Turbulence modelling for low Reynolds number flows

A modification of the k-omega shear-stress-transport turbulence model is presented in this paper with the aim to improve the simulation of flows at a low Reynolds number. For this purpose, the incompressible flow at Reynolds number 6 x10^4 , around the Selig–Donovan 7003 airfoil, is investigated by using several turbulence models. The focus is placed on the k-omega shear-stress-transport turbulence model: a model very reliable for transonic flows but for which the application to low-Reynolds-number flows has been questioned. The limits of this model are analyzed in detail. The simulation of the laminar bubble arising on the SD 7003 airfoil has been remarkably improved by the proposed modification. In addition, the accuracy of the k-omega shear-stress-transport turbulence model has been preserved in the transonic regime at a high Reynolds number, as shown by the simulations of the flow around the RAE M2155 wing. Large-eddy simulations of the flow around the Selig–Donovan 7003 airfoil have also been performed and presented in the paper. The large-eddy-simulation data are used as a reference for the results achieved by the Reynolds-averaged Navier–Stokes method. The computed aerodynamic coefficients are compared with some available experimental data

Model for Enhancing Turbulent Production in Laminar Separation Bubbles

Laminar separation bubbles are one of the main critical aspects of flows at low Reynolds numbers in the range of 104 –105 . The flow separates in the laminar regime, the turbulence developing inside the recirculation region enhances the momentum transport, and the flow can reattach. Models based on the Reynolds-averaged Navier–Stokes equations suffer two of main issues: the determination of the transition onset and the level of the pressure recovery downstream of the reattachment of the flow. A model addressing both issues is presented in this paper. It is based on the γ transition model for the transition detection. The production of the turbulent kinetic energy κ has been properly enhanced thanks to a correlation found between the necessary boosting of κ and the intermittency function behavior within the bubble. The low-Mach-number and Reynolds-number flows around the Selig–Donovan 7003, Eppler 387, and NACA 0015 airfoils are analyzed. The results are compared to experimental data and large-eddy simulations available in the literature. The model can be applied to the analysis of an arbitrary airfoil without need of preliminary calculation of the transition point within the bubble

Effect of body shape on riblets performance

The effect of partial slip flow on airfoil performance at high Reynolds numbers is analyzed in this paper. The link between the physical mechanism of drag reduction attained by many devices and the slip length concept has been well assessed in the literature. A slip length model is therefore here adopted in large eddy simulations to quantify the effect of slip flow on airfoil performance. The possibility to adopt a slip flow boundary condition to simulate riblets on airfoil is verified. Their effectiveness in reducing friction drag in turbulent flow has been well assessed since the end of the last century. Both theory and experiments proved that the effect of riblets only depends on the local Reynolds number. However, some experiments showed an increased effectiveness of riblets in the presence of pressure gradient. This secondary effect is still being debated and a physical explanation has not been found. This paper has the aim to provide a contribution to the understanding of this phenomenon. Large eddy simulations of flows around airfoils are proposed with an extensive analysis of riblet performance, obtained by a proper slip flow boundary condition. It is shown that riblets reduce the boundary layer displacement thickness inducing small but significant modifications to the pressure distribution, in particular in the adverse pressure gradient region. The reduced thickening of the equivalent body is the reason for the reduced form drag