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The effect of rotation-curvature correction and inviscid spatial discretization scheme on the aerodynamic performance and flow characteristics of Darrieus H-type vertical axis wind turbine (VAWT) are investigated based on an in-house solver. This solver is developed on an in-house platform HRAPIF based on the finite volume method (FVM) with the elemental velocity vector transformation (EVVT) approach. The present solver adopts the density-based method with a low Mach preconditioning technique. The turbulence models are the Spalart-Allmaras (SA) model and the k-ω shear stress transport (SST) model. The inviscid spatial discretization schemes are the third-order monotone upstream-centered schemes for conservation laws (MUSCL) scheme and the fifth-order modified weighted essentially non-oscillatory (WENO-Z) scheme. The power coefficient, instantaneous torque of blades, blade wake, and turbine wake are compared and analyzed at different tip speed ratios. The extensive analysis reveals that the density-based method can be applied in VAWT numerical simulation; the SST models perform better than the SA models in power coefficient prediction; the rotation-curvature correction is not necessary and the third-order MUSCL is enough for power coefficient prediction, the high-order WENO-Z scheme can capture more flow field details, the rotation-curvature correction and high-order WENO-Z scheme reduce the length of the velocity deficit region in the turbine wake. [ABSTRACT FROM AUTHOR]

The task of studying the process of icing is important due to the operation of large ships and fishing vessels in the seas of the Arctic Ocean. In this paper the ice accretion simulation for fishing vessel has been done at different Reynolds and Froude numbers. A hybrid approach was used for modeling: first, the approach based on Reynolds-averaged Navier-Stokes (RANS) model and volume of fluid (VOF) method with interDyMFoam solver was used for ship modelling in waves, second, RANS model and Euler-Lagrangian approach with iceFoam solver for ice accretion process modelling for ship with hull was applied. The k-ω SST turbulence model with wall functions was used for the RANS model. With the help of the interDyMFoam solver, the position of water jet and liquid droplets in space were determined during the movement of the ship and taking into account the waves in the sea. With the help of iceFoam solver, the process of ice accretion on the surface of the vessel was modeled. The calculations were run for different number Reynolds, Froude. The analysis of mesh convergency was done. The distribution of ice thickness and water film on the ship's surface is obtained. The simulations were performed on a high-performance cluster in parallel mode. [ABSTRACT FROM AUTHOR]

Rotating separation flow (RSF) in hydraulic machinery is characterized by the large flow separations and complex vortical structures induced by the effects of strong rotation, large curvature and multiple wall surfaces, and conducting efficient engineering computation and putting forward effective control strategy for the RSF are important topics in the inner flow theory. To meet these engineering requirements, the studies on computational method and control strategy of the RSF are conducted in this paper. In terms of the computational method, the time-scale-driven (TSD) hybrid unsteady Reynolds-averaged Navier-Stokes / large eddy simulation (URANS/LES) modelling strategy is clarified, and an adaptive TSD hybrid model is established based on the RSF characteristics in hydraulic machinery, thereby avoiding the problem of non-monotonic grid convergence and improving the robustness. Besides, a novel vortex-feature-driven idea suitable for the RSF is further developed inspired by it. In terms of the control strategy, the secondary flow generation mechanism in a rotor domain is revealed, and the relationship between natural secondary flows and blade loading distributions is grasped. On the basis of it, an active control strategy with general significance is proposed, and a general alternate loading technique (GALT) is established. Both aspects can provide generalized paradigms with expandable potential, which are of benefit to the efficient computation and effective control of the RSF in hydraulic machinery. [ABSTRACT FROM AUTHOR]

A novel Omega (Ω) -driven dynamic partially-averaged Navier-Stokes (PANS) model is proposed in this paper. The ratio of the modeled-to-total turbulent kinetic energies fk is dynamically adjusted by the rigid vorticity ratio (the ratio of the rigid vorticity to the total vorticity), the key parameter of the Ω vortex identification method. Three classical flow cases with rotation and curvature are used to test the model. The results show that the turbulent viscosity is effectively adjusted by the new dynamic fk and the LES-like mode is activated, which can help the revelation of more turbulence information and improve the prediction accuracy. The new PANS model does not contain any explicit dependency on the grid size and enjoys good adaptability to the flow fields, and can be used for efficient engineering computations of the turbulent flows in the hydraulic machinery. [ABSTRACT FROM AUTHOR]

