Hydrodynamics of Accumulators of Compressed Air for an UWCAES Plant

The present document is a manuscript-based dissertation covering Ahmadreza Vasel-Be-Hagh\u27s PhD research from September, 2011 to May, 2015. The research was particularly focused on studying hydrodynamics of underwater accumulators of compressed air for an underwater compressed air energy storage (UWCAES) plant. The accumulator units were floral configurations of droplet-shaped balloons installed close to the bed of deep water. The research was carried out in two major parts: water flow over the balloons and flow produced by the bursting of the balloons. In the first part, three-dimensional simulations were conducted to investigate water flow over accumulators. The simulation was carried out at a free stream Reynolds number of 230,000 using URANS k--omega and LES Dyna--SM turbulence models. The structure of the flow was investigated using iso-surfaces of the second invariant of the velocity gradient and three-dimensional path lines. Several shedding vortex tubes were identified downstream of the balloons. The dynamics of these vortex tubes was further illustrated through time series snapshots containing vorticity lines on two-dimensional planes perpendicular to the flow direction. The frequency of the shedding and the turbulent movements of the vortex tubes were studied through power spectrum analysis of the force coefficients. In the second part, the flow produced by the bursting of balloons was studied experimentally using photographs taken by three cameras with speed of 60 frames per second at a resolution of 1080P. It was observed that if a sufficiently large air-filled balloon quickly burst underwater, a vortex ring bubble was generated. The effect of dimensionless surface tension on general characteristics of the vortex ring bubble including rise velocity, rate of expansion, circulation and trajectory was investigated. It was observed that as the dimensionless surface tension increased, the rise velocity, the circulation and consequently the stability of the vortex ring bubble increased; however, the rate of expansion tends toward constant values. A semi-analytical model was also developed suggesting that the vortex ring expansion is essentially due to the buoyancy force. An expression was also obtained for the circulation in terms of the initial volume of the balloon and the depth at which balloon bursts. Extending from the mentioned semi-analytical model, a perturbation analysis was performed to find an expression for the radius of the buoyant vortex rings. The radius equation includes two terms; the zeroth-order solution representing the effect of buoyancy, and the first-order perturbation correction describing the influence of viscosity. The zeroth-order solution is an explicit function of time; the first-order perturbation modification, however, includes the drag coefficient which is unknown and of interest. Fitting the photographically measured radius into the modified equation yields the time history of the drag coefficient of the corresponding buoyant vortex ring