Journal articles on the topic 'Turbulence Flow'
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1
Sofiadis, G., and I. Sarris. "Reynolds number effect of the turbulent micropolar channel flow." Physics of Fluids 34, no. 7 (July 2022): 075126. http://dx.doi.org/10.1063/5.0098453.
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The turbulent regime of non-Newtonian flows presents a particular interest as flow behavior is directly affected by the internal microstructure type of the fluid. Differences in the dispersed phase of a particle laden flow can either lead to drag reduction and turbulence attenuation or to drag and turbulence enhancement in polymer flows and dense suspensions, respectively. A general concept of non-Newtonian fluid flow may be considered in a continuous manner through the micropolar theory, recognizing the limitations that bound this theory. In recent articles [Sofiadis and Sarris, “Microrotation viscosity effect on turbulent micropolar fluid channel flow,” Phys. Fluids 33, 095126 (2021); Sofiadis and Sarris, “Turbulence intensity modulation by micropolar fluids,” Fluids 6, 195 (2021)], the micropolar viscosity effect of the turbulent channel flow under constant Reynolds number and its turbulent modulation were investigated. The present study focuses on the investigation of the turbulent micropolar regime as the Reynolds number increases in a channel flow. Findings support that the micropolar stress, which was found to assist turbulence enhancement in the present model, attenuates as Re increases. Effects on the friction behavior of the flow, as Reynolds number increases, become more important for cases of higher micropolar viscosity, where a reverse drag behavior is observed as compared to lower micropolar viscosity ones. Finally, turbulence intensification for these cases declines close to the wall in contrast to lower micropolar viscosity flows, which manage to sustain high turbulence and increase drag in the near-wall region along with Re.
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Barkley, D. "Taming turbulent fronts by bending pipes." Journal of Fluid Mechanics 872 (June 4, 2019): 1–4. http://dx.doi.org/10.1017/jfm.2019.340.
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The flow of fluid through a pipe has been instrumental in illuminating the subcritical route to turbulence typical of many wall-bounded shear flows. Especially important in this process are the turbulent–laminar fronts that separate the turbulent and laminar flow. Four years ago Michael Graham (Nature, vol. 526, 2015, p. 508) wrote a commentary entitled ‘Turbulence spreads like wildfire’, which is a picturesque but also accurate characterisation of the way turbulence spreads through laminar flow in a straight pipe. In this spirit, the recent article by Rinaldi et al. (J. Fluid Mech., vol. 866, 2019, pp. 487–502) shows that turbulent wildfires are substantially tamed in bent pipes. These authors find that even at modest pipe curvature, the characteristic strong turbulent–laminar fronts of straight pipe flow vanish. As a result, the propagation of turbulent structures is modified and there are hints that the route to turbulence is fundamentally altered.
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Bech, Knut H., and Helge I. Andersson. "Secondary flow in weakly rotating turbulent plane Couette flow." Journal of Fluid Mechanics 317 (June 25, 1996): 195–214. http://dx.doi.org/10.1017/s0022112096000729.
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As in the laminar case, the turbulent plane Couette flow is unstable (stable) with respect to roll cell instabilities when the weak background angular velocity Ωk is antiparallel (parallel) to the spanwise mean flow vorticity (-dU/dy)k. The critical value of the rotation number Ro, based on 2Ω and dU/dy of the corresponding laminar flow, was estimated as 0.0002 at a low Reynolds number with fully developed turbulence. Direct numerical simulations were performed for Ro = ±0.01 and compared with earlier results for non-rotating Couette flow. At the low rotation rates considered, both senses of rotation damped the turbulence and the number of near-wall turbulence-generating events was reduced. The destabilized flow was more energetic, but less three-dimensional, than the non-rotating flow. In the destabilized case, the two-dimensional roll cells extracted a comparable amount of kinetic energy from the mean flow as did the turbulence, thereby decreasing the turbulent kinetic energy. The turbulence anisotropy was practically unaffected by weak spanwise rotation, while the secondary flow was highly anisotropic due to its inability to contract and expand in the streamwise direction.
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КОZYR, I. E., and I. F. PIKALOVA. "CHARACTERISTICS OF TURBULENT FLOW IN THE PLANNED EXPANSION WITH THE FORMATION OF WHIRLPOOL ZONES." Prirodoobustrojstvo, no. 3 (2021): 111–16. http://dx.doi.org/10.26897/1997-6011-2021-3-111-116.
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Theory of turbulence is an important part of the open flow dynamics. The interes in the research of experimental turbulence increased with the development of mathematical modeling of open flows and the need to obtain materials for the parameters of turbulence models, with numerical modeling of turbulent flows. The purpose of this research was to study characteristics of the turbulent flow during the planned expansion with the formation of whirlpool zones and to obtain practical recommendations for the analysis and calculation of the kinematic structure of such flows. There was used an experimental research method. Experimental graphs for determining the turbulent shear stresses at the interface were obtained and dependences were given for determining the coefficients of turbulent mixing. There were identified zones of intense pulsation of speed which is of a practical interest and allows to predict an increase in the erosion ability of the flow in these places and to obtain recommendations for strengthening the channel and extinguishing energy in the downstream. The results allow us to use this data in further study of the conditions of origination, development and decaying of turbulence, to consider the mechanism of turbulence of energy transformation.
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Čantrak, Đorđe S., and Novica Z. Janković. "High speed stereoscopic PIV investigation of the statistical characteristics of the axially restricted turbulent swirl flow behind the axial fan in pipe." Advances in Mechanical Engineering 14, no. 11 (November 2022): 168781322211305. http://dx.doi.org/10.1177/16878132221130563.
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Investigation of the turbulent swirl flow in the piping system is one of the most complex investigations in the field of energetics and turbulence. Axial fans in a pipe, without guide vanes, are widely used in practice and the problem of their duty point and energy efficiency is still extensively discussed. Analysis of the interaction between axial fans energy and construction parameters is one of the main topics in defining the fans energy efficiency potential. On one side, there is a three-dimensional velocity field in the wall-bounded flow with regions of great turbulence intensity. On the other side, there is a complex blade geometry, which generates the turbulent swirl flow. This paper presents research on the turbulent swirl flow, Rankine type, in an axially restricted system, using high-speed stereo particle image velocimetry (HSS PIV). Axial fan impeller, with outer diameter 0.399 m and nine twisted blades is the flow generator. The Reynolds number Re = 176,529 is achieved in the pipe. Reynolds stresses, statistical moments of higher order, and invariant maps are calculated based on the three component velocity fields. Here, intensive changes of all statistical parameters occur in radial and axial direction. In the flow region, four flow regions can be identified. Interaction of all these four flow regions produces extremely complex turbulent swirl flow, which is generated behind the axial fans. Determined invariant maps reveal turbulence structure. It is shown that the state of turbulence on the pipe axis is three-component isotropic, which is contrary to the case of axially unrestricted turbulent swirl flows. In the rest of the space, in the region up to r/ R = 0.52, the states of turbulence occur in the area in between the boundaries which designate axis-symmetric turbulence (contraction) and axis-symmetric turbulence (expansion), in the vicinity of the state of three-component isotropic turbulence.
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Tai, Jiayan, and Yee Cheong Lam. "Elastic Turbulence of Aqueous Polymer Solution in Multi-Stream Micro-Channel Flow." Micromachines 10, no. 2 (February 7, 2019): 110. http://dx.doi.org/10.3390/mi10020110.
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Viscous liquid flow in micro-channels is typically laminar because of the low Reynolds number constraint. However, by introducing elasticity into the fluids, the flow behavior could change drastically to become turbulent; this elasticity can be realized by dissolving small quantities of polymer molecules into an aqueous solvent. Our recent investigation has directly visualized the extension and relaxation of these polymer molecules in an aqueous solution. This elastic-driven phenomenon is known as ‘elastic turbulence’. Hitherto, existing studies on elastic flow instability are mostly limited to single-stream flows, and a comprehensive statistical analysis of a multi-stream elastic turbulent micro-channel flow is needed to provide additional physical understanding. Here, we investigate the flow field characteristics of elastic turbulence in a 3-stream contraction-expansion micro-channel flow. By applying statistical analyses and flow visualization tools, we show that the flow field bares many similarities to that of inertia-driven turbulence. More interestingly, we observed regions with two different types of power-law dependence in the velocity power spectra at high frequencies. This is a typical characteristic of two-dimensional turbulence and has hitherto not been reported for elastic turbulent micro-channel flows.
