Search

Direct numerical simulation is used to determine the turbulent Prandtl number P r t above cold (isothermal) and hot (adiabatic) walls in a family of low-supersonic channel flows. A range of mean temperature/density variations, corresponding to effective/edge Mach numbers between 1.1 to 2.2, and wall-variable-based Reynolds number R e τ w from 73 to 3800, is considered. The adiabatic condition is a new feature of special interest. The value of P r t away from the wall approaches 0.85 above both the isothermal and adiabatic walls. The variations of the near-wall P r t profiles in both the present and previous, passive-scalar simulations collapse as a function of the semilocal y w ∗ wall scaling proposed in 1995 by [Huang, P. G., G. N. Coleman, and P. Bradshaw. 1995. "Compressible Turbulent Channel Flows: DNS Results and Modelling." Journal of Fluid Mechanics 305: 185–218. doi:.], with only a weak dependence on R e τ w . This leads to a rather simple proposal for a model of heat transfer, attached to an eddy-viscosity model. [ABSTRACT FROM AUTHOR]

This paper presents a low-communication-overhead parallel method for direct numerical simulations of the 3D incompressible wall turbulence. A fully explicit projection method with second-order space-time accuracy is adopted. Combined with fast Fourier transforms, the parallel diagonal dominant (PDD) algorithm for the tridiagonal system is employed to solve the pressure Poisson equation. The number of all-to-all communications is decreased to only two, in a 2D pencil-like domain decomposition. The resulting MPI/OpenMP hybrid parallel code shows excellent strong scalability up to 104 cores and small wall-clock time per timestep. Numerical simulations of turbulent channel flow at different friction Reynolds numbers ( R e τ = 550 , 1000 , 2000) have been conducted and the statistics are in good agreement with the reference data and the recent scaling theories [Chen, X. and K. R. Sreenivasan. 2021. "Reynolds Number Scaling of the Peak Turbulence Intensity in Wall Flows." Journal of Fluid Mechanics 908: R3. doi:10.1017/jfm.2020.991]. [ABSTRACT FROM AUTHOR]

Jameson cell is a flotation device that has unique characteristics for fine grained particles, and thus, it has more than three hundred applications in worldwide. On the contrary, the flotation performance of the cell decreases by coarser feeding, which limits the application of the Jameson cell. It has been figured out that the main reason for the performance decrease for coarse particle is turbulence which occurs at the outlet of the downcomer in the Jameson cell. Therefore, the aim of this study was to prevent the performance decrease in the Jameson cell by modifying the downcomer which was implemented by taking into account the fluid mechanics law and mathematical modelling. The modified downcomer, which can be called as diffuser, can provide better aggregate stability and less turbulence at the end of the downcomer. Thus, theoretically the better performance was obtained by the use of the new form of downcomer. In addition, a series of experiments were carried out to verify the success of the modified downcomer. According to the results, velocity gradient (G) decreased to 640 s−1 from 671 s−1 by using the modified downcomer, and the maximum floatable particle size increased to 255 µm from 225 µm. [ABSTRACT FROM AUTHOR]

The challenge to understanding the fluid mechanics of fish swimming is knowing exactly what the water is doing where the fish swims. Recent field and laboratory observations in box culvert barrel showed that fish tend to swim preferentially close to the channel sidewalls, in regions of slow velocity and often high turbulence intensity. An analogy with human swimming is developed herein. Fish minimise their energy expenditure by swimming in inter-connected low-velocity zones (LVZs) and minimising acceleration-deceleration amplitudes. In a box culvert barrel, the mechanical energy expenditure is drastically reduced in sidewall and corner flow regions, characterised by low velocities and secondary current motion. These regions were “sweet spots” used by small bodied fish to minimise their rate of work. Both bed and sidewall roughness must be scaled to the fish dimensions. More generally, the methodology brings rigorous scientific insights into why certain culvert designs, possibly equipped with baffles and apertures, are more efficient in promoting fish passage. One may foresee an evolution of the scientific approach towards using advanced physics-based theory supported by high-quality data sets. The results also raise questions on limitations of current fish swim tunnel tests, and matching swimming performance data to hydrodynamic measurements. [ABSTRACT FROM AUTHOR]

In numerical simulations of flow over rough surfaces, the roughness is often not resolved but represented by a numerical model. The validity of such an assumption is investigated in this paper by Reynolds- Averaged Navier-Stokes simulations of flow over a surface with a large roughness. The surface was created from a high-resolution laser scanning of a real rock blasted tunnel. By reducing the geometrical resolution of the roughness in two steps, the importance of an appropriate surface description could be examined. The flow fields obtained were compared to a set-up with a geometrical flat surface where the roughness was represented by a modified form of the Launder and Spalding wall-function. The flow field over the surface with the lowest resolution was substantially different from those of the two finer resolutions and rather close to the results from the set-up with the wall-function. The results also yield that the finer the resolution is the more vorticity is formed close to the rough surface and more turbulence is generated. [ABSTRACT FROM AUTHOR]

