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Studies on flow-induced vibrations in large tube bundles are usually focused solely on frequency analysis, without considering the flow patterns which are responsible for the fluid forces. Furthermore, investigations which involve variations in the spacing ratios do not separate transversal and longitudinal proximity effects. The purpose of this article is to separately analyze the influence of the transversal ( T / D ) and longitudinal ( L / D ) spacing ratios of a confined in-line cylinder array with five rows on the flow characteristics and to identify flow patterns. The laser Doppler anemometry technique was employed to acquire the mean velocity and its fluctuations in the transversal and longitudinal directions between the cylinder rows. Strouhal numbers and regimes reported in the literature were identified in the experiments. The same regime did not always persist along all cylinder rows for a given spacing ratio, as a result of the combined longitudinal and transversal proximity effects and also of the generation of turbulence by the array. For the smallest T / D ratio, a quasi-steady behavior associated with the biased flow pattern was noted in the experimental set-up and flip-flopping was observed in one case. Additionally, the flow characteristics in these arrays diverged from tube bundle classifications described in the literature. The behavior of the fluid forces and susceptibility to vibrations in the array were predicted based on the turbulence intensity of the incident flow of the cylinders. The results reinforced the need to extend flow pattern investigations to arrays with more cylinder rows and to consider both transversal and longitudinal proximity effects, when studying flow-induced vibrations. [ABSTRACT FROM AUTHOR]

Three-dimensional direct numerical simulations of a Bingham fluid flowing past a confined circular cylinder have been used in order to investigate viscoplastic effects in the wake-transition regime. The case of a cylinder confined in a plane channel with a fixed blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2 has been studied, for values of the Bingham number and Reynolds numbers (based on the cylinder diameter and bulk flow velocity) in the range 0 < Bn ≤ 5 and 150 ⩽ Re ⩽ 600 , respectively. The critical Reynolds number for the onset of three-dimensional flow regime has been shown to increase linearly with Bingham number, at least in the range of parameters considered. Two distinct modes of three-dimensional instability (modes A and B) have been identified in the flow, and the influence of viscoplastic effects on the evolution of these instabilities have been described. Significant deviations in the structure of the far wake from the Newtonian case have been observed and associated with a disruption of underlying processes that are known to operate in the Newtonian wake. Despite these structural differences, it has been shown that the evolution of the Strouhal number with Reynolds number in the viscoplastic fluid exhibits two discontinuous changes that are associated with the onset of the different instabilities in the flow, a behavior that mirrors the behavior in Newtonian fluids. [ABSTRACT FROM AUTHOR]

Highlights: [•] Flow laminarization in corner region of non-circular ducts is observed. [•] Coexistence of laminar and turbulent flow leads to a reduced friction factor. [•] Analytical considerations for statistically axisymmetric turbulence are used. [•] These considerations show mechanisms responsible for flow laminarization. [•] The DNS results support these findings. [Copyright &y& Elsevier]

Abstract: There are very few established correlations to predict the interfacial area concentration beyond the bubbly flow regime in cap-slug and churn-turbulent flow regimes. Present study shows a systematic approach to estimate the interfacial area concentration in bubbly, cap-slug and churn-turbulent flow regimes. Ishii and Mishima’s (1980) formulation and the two group approach for categorizing bubbles (Group-1: spherical or distorted bubble, Group-2: cap bubble) are used to estimate the interfacial area concentration. The key parameters in this framework are the estimation of Group-1 bubble size and the amount of void in the liquid slug, which is a function of Group-1 void fraction. Hibiki and Ishii’s (2002) correlation is utilized to predict the size of the Group-1 bubbles. A correlation is developed to estimate the Group-1 void fraction. The developed model for the estimation of interfacial area concentration is compared with the three existing datasets. These are data for air–water flow taken in annular geometry and round tube and also for air–NaOH solution taken in round tube. The estimation accuracies for these data sets are ±36.4%, ±26.5% and ±37.4%, respectively. These datasets cover a wide range of flow regimes and different physical properties. [Copyright &y& Elsevier]

Abstract: The success of Large-Eddy Simulations (LES) of wall-bounded turbulence depends strongly on an accurate representation of the flow near the boundaries. Since in implicit LES the truncation error of the numerical discretization itself functions as SGS model, the order of accuracy of the discretization should be maintained near the boundary. In this paper, we analyze the performance of implicit LES for predicting turbulent flows along complex geometries. For the representation of wall-boundaries two different approaches are compared: (A) the case of body-fitted grids, and (B) a novel second-order accurate Conservative Immersed Interface Method (CIIM) for representing boundaries of arbitrary shape on Cartesian grids. The need for mapping domain boundaries on the grid raises the question whether a comparable accuracy with body-fitted grids can be achieved. In both (A) and (B) we employ the Adaptive Local Deconvolution Method (ALDM) for implicit LES. The assessment is carried out on the example of the flow over a circular cylinder at Re =3900. Two simulations with comparable numerical resolution are performed and compared to experimental reference data. Both methods result in similar accuracy for turbulent statistics and mean integral quantities. This demonstrates that ALDM on Cartesian grids is a feasible and efficient approach to simulate turbulent flows on complex domains. [Copyright &y& Elsevier]

