Characteristics of Transitional Separation Bubble Formed Over Negatively and Positively Skewed Anisotropic Rough Surface

Effect of leading-edge anisotropic erosion (negative skewness) and deposition (positive skewness) on a transitional separation bubble (TSB) is investigated using single-wire hot wire anemometer for a Reynolds number (Rec) of \(2.5 \times 10^{5}\) , , based on the model chord and tunnel free stream velocity. Three different models are fabricated for investigation, whose surfaces are smooth (SS), sand blasted (BS) and sand deposited (DS), respectively. The fabricated rough surfaces are three-dimensionally irregular and replicate the anisotropic pattern of an operationally deteriorated turbine blade due to chronic exposure to harsh working environments. Results of positive and negative skewed rough surfaces are compared with that of smooth surface. The bubble is observed to be shortened by about 35% and 10% of that formed over SS for DS and BS, respectively. Premature transition is observed on DS, which is 49% earlier than BS case. Though the transition point varies, it is interesting to note that the chord length occupied by the transition and turbulent region of the bubble is almost same for all the cases. Presumably, more than the negatively skewed anisotropic rough surface, the positive counterpart tends to influence the boundary layer stability and thereby the transition characteristic of separation bubble.