No abstract

No abstract

The deployment of a complete carbon capture and storage chain requires a focus upon the hazards posed by the operation of pipelines transporting carbon dioxide (CO 2 ) at high pressure in a dense-phase (supercritical or liquid state). The consequences of an intentional or accidental release from such pipelines must be considered as an integral part of the design process. There are a number of unique challenges to modelling these releases due to the unusual phase-transition behaviour of CO 2 . Additionally, few experimental observations of large-scale CO 2 releases have been made, and the physics and thermochemistry involved are not fully understood. This work provides an overview of elements of the EC FP7CO2PipeHaz project, whose overall aim is to address these important and pressing issues, and to develop and validate mathematical models for multiphase discharge and dispersion from CO 2 pipelines. These are demonstrated here upon a full-scale pipeline release scenario, in which dense-phase CO 2 is released from a full-bore 36-inch pipeline rupture into a crater, and the resulting multiphase CO 2 plume disperses over complex terrain, featuring hills and valleys. This demonstration case is specifically designed to illustrate the integration of different models for the pipeline outflow, near-field and far-field dispersion.

Vortex shedding behind a tapered circular cylinder with taper ratio 75 placed normal to the inflow has been studied. The Reynolds numbers based on the uniform inflow velocity and the diameter of the cylinder at the wide and narrow ends were 300 and 102, respectively. In the present direct numerical simulation study it was observed that even with a very long time sampling a discrete cellular shedding pattern prevails. This is in contrast to what Parnaudeau et al. ͓J. Turbulence 8, 13 ͑2007͔͒ speculated in their tapered cylinder study, where they suggested that with a longer time sampling a diffused cellular pattern might appear. In the present investigation it was found that streamwise vorticity increases as vortex dislocation occurs, an effect also reported by Piccirillo and Van Atta ͓J. Fluid Mech. 246, 163 ͑1993͔͒ in their experimental study. Flow visualizations revealed that both modes A and B secondary flow structures coexist along the span of the present tapered cylinder. The wavelength of mode B is in surprisingly good agreement with the experimental value found by Williamson ͓J. Fluid Mech. 328, 345 ͑1996͔͒ for uniform circular cylinders. The present numerical calculation revealed a spanwise secondary motion, both in the front stagnation zone and also in the wake of the cylinder. In the front stagnation zone, the secondary flow was driven by a spanwise pressure gradient. The secondary flow pattern in the wake was found to be rather complex. This complex behavior of the secondary motion was attributed to the intrinsic secondary instabilities induced by the transition process itself. This is in contrast to what Parnaudeau et al. ͓J. Turbulence 8, 13 ͑2007͔͒ speculated in their tapered cylinder study, where they attributed this to the oblique and cellular vortex shedding. In spite of this secondary flow in the base region, the local formation length in the present tapered cylinder study is in surprisingly good agreement with the results of uniform circular cylinders.

Direct numerical simulation (DNS) of vortex shedding behind a tapered plate with the taper ratio 20 placed normal to the inflow has been performed. The Reynolds numbers based on the uniform inflow velocity and the width of the plate at the wide and narrow ends were 1000 and 250, respectively. For the first time ever cellular vortex shedding was observed behind a tapered plate in a numerical experiment (DNS). Multiple cells of constant shedding frequency were found along the span of the plate. This is in contrast to apparent lack of cellular vortex shedding found in the high-Reynolds-number experiments by Gaster & Ponsford (Aero. J., vol. 88, 1984, p. 206). However, the present DNS data is in good qualitative agreement with similar high-Reynolds-number experimental data produced by Castro & Watson (Exp. Fluids, vol. 37, 2004, p. 159). It was observed that a tapered plate creates longer formation length coupled with higher base pressure as compared to non-tapered (i.e. uniform) plates. The three-dimensional recirculation bubble was nearly conical in shape. A significant base pressure reduction towards the narrow end of the plate, which results in a corresponding increase in Strouhal number, was noticed. This observation is consistent with the experimental data of Castro & Rogers (Exp. Fluids, vol. 33, 2002, p. 66). Pressure-driven spanwise secondary motion was observed, both in the front stagnation zone and also in the wake, thereby reflecting the three-dimensionality induced by the tapering.