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20 pages, 35745 KiB

Open AccessArticle

Effect of Plasma Actuator on Velocity and Temperature Profiles of High Aspect Ratio Rectangular Jet

by Anh Viet Pham, Kazuaki Inaba, Miyuki Saito and Masaharu Sakai

Fluids 2022, 7(8), 281; https://doi.org/10.3390/fluids7080281 - 16 Aug 2022

Cited by 3 | Viewed by 2265

Abstract

The turbulence jet centerline velocity and temperature decay intensely along the centerline flow direction. Thus, improving it could benefit engineering applications, such as air conditioners. However, active flow control techniques with high-aspect-ratio jets, especially for controlling the temperature, have not been widely investigated. [...] Read more.

The turbulence jet centerline velocity and temperature decay intensely along the centerline flow direction. Thus, improving it could benefit engineering applications, such as air conditioners. However, active flow control techniques with high-aspect-ratio jets, especially for controlling the temperature, have not been widely investigated. This paper presents the velocity and temperature performance of a high-aspect-ratio rectangular jet controlled by two dielectric barrier discharge plasma actuators located on the longer sides of the nozzle and controlled by high-voltage and high-frequency pulse-width modulation sinusoidal waves. The scanning method was used to cover 362 cases as combinations of working parameters (modular frequency vs. duty vs. phase difference) for the velocity and temperature performances of the jets. Results show that plasma actuators can control both velocity and temperature distribution with minor input power compared with the rectangular jet’s kinetic energy and heat flux. The velocity increased up to 4% and decreased to 11%, measured at the interest position where

x / h

= 70 on the centerline. There were a 5% increase and a 4% decrease compared to the temperature-based case. Distinctive velocity and temperature distributions were observed under noteworthy cases, indicating the potential of the actuator to create various flow features without installing new hardware on the flow. Full article

(This article belongs to the Special Issue Turbulent Flow)

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14 pages, 1715 KiB

Open AccessArticle

Observations and Parametrization of the Turbulent Energy Dissipation Beneath Non-Breaking Waves

by Darek J. Bogucki, Brian K. Haus and Mohammad Barzegar

Fluids 2022, 7(7), 216; https://doi.org/10.3390/fluids7070216 - 27 Jun 2022

Cited by 1 | Viewed by 1746

Abstract

Here, for non-breaking short surface waves, we have experimentally determined the value of the turbulent eddy viscosity

ν_{T}

or its ratio

ν_{T}^{*} \equiv ν_{T} / ν

, where

ν

is the water kinematic viscosity. The non-breaking wave-generated turbulent eddy [...] Read more.

Here, for non-breaking short surface waves, we have experimentally determined the value of the turbulent eddy viscosity

ν_{T}

or its ratio

ν_{T}^{*} \equiv ν_{T} / ν

, where

ν

is the water kinematic viscosity. The non-breaking wave-generated turbulent eddy viscosity

ν_{T}

was found to depend on the ratio of the wave period, T, to the microscale Kolmogorov time scale,

τ_{η}

. Our observations were consistent with

ν_{T}^{*} = 1.46 \cdot {(T / τ_{η})}^{- 2.6}

when

(T / τ_{η}) < 0.9

. That implied that the

ν_{T}^{*} \propto ϵ^{- 1.3}

, where

ϵ

is the background turbulent energy dissipation rate. The near-surface turbulent flow associated with non-breaking waves was characterized by a short inertial subrange. The background turbulence appears to modulate the amount of energy the non-breaking waves dissipate locally and, consequently, the wave’s decay rate. Our results imply that the background turbulent flow acts as a lubricant, permitting waves to propagate further when traveling over a more energetic turbulent background flow. Our results have implications for the modeling of oceanic wave propagation or the air–sea exchange processes. Full article

(This article belongs to the Special Issue Turbulent Flow)

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25 pages, 7199 KiB

Open AccessArticle

Coherent Structures of a Turbulent Flow Bounded by a Compact Permeable Wall

by James K. Arthur

Fluids 2022, 7(5), 158; https://doi.org/10.3390/fluids7050158 - 29 Apr 2022

Cited by 2 | Viewed by 2347

Abstract

In order to optimize the use of compact porous media as flow and heat transfer devices, it is imperative to understand those coherent structures of the associated flow that generate and sustain turbulence. Given the deficiency of data regarding this area in the [...] Read more.

