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Numerical and modelling aspects of large-eddy and hybrid simulations of turbulent flows
KTH, School of Engineering Sciences (SCI), Mechanics. (Linne' FLOW Centre, KTH Mechanics)ORCID iD: 0000-0002-9645-5880
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

In this study, the explicit algebraic sub-grid scale (SGS) model (EAM) has been extensively validated in wall-resolved large-eddy simulations (LES) of wall-bounded turbulent flows at different Reynolds numbers and a wide range of resolutions. Compared to eddy-viscosity based models, the formulation of the EAM is more consistent with the physics and allows to accurately capture SGS anisotropy,which is relevant especially close to walls.The present work aims to extend the validation of the EAM to larger Reynolds numbers using codes with different orders of numerical accuracy.The first simulations, performed by using a pseudo-spectral code, show that the use of the EAM, compared to the dynamic Smagorinsky model (DSM), leads to significant improvements in the prediction of the first-and second order statistics of turbulent channel flow.These improvements are observed from relatively low to  reasonably high Reynolds numbers and with coarse grids.The evaluation of the EAM was continued by implementing and testing of the EAM in the general-purpose finite-volume code OpenFOAM.Several tests of LES of turbulent channel flow have shown thatthe use of the Rhie and Chow (R&C) interpolation in OpenFOAM induces significant numerical dissipation.A new custom-built solver has been utilized in order to minimize the dissipation without generating significant adverse effects. The use of the EAM, together with the new solver, gives a substantially improved prediction of the mean velocity profiles as compared to predictions using the DSM, resulting in roughly 50% reduction in the grid point requirements to achieve a given degree of accuracy. In periodic hill flow, LES with the EAM agreed reasonably well with the reference dataat different bulk Reynolds numbers and reduced the misprediction of the first- and second order statistics observed in LES with DSM.The reduction of the R&C filter dissipation was also shown to be beneficial for the prediction of the mean quantities. An analysis of the skin friction along the lower wall reveals spanwise-elongated, almost axi-symmetric vortical structures generated by the Kelvin-Helmholtz instability. The structures introduced a significant amount of anisotropy.The last part of the study involved the development of a novel hybrid RANS-LES model where explicit algebraic Reynolds stress modelling is applied in both RANS and LES regions.Validations have been conducted on turbulent channel and periodic hill flows at different Reynolds numbers.The explicit algebraic Reynolds stress model for improved-delayed-detached-eddy simulation (EARSM-IDDES) gives reasonable predictions of the mean quantities and Reynolds stresses in both the geometries considered.The use of EARSM-IDDES, compared to the k-omega SST-IDDES model, improves the estimation of the quantities close to the wall.The present work has proven that the use of EAM in wall-resolved LES of wall-bounded flows in simple and complex geometries leads to a substantial reduction of  computational requirements both in high-accuracy and general-purpose codes, compared to the use of eddy-viscosity models.In hybrid simulations the EARSM-IDDES shows a clear potential in capturing the physics of wall-bounded flows.

##### Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 60
##### Series
TRITA-SCI-FOU ; 2019:48
##### Keywords [en]
turbulence, wall-bounded flows, large-eddy simulation, SGS modelling, OpenFOAM, hybrid RANS-LES
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
ISBN: 978-91-7873-369-9 (electronic)OAI: oai:DiVA.org:kth-264025DiVA, id: diva2:1371733
##### Public defence
2019-12-19, F3, Lindstedtsvägen 26, Stockholm, 10:15 (English)
##### Funder
Swedish Research Council, 621-2014- 5700Vinnova, 2017-04887
##### Note

QC 20191122

Available from: 2019-11-22 Created: 2019-11-20 Last updated: 2019-11-22Bibliographically approved
##### List of papers
1. Taking large-eddy simulation of wall-bounded flows to higher Reynolds numbers by use of anisotropy-resolving subgrid models
Open this publication in new window or tab >>Taking large-eddy simulation of wall-bounded flows to higher Reynolds numbers by use of anisotropy-resolving subgrid models
2017 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 2, article id 034601Article in journal (Refereed) Published
##### Abstract [en]

