Rafik Absi

An improved near-wall treatment for turbulent channel flows

The success of predictions of wall-bounded turbulent flows requires an accurate description of the flow in the near-wall region. This article presents a comparative study between different near-wall treatments and presents an improved method. The study is applied to fully developed plane channel flow (i.e. the flow between two infinitely large plates). Simulations were performed using Fluent. Near-wall treatments available in Fluent were tested: standard wall functions, non-equilibrium wall function and enhanced wall treatment. A user defined function (UDF), based on an analytical profile for the turbulent kinetic energy (Absi, R., 2008. Analytical solutions for the modeled k-equation. ASME Journal of Applied Mechanics, 75 (4), 044501), is developed and implemented. Predicted turbulent kinetic energy profiles are presented and validated by DNS data.

ENGINEERING MODELING OF WAVE-RELATED SUSPENDED SEDIMENT TRANSPORT OVER RIPPLES

The aim of our study is to improve the description of suspended sediment transport over wave ripples. We will first show the importance of sediment diffusivity with convective transfer (hereafter called) which is different from the sediment diffusivity associated to turbulent flux . It is possible to interpret concentration profiles, in semi-log plots, thanks to a relation between second derivative of the logarithm of concentration and derivative of (Absi, 2010). An analytical profile for will be presented and validated by experimental data obtained by Thorne et al. (2009) for medium sand. The proposed profile allows a good description of suspended sediment concentrations for fine and coarse sediments.

Near-Wall Models for Improved Heat Transfer Predictions in Channel Flow Applications

This work aims to improve the accuracy of the predicted temperature profile in the near-wall region of turbulent boundary layers. It is well known that practical understanding of turbulent boundary layers is central to heat transfer enhancement in many engineering applications. In one case, an eddy viscosity formulation based on a near-wall turbulent kinetic energy k+ function and the van Driest mixing length equation are used. In the second case, the turbulent Prandtl number is not assumed constant but rather is modeled by incorporating improved turbulent Prandtl number relations into existing models. The test case is the fully developed plane channel flow, which is considered to be the simplest and most idealized boundary-layer flow. These new formulations show improved agreement against the temperature profiles of direct numerical simulations of turbulent channel flow versus simulated cases using a commercial computational fluid dynamics package.