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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "J: Thermal Management".

Deadline for manuscript submissions: closed (9 October 2023) | Viewed by 5091

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Prof. F. Xavier Trias

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Heat and Mass Transfer Technological Center, Technical University of Catalonia, ESEIAAT, Colom 11, Terrassa, 08222 Barcelona, Spain
Interests: fluid mechanics; turbulence modeling; CFD; large-eddy simulation; direct numerical simulation; applied mathematics and numerical methods
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Dear Colleagues,

Navier–Stokes (NS) equations are an excellent mathematical model for turbulent flows. Unfortunately, most of practical turbulent flows cannot be computed directly from NS equations because not enough resolution is available to resolve all relevant scales of motion. Hence, in the foreseeable future, numerical simulations of turbulent flows will have to resort to models of smaller scale. We may therefore turn to large-eddy simulation (LES) to predict the large scale behaviour of turbulent flows. In LES, the large scales of motions in a flow are explicitly computed, whereas effects of small-scale motions are modelled. Most of the difficulties in LES are then associated with the presence of walls where SGS activity tends to vanish. This implies an accurate resolution of the near-wall region, which demands an (extremely) high computational force. Wall-modelling techniques for LES or hybrid RANS-LES approaches are attempts to overcome this inherent limitation of LES. On the other hand, the spatial and temporal discretization techniques can also play a very important role in the simulation of turbulent flows; namely, there is a complex interplay between the numeric and the turbulence model. In this context, the objective of this Special Issue in Energies is to bring researchers working on advanced, cutting-edge methods for the simulation of turbulent flows using different turbulence modelling techniques, as well as on the application of such methods to complex engineering systems, together. The scope includes (but is not limited to):

Direct numerical simulation;
LES fundamentals;
Conservative discretizations for CFD problems;
Hybrid RANS-LES methods;
Wall-modelling techniques;
Heat and mass transfer problems;
Multiphase flows;
Combustion;
Environmental and geophysical applications;
Industrial applications.

Prof. F. Xavier Trias
Guest Editor

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Keywords

Turbulence
Numerical methods for turbulent flows
Large-eddy simulation
Turbulence modeling
Subgrid-scale model
Computational fluid dynamics
Wall-modeling
Hybrid RANS-LES

Published Papers (4 papers)

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Research

20 pages, 18436 KiB

Open AccessArticle

A Study and Optimization of the Unsteady Flow Characteristics in the Last Stage Impeller of a Small-Scale Multi-Stage Hydraulic Turbine

by Jun Yang, Tao Peng, Gang Xu, Wenli Hu, Huazhou Zhong and Xiaohua Liu

Energies 2024, 17(1), 107; https://doi.org/10.3390/en17010107 - 24 Dec 2023

Viewed by 684

Abstract

The demand for small-size multi-stage hydraulic turbines is experiencing rapid growth due to the ongoing efforts towards energy conservation and emission reduction. On account of their compact structural design, these turbines feature a more intricate internal flow configuration, rendering them prone to the creation of low-pressure zones, resulting in vapor–liquid two-phase flow, accompanied by the development of intense vibrations and noise, thereby adversely affecting the safety and stability of turbine operations. Concurrently, an innovative method for analyzing flow fields has been formulated combined with two-dimensional frequency domain visualization technology and proper orthogonal decomposition, serving to establish a diagnostic and optimization framework for the unsteady flow structures within rotating machinery by considering the features related to frequency distribution, spatial distribution, and energy contributions. It was found that there are two main unsteady flow structures which are the areas with high risks of vaporization under this study condition. According to the flow characteristics of the analysis, an optimization scheme was proposed to improve the two-phase flow problem in the secondary impeller, and the preliminary results were satisfactory. Full article

(This article belongs to the Special Issue Applied Mathematics and Numerical Methods of Fluid Mechanics and Turbulence Modeling)