In order to optimize the use of compact porous media as flow and heat transfer devices, it is imperative to understand those coherent structures of the associated flow that generate and sustain turbulence. Given the deficiency of data regarding this area in the literature, this study has been carried out to fill this need. To this end, a series of particle image velocimetry measurements were conducted to capture a turbulent flow field bounded by a model permeable medium of 85% porosity. The bulk Reynolds numbers based on the bulk velocity through the entire flow domain and the depth of flow over the permeable boundary are approximately 5.0 × 10³ and 2.0 × 10⁴. By applying velocity gradient eigenanalysis, quadrant decomposition, multi-point correlations, and proper orthogonal decomposition, requisite information about the coherent structures of the flow field is extracted. The results indicate the existence of spatial structures whose order, size, and orientation are dependent on the Reynolds number and location along the permeable boundary. While the largest scales are marked by sweeps, ejections, and high vortex activity, there is evidence of inward and outward interactive events at the upstream portions of the permeable boundary layer flow. This work helps to clarify some observations made on turbulent flow over the compact permeable boundary. Full article

(This article belongs to the Special Issue Turbulent Flow)

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23 pages, 4133 KiB

Open AccessArticle

RANS Modelling of a NACA4412 Wake Using Wind Tunnel Measurements

by Narges Tabatabaei, Majid Hajipour, Fermin Mallor, Ramis Örlü, Ricardo Vinuesa and Philipp Schlatter

Fluids 2022, 7(5), 153; https://doi.org/10.3390/fluids7050153 - 26 Apr 2022

Cited by 7 | Viewed by 3595

Abstract

Wake analysis plays a significant role in wind-farm planning through the evaluation of losses and energy yield. Wind-tunnel tests for wake studies have high costs and are time-consuming. Therefore, computational fluid dynamics (CFD) emerges as an efficient alternative. An especially attractive approach is [...] Read more.

Wake analysis plays a significant role in wind-farm planning through the evaluation of losses and energy yield. Wind-tunnel tests for wake studies have high costs and are time-consuming. Therefore, computational fluid dynamics (CFD) emerges as an efficient alternative. An especially attractive approach is based on the solution of the Reynolds-averaged Navier–Stokes (RANS) equations with two-equation turbulence closure models. The validity of this approach and its inherent limitations, however, remain to be fully understood. To this end, detailed wind-tunnel experiments in the wake of a NACA4412 wing section profile are compared with CFD results. Two- and three-dimensional RANS simulations are carried out for a range of angles of attack up to stall conditions at a chord- and inflow-based Reynolds number of

R e_{c} = 4 \times 10^{5}

. Here, we aim to investigate the wake characteristics and self-similar behaviour, both from the experimental and numerical perspectives. The measurements are carried out by means of hot-wire anemometry capturing the wake pattern in several planes. The sensitivity of the CFD model to different configurations of the setup and the considerations required for reliable simulation are discussed. The agreement between CFD, experiments, and the literature is fairly good in many aspects, including the self-similar behaviour and wake parameters, as well as the flow field. Comparison of experiments with URANS/RANS data indicates that the latter is an adequate methodology to characterize wings and their wakes once the CFD setup is designed appropriately and the limitations due to discretization and turbulence modelling are considered. Full article

(This article belongs to the Special Issue Turbulent Flow)

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14 pages, 2360 KiB

Open AccessArticle

Impact of Porous Media on Boundary Layer Turbulence

by Sutharsan Satcunanathan, Matthias Meinke and Wolfgang Schröder

Fluids 2022, 7(4), 139; https://doi.org/10.3390/fluids7040139 - 13 Apr 2022

Cited by 4 | Viewed by 3231

Abstract

The subsonic flows around NACA 0012 aerofoils with a solid, a porous, and a poro-serrated trailing edge (TE) at a Reynolds number of 1 × 10⁶ are investigated by a hybrid Reynolds-averaged Navier–Stokes (RANS)/large-eddy simulation (LES) approach. The porosity is treated by [...] Read more.

The subsonic flows around NACA 0012 aerofoils with a solid, a porous, and a poro-serrated trailing edge (TE) at a Reynolds number of 1 × 10⁶ are investigated by a hybrid Reynolds-averaged Navier–Stokes (RANS)/large-eddy simulation (LES) approach. The porosity is treated by the method-of-volume averaging. In the RANS, a two-equation low Reynolds number k-ε turbulence model is modified to include the porous treatment. Similarly the equations in the LES are extended by the Darcy–Forchheimer model. The simulation is set up with the broadband turbulent boundary layer trailing edge (TBL-TE) noise prediction as a future objective in mind, i.e., the noise sources in the trailing edge region are captured by the LES. To enforce a physically realistic transition from an averaged RANS solution towards a resolved turbulent flow field, at the inflow of the LES coherent structures are generated by means of the reformulated synthetic turbulence generation (RSTG) method. For the poro-serrated TE, turbulence statistics vary in the spanwise direction between the two extremes of a pure solid and a rectangular porous TE, where porosity locally increases the level of turbulence intensity and alters the near wall turbulence anisotropy. Full article

(This article belongs to the Special Issue Turbulent Flow)

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15 pages, 514 KiB

Open AccessArticle

Drag Reduction of Turbulent Boundary Layers by Travelling and Non-Travelling Waves of Spanwise Wall Oscillations

by Martin Skote

Fluids 2022, 7(2), 65; https://doi.org/10.3390/fluids7020065 - 5 Feb 2022

Cited by 5 | Viewed by 2684

Abstract

Turbulence control in the form of a streamwise travelling wave of transverse wall motion was studied numerically by employing direct numerical simulations (DNS). Both total and phase averaging were utilised to examine the statistical behaviour of the turbulence affected by the wall forcing, [...] Read more.