This paper proposed an optimization design for a ride control system (RCS) for fast catamaran. The ship vertical motions are predicted by 2.5-D theory. The hydrodynamic characteristics of T-foil and trim tab are given, and the dynamic stability of the system is discussed. On this basis, the ride control system optimization model in regular and irregular waves is established for a specific catamaran. The optimal solution is obtained by using non-dominated sorting genetic algorithm-II (NSGA-II). [ABSTRACT FROM AUTHOR]

Recently, the methodology of deep learning is used to improve the calculation accuracy of the Reynolds-averaged Navier-Stokes (RANS) model. In this paper, a neural network is designed to predict the Reynolds stress of a channel flow of different Reynolds numbers. The rationality and the high efficiency of the neural network is validated by comparing with the results of the direct numerical simulation (DNS), the large eddy simulation (LES), and the deep neural network (DNN) of other studies. To further enhance the prediction accuracy, three methods are developed by using several algorithms and simplified models in the neural network. In the method 1, the regularization is introduced and it is found that the oscillation and the overfitting of the results are effectively prevented. In the method 2, y+ is embedded in the input variable while the combination of the invariants is simplified in the method 3. From the predicted results, it can be seen that by using the first two methods, the errors are reduced. Moreover, the method 3 shows considerable advantages in the DNS trend and the smoothness of a curve. Consequently, it is concluded that the DNNs can predict effectively the anisotropic Reynolds stress and is a promising technique of the computational fluid dynamics. [ABSTRACT FROM AUTHOR]

This paper reviews the numerical models of various cavitating flows around hydrofoils. Numerical models relating to cavitation flows, including mass transfer models and turbulence models, are summarized at first. Then numerical results and analysis of flow characteristics for the cavitating flows around twisted hydrofoils, truncated hydrofoil and tip leakage are discussed respectively. For mean flow fields, Reynolds averaged Navier-Stokes (RANS) simulation associated with a kind of nonlinear turbulence model is found to be an economic and robust numerical approach for different kinds of cavitating flows including cloud cavitation, tip cavitation and tip leakage cavitation. To predict the fluctuations of pressure and velocity, large eddy simulation (LES) and detached eddy simulation (DES) are two effective approaches. Finally, a few open questions are proposed for future research. [ABSTRACT FROM AUTHOR]

Hydraulic machinery mainly includes turbine and pump, which is closely related to national economy and people's livelihood involving aerospace industry, marine engineering, hydropower engineering, petroleum industry, chemical industry, mining industry, biomedical engineering, environmental engineering, agricultural water-soil engineering, etc.. The internal flow of hydraulic machinery is extremely complex, and its characteristics can be summarized as high Reynolds number, multi-scales, inhomogeneous and vortex-dominant unsteady turbulence which interact with the rotating dynamic boundary (rotor blade). Based on the analysis of the internal flow characteristics of hydraulic machinery, the author and his research team successively proposed a rotation correction model, a curvature corrected filter-based model, a scalable detached eddy simulation method, and a non-linear hybrid RANS/LES turbulence model to capture unsteady flow structures and then predict hydraulic performance and dynamic characteristics more accurately. According to the analysis on the internal flow, the corresponding flow control measures were put forward. It was verified by experiments that these methods could significantly improve the hydraulic performance, anti-cavitation performance and dynamic characteristics (pressure pulsation and vibration) of hydraulic machinery in a certain range of operating conditions. In addition, the mechanism how flow control measures influence internal flow was analyzed in depth, aiming at finding a feasible and effective way to improve hydraulic performance, anti-cavitation performance and dynamic characteristics of hydraulic machinery. [ABSTRACT FROM AUTHOR]

