Leading edge vortex formation and detachment on a flat plate undergoing simultaneous pitching and plunging motion: Experimental and computational study

This study focuses on the formation and detachment of a leading edge vortex (LEV) appearing on an airfoil when its effective angle of attack is dynamically changed, inducing additional forces and moments on the airfoil. Experimental measurements of the time-resolved velocity field using Particle Image Velocimetry (PIV) are complemented by a computational study using an URANS (Unsteady Reynolds-Averaged Navier–Stokes) framework. In this framework a transition-sensitive Reynolds-stress model of turbulence, proposed by Maduta et al. (2018), which combines the near-wall Reynolds-Stress model by Jakirlic and Maduta (2015) and a phenomenological transition model governing the pre-turbulent kinetic energy by Walters and Cokljat (2008), is employed. Combined pitching and plunging kinematics of the investigated flat plate airfoil enable the effective inflow angle to be arbitrarily prescribed. A qualitative assessment of flow fields and a quantitative comparison of LEV characteristics in terms of its center position and circulation as well as an investigation of the mechanism causing the vortex to stop accumulating circulation revealed close agreement between the experimental and simulation results. Further considerations of the lift contribution from the pressure and suction side of the airfoil to the overall lift indicates that the qualitative lift evolution is reproduced even if the pressure side contribution is neglected. This reveals important characteristics of such airfoil dynamics, which can be exploited in future experimental studies, where direct aerodynamic force and moment measurements are greatly inhibited by dominating inertial forces

Modeling the turbulent wall flows subjected to strong pressure variations

Pressure Gradient and Implications on Turbulence Modeling It is known that in incompressible flow the pressure gradient affects the turbulence properties and their budget only indirectly, through the modulation of the mean strain. Experiments and direct numerical simulations (DNS) in attached boundary layers at moderate-to-strong pressure gradients (e.g., These are only some of the facets of the effects of the mean pressure gradient on the turbulence fluctuations in the near-wall region, indicating a necessity for a turbulence model to account for separate contribution of each stress components to the momentum balance, as well as to the dynamics of the turbulence scale and of the stress tensor itself. Best prospects for accurate predictions of turbulent flows with strong pressure gradients have the second-moment models which can mimic better the dynamics of the turbulent stress field and evolution of each stress component. Furthermore, the departure from local equilibrium and conventional boundary layer scaling, calls for resolving in full the near-wall layer with all necessary implications on model modifications for the wall-proximity and viscous effects. This paper presents some results of a systematic testing of a version of the second-moment closure with low-Re-number and wall-vicinity modifications. Some modifications of £-equations are considered, which are applicable also to the high-Re-number models. Arguments in favor of these modifications, as well as the complete low-Re-number model, will be substantiated by presenting some results for a series of attached and separating wall flows. For illustration, some cases obtained with the highRe-number second-moment and with the standard k -e closures will also be shown. Considered cases include flows in strong favorable pressure gradient (including laminarization), bypass transition on a finite-thickness plate, flows in adverse pressure gradient (nonseparating and separating) and flows subjected to periodic alternation (in time or space) of positive and negative pressure gradients. Also, some results of computational study of mean flow and turbulence field in flows separating on sharp Journal of Fluids Engineerin

13th ERCOFTAC workshop on refined turbulence modelling, 25-26 th September, 2008, Graz University of Technology, Austria

International audienceA report is given of the 13th ERCOFTAC SIG 15 Workshop on Refined Turbulence Modelling, which was held at Graz University of Technology on 25th and 26th September, 2008