Turbulence control in the form of a streamwise travelling wave of transverse wall motion was studied numerically by employing direct numerical simulations (DNS). Both total and phase averaging were utilised to examine the statistical behaviour of the turbulence affected by the wall forcing, with a focus on the skin friction. Comparison with results from pure temporal and spatial wall forcing are conducted, and a compilation of data is used to explore analogies with drag-reduced channel flow. Full article

(This article belongs to the Special Issue Turbulent Flow)

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13 pages, 4556 KiB

Open AccessArticle

Numerical Investigation of Orifice Nearfield Flow Development in Oleo-Pneumatic Shock Absorbers

by Ahmed A. Sheikh Al-Shabab, Bojan Grenko, Dimitrios Vitlaris, Panagiotis Tsoutsanis, Antonios F. Antoniadis and Martin Skote

Fluids 2022, 7(2), 54; https://doi.org/10.3390/fluids7020054 - 25 Jan 2022

Cited by 5 | Viewed by 2986

Abstract

The flow field development through a simplified shock absorber orifice geometry is investigated using a single phase Large Eddy Simulation. Hydraulic oil is used as the working fluid with a constant inlet velocity and an open top boundary to allow the study to [...] Read more.

The flow field development through a simplified shock absorber orifice geometry is investigated using a single phase Large Eddy Simulation. Hydraulic oil is used as the working fluid with a constant inlet velocity and an open top boundary to allow the study to focus on the free shear layer and the flow development in the vicinity of the main orifice. The flow field is validated using standard mixing layer dynamics. The impact of the orifice shape is discussed with regards to the initial free shear layer growth, boundary layer development and the potential appearance of cavitation bubbles. Observations are made regarding the presence of flow field disturbances upstream of and through the orifice, thereby, leading to a notable turbulence intensity level in those regions. Full article

(This article belongs to the Special Issue Turbulent Flow)

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20 pages, 9428 KiB

Open AccessArticle

A Compressible Turbulence Model for Pressure—Strain

by Hechmi Khlifi and Adnen Bourehla

Fluids 2022, 7(1), 34; https://doi.org/10.3390/fluids7010034 - 14 Jan 2022

Cited by 3 | Viewed by 2290

Abstract

This work focuses on the performance and validation of compressible turbulence models for the pressure-strain correlation. Considering the Launder Reece and Rodi (LRR) incompressible model for the pressure-strain correlation, Adumitroaie et al., Huang et al., and Marzougui et al., used different modeling approaches [...] Read more.

This work focuses on the performance and validation of compressible turbulence models for the pressure-strain correlation. Considering the Launder Reece and Rodi (LRR) incompressible model for the pressure-strain correlation, Adumitroaie et al., Huang et al., and Marzougui et al., used different modeling approaches to develop turbulence models, taking into account compressibility effects for this term. Two numerical coefficients are dependent on the turbulent Mach number, and all of the remaining coefficients conserve the same values as in the original LRR model. The models do not correctly predict the compressible turbulence at a high-speed shear flow. So, the revision of these models is the major aim of this study. In the present work, the compressible model for the pressure-strain correlation developed by Khlifi−Lili, involving the turbulent Mach number, the gradient, and the convective Mach numbers, is used to modify the linear mean shear strain and the slow terms of the previous models. The models are tested in two compressible turbulent flows: homogeneous shear flow and the newly developed plane mixing layers. The predicted results of the proposed modifications of the Adumitroaie et al., Huang et al., and Marzougui et al., models and of its universal versions are compared with direct numerical simulation (DNS) and experiment data. The results show that the important parameters of compressibility in homogeneous shear flow and in the mixing layers are well predicted by the proposal models. Full article

(This article belongs to the Special Issue Turbulent Flow)

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12 pages, 6676 KiB

Open AccessArticle

Combined Stereoscopic Particle Image Velocimetry Measurements in a Single Plane for an Impinging Jet around a Thin Control Rod

by Marwan Alkheir, Hassan H. Assoum, Nour Eldin Afyouni, Kamel Abed Meraim, Anas Sakout and Mouhammad El Hassan

Fluids 2021, 6(12), 430; https://doi.org/10.3390/fluids6120430 - 28 Nov 2021

Cited by 5 | Viewed by 2265

Abstract

Impinging jets are of high interest in many industrial applications and their flow dynamics has a complex three-dimensional behavior. These jets can result in a high noise generation leading to acoustic discomfort. Thus, a passive control mechanism which consists of introducing a thin [...] Read more.