The predictive capability of Reynolds-averaged numerical simulation (RANS) models is investigated by simulating the flow in meandering open channel flumes and comparing the obtained results with the measured data. The flow structures of the two experiments are much different in order to get better insights. Two eddy viscosity turbulence models and different wall treatment methods are tested. Comparisons show that no essential difference exists among the predictions. The difference of turbulence models has a limited effect, and the near wall refinement improves the predictions slightly. Results show that, while the longitudinal velocities are generally well predicted, the predictive capability of the secondary flow is largely determined by the complexity of the flow structure. In Case 1 of a simple flow structure, the secondary flow velocity is reasonably predicted. In Case 2, consisting of sharp curved consecutive reverse bends, the flow structure becomes complex after the first bend, and the complex flow structure leads to the poor prediction of the secondary flow. The analysis shows that the high level of turbulence anisotropy is related with the boundary layer separation, but not with the flow structure complexity in the central area which definitely causes the poor prediction of RANS models. The turbulence model modifications and the wall treatment methods barely improve the predictive capability of RANS models in simulating complex flow structures. [ABSTRACT FROM AUTHOR]

A non-linear eddy viscosity model (NLEVM) and a scalable hybrid Reynolds averaged Navier-Stokes/ large eddy simulation (RANS/LES) strategy are developed to improve the capability of the eddy viscosity model (EVM) to simulate complex flows featuring separations and unsteady motions. To study the performance of the NLEVM, numerical simulations around S809 airfoil are carried out and the results show that the NLEVM performs much better when a large separation occurs. Calculated results of the flow around a triangular cylinder show that the NLEVM can improve the precision of the flow fields to some extents, but the error is still considerable, and the small turbulence structures can not be clearly captured as the EVM. Whereas the scalable hybrid RANS/LES model based on the NLEVM is fairly effective on resolving the turbulent structures and can give more satisfactory predictions of the flow fields. [ABSTRACT FROM AUTHOR]

A new Reynolds-averaged Navier-Stokes (RANS) turbulence model is developed in order to correctly predict the mean flow field in a draft tube operating under partial load using 2-D axisymmetric simulations. It is shown that although 2-D axisymme- tric simulations cannot model the 3-D unsteady features of the vortex rope, they can give the average location of the vortex rope in the draft tube. Nevertheless, RANS simulations underpredict the turbulent kinetic energy (TKE) production and diffusion near the center of the draft tube where the vortex rope forms, resulting in incorrect calculation of TKE profiles and, hence, poor prediction of the axial velocity. Based on this observation, a new k-ε turbulence RANS model taking into account the extra production and diffusion of TKE due to coherent structures associated with the vortex rope formation is developed. The new model can successfully predict the mean flow velocity with significant improvements in comparison with the realizable k-ε model. This is attributed to better prediction of TKE production and diffusion by the new model in the draft tube under partial load. Specifically, the new model calculates 31% more production and 46% more diffusion right at the shear layer when compared to the k-ε model. [ABSTRACT FROM AUTHOR]

In this paper, various turbulence closure models for unsteady cavitating flows are investigated. The filter-based model (FBM) and the density correction model (DCM) were proposed to reduce the turbulent eddy viscosities in a turbulent cavitating flow based on the local meshing resolution and the local fluid density, respectively. The effects of the resolution control parameters in the FBM and DCM models are discussed. It is shown that the eddy viscosity near the cavity closure region can significantly influence the cavity shapes and the unsteady shedding pattern of the cavitating flows. To improve the predictions, a Filter-Based Density Correction model (FBDCM) is proposed, which blends the FBM and DCM models according to the local fluid density. The new FBDCM model can effectively represent the eddy viscosity, according to the multi-phase characteristics of the unsteady cavitating flows. The experimental validations regarding the force analysis and the unsteady cavity visualization show that good agreements with experimental visualizations and measurements are obtained by the FBDCM model. For the FBDCM model, the attached cavity length and the resulting hydrodynamic characteristics are subsequently affected by the detail turbulence modeling parameters, and the model is shown to be effective in improving the overall predictive capability. [ABSTRACT FROM AUTHOR]

