The effect of negative shear on the transitional separated flow around a semi-circular leading edge

The effect of a negative free-stream mean-shear velocity distribution on the boundary layer development on a flat plate with a semi-circular leading edge is studied experimentally and computationally. The geometry is the same as in the T3L test case of the ERCOFTAC Special Interest Group on Transition. The existence of a negative shear is related to the transition of the boundary layer from laminar to turbulent through separation. The flow investigated here has the same general characteristics as the one presented in a recent work by the authors, Palikaras et al. [Int. J. Heat Fluid Flow 23 (2002) 455–470], where the boundary layer development has been studied under free-stream conditions of uniform and positive mean-shear velocity distributions. The negative shear flow in the core region of the wind tunnel has a value ∂U/∂y=−27.7 s−1, which is the opposite to the case examined by Palikaras et al. [Int. J. Heat Fluid Flow 23 (2002) 455–470]. In the first part of the paper, a detailed description of the flow is given. The measured quantities are presented, discussed and compared with the computational analysis in order to obtain a complete picture of the investigated flow. For the computations, the non-linear k–ε model of Craft et al. [Int. J. Heat Fluid Flow 17 (1996) 108–115] is used, and satisfactory predictions are reported. In the second part, a detailed comparison of the results with the cases of uniform and positive mean-shear velocity inlet distribution is carried out. In the case of negative mean velocity the separated boundary layer leads to a larger reverse flow region than the two other cases. A relation is observed between the location of the stagnation point at the leading edge and the presence or absence of shear. When mean shear is present, depending on the sign, there is a movement of the stagnation point away from the symmetry line of the flat plate and it is believed that this is the driving mechanism that affects the boundary layer development and the longitudinal size of the reverse flow region. This remark is supported by the observation that for all the three cases studied, the longitudinal RMS distribution above the reverse flow region and in the free-stream region has the same values.