A Comparison of Spreading Angles of Turbulent Wedges in Velocity and Thermal Boundary Layers

Turbulent wedges induced by a three-dimensional surface roughness placed in a laminar boundary layer over a flat plate were visualized for the first time using both shear-sensitive and temperature-sensitive liquid crystals. The experiments were carried out at zero pressure gradient and two different levels of favorable pressure gradients. The purpose of this investigation was to examine the spreading angles of turbulent wedges indicated by their associated surface shear stresses and heat transfer characteristics and hence obtain further insight about the difference in the behavior of transitional momentum and thermal boundary layers when a streamwise pressure gradient exists. It was found that under a zero pressure gradient the spreading angles indicated by the two types of liquid crystals are the same, but the difference increases as the level of favorable pressure gradient increases with the angle indicated by temperature-sensitive liquid crystals being smaller. The results from the present study suggest that the spanwise growth of a turbulent region is smaller in a thermal boundary layer than in its momentum counterpart and this seems to be responsible for the inconsistency in transition zone length indicated by the distribution of heat transfer rate and boundary layer shape factor reported in the literature. This finding would have an important implication to the transition modeling of thermal boundary layers over gas turbine blades.