Comparative experiments were conducted between uniform and non-uniform stepped spillway profiles in a large-size laboratory facility. For each stepped configuration, the air concentration distributions matched the advective diffusion equation and the interfacial velocity was well correlated with a power law. A comparison of the air–water flow properties showed small differences in terms of number of entrained air bubbles, chord sizes and turbulence characteristics between the different configurations. For the non-uniform steps, larger flow instabilities and stronger variation in the air–water flow properties were observed. Further some non-intrusive measurements were performed with acoustic displacement metres to characterize the free-surface profiles, free-surface fluctuations and free-surface wave celerity in both non-aerated and aerated flow regions. The experiments highlighted a close agreement between experimental data and theoretical predictions in the non-aerated flows and with conductivity probe data in the aerated flows. The non-intrusive technique was suitable for measuring the free-surface characteristics on stepped chutes, especially in the non-aerated flow region. [ABSTRACT FROM PUBLISHER]

In the present study, a novel model was established to investigate the enhanced heat transfer to turbulent pipe flow of supercritical pressure fluids. The governing equations for the steady turbulent compressible pipe flow were simplified into the one-dimensional nondimensionalized forms based on the boundary layer theory. A conventional mixing length turbulence model for constant-property pipe flows was modified by introducing the effect of density fluctuations into the equations of turbulent transport, and the modified turbulence model was applicable to both constant-property and variable-property pipe flows. With the suggested model, which was a combination of the nondimensional governing equations and the modified turbulence model, the numerical calculations were carried out for the turbulent convective heat transfer of water in round tubes at supercritical pressures. The results showed that the present model can provide a relatively precise prediction about the effect of pressure, mass flux, and wall heat flux on heat transfer for supercritical fluid flows and greatly reduce the calculation workload. The modified turbulence model showed a much better agreement with the experimental results than the original turbulence model. [ABSTRACT FROM AUTHOR]

The identification of the spatial characteristics and the shedding frequencies of coherent structures in shallow flows usually involves the use of sophisticated equipment for velocity measurements such as laser Doppler anemometry or particle image velocimetry. In this work, a simple and low-cost alternative for the quantitative characterization of quasi two-dimensional shallow coherent structures is presented. The technique is based on the image pre-processing of flow visualizations and post-processing by means of the proper orthogonal decomposition. As an illustration of the method, the analysis of a shallow flow in the wake of a cylinder with a vortex-street pattern is presented. The method is able to discriminate concentration patches attached to the vortices and their advection, providing frequencies in excellent agreement with a previous study. [ABSTRACT FROM PUBLISHER]

The success of predictions of wall-bounded turbulent flows requires an accurate description of the flow in the near-wall region. This article presents a comparative study between different near-wall treatments and presents an improved method. The study is applied to fully developed plane channel flow (i.e. the flow between two infinitely large plates). Simulations were performed using Fluent. Near-wall treatments available in Fluent were tested: standard wall functions, non-equilibrium wall function and enhanced wall treatment. A user defined function (UDF), based on an analytical profile for the turbulent kinetic energy (Absi, R., 2008. Analytical solutions for the modeled k-equation. ASME Journal of Applied Mechanics, 75 (4), 044501), is developed and implemented. Predicted turbulent kinetic energy profiles are presented and validated by DNS data. [ABSTRACT FROM AUTHOR]

Universal probability density functions (PDFs) of two-dimensional turbulent velocity fluctuations, Reynolds shear stress and conditional Reynolds shear stresses in flows over smooth and rough beds are obtained using a Gram-Charlier series expansion based on the exponential distribution. To include skewness and kurtosis, the series is truncated up to moments of Order 4. The distributions of PDFs obtained theoretically and from the experimental data are in agreement. The conditional Reynolds shear stresses related to the ejections and sweeps are well represented by the exponential distribution, but those related to the outward and inward interactions depart from the theoretical distributions. [ABSTRACT FROM AUTHOR]

This note presents experimental measurements on the turbulence characteristics in the transition region from the end of a hydraulic jump to open channel flow in a rectangular channel. An acoustic Doppler velocimeter was used to measure mean velocities, turbulent intensities, Reynolds stress and turbulent kinetic energy for two supercritical Froude numbers. General correlations for the variation of the normalized maximum values of the Reynolds normal stresses as well as the shear stress and the turbulent kinetic energy have been developed, using the present results and those from the literature. The supercritical Froude number was found to be an effective scale in addition to the supercritical approach flow velocity in obtaining the general correlations. [ABSTRACT FROM AUTHOR]

The de-oiling hydrocyclone is a device for liquid-liquid separation and production water cleanup. Significant progress has been made in the development of such a device since its first introduction and use in the late 1970s. The present article is a literature review of development and research work performed so far on de-oiling hydrocyclones. It reviews the performance parameters affecting the de-oiling hydrocyclone operation; namely, the inlet oil concentration and drop size distribution, the turn-down ratio, the pressure drop ratio, the flow split, and the geometrical parameters. The article addresses work done to elucidate the internal flow structure and performance of de-oiling hydrocylones in both experimental and computational fluid dynamics areas. Challenges and remaining research issues are also identified. [ABSTRACT FROM AUTHOR]

