Near-wake characteristics of various V-shaped bluff bodies

Abstract

Mechanism of vortex shedding and turbulent flow features of the near-wake flow behind regular/irregular v-shaped bluff bodies are experimentally investigated at various airflow speeds between 10-60 m/s. With the aid of schlieren photography and a three-beam, two-component backward-scattering LDA system, the phenom- ena of vortex shedding and flow recirculation behind the flameholder are well illustrated. Results show Strouhal numbers, based on vortex shedding frequencies, being independent of gutter shape and within a range of 0.23- 0.25. A similar flow structure of flow exists among near wake flows of v-gutters with different span angles. Increase of Reynolds number monotonically reduces the size of the recirculation zone. Variation in attack angle only slightly changes mean flow features, but enhances normalized Reynolds shear stresses in the near-wake. NTRODUCING a t?luff body into the airstream is an ef- fective way of holding the flame in ramjet combustors and thrust augmenters. Ample evidence has shown aerodynamic characteristics of the near-wake flow behind a v-gutter having crucial influence on flame structures and flameholding mech- anisms. This type of flow, however, is associated with com- plicated phenomena such as separation and recirculation, mass entrainment through the shear layers, and vortex shedding. Many of these detailed mechanisms have not yet been well understood. Extensive research14 was conducted on ranges of fuel con- centration, flow velocity, fuel air ratio, and blockage ratio with the flame stably sustained. Nicholson and Field5 com- pared various experimental methodologies; they also used a high-speed camera for recording the wake flow patterns. Longwell6 and William et al.7 investigated effects of the mixing on oscillation of flow velocity and pressure. Rao and Lefebvre8 and Stwalley and Lefebvre9 measured effects of flameholder shape on limits of stabilization . Previous research primarily focused on overall performance of flame stabilization of the gutter and flame stability limits. Little research concentrated on aerodynamical aspect. Vortex shedding process in shear layers supposedly has a close interaction with wake flow structures. Roshko10 used a hot wire anemometer and a pitot tube in investigating the flow past a circular cylinder at a very high Reynolds number of 10 7. Strouhal number increased with Reynolds number when the Reynolds number was less than 3.5 x 10 6. Future research on recirculation zone was proposed. Mechanism of formation and shedding of vortices in wake region were stud- ied by Gerrad.11 He used wake width as the characteristic length, determining the dimensionless vortex shedding fre- quency. Unal and Rockwell12 used a water channel in ex- ploring the wake flow with Reynolds number less than 5000. Good quality photographs of wake flow pattern at a low Rey- nolds number were obtained, reporting Strouhal number as remaining at 0.2, while the Reynolds number was higher than '1000.