Abstract: The study of mixing in coastal area is a key issue in environmental fluid mechanics. In most applications it requires a three-dimensional approach in order to take into account phenomena related to buoyancy effects and to complex geometry. In a coastal shallow-water environment the wind-driven upper boundary layer may encroach upon the bottom boundary layer creating a complex turbulent field, making difficult proper modelization with simple RANS-like methodologies. A LES methodology is presented aimed at studying turbulent mixing in coastal areas under general forcing and geometric configuration. The strong grid anisotropy, between the horizontal length scale of kilometers and the vertical one of about 10–100m, is handled expressing the subgrid stress by means of directional eddy viscosities. Geometry is modeled using a combination of curvilinear grid and the Immersed Boundary Method. Proper validation of the model has been carried out and the superiority with respect to the standard one-coefficient version clearly shown. The model has been applied to the investigation of mixing within the Muggia bay under wind forcing. Overall the model has shown to be able to reproduce the peculiarities of coastal dynamics. The model proved to be successful for the study of shallow-water in-shore fundamental processes. [Copyright &y& Elsevier]

Abstract: A coupling methodology between an upstream Reynolds Averaged Navier–Stokes (RANS) simulation and a Large Eddy Simulation (LES) further downstream is presented. The focus of this work is on the RANS-to-LES interface inside an attached turbulent boundary layer, where an unsteady LES content has to be explicitly generated from a steady RANS solution. The performance of the Synthetic-Eddy Method (SEM), which generates realistic synthetic eddies at the inflow of the LES, is investigated on a wide variety of turbulent flows, from simple channel and square duct flows to the flow over an airfoil trailing edge. The SEM is compared to other existing methods of generation of synthetic turbulence for LES, and is shown to reduce substantially the distance required to develop realistic turbulence downstream of the inlet. [Copyright &y& Elsevier]

Abstract: The present work focuses on the application of two near-wall, wall-normal free, second-moment closure (SMC) models in the RANS (Reynolds-Averaged Navier–Stokes) framework in computing the (compressible) transonic flow past aircraft configurations and their elements (airfoils and wings). The flow geometries considered in this work include the transonic RAE 2822 profiles (cases 9 and 10), the ONERA M6 wing and the DLR-ALVAST wing-body configuration. The model results are analysed and discussed in conjunction with an available experimental database and the results of two widely used eddy-viscosity-based models, the one-equation Spalart–Allmaras model [Spalart, P.R., Allmaras, S.R., 1994. A one-equation turbulence model for aerodynamic flows. La Recherche Aérospatiale 1, 5–21] and the two-equation k–ω model of Wilcox [Wilcox, D.C., 1988. Reassessment of the scale-determining equation for advanced turbulence models. AIAA Journal 26, 1299–1310]. The SMC predictions show encouraging results with respect to the shock position, shock-affected flow structure and the strength of the wing-tip vortex. [Copyright &y& Elsevier]

Abstract: Strong interaction among tip-leakage vortex, mainstream, passage vortex and turbulence in and behind the blade tip and casing in compressors is one of the major causes of energy losses, affecting the overall gas turbine performance. A number of experiments and recent large-eddy simulations of simplified generic tip-clearance configurations have substantially advanced the understanding of the complex vortical and turbulence structures and energy conversion in and behind a very narrow clearance space. However, the prediction of phenomena in real turbomachinery still relies on the one-point (Reynolds-averaged Navier–Stokes, RANS) Computational Fluid Dynamics which is still burdened by uncertainties related primarily to inadequate turbulence modelling, and also to insufficient accuracy of the numerical algorithms. In this paper we address both these issues by testing an advanced turbulence model and a numerical method in the computations of a tip-clearance flow in a three-dimensional linear compressor cascade. For closing the mean momentum equations we adopted a Second Moment Turbulence Closure (SMC) incorporating low-Re-number and wall proximity effects, which has earlier proved to reproduce accurately a series of generic two- and three-dimensional anisotropic and non-equilibrium flows relevant to turbomachinery. In order to diminish numerical uncertainties, the computations are performed using Petrov-Galerkin stabilized finite element code featuring equal order Q1–Q1 pairs. The solution scheme is based on an in-house parallel Overlapping Domain Decomposition approach featuring a FGMRes linear solver. The results, compared with experiments and the conventional linear low-Re-number eddy viscosity model demonstrate that the SMC here adopted is capable to reproduce all the phenomenological features related to tip-leakage flows, especially the evolution of the strong three-dimensionality and turbulence anisotropy, and can be considered a prospective model for predicting real turbomachinery flows. [Copyright &y& Elsevier]