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XU, HONGYI. "Direct numerical simulation of turbulence in a square annular duct." Journal of Fluid Mechanics 621 (February 12, 2009): 23–57. http://dx.doi.org/10.1017/s0022112008004813.
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Direct numerical simulation (DNS) is performed to investigate the fully developed turbulence in a straight square annular duct. The mean flow field and the turbulent statistics are compared with existing experiments and numerical results. The comparisons and the analysis of the DNS data led to the discovery of the turbulent boundary layers of concave and convex 90° corners, a corner flow similarity and the scaling characteristics of corner turbulence. Analysis of the mean streamwise velocity near the concave and convex 90° corners resulted in establishing the ‘law-of-the-corner’ formulations. Comparing these formulations with the ‘law-of-the-wall’ relation, both damping and enhancing mechanisms analytically represented by the van Driest damping function, and the enhancement function were revealed for the concave and convex corner turbulence. The investigation captures the distinctive turbulence-driven secondary flows for both convex and concave 90° corners, and a corner flow similarity rule is discovered, which is associated with the pattern of these secondary flows. A turbulence energy spectrum analysis provides the distinctive features of the fully developed turbulence in the wall and corner regions. The validity of the turbulence eddy viscosity concept is evaluated based on these turbulence energy spectra. The turbulence-driven secondary-flow generation mechanisms are investigated by analysing the anisotropy of the Reynolds stresses.
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Chandler, Gary J., and Rich R. Kerswell. "Invariant recurrent solutions embedded in a turbulent two-dimensional Kolmogorov flow." Journal of Fluid Mechanics 722 (March 28, 2013): 554–95. http://dx.doi.org/10.1017/jfm.2013.122.
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AbstractWe consider long-time simulations of two-dimensional turbulence body forced by $\sin 4y\hat {\boldsymbol{x}} $ on the torus $(x, y)\in \mathop{[0, 2\mathrm{\pi} ] }\nolimits ^{2} $ with the purpose of extracting simple invariant sets or ‘exact recurrent flows’ embedded in this turbulence. Each recurrent flow represents a sustained closed cycle of dynamical processes which underpins the turbulence. These are used to reconstruct the turbulence statistics using periodic orbit theory. The approach is found to be reasonably successful at a low value of the forcing where the flow is close to but not fully in its asymptotic (strongly) turbulent regime. Here, a total of 50 recurrent flows are found with the majority buried in the part of phase space most populated by the turbulence giving rise to a good reproduction of the energy and dissipation p.d.f. However, at higher forcing amplitudes now in the asymptotic turbulent regime, the generated turbulence data set proves insufficiently long to yield enough recurrent flows to make viable predictions. Despite this, the general approach seems promising providing enough simulation data is available since it is open to extensive automation and naturally generates dynamically important exact solutions for the flow.
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Varaksin, Aleksey Yu, and Sergei V. Ryzhkov. "Turbulence in Two-Phase Flows with Macro-, Micro- and Nanoparticles: A Review." Symmetry 14, no. 11 (November 16, 2022): 2433. http://dx.doi.org/10.3390/sym14112433.
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Turbulent flows are nonstationary in nature. Since the turbulent fluctuations of most flow parameters satisfy a symmetric Gaussian distribution, the turbulent characteristics have the property of symmetry in the statistical meaning. A widespread simplest model of turbulent flows is the model of “symmetric” turbulence, namely, homogeneous isotropic turbulence (HIT). The presence of particles with non-uniform distribution of their concentration in the turbulent flow, even under HIT conditions, can lead to redistribution of different components of fluctuation velocities of the carrier gas, i.e., to the appearance of asymmetry. The subject of the review is turbulent flows of gas with solid particles. Particular attention is paid to the problem of the back influence of particles on carrier gas characteristics (first of all, on the turbulent kinetic energy). A review of the results of experimental and computational-theoretical studies of the effect of the presence of the dispersed phase in the form of particles on the parameters of the turbulent flow of the carrier gas phase has been carried out. The main physical mechanisms and dimensionless criteria determining the direction and magnitude of the impact of particles of different inertia on the carrier gas phase turbulence energy are described and analyzed. The peculiarities of the influence of particles on the turbulence energy of the gas for different classes of flows: homogeneous isotropic turbulence, homogeneous shear flow, and wall turbulence in a pipe (channel) have been considered. For the near-wall flow in the pipe, it is shown that the turbulizing effect of extremely low-inertia particles of sub-micrometer size (nanoparticles) is replaced by the laminarizing effect of low-inertia particles of micrometer size (microparticles), and then again it is replaced by turbulizing due to additional generation of turbulence in the wakes of large particles of millimeter size (macroparticles). The review is intended to some extent to fill in the currently existing gap associated with the absence of dimensionless criteria (or complexes of physical parameters) responsible for the direction (attenuation or enhancement) of turbulence modification, and the value of this change. Possible directions for further researches are given in the conclusion of the review.
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Nazarov, F. Kh. "Comparing Turbulence Models for Swirling Flows." Herald of the Bauman Moscow State Technical University. Series Natural Sciences, no. 2 (95) (April 2021): 25–36. http://dx.doi.org/10.18698/1812-3368-2021-2-25-36.
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The paper considers a turbulent fluid flow in a rotating pipe, known as the Taylor --- Couette --- Poiseuille flow. Linear RANS models are not suitable for simulating this type of problems, since the turbulence in these flows is strongly anisotropic, which means that solving these problems requires models accounting for turbulence anisotropy. Modified linear models featuring corrections for flow rotations, such as the SARC model, make it possible to obtain satisfactory solutions. A new approach to turbulence problems has appeared recently. It allowed a novel two-fluid turbulence model to be created. What makes this model different is that it can describe strongly anisotropic turbulent flows; moreover, it is easy to implement numerically while not being computationally expensive. We compared the results of solving the Taylor --- Couette --- Poiseuille flow problem using the novel two-fluid model and the SARC model. The numerical investigation results obtained from the novel two-fluid model show a better agreement with the experimental data than the results provided by the SARC model
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LEVICH, E. "NEW DEVELOPMENTS AND CLASSICAL THEORIES OF TURBULENCE." International Journal of Modern Physics B 10, no. 18n19 (August 30, 1996): 2325–92. http://dx.doi.org/10.1142/s0217979296001057.
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In this paper we review certain classical and modern concepts pertinent for the theory of developed turbulent flows. We begin by introducing basic facts concerning the properties of the Navier-Stokes equation with the emphasis on invariant properties of the vorticity field. Then we discuss classical semiempirical approaches to developed turbulence which for a long time have constituted a basis for engineering solutions of turbulent flows problems. We do it for two examples, homogeneous isotropic turbulence and flat channel turbulent flow. Next we discuss the insufficiency of classical semi-empirical approaches. We show that intermittency is an intrinsic feature of all turbulent flows and hence it should be accounted for in any reasonable theoretical approach to turbulence. We argue that intermittency in physical space is in one to one correspondence with certain phase coherence of turbulence in an appropriate dual space, e.g. Fourier space for the case of homogeneous isotropic turbulence. In the same time the phase coherence has its origin in invariant topological properties of vortex lines in inviscid flows, modified by the presence of small molecular viscosity. This viewpoint is expounded again using the examples of homogeneous isotropic turbulence and channel flow turbulence. Finally we briefly discuss the significance of phase coherence and intermittency in turbulence for the fundamental engineering challenge of turbulence control.
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Kim, Doohyeon, Jihun Kang, Ehsan Adeeb, Gyu-Han Lee, Dong Hyun Yang, and Hojin Ha. "Comparison of Four-Dimensional Flow Magnetic Resonance Imaging and Particle Image Velocimetry to Quantify Velocity and Turbulence Parameters." Fluids 6, no. 8 (August 6, 2021): 277. http://dx.doi.org/10.3390/fluids6080277.
