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Showing papers on "Turbulence published in 2018"


Book
28 Mar 2018
TL;DR: In this article, a statistical analysis of homogeneous turbulent flows is presented, including the effects of rotation, stratification, strain, buoyancy, and shear on anisotropic isotropic turbulence.
Abstract: 1. Introduction 2. Statistical analysis of homogeneous turbulent flows: reminders 3. Incompressible homogeneous isotropic turbulence 4. Incompressible homogeneous anisotropic turbulence: pure rotations 5. Incompressible homogeneous anisotropic turbulence: strain 6. Incompressible homogeneous anisotropic turbulence: pure shear 7. Incompressible homogeneous anisotropic turbulence: buoyancy and stable stratification 8. Coupled effects: rotations, stratification, strain and shear 9. Compressible homogeneous isotropic turbulence 10. Compressible homogeneous anisotropic turbulence 11. Isotropic turbulence/shock interaction 12. Linear interaction approximation for shock/perturbation interaction 13. Linear theories - from rapid distortions theory to WKB variants 14. Anisotropic nonlinear triadic closures 15. Conclusions and perspectives.

409 citations


Journal ArticleDOI
TL;DR: In this article, the authors provide a critical summary of recent work on turbulent flows from a unified point of view and present a classification of all known transfer mechanisms, including direct and inverse energy cascades.

315 citations


Journal ArticleDOI
TL;DR: In this paper, the structure of turbulence in jets in the subsonic, transonic and supersonic regimes was examined by large-eddy simulation (LES) data and resolvent analysis of the mean flow.
Abstract: Informed by large-eddy simulation (LES) data and resolvent analysis of the mean flow, we examine the structure of turbulence in jets in the subsonic, transonic and supersonic regimes. Spectral (frequency-space) proper orthogonal decomposition is used to extract energy spectra and decompose the flow into energy-ranked coherent structures. The educed structures are generally well predicted by the resolvent analysis. Over a range of low frequencies and the first few azimuthal mode numbers, these jets exhibit a low-rank response characterized by Kelvin–Helmholtz (KH) type wavepackets associated with the annular shear layer up to the end of the potential core and that are excited by forcing in the very-near-nozzle shear layer. These modes too have been experimentally observed before and predicted by quasi-parallel stability theory and other approximations – they comprise a considerable portion of the total turbulent energy. At still lower frequencies, particularly for the axisymmetric mode, and again at high frequencies for all azimuthal wavenumbers, the response is not low-rank, but consists of a family of similarly amplified modes. These modes, which are primarily active downstream of the potential core, are associated with the Orr mechanism. They occur also as subdominant modes in the range of frequencies dominated by the KH response. Our global analysis helps tie together previous observations based on local spatial stability theory, and explains why quasi-parallel predictions were successful at some frequencies and azimuthal wavenumbers, but failed at others.

278 citations


Book ChapterDOI
TL;DR: In this article, the authors reviewed the current knowledge about some particular kinds of coherent structures in the logarithmic and outer layers of wall-bounded turbulent flows and argued that a concerned effort is required to quantitatively identify which one (or ones) of the plausible available dynamical models is a better representation of the observed behaviour.
Abstract: The current knowledge about some particular kinds of coherent structures in the logarithmic and outer layers of wall-bounded turbulent flows is briefly reviewed. It is shown that a lot has been learned about their geometry, flow properties and temporal behaviour. It is also shown that, although the wall-attached structures carry the largest fraction of most flow properties, they are only extreme cases of smaller wall-detached eddies, and that the latter connect with the more classical behaviour of homogeneous turbulence away from walls. Nevertheless, it is argued that little is known about the dynamical origin of these structures, and that a concerned effort is required to quantitatively identify which one (or ones) of the plausible available dynamical models is a better representation of the observed behaviour.