Three-dimensional simulation of incompressible flow in rotating tubes for both laminar and turbulent flows has been performed using a finite-volume method for elliptic flows. The influence of Reynolds number on the velocity field and the effects of temperature gradient on temperature profiles have been presented by numerical simulations. Also the effects of velocity field, flow regime, and temperature distribution along the tube have been studied from different points of view. The results have been calculated for rotational Reynolds numbers ranging from 1000 to 320,000. The comparisons between numerically calculated velocity field and the Nusselt number have shown satisfactory agreement with the experimental data. [ABSTRACT FROM AUTHOR]

A scroll is used to collect and transport swirling fluid produced by impeller and diffuser. Scroll or volute is one of the key components of centrifugal compressors. Design of the scroll not only impacts compressor efficiency but also influences operating range of the compressor. In this study, Navier-Stokes equations with both the zero-equation turbulence model and the k-ε turbulence model were used to simulate flows inside a single stage compressor. Detailed flow simulations for a large cut back tongue scroll are presented and discussed. Studies showed that a large cut back rounded tongue scroll provided good operating range without dropping compressor peak efficiency dramatically. The turbulence model influences to the calculation were discussed and some suggestions for scroll flow modeling are made. The numerical results obtained using two turbulence models were compared and showed agreement reasonably well with experiments in overall compressor performance. Although the k-ε model behaves well inside the boundary layer, it was not decisively better than the Zero-equation model for compressor stage performance predictions. [ABSTRACT FROM AUTHOR]

A fluid-flow model which automatically determines the flow regime was used to analyze a timewise-periodic pipe flow. Numerical simulation was employed to implement the model. The range of the instantaneous Reynolds number gave rise to four distinct flow regimes: laminarizing, fully laminar, turbulentizing, and fully turbulent. The period of the imposed harmonic oscillations was varied over a very wide range, and the magnitude of the oscillations was of the same order as that of the steady flow on which the oscillations were superimposed. A large-period limit at which the flow is quasi-steady was identified. The predicted quasi-steady fully developed friction factor for each regime was found to be in excellent agreement with steady-state results applied instantaneously. A metric in the form of the ratio of the turbulence production to turbulence destruction was used to exhibit the turbulence characteristics of each of the four flow regimes. The value of this metric was somewhat different in the laminarization and turbulentization regimes at the same instantaneous Reynolds number. This outcome suggests that the flow has memory. [ABSTRACT FROM AUTHOR]

Flow pattern, bubble size distribution, and gas holdup are investigated in the bubble columns. Simulated results by the standard k-ε turbulence model together with Grace drag model in the commercial computational fluid dynamics software platform (ANSYS CFX 10.0) show good agreement with experimental measurement within the 2D and 3D columns. Three flow regimes (homogeneous bubbly, transitional, and turbulent flows) are identified by the instantaneous velocity profiles of gas and liquid phases, and general flow patterns are summarized under different superficial gas velocities. The findings are useful for designing the slurry bubble columns for synthesizing liquid hydrocarbons and alcohols. [ABSTRACT FROM AUTHOR]

The objective of the present work is to analyse the behaviour of a turbulent decelerating boundary layer under the effect of both passive and active jets vortex generators (VGs). The stereo PIV database of Godard and Stanislas [1,2] obtained in an adverse pressure gradient boundary layer is used for this study. After presenting the effect on the mean velocity field and the turbulent kinetic energy, the line of analysis is extended with two points spatial correlations and vortex detection in instantaneous velocity fields. It is shown that the actuators concentrate the boundary layer turbulence in the region of upward motion of the flow, and segregate the near-wall streamwise vortices of the boundary layer based on their vorticity sign. [ABSTRACT FROM AUTHOR]

The objective of the present work is to evaluate the steady and unsteady Reynolds-averaged Navier-Stokes (RANS) approach turbulence model for separated flow applications. A υ2- f model based on the standard k - ω model is employed and referred to as the k - ω - υ2- f model. This model is validated by flow around two- and three-dimensional bluff bodies. The selected configurations are a single square cylinder, tandem square cylinders and cube. The flow predictions are discussed and compared with available experimental and numerical data. For these cases, none of the previously published numerical predictions obtained by steady-state RANS produced a good match with experimental data. Because these flow cases are inherently unsteady at high Reynolds numbers, unsteady RANS computations should be employed. It is found that the results obtained with this mode are in agreement with the available experimental and numerical data. [ABSTRACT FROM AUTHOR]

Regularization-based subgrid-scale (SGS) turbulence models for large eddy simulations (LES) are quantitatively assessed for decaying homogeneous turbulence (DHT) and transition to turbulence for the Taylor-Green vortex (TGV) through comparisons to laboratory measurements and direct numerical simulations (DNS). LES predictions using the Leray-α, LANS-α, and Clark-α regularization-based SGS models are compared to the classic nondynamic Smagorinsky model. Regarding the regularization models, this work represents their first application to relatively high-Re decaying turbulence with comparison to the active-grid-generated decaying turbulence measurements of Kang et al. (J. Fluid Mech., 2003) at Reλ ≈ 720 and the Re = 3000 DNS of transition to turbulence in the TGV of Drikakis et al. (J. Turbulence, 2007). For DHT the nondynamic Smagorinsky model is in excellent agreement with measurements for the turbulent kinetic energy and energy spectra but higher-order moments show slight discrepancies. For TGV too, the energy decay rates of Smagorinsky agree reasonably well with DNS. Regarding the regularization models stable results are not obtained as compared to the Smagorinsky model at the same grid resolution for various values of α; and at higher grid resolutions, they are in worse agreement. However, with additional dissipation such as in mixed α-Smagorinsky models, results are acceptable and show only slight deviations from Smagorinsky. [ABSTRACT FROM AUTHOR]