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Although recent advances of four-dimensional (4D) flow magnetic resonance imaging (MRI) has introduced a new way to measure Reynolds stress tensor (RST) in turbulent flows, its measurement accuracy and possible bias have remained to be revealed. The purpose of this study was to compare the turbulent flow measurement of 4D flow MRI and particle image velocimetry (PIV) in terms of velocity and turbulence quantification. Two difference flow rates of 10 and 20 L/min through a 50% stenosis were measured with both PIV and 4D flow MRI. Not only velocity through the stenosis but also the turbulence parameters such as turbulence kinetic energy and turbulence production were quantitatively compared. Results shows that 4D flow MRI velocity measurement well agreed with the that of PIV, showing the linear regression slopes of two methods are 0.94 and 0.89, respectively. Although turbulence mapping of 4D flow MRI was qualitatively agreed with that of PIV, the quantitative comparison shows that the 4D flow MRI overestimates RST showing the linear regression slopes of 1.44 and 1.66, respectively. In this study, we demonstrate that the 4D flow MRI visualize and quantify not only flow velocity and also turbulence tensor. However, further optimization of 4D flow MRI for better accuracy might be remained.
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Liu, Zhiyu, S. A. Thorpe, and W. D. Smyth. "Instability and hydraulics of turbulent stratified shear flows." Journal of Fluid Mechanics 695 (February 20, 2012): 235–56. http://dx.doi.org/10.1017/jfm.2012.13.
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AbstractThe Taylor–Goldstein (T–G) equation is extended to include the effects of small-scale turbulence represented by non-uniform vertical and horizontal eddy viscosity and diffusion coefficients. The vertical coefficients of viscosity and diffusion, ${A}_{V} $ and ${K}_{V} $, respectively, are assumed to be equal and are expressed in terms of the buoyancy frequency of the flow, $N$, and the dissipation rate of turbulent kinetic energy per unit mass, $\varepsilon $, quantities that can be measured in the sea. The horizontal eddy coefficients, ${A}_{H} $ and ${K}_{H} $, are taken to be proportional to the dimensionally correct form, ${\varepsilon }^{1/ 3} {l}^{4/ 3} $, found appropriate in the description of horizontal dispersion of a field of passive markers of scale $l$. The extended T–G equation is applied to examine the stability and greatest growth rates in a turbulent shear flow in stratified waters near a sill, that at the entrance to the Clyde Sea in the west of Scotland. Here the main effect of turbulence is a tendency towards stabilizing the flow; the greatest growth rates of small unstable disturbances decrease, and in some cases flows that are unstable in the absence of turbulence are stabilized when its effects are included. It is conjectured that stabilization of a flow by turbulence may lead to a repeating cycle in which a flow with low levels of turbulence becomes unstable, increasing the turbulent dissipation rate and so stabilizing the flow. The collapse of turbulence then leads to a condition in which the flow may again become unstable, the cycle repeating. Two parameters are used to describe the ‘marginality’ of the observed flows. One is based on the proximity of the minimum flow Richardson number to the critical Richardson number, the other on the change in dissipation rate required to stabilize or destabilize an observed flow. The latter is related to the change needed in the flow Reynolds number to achieve zero growth rate. The unstable flows, typical of the Clyde Sea site, are relatively further from neutral stability in Reynolds number than in Richardson number. The effects of turbulence on the hydraulic state of the flow are assessed by examining the speed and propagation direction of long waves in the Clyde Sea. Results are compared to those obtained using the T–G equation without turbulent viscosity or diffusivity. Turbulence may change the state of a flow from subcritical to supercritical.
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Nejad, A. S., S. P. Vanka, S. C. Favaloro, M. Samimy, and C. Langenfeld. "Application of Laser Velocimetry for Characterization of Confined Swirling Flow." Journal of Engineering for Gas Turbines and Power 111, no. 1 (January 1, 1989): 36–45. http://dx.doi.org/10.1115/1.3240225.
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A two-component LDV was used in a cold flow dump combustor model to obtain detailed mean and turbulence data for both swirling and nonswirling inlet flows. Large samples were collected to resolve the second and third-order products of turbulent fluctuations with good accuracy. Particle interarrival time weighting was used to remove velocity bias from the data. The swirling flows, with and without vortex breakdown, exhibited significantly different mean flow and turbulent field behavior. A numerical scheme with the k–ε closure model was used to predict the flow fields. Comparison of the numerical and experimental results showed that the k–ε turbulence model is inadequate in representing the complex turbulent structure of confined swirling flows.
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Blair, M. F. "Boundary-Layer Transition in Accelerating Flows With Intense Freestream Turbulence: Part 2—The Zone of Intermittent Turbulence." Journal of Fluids Engineering 114, no. 3 (September 1, 1992): 322–32. http://dx.doi.org/10.1115/1.2910033.
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Hot-wire anemometry was employed to examine the laminar-to-turbulent transition of low-speed, two-dimensional boundary layers for two (moderate) levels of flow acceleration and various levels of grid-generated freestream turbulence. Flows with an adiabatic wall and with uniform-flux heat transfer were explored. Conditional discrimination techniques were employed to examine the zones of flow within the transitional region. This analysis demonstrated that as much as one-half of the streamwise-component unsteadiness, and much of the apparent anisotropy, observed near the wall was produced, not by turbulence, but by the steps in velocity between the turbulent and inter-turbulent zones of flow. Within the turbulent zones u′/v′ ratios were about equal to those expected for equilibrium boundary-layer turbulence. Near transition onset, however, the turbulence kinetic energy within the turbulent zones exceeded fully turbulent boundary-layer levels. Turbulent-zone power-spectral-density measurements indicate that the ratio of dissipation to production increased through transition. This suggests that the generation of the full equilibrium turbulent boundary-layer energy cascade required some time (distance) and may explain the very high TKE levels near onset.
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NIKITIN, NIKOLAY, HENGLIANG WANG, and SERGEI CHERNYSHENKO. "Turbulent flow and heat transfer in eccentric annulus." Journal of Fluid Mechanics 638 (September 18, 2009): 95–116. http://dx.doi.org/10.1017/s002211200900812x.
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A detailed statistical analysis of turbulent flow and heat transfer in eccentric annular duct was performed via direct numerical simulations (DNS) with particular emphasis on the needs of turbulence closure models. A large number of flow characteristics such as components of the Reynolds stress tensor, temperature–velocity correlations and some others were obtained for the first time for such kind of a flow. The results of the paper will serve as a benchmark test case for turbulence modelling, specifically for models intended to be used for flows with partly turbulent regimes.
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Baumert, H. Z., and H. Peters. "Turbulence closure: turbulence, waves and the wave-turbulence transition – Part 1: Vanishing mean shear." Ocean Science Discussions 5, no. 4 (November 14, 2008): 545–80. http://dx.doi.org/10.5194/osd-5-545-2008.
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Abstract. A new two-equation, closure-like turbulence model for stably stratified flows is introduced which uses the turbulent kinetic energy (K) and the turbulent enstrophy (Ω) as primary variables. It accounts for mean shear – and internal wave-driven mixing in the two limits of mean shear and no waves and waves but no mean shear, respectively. The traditional TKE balance is augmented by an explicit energy transfer from internal waves to turbulence. A modification of the Ω-equation accounts for the effect of the waves on the turbulence time and space scales. The latter is based on the assumption of a non-zero constant flux Richardson number in the limit of vanishing mean-flow shear when turbulence is produced exclusively by internal waves. The new model reproduces the wave-turbulence transition analyzed by D'Asaro and Lien (2000). At small energy density E of the internal wave field, the turbulent dissipation rate (ε) scales like ε~E2. This is what is observed in the deep sea. With increasing E, after the wave-turbulence transition has been passed, the scaling changes to ε~E1. This is observed, for example, in the swift tidal flow near a sill in Knight Inlet. The new model further exhibits a turbulent length scale proportional to the Ozmidov scale, as observed in the ocean, and predicts the ratio between the turbulent Thorpe and Ozmidov length scales well within the range observed in the ocean.
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Shaheed, Rawaa, Xiaohui Yan, and Abdolmajid Mohammadian. "Review and Comparison of Numerical Simulations of Secondary Flow in River Confluences." Water 13, no. 14 (July 11, 2021): 1917. http://dx.doi.org/10.3390/w13141917.
