Turbulence Model

Application of night cooling to educational buildings looks very promising in mild climates. Night cooling strategy performance in a school building to be realised in Italy during 2008 has been studied by means of a commercial CFD code. Due to the symmetry of the building, first a 2D numerical model consisting of a vertical section through the three-storey building has been developed. The CFD model includes the solid zone describing the concrete structure. A conjugate heat transfer transient simulation has been implemented to evaluate the natural ventilation flow and the cooling power during a typical mid-season and summer night cooling cycle . A similar 3D model has been used to obtain a more detailed description of the air distribution in the room and to test the results of different turbulence modeling approaches: Reynolds Average Navier Stokes (RANS) and Large Eddy Simulation (LES). Some critical aspects of turbulence modeling in buoyancy multi-scale model have been detected and...

Quantification of spatial and temporal concentration profiles of vapor clouds resulting from accidental loss of containment of toxic and/or flammable substances is of great importance as correct prediction of spatial and temporal profiles can not only help in designing mitigation/prevention equipment such as gas detection alarms and shutdown procedures but also help decide on modifications that may help prevent any

The paper describes an investigation into the predictive performance of linear and non-linear eddy-viscosity models and differential stress-transport closures for separated flow in a nominally two-dimensional, asymmetric diffuser. The test case forms part of a broader collaborative exercise between academic and industrial partners. It is demonstrated that advanced turbulence models using strain-dependent coefficients and anisotropy-resolving closure offer tangible advantages in predictive capability, although the quality of their performance can vary significantly, depending on the details of closure approximations adopted. Certain features of the flow defy resolution by any of the closures investigated. In particular, no model resolves correctly the flow near the diffuser's inclined wall immediately downstream of the inlet corner, which may reflect the presence of a “flapping” motion associated with a highly-localised process of unsteady separation and reattachment.

Literature on turbulence modeling is rich in empirical, semi-empirical and theoretical spectral equations whose parameters assume deterministic values. Starting from a critical review of the state of the art, this paper proposes a unified model of atmospheric turbulence especially suited to determine the 3-D gust-excited response of structures. Unlike classical models, all parameters are assigned through first and second order statistical moments derived from a wide set of selected experimental measurements. A general discussion is ...

The Venturi tube is a device used to measure the flow
rate in different industrial processes. In the present
work, a study is carried out for two cases, one experimental
and another numerical of the pressure exerted
by the flow of water on the walls of a Venturi tube.
In the first case, five experiments with different flow
rates are carried out. In the second, the flow is simulated
for two types of meshes and two turbulence
models, using the code COMSOL Multhiphysics 4.3.
The experimental and numerical results showed that
the pressures of the flow on the walls in two references
identified as C and G keep their magnitude constant;
in addition, the numerical profiles showed that the
lowest pressure drop occurs in the wall at the inlet
and outlet of the throat section. It is concluded that,
the distribution of the flow pressure in the wall of the
throat section has a convex profile, and the results of
pressures obtained for the standard k − e turbulence
model are more adjusted to the experimental data.

Until recently, urban air quality modelling has been based on operational models of an integral nature. The use of computational fluid dynamics (CFD) models to address the same problems is increasing rapidly. Operational models e.g. OSPM, AERMOD, ADMS-Urban have undergone many comprehensive formal evaluations as to their “fitness for purpose” while CFD models do not have such an evaluation record in the urban air quality context. This paper looks at the application of both approaches to common problems. In particular, pollutant dispersion from point and line sources in the simplest neutral atmospheric boundary layer and line sources placed within different regular building geometries is studied with the CFD code FLUENT and the atmospheric dispersion model ADMS-Urban. Both the effect of street canyons of different aspect ratios and various obstacle array configurations consisting of cubical buildings are investigated. The standard k–εk–ε turbulence model and the advection–diffusion (AD) method (in contrast to the Lagrangian particle tracking method) are used for the CFD simulations. Results from the two approaches are compared. Overall CFD simulations with the appropriate choice of coefficients produce similar concentration fields to those predicted by the integral approach. However, some quantitative differences are observed. These differences can be explained by investigating the role of the Schmidt number in the CFD simulations. A further interpretation of the differences between the two approaches is given by quantifying the exchange velocities linked to the mass fluxes between the in-canopy and above-canopy layers.

Finding an appropriate turbulence model for a given flow case usually calls for extensive experimentation with both models and numerical solution methods. This work presents the design and implementation of a flexible, programmable software framework for assisting with numerical experiments in computational turbulence. The framework targets Reynolds-averaged Navier-Stokes models, discretized by finite element methods. The novel implementation makes use of Python and the FEniCS package, the combination of which leads to compact and reusable code, where model- and solver-specific code resemble closely the mathematical formulation of equations and algorithms. The presented ideas and programming techniques are also applicable to other fields that involve systems of nonlinear partial differential equations. We demonstrate the framework in two applications and investigate the impact of various linearizations on the convergence properties of nonlinear solvers for a Reynolds-averaged Navier-Stokes model.

The paper investigates the potential of a courtyard for passive cooling in a single storey
high mass building in a warm humid climate. The inclusion of an internal courtyard in building
design is attributed to the optimization of natural ventilation in order to minimize indoor
overheating conditions. However, the efficiency of this strategy greatly depends on the design
details of the building composition in providing appropriate airflow pattern to the courtyard.
From the results of thermal measurements, a significant correlation between wall surface temperatures
and indoor air temperatures is evident. A reduction of indoor air temperature below
the levels of ambient is seen as a function of heat exchange between the indoor air and high
thermal mass of the building fabric. However, this behavior is affected by indoor airflow
patterns, which are controlled through the composition between envelope openings and the
courtyard of the building.
From a computational analysis, several airflow patterns are identified. A relatively better
indoor thermal modification is seen when the courtyard acts as an air funnel discharging indoor
air into the sky, than the courtyard acts as a suction zone inducing air from its sky opening.
The earlier pattern is promoted when the courtyard is ventilated through openings found in
the building envelope. The computational simulation utilizing the standard k-
turbulent model
with isothermal condition agrees closely with the measurements taken from the field investigation.