231 citations


Journal ArticleDOI
TL;DR: In this paper, two machine learning models, namely the convolutional neural network (CNN) and the hybrid Downsampled Skip-Connection Multi-Scale (DSC/MS) models, were developed to reconstruct turbulent flows from extremely coarse flow field images with remarkable accuracy.
Abstract: We use machine learning to perform super-resolution analysis of grossly under-resolved turbulent flow field data to reconstruct the high-resolution flow field. Two machine-learning models are developed; namely the convolutional neural network (CNN) and the hybrid Downsampled Skip-Connection Multi-Scale (DSC/MS) models. These machine-learning models are applied to two-dimensional cylinder wake as a preliminary test and show remarkable ability to reconstruct laminar flow from low-resolution flow field data. We further assess the performance of these models for two-dimensional homogeneous turbulence. The CNN and DSC/MS models are found to reconstruct turbulent flows from extremely coarse flow field images with remarkable accuracy. For the turbulent flow problem, the machine-leaning based super-resolution analysis can greatly enhance the spatial resolution with as little as 50 training snapshot data, holding great potential to reveal subgrid-scale physics of complex turbulent flows. With the growing availability of flow field data from high-fidelity simulations and experiments, the present approach motivates the development of effective super-resolution models for a variety of fluid flows.

191 citations


Journal ArticleDOI
TL;DR: In this article, the efficiency of a parabolic trough solar collector (PTSC) was enhanced by using TiO2/DI-H2O (De-Ionized water) nanofluid.

180 citations


Journal ArticleDOI
TL;DR: In this article, the effects of width ratio, Reynolds number, and pitch ratio on nanofluid hydrothermal behavior were illustrated in a heat exchanger equipped with a helical twisted tape turbulator.

179 citations


Journal ArticleDOI
TL;DR: In this paper, large-eddy simulations of an isothermal Mach 0.9 jet issued from a convergent-straight nozzle are performed at a diameter-based Reynolds number of 1 x 10^6.
Abstract: To investigate the effects of the nozzle-exit conditions on jet flow and sound fields, large-eddy simulations of an isothermal Mach 0.9 jet issued from a convergent-straight nozzle are performed at a diameter-based Reynolds number of 1 x 10^6. The simulations feature near-wall adaptive mesh refinement, synthetic turbulence and wall modelling inside the nozzle. This leads to fully turbulent nozzle-exit boundary layers and results in significant improvements for the flow field and sound predictions compared with those obtained from the typical approach based on laminar flow in the nozzle. The far-field pressure spectra for the turbulent jet match companion experimental measurements, which use a boundary-layer trip to ensure a turbulent nozzle-exit boundary layer to within 0.5 dB for all relevant angles and frequencies. By contrast, the initially laminar jet results in greater high-frequency noise. For both initially laminar and turbulent jets, decomposition of the radiated noise into azimuthal Fourier modes is performed, and the results show similar azimuthal characteristics for the two jets. The axisymmetric mode is the dominant source of sound at the peak radiation angles and frequencies. The first three azimuthal modes recover more than 97 % of the total acoustic energy at these angles and more than 65 % (i.e. error less than 2 dB) for all angles. For the main azimuthal modes, linear stability analysis of the near-nozzle mean-velocity profiles is conducted in both jets. The analysis suggests that the differences in radiated noise between the initially laminar and turbulent jets are related to the differences in growth rate of the Kelvin–Helmholtz mode in the near-nozzle region.

162 citations


Journal ArticleDOI
TL;DR: Direct time-resolved imaging of melt pool flow dynamics from a high-energy synchrotron radiation experiment is presented to show surface tension affects flow speed, orientation and surface turbulence.
Abstract: Internal flow behaviour during melt-pool-based metal manufacturing remains unclear and hinders progression to process optimisation. In this contribution, we present direct time-resolved imaging of melt pool flow dynamics from a high-energy synchrotron radiation experiment. We track internal flow streams during arc welding of steel and measure instantaneous flow velocities ranging from 0.1 m s−1 to 0.5 m s−1. When the temperature-dependent surface tension coefficient is negative, bulk turbulence is the main flow mechanism and the critical velocity for surface turbulence is below the limits identified in previous theoretical studies. When the alloy exhibits a positive temperature-dependent surface tension coefficient, surface turbulence occurs and derisory oxides can be entrapped within the subsequent solid as result of higher flow velocities. The widely used arc welding and the emerging arc additive manufacturing routes can be optimised by controlling internal melt flow through adjusting surface active elements. Understanding what happens to the liquid in melt pools during welding and metal-based additive manufacturing remains a challenge. Here, the authors directly image internal melt pool dynamics using synchrotron radiation to show surface tension affects flow speed, orientation and surface turbulence.