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River confluences are a common feature in natural water resources. The flow characteristics in confluences are complicated, especially at junction areas between tributaries and the main river. One of the typical characteristics of confluences is secondary flow, which plays an important role in mixing, velocity, sediment transport, and pollutant dispersion. In addition to the experimental and field studies that have been conducted in this area, the development of computational fluid dynamics has allowed researchers in this field to use different numerical models to simulate turbulence properties in rivers, especially secondary flows. Nowadays, the hydrodynamics of flows in confluences are widely simulated by using three-dimensional models in order to fully capture the flow structures, as the flow characteristics are considered to be turbulent and three-dimensional at river junctions. Several numerical models have been recommended for this purpose, and various turbulence models have been used to simulate the flows at confluences. To assess the accuracy of turbulence models, flows have been predicted by applying different turbulence models in the numerical model and the results have been compared with other data, such as field, laboratory, and experimental data. The purpose behind these investigations was to find the suitable model for each case of turbulent flow and for different types of confluences. In this study, the performances of turbulence models for confluences are reviewed for different numerical simulation strategies.
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Tuckerman, Laurette S., Matthew Chantry, and Dwight Barkley. "Patterns in Wall-Bounded Shear Flows." Annual Review of Fluid Mechanics 52, no. 1 (January 5, 2020): 343–67. http://dx.doi.org/10.1146/annurev-fluid-010719-060221.
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Experiments and numerical simulations have shown that turbulence in transitional wall-bounded shear flows frequently takes the form of long oblique bands if the domains are sufficiently large to accommodate them. These turbulent bands have been observed in plane Couette flow, plane Poiseuille flow, counter-rotating Taylor–Couette flow, torsional Couette flow, and annular pipe flow. At their upper Reynolds number threshold, laminar regions carve out gaps in otherwise uniform turbulence, ultimately forming regular turbulent–laminar patterns with a large spatial wavelength. At the lower threshold, isolated turbulent bands sparsely populate otherwise laminar domains, and complete laminarization takes place via their disappearance. We review results for plane Couette flow, plane Poiseuille flow, and free-slip Waleffe flow, focusing on thresholds, wavelengths, and mean flows, with many of the results coming from numerical simulations in tilted rectangular domains that form the minimal flow unit for the turbulent–laminar bands.
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Yao, Jianfeng, Wenjuan Lou, Guohui Shen, Yong Guo, and Yuelong Xing. "Influence of Inflow Turbulence on the Flow Characteristics around a Circular Cylinder." Applied Sciences 9, no. 17 (September 2, 2019): 3595. http://dx.doi.org/10.3390/app9173595.
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To study the influence of turbulence on the wind pressure and aerodynamic behavior of smooth circular cylinders, wind tunnel tests of a circular cylinder based on wind pressure testing were conducted for different wind speeds and turbulent flows. The tests obtained the characteristic parameters of mean wind pressure coefficient distribution, drag coefficient, lift coefficient and correlation of wind pressure for different turbulence intensities and of Reynolds numbers. These results were also compared with those obtained by previous researchers. The results show that the minimum drag coefficient in the turbulent flow is basically constant at approximate 0.4 and is not affected by the turbulence intensity. When the Reynolds number is in the critical regime, the lift coefficient increased sharply to 0.76 in the smooth flow, indicating that flow separation has an asymmetry; however, the asymmetry does not appear in the turbulent flow. Drag coefficient decreases sharply at a lower critical Reynolds number in the turbulent flow than in the smooth flow. In the smooth flow, the separation point is about 80° in the subcritical regime; it suddenly moves backwards in the critical regime and remains almost unchanged at about 140° in the supercritical regime. However, the angular position of the separation point will always be about 140° for turbulent flow for the Reynolds number in these three regimes. Turbulence intensity and Reynolds number have a significant effect on the correlation of wind pressures around the circular cylinder. Turbulence will weaken the positive correlation of the same side and also reduce the negative correlation between the two sides of the circular cylinder.
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NAKABAYASHI, Koichi, Osami KITOH, and Yoshitaka KATOU. "Turbulence Statistics of CouettePoiseuille Turbulent Flow. 1st Report. Turbulence Intensities." Transactions of the Japan Society of Mechanical Engineers Series B 64, no. 626 (1998): 3272–78. http://dx.doi.org/10.1299/kikaib.64.3272.
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Volino, R. J., and T. W. Simon. "Boundary Layer Transition Under High Free-Stream Turbulence and Strong Acceleration Conditions: Part 2—Turbulent Transport Results." Journal of Heat Transfer 119, no. 3 (August 1, 1997): 427–32. http://dx.doi.org/10.1115/1.2824115.
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Measurements from heated boundary layers along a concave-curved test wall subject to high (initially 8 percent) free-stream turbulence intensity and strong (K = (ν/U∞2 dU∞/dx, as high as 9 × 10−6) acceleration are presented and discussed. Conditions for the experiments were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. Turbulence statistics, including the turbulent shear stress, the turbulent heat flux, and the turbulent Prandtl number are presented. The transition zone is of extended length in spite of the high free-stream turbulence level. Turbulence quantities are strongly suppressed below values in unaccelerated turbulent boundary layers. Turbulent transport quantities rise with the intermittency, as the boundary layer proceeds through transition. Octant analysis shows a similar eddy structure in the present flow as was observed in transitional flows under low free-stream turbulence conditions. To the authors’ knowledge, this is the first detailed documentation of a high-free-stream-turbulence boundary layer flow in such a strong acceleration field.
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Donnelly, Russell J., and Charles E. Swanson. "Quantum turbulence." Journal of Fluid Mechanics 173 (December 1986): 387–429. http://dx.doi.org/10.1017/s0022112086001210.
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We present a review of quantum turbulence, that is, the turbulent motion of quantized vortex lines in superfluid helium. Our discussion concentrates on the turbulence produced by steady, uniform heat flow in a pipe, but touches on other turbulent flows as well. We have attempted to motivate the study of quantum turbulence and discuss briefly its connection with classical turbulence. We include background on the two-fluid model and mutual friction theory, examples of modern experimental techniques, and a brief survey of the phenomenology. We discuss the important recent insights that vortex dynamics has provided to the understanding of quantum turbulence, from simple scaling arguments to detailed numerical simulations. We conclude with a discussion of open questions in this field.
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Enders, Eva C., Daniel Boisclair, and André G. Roy. "The effect of turbulence on the cost of swimming for juvenile Atlantic salmon (Salmo salar)." Canadian Journal of Fisheries and Aquatic Sciences 60, no. 9 (September 1, 2003): 1149–60. http://dx.doi.org/10.1139/f03-101.
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Fish activity costs are often estimated by transforming their swimming speed in energy expenditures with respirometry models developed while forcing fish to swim against a flow of constant velocity. Forced swimming models obtained using a procedure that minimizes flow heterogeneity may not represent the costs of swimming in rivers characterized by turbulence and by a wide range of instantaneous flow velocities. We assessed the swimming cost of juvenile Atlantic salmon (Salmo salar) in turbulent flows using two means (18 and 23 cm·s1) and two standard deviations of flow velocity (5 and 8 cm·s1). Twenty respirometry experiments were conducted at 15 °C with fish averaging 10 g. Our results confirmed that swimming costs are affected by the level of turbulence. For a given mean flow velocity, swimming costs increased 1.3- to 1.6-fold as turbulence increased. Forced swimming models under estimated actual swimming costs in turbulent flow by 1.9- to 4.2-fold. Spontaneous swimming models overestimated the real cost of swimming in turbulent flow by 2.8- to 6.6-fold. Our analyses suggest that models in which both the mean and the standard deviation of flow velocity are explicitly represented are needed to adequately estimate the costs of swimming against turbulent flows.
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Ura, Masaru, and Nobuhiro Matsunaga. "ENTRAINMENT DUE TO MEAN FLOW IN TWO-LAYERED FLUID." Coastal Engineering Proceedings 1, no. 21 (January 29, 1988): 189. http://dx.doi.org/10.9753/icce.v21.189.
