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Entropy
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Hybrid LES/RANS Simulations in Fundamental, Environmental, and Industrial Applications
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Hybrid LES/RANS Simulations in Fundamental, Environmental, and Industrial Applications

A special issue of Entropy (ISSN 1099-4300). This special issue belongs to the section "Statistical Physics".

Deadline for manuscript submissions: closed (10 July 2024) | Viewed by 5135

Special Issue Editor

Dr. Amir E. Fard

E-Mail Website1 Website2
Guest Editor
School of Engineering, Newcastle University, Newcastle NE1 7RU, UK
Interests: hybrid LES/RANS; turbulence; CFD; wall-modelled LES; multiphase flows; Lattice Boltzmann method
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The developments in computational power and resources over the past few decades has led to widespread adoption of numerical simulation of turbulent flows. Due to the relatively large number of grid points needed to capture all eddies up to the Kolmogorov scale, the direct numerical simulation (DNS) of large Reynolds number flows is currently restricted to small geometries. While using coarser grids in large eddy simulation (LES), we must come up with an adequate method for modelling in order to describe dissipation. Therefore, resolved-LES is still not the best option for flows near walls. Wall-modelled LES (WMLES) is an alternative method, modelling the near-wall area with Reynolds averaged Navier–Stokes (RANS) and resolving the outer region with LES. This Special Issue aims to demonstrate recent advances in wall-modelled LES and in any other approaches that address turbulence simulation of wall-bounded flows with reasonable computational resources. Manuscripts covering the range of "hybrid" models, "wall-stress models (WSM)," and other cutting-edge approaches are encouraged.

Dr. Amir E. Fard
Guest Editor

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Published Papers (4 papers)

