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

Simulation of Three-Dimensional Free-Surface Dam-Break Flows over a Cuboid, Cylinder, and Sphere

A three-dimensional (3D) numerical study is undertaken to investigate dam-break flows over 3D structures. A two-phase flow model has been developed within the large-eddy simulation (LES) framework. The governing equations have been discretized using the finite-volume method, with the air-water interface being captured using a volume-of-fluid method while the Cartesian cut-cell method deals with complex geometries. The robustness and versatility of the proposed numerical approach are demonstrated first by applying it to a 3D dam-break flow over a cuboid. Good agreement is obtained between the simulation results and the corresponding experimental data and other numerical solutions. Then, a horizontal cylinder and a sphere are subjected to the same dam-break flow. Snapshots of water surface profiles are presented and discussed, and turbulent vortical structures are identified in the flow. In addition, the internal kinematics, hydrodynamic loading on the structure, and energy dissipation during dam-break flow impact are analyzed and discussed, providing more insight into such flows

Impact of turbulence and secondary flow on the water surface in partially filled pipes

Large eddy simulations (LESs) of turbulent flow in partially filled pipes at various filling degrees are conducted to investigate the response of the water surface to the turbulence and the secondary flow below it. LESs are validated first using experimental and direct numerical simulation data. At increasing water depth, the magnitude of water surface fluctuations increases with increasing strength of the main secondary flow. Visualizations of the instantaneous water surface and the turbulent flow underneath reveal that thin surface waves are generated by flow meandering in the shallower case, whereas surface waves in the deeper cases are influenced by the vertical velocity fluctuation. Pre-multiplied spectra of the water surface fluctuation, h′, provide further evidence of the origin of the surface waves. In the shallow flow, the peak frequency of the h′ spectra is consistent with the peak frequency of the u′ and v′ spectra, while for deeper flows, it agrees more with the w′ spectra. Furthermore, the transport patterns of the surface waves are investigated by the wavenumber-frequency spectra. Three types of surface waves are observed in the wavenumber-frequency spectra, i.e., (1) convective waves with phase velocity equaling the surface velocity, (2) irrotational dispersive gravity-capillary waves, and (3) stationary waves caused by secondary currents

Effect of secondary currents on the flow and turbulence in partially filled pipes

Large-eddy simulations of turbulent flow in partially filled pipes are conducted to investigate the effect of secondary currents on the friction factor, first- and second-order statistics and large-scale turbulent motion. The method is validated first and simulated profiles of the mean streamwise velocity, normal stresses and turbulent kinetic energy (TKE) are shown to be in good agreement with experimental data. The secondary flow is stronger in half- and three-quarters full pipes compared with quarter full or fully filled pipe flows, respectively. The origin of the secondary flow is examined by both the TKE budget and the steamwise vorticity equation, providing evidence that secondary currents originate from the corner between the free surface and the pipe walls, which is where turbulence production is larger than the sum of the remaining terms of the TKE budget. An extra source of streamwise vorticity production is found at the free surface near the centreline bisector, due to the two-component asymmetric turbulence there. The occurrence of dispersive stresses (due to secondary currents) reduces the contribution of the turbulent shear stress to the friction factor, which results in a reduction of the total friction factor of flows in half and three-quarters full pipes in comparison to a fully filled pipe flow. Furthermore, the presence of significant secondary currents inhibits very-large-scale motion (VLSM), which in turn reduces the strength and scales of near-wall streaks. Subsequently, near-wall coherent structures generated by streak instability and transient growth are significantly suppressed. The absence of VLSM and less coherent near-wall turbulence structures is supposedly responsible for the drag reduction in partially filled pipe flows relative to a fully filled pipe flow at an equivalent Reynolds number

A large-eddy-simulation-based numerical wave tank for three-dimensional wave-structure interaction

A three-dimensional numerical wave tank (NWT) based on the open-source large eddy simulation (LES) code Hydro3D is introduced. The code employs the level set and immersed boundary methods to enable accurate computations of the deformation of the water surface and to account for solid structures in the fluid domain, respectively. The spatially-filtered Navier–Stokes (N–S) equations are solved on a staggered Cartesian grid using the finite difference method while time advancement is achieved using the fractional-step method based with a three-step Runge–Kutta scheme. Velocities and pressure are coupled with the Poisson equation and its solution is obtained via a multi-grid technique. The code is then applied to predict the progression and damping of monochromatic waves and the interaction of non-linear waves with various submerged obstacles. The accuracy of Hydro3D is confirmed by comparing numerical results with data of previously reported laboratory experiments. Comparisons of numerically predicted and measured water-levels, local velocity and pressure fields and forces acting on structures under the influence of incoming waves with laboratory data are convincing and confirm that the code is able to predict accurately three-dimensional wave-structure interaction

