Y. Nagano

A New Low-Reynolds-Number One-Equation Model of Turbulence

AbstractIn this study, we propose a new Low-Reynolds-Number (LRN)one-equation model, which is derived from an LRN two-equation(k-ε) model. The derivation of the transport equation, in principle, is based on the assumption that the turbulent structure parameter remains constant. However, the relation for the turbulent structure parameter a1(=|−n

DNS and Turbulence Modeling for Turbulent Boundary Layers With Various Thermal Stratifications

bar ubar v

Direct numerical simulation of stable and unstable turbulent thermal boundary layers

n|/k) is modified to account for near-wall turbulence. As a result, the present one-equation model contains a term which takes the near-wall limiting behavior explicitly into account. Thus, the present model provides the correct wall-limiting behavior of turbulence in the vicinity of the wall and can be applied to the analysis of heat transfer. The validity of the present model is tested in channel flows, boundary layer flows with and without pressure gradient, plane wall jet, and flow with separation and reattachment. The calculated results showed good agreement with the direct numerical simulation (DNS) and experimental data.

Effects of adverse pressure gradient on heat transfer mechanism in thermal boundary layer

The mechanisms of laminarization in wall-bounded flows have been investigated by performing direct numerical simulations (DNS) of turbulent channel flows. By decreasing Reynolds numbers systematically, the effects of the low Reynolds number are studied in connection with the near-wall turbulent structure and turbulent statistics. At approximately the critical Reynolds number, the turbulent skin friction is reduced, and the turbulent structure changes qualitatively in the very near-wall region. Instantaneous turbulent structures reveal that streamwise vortices, the cores of which are at y+ ≃ 10, disappear, although low speed streaks and Reynolds shear stress are still produced by larger streamwise vortices located in the buffer region y+ > 10. Sweep motions induced by these vortical structures are shifted toward the center of a channel and also significantly deterred, which may heighten the effects of the viscous sublayer over most of the channel section and suppress the regeneration mechanisms of new streamwise vortices in the very near-wall region. To investigate the details of how large-scale coherent vortices affect the viscous sublayer and the relevant small-scale streamwise vortices, a body force is virtually imposed in the wall-normal direction to enhance the large streamwise vortices. As a result, it is found that when they are sufficiently enhanced, the small-scale vortices reappear, and the sweep events are again dominant in the viscous sublayer.

Nonlinear eddy diffusivity models reflecting buoyancy effect for wall-shear flows and heat transfer

Direct numerical simulations (DNS) of boundary layers with various thermal stratifications are carried out to investigate the turbulent structures of these flows. The present DNSs quantitatively provide the characteristics of thermally stratified turbulent boundary layers. In particular, the counter gradient diffusion phenomenon is found in a strong, stable stratified boundary layer. On the other hand, in order to adequately predict turbulent boundary layers with various thermal stratifications, an appropriate turbulence model should be employed in the calculation. Thus, using a database obtained by DNS, the strict assessment of turbulent heat transfer model is made so as to construct a reliable advanced turbulence model. The results of in-depth turbulent model evaluation are indicated, in which we have explored the prediction potential of the proposed nonlinear eddy diffusivity models for momentum and heat in both stable and unstable stratified boundary layers.Copyright

Spatio-temporal turbulent structures of thermal boundary layer subjected to non-equilibrium adverse pressure gradient

Characteristics of turbulent boundary layer flows with adverse pressure gradients (APG) differ significantly from those of canonical boundary layers. We have experimentally investigated the effects of APG on the heat transfer mechanism in a turbulent boundary layer developing on the uniformly heated plate. It is found that in the APG boundary layer the Stanton number follows the correlation curve for a flat plate, although the skin friction coefficient decreases drastically in comparison with zero-pressure-gradient flow. The mean temperature profiles in APG flows lie below the conventional thermal law of the wall in the fully turbulent region. Moreover, the quadrant splitting and trajectory analyses reveal that the effects of APG on the thermal field are not similar to those on the velocity field. The structural change in APG flow causes the non-local interactions between the temperature fluctuations and the wall-normal motions. However, the situation is fairly complex because the heat transport is mainly determined by the ejection motions, which are not significant contributors to the momentum transport in the APG flow.