A hybrid RANS/LES model for high Reynolds number wall-bounded flows is presented, in which individual Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES) are computed in parallel on two fully overlapping grids. The instantaneous, fluctuating subgrid-scale stresses are blended with a statistical eddy viscosity model in regions where the LES grid is too coarse. In the present case, the hybrid model acts as a near-wall correction to the LES, while it retains the fluctuating nature of the flow field. The dual computation enables the LES to be run on isotropic grids with very low wall-normal and wall-parallel resolution, while the auxiliary RANS simulation is conducted on a wall-refined high-aspect ratio grid. Running distinct, progressively corrected simulations allows a clearer separation of the mean and instantaneous flow fields, compliant with the fundamentally dissimilar nature of RANS and LES. Even with the wall-nearest grid point lying far in the logarithmic layer, velocity and temperature predictions of a heated plane channel flow are corrected. For a periodic hill flow, the dual-grid system improves the boundary layer separation and velocity field prediction both for a constant-spaced and a wall-refined LES grid.

/pdf/a-dual-grid-hybrid-rans-les-model-for-under-resolved-near-1c3ffwltko.pdf

A Dual-Grid Hybrid RANS/LES Model for Under-Resolved Near-Wall Regions and its Application to Heated and Separating Flows

This paper presents the results of a numerical study on the thermal–hydraulic and entropy generation analysis for a nanofluid flow inside a plate-fin triangular duct. Vortex generators are placed in the duct to improve the heat transfer rate and fluid mixing. Reynolds number (Re), volume fraction of nanoparticles (α) and vortex generators’ angle of attack (θ) are selected as the pertinent parameters. All simulations are performed for the following range of parameters: 200   40°, increasing θ leads to an increase in the thermal entropy generation. Moreover, the friction entropy generation was found to increase by 8% when the angle of attack is increased from 0 to 40°.

Nanofluid heat transfer and entropy generation inside a triangular duct equipped with delta winglet vortex generators

When a cryogenic fluid initially at a subcritical temperature is injected into a supercritical environment, it will experience a process across a pseudo-boiling point, at which the specific heat reaches its maximum value under the corresponding pressure. Large eddy simulation (LES) is conducted to explore the effects of pseudo-vaporization phenomenon around the pseudo-critical temperature on fluid jet evolution. To highlight the pseudo-vaporization effect, a cryogenic nitrogen jet with different injection temperatures, which correspond to transcritical and supercritical conditions, respectively, is injected into a chamber with same supercritical conditions. All of the thermophysical and transport properties are determined directly from fundamental theories combined with a real fluid equation of state. It is found that when the fluid transits through the pseudo-boiling point, the constant-pressure specific heat reaches a local maximum, while the thermal conductivity and viscosity become minimum. Th...

Large eddy simulation of fluid injection under transcritical and supercritical conditions

Due to a rapid increase in urbanisation, accurate wind microclimate assessment is of crucial importance. Evaluating wind flows around buildings is part of the planning application process in the design of new developments. In this study, computational fluid dynamics (CFD) simulations are carried out for a case study, representing the East Village in the London Olympic Park. Following a validation test against experimental data for a simpler urban configuration, the key input parameters, including appropriate boundary conditions, mesh setting and type of turbulence model, are selected for the Olympic Park model. All the simulations are conducted using the commercial code STARCCM+ under steady-state conditions with the Reynolds-averaged Navier–Stokes (RANS) method. The turbulence is modelled using different common variants of eddy-viscosity models (EVMs) including standard k-e, realizable k-e and standard and shear stress transport (SST) k-ω. The results demonstrate that standard and realisable k-e models correlate very well with the experimental data, while some discrepancies are found with standard and SST k-ω. Following the determination of areas of high velocity, appropriate tree planting is proposed to overcome the effect of corner and downwash acceleration. With the optimised arrangement of trees and using specific types of tree (e.g., birch), wind speeds at the pedestrian level are reduced by 3.5, 25 and 66% in three main regions of interest. Moreover, we investigate the effects of tree heights. The obtained results illustrate that the wind velocity reduces when the crowns of the trees are located closer to the buildings and the ground. Our high-resolution CFD simulation and results offer a quantitative tool for wind microclimate assessment and optimised design and arrangement of trees around buildings to improve pedestrian comfort.

https://www.mdpi.com/2075-5309/11/3/112/pdf

Computational Simulation of Wind Microclimate in Complex Urban Models and Mitigation Using Trees

Scale-up and turbulence modelling in pipes

Rough surfaces are widely used to enhance convective heat transfer by the promotion of higher turbulence levels. The present article reports simulations of the flow and heat transfer in a 2-D rib-roughened passage using a number of advanced Reynolds-averaged Navier-Stokes (RANS) turbulence models including eddy-viscosity models (EVM) and a Reynolds stress model (RSM). Large eddy simulation (LES) is also conducted and results are compared against experimental measurements. In addition, the effects of rib thermal boundary condition on heat transfer are also investigated. In the present work, the blockage ratio of the transversely mounted rectangular ribs is 10% and the rib pitch-to-height ratio of 9 is selected. The Reynolds number, based on the channel bulk velocity and hydraulic diameter, is 30,000. The RANS-based turbulence models investigated here are the k-ω-SST, the v2-f, the ϕ-f, and the elliptic blending RSM. All computations are undertaken using the commercial and industrial CFD codes STAR-...

/pdf/assessment-of-advanced-rans-models-against-large-eddy-3g6iy3j8vw.pdf

Assessment of advanced RANS models against large eddy simulation and experimental data in the investigation of ribbed passages with passive heat transfer

Adaptive wall-functions are developed for the near-wall elliptic-blending eddy-viscosity model (Billard and Laurence, Int. J. Heat Fluid Flow 33(1), 45–58, 2012). The aim of the new methodology is to make the predictions of this turbulence model less dependent on the spatial resolution in the viscous, buffer and logarithmic layer of the near-wall flow. It can be used in meshes for which localized areas with fine resolution are embedded into the coarsely meshed full domain. This allows reducing the computational cost by focusing the grid points density in critical non-equilibrium flow regions. It is based on Kalitzin et al. (J. Comput. Phys. 204(1), 265–291, 2005) but unlike the latter the dimensionless quantities provided by the wall-functions are converted into physical values by means of a turbulence rather than a mean-flow velocity scale. To justify this choice, the wall-function correction of wall shear-stress and heat-flux is presented as a Nusselt number linearization. Two wall-treatments are proposed for use in unstructured mesh finite-volume schemes: 1/ the wall-function values are applied as boundary conditions or 2/ the wall-function profiles are applied over a cluster of cells in the wall vicinity. The latter also addresses the issue of inner cell-size jumps as often produced by automatic mesh generators. The method is validated on channel flows for several types of meshes and Reynolds numbers. Heat-transfer application on a ribbed wall duct test-case is presented. The grid consists in a very coarsely meshed region upstream of a localized focus zone with classical down-to-the-wall resolution, and the heat-transfer and velocity profile are still accurately predicted.

Adaptive Wall Functions for an Elliptic Blending Eddy Viscosity Model Applicable to Any Mesh Topology

Comparison of Advanced RANS Models Against Large Eddy Simulation and Experimental Data in Investigation of Ribbed Passages with Heat Transfer

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Assessment of advanced RANS models against large eddy simulation and experimental data in the investigation of ribbed passages with passive heat transfer

Adaptive Wall Functions for an Elliptic Blending Eddy Viscosity Model Applicable to Any Mesh Topology

Comparison of Advanced RANS Models Against Large Eddy Simulation and Experimental Data in Investigation of Ribbed Passages with Heat Transfer