Tackling complex turbulent flows with transient RANS

This article reviews some recent applications of the transient-Reynoldsaveraged Navier–Stokes (T-RANS) approach in simulating complex turbulent flows dominated by externally imposed body forces, primarily by thermal buoyancy and the Lorentz force. The T-RANS aims at numerical resolving unsteady (semi-) deterministic vortical structures in flows with sufficiently strong internal forcing. With a well-tested RANS model to account for the unresolved ‘subscale’ motion, the T-RANS is considered as a tool for solving large-scale high Rayleigh and Reynolds numbers, which are inaccessible to the conventional large-eddy simulation (LES) or any other numerical simulation approach. First, a brief outline of the T-RANS rationale is presented and its potential illustrated in the simulation of Rayleigh–Bérnard convection in an infinite domain for over a ten-decade range of Rayleigh numbers (106–2×1016). The accurate prediction of heat transfer over a wide range of Rayleigh numbers provided sufficient credibility in the approach and its application to a variety of real-life flows dominated by body forces. This is illustrated by three examples of complex environmental and multi-physics phenomena: dynamics of a fuel-oil cooling inside a sunken tanker wreck, diurnal variations of air-movement and pollutant spreading over a mesoscale mountain city in a valley capped by a thermal inversion layer, and finally in the generation and self-sustenance of a magnetic field by a highly turbulent helical sodium movement. The simulated results agree well with the experimental data where available.MSP/Multi-Scale PhysicsApplied Science

'T-RANS' simulation of deterministic eddy structure in flows driven by thermal buoyancy and Lorentz force

The paper reports on the application of the Time-dependent Reynolds-Averaged Navier-Stokes (T-RANS) approach to analysing the effects of magnetic force and bottom-wall configuration on the reorganisation of a large coherent structure and its role in the transport processes in Rayleigh-Benard convection. The large-scale deterministic motion is fully resolved in time and space, whereas the unresolved stochastic motion is modelled by a `subscale' model for which the conventional algebraic stress/flux expressions were used, closed with the low-Re number -- three-equation model. The applied method reproduces long-term averaged mean flow properties, turbulence second moments, and all major features of the coherent roll/cell structure in classic Rayleigh-Benard convection in excellent agreement with the available DNS and experimental results. Application of the T-RANS approach to Rayleigh-Benard convection with wavy bottom walls and a superimposed magnetic field yielded the expected effects on the reorganisation of the eddy structure and consequent modifications of the mean and turbulence parameters and wall heat transfer.MSP/Multi-Scale PhysicsApplied Science

Numerical simulation of a turbulent magnetic dynamo

Applied Science

Compound wall treatment for RANS computation of complex turbulent flows and heat transfer

Applied Science

River-induced anomalies in seasonal variation of traffic-emitted CO distribution over the City of Krasnoyarsk

Seasonal variation of air quality in a city with a large river was investigated by means of numerical simulations of air movement and pollutant dispersion over inversion-capped diurnal cycles using a Reynolds-averaged Navier-Stokes (RANS) approach with algebraic turbulent flux model. The study accounts for the effects of urban heat island (UHI), terrain orography and high thermal inertia of the river body. The case mimics the real environment of the Krasnoyarsk region with the river Yenisei (Russia). Two scenarios were considered typical of the winter and summer seasons. The study is focused on the dynamics of dispersion of CO emanating mainly from road traffic, which remains fairly uniform throughout the year. The simulation starts from a mild low-altitude inversion with penetrative convection gradually developing over the daytime and attenuating during the night. The main difference between the two cases is in the temperature of the river surface relative to the ambient air. In winter, the non-freezing river acts as a source of positive thermal buoyancy, while in summer the cool river at the daytime acts in the opposite way, as a heat sink. The effect of the river-induced air circulation appears significant enough to account for the observed winter accumulation of the pollutant in the city center.ChemE/Transport Phenomen

Numerical insights into magnetic dynamo action in a turbulent regime

We report on hybrid numerical simulations of a turbulent magnetic dynamo. The simulated set-up mimics the Riga dynamo experiment characterized by Re ? 3.5 × 106 and (Gailitis et al 2000 Phys. Rev. Lett. 84 4365–8). The simulations were performed by a simultaneous fully coupled solution of the transient Reynolds-averaged Navier–Stokes (T-RANS) equations for the fluid velocity and turbulence field, and the direct numerical solution (DNS) of the magnetic induction equations. This fully integrated hybrid T-RANS/DNS approach, applied in the finite-volume numerical framework with a multi-block-structured nonorthogonal geometry-fitted computational mesh, reproduced the mechanism of self-generation of a magnetic field in close accordance with the experimental records. In addition to the numerical confirmation of the Riga findings, the numerical simulations provided detailed insights into the temporal and spatial dynamics of flow, turbulence and electromagnetic fields and their reorganization due to mutual interactions, revealing the full four-dimensional picture of a dynamo action in the turbulent regime under realistic working conditions.Applied Science

Reassessment of modeling turbulence via Reynolds averaging: A review of second-moment transport strategy

This paper examines the evolution of closing the Reynolds-averaged Navier-Stokes equations by approximating the Reynolds stresses via the second-moment transport equations themselves. This strategy first proposed by Rotta is markedly in contrast to the more usual approach of computing an effective “turbulent viscosity” to deduce the turbulent stresses as in a Newtonian fluid in laminar motion. This paper covers the main elements in the development of this approach and shows examples of applications in complex shear flows that collectively include the effects of three-dimensional straining, force fields, and time dependence that affect the flow evolution in ways that cannot be readily mimicked with an eddy viscosity model.ChemE/Transport Phenomen