Transport of a passive scalar at a shear-free boundary in fully developed turbulent open channel flow

Direct numerical simulations of fully developed turbulence in an open chann el geometry were performed in which a passive scalar was introduced. The si mulations were intended to explore transport at free surfaces in two cases for which (1) the free surface was maintained at constant temperature and ( 2) the interfacial flux was fixed. These cases can be considered models for mass and evaporative heat transport where buoyancy and surface deformation effects are negligible. Significant differences were found in the thermal fields in these two cases. The turbulent statistics reveal that the surface flux in the constant temperature case was significantly more intermittent compared to the surface temperature field in the constant flux case. The su rface temperature field in the latter case formed large patches of warm flu id, reminiscent of the so-called fish scale patterns revealed in recent inf rared imagery of the air-water interface. The wake-like structure of the pa tches was evident despite the absence of surface shear. A model of surface renewal based on the existence of two disparate time scales (a fast hydrody namic scale, and a slow, diffusional scale) was introduced to explain these differences in a heuristic manner. The model appears successful in explain ing, in a qualitative sense, the surface thermal structure in each case. Co rrelations between the surface thermal fields (flux or temperature) and the subsurface hydrodynamics were also computed. A model based on the hypothes is that hairpin eddies are the dominant kinematic structure responsible for surface renewal is consistent with the correlations. However, these result s cannot rule out the importance of other turbulent structures in free surf ace heat and mass transport processes. (C) 1999 American Institute of Physi cs. [S1070-6631(99)00309-8].