The present paper focuses on a dynamic version of the variational multiscale model and investigates its performance in large eddy simulations (LES) of turbulent channel flow at Reynolds numbers (based on friction velocity) of 180, 395, and 590. The dynamic procedure was implemented for the variational multiscale model on the basis of the classical Smagorinsky model and the wall-adapting local eddy-viscosity (WALE) model. One base regular grid and three different distorted grids are employed in the evaluation. The dynamic multiscale models are compared with the direct numerical simulation (DNS) data and the conventional dynamic models. Next, the error components involved in LES are separated and the grid irregularity effect on the shortcoming of the subgrid scale models and the error of numerical approximation is assessed. A main outcome is that the dynamic multiscale models in comparison with the dynamic Smagorinsky and the dynamic WALE are less affected by grid distortion, leading to better behavior on the irregular grids. [ABSTRACT FROM AUTHOR]

The purpose of this article is to offer some insights, suggestions and recommendations on usage of current statistical models of turbulence. The issues discussed have arisen during several decades' worth of combined research experience and through direct support of numerous commercial computational fluid dynamics (CFD) customer projects covering a broad range of aerospace, automotive, marine and manufacturing applications. The discussions and ideas presented pertain to how existing algorithms and phenomenological models of turbulence can best be used with current computer hardware. [ABSTRACT FROM AUTHOR]

The characteristics of time-dependent swirling flows in a von Karman configuration are investigated numerically up to Reynolds number Re = 3000 (based on the angular velocity and the radius of the cylinder), and experimentally within turbulent regimes. Experimental results are analyzed together with the periodic and aperiodic flow obtained numerically. In the present configuration, the fluid is contained in a cylindrical cavity with aspect ratio (height to radius) Γ = 2 and the motion is driven by the exact counter rotation of the end walls while the sidewall is at rest. Spectral direct numerical simulations show that for this geometry the axisymmetric base flow becomes unstable to non-axisymmetric perturbations with azimuthal wavenumber m = 1 through a subcritical bifurcation, and the corresponding flow exhibits a pattern with one cat's eye in the axial-azimuthal planes. Increasing the Reynolds number the flow becomes unstable to non-axisymmetric steady perturbations with even azimuthal wavenumbers, and the corresponding flows exhibits a two cat's eyes pattern. The occurrence of cat's eye pattern in radial-azimuthal surfaces was observed in this and other aspect ratio cavities and is associated with vortices in 3D steady flows with characteristic azimuthal modes [Nore, C., Tuckerman, L.S., Daube, O. and Xin, S., The 1 : 2 mode interaction in exactly counter-rotating von Karman swirling flow, J. Fluid Mech., 2003, 477, 51-88; Lackey, T.C. and Sotiropoulos, F., Relationship between stirring rate and Reynolds number in the chaotically advected steady flow in a container with exactly counter-rotating lids, Phys. Fluids, 2006, 18, 1-14]. Time-dependent regimes are obtained numerically when the value of the Reynolds number is Re ≥ 1500. The time dependency is associated with a pulsation of the two vortices found in the steady regime. Experimental visualizations and measurements show that in turbulent regimes the flow also exhibits two vortices, but in this case they travel in the azimuthal direction with a frequency compatible with the frequency obtained in the numerical simulations at much lower Reynolds number. The azimuthal drift of these vortices is associated with the asymmetry of the mean azimuthal flow with respect to the equatorial plane. [ABSTRACT FROM AUTHOR]

Tracer transport in two-dimensional incompressible turbulent fluids is studied using the nested subensemble method, a semi-analytical statistical approach. Quasi-coherent vortical structures of trajectories are shown to appear when the stream function has slow time variation. In the presence of weak perturbation the structures are not destroyed and they determine anomalous transport regimes. [ABSTRACT FROM AUTHOR]

In the current study, the unsteady flow in a centrifugal fan is carried out using Computational Fluid Dynamics calculation based on the Scale Adaptive Simulation (SAS) approach to model the turbulence phenomenon. The SAS concept is based on the introduction of the von Karman length scale into the turbulence scale equation. The information provided by the von Karman length scale allows SAS models to dynamically adjust to resolved structures in an Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation, which results in a Large Eddy Simulation-like behaviour in unsteady regions of the flow field. At the same time, the model provides standard RANS capabilities in stable flow regions. The introduction of the von Karman length scale is based on the reformulation of Rottas's equation for the integral length scale. To validate the numerical results, the overall performances of the fan and the wall pressure fluctuations computed upon the volute casing surface are compared with the unsteady measured data. [ABSTRACT FROM AUTHOR]

