Flow Separation Dynamics in Three-Dimensional Asymmetric Diffusers

The mean and instantaneous flow separation of two different three-dimensional asymmetric diffusers is analysed using the data of large-eddy simulations. The geometry of both diffusers under investigation is based on the experimental configuration of Cherry et al. (Int J Heat Fluid Flow 29(3):803–811, 2008). The two diffusers feature similar area ratios of AR=4.8 A R = 4.8 and AR=4.5 A R = 4.5 while exhibiting differing asymmetric expansion ratios of AER=4.5 A E R = 4.5 or AER=2.0 A E R = 2.0 , respectively. The Reynolds number based on the averaged inlet velocity and height of the inlet duct is approximately Re=10,000 Re = 10,000 . The time-averaged flow in both diffusers in terms of streamwise velocity profiles or the size and location of the mean backflow region are validated using experimental data. In general good agreement of simulated results with the experimental data is found. Further quantification of the flow separation behaviour and unsteadiness using the backflow coefficient reveals the volume portion in which the instantaneous reversal flow evolves. This new approach investigates the cumulative fractional volume occupied by the instantaneous backflow throughout the simulation, a power density spectra analysis of their time series reveals the periodicity of the growth and reduction phases of the flow separation within the diffusers. The dominating turbulent events responsible for the formation of the energy-containing motions including ejection and sweep are examined using the quadrant analysis at various locations. Finally, isourfaces of the Q-criterion visualise the instantaneous flow and the origin and fate of coherent structures in both diffusers

Evolution of high-density submarine turbidity current and its interaction with a pair of parallel suspended pipes

The method of large-eddy simulation (LES) coupled with the density transport equation is employed to simulate the evolution of a gravity-driven high-density turbidity current and its interaction with a pair of parallel suspended pipes. The LES method is validated first using data of a non-Boussinesq lock-exchange experiment and satisfying agreement between LES and experiment is achieved. The simulations reveal that a shear region forms between high- and low-density fluids each moving in opposite directions which lead to the generation of a series of vortices and a substantial mixing region. Close to the bottom boundary, low-density fluid is entrained near the head of the high-density turbidity current, forming a thin water cushion that separates the turbidity current's head from the seabed, the so-called hydroplaning effect, thereby reducing the density of the head and bottom friction. The current study suggests that the effect of hydroplaning phenomena leads to high speed and long distance of the turbidity current. Further, LES simulations of a turbidity current impacting a pair of parallel suspended pipes with different streamwise spacings are performed and impact forces are quantified. The turbulent wake generated by high-density fluid bypassing pipe 1 promotes velocity fluctuations leading to increased impact forces on pipe 2 with increasing streamwise spacing up to 8 times the pipeline diameter (8D). The results suggest that the streamwise spacing between two parallel pipes should be less than 2D to minimize hydrodynamic loads on pipe 2