156 citations


Journal ArticleDOI
TL;DR: Numerical simulations of turbulent convection in fluids at different Prandtl number levels suggest a scale separation and thus the existence of a simplified description of the turbulent superstructures in geo- and astrophysical settings.
Abstract: Turbulent Rayleigh-Benard convection displays a large-scale order in the form of rolls and cells on lengths larger than the layer height once the fluctuations of temperature and velocity are removed. These turbulent superstructures are reminiscent of the patterns close to the onset of convection. Here we report numerical simulations of turbulent convection in fluids at different Prandtl number ranging from 0.005 to 70 and for Rayleigh numbers up to 107. We identify characteristic scales and times that separate the fast, small-scale turbulent fluctuations from the gradually changing large-scale superstructures. The characteristic scales of the large-scale patterns, which change with Prandtl and Rayleigh number, are also correlated with the boundary layer dynamics, and in particular the clustering of thermal plumes at the top and bottom plates. Our analysis suggests a scale separation and thus the existence of a simplified description of the turbulent superstructures in geo- and astrophysical settings.

150 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that most commonly used two-equation turbulence closure models are unconditionally, rather than conditionally, unstable in regions of nearly potential flow with finite strain, resulting in exponential growth of the turbulent kinetic energy and eddy viscosity.
Abstract: In previous computational fluid dynamics studies of breaking waves, there has been a marked tendency to severely over-estimate turbulence levels, both pre- and post-breaking. This problem is most likely related to the previously described (though not sufficiently well recognized) conditional instability of widely used turbulence models when used to close Reynolds-averaged Navier–Stokes (RANS) equations in regions of nearly potential flow with finite strain, resulting in exponential growth of the turbulent kinetic energy and eddy viscosity. While this problem has been known for nearly 20 years, a suitable and fundamentally sound solution has yet to be developed. In this work it is demonstrated that virtually all commonly used two-equation turbulence closure models are unconditionally, rather than conditionally, unstable in such regions. A new formulation of the is the dissipation.)

Journal ArticleDOI
TL;DR: In this paper, a wake model for wind turbines considering ambient turbulence intensity and thrust coefficient effects is proposed by numerical and analytical studies, which is derived based on the axial symmetry and self-similarity assumption for wake deficit and added turbulence intensity.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of combinational opposing jet and spike concept on the aerodynamic drag and heat properties of a single spiked blunt body, and showed that the combinatorial thermal protection system has a great contribution to reduce the drag.

Journal ArticleDOI
TL;DR: In this paper, a k−3 spectral subrange and exponential probability density function for bubble-induced agitation (BIA) was derived from a simulation of bubbles randomly distributed within a liquid.
Abstract: Bubbly flows involve bubbles randomly distributed within a liquid At large Reynolds number, they experience an agitation that can combine shear-induced turbulence (SIT), large-scale buoyancy-driven flows, and bubble-induced agitation (BIA) The properties of BIA strongly differ from those of SIT They have been determined from studies of homogeneous swarms of rising bubbles Regarding the bubbles, agitation is mainly caused by the wake-induced path instability Regarding the liquid, two contributions must be distinguished The first one corresponds to the anisotropic flow disturbances generated near the bubbles, principally in the vertical direction The second one is the almost isotropic turbulence induced by the flow instability through a population of bubbles, which turns out to be the main cause of horizontal fluctuations Both contributions generate a k−3 spectral subrange and exponential probability density functions The subsequent issue will be to understand how BIA interacts with SIT

Journal ArticleDOI
TL;DR: It is shown that, for an appropriate choice of parameters, polymers can reduce the drag beyond the suggested asymptotic limit, eliminating turbulence and giving way to laminar flow.
Abstract: The drag of turbulent flows can be drastically decreased by adding small amounts of high molecular weight polymers. While drag reduction initially increases with polymer concentration, it eventually saturates to what is known as the maximum drag reduction (MDR) asymptote; this asymptote is generally attributed to the dynamics being reduced to a marginal yet persistent state of subdued turbulent motion. Contrary to this accepted view, we show that, for an appropriate choice of parameters, polymers can reduce the drag beyond the suggested asymptotic limit, eliminating turbulence and giving way to laminar flow. At higher polymer concentrations, however, the laminar state becomes unstable, resulting in a fluctuating flow with the characteristic drag of the MDR asymptote. Our findings indicate that the asymptotic state is hence dynamically disconnected from ordinary turbulence.

