Large Eddy Simulation of Buoyant Flows

Researcher: Dalila Ammour

Supervisor(s): Dr.Tim Craft, Prof. Hector Iacovides
Sponsor: School of MACE/EPSRC & EDF Energy (Barnwood)
Start Date: October 2009 End Date: October 2012
Keywords: Buoyant flows, Heat transfer, Natural convection, Unsteady RANS, LES, Code_Saturne

Overall Research Aim

Buoyant flows occur in a variety of engineering cooling systems. It is therefore essential to have reliable and efficient numerical simulation models. Buoyant flows even in simple geometries tend to be physically complex, which poses severe challenges to RANS turbulence models used widely by industry. The objective of this research is to employ the Large Eddy Simulation method to produce accurate simulations of buoyant flows in complex internal systems found in engineering applications. These solutions will on the one hand advance our understanding of the flow physics involved and on the other hand provide validation data for the development of reliable Unsteady RANS models.

Research Progress

Non isothermal turbulent convection flows are encountered in nature and in many engineering applications involving either buoyancy or thermal stratification. Our focus in on an enclosure with the differentially heated vertical walls; i.e. the so-called “Cavity flows”.

The development of method of calculations of turbulent natural convection in cavities heated on the side still remains an urgent problem from the stand-point of both basic research and practical applications. Until recently, numerical investigations of this problem involved the use of RANS models. In so doing the calculation results depended significantly on the choice of the closing model of turbulence and acceptable agreement with experimental data.

The calculations, whose results are given below, were performed for the condition of a recent experimental investigation. Two dimensional natural convection of air in vertical and inclined rectangular cavities is computed,as a first test case, using CFD finite volume Code named: Code_Saturne. The unsteady RANS models tested in the present study are: standard wall function $k-\varepsilon$ of Lauder, $k-\omega$ of Menter, $\overline{v^2}-f $ of Durbin and also Scalable wall functions with conjunction of low Re $k-\varepsilon$ Launder-Sharma.

The present numerical investigation focusses on the effects of angle of inclination on buoyancy driven flows inside cavity. Some 2D and 3D Results, for the flow and thermal fields from a 2.18mx0.52mx0.0762m cavity, are presented for a vertical cavity 90, inclined cavity at 60 to the horizontal under stable stratification, with the hot surface the upper one (Stable case) and also a 15 inclination angle, with the hot surface the lower one (Unstable case). Results are compared with both recent Experimental investigations : Experiment of Betts & Bokhari (vertical cavity) , K.Esteifi (inclined cavities) [1]

First test cases (Buoyant flow inside 2-D and 3-D differentially heated vertical and inclined cavities)

2-D and 3-D coarse and fine Grids used for RANS and LES respectively: 1-geometrygrids.jpg

Modifications due to buoyancy: equs.jpg

Tests on Standard wall functions for k-epsilon and wall echo terms for RSM (LRR) model WF-Wecho.jpg

Turbulent kinetic energy inside 2-D cavity resulted from computation using RSM (SSG) (Comparison of different angle of inclinations) k2D-3angles.jpg

Temperature distribution inside inclined cavity at 15 heated from the lower side (comparison between 2-D and 3-D results) 2D-vs-3D.jpg

Snapshot of the velocity and temperature distributions inside 3-D inclined cavity at 15 under unstable stratification obtained from computation using RSM (SSG) model

animationV-SSG.gif animationT-SSG.gif

LES results (3-D inclined cavity at 15 under unstable stratification)

Qualitative results

T-inst.jpg V-midxy-mean.jpg V-midxz-mean.jpg

Time-averaged temperature and velocity distribution 3-D inclined cavity at 15 heated from the lower side obtained from LES

t-15-unstable.jpg v-15-unstable.jpg u-15-unstable.jpg

Time-averaged Resolved streamwise velocity fluctuations resulted from LES computation of 3-D inclined cavity at 15 (unstable stratification)

uu1-15unstable.jpg uu5-15-unstable.jpg uu9-15-unstable.jpg

Time-averaged Resolved vertical velocity fluctuations resulted from LES computation of 3-D inclined cavity at 15 (unstable stratification)

vv1-15-unstable.jpg vv5-15unstable.jpg vv9-15unstable.jpg

Time-averaged Resolved temperature fluctuations resulted from LES computation of 3-D inclined cavity at 15 (stable stratification)

tt1-15unstable.jpg tt5-15unstable.jpg tt9-15unstable.jpg

Snapshot of Iso-Q parameter coloured by temperature resulted from LES computation of 3-D inclined cavity at 15 (unstable stratification) Q-criteria-colored-by-T.gif

LES results (3-D inclined cavity at 15 under stable stratification)

Qualitative results

T-15-stable.jpg V4-15-stable.jpg U5-15-stable.jpg

Time-averaged temperature and velocity distribution 3-D inclined cavity at 15 under stable stratification obtained from LES

t-15stable.jpg v-15stable.jpg u-15stable.jpg

Time-averaged Resolved streamwise velocity fluctuations resulted from LES computation of 3-D inclined cavity at 15 (stable stratification)

uu1-15stable.jpg uu5-15stable.jpg uu9-15stable.jpg

Time-averaged Resolved vertical velocity fluctuations resulted from LES computation of 3-D inclined cavity at 15 (stable stratification)

vv1-15stable.jpg vv5-15stable.jpg vv9-15stable.jpg

Time-averaged Resolved temperature fluctuations resulted from LES computation of 3-D inclined cavity at 15 (stable stratification)

tt-1-15stable.jpg tt-5-15-stable.jpg tt-9-15stable.jpg

New test case (Buoyant flow inside penetration cavity)

Geometry of the penetration cavity
Geometry-penet.jpg

Coarse Grid Grid for LES
coarse-mesh.jpg Mexh-LES-penet.jpg

Time-averaged temperature distribution at different positions inside the penetration obtained from computation using k-ε model with standard wall-function

Tm-0875.jpg Tm-075.jpg Tm-05.jpg Tm-025.jpg Tm-0125.jpg

Time-averaged parallel velocity distribution at different positions inside the penetration obtained from computation using k-ε model with standard wall-function

um-ad-0875.jpg um-ad-075.jpg um-ad-05.jpg um-ad-025.jpg um-ad-0125.jpg

Time-averaged vertical velocity distribution at different positions inside the penetration obtained from computation using k-ε model with standard wall-function

Vm-ad-0875.jpg Vm-ad-075.jpg Vm-ad-05.jpg Vm-ad-025.jpg Vm-ad-0125.jpg

Current work

LES of penetration cavity in progress...



Last Modification: r30 - 2011-08-25 - 22:48:50 - DalilaAmmour


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