| Résumé | Many authors refer to a threshold of surface roughness that can be considered 'Hydrodynamically Smooth' but few to none actually quantify this level of roughness although they preach its importance. The purpose of this experiment was to try and determine a quantifiable roughness level that actually is 'Hydrodynamically Smooth' in that any progression to a smoother surface offers not gains in performance. To attain this goal, three foils were tested in the Institute for Marine Dynamics' cavitation tunnel using the previously developed Viscous Drag dynamometer. The first fin tested was an existing control foil used on the C-Scout Autonomous Underwater Vehicle project. This fin was built using an injection-molded plastic stiffened with steel rods. Although the liquid plastic had been subjected to high vacuum before injection, tiny air bubbles persisted throughout the finished foil. When the surface of the foil was sanded to smoother surface roughnesses, these bubbles became apparent in the form of pits in the surface which confused the data, throwing doubt on whether the differences in observed hydrodynamic loading were due to the overall smoother finish of the bubbles. To rectify this problem, two new prismatic foils were built using very uniform high-density foam. These foils were again tested in the cavitation tunnel with specific attention being paid to the effects of roughness on Drag, Lift and stall angle. The results from this experiment look promising and, although more research is needed, several interesting conclusions can be drawn. Sanding a foil to a smoother finish can enhance performance in several ways: Firstly, a smoother finish decreased drag or retarding force when the foil is yawed with respect to the flow direction. In any design application, foils spend the bulk of their service life at some angle of yaw to the stream. A smooth surface can also increase lift and delay stall, a result that could certainly help to improve the versatility and efficiency of foils. Interesting data was also collected with respect to the roughness levels generated with sandpaper. A roughness meter was used to empirically determine the actual surface roughness of the foils after treatment with different grades of sandpaper. It was found that the obtained surface is not necessarily smoother for a finer grade of sandpaper. Also, the finish obtained by a given grade of paper depends entirely on the material being treated. |
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