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14 pages, 1158 KiB

Open AccessArticle

Robust Eddy Viscosity Turbulence Modeling with Elliptic Relaxation and Compound Wall Treatment

by Mirza Popovac

Energies 2023, 16(9), 3685; https://doi.org/10.3390/en16093685 - 25 Apr 2023

Cited by 1 | Viewed by 912

Abstract

This paper presents a holistic Reynolds-averaged Navier–Stokes (RANS) turbulence modeling framework for the computational fluid dynamics (CFD) simulations of complex wall-bounded turbulent flows. Based on the elliptic relaxation idea, the deployed eddy viscosity turbulence model reconstructs the near-wall stress anisotropy and nonviscous effects. The appropriate selection of the turbulent quantities that are being solved for, together with the zero value wall boundary condition for the related turbulent quantities, renders the model less sensitive to the near-wall grid nonuniformities and resolution. The unified near-wall velocity profile, obtained based on the boundary layer theory, is used to devise the compound near-wall treatment that ensures the robustness of the numerical simulation. The proposed turbulence modeling framework is implemented into the general-purpose open-source CFD code and validated against the generic test cases with satisfactory agreement. Full article

(This article belongs to the Special Issue Applied Mathematics and Numerical Methods of Fluid Mechanics and Turbulence Modeling)

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26 pages, 8491 KiB

Open AccessArticle

Direct Numerical Simulation of a Turbulent Boundary Layer Encountering a Smooth-to-Rough Step Change

by Umair Ismail

Energies 2023, 16(4), 1709; https://doi.org/10.3390/en16041709 - 08 Feb 2023

Cited by 3 | Viewed by 1474

Abstract

Using a direct numerical simulation (DNS), we investigate the onset of non-equilibrium effects and the subsequent emergence of a self-preserving state as a turbulent boundary layer (TBL) encounters a smooth-to-rough (STR) step change. The rough surface comprises over 2500 staggered cuboid-shaped elements where [...] Read more.

50 θ_{0}

from the inflow. A

R e_{θ} = 4500

value is attained along with

\frac{δ}{k} \approx 35

as the TBL develops. While different flow parameters adjust at dissimilar rates that further depend on the vertical distance from the surface and perhaps on

δ_{S T R} / k

, an equilibrium for wall stress, mean velocity, and Reynolds stresses exists across the entire TBL by

35 δ_{S T R}

after the step change. First-order statistics inside the inner layer adapt much earlier, i.e., at

10

–

15 δ_{S T R}

after the step change. Like rough-to-smooth (RTS) scenarios, an equilibrium layer develops from the surface. Unlike RTS transitions, a nascent logarithmic layer is identifiable much earlier, at

4 δ_{S T R}

after the step change. The notion of equivalent sandgrain roughness does not apply upstream of this fetch because non-equilibrium advection effects permeate into the inner layer. The emergent equilibrium TBL is categorized by a fully rough state (

k_{s}^{+} \approx 120

–

130

;

k_{s} / k \approx 2.8

). Decomposition of wall stress into constituent parts reveals no streamwise dependence. Mean velocity in the outer layer is well approximated by Coles’ wake law. The wake parameter and shape factor are enhanced above their smooth-wall counterparts. Quadrant analysis shows that shear-stress-producing motions adjust promptly to the roughness, and the balance between ejections and sweeps in the outer layer remains impervious to the underlying surface. Full article

(This article belongs to the Special Issue Applied Mathematics and Numerical Methods of Fluid Mechanics and Turbulence Modeling)

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16 pages, 6574 KiB

Open AccessArticle

Numerical and Experimental Investigation of a Non-Premixed Double Swirl Combustor

by Jiming Lin, Ming Bao, Feng Zhang, Yong Zhang and Jianhong Yang

Energies 2022, 15(2), 458; https://doi.org/10.3390/en15020458 - 10 Jan 2022

Cited by 1 | Viewed by 1290

Abstract

This paper focuses on a detailed numerical investigation combined with experimental research for a non-premixed swirl combustor, which aims to analyze the effects of the blade angle of the outer swirler and equivalence ratio on flow and combustion characteristics. In the experiment, the temperature in the furnace was obtained with a thermocouple, while a realizable k-ε turbulence model and two-step reaction mechanism of methane and air are used in the numerical method. The calculation results are in good agreement with the experimental data. The results reveal that the air flow rate through the swirler accounts for a small amount of the total air due to the influence of the draft fan, and there is no central recirculation zone (CRZ) despite the presence of the swirler. It was also found that NO emissions gradually decrease as the blade angle of the outer swirler increases. It was also indicated that the average temperature is 100 K higher than the general combustor with a 58° blade angle in the furnace by increasing the equivalent ratio of the tertiary air area, and the NO emissions reduced by approximately 25%. This study can provide guidance for the operation and structural design of non-premixed swirl combustors. Full article

(This article belongs to the Special Issue Applied Mathematics and Numerical Methods of Fluid Mechanics and Turbulence Modeling)

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