Impinging jets are of high interest in many industrial applications and their flow dynamics has a complex three-dimensional behavior. These jets can result in a high noise generation leading to acoustic discomfort. Thus, a passive control mechanism which consists of introducing a thin rod in the flow of the jet is proposed in order to reduce the noise generation. The stereoscopic particle image velocimetry (SPIV) technique is employed to measure the three velocity components in a plane. An experimental difficulty is encountered to acquire images of the flow in the shadow of the rod which block a part of the field of interest. In this paper, an experimental arrangement is proposed in order to overcome this experimental difficulty using a combined SPIV technique denoted by (C-SPIV). This technique consists of using an inclined mirror to illuminate the area under the rod by reflecting the laser light and two independent systems of SPIV synchronized and correlated together in order to obtain the combined field of velocity in the same plane above and below the rod. The C-SPIV measurements allowed to obtain the kinematic field in the whole area of interest. Thus, vortex shedding frequency, Turbulent Kinetic Energy were calculated and analyzed along with the acoustic signal. These results are of high interest when seeking for noise reduction in such jet configuration. Full article

(This article belongs to the Special Issue Turbulent Flow)

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10 pages, 3764 KiB

Open AccessArticle

Tomographic Particle Image Velocimetry and Dynamic Mode Decomposition (DMD) in a Rectangular Impinging Jet: Vortex Dynamics and Acoustic Generation

by Hassan H. Assoum, Jana Hamdi, Marwan Alkheir, Kamel Abed Meraim, Anas Sakout, Bachar Obeid and Mouhammad El Hassan

Fluids 2021, 6(12), 429; https://doi.org/10.3390/fluids6120429 - 27 Nov 2021

Cited by 5 | Viewed by 2825

Abstract

Impinging jets are encountered in ventilation systems and many other industrial applications. Their flows are three-dimensional, time-dependent, and turbulent. These jets can generate a high level of noise and often present a source of discomfort in closed areas. In order to reduce and [...] Read more.

Impinging jets are encountered in ventilation systems and many other industrial applications. Their flows are three-dimensional, time-dependent, and turbulent. These jets can generate a high level of noise and often present a source of discomfort in closed areas. In order to reduce and control such mechanisms, one should investigate the flow dynamics that generate the acoustic field. The purpose of this study is to investigate the flow dynamics and, more specifically, the coherent structures involved in the acoustic generation of these jets. Model reduction techniques are commonly used to study the underlying mechanisms by decomposing the flow into coherent structures. The dynamic mode decomposition (DMD) is an equation-free method that relies only on the system’s data taken either through experiments or through numerical simulations. In this paper, the DMD technique is applied, and the spatial modes and their frequencies are presented. The temporal content of the DMD’s modes is then correlated with the acoustic signal. The flow is generated by a rectangular jet impinging on a slotted plate (for a Reynolds number Re = 4458) and its kinematic field is obtained via the tomographic particle image velocimetry technique (TPIV). The findings of this research highlight the coherent structures signature in the DMD’s spectral content and show the cross correlations between the DMD’s modes and the acoustic field. Full article

(This article belongs to the Special Issue Turbulent Flow)

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16 pages, 21271 KiB

Open AccessArticle

Turbulent Characteristics and Air Entrainment Patterns in Breaking Surge Waves

by Zhuoran Li, Akash Venkateshwaran and Shooka Karimpour

Fluids 2021, 6(12), 422; https://doi.org/10.3390/fluids6120422 - 23 Nov 2021

Cited by 3 | Viewed by 2940

Abstract

Breaking surge waves are highly turbulent three-dimensional (3D) flows, which occur when the water flow encounters a sudden change in depth or velocity. The 3D turbulent structures across a breaking surge are induced by the velocity gradient across the surge and phase discontinuity [...] Read more.

Breaking surge waves are highly turbulent three-dimensional (3D) flows, which occur when the water flow encounters a sudden change in depth or velocity. The 3D turbulent structures across a breaking surge are induced by the velocity gradient across the surge and phase discontinuity at the front. This paper examined the turbulent structures in breaking surge waves with Froude numbers of 1.71 and 2.13 by investigating the air entrainment and perturbation patterns across the surge front. A combination of the Volume Of Fluid (VOF) method and Large Eddy Simulation (LES) was utilized to capture air entrainment and turbulent structures simultaneously. The 3D nature of the vortical structures was simulated by implementing a spanwise periodic boundary. The water surface perturbation and air concentration profiles were extracted, and the averaged air concentration profiles obtained from the numerical simulations were consistent with laboratory observations reported in the literature. The linkage between turbulent kinetic energy distribution and air entrainment was also explored in this paper. Finally, using quadrant analysis and the Q-criterion, this paper examined the role of the spanwise perturbations in the development of turbulent structures in the surge front. Full article

(This article belongs to the Special Issue Turbulent Flow)

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19 pages, 22072 KiB

Open AccessArticle

Testing Basic Gradient Turbulent Transport Models for Swirl Burners Using PIV and PLIF

by Alexey Savitskii, Aleksei Lobasov, Dmitriy Sharaborin and Vladimir Dulin

Fluids 2021, 6(11), 383; https://doi.org/10.3390/fluids6110383 - 25 Oct 2021

Cited by 2 | Viewed by 1753

Abstract

The present paper reports on the combined stereoscopic particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) measurements of turbulent transport for model swirl burners without combustion. Two flow types were considered, namely the mixing of a free jet with surrounding air [...] Read more.

