Self-sustained oscillations in flows around long blunt plates

The presence of flow separation from both leading and trailing edges of elo ngated bluff bodies leads to vortex interactions and resonances not observe d in shorter bodies such as circular and square cylinders. Stepwise behavio ur in the Strouhal number with increasing plate chord-to-thickness ratio ha s been observed for long bodies in a number of different situations: natura l shedding, under transverse forcing, and with excited duct modes. In the p resent study, an investigation is made of the predicted unforced laminar fl ow around long plates (up to chord, c, to thickness, t, ratio c/t = 16). Th e two main types of plate geometry considered are rectangular plates and pl ates with an aerodynamic leading edge. The rectangular plate represents a g eometrical extension of the normal flat and square plates. The aerodynamic leading-edge plate is a natural precursor to the rectangular plate because the vortex shedding is only from the trailing edge. The natural flow around rectangular plates is of greater complexity due to the interaction between the leading- and trailing-edge shedding. The previously neglected influenc e of the trailing-edge vortex shedding is found to play an important role i n the stepwise progression of the Strouhal number with chord-to-thickness r atio. In addition, the formation of three-dimensional patterns in the bound ary layer along the plate and in the trailing-edge wake is predicted. The p redicted boundary layer hairpin vortices are compared with previous observa tions and the predicted streamwise modes in the wake are compared with thos e found in the case of circular cylinders. (C) 2001 Academic Press.