Properly resolved large-eddy simulations of wall-bounded high Reynolds number flows using standard subgrid-scale (SGS) models requires high spatial and temporal resolution. We have shown that a more elaborate SGS model taking into account the SGS Reynolds stress anisotropies can relax the requirement for the number of grid points by at least an order of magnitude for the same accuracy. This was shown by applying the recently developed explicit algebraic subgrid-scale model (EAM) to fully developed high Reynolds number channel flows with friction Reynolds numbers of 550, 2000, and 5200. The near-wall region is fully resolved, i.e., no explicit wall modeling or wall functions are applied. A dynamic procedure adjusts the model at the wall for both low and high Reynolds numbers. The resolution is reduced, from the typically recommended 50 and 15 wall units in the stream-and spanwise directions respectively, by up to a factor of 5 in each direction. It was shown by comparison with direct numerical simulations that the EAM is much less sensitive to reduced resolution than the dynamic Smagorinsky model. Skin friction coefficients, mean flow profiles, and Reynolds stresses are better predicted by the EAM for a given resolution. Even the notorious overprediction of the streamwise fluctuation intensity typically seen in poorly resolved LES is significantly reduced whenEAMis used on coarse grids. The improved prediction is due to the capability of the EAM to capture the SGS anisotropy, which becomes significant close to the wall.

##### Place, publisher, year, edition, pages
American Physical Society, 2017
Natural Sciences
##### Identifiers
urn:nbn:se:kth:diva-220931 (URN)10.1103/PhysRevFluids.2.034601 (DOI)000396070400001 ()2-s2.0-85028541529 (Scopus ID)
##### Note

QC 20180110

Available from: 2018-01-09 Created: 2018-01-09 Last updated: 2019-11-20Bibliographically approved
2. Improving LES with OpenFOAM by minimizing numerical dissipation and use of Explicit Algebraic SGS stress model
Open this publication in new window or tab >>Improving LES with OpenFOAM by minimizing numerical dissipation and use of Explicit Algebraic SGS stress model
(English)In: Article in journal (Refereed) Submitted
##### Abstract [en]

There is a rapidly growing interest in using general-purpose CFD codes based on second order finite volume methods for Large-Eddy Simulation (LES) in a wide range of applications,and in many cases involving wall-bounded flows. However, such codes are strongly affected by numerical dissipation and the accuracy obtained for typical LES-resolutions is often poor. In the present study we approach the problem of improving LES capability of such codes by reduction of the numerical dissipation and use of an anisotropy-capturing sub-grid scale (SGS) stress model. The latter is of special importance for wall-resolved LES with resolutions where the SGS anisotropy can be substantial. Here we use the Explicit Algebraic (EA) SGS model (L. Marstorp, et al., J. Fluid Mech. {\bf 639}, (2009)), and comparisons are made for channel flow at friction Reynolds numbers up to 934 with the dynamic Smagorinsky model.The numerical dissipation is reduced byusing an OpenFOAM based custom-built flow solver that modifies the  Rhie \& Chow interpolation and allows to control and minimize its effects without causing numerical instability (in viscous, fully turbulent flows). Different resolutions were used and large improvements of the LES accuracy were demonstrated for skin friction, mean velocity and other flow statistics by use of the new solver in combination with the EA SGS model.By reducing the numerical dissipation and using the EA SGS model the resolution requirements for wall-resolved LES can be significantly reduced.

##### Keywords
wall-bounded flows, OpenFOAM, SGS modelling
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
urn:nbn:se:kth:diva-264013 (URN)
##### Funder
Swedish Research Council, 621-2014- 5700
##### Note

QC 20191129

Available from: 2019-11-20 Created: 2019-11-20 Last updated: 2019-11-29Bibliographically approved
3. Capturing Reynolds number effects in the periodic hill flow by using LES with anisotropy-resolving sub-grid scale model
Open this publication in new window or tab >>Capturing Reynolds number effects in the periodic hill flow by using LES with anisotropy-resolving sub-grid scale model
2019 (English)In: 11th International Symposium on Turbulence and Shear Flow Phenomena (TSFP11), 2019Conference paper, Published paper (Refereed)
##### Abstract [en]

Concerning wall resolved large-eddy simulation (LES), a considerable reduction of computational resources is achievable by employing the Explicit Algebraic subgrid scale model (EAM) (\cite{marstorp2009explicit}).LES of periodic hill is carried out using OpenFOAM with the EAM and a low-diffusive implementation that has been previously tested on a turbulent channel flow. The aim of the present study is to evaluate in a broad sense the influence of  the Reynolds number on the flow quantities.

