Trips and Surface Roughness
As amply demonstrated in the airfoil polars, turbulators placed upstream of the laminar separation bubble may improve performance. While it is known that some airfoils benefit from trips at and above a Rn of 300k, for the airfoils tested in this study, trips were generally effective only below 300k. It is possible that different heights and chordwise locations might further improve the airfoils tested, but this was not systematically studied in this project.
For Rn below 300k performance gains were achieved, but these gains were highly dependent on the trip height and position. In addition, no “miracle” trip shape was found. Rather, it appears that if the boundary layer was sufficiently tripped, regardless of how, the performance gain was independent of the particular method. The conclusion is that elaborate turbulator devices are not warranted; the simple two-dimensional trip strip is satisfactory. We should point out that other investigators31 have found differences in performance and recommend specific types of trips.
The MILEY airfoil and the SD7003 are two extreme examples of the effectiveness of trips. The MILEY airfoil, as may be deduced from the upper-surface shape, has a steep pressure recovery region. Consequently the laminar flow separates to form a large bubble. The polars show that trips, when placed upstream of this point, dramatically improve the performance.
The SD7003 represents the other extreme. On this airfoil the pressure recovery region is so gradual that most of the upper surface may be considered a bubble ramp. As a result, the bubble is so shallow and reattaches so quickly that the drag of a trip is about the same as the drag of the bubble it might prevent. At a Rn of 60k, where one would normally expect bubble drag to be high, the SD7003 had the lowest drag of any airfoil tested, and over the entire Rn range the presence of a bubble cannot be deduced from the polars alone (which is in sharp contrast to the MILEY airfoil).
Finally, the usefulness of a trip (on either surface) depends on the severity of the pressure recovery. Since the newer, low-drag airfoils presented here generally have a gradual recovery, one would expect trips to be of marginal value. Nonetheless trips are useful to “repair” an otherwise poor performer, typically an airfoil designed for a higher Rn. When they are properly used in this context they can produce substantial improvements. Furthermore some low-fEn airfoils are designed expressly for use with trips31. In these cases trips are, of course, essential.
The effects of a discrete trip can also be produced by distributed roughness. Much like single roughness elements—trips strips, bumps, blowing through holes—distributed roughness acts to promote transition. In these experiments “sheet balsa roughness” (no plastic film covering) was tested and, as expected, produced results qualitatively like that found for the trip strips. However, distributed roughness apparently does not provide any advantage over discrete roughness elements.