The present paper reports on the combined stereoscopic particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) measurements of turbulent transport for model swirl burners without combustion. Two flow types were considered, namely the mixing of a free jet with surrounding air for different swirl rates of the jet (Re = 5 × 10³) and the mixing of a pilot jet (Re = 2 × 10⁴) with a high-swirl co-flow of a generic gas turbine burner (Re = 3 × 10⁴). The measured spatial distributions of the turbulent Reynolds stresses and fluxes were compared with their predictions by gradient turbulent transport models. The local values of the turbulent viscosity and turbulent diffusivity coefficients were evaluated based on Boussinesq’s and gradient diffusion hypotheses. The studied flows with high swirl were characterized by a vortex core breakdown and intensive coherent flow fluctuations associated with large-scale vortex structures. Therefore, the contribution of the coherent flow fluctuations to the turbulent transport was evaluated based on proper orthogonal decomposition (POD). The turbulent viscosity and diffusion coefficients were also evaluated for the stochastic (residual) component of the velocity fluctuations. The high-swirl flows with vortex breakdown for the free jet and for the combustion chamber were characterized by intensive turbulent fluctuations, which contributed substantially to the local turbulent transport of mass and momentum. Moreover, the high-swirl flows were characterized by counter-gradient transport for one Reynolds shear stress component near the jet axis and in the outer region of the mixing layer. Full article

(This article belongs to the Special Issue Turbulent Flow)

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27 pages, 7017 KiB

Open AccessArticle

Comparison of Techniques for the Estimation of Flow Parameters of Fan Inflow Turbulence from Noisy Hot-Wire Data

by Luciano Caldas, Carolin Kissner, Maximilian Behn, Ulf Tapken and Robert Meyer

Fluids 2021, 6(11), 372; https://doi.org/10.3390/fluids6110372 - 20 Oct 2021

Cited by 7 | Viewed by 2243

Abstract

Turbulence parameters, in particular integral length scale (ILS) and turbulence intensity (Tu), are key input parameters for various applications in aerodynamics and aeroacoustics. The estimation of these parameters is typically performed using data obtained via hot-wire measurements. On the one hand, hot-wire measurements [...] Read more.

Turbulence parameters, in particular integral length scale (ILS) and turbulence intensity (Tu), are key input parameters for various applications in aerodynamics and aeroacoustics. The estimation of these parameters is typically performed using data obtained via hot-wire measurements. On the one hand, hot-wire measurements are affected by external disturbances resulting in increased measurement noise. On the other hand, commonly applied turbulence parameter estimators lack in robustness. If not addressed correctly, both issues may impede the accuracy of the turbulence parameter estimation. In this article, a procedure consisting of several signal processing steps is presented to filter non-turbulence related disturbances from the unsteady velocity data. The signal processing techniques comprise time- and frequency-domain approaches. For the turbulence parameter estimation, two different models of the turbulence spectra—the von Kármán model and the Bullen model—are fitted to match the spectrum of the measured data. The results of several parameter estimation techniques are compared. Computational Fluid Dynamics (CFD) data are used to validate the estimation techniques and also to assess the influence of the variation in window size on the estimated parameters. Additionally, hot-wire data from a high-speed fan rig are analyzed. ILS and Tu are assessed at several radial positions for two fan speeds. It is found that most techniques yield similar values for ILS and Tu. The comparison of the fitted spectra with the spectra of the measured data shows a good agreement in most cases provided that a sufficiently fine frequency resolution is applied. The ratio of ILS and Tu of the velocity components in longitudinal and transverse direction allows the assessment of flow-isotropy. Results indicate that the turbulence is anisotropic for the investigated flow fields. Full article

(This article belongs to the Special Issue Turbulent Flow)

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18 pages, 2650 KiB

Open AccessArticle

Velocity Profile Representation for Fully Developed Turbulent Flows in Pipes: A Modified Power Law

by Amgad Salama

Fluids 2021, 6(10), 369; https://doi.org/10.3390/fluids6100369 - 19 Oct 2021

Cited by 27 | Viewed by 14157

Abstract

In the design practices of many engineering applications, gross information about the flow field may suffice to provide magnitudes of the parameters that are essential to complete the design with reasonable accuracy. If such design parameters can be estimated following simpler steps, it [...] Read more.

In the design practices of many engineering applications, gross information about the flow field may suffice to provide magnitudes of the parameters that are essential to complete the design with reasonable accuracy. If such design parameters can be estimated following simpler steps, it may be possible to abandon the need to conduct expensive numerical and/or experimental works to produce them. In this work, we are interested in providing a generalized power law that depicts the velocity profile for fully developed turbulent flows. This law incorporates two fitting parameters m and n that represent the exponents of (1) a nondimensional length scale and (2) an overall exponent, respectively. These two parameters may be determined by fitting the experimental and/or computational data. In this work, fitting benchmark experimental and computational fluid dynamics (CFD) data found in the literature reveals that the parameter m changes over a relatively smaller range (between 1 and 2), while the parameter n changes over a wider range (between 1 and 12 for the range of Reynolds number considered). These two parameters (m and n) are, generally, not universal, and they depend on the Reynolds number (Re). A correlation was also developed to correlate n and Re in the turbulent flow region. In order to preserve the continuity of the derivative of the velocity profile at the centerline, a value of m equals 2 over the whole range of Re is recommended. Apart from the near wall area, the new law fits the velocity profile reasonably well. This generalized law abides to a number of favorable stipulations for the velocity profile, namely the continuity of derivatives and reduction to the laminar flow velocity profile for lower values of Re. Full article

