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Turbulence and Transitional Modeling of Aerodynamic Flows

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A special issue of Fluids (ISSN 2311-5521).

Deadline for manuscript submissions: closed (30 June 2019) | Viewed by 45215

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Dear Colleagues,

Over the last four decades, a great deal of progress has been made in the prediction of a wide variety of turbulent flows using physical models based on Reynolds-Averaged Navier-Stokes (RANS) equations, Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS). The solution of RANS equations via modelling of turbulent stresses remains, by far, the most widely-used approach for the prediction of turbulent flows encountered in aerospace and other industrial applications. Recent developments include the hybrid RANS/LES models, known as the Detached Eddy Simulation (DES), and its variants, the Delayed Detached Eddy Simulation (DDES) and Improved Delayed Detached Eddy Simulation (IDDES). Wall-Model LES (WM-LES) and Wall Resolved LES (WR-LES) models have also been developed to improve accuracy and efficiency. Somewhat more recently, there have been efforts to use machine learning and tools of uncertainty quantification, and data driven modelling to improve the model constants in existing models, as well as to pave the way for the development of new models. There has been parallel effort to augment turbulence models to account for intermittency and transition. A critical review of all these topics, as well the traditional topics in improvement and development of one-, two- and RSM models, are needed by researchers and CFD practitioners working in academia, government and industry. The goal of this Special Issue is to provide a state-of-the-art summary of recent developments in the modelling of turbulent and transitional flows. Papers describing complex applications in aerospace, turbomachinery, automobile and other industries are also welcome. The Guest Editor is open to considering in any paper relevant to the subject matter of the Special Issue.

Prof. Ramesh Agarwal
Guest Editor

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Keywords

Reynolds-Averaged Navier-Stokes (RANS) Models of all types and categories
Large Eddy Simulation including Wall-Modeled (WM) LES
Hybrid RANS/LES (DES, DDES, IDDES,SBES)*
Wall-Resolved (WR) Models
Data Driven Modeling including improvement of Turbulence Models using Uncertainty Quantification (UQ) and Machine Learning
Intermittency and Transition Modeling
Applications to Aircraft, Turbomachinery, Automobiles and other industrial products

*DES= Detached Eddy Simulation, DDES = Delayed Detached Eddy Simulation
IDDES= Improved Detached Eddy Simulation, SBES= Stress Blended Eddy Simulation

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Published Papers (10 papers)

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Research

19 pages, 8579 KiB

Open AccessArticle

Analysis of Transition for a Flow in a Channel via Reduced Basis Methods

by Gaetano Pascarella, Ioannis Kokkinakis and Marco Fossati

Fluids 2019, 4(4), 202; https://doi.org/10.3390/fluids4040202 - 5 Dec 2019

Cited by 5 | Viewed by 3661

Abstract

The study of the flow mechanisms leading to transition in a planar channel flow is investigated by means of a reduced basis method known as Dynamic Mode Decomposition (DMD). The problem of identification of the most relevant DMD modes is addressed in terms of the ability to (i) provide a fairly accurate reconstruction of the flow field, and (ii) match the most relevant flow structures at the beginning of the transition region. A comparative study between a natural method of selection based on the energetic content of the modes and a new one based on the temporal dynamics of the modes is here presented. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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18 pages, 3982 KiB

Open AccessArticle

Heat Convective Effects on Turbulence and Airflow inside an B767 Aircraft Cabin

by Maher Shehadi

Fluids 2019, 4(3), 167; https://doi.org/10.3390/fluids4030167 - 8 Sep 2019

Cited by 1 | Viewed by 2516

Abstract

Thermal plumes generated by human bodies can affect the temperature and humidity of the surrounding environment. An experimental study investigated the effects of thermal plumes formed by aircraft passengers on airflow and turbulence characteristics inside aircraft-cabins. An 11-row, wide-body B767 cabin mockup was used with actual seats, air diffusers and cabin profile. Thermal manikins were used simulating passengers in the cabin. Tracer gas and air speed inside the cabin were measured while the heat from the manikins was turned on and off to help understand the effects of the thermal heat released by the manikins. Results showed that tracer gas distribution were more uniformly and equally distributed around the release source and the air speed fluctuation were lower under cooler environments when the thermal manikins were turned off. Heated environments increased the values of turbulence kinetic energy and the turbulence intensity levels. However, the effects on the turbulence intensity were less significant compared to the turbulence kinetic energy. On the other hand, the dissipation rates were higher for unheated cases in the front and back sections of the mockup cabin. The relative uncertainty for tracer gas sampling ranged between ±5–14% for heated manikins versus ±8–17% for unheated manikins. Higher uncertainty levels accompanied the turbulence measurements due to the highly chaotic nature of the flow inside the cabin. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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28 pages, 3879 KiB

