Showing papers by "Bart Merci published in 2006"

Intermittency based RANS bypass transition modelling

Abstract: A transition model for describing bypass transition is presented. It is based on a two-equations k-ω model and a dynamic equation for intermittency factor. This intermittency factor is a multiplier of the turbulent viscosity computed by the turbulence model. Following a suggestion by Menter et al. (2002), the start of transition is computed based on local variables. The quality of the transition model, developed on flat plate test cases is illustrated on turbine cascades.

Application of a RG hybrid RANS/LES model to swirling confined turbulent jets

Abstract: A renormalization group (RG) based hybrid RANS/LES model is validated for turbulent swirling confined jets. The results are compared with the experimental data of Dellenback et al. (1988, Measurements in turbulent swirling flow through an abrupt axisymmetric expansion. AIAA Journal, 26(6), 669–681) and results for the same flows of an unsteady second-moment closure RANS simulation. A general quality/cost comparison is made between the hybrid RANS/LES and the second-moment closure simulations. In the final section, the hybrid RANS/LES result is further compared to a detached-eddy simulation, dynamic -equation LES and dynamic Smagorinsky LES for one of the flows, and the overall good quality of the RG hybrid RANS/LES model demonstrated.

A stable pressure-correction algorithm for low-speed turbulent combustion simulations

Abstract: To perform accurate LES-calculations of low Mach number flows with non-premixed combustion, an efficient, robust and accurate algorithm is needed. So-called pressure-correction methods are efficient algorithms for this purpose. Those methods are well elaborated for flows with constant density. However, in non-premixed combustion simulations, the density is variable in time and space, leading to instabilities. In this paper we provide insight in the origin of the instability by examining the test case of a density jump. Further, we propose improvements to existing schemes in order to increase robustness.

Impingement Heat Transfer with a Nonlinear First-Order k-e Model

Abstract: D = nozzle diameter Dh = hydraulic diameter fRy = blending function H = nozzle-plate distance h = convection coefficient k = turbulent kinetic energy Nu = Nusselt number n = unit normal vector P = impingement detector Re = Reynolds number Ry = dimensionless distance from solid boundary, = ρ√ky/μ S = strain rate, = (2Si j Si j )1/2 S = strain-rate tensor W = nozzle width/impingement detector y = normal distance from nearest solid boundary e = turbulent dissipation rate η = dimensionless strain rate, = τt (S2 + 2)1/2 κ = molecular thermal conductivity μ = molecular viscosity μt = turbulent or eddy viscosity ρ = density τt = turbulence timescale = vorticity, = (2 i j i j )1/2

Scalar PDF and velocity-scalar PDF modelling of the bluff-body stabilised flame HM1 using ILDM

Abstract: Velocity-scalar and scalar PDF results are compared for th e bluff-body stabilised flame HM1 using ILDM chemistry based on mixture fraction, and CO 2 and H2O mass fractions. The same Reynolds stress turbulence model and the same modified Curl m ixing model are used. No effect of radiative heat loss is included. The results for mean veloci ty and Reynolds stresses are satisfactory and very similar for both calculations. Each PDF modelling appr oach implies a different closure for the velocity-scalar correlation. In the present calculations thi leads to significant differences in the radial profiles of mean scalars and of mixture fraction variance (di fferent scalar flux modelling): velocity-scalar PDF results (differential scalar flux model) are better than scalar PDF results (gradient diffusion). Results in composition space (scatter plots) confirm the higher qual ity of the velocity-scalar PDF.

Importance of Buoyancy and Chemistry Modelling in Steady RANS Simulations of Well-Ventilated Tunnel Fires

Abstract: Numerical CFD simulation results are presented for well-ventilated fires in horizontal tunnels. Simulations are performed in the framework of steady Reynolds-Averaged Navier-Stokes modelling. As a turbulence model, a ‘realisable’ k-e model is applied. Both the simple and generalised gradient diffusion hypotheses are compared as models for the buoyancy production of turbulent kinetic energy. Within the framework of the conserved scalar approach (with the mixture fraction as conserved scalar), with pre-assumed β-PDF modelling for the turbulence-chemistry interaction, 2 combustion models are compared as well: a steady laminar flamelet model with a constant strain rate and the full chemical equilibrium model. The simple and the generalised gradient diffusion approach for buoyancy modelling give similar results for the global flow field. However, large differences are visible in the region of smoke reversal. This is important with respect to the prediction of the critical ventilation velocity, an important design parameter for tunnels that should be correctly predicted by numerical simulations. Details of the chemistry model have only a small influence on the prediction of the global flow field. Even for the quantitative determination of the critical ventilation velocity, the differences between the combustion models considered are small. The realisable k-e model, with the generalised gradient diffusion hypothesis for the buoyancy source term, gives satisfactory results for the prediction of the critical velocity, with both chemistry models applied.