(This article belongs to the Special Issue Turbulent Flow)

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12 pages, 3786 KiB

Open AccessArticle

Development of an Analytical Wall Function for Bypass Transition

by Ekachai Juntasaro, Kiattisak Ngiamsoongnirn, Phongsakorn Thawornsathit and Kazuhiko Suga

Fluids 2021, 6(9), 328; https://doi.org/10.3390/fluids6090328 - 14 Sep 2021

Cited by 1 | Viewed by 2366

Abstract

The objective of the present work is to propose an extended analytical wall function that is capable of predicting the bypass transition from laminar to turbulent flow. The algebraic

γ

transition model, the

k - ω

turbulence model and the analytical wall function [...] Read more.

The objective of the present work is to propose an extended analytical wall function that is capable of predicting the bypass transition from laminar to turbulent flow. The algebraic

γ

transition model, the

k - ω

turbulence model and the analytical wall function are integrated together in this work to detect the transition onset and start the transition process. The present analytical wall function is validated with the experimental data, the Blasius solution and the law of the wall. With this analytical wall function, the transition onset in the skin friction coefficient is detected and the growth rate of transition is properly generated. The predicted mean velocity profiles are found to be in good agreement with the Blasius solution in the laminar flow, the experimental data in the transition zone and the law of the wall in the fully turbulent flow. Full article

(This article belongs to the Special Issue Turbulent Flow)

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15 pages, 899 KiB

Open AccessArticle

Cutting-Edge Turbulence Simulation Methods for Wind Energy and Aerospace Problems

by Stefan Heinz, Joachim Peinke and Bernhard Stoevesandt

Fluids 2021, 6(8), 288; https://doi.org/10.3390/fluids6080288 - 16 Aug 2021

Cited by 8 | Viewed by 2564

Abstract

The availability of reliable and efficient turbulent flow simulation methods is highly beneficial for wind energy and aerospace developments. However, existing simulation methods suffer from significant shortcomings. In particular, the most promising methods (hybrid RANS-LES methods) face divergent developments over decades, there is [...] Read more.

The availability of reliable and efficient turbulent flow simulation methods is highly beneficial for wind energy and aerospace developments. However, existing simulation methods suffer from significant shortcomings. In particular, the most promising methods (hybrid RANS-LES methods) face divergent developments over decades, there is a significant waste of resources and opportunities. It is very likely that this development will continue as long as there is little awareness of conceptional differences of hybrid methods and their implications. The main purpose of this paper is to contribute to such clarification by identifying a basic requirement for the proper functioning of hybrid RANS-LES methods: a physically correct communication of RANS and LES modes. The state of the art of continuous eddy simulations (CES) methods (which include the required mode communication) is described and requirements for further developments are presented. Full article

(This article belongs to the Special Issue Turbulent Flow)

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16 pages, 4285 KiB

Open AccessArticle

Aerodynamic Free-Flight Conditions in Wind Tunnel Modelling through Reduced-Order Wall Inserts

by Narges Tabatabaei, Ramis Örlü, Ricardo Vinuesa and Philipp Schlatter

Fluids 2021, 6(8), 265; https://doi.org/10.3390/fluids6080265 - 27 Jul 2021

Cited by 4 | Viewed by 4399

Abstract

Parallel sidewalls are the standard bounding walls in wind tunnels when making a wind tunnel model for free-flight condition. The consequence of confinement in wind tunnel tests, known as wall-interference, is one of the main sources of uncertainty in experimental aerodynamics, limiting the [...] Read more.

Parallel sidewalls are the standard bounding walls in wind tunnels when making a wind tunnel model for free-flight condition. The consequence of confinement in wind tunnel tests, known as wall-interference, is one of the main sources of uncertainty in experimental aerodynamics, limiting the realizability of free-flight conditions. Although this has been an issue when designing transonic wind tunnels and/or in cases with large blockage ratios, even subsonic wind tunnels at low-blockage-ratios might require wall corrections if a good representation of free-flight conditions is intended. In order to avoid the cumbersome streamlining methods especially for subsonic wind tunnels, a sensitivity analysis is conducted in order to investigate the effect of inclined sidewalls as a reduced-order wall insert in the airfoil plane. This problem is investigated via Reynolds-averaged Navier–Stokes (RANS) simulations, and a NACA4412 wing at the angles of attack between 0 and 11 degrees at a moderate Reynolds number (400 k) is considered. The simulations are validated with well-resolved large-eddy simulation (LES) results and experimental wind tunnel data. Firstly, the wall-interference contribution in aerodynamic forces, as well as the local pressure coefficients, are assessed. Furthermore, the isolated effect of confinement is analyzed independent of the boundary-layer growth. Secondly, wall-alignment is modified as a calibration parameter in order to reduce wall-interference based on the aforementioned assessment. In the outlined method, we propose the use of linear inserts to account for the effect of wind tunnel walls, which are experimentally simple to realize. The use of these inserts in subsonic wind tunnels with moderate blockage ratio leads to very good agreement between free-flight and wind tunnel data, while this approach benefits from simple manufacturing and experimental realization. Full article

