Collision of inwardly propagating axisymmetric gravity currents was investigated by means of three-dimensional high-resolution simulations. Our goal is to deepen our understanding of the flow structures, energy budgets, and the mean flow and turbulence characteristics in the axisymmetric collision in light of our understanding of the collision of two planar counterflowing gravity currents. The lifetime of axisymmetric collision is approximately $4\tilde{H}/{\tilde{u}}_{N,\text{max}}$, where $\tilde{H}$ is the depth of heavy and ambient fluids and ${\tilde{u}}_{N,\text{max}}$ is the maximum inward front speed before the collision, and the lifetime of axisymmetric collision can be divided into three phases. During Phase I, $0\le (\tilde{t}-{\tilde{t}}_c){\tilde{u}}_{N,\text{max}}/\tilde{H}\lesssim 0.7$, where $\tilde{t}$ is the time and ${\tilde{t}}_c$ is the time instance at which the “averaged” vertical acceleration of the density front from collision reaches its maximum value, a circular array of radially elongated vortices surrounding the center of the flow domain develop close to the bottom boundary and create strong turbulence and dissipation of turbulent kinetic energy. The radial vorticity is generated from the tilting of azimuthal vorticity due to the axisymmetric geometry and this mechanism for generation of vorticity is essentially different from the collision of two planar counterflowing gravity currents. During Phase II, $0.7\lesssim (\tilde{t}-{\tilde{t}}_c){\tilde{u}}_{N,\text{max}}/\tilde{H}\lesssim 1.26$, the deflected updraught of collided heavy fluid expands radially outwards and there exists another circular array of radially elongated vortices close to the top boundary. Both baroclinic production of vorticity and diffusion of vorticity act to stabilize the flow from further intensifying radial vorticity close to the top boundary. During Phase III, $1.26\lesssim (\tilde{t}-{\tilde{t}}_c){\tilde{u}}_{N,\text{max}}/\tilde{H}\lesssim 4$, the radially elongated vortices observed in Phase II break up into smaller structures while the collided heavy fluid slumps away from the center of the flow domain. In Phase I and Phase II, the primary source of turbulent kinetic energy is production of turbulent kinetic energy, while in Phase III, the primary source is due to turbulent buoyancy flux. Our space-time averaged eddy diffusivity data support the experimental data, numerical simulation data and the guide for parameterization of eddy diffusivity proposed for the collision of two planar counterflowing gravity currents of equal strengths in the literature.

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• Geophysical fluid dynamics
• Turbulence