Journal ArticleDOI
TL;DR: It is found that the generation of a power-law particle energy spectrum is a generic by-product of relativistic turbulence in magnetically dominated (or, equivalently, "relativistic") pair plasmas.
Abstract: Due to its ubiquitous presence, turbulence is often invoked to explain the origin of nonthermal particles in astrophysical sources of high-energy emission. With particle-in-cell simulations, we study decaying turbulence in magnetically dominated (or, equivalently, "relativistic") pair plasmas. We find that the generation of a power-law particle energy spectrum is a generic by-product of relativistic turbulence. The power-law slope is harder for higher magnetizations and stronger turbulence levels. In large systems, the slope attains an asymptotic, system-size-independent value, while the high-energy spectral cutoff increases linearly with system size; both the slope and the cutoff do not depend on the dimensionality of our domain. By following a large sample of particles, we show that particle injection happens at reconnecting current sheets; the injected particles are then further accelerated by stochastic interactions with turbulent fluctuations. Our results have important implications for the origin of nonthermal particles in high-energy astrophysical sources.

Journal ArticleDOI
TL;DR: In this paper, the authors investigate the heat transfer capability of Mg (OH)2/MWCNT- engine oil hybrid nano-lubricant and propose two new trustworthy correlations to predict the dynamic viscosity and thermal conductivity.

Journal ArticleDOI
06 Apr 2018
TL;DR: In this article, the authors report the observation of very large-scale and long living coherent structures in highly turbulent Rayleigh-Benard convection up to Rayleigh Ra=109.
Abstract: We report the observation of superstructures, i.e., very large-scale and long living coherent structures in highly turbulent Rayleigh-Benard convection up to Rayleigh Ra=109. We perform direct numerical simulations in horizontally periodic domains with aspect ratios up to Γ=128. In the considered Ra number regime the thermal superstructures have a horizontal extend of six to seven times the height of the domain and their size is independent of Ra. Many laboratory experiments and numerical simulations have focused on small aspect ratio cells in order to achieve the highest possible Ra. However, here we show that for very high Ra integral quantities such as the Nusselt number and volume averaged Reynolds number only converge to the large aspect ratio limit around Γ≈4, while horizontally averaged statistics such as standard deviation and kurtosis converge around Γ≈8, the integral scale converges around Γ≈32, and the peak position of the temperature variance and turbulent kinetic energy spectra only converge around Γ≈64.

Journal ArticleDOI
TL;DR: In this paper, the effect of attack angle of inclined rectangular rib, Reynolds number and volume fraction of nanoparticles on heat transfer enhancement has been investigated, and the results show that, in high Reynolds numbers, by using ribs and nanofluid, the performance evaluation criterion improves.
Abstract: In present study, the turbulent flow and heat transfer of Water/Al2O3 nanofluid inside a rectangular channel have been numerically simulated. The main purpose of present study is investigating the effect of attack angle of inclined rectangular rib, Reynolds number and volume fraction of nanoparticles on heat transfer enhancement. For this reason, the turbulent flow of nanofluid has been simulated at Reynolds numbers ranging from 15000 to 30000 and volume fractions of nanoparticles from 0 to 4%. The changes attack angle of ribs have been investigated ranging from 0 to 180°. The results show that, the changes of attack angle of ribs, due to the changes of flow pattern and created vortexes inside the channel, have significant effect on fluid mixing. Also, the maximum rate of heat transfer enhancement accomplishes in attack angle of 60°. In Reynolds numbers of 15000, 20000 and 30000 and attack angle of 60°, comparing to the attack angle of 0°, the amount of Nusselt number enhances to 2.37, 1.96 and 2 times, respectively. Also, it can be concluded that, in high Reynolds numbers, by using ribs and nanofluid, the performance evaluation criterion improves.