(This article belongs to the Special Issue Turbulent Flow)

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12 pages, 19209 KiB

Open AccessArticle

Assessing IDDES-Based Wall-Modeled Large-Eddy Simulation (WMLES) for Separated Flows with Heat Transfer

by Rozie Zangeneh

Fluids 2021, 6(7), 246; https://doi.org/10.3390/fluids6070246 - 5 Jul 2021

Cited by 1 | Viewed by 2750

Abstract

The Wall-modeled Large-eddy Simulation (WMLES) methods are commonly accompanied with an underprediction of the skin friction and a deviation of the velocity profile. The widely-used Improved Delayed Detached Eddy Simulation (IDDES) method is suggested to improve the prediction of the mean skin friction [...] Read more.

The Wall-modeled Large-eddy Simulation (WMLES) methods are commonly accompanied with an underprediction of the skin friction and a deviation of the velocity profile. The widely-used Improved Delayed Detached Eddy Simulation (IDDES) method is suggested to improve the prediction of the mean skin friction when it acts as WMLES, as claimed by the original authors. However, the model tested only on flow configurations with no heat transfer. This study takes a systematic approach to assess the performance of the IDDES model for separated flows with heat transfer. Separated flows on an isothermal wall and walls with mild and intense heat fluxes are considered. For the case of the wall with heat flux, the skin friction and Stanton number are underpredicted by the IDDES model however, the underprediction is less significant for the isothermal wall case. The simulations of the cases with intense wall heat transfer reveal an interesting dependence on the heat flux level supplied; as the heat flux increases, the IDDES model declines to predict the accurate skin friction. Full article

(This article belongs to the Special Issue Turbulent Flow)

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11 pages, 244 KiB

Open AccessArticle

Statistical Model of Turbulent Dispersion Recapitulated

by J. J. H. Brouwers

Fluids 2021, 6(5), 190; https://doi.org/10.3390/fluids6050190 - 18 May 2021

Cited by 1 | Viewed by 1836

Abstract

A comprehensive summary and update is given of Brouwers’ statistical model that was developed during the previous decade. The presented recapitulated model is valid for general inhomogeneous anisotropic velocity statistics that are typical of turbulence. It succeeds and improves the semiempirical and heuristic [...] Read more.

A comprehensive summary and update is given of Brouwers’ statistical model that was developed during the previous decade. The presented recapitulated model is valid for general inhomogeneous anisotropic velocity statistics that are typical of turbulence. It succeeds and improves the semiempirical and heuristic models developed during the previous century. The model is based on a Langevin and diffusion equation of which the derivation involves (i) the application of general principles of physics and stochastic theory; (ii) the application of the theory of turbulence at large Reynolds numbers, including the Lagrangian versions of the Kolmogorov limits; and (iii) the systematic expansion in powers of the inverse of the universal Lagrangian Kolmogorov constant

C_{0}

,

C_{0}

about 6. The model is unique in the collected Langevin and diffusion models of physics and chemistry. Presented results include generally applicable expressions for turbulent diffusion coefficients that can be directly implemented in numerical codes of computational fluid mechanics used in environmental and industrial engineering praxis. This facilitates the more accurate and reliable prediction of the distribution of the mean concentration of passive or almost passive admixture such as smoke, aerosols, bacteria, and viruses in turbulent flow, which are all issues of great societal interest. Full article

(This article belongs to the Special Issue Turbulent Flow)

22 pages, 13690 KiB

Open AccessArticle

Computational Study of Three-Dimensional Flow Past an Oscillating Cylinder Following a Figure Eight Trajectory

by Sofia Peppa, Lambros Kaiktsis, Christos E. Frouzakis and George S. Triantafyllou

Fluids 2021, 6(3), 107; https://doi.org/10.3390/fluids6030107 - 5 Mar 2021

Cited by 2 | Viewed by 2677

Abstract

The paper presents a computational study of three-dimensional flow past a cylinder forced to oscillate in a uniform stream, following a figure-eight trajectory. Flow simulations were performed for Re = 400, for different cases, defined in terms of the oscillation mode (‘counter-clockwise’ or [...] Read more.

