Abstract in atti di convegno, 1991, ENG

Vorticity shedding and wakes interaction past a lentil shaped body

G. Riccardi, A. Iafrati, R. Piva

Dip. Meccanica e Aeronautica, Univ. La Sapienza, Roma

In this work we develop an analysis of the two dimensional inviscid flow past a lentil, focusing our attention on the vorticity production and convection. The study, carried out numerically, shows several interesting features related to the wakes interaction which induces a periodic vorticity shedding (see fig.1). Due to the flow direction on the back side of the lentil two different breaking mechanisms may be observed for the upper and lower wake. In fact, for the case presented in fig. 2, the upper wake breaking is forced by a massive penetration of the lower wake into the region between the first wake and the body (fig. 2a), while the lower wake breaking is caused by a convection of a few vortices detached by the upper wake (fig. 2b). This behaviour of the wakes leads to a significant perturbation in the vorticity generation mechanism. As a consequence several wiggles appear in the time history of d Gamma/dt (see fig. 3), while a negligible influence is observed for the time history of Gamma and for the Strouhal number. The difference between the upper and the lower wake seems to disappear for the lentil thickness going to zero, as experimented for thinner lentils and for a flat plate at the same incidence. A comparison performed with a solution obtained by a conformal mapping technique shows a quite satisfactory agreement, at least for the global flow structure, even if a completely different method for the vorticity generation is adopted. With regard to the numerical model, we use for the velocity in the body frame of reference a Poincare representation that gives an integral equation in terms of the tangential velocity on the body boundary, to be solved according to the Kelvin theorem. The shed vorticity is discretized by the vortex method which facilitates the study of the wakes interaction with respect to a boundary elements approach. The production of vorticity is actually modelled in the following approximate way. Between two consecutive generation times we follow a neutral particle which leaves the edge with an initial velocity given by the vectorial mean on the two sides of the edge. At the new generation time we assign to this particle a circulation given by the conservation of the vorticity flux across the edge. A deeper comprehension of the local flow structure and its influence on the observed dGamma/dt oscillations is required and a more satisfactory physical modelling of the vorticity shedding should be provided. A different approach accounting locally for the mass, vorticity and momentum fluxes (from the edge to the wake) is discussed together with its main effects on the numerical results.

1st European Fluid Mechanics Conference, Cambridge (UK), 16-20 September 1991

Keywords

Vortex dynamics, Vortex method, Bluff-body hydrodynamics

CNR authors

Iafrati Alessandro

CNR institutes

INM – Istituto di iNgegneria del Mare