As described in Chapter 1, laminar separation takes place at low Rn due to the reluctance of the boundary layer to make a natural transition from laminar to turbulent flow on the airfoil surface. This type of separation and the subse­quent formation of a laminar separation bubble are the principal reasons for the degradation in airfoil performance with decreasing Rn.

At high Rn (greater than 1 million) a graph of С/ vs Cd for most airfoils shows a rounded appearance with the convex side towards the C axis. See References 15 and 17 for examples of polars at higher Rn’s. At low Rn the situation is often markedly different. Here, separation is a major factor and can contribute a large increment of drag not normally found at higher Rn’s. The effect on the shape of the polar is to produce a bulge in the mid-Cf range that is concave towards the C axis (see Fig. 12.13). Interestingly, with increasing Ci the drag decreases again just before stall.

Because of these effects, the shape of the polar clearly reveals the severity of the laminar separation bubble. For example, the E214 has a major problem with this at Rn’s of 60k and 100k. (This will be discussed in Section 5.1.) If one compares the tripped and untripped cases (Figs. 12.19 and 12.22) the difference in shape illustrates the difference in the separation—the polar changes from concave for the untripped (separated) case, to a more favorable convex shape for the tripped case.

Separation can, however, be minimized by proper design. For such airfoils, e. g. the SD7003, the graphs, even without trips, are more typical of those for higher Rn’s.