High Reynolds Number Channel Flows Research Articles

On the Large-Scale Structure of Flow in Far Layer of Wall Turbulence.

Comparatively high Reynolds number channel flows up to Reτ=640 are simulated by DNS. Noticeable property of the high Reynolds number flow is large-scale streaks which are larger not only in scale but also in their spanwise separation than those well-known in buffer layer. It is revealed that low speed streaks are the region of high turbulent activity and accordingly of high turbulent shear stress with densely populated elementary vortices. These properties are little influenced by wall condition, such as drag controlled or not. LSE (Linear Stochastic Estimation) technique is applied to extract elementary vortex pairs. Time evolution of these vertices submerged in a laminar flow but having velocity distribution of turbulent one reveals the process of generation of large-scale streaks and accompanying densely populated elementary vortices. The formation of large-scale streaks by the model simulation is also little influenced by wall condition and this modeled process is expected to mimic that taking place in the near-wall layer of practical channel flow. The existence of low speed streaks itself is the key for the formation of the structure, thus suggesting the self-sustenance of turbulence by large-scale streaks and vortices concentrated area.

Wall-boundary conditions in probability density function methods and application to a turbulent channel flow

An application of a probability density function (PDF), or Lagrangian stochastic, approach to the case of high-Reynolds number wall-bounded turbulent flows is presented. The model simulates the instantaneous velocity and dissipation rate attached to a large number of particles and the wall-boundary conditions are formulated directly in terms of the particle properties. The present conditions aim at reproducing statistical results of the logarithmic region and are therefore in the spirit of wall functions. A new derivation of these boundary conditions and a discussion of the resulting behavior for different mean variables, such as the Reynolds stress components, is proposed. Thus, the present paper complements the work of Dreeben and Pope [Phys. Fluids 9, 2692 (1997)] who proposed similar wall-boundary particle conditions. Numerical implementation of these conditions in a standalone two-dimensional PDF code and a pressure-correction algorithm are detailed. Moments up to the fourth order are presented for a high-Reynolds number channel flow and are analyzed. This case helps to clarify how the method works in practice, to validate the boundary conditions and to assess the model and the code performance.

Modeling near-wall effects in second-moment closures by elliptic relaxation

The elliptic relaxation method for modeling near-wall turbulence via second-moment closures (SMC) is compared to direct numerical simulation (DNS) data for channel flow at Re τ=395. The agreement for second-order statistics, and the terms in their balance equation is quite satisfactory, confirming that essential kinematic effects of the solid boundary on near-wall turbulence are accurately modeled by an elliptic operator. Additional viscous effects, immediately next to the surface, can be added via Kolmogoroff scales. In combination, elliptic relaxation and Kolmogorov scaling provide a general formulation to extend high Reynolds number SMC to wall-bounded flows. This formulation was easily applied to the nonlinear Craft-Launder and Speziale-Sarkar-Gatski (SSG) pressure-strain models. It is observed that the boundary conditions of the relaxation operator dominate the homogeneous pressure-strain model in the near-wall region. While looking at high-Reynolds number channel flows, it was found necessary to modify the effect of the relaxation operator throughout the log-layer by accounting for gradients of the turbulent lengthscale; this brings the velocity gradient into perfect agreement with the von Kármán constant. The final form of the model based on the SSG homogeneous closure was then successfully applied to rotating channel flows, including relaminarization. The paper merges and updates two contributions by the five authors to the 10th Turbulent Shear Flow Conference.