Experiments were conducted to understand the physical mechanisms that lead to the formation of scallops on swept wings. Icing runs were performed on a NACA 0012 swept wing tip at 45{dollar}spcirc{dollar}, 30{dollar}spcirc{dollar}, and 15{dollar}spcirc{dollar} sweep angles. A baseline case was chosen and direct measurements of scallop height and spacing, castings, video data. and close-up photographic data were obtained. The results show that the scallops are made of glaze ice feathers that grow from roughness elements that have reached a minimum height and are located beyond a given distance from the attachment line. This distance depends on tunnel conditions and sweep angle, and is the critical parameter in the formation of scallops. It determines (together with the shape of the streamlines) whether complete scallops, incomplete scallops or no scallops are going to be formed. The mechanisms of growth for complete and incomplete scallops are identified. The effects of velocity, temperature and liquid water content on scallop formation were studied. The possibility that cross flow instability may be the physical mechanism that triggers the growth of roughness elements into glaze ice feathers is examined. Additional icing runs were carried out with a NLF(2)-0415 airfoil at a 45{dollar}spcirc{dollar} sweep angle, and angles of attack of {dollar}-{dollar}4{dollar}spcirc{dollar}, 0{dollar}spcirc{dollar}, and +4{dollar}spcirc{dollar}, to study scallop formation and the change of scallop spacing and height with LWC and ice accretion time. Flow visualization runs using a chemical sublimation technique that employed Naphthalene were conducted on the NACA 0012 swept wing tip airfoil and on the NLF(2)-0415 airfoil. In each case transition location was measured and found to decrease with Reynolds number.
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