The objectives of this study are to reconstruct a turbulence model of both the large Eddy simulation (LES) and the Reynolds-averaged Navier–Stokes simulation (RANS) which can predict wind synopsis in various thermally stratified turbulent boundary layers over any obstacles. Hence, the direct numerical simulation (DNS) of various thermally stratified turbulent boundary layers with/without forward-step, two-dimensional block, or two-dimensional hill is carried out in order to obtain detailed turbulent statistics for the construction of a database for the evaluation of a turbulence model. Also, DNS clearly reveals the characteristics of various thermally stratified turbulent boundary layers with/without forward-step, two-dimensional block, or two-dimensional hill. The turbulence models employed in LES and RANS are evaluated using the DNS database we obtained. In the LES, an evaluated turbulence model gives proper predictions, but the quantitative agreement of Reynolds shear stress with DNS results is difficult to predict. On the other hand, the nonlinear eddy diffusivity turbulence models for Reynolds stress and turbulent heat flux are also evaluated using DNS results of various thermally stratified turbulent boundary layers over a forward-step in which the turbulence models are evaluated using an a priori method. Although the evaluated models do not make it easy to properly predict the Reynolds shear stresses in all cases, the turbulent heat fluxes can be qualitatively predicted by the nonlinear eddy diffusivity for a heat turbulence model. Therefore, the turbulence models of LES and RANS should be improved in order to adequately predict various thermally stratified turbulent boundary layers over an obstacle.
The drag and lift forces acting on a droplet moving near a plane wall were numerically investigated using a threedimensional direct numerical simulation (DNS) based on the marker and cell (MAC) method. The numerical results showed that the presence of the wall causes an increase in the drag force. The less viscosity ratio of the internal fluid to the external fluid caused the decrease in the wall-induced drag coefficient. The direction of the wall-induced lift force acting on a droplet near the wall changed depending on the viscosity ratio in the high Reynolds number cases. At low Reynolds numbers, the wall-induced drag and lift coefficients of a droplet had the similarity for different viscosity ratios, respectively.
The objective of this study is to investigate turbulent heat transfer phenomena in a thermally-stratified turbulent boundary layer over a 2-dimensional hill (2DH) by means of direct numerical simulation (DNS). Considering the effect of thermal stratification and wall shape to velocity and thermal fields, in order to investigate and obtain the near-wall distributions of turbulent quantities in various thermally-stratified turbulent boundary layers over a 2DH in detail, DNS is carried out, in which the thermal stratifications are set to stable, neutral and unstable conditions. The present DNS indicates the fundamental and detailed characteristics of turbulent boundary layers with various thermal stratifications over a 2DH, and shows effects of the wall shape and the thermal stratifications to the turbulent boundary layer. Also, in order to reveal the distribution of turbulence energy around a 2DH, the budget of turbulence energy are shown at the separation and reattachment points both in front and rear of the 2DH.
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