Open AccessArticle

Statistical Structure and Deviations from Equilibrium in Wavy Channel Turbulence

by Saadbin Khan and Balaji Jayaraman

Fluids 2019, 4(3), 161; https://doi.org/10.3390/fluids4030161 - 27 Aug 2019

Cited by 8 | Viewed by 3736

Abstract

The structure of turbulent flow over non-flat surfaces is a topic of major interest in practical applications in both engineering and geophysical settings. A lot of work has been done in the fully rough regime at high Reynolds numbers where the effect on the outer layer turbulence structure and the resulting friction drag is well documented. It turns out that surface topology plays a significant role on the flow drag especially in the transitional roughness regime and therefore, is hard to characterize. Survey of literature shows that roughness function depends on the interaction of roughness height, flow Reynolds number, and topology shape. In addition, if the surface topology contains large enough scales then it can impact the outer layer dynamics and in turn modulate the total frictional force. Therefore, it is important to understand the mechanisms underlying drag increase from systematically varied surface undulations in order to better interpret quantifications based on mean statistics such as roughness function. In this study, we explore the mechanisms that modulate the turbulence structure over a two-dimensional (2D) sinusoidal wavy surface with a fixed amplitude, but varying slopes that are sufficiently small to generate only intermittent flow separation. To accomplish this, we perform a set of highly resolved direct numerical simulations (DNS) to model the turbulent flow between two infinitely wide 2D wavy plates at a friction Reynolds number,

R e_{τ} = 180

, which represents modest scale separation. We pursue two different but related flavors of analysis. The first one adopts a roughness characterization flavor of such wavy surfaces. The second one focuses on understanding the nonequilibrium near-surface turbulence structure and their impact on roughness characterization. Analysis of the different statistical quantifications show strong dependence on wave slope for the roughness function indicating drag increase due to enhanced turbulent stresses resulting from increased production of vertical velocity variance from the surface undulations. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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20 pages, 8179 KiB

Open AccessArticle

Impact of Longitudinal Acceleration and Deceleration on Bluff Body Wakes

by Brett Peters and Mesbah Uddin

Fluids 2019, 4(3), 158; https://doi.org/10.3390/fluids4030158 - 18 Aug 2019

Cited by 4 | Viewed by 4132

Abstract

This study investigated the unsteady acceleration aerodynamics of bluff bodies through the study of a channel mounted square cylinder undergoing free-stream acceleration of ±20 ms⁻² with Reynolds numbers spanning 3.2 × 10⁴ to 3.6 × 10⁵. To achieve this, a numerical simulation was created with a commercial finite volume unstructured computational fluid dynamics code, which was first validated using Improved Delayed Detached Eddy Simulation against experimental and direct numerical simulated results. Then, the free stream conditions were subjected to a periodic velocity signal where data were recorded and ensemble averaged over at least 30 distinct acceleration and deceleration data points. This enabled the comparison of body forces and flow field variations among accelerating, steady and decelerating free-stream conditions. Body force analysis determined that decelerating and accelerating drag forces varied −47% and 44%, respectively, in comparison to steady free-stream conditions. In addition, several differences were also observed and explored such as near-body flow structures, wake dynamics, Kármán vortices and vorticity production during the aforementioned conditions. The primary interest of this study was for the future application towards road vehicles for predictive dynamic modeling and aerodynamic development. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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28 pages, 6385 KiB