Characteristics of the process of entrainment in plane mixing layers, and the changes with compressibility and heat release, were studied using temporal DNS with simultaneous fluid packet tracking. Convective Mach numbers of the simulations are 0.15, 0.7 and 1.1. The Reynolds number is quite high (between 11 000 and 37 000 based on layer width and velocity difference), and is above the mixing transition. The study agrees with recent findings in round jets: first, engulfed fluid volume and its growth rate are both very small compared with the volume of the turbulent region and its growth rate, respectively. Secondly, most often, the process occurs close to the turbulent-nonturbulent boundaries. A new finding is that both compressibility and heat release retard the entrainment process so that it takes an O(1) time for vorticity or scalar levels to grow even after growth has been initiated. This delay is manifested as the fall in mixing layer growth rates as compressibility and heat release levels increase. [ABSTRACT FROM AUTHOR]

This paper aims at determining the most influential inlet turbulence parameters on the downstream transitional mixing region. To this end, a stochastic method is developed to generate a divergence-free random velocity field with a prescribed energy distribution in physical and wave-number space. In addition, predetermined integral length scales can be established. Ten direct numerical simulations of a temporally evolving transitional turbulent mixing layer are examined in detail. Simulation results show a large disparity in mean and instantaneous turbulent quantities, mainly effected by the energy spectrum, the integral length scale and the divergence freeness of the initial field. [ABSTRACT FROM AUTHOR]

End-wall effects for high aspect ratio (AR) turbulent duct flows under moderate spanwise rotation are investigated using Reynolds-Averaged Navier-Stokes (RANS) calculations with a Reynolds stress turbulence closure model. It is shown that despite an important uniformisation of the mean streamwise flow compared to the non-rotating case, the channel flow solution (AR = ∞) is not recovered in practical high AR ducts used in experiments. The unavoidable end-wall generated secondary flow causes transverse advection which is capable of altering the mean velocity profile, even for AR as high as 22. In addition, for Re = 40,000 and Ro = 0.22, persistent longitudinal roll cells are found in the RANS solutions. The results suggest that their interaction with the secondary flow may challenge the prospect of formally reaching a steady, streamwise invariant regime in actual rotating duct experiments. [ABSTRACT FROM AUTHOR]

The fluid flows going through the Canada Deuterium Uranium (CANDU) moderator inlet diffuser assembly consist of a pipe flow, a curved pipe flow, and an impinging jet. For predicting the velocity profile at the diffuser outlet faces, a computational fluid dynamics ( CFD) analysis has been performed to simulate the internal flow in the diffuser assembly. For the validation of a CFD code, some experimental data were chosen for each flow, and various turbulence models were examined. The shear stress transport turbulence model was proven to be the most appropriate for a prediction of the impinging jets and to give better predictions for a curved pipe flow compared to the standard k-e turbulence model. As a result of the investigation, detailed velocity profiles and turbulent parameters at the real diffuser outlets were obtained, which can be applied as an inlet boundary condition for the CANDU moderator analysis. [ABSTRACT FROM AUTHOR]

A set of predictive models is formulated for computing suspended sediment concentration in and outside the surf zone under three different mechanisms: (1) suspension due to turbulence motion over sand ripples, (2) suspension from sheet flow layer, and (3) suspension due to turbulence motion under breaking waves Dimensional analysis and best-fit technique are the main methods for the formulation of reference concentration and diffusion coefficient and the model parameters are calibrated with the help of a large amount of published data. Time-averaged concentration profiles are derived from the steady diffusion equation. Published experimental data from 19 sources from 1977 to 1996 are better explained by the proposed formulae than the preceding formulae of Sleath [1982], Nielsen [1986 and 1988] and, Rattanapitikon and Shibayama [1994]. Due to scale effects, models are given for small-scale and large-scale cases separately. Predominance of each suspension mechanism is verified with a newly developed explicit formula. Finally, the applicability of the above sub models for irregular waves is also confirmed by the measured data sets. [ABSTRACT FROM AUTHOR]

This paper gives a brief summary of computational methods in heat transfer equipment, such as CFD (computational fluid dynamics), methods for thermal problems (computational heat transfer), and turbulence modeling for single-phase applications in the design, research and development of heat exchangers. Details of the finite volume method and their extension to arbitrary geometries are presented. An overall presentation of turbulence modeling is provided. Some of the most common commercially available computer codes are listed. Ways to apply CFD to heat exchangers are discussed. The paper presents results from some examples of application of CFD methods for real heat exchangers in order to demonstrate how such methods can be used in the research, development, and design of heat transfer equipment. Limitations and shortcomings as well as needs for further research are also highlighted. [ABSTRACT FROM AUTHOR]

