The performance of smooth-wall drag reducing outer-layer devices in rough-wall boundary layers
Abstract
The boundary layer drag on a 0.28 m x 0.91 m flat-plate was measured to evaluate the performance of smooth-wall drag reducing outer-layer devices in rough-wall boundary layers up to a chord Reynolds number, Re(c) of 17,000. The devices were a pair of thin flat ribbons placed in tandem. The wall conditions examined were smooth, transverse-groove roughnesses having cavity width/depth ratios of 0.7 (d-type) and 3 (k-type), and sandpaper roughness (k-type) of grit sizes 180, 50 and 36. The area-averaged wall-drag was reduced compared to the respective normal levels for all wall conditions. All k-type rough walls exhibited a comparable level of drag reduction (about 7 percent) which was lower than that in the smooth-wall. The d-type rough-wall exhibited a transitional behavior in that the drag reduction dropped from the smooth-wall level to that of the k-type roughnesses with increase in the flow speed. In further experiments on a 6 meter long sandpaper roughened wall and with a pair of symmetric airfoil devices in tandem at an Re(c) of 77,500, it was possible to recover the device drag penalty fully, but no net drag reduction was achieved. The apparent loss of performance of the devices in a rough-wall is explained in terms of their inability to alter the pressure component of the drag in a significant way.
- Publication:
-
American Institute of Aeronautics and Astronautics
- Pub Date:
- March 1985
- Bibcode:
- 1985shfc.confR....B
- Keywords:
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- Airfoils;
- Boundary Layer Flow;
- Drag Reduction;
- Low Speed Wind Tunnels;
- Surface Roughness Effects;
- Wall Flow;
- Angle Of Attack;
- Data Acquisition;
- Flat Plates;
- Reynolds Number;
- Fluid Mechanics and Heat Transfer