Open AccessArticle

Turbulence Modeling Effects on the CFD Predictions of Flow over a Detailed Full-Scale Sedan Vehicle

by Chunhui Zhang, Charles Patrick Bounds, Lee Foster and Mesbah Uddin

Fluids 2019, 4(3), 148; https://doi.org/10.3390/fluids4030148 - 1 Aug 2019

Cited by 52 | Viewed by 8709

Abstract

In today’s road vehicle design processes, Computational Fluid Dynamics (CFD) has emerged as one of the major investigative tools for aerodynamics analyses. The age-old CFD methodology based on the Reynolds Averaged Navier–Stokes (RANS) approach is still considered as the most popular turbulence modeling approach in automotive industries due to its acceptable accuracy and affordable computational cost for predicting flows involving complex geometries. This popular use of RANS still persists in spite of the well-known fact that, for automotive flows, RANS turbulence models often fail to characterize the associated flow-field properly. It is even true that more often, the RANS approach fails to predict correct integral aerodynamic quantities like lift, drag, or moment coefficients, and as such, they are used to assess the relative magnitude and direction of a trend. Moreover, even for such purposes, notable disagreements generally exist between results predicted by different RANS models. Thanks to fast advances in computer technology, increasing popularity has been seen in the use of the hybrid Detached Eddy Simulation (DES), which blends the RANS approach with Large Eddy Simulation (LES). The DES methodology demonstrated a high potential of being more accurate and informative than the RANS approaches. Whilst evaluations of RANS and DES models on various applications are abundant in the literature, such evaluations on full-car models are relatively fewer. In this study, four RANS models that are widely used in engineering applications, i.e., the realizable

k - ε

two-layer, Abe–Kondoh–Nagano (AKN)

k - ε

low-Reynolds, SST

k - ω

, and V2F are evaluated on a full-scale passenger vehicle with two different front-end configurations. In addition, both cases are run with two DES models to assess the differences between the flow predictions obtained using RANS and DES. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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22 pages, 2030 KiB

Open AccessArticle

Stability Analysis on Nonequilibrium Supersonic Boundary Layer Flow with Velocity-Slip Boundary Conditions

by Xin He, Kai Zhang and Chunpei Cai

Fluids 2019, 4(3), 142; https://doi.org/10.3390/fluids4030142 - 31 Jul 2019

Cited by 10 | Viewed by 3323

Abstract

This paper presents our recent work on investigating velocity slip boundary conditions’ effects on supersonic flat plate boundary layer flow stability. The velocity-slip boundary conditions are adopted and the flow properties are obtained by solving boundary layer equations. Stability analysis of two such boundary layer flows is performed by using the Linear stability theory. A global method is first utilized to obtain approximate discrete mode values. A local method is then utilized to refine these mode values. All the modes in these two scenarios have been tracked upstream-wisely towards the leading edge and also downstream-wisely. The mode values for the no-slip flows agree well with the corresponding past results in the literature. For flows with slip boundary conditions, a stable and an unstable modes are detected. Mode tracking work is performed and the results illustrate that the resonance phenomenon between the stable and unstable modes is delayed with slip boundary conditions. The enforcement of the slip boundary conditions also shortens the unstable mode region. As to the conventional second mode, flows with slip boundary conditions can be more stable streamwisely when compared with the results for corresponding nonslip flows. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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28 pages, 16182 KiB

Open AccessArticle

Hybrid RANS/LES Turbulence Model Applied to a Transitional Unsteady Boundary Layer on Wind Turbine Airfoil

by Di Zhang, Daniel R. Cadel, Eric G. Paterson and K. Todd Lowe

Fluids 2019, 4(3), 128; https://doi.org/10.3390/fluids4030128 - 11 Jul 2019

Cited by 3 | Viewed by 4407

Abstract

A hybrid Reynolds-averaged Navier Stokes/large-eddy simulation (RANS/LES) turbulence model integrated with a transition formulation is developed and tested on a surrogate model problem through a joint experimental and computational fluid dynamic approach. The model problem consists of a circular cylinder for generating coherent unsteadiness and a downstream airfoil in the cylinder wake. The cylinder flow is subcritical, with a Reynolds number of 64,000 based upon the cylinder diameter. The quantitative dynamics of vortex shedding and Reynolds stresses in the cylinder near wake are well captured, owing to the turbulence-resolving large eddy simulation mode that was activated in the wake. The hybrid model switched between RANS and LES modes outside the boundary layers, as expected. According to the experimental and simulation results, the airfoil encountered local flow angle variations up to ±50°. Further analysis through a phase-averaging technique found phase lags in the airfoil boundary layer along the chordwise locations, and both the phase-averaged and mean velocity profiles collapsed into the Law-of-the-wall in the range of

0 < y^{+} < 50

. The features of high blade-loading fluctuations due to unsteadiness and transitional boundary layers are of interest in the aerodynamic studies of full-scale wind turbine blades, making the current model problem a comprehensive benchmark case for future model development and validation. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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22 pages, 9201 KiB