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The entrainment phenomena have been investigated across an interface between two-layered stratified flow induced by wind shear stress. The velocities of mean flow, turbulence and entrainment have been measured under three different conditions of water surface by using a wind-wave tank. When the entrainment velocity ue is expressed on the basis of the turbulent quantities at the interface, the turbulent entrainment coefficient E ( = ue/u) is given by E = A-(egl/u2)-3I1 ( A = 0.7). Here Eg, u and 1 are the effective buoyancy, the turbulence intensity and the integral lengthscale of turbulence at the interface, respectively. This result coincides with the relationship of entrainment due to oscillating grid turbulence, in which the mean flow does not exist. When, for the practical purpose, the estimation of ue is made by using the mean velocity Um and the depth h of mixed layer, Em ( - Ue/Um ) = Am•(egh/Um 2)"3/2 is derived from the transformation of E = A-(egl/u2)-3/2. There holds Am = A-Tf between Am and Tf, Tf being a turbulence factor given by (u/Um)4•(1/h)-3/2. It has been found that this relationship is also valid in various types of two-layered stratified flows as well as the wind-induced two-layered flows.
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Malikov, Zafar M., Farrukh Kh Nazarov, and Murodil E. Madaliev. "Comparison of advanced turbulence models for the Taylor-Couette flow." Vestnik Tomskogo gosudarstvennogo universiteta. Matematika i mekhanika, no. 78 (2022): 125–42. http://dx.doi.org/10.17223/19988621/78/10.
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Swirling flows of fluids and gases are an integral part of many complex flows which are widely encountered in nature and technology. The working process of numerous technical devices (cyclones, vortex combustion chambers, air separators, gas and steam turbines, electric machines and generators, etc.) is generally determined by the laws of hydrodynamics and heat exchange of rotating flows. The problem of deriving general laws for a turbulent flow in the field of centrifugal forces provokes considerable scientific interest since it belongs to an underdeveloped field of hydromechanics. Therefore, mathematical modeling of swirling turbulent flows is still an urgent problem. In this paper, a comparative analysis of the advanced turbulence models for the Taylor -Couette flow is carried out. For this purpose, the linear turbulence models (SARC and SST-RC), the Reynolds stress method SSG/LRR-RSM-w2012, and a two-fluid model are used. The results obtained using these models are compared with each other and with known experimental data and direct numerical simulation results. The numerical results calculated with the use of turbulence models for the Taylor-Couette flow confirm that almost all the models adequately describe velocity profiles. However, they yield different turbulent viscosity values and, as a result, different friction coefficients. A comparison of the numerical results shows that the friction coefficient calculated using a two-fluid turbulence model is the closest to that obtained experimentally.
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Carosi, G., and H. Chanson. "Turbulence characteristics in skimming flows on stepped spillways." Canadian Journal of Civil Engineering 35, no. 9 (September 2008): 865–80. http://dx.doi.org/10.1139/l08-030.
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The stepped spillway design is characterized by an increase in the rate of energy dissipation on the chute associated with a reduction of the size of the downstream energy dissipation system. This study presents a thorough investigation of the air–water flow properties in skimming flows with a focus on the turbulent characteristics. New measurements were conducted in a large-size facility (θ = 22°; step height, h = 0.1 m) with several phase-detection intrusive probes. Correlation analyses were applied to estimate the integral turbulent length and time scales. The skimming flow properties presented some basic characteristics that were qualitatively and quantitatively in agreement with previous air–water flow measurements in skimming flows. Present measurements showed some relatively good correlation between turbulence intensities T u and turbulent length and time scales. These measurements also illustrated large turbulence levels and large turbulent time and length scales in the intermediate region between the spray and bubbly flow regions.
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Ye, Weixiang, Xianwu Luo, and Ying Li. "Modified partially averaged Navier–Stokes model for turbulent flow in passages with large curvature." Modern Physics Letters B 34, no. 23 (May 30, 2020): 2050239. http://dx.doi.org/10.1142/s0217984920502395.
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This study presents a partially averaged Navier–Stokes model, MSST PANS, based on a modified SST [Formula: see text] turbulence model to predict turbulent flows with large streamline curvature. The model was validated for turbulent flow in a [Formula: see text] curved rectangular duct (Re = 224,000) to assess the MSST PANS capabilities. The predictions are compared against flow simulations for the same curved rectangular duct using four turbulence models including the standard [Formula: see text] model, SST [Formula: see text] model, [Formula: see text] PANS model and SST [Formula: see text] PANS model. Comparisons among those numerical results and available experimental data show that the MSST PANS model more accurately predicts the velocity components in all three directions, especially in the wall-bounded region than the other models. The study also shows the advantages of the MSST PANS model for predicting the Reynolds stresses, vorticity, and smaller scale turbulent structures in the wall-bounded region not only qualitatively but quantitatively. Furthermore, the MSST PANS model requires fewer computations than the SST PANS model, indicating that this turbulence model, which takes large streamlines curvature effects into consideration, is an effective alternative for capturing the small-scale turbulence flow structures. This turbulence model is expected to be very useful for engineering applications, especially for flows in turbomachinery.
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SHATS, MICHAEL, HUA XIA, and DAVID BYRNE. "TURBULENCE IN THICK LAYERS." International Journal of Modern Physics: Conference Series 19 (January 2012): 390–95. http://dx.doi.org/10.1142/s2010194512008987.
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Properties of turbulence, such as the direction of the energy cascade depend on the flow dimensionality. Recent experimental results reveal new physics understanding of flows in thick layers. We show that the flow dimensionality can be characterized by the flow damping and that a comparison of the decay rate with that of a quasi-2D flow can be used as a measure of dimensionality of a turbulent flow in a layer. This dimensionality, however can be strongly affected if large scale coherent vortices are present in the flow. These vortices can imposed twodimensionality and reduce the damping rate. Finally we show that even in thick layers with unperturbed free surface, turbulent flow can be viewed as co-existing 2D top sub-layer and the bulk 3D flow.
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Mirabi, Mohammad Hossein, Ehsan Jabbari, Taher Rajaee, and Keivan Seiiedi Niaki. "Experimental investigation of turbulent flow in a rectangular bonneted slide gate and eliminating random fluctuating loads." Physics of Fluids 35, no. 1 (January 2023): 015155. http://dx.doi.org/10.1063/5.0134664.
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Bonneted slide gates are widely used in dam bottom outlets for flow regulation. High kinetic energy flow generates turbulence, which induces vibrations in bonneted slide gates under partially open conditions. Significant vibrations can indicate problems and cause damage or expedited deterioration of the gate if left unchecked. This experimental study focuses on two aspects: turbulent flow formation due to rectangular bonneted slide gates and elimination of random fluctuating loads on the gate using a turbulence inhibitor. Five reservoir heads were combined with nine gate openings (10%–90%) to produce 45 different discharges of 12–182 l/s. The different types of the gate flow create different turbulence and random loads based on the water–air mixture and the kinetic energy of the flow. Flow analysis indicates considerable random static pressure fluctuations under the slide gate. The results show the formation of swirling secondary flows in the gate's side guide slots, and their movement and collision with the gate are the leading cause of the turbulent multiphase flow and random fluctuations in static pressure. Also, the use of turbulence inhibitors on the gate can prevent the formation of secondary flows, which results in a 98.92% reduction in the amplitude of random static pressure fluctuations and the elimination of the random fluctuating loads on the slide gate. After removing the swirling secondary flows, a stable air boundary layer was formed under the gate. Finally, the gate flow changed from a turbulent two-phase flow to a steady single-phase jet of water.
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MARUSIC, IVAN. "Unravelling turbulence near walls." Journal of Fluid Mechanics 630 (July 10, 2009): 1–4. http://dx.doi.org/10.1017/s0022112009007708.
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Turbulent flows near walls have been the focus of intense study since their first description by Ludwig Prandtl over 100 years ago. They are critical in determining the drag and lift of an aircraft wing for example. Key challenges are to understand the physical mechanisms causing the transition from smooth, laminar flow to turbulent flow and how the turbulence is then maintained. Recent direct numerical simulations have contributed significantly towards this understanding.
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Cimarelli, A., A. Fregni, J. P. Mollicone, M. van Reeuwijk, and E. De Angelis. "Structure of turbulence in temporal planar jets." Physics of Fluids 34, no. 4 (April 2022): 045109. http://dx.doi.org/10.1063/5.0085091.