##### Keywords
SGS modelling, Periodic hill
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
urn:nbn:se:kth:diva-264019 (URN)
##### Conference
11th International Symposium on Turbulence and Shear Flow Phenomena (TSFP11), July 30 to August 2, 2019, Grand Harbour Hotel, Southampton, UK
##### Funder
Swedish Research Council, 621-2014- 5700
##### Note

QC 20191122

Available from: 2019-11-20 Created: 2019-11-20 Last updated: 2019-11-22Bibliographically approved
4. Reynolds number effects in periodic hill flow: an LES study using OpenFOAM and the Explicit Algebraic SGS stress model
Open this publication in new window or tab >>Reynolds number effects in periodic hill flow: an LES study using OpenFOAM and the Explicit Algebraic SGS stress model
(English)In: Article in journal (Refereed) Submitted
##### Abstract [en]

Periodic hill channel flow at two different bulk Reynolds numbers of 10595 and 37000 is studied by wall-resolved large-eddy simulations (LES) to investigate the detailed Reynolds number effects of the separation bubble and the associated flow physics.The capability of OpenFOAM is here extended by using a modified solver which considerably reduces the Rhie and Chow (R\&C) interpolation-induced dissipation.The capability of the code is further enhanced by use of  the Explicit Algebraic SGS stress model (EAM) (L. Marstorp {\itshape et al.}, J. Fluid Mech. {\bf 639}, (2009)).The EAM was shown to be instrumental for accurate prediction of turbulent structures and anisotropy while still maintaining a moderate amount of grid points.The generation of large-scale, spanwise oriented structures by the shear layer instability and the subsequent breakdown of these structures give an anisotropy state close to the axisymmetry limit followed by a transition to a much more isotropic state. These variations are well captured with the LES with EAM. The Reynolds number dependency of the separation bubble size is also well captured. For both the Reynolds numbers considered, the use of the EAM has significantly enhanced the prediction accuracy of the skin friction and the mean quantities, compared to LES with the Dynamic Smagorinsky model.

##### Keywords
Periodic hill, SGS modelling, OpenFOAM
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
urn:nbn:se:kth:diva-264022 (URN)
##### Funder
Vinnova, 2017-04887
##### Note

QC 20191126

Available from: 2019-11-20 Created: 2019-11-20 Last updated: 2019-11-26Bibliographically approved
5. Development and testing of an anisotropy-resolving model for hybrid RANS-LES computations of turbulent incompressible flows
Open this publication in new window or tab >>Development and testing of an anisotropy-resolving model for hybrid RANS-LES computations of turbulent incompressible flows
2019 (English)Report (Other academic)
##### Abstract [en]

In the present work a new model for hybrid RANS-LES computations based on the improved-delayed-detached eddy simulation (IDDES) approach is proposed.The model combines the Explicit Algebraic Reynolds Stress model for RANS and the Explicit Algebraic sub-grid scale stress model.Several tests using the present model have been conducted for a turbulent plane channel flow and periodic hill flow adopting the general-purpose finite-volume code OpenFOAM. For each geometry two different Reynolds numbers are investigated. For both the Reynolds numbers considered, hybrid computations using the EARSM-IDDES model reasonably predict the mean flow quantities and the Reynolds stresses, compared to the established $k-\omega$ SST-IDDES.The ability of the EARSM to capture flow anisotropy is advantageous especially close to the lower wall of the periodic hill flow. The proposed model induces a steeper RANS to LES transition, with a resulting reduction of the extent of the transitional region, but with a less smooth solution through the transition.

##### Keywords
hybrid modelling, turbulent flows, OpenFOAM
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
urn:nbn:se:kth:diva-264023 (URN)
##### Funder
Vinnova, 2017-04887
##### Note

QC 20191126

Available from: 2019-11-20 Created: 2019-11-20 Last updated: 2019-11-26Bibliographically approved

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