Instantaneous transport of a passive scalar in a turbulent separated flow

Abstract

The results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180°bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent largescale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection.

Characteristics of Flow Movement in Complex Canal System and Its Influence on Sudden Pollution Accidents

“…However, it is also evident that the pollutant concentration at the bend (WCS11 section) is higher than that behind the bend (WCS12 section) because of the influence of transverse circulation at the bend. erefore, it can be found that the dead water zone caused by vortex and circulation will delay the diffusion of pollutants, which is consistent with the results of Ouro [35].…”

Drivers for mass and momentum exchange between the main channel and river bank lateral cavities

“…High-fidelity numerical simulations are performed using the in-house code Hydro3D (Ouro et al [36]), which resolves the flow dynamics by means of Large-Eddy Simulation (LES) (Liu et al [37], Ouro et al [31,38], Stoesser [39], Stoesser et al [40]). In the framework of LES, the flow structures larger than a given filter size, e.g.…”

“…DES or LES not just provide the required resolution of the turbulent structures but they can accurately predict areas of flow separation of high importance in open channel and river flows, proving their superiority to Reynolds Averaged Navier-Stokes (RANS) computations (van Balen et al [28], Juez et al [29], Navas-Montilla and Murillo [30]). Thus, these eddy-resolving techniques provide valuable and complete information of the turbulent flow dynamics (Ouro et al [31]), which ultimately, govern mass and momentum exchange processes (McCoy et al [32], Gualtieri [33]). This paper conducted numerical simulations by means of LES to reveal 3D flow hydrodynamics (i.e.…”

Modelling solute transport in water disinfection systems: Effects of temperature gradient on the hydraulic and disinfection efficiency of serpentine chlorine contact tanks

“…The comparison of solute transport in contact tank using different turbulence models such as RANS and LES models (e.g., Zhang et al [31] and Ouro et al [30]) showed slight effects of flow unsteadiness on scalar dispersion. Despite RANS models can fail to accurately resolve flow structure with anisotropic turbulence and large-scale flow structures, for the case of flow in serpentine chlorine contact tank in this study, time-averaged flow properties (quasi-steady flow, short-circuiting and dead zones) are well-described by RANS equations along with turbulence closure model, resulting to robust and accurate prediction of hydraulic efficiency of the tank.…”

“…Kizilaslan et al [29] investigated the impacts of seasonal variations in the water supply on mixing and disinfection efficiency of the chlorine contact tank and reported significant effects of temporal variations of flow conditions on pathogen inactivation and chlorine concentrations. The effects of flow unsteadiness and large-scale turbulent flow structures on scalar transport was investigated by Ouro et al [30] and Zhang et al [31] using high resolution turbulence models (i.e., Large Eddy Simulation) [30,31]. Long et al [32] developed a CFD model to simulate a UV disinfectant tank and demonstrated the ability of CFD models to predict flow, radiation intensity, and concentration of microorganisms in UV reactors.…”