Slawomir Kubacki

TRANSITION MODELLING BASED ON LOCAL VARIABLES

An engineering transition model has been developed which allows to couple general experimental transition criteria to general-purpose CFD codes. The model is based on a transport equation for a generalized intermittency variable and is formulated without the use of integral parameters. The model is coupled with the SST turbulence model and tested against a series of 2D testcases. The simulatiosn were carried out with CFX-TASCflow.

Convective heat transfer prediction for an axisymmetric jet impinging onto a flat plate with an improved k‐ ω model

Purpose – This paper aims to provide improvements to the newest version of the k‐ ω turbulence model of Wilcox for convective heat transfer prediction in turbulent axisymmetric jets impinging onto a flat plate.Design/methodology/approach – Improvements to the heat transfer prediction in the impingement zone are obtained using the stagnation flow parameter of Goldberg and the vortex stretching parameter of Wilcox. The third invariant of the strain rate tensor in the form of Shih et al. and the blending function of Menter are applied in order make negligible the influence of the impingement modifications in the benchmark flows for turbulence models. Further, it is demonstrated that for two‐dimensional jets impinging onto a flat plate the stagnation region Nusselt number predicted by the original k‐ ω model is in good agreement with direct numerical simulation (DNS) and experimental data. Also for two‐dimensional jets, the proposed modification is deactivated.Findings – The proposed modification has been app...

Hybrid RANS/LES of Flow in a Rib-Roughened Rotating Channel

The aim of the present study is to verify the ability of a hybrid RANS/LES model, based on the newest version of the RANS k-ω model of Wilcox (2008), to reproduce the flow in a rib-roughened rotating channel. The numerical results are compared to experimental data by Coletti and Arts (2011), and Coletti et al. (2012). Results obtained with the basic RANS k-ω model are analysed as well, with and without the correction term for frame rotation by Hellsten (1998). It is demonstrated that the hybrid RANS/LES k-ω model produces accurate results for the rotating ribbed duct flow. However, the RANS model with the correction term for frame rotation produces quite good results as well for mean velocity profiles, although not as good as with the hybrid model. Further, the RANS model has lower quality in the representation of the small-scale fluctuations.

Hybrid RANS/LES of plane impinging jets with k-omega based models

Plane impinging jets with nozzle-plate distances H/B=10 at Re=13500 and H/B=9.2 at Re=20000 are simulated with k-omega based hybrid RANS/LES models. Three ways of substitution of the turbulent length scale by the local grid size in the LES mode of the hybrid RANS/LES models are tested. The results show that the hybrid models give much better prediction of wall shear stress and heat transfer rate along the impingement plate than the RANS model. The good performance of the hybrid models is due to their ability to resolve the evolution and break-up of the vortices in the shear layer of the jet, which strongly affects the turbulent flow and convective heat transfer in the stagnation region and the developing wall-jet region.

Convective heat transfer predictions in an axisymmetric jet impinging onto a flat plate using an improved k-ω model

The paper discusses the convective heat transfer prediction in turbulent axisymmetric jets impinging onto a flat plate using the k-@wmodel of Wilcox (2006) [14]. Improvements to the heat transfer predictions are obtained in flow regions characterized by large level of strain using an impingement invariant proposed by Manceau (2003) [19]. The Manceau term is carefully introduced in order to leave the k-@wmodel predictions unmodified for reference flows. This is obtained using a blending function of Menter (1994) [12]. As an alternative, a modification based on the von Karman length scale is also discussed. Both modifications do not lead to stability problems in flow regions characterized by large levels of strain, preserving the robustness of the k-@wmodel. Improvements have been obtained for convective heat transfer prediction in stagnation flow regions of axisymmetric jets impinging onto a flat plate with nozzle-plate distances H/D=2,6,10 and Reynolds numbers Re=23000,70000.

Predictions of round impinging jet heat transfer with two k‐ω hybrid RANS/LES models

Purpose – Applicability of two k‐ω hybrid RANS/LES and a k‐ω RANS models is studied for simulation of round impinging jets at nozzle‐plate distance H/D=2 with Reynolds number 70000, H/D=2 with Reynolds number 5000 and H/D=10 with Reynolds number 5000 (D is the nozzle exit diameter). The aim is to verify two concepts of unified hybrid RANS/LES formulations, one of DES (Detached Eddy Simulation) type and one of LNS (Limited Number Scales) type in analysis of impinging jet flow and heat transfer. The grid resolution requirements are also discussed.Design/methodology/approach – The simulations are performed with two k‐ω based hybrid RANS/LES models of very different nature, one of DES type and one of LNS type, and the RANS k‐ω model. For the lower Reynolds number (5000), also dynamic Smagorinsky LES is done. Both hybrid model formulations converge to the same RANS k‐ω model in the near‐wall region and have the same Smagorinsky limit on fine isotropic grids in the LES mode of the hybrid models.Findings – With ...

Hybrid RANS/LES of plane impinging jets with k-ω based models

Plane impinging jets with nozzle-plate distances H/B=10 at Re=13500 and H/B=9.2 at Re=20000 are simulated with k-ω based hybrid RANS/LES models. Three ways of substitution of the turbulent length scale by the local grid size in the LES mode of the hybrid RANS/LES models are tested. The results show that the hybrid models give much better prediction of wall shear stress and heat transfer rate along the impingement plate than the RANS model. The good performance of the hybrid models is due to their ability to resolve the evolution and break-up of the vortices in the shear layer of the jet, which strongly affects the turbulent flow and convective heat transfer in the stagnation region and the developing wall-jet region.

Robust Optimization with Gaussian Process Models

In this chapter, the application of the Gaussian regression models in the robust design and uncertainty quantification is demonstrated. The computationally effective approach based on the Kriging method and relative expected improvement concept is described in detail. The new sampling criterion is proposed which leads to localization of the robust optimum in a limited number of steps. The methodology is employed to the optimal design process of the intake channel of the small turboprop engine.

Simulation of impinging jet mass transfer at high Schmidt number with algebraic models

A simple method is proposed for simulation of impinging jet mass transfer at high Schmidt numbers. Two variants of algebraic models, based on the limiting behaviour of the turbulent diffusivity Dt for y+ → 0, are employed in order to provide Dt in the near-wall region while away from walls a formula based on the ratio of turbulent to molecular viscosity (νt/ν) is applied for determination of the turbulent Schmidt number. Blending between the algebraic expressions and the formula for νt/ν is performed in the buffer region of the boundary layer, making the model independent of y+ at larger distances from the walls. With the proposed models, very good results have been obtained for the mass transfer at high Schmidt number in impinging jet flows. The model also functions well at low Schmidt number.

Hybrid RANS/LES of round impinging jets

Fluid flow and heat transfer characteristics are presented for simulations of round impinging jets at two nozzle-plate distances H/D = 2 and 10 (D is the nozzle exit diameter) and two Reynolds numbers Re = 5000 and 70,000 with hybrid RANS/LES (Reynolds-averaged Navier-Stokes/Large Eddy Simulation), dynamic Smagorinsky LES and RANS k−ω models. Three k−ω based hybrid RANS/LES models are analyzed. With the hybrid RANS/LES models, improved heat transfer results are obtained, when compared to RANS, in the impact region and in the developing wall-jet region. For accurate predictions at low nozzle-plate distance, it is necessary to sufficiently resolve the formation and development of the near-wall vortices in the jet impingement region. At high nozzle-plate distance, it is important to capture the evolution and breakup of the unsteady vortices in the shear layer of the jet, so that realistic mean and fluctuating velocity profiles are obtained in the impact jet region.