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A detailed analysis of the structure of turbulence in a temporal planar turbulent jet is reported. Instantaneous snapshots of the flow and three-dimensional spatial correlation functions are considered. It is found that the flow is characterized by large-scale spanwise vortices whose motion is felt in the entire flow field. Superimposed to this large-scale motion, a hierarchy of turbulent structures is present. The most coherent ones take the form of quasi-streamwise vortices and high and low streamwise velocity streaks. The topology of these interacting structures is analyzed by quantitatively addressing their shape and size in the different flow regions. Such information is recognized to be relevant for a structural description of the otherwise disorganized motion in turbulent free-shear flows and can be used for the assessment of models based on coherent structure assumptions. Finally, the resulting scenario provides a phenomenological description of the elementary processes at the basis of turbulence in free-shear flows.
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Owolabi, Bayode E., Robert J. Poole, and David J. C. Dennis. "Experiments on low-Reynolds-number turbulent flow through a square duct." Journal of Fluid Mechanics 798 (June 3, 2016): 398–410. http://dx.doi.org/10.1017/jfm.2016.314.
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Previous experimental studies on turbulent square duct flow have focused mainly on high Reynolds numbers for which a turbulence-induced eight-vortex secondary flow pattern exists in the cross-sectional plane. More recently, direct numerical simulations (DNS) have revealed that the flow field at Reynolds numbers close to transition can be very different; the flow in this ‘marginally turbulent’ regime alternating between two states characterised by four vortices. In this study, we experimentally investigate the onset criteria for transition to turbulence in square ducts. In so doing, we highlight the potential importance of Coriolis effects on this process for low-Ekman-number flows. We also present experimental data on the mean flow properties and turbulence statistics in both marginally and fully turbulent flow at relatively low Reynolds numbers using laser Doppler velocimetry. Results for both flow categories show good agreement with DNS. The switching of the flow field between two flow states at marginally turbulent Reynolds numbers is confirmed by bimodal probability density functions of streamwise velocity at certain distances from the wall as well as joint probability density functions of streamwise and wall normal velocities which feature two peaks highlighting the two states.
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Sarkar, S. "The stabilizing effect of compressibility in turbulent shear flow." Journal of Fluid Mechanics 282 (January 10, 1995): 163–86. http://dx.doi.org/10.1017/s0022112095000085.
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Direct numerical simulation of turbulent homogeneous shear flow is performed in order to clarify compressibility effects on the turbulence growth in the flow. The two Mach numbers relevant to homogeneous shear flow are the turbulent Mach number Mt and the gradient Mach number Mg. Two series of simulations are performed where the initial values of Mg and Mt are increased separately. The growth rate of turbulent kinetic energy is observed to decrease in both series of simulations. This ‘stabilizing’ effect of compressibility on the turbulent energy growth rate is observed to be substantially larger in the DNS series where the initial value of Mg is changed. A systematic comparison of the different DNS cases shows that the compressibility effect of reduced turbulent energy growth rate is primarily due to the reduced level of turbulence production and not due to explicit dilatational effects. The reduced turbulence production is not a mean density effect since the mean density remains constant in compressible homogeneous shear flow. The stabilizing effect of compressibility on the turbulence growth is observed to increase with the gradient Mach number Mg in the homogeneous shear flow DNS. Estimates of Mg for the mixing layer and the boundary layer are obtained. These estimates show that the parameter Mg becomes much larger in the high-speed mixing layer relative to the high-speed boundary layer even though the mean flow Mach numbers are the same in the two flows. Therefore, the inhibition of turbulent energy production and consequent ‘stabilizing’ effect of compressibility on the turbulence (over and above that due to any mean density variation) is expected to be larger in the mixing layer relative to the boundary layer, in agreement with experimental observations.
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Le, Thai-Hoa, and Dong-Anh Nguyen. "TEMPORO-SPECTRAL COHERENT STRUCTURE OF TURBULENCE AND PRESSURE USING FOURIER AND WAVELET TRANSFORMS." ASEAN Journal on Science and Technology for Development 25, no. 2 (November 22, 2017): 405–17. http://dx.doi.org/10.29037/ajstd.271.
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Studying the spatial distribution in coherent fields such as turbulent and turbulent-induced force ones is important to model and evaluate turbulent-induced forces and response of structures on the turbulent flows. Turbulent field-based coherent function is commonly used for the spatial distribution characteristic of induced forces in the frequency domain. This paper will focus to study spectral coherent structure of turbulence and forces in not only the frequency domain using conventional Fourier transform-based coherence, but also temporo-spectral coherent one in the time-frequency plane thanks to wavelet transform-based coherence for more understanding of the turbulence and force coherences and their spatial distributions. Effects of spanwise separations, bluff body flow and flow conditions on coherent structures of turbulence and induced pressure, comparison between turbulence and pressure coherences as well as intermittency of coherent structure in the time-frequency plane will be investigated here.
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ZHOU, J. G. "A LATTICE BOLTZMANN MODEL FOR THE SHALLOW WATER EQUATIONS WITH TURBULENCE MODELING." International Journal of Modern Physics C 13, no. 08 (October 2002): 1135–50. http://dx.doi.org/10.1142/s0129183102003814.
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A lattice Boltzmann model for the shallow water equations with turbulence modeling (LABSWETM) is developed. The flow turbulence is efficiently and naturally taken into account by incorporating the standard subgrid-scale stress model into the lattice Boltzmann equation in a consistent manner with the lattice gas dynamics. The model is applied to solve two flow problems and is verified by comparing numerical predictions with analytical solutions and available experimental data. The results show that the LABSWETM is able to provide basic features of flow turbulence and produce good predictions for turbulent shallow water flows.
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Mazouz, A., L. Labraga, and C. Tournier. "Anisotropy Invariants of Reynolds Stress Tensor in a Duct Flow and Turbulent Boundary Layer." Journal of Fluids Engineering 120, no. 2 (June 1, 1998): 280–84. http://dx.doi.org/10.1115/1.2820645.
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The present study shows that the Reynolds stress anisotropy tensor for turbulent flow depends both on the nature of the surface and the boundary conditions of the flow. Contrary to the case of turbulent boundary layers with k-type surface roughness, the measured anisotropy invariants of the Reynolds stress tensor over a series of spanwise square bars separated by rectangular cavities (k-type) in duct flows show that roughness increases the anisotropy. There is a similarity between the effect of roughness on channel flow turbulence and that on pipe flow turbulence. The present data show that the effect of introducing a surface roughness significantly perturbs the entire thickness of the turbulent flow.
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Xi, Li, and Michael D. Graham. "Intermittent dynamics of turbulence hibernation in Newtonian and viscoelastic minimal channel flows." Journal of Fluid Mechanics 693 (January 17, 2012): 433–72. http://dx.doi.org/10.1017/jfm.2011.541.
Full textAbstract:
AbstractMaximum drag reduction (MDR), the asymptotic upper limit of reduction in turbulent friction drag by polymer additives, is the most important unsolved problem in viscoelastic turbulence. Recent studies of turbulence in minimal flow units have identified time intervals showing key features of MDR. These intervals, denoted ‘hibernating turbulence’ are found in both Newtonian and viscoelastic flows. The present study provides a comprehensive examination of this turbulence hibernation phenomenon in the minimal channel geometry, and discusses its impact on the turbulent dynamics and drag reduction. Similarities between hibernating turbulence and MDR are established in terms of both flow statistics (an intermittency factor, mean and fluctuating components of velocity) and flow structure (weak vortices and nearly streamwise-invariant kinematics). Hibernation occurs more frequently at high levels of viscoelasticity, leading to flows that increasingly resemble MDR. Viscoelasticity facilitates the occurrence of hibernation by suppressing the conventional ‘active’ turbulence, but has little influence on hibernation itself. At low Weissenberg number $\mathit{Wi}$, the average duration of active turbulence intervals is constant, but above a critical value of $\mathit{Wi}$, the duration decreases dramatically, and accordingly, the fraction of time spent in hibernation increases. This observation can be explained with a simple mathematical model that posits that the lifetime of an active turbulence interval is the time that it takes for the turbulence to stretch polymer molecules to a certain threshold value; once the molecules exceed this threshold, they exert a large enough stress on the flow to suppress the active turbulence. This model predicts an explicit form for the duration as a function of $\mathit{Wi}$ and the simulation results match this prediction very closely. The critical point where hibernation frequency becomes substantially increased coincides with the point where qualitative changes are observed in overall flow statistics – the transition between ‘low-drag-reduction’ and ‘high-drag-reduction’ regimes. Probability density functions of important variables reveal a much higher level of intermittency in the turbulent dynamics after this transition. It is further confirmed that hibernating turbulence is a Newtonian structure during which polymer extension is small. Based on these results, a framework is proposed that explains key transitions in viscoelastic turbulence, especially the convergence toward MDR.
