No abstract

Turbulent flows around bluff bodies are present in a large number of aeronautical, civil, mechanical, naval and oceanic engineering problems and still need comprehension. This paper provides a detailed investigation of turbulent boundary layer flows past a bluff body. The flows are disturbed by superficial roughness effect, one of the most influencing parameters present in engineering applications. A roughness model, recently developed by the authors, is here employed in order to capture the main features of these complex flows. Starting from subcritical Reynolds number simulations (Re = 1.0 × 105), typical phenomena found on critical and supercritical flow regimes are successfully captured, like non-zero lift force and its direction change, drag crisis followed by a gradual increase on this force, and separation and stagnation points displacement. The main contribution of this paper is to present a wide discussion related with the temporal history of aerodynamic loads of a single rough circular cylinder capturing the occurrence of asymmetric separation bubbles generation. The formation of asymmetric separation bubbles is an intrinsic phenomenon of the physical problem, which is successfully reported by our work. Unfortunately, there is a lack of numerical results available in the literature discussing the problem, which has also motivated the present paper. Previous study of our research group has only discussed the drag crisis, without to investigate its gradual increase and the change on lift force direction. Our results again confirm that the Lagrangian vortex method in association with Large-Eddy Simulation (LES) theory enables the development of two-dimensional roughness models.

In this paper a method for the simulation of convection and diffusion of the vorticity, generated on a body surface is presented. A purely Lagrangian scheme is used for the vorticity convection, and thereby avoiding mesh associated problems. The body surface is simulated by straight-line panels, with constant-strength vortex distribution. The strength of the discrete vortices is obtained directly without going through any additional calculation. Using a primary diffusion process this vorticity is replaced by Lamb vortices located nearby the body surface. The diffusion process of the vorticity is simulated using the random walk scheme. Turbulence sub-grid scale modelling employs a Second Order Velocity Structure Function model adapted to the Lagrangian scheme. With a cloud of discrete vortices this velocity structure function model is employed to simulate the micro structures of the flow. The flow over a circular cylinder and a NACA 0012 aerofoil are considered to evaluate the integrated aerodynamic loads. Comparisons are made with previous theoretical and experimental studies

No abstract

a b s t r a c tThe two-dimensional viscous incompressible fluid flow around a circular cylinder near a moving ground is numerically simulated. In a moving ground one eliminates the influence of the ground boundary layer, which is a crucial factor in the numerical simulation of the flow around a body moving in a close vicinity to a flat ground. A Lagrangian mesh-free vortex method is used to calculate global and local quantities of high Reynolds number flow of 1.0 Â 10 5 . This method is modified to take into account the sub-grid scale phenomena through a second-order velocity structure function model adapted to the Lagrangian scheme. In the present algorithm vortices with a Lamb core are generated only on the circular cylinder surface to ensure that the no-slip condition is satisfied and that the circulation is conserved. On the ground it is only sufficient to ensure the impermeability condition. Based on the experimental results available in the literature, the critical drag behaviour was found to be directly related to a global change in the near wake structure of the cylinder.

No abstract

No abstract

No abstract