Trips and Bubble Ramps

Several means can be used to destabilize the laminar boundary layer and promote an early transition. The most direct means of doing this is through the airfoil shape. At higher Rni than those found on model aircraft, a short, gradual pressure recovery, called a transition or instability ramp, is sometimes used before the steeper main recovery. The purpose of the ramp in this case is to ensure that the boundary layer is fully turbulent and energetic before reach­ing the main pressure recovery. At low Rn, the transition ramp is still useful, although it needs to be longer, and may be more appropriately called a “bub­ble” ramp. The gradual pressure recovery of the ramp in this case shortens the length of the bubble by shortening the distance required for reattachment. As the Rn decreases, more and more of the airfoil surface is required for the bubble ramp18’19. In fact, for a Rn near 60k and at moderate lift coefficients, almost the entire upper surface of the airfoil is needed to ensure transition and subsequent reattachment.

Another method of inducing transition is through the use of a turbulator or trip. Typically, these are external ridges or bumps applied to the surface of the airfoil in a direction parallel to the span. They protrude into the boundary layer in such a way as to energize it sufficiently to promote transition. Many free-flight model aircraft employ turbulators to improve performance. Several detailed experiments, as summarized by Mueller4, have amply demonstrated that if an airfoil has high drag and hysteresis owing to a laminar separation bubble, a turbulator often alleviates these adverse effects by shortening the length of the bubble.

Currently at issue in the design of low Reynolds number airfoils are the fac­tors governing the use of bubble ramps and turbulators. At very low Reynolds numbers, turbulators may be better them bubble ramps, but as the Rn increases, the bubble shortens naturally. Thus, the turbulator becomes unnecessary and handicaps the performance by making transition happen too early. In this case the ramp would probably be better. In the middle of the low-iEn range, the ques­tion remains unclear. Is it better to use only a bubble ramp or only a turbulator or a combination of both? Usually airfoil turbulators are simple two-dimensional strips, but some three-dimensional trips (such as zig-zag tape and bump tape) have proved highly successful in application to modern, full-size sailplane airfoils. In our research, trips and ramps were used to begin to explore their respective benefits and operating regimes.