The paper presents a computational study of three-dimensional flow past a cylinder forced to oscillate in a uniform stream, following a figure-eight trajectory. Flow simulations were performed for Re = 400, for different cases, defined in terms of the oscillation mode (‘counter-clockwise’ or ‘clockwise’), for values of the ratio, F, of the transverse oscillation frequency to the Strouhal frequency close to 1.0. The results demonstrate that, for F ≤ 1.0, counter-clockwise cylinder motion is associated with positive power transfer from the flow to the cylinder, corresponding to excitation; for the clockwise motion, power transfer is negative at intermediate to high amplitudes, corresponding to damping. For the clockwise mode, in the range F = 0.9–1.1, a transition to two-dimensional vortex street is identified for transverse oscillation amplitude exceeding a critical value. This results from the induced suction of vortices, which moves vortex formation and shedding closer to the cylinder surface, thus resulting in a narrower wake, characterized by an effective lower Reynolds number. Both oscillation modes are characterized by higher harmonics in the lift force spectrum, with the third harmonic being very pronounced, while even harmonics are present for the case of clockwise mode, resulting from a wake transition to a “S + P” mode. Full article

(This article belongs to the Special Issue Turbulent Flow)

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24 pages, 3163 KiB

Open AccessArticle

Coherent Streamwise Vortex Structure of a Three-Dimensional Wall Jet

by Lhendup Namgyal and Joseph W. Hall

Fluids 2021, 6(1), 35; https://doi.org/10.3390/fluids6010035 - 11 Jan 2021

Viewed by 2230

Abstract

The dynamics of the coherent structures in a turbulent three-dimensional wall jet with an exit Reynolds number of 250,000 were investigated using the Snapshot Proper Orthogonal Decomposition (POD). A low-dimensional reconstruction using the first 10 POD modes indicates that the turbulent flow is [...] Read more.

The dynamics of the coherent structures in a turbulent three-dimensional wall jet with an exit Reynolds number of 250,000 were investigated using the Snapshot Proper Orthogonal Decomposition (POD). A low-dimensional reconstruction using the first 10 POD modes indicates that the turbulent flow is dominated by streamwise vortex structures that grow in size and relative strength, and that are often accompanied by strong lateral sweeps of fluid across the wall. This causes an increase in the bulging and distortions of streamwise velocity contours as the flow evolves downstream. The instantaneous streamwise vorticity computed from the reconstructed instantaneous velocities has a high level of vorticity associated with these outer streamwise vortex structures, but often has a persistent pair of counter-rotating regions located close to the wall on either side of the jet centerline. A model of the coherent structures in the wall jet is presented. In this model, streamwise vortex structures are produced in the near-field by the breakdown of vortex rings formed at the jet outlet. Separate structures are associated with the near-wall streamwise vorticity. As the flow evolves downstream, the inner near-wall structures tilt outward, while the outer streamwise structures amalgamate to form larger streamwise asymmetric structures. In all cases, these streamwise vortex structures tend to cause large lateral velocity sweeps in the intermediate and far-field regions of the three-dimensional wall jet. Further, these structures meander laterally across the jet, causing a strongly intermittent jet flow. Full article

(This article belongs to the Special Issue Turbulent Flow)

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31 pages, 452 KiB

Open AccessConcept Paper

Reduced Numerical Modeling of Turbulent Flow with Fully Resolved Time Advancement. Part 1. Theory and Physical Interpretation

by Alan R. Kerstein

Fluids 2022, 7(2), 76; https://doi.org/10.3390/fluids7020076 - 13 Feb 2022

Cited by 6 | Viewed by 2275

Abstract

A multiscale modeling concept for numerical simulation of multiphysics turbulent flow utilizing map-based advection is described. The approach is outlined with emphasis on its theoretical foundations and physical interpretations in order to establish the context for subsequent presentation of the associated numerical algorithms [...] Read more.

A multiscale modeling concept for numerical simulation of multiphysics turbulent flow utilizing map-based advection is described. The approach is outlined with emphasis on its theoretical foundations and physical interpretations in order to establish the context for subsequent presentation of the associated numerical algorithms and the results of validation studies. The model formulation is a synthesis of existing methods, modified and extended in order to obtain a qualitatively new capability. The salient feature of the approach is that time advancement of the flow is fully resolved both spatially and temporally, albeit with modeled advancement processes restricted to one spatial dimension. This one-dimensional advancement is the basis of a bottom-up modeling approach in which three-dimensional space is discretized into under-resolved mesh cells, each of which contains an instantiation of the modeled one-dimensional advancement. Filtering is performed only to provide inputs to a pressure correction that enforces continuity and to obtain mesh-scale-filtered outputs if desired. The one-dimensional advancement, the pressure correction, and coupling of one-dimensional instantiations using a Lagrangian implementation of mesh-resolved volume fluxes is sufficient to advance the three-dimensional flow without time advancing coarse-grained equations, a feature that motivates the designation of the approach as autonomous microscale evolution (AME). In this sense, the one-dimensional treatment is not a closure because there are no unclosed terms to evaluate. However, the approach is additionally suitable for use as a subgrid-scale closure of existing large-eddy-simulation methods. The potential capabilities and limitations of both of these implementations of the approach are assessed conceptually and with reference to demonstrated capabilities of related methods. Full article

(This article belongs to the Special Issue Turbulent Flow)

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