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Research

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21 pages, 11202 KiB  
Article
Simulation of Flow Around a Finite Rectangular Prism: Influence of Mesh, Model, and Subgrid Length Scale
by Xutong Zhang, Maxime Savoie, Mark K. Quinn, Ben Parslew and Alistair Revell
Entropy 2025, 27(1), 65; https://doi.org/10.3390/e27010065 - 13 Jan 2025
Viewed by 843
Abstract
This study investigates the flow field around a finite rectangular prism using both experimental and computational methods, with a particular focus on the influence of the turbulence approach adopted, the mesh resolution employed, and different subgrid length scales. Ten turbulence modelling and simulation [...] Read more.
This study investigates the flow field around a finite rectangular prism using both experimental and computational methods, with a particular focus on the influence of the turbulence approach adopted, the mesh resolution employed, and different subgrid length scales. Ten turbulence modelling and simulation approaches, including both ‘scale-modelling’ Reynolds-Averaged Navier–Stokes (RANS) models and ‘scale-resolving’ Delayed Detached Eddy Simulation (DDES), were tested across six different mesh resolutions. A case with sharp corners allows the location of the flow separation to be fixed, which facilitates a focus on the separated flow region and, in this instance, the three-dimensional interaction of three such regions. The case, therefore, readily enables an assessment of the ‘grey-area’ issue, whereby some DDES methods demonstrate delayed activation of the scale-resolving model, impacting the size of flow recirculation. Experimental measurements were shown to agree well with reference data for the same geometry, after which particle image velocimetry (PIV) data were gathered to extend the reference dataset. Numerical predictions from the RANS models were generally quite reasonable but did not show improvement with further refinement, as one would expect, whereas DDES clearly demonstrated continuous improvement in predictive accuracy with progressive mesh refinement. The shear-layer-adapted (SLA) subgrid length scale (ΔSLA) displayed consistently superior performance compared to the more widely used length scale based on local cell volume, particularly for moderate mesh resolutions commonly employed in industrial settings with limited resources. In general, front-body separation and reattachment exhibited greater sensitivity to mesh refinement than wake resolution. Finally, in order to correlate the observed DDES mesh requirements with the observations from the converged RANS solutions, an approximation for the Taylor microscale was explored as a potential tool for mesh sizing. Full article
(This article belongs to the Special Issue Hybrid LES/RANS Simulations in Fundamental, Environmental, and Industrial Applications)
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20 pages, 3282 KiB  
Article
A Near-Wall Methodology for Large-Eddy Simulation Based on Dynamic Hybrid RANS-LES
by Michael Tullis and D. Keith Walters
Entropy 2024, 26(12), 1095; https://doi.org/10.3390/e26121095 - 14 Dec 2024
Viewed by 954
Abstract
Attempts to mitigate the computational cost of fully resolved large-eddy simulation (LES) in the near-wall region include both the hybrid Reynolds-averaged Navier–Stokes/LES (HRL) and wall-modeled LES (WMLES) approaches. This paper presents an LES wall treatment method that combines key attributes of the two, [...] Read more.
Attempts to mitigate the computational cost of fully resolved large-eddy simulation (LES) in the near-wall region include both the hybrid Reynolds-averaged Navier–Stokes/LES (HRL) and wall-modeled LES (WMLES) approaches. This paper presents an LES wall treatment method that combines key attributes of the two, in which the boundary layer mesh is sized in the streamwise and spanwise directions comparable to WMLES, and the wall-normal mesh is comparable to a RANS simulation without wall functions. A mixing length model is used to prescribe an eddy viscosity in the near-wall region, with the mixing length scale limited based on local mesh size. The RANS and LES regions are smoothly blended using the dynamic hybrid RANS-LES (DHRL) framework. The results are presented for the turbulent channel flow at two Reynolds numbers, and comparison to the DNS results shows that the mean and fluctuating quantities are reasonably well predicted with no apparent log-layer mismatch. A detailed near-wall meshing strategy for the proposed method is presented, and estimates indicate that it can be implemented with approximately twice the number of grid points as traditional WMLES, while avoiding the difficulties associated with analytical or numerical wall functions and modified wall boundary conditions. Full article
(This article belongs to the Special Issue Hybrid LES/RANS Simulations in Fundamental, Environmental, and Industrial Applications)
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17 pages, 10130 KiB  
Article
Study on the Tip Leakage Loss Mechanism of a Compressor Cascade Using the Enhanced Delay Detached Eddy Simulation Method
by Shiyan Lin, Ruiyu Li and Limin Gao
Entropy 2024, 26(4), 295; https://doi.org/10.3390/e26040295 - 28 Mar 2024
Viewed by 1465
Abstract
The leakage flow has a significant impact on the aerodynamic losses and efficiency of the compressor. This paper investigates the loss mechanism in the tip region based on a high-load cantilevered stator cascade. Firstly, a high-fidelity flow field structure was obtained based on [...] Read more.
The leakage flow has a significant impact on the aerodynamic losses and efficiency of the compressor. This paper investigates the loss mechanism in the tip region based on a high-load cantilevered stator cascade. Firstly, a high-fidelity flow field structure was obtained based on the Enhanced Delay Detached Eddy Simulation (EDDES) method. Subsequently, the Liutex method was employed to study the vortex structures in the tip region. The results indicate the presence of a tip leakage vortex (TLV), passage vortex (PV), and induced vortex (IV) in the tip region. At i=4°,8°, the induced vortex interacts with the PV and low-energy fluid, forming a “three-shape” mixed vortex. Finally, a qualitative and quantitative analysis of the loss sources in the tip flow field was conducted based on the entropy generation rate, and the impact of the incidence on the losses was explored. The loss sources in the tip flow field included endwall loss, blade profile loss, wake loss, and secondary flow loss. At i=0°, the loss primarily originated from the endwall and blade profile, accounting for 40% and 39%, respectively. As the incidence increased, the absolute value of losses increased, and the proportion of loss caused by secondary flow significantly increased. At i=8°, the proportion of secondary flow loss reached 47%, indicating the most significant impact. Full article
(This article belongs to the Special Issue Hybrid LES/RANS Simulations in Fundamental, Environmental, and Industrial Applications)
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Review

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20 pages, 4055 KiB  
Review
Physically Consistent Resolving Simulations of Turbulent Flows
by Stefan Heinz
Entropy 2024, 26(12), 1044; https://doi.org/10.3390/e26121044 - 30 Nov 2024
Cited by 3 | Viewed by 909
Abstract
Usually applied simulation methods for turbulent flows as large eddy simulation (LES), wall-modeled LES (WMLES), and detached eddy simulation (DES) face significant challenges: they are characterized by improper resolution variations and essential practical simulation problems given by huge computational cost, imbalanced resolution transitions, [...] Read more.
Usually applied simulation methods for turbulent flows as large eddy simulation (LES), wall-modeled LES (WMLES), and detached eddy simulation (DES) face significant challenges: they are characterized by improper resolution variations and essential practical simulation problems given by huge computational cost, imbalanced resolution transitions, and resolution mismatch. Alternative simulation methods are described here. By using an extremal entropy analysis, it is shown how minimal error simulation methods can be designed. It is shown that these methods can overcome the typical shortcomings of usually applied simulation methods. A crucial ingredient of this analysis is the identification of a mathematically implied general hybridization mechanism, which is missing in existing methods. Applications to several complex high Reynolds number flow simulations reveal essential performance, functionality, and computational cost advantages of minimal error simulation methods. Full article
(This article belongs to the Special Issue Hybrid LES/RANS Simulations in Fundamental, Environmental, and Industrial Applications)
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