Vortical and thermal interfacial layers in wall-bounded turbulent flows under transcritical conditions

The outer region of fully developed turbulent boundary layers can be viewed as a collection of uniform momentum zones separated by thin (but finite-thickness) shear layers referred to as momentum internal interface layers (MIILs). We first show the existence of such interfacial layers under transcritical thermodynamic conditions and introduce their thermal counterpart, named uniform thermal zones (UTZs), based on temperature. The UTZs, and the corresponding thermal internal interfacial layers (TIILs), are studied for a database of turbulent channel flow at transcritical thermal conditions [K. Kim et al., J. Fluid Mech. 871, 52 (2019)]. The thermal and vortical interfaces are identified using a recently proposed clustering approach by Fan et al. [D. Fan et al., J. Fluid Mech. 872, 198 (2019)]. It is shown that the MIILs and TIILs are correlated, but not collocated; their location is related to the underlying turbulent structures in the flow. On average, the TIILs are located about halfway between the wall and the MILL, and the relationship between these layers is studied from the perspective of the attached eddy model. Under high near-wall thermal gradients the pseudoboiling line and the outer MIIL are collocated, which is explained using a shear stress balance analysis. Ultimately, the study of the thermal layering permits a simplification of the wall scaling in the presence of complex transcritical thermodynamics.