Direct numerical simulations (DNS) are performed to investigate mixing in free round coaxial jets. A great attention has been put on the influence of upstream conditions upon the global flow structure and the mixing process. The mixing behavior is studied through the spatial and temporal development of the mixture fraction of the annular and the inner fluids, and examined by means of flow visualization and statistics. It is shown that the turbulent mixing process and the mixture fraction field in coaxial jets depend on the upstream conditions, even though a quasi self-similar state is reached. The mixing alterations are explained by the understanding of the flow dynamics modifications implied by the different upstream conditions. These alterations are mainly due to the intense generation of streamwise vortices, favored by high inlet velocity gradients and velocity ratios, as well as low ratios between the inner and the outer jet diameters. This is associated with a high quality of mixing, as far as global mixedness is concerned. It is also shown that the annular fluid reaches the inner fluid and mixes swiftly into it. Conversely, the latter remains confined. Additionally, spots of pure unmixed species are observed at the end of the computational domain, and shown to be due to the annular jet. [ABSTRACT FROM AUTHOR]

Conventional large-eddy simulation (LES) and monotone integrated LES (MILES) are tested in emulating the dynamics of transition to turbulence in the Taylor-Green vortex (TGV). A variety of subgrid scale (SGS) models and high-resolution numerical methods are implemented in the framework of both incompressible and compressible fluid flow equations. Comparisons of the evolution of characteristic TGV integral measures are made with previously reported and new direct numerical simulation (DNS) data. The computations demonstrate that the convective numerical diffusion effects in the MILES methods can consistently capture the physics of flow transition and turbulence decay without resorting to an explicit SGS model, while providing accurate prediction of established theoretical findings for the kinetic energy dissipation, energy spectra, enstrophy and kinetic energy decay. All approaches tested provided fairly robust computational frameworks. [ABSTRACT FROM AUTHOR]

A direct numerical simulation has been carried out for turbulent flow around a periodically deforming film. The film deformation is sinusoidal in the streamwise direction, whose nodes are fixed in space. The maximum amplitude of the deformation is limited within the linear sublayer. An unsteady generalized curvilinear coordinate and the collocated grid system are applied to the finite differencing of the NS equation. The computational results show that the turbulence intensities, the Reynolds shear stress and the wall-shear stress increase, in the case where the period of wall deformation is longer than the ratio of the wavelength of deformation to the fluid velocity in the buffer region. In contrast to this, these statistical quantities do not increase, in the case where the period of wall deformation is slightly shorter than the ratio. This difference is caused by the enhancement of mixing in the wall-normal direction and the modifications of the Q4 event due to the deformation. [ABSTRACT FROM AUTHOR]

This paper presents a manufactured solution (MS), resembling a two-dimensional, steady, wall-bounded, incompressible, turbulent flow for RANS codes verification. The specified flow field satisfies mass conservation, but requires additional source terms in the momentum equations. To also allow verification of the correct implementation of the turbulence models transport equations, the proposed MS exhibits most features of a true near-wall turbulent flow. The model is suited for testing six eddy-viscosity turbulence models: the one-equation models of Spalart and Allmaras and Menter; the standard two-equation k-ε model and the low-Reynolds version proposed by Chien; the TNT and BSL versions of the k-ω model. [ABSTRACT FROM AUTHOR]

We demonstrate that the spacetime statistics of the birth of turbulent spots in boundary layers can be reconstructed qualitatively from the average behaviour of macroscopic measures in the transition zone. The conclusion in [1] that there exists a connection between the patterns in laminar instability and the birth of turbulent spots is strengthened. We examine why the relationship between instability and transition to turbulence is manifest in some cases and appears to be totally absent in others. Novel cellular automaton-type simulations of the transition zone are conducted, and the pattern of spot birth is obtained from secondary instability analysis. The validity of the hypothesis of concentrated breakdown, according to which most turbulent spots originate at a particular streamwise location, is assessed. The predictions made lend themselves to straightforward experimental verification. [ABSTRACT FROM AUTHOR]

The first direct numerical simulation that is sufficiently large to study the self-similar behaviour of a turbulent wall jet is performed. The investigation is an extension of the simulation performed by Levin et al. (2005, A study of the Blasius wall jet. Journal of Fluid Mechanics, 539, 313–347). The same numerical method is used, but a significantly larger computational domain enables us to follow the development of the flow throughout the transition into its early turbulent evolution. Two-dimensional waves and streamwise elongated streaks, matched to measured disturbances, are introduced in the flow to trigger a natural transition mechanism. The Reynolds number is 3090 based on the inlet velocity and the nozzle height. The simulation provides detailed visualisations of the flow structures and statistics of mean flow and turbulent stresses. A weak subharmonic behaviour in the transition region is revealed by animations of the flow. The averaged data are presented in both inner and outer scalings in order to identify self-similar behaviour. Despite the low Reynolds number and the short computational domain, the turbulent flow exhibits a reasonable self-similar behaviour, which is most pronounced with inner scaling in the near-wall region. [ABSTRACT FROM AUTHOR]

This paper introduces a two-equation turbulence model sensitized to deviations from simple shear flows. The closure is topography-parameter-free and is based on solving transport equations for the turbulence kinetic energy (k) and the turbulence length-scale (ℓ). Brief model derivation details are given and test cases are presented to compare the model's performance to other closures and to experimental data. The flow examples demonstrate the advantage of the k-ℓ model in non-simple shear flows. [ABSTRACT FROM AUTHOR]