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Yang, Xingtuan, Nan Gui, Gongnan Xie, Jie Yan, Jiyuan Tu, and Shengyao Jiang. "Anisotropic Characteristics of Turbulence Dissipation in Swirling Flow: A Direct Numerical Simulation Study." Advances in Mathematical Physics 2015 (2015): 1–9. http://dx.doi.org/10.1155/2015/657620.
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This study investigates the anisotropic characteristics of turbulent energy dissipation rate in a rotating jet flow via direct numerical simulation. The turbulent energy dissipation tensor, including its eigenvalues in the swirling flows with different rotating velocities, is analyzed to investigate the anisotropic characteristics of turbulence and dissipation. In addition, the probability density function of the eigenvalues of turbulence dissipation tensor is presented. The isotropic subrange of PDF always exists in swirling flows relevant to small-scale vortex structure. Thus, with remarkable large-scale vortex breakdown, the isotropic subrange of PDF is reduced in strongly swirling flows, and anisotropic energy dissipation is proven to exist in the core region of the vortex breakdown. More specifically, strong anisotropic turbulence dissipation occurs concentratively in the vortex breakdown region, whereas nearly isotropic turbulence dissipation occurs dispersively in the peripheral region of the strong swirling flows.
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Radomsky, R. W., and K. A. Thole. "Measurements and Predictions of a Highly Turbulent Flowfield in a Turbine Vane Passage." Journal of Fluids Engineering 122, no. 4 (July 10, 2000): 666–76. http://dx.doi.org/10.1115/1.1313244.
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As highly turbulent flow passes through downstream airfoil passages in a gas turbine engine, it is subjected to a complex geometry designed to accelerate and turn the flow. This acceleration and streamline curvature subject the turbulent flow to high mean flow strains. This paper presents both experimental measurements and computational predictions for highly turbulent flow as it progresses through a passage of a gas turbine stator vane. Three-component velocity fields at the vane midspan were measured for inlet turbulence levels of 0.6%, 10%, and 19.5%. The turbulent kinetic energy increased through the passage by 130% for the 10% inlet turbulence and, because the dissipation rate was higher for the 19.5% inlet turbulence, the turbulent kinetic energy increased by only 31%. With a mean flow acceleration of five through the passage, the exiting local turbulence levels were 3% and 6% for the respective 10% and 19.5% inlet turbulence levels. Computational RANS predictions were compared with the measurements using four different turbulence models including the k-ε, Renormalization Group (RNG) k-ε, realizable k-ε, and Reynolds stress model. The results indicate that the predictions using the Reynolds stress model most closely agreed with the measurements as compared with the other turbulence models with better agreement for the 10% case than the 19.5% case. [S0098-2202(00)00804-X]
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Jovanović, J., M. Pashtrapanska, B. Frohnapfel, F. Durst, J. Koskinen, and K. Koskinen. "On the Mechanism Responsible for Turbulent Drag Reduction by Dilute Addition of High Polymers: Theory, Experiments, Simulations, and Predictions." Journal of Fluids Engineering 128, no. 1 (August 2, 2005): 118–30. http://dx.doi.org/10.1115/1.2073227.
Full textAbstract:
Turbulent drag reduction by dilute addition of high polymers is studied by considering local stretching of the molecular structure of a polymer by small-scale turbulent motions in the region very close to the wall. The stretching process is assumed to restructure turbulence at small scales by forcing these to satisfy local axisymmetry with invariance under rotation about the axis aligned with the main flow. It can be shown analytically that kinematic constraints imposed by local axisymmetry force turbulence near the wall to tend towards the one-component state and when turbulence reaches this limiting state it must be entirely suppressed across the viscous sublayer. For the limiting state of wall turbulence, the statistical dynamics of the turbulent stresses, constructed by combining the two-point correlation technique and invariant theory, suggest that turbulent drag reduction by homogeneously distributed high polymers, cast into the functional space which emphasizes the anisotropy of turbulence, resembles the process of reverse transition from the turbulent state towards the laminar flow state. These findings are supported by results of direct numerical simulations of wall-bounded turbulent flows of Newtonian and non-Newtonian fluids and by experiments carried out, under well-controlled laboratory conditions, in a refractive index-matched pipe flow facility using state-of-the art laser-Doppler anemometry. Theoretical considerations based on the elastic behavior of a polymer and spatial intermittency of turbulence at small scales enabled quantitative estimates to be made for the relaxation time of a polymer and its concentration that ensure maximum drag reduction in turbulent pipe flows, and it is shown that predictions based on these are in very good agreement with available experimental data.
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Buice, C. U., and J. K. Eaton. "Turbulent Heat Transport in a Perturbed Channel Flow." Journal of Heat Transfer 121, no. 2 (May 1, 1999): 322–25. http://dx.doi.org/10.1115/1.2825983.
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The recovering boundary layer downstream of a separation bubble is known to have a highly perturbed turbulence structure which creates difficulty for turbulence models. The present experiment addressed the effect of this perturbed structure on turbulent heat transport. The turbulent diffusion of heat downstream of a heated wire was measured in a perturbed channel flow and compared to that in a simple, fully developed channel flow. The turbulent diffusivity of heat was found to be more than 20 times larger in the perturbed flow. The turbulent Prandtl number increased to 1.7, showing that the turbulent eddy viscosity was affected even more strongly than the eddy thermal diffusivity. This result corroborates previous work showing that boundary layer disturbances generally have a stronger effect on the eddy viscosity, rendering prescribed turbulent Prandtl number models ineffective in perturbed flows.
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He, S., C. Ariyaratne, and A. E. Vardy. "Wall shear stress in accelerating turbulent pipe flow." Journal of Fluid Mechanics 685 (September 21, 2011): 440–60. http://dx.doi.org/10.1017/jfm.2011.328.
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AbstractAn experimental study of wall shear stress in an accelerating flow of water in a pipe ramping between two steady turbulent flows has been undertaken in a large-scale experimental facility. Ensemble averaged mean and r.m.s. of the turbulent fluctuations of wall shear stresses have been derived from hot-film measurements from many repeated runs. The initial Reynolds number and the acceleration rate were varied systematically to give values of a non-dimensional acceleration parameter $k$ ranging from 0.16 to 14. The wall shear stress has been shown to follow a three-stage development. Stage 1 is associated with a period of minimal turbulence response; the measured turbulent wall shear stress remains largely unchanged except for a very slow increase which is readily associated with the stretching of existing turbulent eddies as a result of flow acceleration. In this condition of nearly ‘frozen’ turbulence, the unsteady wall shear stress is driven primarily by flow inertia, initially increasing rapidly and overshooting the pseudo-steady value, but then increasing more slowly and eventually falling below the pseudo-steady value. This variation is predicted by an analytical expression derived from a laminar flow formulation. The start of Stage 2 is marked by the generation of new turbulence causing both the mean and turbulent wall shear stress to increase rapidly, although there is a clear offset between the responses of these two quantities. The turbulent wall shear, reflecting local turbulent activities near the wall, responds first and the mean wall shear, reflecting conditions across the entire flow field, responds somewhat later. In Stage 3, the wall shear stress exhibits a quasi-steady variation. The duration of the initial period of nearly frozen turbulence response close to the wall increases with decreasing initial Reynolds number and with increasing acceleration. The latter is in contrast to the response of turbulence in the core of the flow, which previous measurements have shown to be independent of the rate of acceleration.
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Blaisdell, G. A., N. N. Mansour, and W. C. Reynolds. "Compressibility effects on the growth and structure of homogeneous turbulent shear flow." Journal of Fluid Mechanics 256 (November 1993): 443–85. http://dx.doi.org/10.1017/s0022112093002848.
