Koen Lodefier

One-equation RG hybrid RANS/LES computation of a turbulent impinging jet

A one-equation variant of a previously developed two-equation, renormalization group based, hybrid RANS/LES model is presented. The model consists of a transport equation for the mean dissipation rate and an algebraically prescribed length scale. The length scale is proportional to the wall-distance in RANS regions of the flow and to the filter width in LES regions. A very simple, but efficient, near-wall model is also presented, in the manner of Yakhot and Orszags RNG modeling. The only free parameter entering the low-Reynolds formulation has been calibrated against channel flow, and the value corresponds well with experimental values for the same parameter obtained for homogeneous isotropic turbulence. The model is then validated for a turbulent impinging jet heat transfer problem. Results are satisfactory and compare very favorably against DES and dynamic LES for the same computation.

Hybrid RANS/LES modelling with an approximate renormalization group. II: Applications

The hybrid RANS/LES model developed previously is extended with near-wall modifications. The model is validated, in its low-Reynolds form, for channel flow and for flow over a periodic hill. The results for the channel flow are (indirectly) compared with DES results for the same flow, and are qualitatively similar. The separated flow over the periodic hill shows good agreement with the reference LES data, although performed with about 20 times less grid points. Finally the model is applied in high-Reynolds form with wall functions to a sudden pipe expansion; good agreement with experimental data is also obtained.

Axisymmetric Impingement Heat Transfer with a Nonlinear k-e Model

A nonlinear κ-e model is applied to predict local convective heat transfer in impinging turbulent axisymmetric jets onto a flat plate. Both the nonlinear constitutive law for the turbulent stresses and the e transport equation improve the results. Accurate results are obtained for different geometrical setups (in particular a different nozzle-plate distance), both in terms of heat transfer and of flowfield predictions. Reynolds-number dependence of the results is discussed. Second, it is illustrated that the second-order terms in the constitutive law have a negligible influence on the results, except for the turbulent normal stress profiles. Consequently, a first-order model, with locally flow-dependent eddy viscosity, is sufficient to obtain accurate results for the complex test case under study

One-Equation RG Hybrid RANS/LES Modelling

A one-equation variant of a previously developed two-equation, renormalization group (RG) based, hybrid RANS/LES model is presented. The model has a transport equation for the mean dissipation rate and an algebraically prescribed length scale. The length scale is proportional to the wall distance in RANS regions of the flow and to the filter width in LES regions. A near-wall formulation is used in the manner of Yakhot and Orszag’s RNG modelling. The only free parameter entering the low-Reynolds formulation has been calibrated against channel flow, and the value corresponds well with experimental values for the same parameter obtained for decaying homogeneous isotropic turbulence. Applications to an impinging jet and a plane asymmetric diffuser are presented.

Intermittency based RANS bypass transition modelling

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.

Transition modelling with the SST turbulence model and an intermittency transport equation

In gas turbine engines, laminar-turbulent transition occurs. However, generally, the turbulence models to describe such transition results in too early and too short transition. Combining a turbulence model with a description of intermittency, i.e. the fraction of time the flow is turbulent during the transition phase, can improve it. By letting grow the intermittency from zero to unity, start and evolution of transition can be imposed. In this paper, a method where a dynamic equation of intermittency combining with a two-equation k-ω turbulence model is described. This intermittency factor is a premultiplicator of the turbulent viscosity computed by the turbulence model. Following a suggestion by Menter et al.[1], the start of transition is computed based on local variables.

An Unsteady RANS Transition Model With Dynamic Description of Intermittency

A transition model for describing wake-induced transition is presented. It is based on the SST turbulence model by Menter, with the k–ω part in low-Reynolds form according to Wilcox, and two dynamic equations for intermittency: one for near-wall-intermittency and one for free-stream-intermittency. The total intermittency factor, which is the sum of the two, multiplies the turbulent viscosity computed by the turbulence model. The quality of the transition model is illustrated on the T106a test cascade using experimental results for flow with low free-stream turbulence intensity and transition in separated state and for flow with high free-stream turbulence intensity and transition in attached state. The unsteady results are presented in S–T diagrams of the shape factor and wall shear stress on the suction side. Results show the capability of the model to capture the basics of unsteady transition.© 2005 ASME

Rans modelling of wake induced transition with the dynamic intermittency concept

A transition model for describing wake-induced transition is presented based on the SST turbulence model by Menter and two dynamic equations for intermittency: one for near-wall intermittency and one for free-stream intermittency. In the Navier-Stokes equations, the total intermittency factor, which is the sum of the two, multiplies the turbulent viscosity computed by the turbulence model. The quality of the transition model is illustrated on the T106A test cascade for different levels of inlet free-stream turbulence intensity. The unsteady results are presented in space-time diagrams of shape factor, wall shear stress, momentum thickness and intermittency on the suction side. Results show the capability of the model to capture the physics of unsteady transition. Inevitable shortcomings are also revealed.Copyright

Modelling of Unsteady Transition with a Dynamic Intermittency Equation

ABSTRACT A transition model for describing wake-induced transition is presented based on the SST-model by Menter, with the k - ω part in low-Reynolds form according to Wilcox, and two dynamic equations for intermittency: one for near-wall-intermittency and one for free-stream-intermittency. The total intermittency factor, which is the sum of the two, multiplies the turbulent viscosity computed by the turbulence model. The quality of the transition model is illustrated on the T106a test cascade using experimental results by Stieger and Hodson for transition in separated state and on the T106d test cascade using experimental results from Hilgenfeld, Stadtmuller and Fottner for transition in attached state. The test cases differ in pitch to chord ratio, Reynolds number and inlet free-stream turbulence intensity. The unsteady results are presented in S - T diagrams of the shape factor and wall shear stress on the suction side. Results show the capability of the model to capture the basics of unsteady transition.