Abstract: The performance of ultraviolet (UV) reactors used for water treatment is greatly influenced by the reactor hydrodynamics due to the non-homogeneity of the radiation field. Reliable modeling of the reactor flow structures is therefore crucial for the design process. In this study, the turbulent flow through two characteristic annular UV-reactor configurations, with inlets concentric (L-shape) and normal (U-shape) to the reactor axis, was investigated through computational fluid dynamic (CFD). The modeling results were evaluated with the velocity profiles from particle image velocimetry (PIV) experiments. The influence of mesh structure and density, as well as three turbulence models: Standard κ–ϵ, Realizable κ–ϵ, and Reynolds stress model (RSM), on the simulation results were evaluated. Mesh-independent solutions were achieved at mean cell volumes of 5×10−9 m3. The Realizable κ–ϵ displayed the best overall match to the experimental PIV measurements. In general, the CFD models showed a close agreement with the experimental data for the majority of the reactor domain and captured the influences of reactor configuration and internal reactor structures on the flow distribution. The validated CFD hydrodynamic models could be integrated with kinetic and radiation distribution models for UV-reactor performance simulation. [Copyright &y& Elsevier]

Abstract: Laser-Doppler velocimeter measurements of a wing/body junction flow field made within a plane to the side of the wing/wall junction and perpendicular both to a 3:2 elliptical nose—NACA 0020 tail wing, and a flat wall are presented. Reynolds number of the approach boundary layer was, Re θ =5940, and free-stream air velocity was, U ref =27.5m/s. A large vortical structure residing in the outer region redirects the low-turbulence free-stream flow to the vicinity of the wing/wall junction, resulting in thin boundary layers with velocity magnitudes higher than free-stream flow. Lateral pressure gradients result in a three-dimensional separation on the uplifting side of the vortex. Additionally, a high vorticity vortical structure with opposite sense to the outer-layer vortex forms beneath the outer-layer vortex. Normal and shear stresses increase to attain values an order of magnitude larger compared to values measured in a three-dimensional boundary layer just outside the junction vortex. Bimodal histograms of the w fluctuating velocity occur under the outer-layer vortex near the wall due to the time-dependent nature of the horseshoe vortex. In such a flow the shear-stress angle (SSA) highly lags the flow-gradient angle (FGA), and the turbulence diffusion is highly altered due to presence of vortical structures. [Copyright &y& Elsevier]

Abstract: Hybrid RANS/LES simulations, in which the attached boundary layers are computed using the Reynolds-averaged Navier–Stokes (RANS) equations, and the non-equilibrium regions of the flow by large-eddy simulation (LES), have received considerable attention over recent years. One issue that may affect (sometimes significantly) the accuracy of the results of hybrid methods is the generation from a smooth RANS field (in which the Reynolds stress is due entirely to the model) of turbulent eddies capable of supporting the Reynolds stresses in the LES region. In this paper, we perform hybrid RANS/LES of turbulent boundary-layer flows in zero, favorable and adverse pressure-gradients. The RANS equations are solved in regions of the flow that are at or near equilibrium, while LES is used elsewhere. Synthetic turbulence and controlled forcing are used to generate turbulent structures in the LES region. When only synthetic turbulence is used at the inflow of the LES region, some distance is required before the proper phase relationships between the modes introduced at the inlet are established; the turbulent statistics are unrealistic in this region, which can extend for 10–20 boundary layer thicknesses. When the forcing is used, this development region is significantly shortened, and realistic statistics are obtained 1–2 boundary-layer thicknesses downstream of the interface. The accuracy of the method, however, was found to depend critically on the accuracy of the RANS solution. [Copyright &y& Elsevier]

Abstract: The present study deals with recent numerical results from on-going research conducted at ONERA/DMAE regarding the prediction of transonic flows, for which shock wave/boundary layer interaction is important. When this interaction is strong enough (M ⩾1.3), shock induced oscillations (SIO) appear at the suction side of the airfoil and lead to the formation of unsteady separated areas. The main issue is then to perform unsteady computations applying appropriate turbulence modelling and relevant boundary conditions with respect to the test case. Computations were performed with the ONERA elsA software and the URANS-type approach, closure relationships being achieved from transport-equation models. Applications are provided for the OAT15A airfoil data base, well documented for unsteady CFD validation (mean and r.m.s. pressure, phase-averaged LDA data, …). In this paper, the capabilities of turbulence models are evaluated with two 2D URANS strategies, under free-stream or confined conditions. The latter takes into account the adaptive upper and lower wind-tunnel walls. A complete 3D URANS simulation was then performed to demonstrate the real impact of all lateral wind-tunnel walls on such a flow. [Copyright &y& Elsevier]