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Compressibility effects within decaying isotropic turbulence and homogeneous turbulent shear flow have been studied using direct numerical simulation. The objective of this work is to increase our understanding of compressible turbulence and to aid the development of turbulence models for compressible flows. The numerical simulations of compressible isotropic turbulence show that compressibility effects are highly dependent on the initial conditions. The shear flow simulations, on the other hand, show that measures of compressibility evolve to become independent of their initial values and are parameterized by the root mean square Mach number. The growth rate of the turbulence in compressible homogeneous shear flow is reduced compared to that in the incompressible case. The reduced growth rate is the result of an increase in the dissipation rate and energy transfer to internal energy by the pressure–dilatation correlation. Examination of the structure of compressible homogeneous shear flow reveals the presence of eddy shocklets, which are important for the increased dissipation rate of compressible turbulence.
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Hu, Xu Yue, Kun Jiang, Hua Qiang Ren, and Xiao Xiong Shen. "An Experimental Study of the Flow Structures in Vegetated Open Channels." Applied Mechanics and Materials 522-524 (February 2014): 941–49. http://dx.doi.org/10.4028/www.scientific.net/amm.522-524.941.
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To investigate the effects of vegetation on flow Reynolds stress and turbulence intensity,an experiment was performed with plastic rods and artificial waterweeds in a slope-variable laboratory flume; an acoustic Doppler velocimeter was used to measure the instantaneous velocity at different points on the vertical line under different conditions; Turbulence parameters at each measuring point were calculated, such as Reynolds stress and turbulence intensity; The effects of vegetation on flow structures were analyzed through comparison with the turbulence characteristics of uniform open channel flows without vegetation distribution. The experimental results show that the turbulent constant Reynolds stress layer exists in water flows with vegetation distribution compared with the water flows without vegetation distribution. Without vegetation distribution, the viscous shear stress at the flume bed mainly affects the area between the bed and the level at a depth about 30% of the water depth. With vegetation distribution, the effect of the viscous shear stress at the bed weakens.The highest flow turbulence intensity with vegetation distribution occurs within the range of vegetation height.
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Chagelishvili, G., G. Khujadze, H. Foysi, and M. Oberlack. "Spanwise reflection symmetry breaking and turbulence control: plane Couette flow." Journal of Fluid Mechanics 745 (March 19, 2014): 300–320. http://dx.doi.org/10.1017/jfm.2014.99.
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AbstractWe propose and analyse a new strategy of shear flow turbulence control that can be realized by the following steps: (i) imposing specially designed seed velocity perturbations, which are non-symmetric in the spanwise direction, at the walls of a flow; (ii) the configuration of the latter ensures a gain of shear flow energy and the breaking of turbulence spanwise reflection symmetry: this leads to the generation of spanwise mean flow; (iii) that changes the self-sustained dynamics of turbulence and results in a considerable reduction of the turbulence level and the production of turbulent kinetic energy. In fact, by this strategy the shear flow transient growth mechanism is activated and the formed spanwise mean flow is an intrinsic, nonlinear composition of the controlled turbulence and not directly introduced in the system. In the present paper, a weak near-wall volume forcing is designed to impose the velocity perturbations with required characteristics in the flow. The efficiency of the proposed scheme has been demonstrated by direct numerical simulation using plane Couette flow as a representative example. A promising result was obtained: after a careful parameter selection, the forcing reduces the turbulence kinetic energy and its production by up to one-third. The strategy can be naturally applied to other wall-bounded flows, e.g. channel and boundary-layer flows. Of course, the considered volume force is theoretical and hypothetical. Nevertheless, it helps to gain knowledge concerning the design of the seed velocity field that is necessary to be imposed in the flow to achieve a significant reduction of the turbulent kinetic energy. This is convincing with regard to a new control strategy, which could be based on specially constructed blowing/suction or riblets, by employing the insight gained by the comprehension of the results obtained using the investigated methodology in this paper.
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Liu, Zhenchen, Peiqing Liu, Hao Guo, and Tianxiang Hu. "Experimental investigations of turbulent decaying behaviors in the core-flow region of a propeller wake." Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 234, no. 2 (August 1, 2019): 319–29. http://dx.doi.org/10.1177/0954410019865702.
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This work investigates the turbulent decaying behaviors downstream of a propeller in the core-flow region. Both axial and tangential velocity fluctuations behind a two-bladed propeller were measured using a stationary hot-wire probe. Unexpectedly, the complex near-wake core-flow of the propeller is found to show a similar decay characteristic of homogeneous turbulence, such as grid turbulence. The decay of turbulence intensity is found to be dominated by the level of periodic velocity fluctuations, showing a similar behavior of the homogenous and isotropic turbulence. This turbulent decaying behavior of the core-flow can be adopted for future turbulent modeling techniques.
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George, S. G., and A. R. L. Tatnall. "Measurement of turbulence in the oceanic mixed layer using Synthetic Aperture Radar (SAR)." Ocean Science Discussions 9, no. 5 (September 13, 2012): 2851–83. http://dx.doi.org/10.5194/osd-9-2851-2012.
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Abstract. Turbulence in the surface layer of the ocean contributes to the transfer of heat, gas and momentum across the air-sea boundary. As such, study of turbulence in the ocean surface layer is becoming increasingly important for understanding its effects on climate change. Direct Numerical Simulation (DNS) techniques were implemented to examine the interaction of small-scale wake turbulence in the upper ocean layer with incident electromagnetic radar waves. Hydrodynamic-electromagnetic wave interaction models were invoked to demonstrate the ability of Synthetic Aperture Radar (SAR) to observe and characterise surface turbulent wake flows. A range of simulated radar images are presented for a turbulent surface current field behind a moving surface vessel, and compared with the surface flow fields to investigate the impact of turbulent currents on simulated radar backscatter. This has yielded insights into the feasibility of resolving small-scale turbulence with remote-sensing radar and highlights the potential for extracting details of the flow structure and characteristics of turbulence using SAR.
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Nagano, Y., and M. Hishida. "Improved Form of the k-ε Model for Wall Turbulent Shear Flows." Journal of Fluids Engineering 109, no. 2 (June 1, 1987): 156–60. http://dx.doi.org/10.1115/1.3242636.
Full textAbstract:
An improved k-ε turbulence model for predicting wall turbulence is presented. The model was developed in conjunction with an accurate calculation of near-wall and low-Reynolds-number flows to meet the requirements of the Evaluation Committee report of the 1980–1981 Stanford Conference on Complex Turbulent Flows. The proposed model was tested by application to turbulent pipe and channel flows, a flat plate boundary layer, a relaminarizing flow, and a diffuser flow. In all cases, the predicted values of turbulent quantities agreed almost completely with measurements, which many previously proposed models failed to predict correctly, over a wide range of the Reynolds number.
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Kestoras, M. D., and T. W. Simon. "Effects of Free-Stream Turbulence Intensity on a Boundary Layer Recovering From Concave Curvature Effects." Journal of Turbomachinery 117, no. 2 (April 1, 1995): 240–47. http://dx.doi.org/10.1115/1.2835652.
Full textAbstract:
Experiments are conducted on a flat recovery wall downstream of sustained concave curvature in the presence of high free-stream turbulence (TI ∼ 8%). This flow simulates some of the features of the flow on the latter parts of the pressure surface of a gas turbine airfoil. The combined effects of concave curvature and TI, both present in the flow over a turbine airfoil, have so far been little studied. Computation of such flows with standard turbulence closure models has not been particularly successful. This experiment attempts to characterize the turbulence characteristics of this flow. In the present study, a turbulent boundary layer grows from the leading edge of a concave wall, then passes onto a downstream flat wall. Results show that turbulence intensities increase profoundly in the outer region of the boundary layer over the recovery wall. Near-wall turbulent eddies appear to lift off the recovery wall and a “stabilized” region forms near the wall. In contrast to a low-free-stream turbulence intensity flow, turbulent eddies penetrate the outer parts of the “stabilized” region where sharp velocity and temperature gradients exist. These eddies can more readily transfer momentum and heat. As a result, skin friction coefficients and Stanton numbers on the recovery wall are 20 and 10 percent, respectively, above their values in the low-free-stream turbulence intensity case. Stanton numbers do not undershoot flat-wall expectations at the same Reδ2 values as seen in the low-TI case. Remarkably, the velocity distribution in the core of the flow over the recovery wall exhibits a negative gradient normal to the wall under high-free-stream turbulence intensity conditions. This velocity distribution appears to be the result of two effects: (1) cross transport of kinetic energy by boundary work in the upstream curved flow and (2) readjustment of static pressure profiles in response to the removal of concave curvature.