A conservative and consistent implicit Cartesian cut-cell method for moving geometries with reduced spurious pressure oscillations

A conservative and consistent three-dimensional Cartesian cut-cell method is presented for reducing the spurious pressure oscillations often observed in moving body simulations in sharp-interface Cartesian grid methods. By analysing the potential sources of the oscillation in the cut-cell framework, an improved moving body algorithm is proposed for the cut-cell method for the temporal discontinuity of the solid volume change. Strict conservation of mass and momentum for both fluid and cut cells is enforced through pressure-velocity coupling to reduce local mass conservation errors. A consistent mass and momentum flux computation is employed in the finite volume method. In contrary to the commonly cut-cell methods, an implicit time integration scheme is employed in the present method, which prevents numerical instability without any additional small cut-cell treatment. The effectiveness of the present cut-cell method for reducing spurious pressure oscillations is demonstrated by simulating various two- and three-dimensional benchmark cases (in-line and transversely oscillating cylinder, oscillating and free-falling sphere), with good agreement with previous experimental measurements and other numerical methods available in the literature

Influence of boulder concentration on turbulence and sediment transport in open-channel flow over submerged boulders

In this paper the effects of boulder concentration on hydrodynamics and local and reach-averaged sediment transport properties with a flow over submerged boulder arrays are investigated. Four numerical simulations are performed in which the boulders' streamwise spacings are varied. Statistics of near-bed velocity, Reynolds shear stresses, and turbulent events are collected and used to predict bed load transport rates. The results demonstrate that the presence of boulders at various interboulder spacings altered the flow field in their vicinity causing (1) flow deceleration, wake formation, and vortex shedding; (2) enhanced outward and inward interaction turbulence events downstream of the boulders; and (3) a redistribution of the local bed shear stress around the boulder consisting of pockets of high and low bed shear stresses. The spatial variety of the predicted bed load transport rate qs based on local bed shear stress is visualized and is shown to depend greatly on the boulder concentration. Quantitative bed load transport calculations demonstrate that the reach-averaged bed load transport rate may be overestimated by up to 25 times when including the form-drag-generated shear stress of the immobile boulders in the chosen bed load formula. Further, the reach-averaged bed load transport rate may be underestimated by 11% if the local variability of the bed shear stress is not accounted for. Finally, it is shown that for the small-spaced boulder array, the bed load transport rates should no longer be predicted using a normal distribution with standard deviation of the shear stress distribution σ

Contact Tank Design Impact on Process Performance

In this study three-dimensional numerical models were refined to predict reactive processes in disinfection contact tanks (CTs). The methodology departs from the traditional performance assessment of contact tanks via hydraulic efficiency indicators, as it simulates directly transport and decay of the disinfectant, inactivation of pathogens and accumulation of by-products. The method is applied to study the effects of inlet and compartment design on contact tank performance, with special emphasis on turbulent mixing and minimisation of internal recirculation and short-circuiting. In contrast to the conventional approach of maximising the length-to-width ratio, the proposed design changes are aimed at addressing and mitigating adverse hydrodynamic structures, which have historically led to poor hydraulic efficiency in many existing contact tanks. The results suggest that water treatment facilities can benefit from in-depth analyses of the flow and kinetic processes through computational fluid dynamics, resulting in up to 38 % more efficient pathogen inactivation and 14 % less disinfection by-product formation

Water surface response to turbulent flow over a backward-facing step

The water surface response to subcritical turbulent flow over a backward-facing step (BFS) is studied via high-resolution large-eddy simulation (LES). The LES method is validated first using data of previously reported experiments. The LES-predicted water surface is decomposed into different types of gravity waves as well as turbulence-driven forced waves. Analysis of the LES data reveals the interplay between low-frequency large-scale turbulence structures, which are the result of flow separation from the step and reattachment behind the step, and the dynamics of the water surface. The water surface deformation is mainly the result of freely propagating gravity waves and forced waves, owing to turbulence in the form of rollers and/or hairpin vortices. Gravity waves with zero group velocity define the characteristic spatial and temporal scales of the surface deformations at higher frequencies, while large eddies determine their low-frequency modulation. These deformations are mainly confined in lateral bands that propagate downstream following the advection of the near-surface streamwise vortices (rollers) that are shed from the step. Steeper surface waves are observed in regions of negative perturbation velocity gradient and down-welling, downstream of the larger rollers, and are associated with thin isolated regions of high vorticity near the surface. The investigation of such a complex flow has shown that the decomposition of the water surface fluctuations into its different physical components may be used to identify the dynamics of the underlying flow structure