The steady-state flow field characteristics have important effects on the stability of the throttled surge tank with the standpipe. This paper analyzes these effects on the basis of the numerical simulation of the flow field by using the Computational Fluid Dynamics (CFD) method. It is shown that the anticlockwise recirculation zone is formed in the standpipe, which affects the local head loss at the junction of the standpipe with the pipeline. The variation of the head loss coefficient at the junction is linearly related with the diameter ratio of the standpipe to the pipeline. The dimensionless recirculation flow rate is proportional to the square of the diameter ratio. Considering the effects of the recirculation zone, an empirical expression of the critical stability area is obtained. Comparing with the Thoma critical area, the area obtained by the present method is smaller, and the reduction depends on the diameter ratio and the ratio of the velocity head to the head losses in the tunnel. [ABSTRACT FROM AUTHOR]

The performance of the standard k-ɛ model, the high-Reynolds-number k-ω model, the 1-equation k model, and the non-friction Euler model is examined against the case of wave run-up on a mild slope. A numerical model based on N-S equations and Volume Of Fluids (VOF) method is employed. Comparisons of elevation, velocity and shear stress are made among the four turbulence models against experimental data of wave run-up on a mild slope beach. It is found that before wave breaking on the slope, the outputs of the four different turbulence models agree reasonably well with each other. This suggests that during the run-up process the turbulence effect is negligible before wave breaking. Moreover, in the wave breaking zone, both the standard k-ɛ model and the high-Reynolds-number k-ω model predict the mean velocity field quite well, but generally under-predict the velocity and turbulent kinetic energy using wall functions on the solid slope surface. [ABSTRACT FROM AUTHOR]

Based on the analysis of transportation equation of turbulence fluctuant velocity in the rotating frame of reference, it is the first time that the extended intrinsic mean spin tensor is instead of mean vorticity tensor in the well-known Richardson number and applied to improve the A. Hellsten's correction of system rotation to Menter's κ – ω SST turbulence model. The modified model is more applicable for turbulence simulation in the rotating frame of reference. It is used for turbulence simulation of flow in the centrifugal pump impeller in this paper. Compared with Menter's κ – ω SST and original rotation correction, this model can get better result of effect of system rotation and its induced to the turbulence in the centrifugal impeller. [ABSTRACT FROM AUTHOR]

The paper presents a study of using a modified k-ω model to predict the unsteady cavitating flows around 2D and 3D hydrofoils in the framework of multi-phase mixture flow RANS approach. The cavitation is modeled by Schnerr-Sauer's cavitation model. A 2D NACA0015 foil at cavitation number σ=1.0 (unsteady with cloud shedding) is studied first, followed by the Delft twisted 3D foil. It is found that the present RANS method is able to predict the essential features like re-entrant jets, the periodic shearing and shedding of cloud cavities. Two distinct shedding dynamics are noted for the 2D foil: (a) Shedding of medium to large scale structures (at low frequency); (b) Shedding of a series of secondary vortex cavities (at high frequency). For the 3D twisted foil, the collaborated effect of re-entrant jets from the curved closure line to break up a primary cavity, as well as the formation, roll-up and transport of cavitation vortices that are observed in the experiment are truly reproduced in the simulation. The method is found to have a tendency to under-predict the lift coefficients. [ABSTRACT FROM AUTHOR]

A compressible, multiphase, one-fluid RANS solver was developed to study turbulent cavitating flows. The interaction between turbulence and two-phase structures is complex and not well known. This constitutes a critical point to accurately simulate unsteady behaviours of cavity sheets. In the present study, different turbulence transport-equation models are investigated. Numerical results are given for a Venturi geometry and comparisons are made with experimental data. [ABSTRACT FROM AUTHOR]