Abstract: The generation of inflow data for spatially developing turbulent flows is one of the challenges that must be addressed prior to the application of LES to industrial flows and complex geometries. A new method of generation of synthetic turbulence, suitable for complex geometries and unstructured meshes, is presented herein. The method is based on the classical view of turbulence as a superposition of coherent structures. It is able to reproduce prescribed first and second order one point statistics, characteristic length and time scales, and the shape of coherent structures. The ability of the method to produce realistic inflow conditions in the test cases of a spatially decaying homogeneous isotropic turbulence and of a fully developed turbulent channel flow is presented. The method is systematically compared to other methods of generation of inflow conditions (precursor simulation, spectral methods and algebraic methods). [Copyright &y& Elsevier]

Abstract: The complex transition flow physics associated with the merging of turbulent spots in a Mach 2 boundary-layer has been studied using direct numerical simulation. Dynamics of an isolated turbulent spot, merging of laterally displaced spots, and merging of two spots in tandem are considered. The coherent structures associated with the wingtip region of the spot are found to play a major role in destabilising the surrounding laminar fluid. In the merging of laterally displaced spots a strong velocity defect, resulting in unstable inflectional velocity profiles, is observed in the interaction zone. These local inflectional instabilities within the interaction region trigger new large scale coherent structures. During the inline merging, the calmed region behind the tail of the downstream spot is found to suppress the growth of the upstream spot. The upstream spot is ultimately engulfed by the downstream spot. [Copyright &y& Elsevier]

Abstract: Measurements of the mean concentration of source fluid and mean velocity fields were obtained for the first time in the self-preserving region of steady round buoyant turbulent plumes in uniform crossflows using Planar-Laser-Induced-Fluorescence (PLIF) and Particle-Image-Velocimetry (PIV), respectively. The experiments involved salt water sources injected into water/ethanol crossflows within a water channel. Matching the index of refraction of the source and ambient fluids was required in order to avoid image distortion and laser intensity nonuniformities. Further experimental methods and procedures are explained in detail. The self-preserving structure properties of the flow were correlated successfully based on the scaling analysis of [Fischer, H.B., List, E.J., Koh, R.C., Imberger, J., Brooks, N.H., 1979. Mixing in Inland and Coastal Waters, Academic Press, New York, pp. 315–389]. The resulting self-preserving structure consisted of two counter-rotating vortices having their axes nearly aligned with the crossflow direction that move away from the source in the streamwise (vertical) direction due to the action of buoyancy. This alignment, was a strong function of the source/crossflow velocity ratio, u 0/v ∞. Finally, the counter-rotating vortex system was responsible for substantial increases in the rate of mixing of the source fluid with the ambient fluid compared to axisymmetric round buoyant turbulent plumes in still environments, e.g., transverse dimensions in the presence of the self-preserving counter-rotating vortex system were 2–3 times larger than the transverse dimensions of self-preserving axisymmetric plumes at similar streamwise distances from the source. [Copyright &y& Elsevier]

Abstract: Large eddy simulation (LES) has been applied to turbulent thermal fields in a channel having one wavy wall for Prandtl number=0.7. Wall wave amplitude is changed in three steps. Increasing the wall wave amplitude, a flow separation bubble comes to appear and a separated turbulent shear layer develops above the separation bubble. Additionally, in the up-slope region of the bottom wavy wall, near-wall streamwise vortices are generated with larger population. These two turbulence features, i.e., the separated shear layer and the near-wall streamwise vortex, play an important role for the heat and momentum transfer near the wavy wall. So, the characteristics of the separated shear layer and the near-wall streamwise vortices are discussed in relation to the turbulent heat transfer. For this purpose, vortical flow structures aligned toward the streamwise direction and turbulent thermal fields around the vortices were captured and the statistical quantities relating to the turbulent thermal fields were scrutinized. Especially, attention is paid to the effect of the near-wall streamwise vortices on the turbulent heat transfer and the conditionally averaged patterns of turbulent thermal fields around the streamwise vortices were established. The results obtained indicate that the near-wall streamwise vortices produce the strong events of Q T4 and Q T6. Here, Q T4 and Q T6 are cold sweep-like motion and hot ejection-like motion, respectively. Especially, Q T4 event makes the gradient of fluid temperature near the wall steeper in the up-slope region and consequently enhances the heat transfer at the bottom wavy wall. Furthermore, with increasing the wall wave amplitude, the near-wall streamwise vortices were strengthened and as a result the wavy wall heat transfer is more noticeably enhanced. In practical view point, if pumping power is kept the same, better heat transfer performance can be achieved with increasing wall wave amplitude. [Copyright &y& Elsevier]