Journal ArticleDOI
TL;DR: In this paper, the authors examined four different, mechanically durable, large-scale superhydrophobic surfaces (SHSs) and found that significant drag reduction was observed on some of the surfaces, dependent on their exact morphology.
Abstract: A significant amount of the fuel consumed by marine vehicles is expended to overcome skin-friction drag resulting from turbulent boundary layer flows. Hence, a substantial reduction in this frictional drag would notably reduce cost and environmental impact. Superhydrophobic surfaces (SHSs), which entrap a layer of air underwater, have shown promise in reducing drag in small-scale applications and/or in laminar flow conditions. Recently, the efficacy of these surfaces in reducing drag resulting from turbulent flows has been shown. In this work we examine four different, mechanically durable, large-scale SHSs. When evaluated in fully developed turbulent flow, in the height-based Reynolds number range of 10 000 to 30 000, significant drag reduction was observed on some of the surfaces, dependent on their exact morphology. We then discuss how neither the roughness of the SHSs, nor the conventional contact angle goniometry method of evaluating the non-wettability of SHSs at ambient pressure, can predict their drag reduction under turbulent flow conditions. Instead, we propose a new characterization parameter, based on the contact angle hysteresis at higher pressure, which aids in the rational design of randomly rough, friction-reducing SHSs. Overall, we find that both the contact angle hysteresis at higher pressure, and the non-dimensionalized surface roughness, must be minimized to achieve meaningful turbulent drag reduction. Further, we show that even SHSs that are considered hydrodynamically smooth can cause significant drag increase if these two parameters are not sufficiently minimized.

Journal ArticleDOI
TL;DR: A direct numerical simulation database of high-speed zero-pressure-gradient turbulent boundary layers developing spatially over a flat plate with nominal freestream Mach number ranging from 2.5 to 14 and wall-to-recovery temperature ranging from 0.18 to 1.0 is presented.
Abstract: In this paper, we present a direct numerical simulation database of high-speed zero-pressure-gradient turbulent boundary layers developing spatially over a flat plate with nominal freestream Mach number ranging from 2.5 to 14 and wall-to-recovery temperature ranging from 0.18 to 1.0. The flow conditions of the DNS are representative of the operational conditions of the Purdue Mach 6 quiet tunnel, the Sandia Hypersonic Wind Tunnel at Mach 8, and the AEDC Hypervelocity Tunnel No. 9 at Mach 14. The DNS database is used to gauge the performance of compressibility transformations, including the classical Morkovin's scaling and strong Reynolds analogy as well as the newly proposed mean velocity and temperature scalings that explicitly account for wall heat flux. Several insights into the effect of direct compressibility are gained by inspecting the thermodynamic fluctuations and the Reynolds stress budget terms. Precomputed flow statistics, including Reynolds stresses and their budgets, will be available at the website of the NASA Langley Turbulence Modeling Resource, allowing other investigators to query any property of interest.

Journal ArticleDOI
TL;DR: In this article, the effect of disk turbulence on the pebble isolation mass (PIM) was studied and its dependence on the gas turbulent viscosity, aspect ratio, and particles Stokes number was found.
Abstract: Context. When a planet becomes massive enough, it gradually carves a partial gap around its orbit in the protoplanetary disk. A pressure maximum can be formed outside the gap where solids that are loosely coupled to the gas, typically in the pebble size range, can be trapped. The minimum planet mass for building such a trap, which is called the pebble isolation mass (PIM), is important for two reasons: it marks the end of planetary growth by pebble accretion, and the trapped dust forms a ring that may be observed with millimetre observations.Aims. We study the effect of disk turbulence on the PIM and find its dependence on the gas turbulent viscosity, aspect ratio, and particles Stokes number.Methods. By means of 2D gas hydrodynamical simulations, we found the minimum planet mass to form a radial pressure maximum beyond the orbit of the planet, which is the necessary condition to trap pebbles. We then carried out 2D gas plus dust hydrodynamical simulations to examine how dust turbulent diffusion impacts particles trapping at the pressure maximum. We finally provide a semi-analytical calculation of the PIM based on comparing the radial drift velocity of solids and the root mean square turbulent velocity fluctuations around the pressure maximum.Results. From our results of gas simulations, we provide an expression for the PIM vs. disk aspect ratio and turbulent viscosity. Our gas plus dust simulations show that the effective PIM can be nearly an order of magnitude larger in high-viscosity disks because turbulence diffuse particles out of the pressure maximum. This is quantified by our semi-analytical calculation, which gives an explicit dependence of the PIM with Stokes number of particles.Conclusions. Disk turbulence can significantly alter the PIM, depending on the level of turbulence in regions of planet formation.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the transport equations for velocity variances using data from DNS of incompressible channel flows at up to 5200, and showed that the energy is transferred from the streamwise elongated modes to modes with a range of orientations through nonlinear interactions.
Abstract: The transport equations for velocity variances are investigated using data from DNS of incompressible channel flows at $Re_\tau$ up to 5200. Each term in the transport equation has been spectrally decomposed to expose the contribution of turbulence at different length scales to the processes governing the flow of energy in the wall-normal direction, in scale and among components. The outer-layer turbulence is dominated by very large-scale streamwise elongated modes. Away from the wall, production occurs primarily in these large-scale streamwise-elongated modes in the streamwise velocity, but dissipation occurs nearly isotropically in both velocity components and scale. For this to happen, the energy is transferred from the streamwise elongated modes to modes with a range of orientations through non-linear interactions, and then transferred to other velocity components. This allows energy to be transferred more-or-less isotropically from these large scales to the small scales at which dissipation occurs. The VLSMs also transfer energy to the wall-region resulting in a modulation of the autonomous near-wall dynamics. The near-wall energy flows are consistent with the well-known autonomous near-wall dynamics. Through the overlap region between outer and inner layer turbulence, there is a self-similar structure to the energy flows. The VLSM production occurs at spanwise scales that grow with $y$. There is transport of energy away from the wall over a range of scales that grows with $y$. And, there is transfer of energy to small dissipative scales which grow like $y^{1/4}$. Finally, the small-scale near-wall processes characterised by wavelengths less than 1000 wall units are largely Reynolds number independent, while the larger-scale outer layer process are strongly Reynolds number dependent. The interaction between them appears to be relatively simple.