The separated turbulent flow around a circular cylinder is investigated using Large-Eddy Simulation (LES), Detached-Eddy Simulation (DES, or hybrid RANS/LES methods), and Unsteady Reynolds-Averaged Navier-Stokes (URANS). The purpose of this study is to examine some typical simulation approaches for the prediction of complex separated turbulent flow and to clarify the capability of applying these approaches to a typical case of the separated turbulent flow around a circular cylinder. Several turbulence models, i.e. dynamic Sub-grid Scale (SGS) model in LES, the DES-based Spalart-Allmaras (S-A) and k-ω Shear-Stress-Transport (SST) models in DES, and the S-A and SST models in URANS, are used in the calculations. Some typical results, e.g., the mean pressure and drag coefficients, velocity profiles, Strouhal number, and Reynolds stresses, are obtained and compared with previous computational and experimental data. Based on our extensive calculations, we assess the capability and performance of these simulation approaches coupled with the relevant turbulence models to predict the separated turbulent flow. [ABSTRACT FROM AUTHOR]

A hybrid model that combines both physical and numerical models was employed to simulate the velocity field in a river area in complex geometry with multiple plunging jets. The simulation was based on experiments concerning energy dissipation and scour prevention at the Xiluodu Hydropower Station on the Yangtze River. The calculated results indicate that the complex geometry of the river area has a significant influence on the velocity field, especially on the circulation flow pattern at upstream and downstream of the plunging area and on the asymmetric characteristics of the spiral flow near both banks. The scour characteristics of the downstream river bed caused by the multiple jets were also predicted and analyzed according to the characteristics of the calculated velocity field. The good agreement between the simulated and experimental results indicates that the hybrid model can be used to effectively solve complicated 3-D problems with complex geometric and inlet conditions. Such problems may not easily be solved by using either a physical or a numerical model alone, and therefore the method presented in this article is considered to be a practical and effective way of dealing with this kind of problems. [ABSTRACT FROM AUTHOR]

ABSTRACT: The paper presents a validation study of a RANS prediction of open water characteristics of a highly skewed propeller in full range advance-ratio. Computational results are compared with open water data, and extensively validated at one loading with paint test, cavitation inception test and PIV measurements. Except at two extreme off-design loads, the difference between prediction and measurement is less than 3% for KT and less than 5% for KQ at a wide range of advance ratios. While this can be considered as good agreement with experiment for KT and KQ, an attempt is made to critically analyze the predicted flow detail and some possible deficiency of the existing RANS method is discussed. Part of the work was carried out within the EU-funded research project “LEADING EDGE 2 [2] Leading Edge: Prediction of Leading Edge and Tip Flow for the Design of Quiet and Efficient Screw Propellers (Contract No: G3RD-CT-2002-00818). ”. [Copyright &y& Elsevier]

ABSTRACT: In this paper, Numerical simulations of mean velocity and turbulent kinetic energy fields are presented for three-dimensional lateral jet in crossflow, at the injection angles of −60° and −30°. The RNG k-ɛ turbulence model, with the two-layer wall function method, is adopted to simulate the characteristics of this flow at the jet-to-crossflow velocity ratios, 1, 2 and 4. The results show that the injection angle and jet-to-crossflow velocity ratio can change the flow fields, and the range upstream affected by jet injected laterally increase and the curvature of jet trajectories varies along the flow direction. Furthermore, the separation events in the lee of the jet exit and behind the jet bending-segment have been found, and the mechanisms of two vortex systems are analyzed. [Copyright &y& Elsevier]

ABSTRACT: In this paper, Numerical simulations of mean velocity and turbulent kinetic energy fields are presented for three-dimensional lateral jet in crossflow, at the injection angles of −60° and −30°. The RNG k-ɛ turbulence model, with the two-layer wall function method, is adopted to simulate the characteristics of this flow at the jet-to-crossflow velocity ratios, 1, 2 and 4. The results show that the injection angle and jet-to-crossflow velocity ratio can change the flow fields, and the range upstream affected by jet injected laterally increase and the curvature of jet trajectories varies along the flow direction. Furthermore, the separation events in the lee of the jet exit and behind the jet bending-segment have been found, and the mechanisms of two vortex systems are analyzed. [Copyright &y& Elsevier]