We discuss the effect of large-scale anisotropy of the shear type on the small-scale structure of turbulence. Our analysis is based on numerical solutions of the Lagrangian tetrad model of Chertkov M., Pumir A. and Shraiman B.I. (1999, Physics of Fluids , 11, 2394) adapted to the model case with large-scale anisotropy. The model, formulated in terms of a set of stochastic differential equations for the coarse-grained velocity gradient and tensor of inertia of a typical shape, naturally connects Lagrangian and Eulerian parameterizations of turbulence. We use diagnostics of Chertkov al. (1999) which allows us to analyse and interpret different correlation functions at the resolved scale in terms of the flow geometry. Our main conclusion, concerning the issue of anisotropy, is that even though overall the local isotropy is restored with the scale decrease, the particular pace of the isotropy restoration depends very much on the object analysed. We found that the vorticity-dominated objects, such as enstrophy, tend to restore the isotropy much faster than their strain-dominated counterparts, e.g. energy flux and strain variance. [ABSTRACT FROM AUTHOR]

We describe how outer flow turbulence phenomena depend on the interaction with the wall. We quantify the influence of highly rough surfaces on turbulence statistics, eigenmodes and spectra of eigenvalues in the outer part of the wall shear layer of an isothermal turbulent flow. Surface geometries with different amplitude-to-wavelength ratios can be regarded as surfaces of defined wall roughness. The wavelength, ?, and the wave amplitude, 2 a , of a water channel are varied by changing the bottom wall in the test section. We consider three sinusoidal wall profiles defined by the amplitude-to-wavelength ratio a = 2 a /? = 0.2 (? = 30 mm), a = 0.2 (? = 15 mm), and a = 0.1 (? = 30 mm). Digital particle image velocimetry is performed to examine the spatial variation of the streamwise, spanwise and wall-normal velocity components. Statistical quantities that are determined are the Reynolds stresses, turbulent kinetic energy, and turbulent energy production. Measurements are performed at a Reynolds number of 11200, defined by the half channel height and the bulk velocity. A characteristic length scale in the most dominant eigenmodes can be found that agrees for all wall profiles and is independent of the amplitude-to-wavelength ratio. The turbulence quantities compared at defined streamwise coordinates in wall-normal direction are independent of the wavy wall profile in the outer region when scaled with the friction velocity, u * w . [ABSTRACT FROM AUTHOR]

This study presents an initial scoping analysis of the application of computational fluid dynamics (CFD) to modeling the flow in an annular centrifugal contactor. The unsteady, turbulent nature of this multi‐phase flow presents significant challenges to quantitative CFD modeling. Existing methods for confronting these obstacles are considered and initial results of the steady‐state flow of a single liquid phase in the annular mixing zone are presented. The flow of particulates and the effects of changes in geometric and operational parameters were also evaluated. Even with simplifying assumptions qualitatively accurate results could be obtained using widely available CFD models. [ABSTRACT FROM AUTHOR]

Experimental and numerical studies on the flow behavior in a prototypical configuration of spallation targets have been performed, with the main purpose being to support the target design and to assess the computational fluid dynamics application. The effects of flow direction, presence of a perforated plate, and turbulence models on the flow behavior are investigated. Good agreement is obtained between the experimental data and the numerical results, except for the case of downward flow without a perforated plate, where large flow recirculation occurs beneath the window. For the numerical simulation of the flow behavior in the complex target geometries investigated, the shear stress transport model does not show advantages over the k-ε model. [ABSTRACT FROM AUTHOR]

Combining Higbie's penetration theory and Kolmogoroff's theory of isotropic turbulence, a small-scale isotropic mass transfer model for the local liquid side mass transfer coefficients in a gas-liquid two-phase airlift loop reactor and a self-aspirated reversed flow jet loop reactor has been developed. Good agreement with the experimental data suggests a general applicability of the proposed model in these reactors. [ABSTRACT FROM AUTHOR]

The flow and thermal fields in a rectangular-sectioned 90° bend with a cross-section aspect ratio of 6 were investigated using four turbulence models. All models managed to reproduce the general flow and thermal patterns. Chen's high-Re k –ε model and Suga's cubic low-Re k –ε model performed well. The V2F k –ε model was found to be the least diffusive model and delivered good results. The RSM-GGDH model showed convergence difficulties and gave poor results. It was found that the boundary-layer thickness and the flow upstream of the bend are crucial for the character of the secondary flow, velocity profile, turbulence level, and heat transfer in the bend. [ABSTRACT FROM AUTHOR]

Many papers have reported that the standard k - ε model fails to accurately predict the mean velocity profile of turbulent axisymmetric jets (Thies and Tam, Computation of turbulent axisymmetric and nonaxisymmetric jet flows using the K - ε model, AIAA J. , 1996, 34(2), 309–316; Pope, Turbulent Flows , 2002 (Cambridge University press: Cambridge). As the jet velocity increases, the deviation of the model with respect to the experimental measurements also increases. This work is aimed at the development of a modified k - ε model that can be used to predict the mean properties of an axisymmetric jet as it (i) flows as a free jet, (ii) propagates between walls, and (iii) impinges on a solid object. Three additional terms are proposed to improve the standard k - ε model predictions. They are Durbin realizable, Heinz turbulence production and Sarkar compressibility correction terms. The performance of the modified model in predicting the velocity and the impact pressure profiles of a free jet with an exit Mach number range of 0.6–2.8 has been confirmed by its close agreement with the experimental measurements. In addition, the study suggests that the model is also capable of predicting the impact pressure of a supersonic jet propagating between smooth walls and impinging on the front edge of the wall in various degrees of intensity. [ABSTRACT FROM AUTHOR]