Open AccessArticle

Unsteady RANS Simulations of Strong and Weak 3D Stall Cells on a 2D Pitching Aerofoil

by Dajun Liu and Takafumi Nishino

Fluids 2019, 4(1), 40; https://doi.org/10.3390/fluids4010040 - 2 Mar 2019

Cited by 3 | Viewed by 4609

Abstract

A series of three-dimensional unsteady Reynolds-averaged Navier–Stokes (RANS) simulations are conducted to investigate the formation of stall cells over a pitching NACA 0012 aerofoil. Periodic boundary conditions are applied to the spanwise ends of the computational domain. Several different pitching ranges and frequencies are adopted. The influence of the pitching range and frequency on the lift coefficient (C_L) hysteresis loop and the development of leading-edge vortex (LEV) agrees with earlier studies in the literature. Depending on pitching range and frequency, the flow structures on the suction side of the aerofoil can be categorized into three types: (i) strong oscillatory stall cells resembling what are often observed on a static aerofoil; (ii) weak stall cells which are smaller in size and less oscillatory; and (iii) no stall cells at all (i.e., flow remains two-dimensional) or only very weak oval-shaped structures that have little impact on C_L. A clear difference in C_L during the flow reattachment stage is observed between the cases with strong stall cells and with weak stall cells. For the cases with strong stall cells, arch-shaped flow structures are observed above the aerofoil. They resemble the Π-shaped vortices often observed over a pitching finite aspect ratio wing. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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10 pages, 786 KiB

Open AccessArticle

Assessment of Turbulence Models in a Hypersonic Cold-Wall Turbulent Boundary Layer

by Junji Huang, Jorge-Valentino Bretzke and Lian Duan

Fluids 2019, 4(1), 37; https://doi.org/10.3390/fluids4010037 - 26 Feb 2019

Cited by 14 | Viewed by 4780

Abstract

In this study, the ability of standard one- or two-equation turbulence models to predict mean and turbulence profiles, the Reynolds stress, and the turbulent heat flux in hypersonic cold-wall boundary-layer applications is investigated. The turbulence models under investigation include the one-equation model of [...] Read more.

k - ω

model by Menter, as well as the shear-stress transport

k - ω

model by Menter. Reynolds-Averaged Navier-Stokes (RANS) simulations with the different turbulence models are conducted for a flat-plate, zero-pressure-gradient turbulent boundary layer with a nominal free-stream Mach number of 8 and wall-to-recovery temperature ratio of

0.48

, and the RANS results are compared with those of direct numerical simulations (DNS) under similar conditions. The study shows that the selected eddy-viscosity turbulence models, in combination with a constant Prandtl number model for turbulent heat flux, give good predictions of the skin friction, wall heat flux, and boundary-layer mean profiles. The Boussinesq assumption leads to essentially correct predictions of the Reynolds shear stress, but gives wrong predictions of the Reynolds normal stresses. The constant Prandtl number model gives an adequate prediction of the normal turbulent heat flux, while it fails to predict transverse turbulent heat fluxes. The discrepancy in model predictions among the three eddy-viscosity models under investigation is small. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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18 pages, 8730 KiB

Open AccessArticle

CFD Analysis of Twin Turbulent Impinging Round Jets at Different Impingement Angles

by Raj Narayan Gopalakrishnan and Peter J. Disimile

Fluids 2018, 3(4), 79; https://doi.org/10.3390/fluids3040079 - 23 Oct 2018

Cited by 3 | Viewed by 4353

Abstract

Round jets impinging at multiple impingement angles were considered for this study to gain better understanding of the parameters affecting resultant jet growth and velocity distribution. Work done by the authors previously on single jet has helped to establish that the SST (Shear Stress Transport) k-ω model is the ideal turbulence model for predicting flow characteristics of jets exiting a fully developed pipe at low Reynolds number. Hence, for the study of impinging jets, SST k-ω turbulence model was used to study the velocity and jet growth characteristics. Based on the mesh obtained from the grid sensitivity study, jets impinging at 30, 45 and 60 degrees at Reynolds number of 7500 were numerically analyzed. It was observed that the profile of the resultant jet closely matched with the prediction of elliptical profile predicted by past researchers. In addition, it was seen that higher jet growth was predicted in the case of jets impinging at a higher impingement angle. Full article

(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)

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