Experimental study on the intrusion and stratification produced by confined laminar and turbulent round fountains

Highlights: • Behavior of round fountains in a cylindrical container is studied experimentally. • Transition of flow behavior and secondary flows increase with Froude number. • Secondary flows change from buoyancy-inertial regime to buoyancy-viscous regime. • Stratification rate increases with Froude number, but decreases with Reynolds number. In this paper, high-speed cameras and flow visualization techniques are used to investigate the behavior of the 'fountain filling box' flow resulted from releasing a round fountain in a homogeneous quiescent fluid in a cylindrical container over the ranges of $1.0⩽\mathrm{Fr}⩽20.0,102⩽\mathrm{Re}⩽1502$, and $27.9⩽\lambda ⩽48.75$, where $\mathrm{Fr},\mathrm{Re}$ and $\lambda$ are the Froude number, the Reynolds number, and the dimensionless radius of the container, respectively, with $\lambda$ non-dimensionalized by the fountain source radius. The results show the transition of the flow behavior of the fountain and its secondary flows (i.e., the intrusion, reversed flow and stratification) from laminar to turbulent with increasing Fr, and turbulence of the flow strengthened with increasing Re. For intermediate (e.g., $\mathrm{Fr}=3.0$) and forced turbulent fountains (e.g., $\mathrm{Fr}=5.0$, 8.0, 15.0) with a specific $\lambda$, the non-dimensionalized time-scale for the intrusion front to impinge upon the sidewall, ${\tau }_w$, is nearly constant for $\mathrm{Re}\gtrsim 500$. This is because the secondary intrusion flow is dominated by the wall-jet and buoyancy-inertial regimes where the non-dimensionalized intrusion front velocity, $v_i$, is only time-dependent ($v_i~{\tau }^{-1/2}$) or time-dependent but also under the influence of Fr ($v_i~{\mathrm{Fr}}^{-1/2}{\tau }^{-1/4}$). However, ${\tau }_w$ for the fountains of $\mathrm{Re}\lesssim 204$ is significantly different, which may result from the change in the dominant regime for the intrusion or the interaction between the upflow, downflow of the fountain and the ambient fluids. Furthermore, it is found that the non-dimensionalized quasi-steady development rate of the stratification, $v_s$, increases with Fr, but decreases with Re, since the diffusion effect is suppressed with decreasing Fr or increasing Re.