This paper provides an overview of recent advances in the theoretical modeling and numerical simulation of cryogenic fluid injection and mixing in transcritical and supercritical environments. The basis of the analysis is a general theoretical and numerical framework that accommodates full conservation laws and real-fluid thermodynamic and transport phenomena. All of the thermophysical properties are determined directly from fundamental thermodynamics theories, along with the use of corresponding-state principles. Turbulence closure is achieved using a large-eddy-simulation technique, in which large-scale motions are calculated explicitly and the effects of unresolved small-scale turbulence are modeled either analytically or empirically. The analysis has been applied to study: (1) fluid jet dynamics, (2) swirl injection of liquid oxygen through a simplex swirl injector, and (3) shear co-axial injection and mixing of liquid oxygen and methane. Various effects, including density stratification, shear-layer instability, volume dilatation, and property variations, dictating the evolution of cryogenic jets and mixing layers, are identified and analyzed in depth. The jet dynamics are found to be largely determined by the local thermodynamic state through its influence on the thermophysical properties of the fluid. The impact of injector configuration and operating conditions on the swirl injector behavior are also highlighted. These results not only shed light on the subject problems, but also provide a quantitative basis for identifying the design parameters and flow variables that exert the strongest influence on the underlying processes. [ABSTRACT FROM AUTHOR]

This paper presents a detailed characterization of the local and global errors associated with the in situ adaptive tabulation (ISAT) algorithm, which is used in conjunction with a transported PDF method. Calculations of a non-premixed turbulent methane/air piloted jet flame (Sandia flame D) using a skeletal chemical mechanism were performed using ISAT coupled with the computational fluid dynamics (CFD) code FLUENT. The three strategies implemented in ISAT for the growing of the ellipsoids of accuracy (EOAs) are discussed, and the cumulative distribution function (CDF) of the local error is presented for each of the three growing strategies. Computations are also performed to characterize the global error in the ISAT/PDF calculation. The computations used to characterize the global error were performed in parallel to achieve substantial savings in computational time. In general the local error is well controlled, but there is a small probability of relatively large errors. Results from the investigation suggest that large retrieve errors are due to the region of accuracy (ROA) being non-convex, where the ROA is the connected region for which the error does not exceed the error tolerance, ε tol . The global error in ISAT is found to be small compared to statistical error for ε tol ≤ 10 -4 , and is found to vary linearly with ε tol . [ABSTRACT FROM AUTHOR]

The propagation of a passive admixture from a local instantaneous source in a turbulent mixing zone in a stable stratified fluid (two-dimensional problem) is simulated numerically. The location of the source does not coincide with the center of the turbulent zone. The calculation results indicate that the average admixture concentration distribution depends significantly on the initial data. The location of the maximum concentration in homogeneous and linearly stratified fluid is fairly slowly displaced toward the center of the zone. Calculations in a pycnocline show that situations are possible when the propagation of a passive admixture is largely determined by a convective flow generated by the turbulent mixing zone. [ABSTRACT FROM AUTHOR]

A comparison of near-wall treatment methods using different turbulence models for flow over a backward-facing step is presented. A Reynolds number ( Re ) of about 38,000 ( U 8 = 44.2 m/s), based on the step height and the mean stream velocity, was considered. An appropriate near-wall treatment method is critical to the choice of turbulence model used to predict wall-bounded flow. Predictions were obtained by applying standard wall functions, non-equilibrium wall functions and a two-layer model with six different turbulence models. These results were compared with data by Driver and Seegmiller (“Backward-facing step with inclined opposite wall—experiments by driver and seegmiller”, 1985a, http://cfd.me.umist.ac.uk/ercoftac [2003, Jan 31]). Non-equilibrium wall functions with modified k - e models predicted the closest reattachment length. However, the two-layer model gave results more representative of the entire flow pattern. The predictions show that a proper combination of turbulence models and near-wall treatment methods give reliable results. [ABSTRACT FROM AUTHOR]

We report results from a high-resolution numerical study of fluid particles transported by a fully developed turbulent flow at Rλ = 280. Single-particle trajectories were followed for a time range spanning more than three decades, from less than a tenth of the Kolmogorov timescale up to one large-eddy turnover time. We present results concerning acceleration statistics and the statistics of trapping by vortex filaments conditioned to the local values of vorticity and enstrophy. We found that a suitable observable, a ⊥ = a × v, connected to the centripetal acceleration, is strongly sensitive to the trapping of particles into vortex filaments. In particular, we found that its sign may persist up to 10–15 τη, while the typical oscillation of the longitudinal acceleration happens on a time lag of a few τη. [ABSTRACT FROM AUTHOR]