Journal ArticleDOI
TL;DR: In this paper, a dynamical system approach is used to devise a linear estimation tool for channel flow at a friction Reynolds number of. The estimator uses time-resolved velocity measurements at a single wall normal location to estimate the velocity field at other wall-normal locations (the data coming from direct numerical simulations).
Abstract: A dynamical systems approach is used to devise a linear estimation tool for channel flow at a friction Reynolds number of . The estimator uses time-resolved velocity measurements at a single wall-normal location to estimate the velocity field at other wall-normal locations (the data coming from direct numerical simulations). The estimation tool builds on the work of McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382) by using a Navier–Stokes-based linear model and treating any nonlinear terms as unknown forcings to an otherwise linear system. In this way nonlinearities are not ignored, but instead treated as an unknown model input. It is shown that, while the linear estimator qualitatively reproduces large-scale flow features, it tends to overpredict the amplitude of velocity fluctuations – particularly for structures that are long in the streamwise direction and thin in the spanwise direction. An alternative linear model is therefore formed in which a simple eddy viscosity is used to model the influence of the small-scale turbulent fluctuations on the large scales of interest. This modification improves the estimator performance significantly. Importantly, as well as improving the performance of the estimator, the linear model with eddy viscosity is also able to predict with reasonable accuracy the range of wavenumber pairs and the range of wall-normal heights over which the estimator will perform well.

Journal ArticleDOI
TL;DR: In this paper, a 50-year journey through flow control strategies that seek to achieve viscous drag reduction in turbulent boundary layers is presented, focusing on different mechanisms underlying flow control.
Abstract: A 50 year journey through flow control strategies that seek to achieve viscous drag reduction in turbulent boundary layers is presented. These are shown to focus on different mechanisms underlying ...

Journal ArticleDOI
TL;DR: In this article, two types of WVGs are introduced: rectangular (RWVG) and trapezoidal (TWVG) to create multiple vortex flows along the duct in order to create turbulent convective heat transfer in a solar air heater duct with winglet-type vortex generators placed on the absorber plate.

