DNS of turbulent transport of scalar concentration in various thermally stratified boundary layers

Abstract

This paper presents phenomena involving the turbulent transport of concentration of passive contaminants in various thermally-stratified boundary layers by means of direct numerical simulation (DNS), because it is essential to know such detailed phenomena in a spatially-developing and buoyancyaffected boundary layer. The various thermally-stratified boundary layers have been simulated by DNS [1, 2]. The DNS of thermally-stratified boundary layers shows intriguing results, in which the counter-gradient diffusion phenomena (CDP) in turbulent momentum and heat transfer are observed in the stronglystratified stable boundary layer (SSBL), i.e., Reynolds shear stress and the wall-normal turbulent heat flux indicate a negative sign despite the positive gradients of mean velocity and temperature. It is well-known that CDP disturb the transport of turbulent quantities due to reductions of the effective diffusivities for momentum, heat and passive contaminants. On the other hand, the transport phenomena of the unstable boundary layer (UBL) is also revealed in the DNS, in which an enhancement of near-wall turbulence is obviously shown in the UBL. Meanwhile, passive contaminants such as suspended particulate matter (SPM) and smog which lead to environmental deterioration infrequently occur in the natural environment, or environmentally-safe passive contaminants are utilized for various purposes in industrial machinery. Thus, passive contaminants must be controlled in various situations, it is important to know that the concentration of passive contaminants is transported in the thermally-stratified boundary layer. Therefore, in the present study, the turbulent transport phenomena of concentrations of passive contaminants in various thermally-stratified boundary layers are investigated and explored using DNS. Two cases of discharge methods for passive contaminants are carried out in the DNS. In the first case, passive contaminants are provided from the wall in a short region. In the second case, the contaminants are supplied at the outer region of the turbulent boundary layer. Thus, the transport phenomena of concentration of passive contaminants affected by the thermally-stratified turbulent boundary layer are investigated in detail. Also, the effects of Schmidt numbers are revealed in the present DNS.KeywordsBoundary LayerDirect Numerical SimulationSuspended Particulate MatterTurbulent Boundary LayerSchmidt NumberThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.