Anne Sergent

Development of a local subgrid diffusivity model for large-eddy simulation of buoyancy-driven flows: Application to a square differentially heated cavity

We present a local subgrid diffusivity model for the large-eddy simulation of natural-convection flows in cavities. This model, which does not make use of the Reynolds analogy with a constant subgrid Prandtl number, computes the subgrid diffusivity independently from the subgrid viscosity along the lines of the mixed scale model for eddy viscosity. First, an a-priori test is performed from a direct numerical simulation (DNS) approach in order to compare the respective effects of the subgrid viscosity model and that introduced by the QUICK scheme used to discretize the convective terms in the momentum equations. Then the subgrid diffusivity model is applied to the case of a two-dimensional square cavity filled with air for a Rayleigh number of 5 2 10 10 . Comparisons with DNS reference results demonstrate significant improvements in capturing the general pattern of the flow and particularly in predicting the transition to turbulence in the boundary layers as compared with Reynolds analogy results. The influence of subgrid diffusivity on the local heat transfer is also examined.

Proper orthogonal decomposition investigation of turbulent Rayleigh-Bénard convection in a rectangular cavity

We consider the large-eddy simulation (LES) of turbulent Rayleigh-Benard convection for air in a parallepipedic cavity of ratio (1:5:1) over the range Ra = 6 × 108 up to Ra = 1010 previously studied in Sergent and Le Quere (Proceedings of the 13th European Turbulence Conference, 2011). Using proper orthogonal decomposition (POD) analysis, we confirm the existence of a large-scale circulation (LSC) consisting of quasi-stationary cross-stream rolls (y-rolls) which are aligned with the small direction of the box. Strong changes in the LSC are observed to take place over a few hundred convective time units, defined as κ/(Lx2Ra1/2), where κ is the fluid diffusivity, Lx is the height of the box and Ra is the Rayleigh number. We also show the existence of a secondary flow, which consists of horizontal rolls (z-rolls) surrounding the core of the cavity and orthogonal to the cross-stream rolls. The amplitude of these longitudinal rolls oscillates on a time scale of 50 convective units. The longitudinal rolls are a...

Automatically generated zonal models for building air flow simulation : Principles and applications

In our formulation of zonal models for calculating room air temperature and flow distributions, the behavior of a room is represented by the connection of SPARK calculation modules. Modules to describe the building walls and various systems have been created. They form the models library. By assembling the appropriate modules, a zonal model of an entire building can be constructed. A model-generating tool called GenSPARK automates this process. SPARK solves the set of equations resulting from this construction to obtain the air flow and temperature distribution in the building. We describe our formulation of zonal models, show how GenSPARK works and give examples of configurations we are able to analyze.