Skin friction fields from TSP surface patterns on an underwater cylinder in crossflow

The relationship between the Temperature Sensitive Paints (TSP) surface pattern and the corresponding relative skin-friction field is investigated for an underwater cylinder in crossflow at subcritical regime. The temperature map evolution can be decomposed in the contribution of a time-averaged, a phase-averaged and a random component. The first two contributions are discussed here. The asymptotic form of the energy equation at wall, solved by following the variational approach, provides relative skin friction fields from the TSP surface temperature maps. Results about the spatial distribution of the time averaged relative skin-friction profiles are compared with literature data. The comparison shows an excellent agreement on the whole laminar boundary layer up to the laminar separation line. Downstream, TSP-related results capture the secondary reattachment/separation events, which are lost in the literature data. Phase-averaged skin friction sequences capture the evolution of the laminar separation, of the turbulent reattachment and of the final separation, providing a description of the Laminar Separation Bubble (LSB) over the whole shedding period. Evolution with Re of the time averaged position of the LSB is in agreement with literature data