Numerical simulation of flow past an underwater energy storage balloon

A three-dimensional simulation was conducted to investigate water flow over the accumulator unit of an underwater compressed air energy storage system. The accumulator unit, which is a droplet shaped balloon, was installed close to the bed of deep water. The simulation was carried out at a free stream Reynolds number of 2.3 × 105 using URANS k-ω and LES Dyna-SM turbulence models. The URANS model failed to capture the turbulent nature of the flow; however, its predictions of the mean values were in reasonable agreement with those of the LES model. The time-averaged force coefficients of the balloon were found to be greater compared to the literature for spherical bluff bodies and smaller than those of circular cylinders. The structure of the flow was closely investigated using isosurfaces of the second invariant of the velocity gradient and three-dimensional path lines. Several shedding vortex tubes were identified downstream of the balloon. 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.