Journal ArticleDOI
TL;DR: In this article, the effect of twist ratio of twisted-tape inserts from 2.5 to 4 and counter-swirl flow and volume fraction of nanofluid from 1 to 4 on the average Nusselt number was investigated.
Abstract: In the present study, the turbulent flow of water/Al2O3 nanofluid in a tubular heat exchanger with two twisted-tape inserts has been numerically investigated in the three-dimensional coordinate. This numerical simulation has been done by using FVM, and all of the equations have been discretized by second-order upwind method. For coupling velocity–pressure equations, SIMPLEC algorithm has been used. The investigated parameters of the present study are Reynolds numbers at the range of 10,000–30,000, the effect of twist ratio of twisted-tape inserts from 2.5 to 4, co-swirl flow and counter-swirl flow of two twisted-tapes inside the tube and volume fractions of nanofluid from 1 to 4%. The results of this research revealed that, by decreasing the twist ratio, the counter-swirl flow twisted-tape and the enhancement of volume fraction of Al2O3 nanoparticles in the base fluid, the amount of average Nusselt number increases. In twist ratios of 3.25 and 2.5, the maximum amounts of performance evaluation criterion (PEC) are 1.6 and 1.55, respectively. Also, the amount of PEC improvement in counter-swirl flow state is considerably more than co-swirl flow state. Therefore, the maximum enhancements of PEC improvement in three twist ratios of 4, 3.25 and 2.5 are, respectively, 40.8, 47 and 51% and the maximum enhancements in three mentioned ratios are 26.5, 28.3 and 30.6%, respectively.

Journal ArticleDOI
TL;DR: In this article, epsilon (e) in the Omega vortex identification criterion (Ω method) is defined as an explicit function in order to apply the Ω method to different cases and even different time steps for the unsteady cases.
Abstract: In the present paper, epsilon (e) in the Omega vortex identification criterion (Ω method) is defined as an explicit function in order to apply the Ω method to different cases and even different time steps for the unsteady cases. In our method, e is defined as a function relating with the flow without any subjective adjustment on its coefficient. The newly proposed criteria for the determination of e is tested in several typical flow cases and is proved to be effective in the current work. The test cases given in the present paper include boundary layer transition, shock wave and boundary layer interaction, and channel flow with different Reynolds numbers.

Journal ArticleDOI
TL;DR: In this article, a KD2 Pro thermal analyzer was used to measure the thermal conductivity of the samples and the results showed an increasing trend for thermal conductivities of the nanofluid by increasing the mass concentration and temperature.
Abstract: The present work aims to study heat transfer performance and pumping power of MgO–MWCNT/ thermal oil hybrid nanofluid. Using a KD2 Pro thermal analyzer, the thermal conductivity of the samples has been measured. The results showed an increasing trend for the thermal conductivity of the nanofluid by increasing the mass concentration and temperature, in which the maximum enhancement of thermal conductivity was approximately 65%. Predicting the thermal conductivity of the nanofluid, a highly accurate correlation in terms of solid concentration and temperature has been proposed. Moreover, the heat transfer efficiency and pumping power in all the studied range of solid concentrations and temperatures have been theoretically investigated, based on the experimental data of dynamic viscosity and thermal conductivity, for both the internal laminar and turbulent flow regimes. It was observed that the studied nanofluid is highly efficient in heat transfer applications as a coolant fluid in both the laminar and turbulent flow regimes, although it causes a certain penalty in the pumping power.

Journal ArticleDOI
TL;DR: In this paper, the authors used the nano-scale thermal anemometry probe (NSTAP), developed at Princeton University to conduct velocity measurements in the high Reynolds number boundary layer facility at the University of Melbourne.
Abstract: Fully resolved measurements of turbulent boundary layers are reported for the Reynolds number range . Despite several decades of research in wall-bounded turbulence there is still controversy over the behaviour of streamwise turbulence intensities near the wall, especially at high Reynolds numbers. Much of it stems from the uncertainty in measurement due to finite spatial resolution. Conventional hot-wire anemometry is limited for high Reynolds number measurements due to limited spatial resolution issues that cause attenuation in the streamwise turbulence intensity profile near the wall. To address this issue we use the nano-scale thermal anemometry probe (NSTAP), developed at Princeton University to conduct velocity measurements in the high Reynolds number boundary layer facility at the University of Melbourne. The NSTAP has a sensing length almost one order of magnitude smaller than conventional hot-wires. This enables us to acquire fully resolved velocity measurements of turbulent boundary layers up to . Results show that in the near-wall region, the viscous-scaled streamwise turbulence intensity grows with in the Reynolds number range of the experiments. A second outer peak in the streamwise turbulence intensity is also shown to emerge at the highest Reynolds numbers. Moreover, the energy spectra in the near-wall region show excellent inner scaling over the small to moderate wavelength range, followed by a large-scale influence that increases with Reynolds number. Outer scaling in the outer region is found to collapse the energy spectra over high wavelengths across various Reynolds numbers.