Search

This article details the development and evaluation of a novel wave energy converter (WEC) aimed at efficiently capturing wave energy for electricity production. The study employs Computational Fluid Dynamics (CFD) techniques, specifically the URANS method and the k-ω SST turbulence model, to solve the Navier-Stokes equations and capture the free surface using the Volume of Fluid (VOF) model. The CFD results are validated against experimental data to ensure accuracy. Various design parameters of the proposed device were tested, revealing that the arms and bottom angle significantly affect its performance. Unlike the floating Wave Dragon (WD) device, which utilises potential energy and is set in deep water, the new fixed-seabed device is positioned in the transitional wave region near the shore, where waves retain 80% of their energy. It can be constructed from environmentally friendly cement, making it resistant to hurricanes and suitable for any wave turbine in the open sea. The MP687 turbine was used to capture the wave energy in the proposed device, testing its performance in three positions: in the open sea, in the middle of the device, and at the device's outlet. The results show that the device significantly enhances wave energy concentration, especially when the turbine is placed at the outlet. The proposed device offers numerous advantages, including its fixed position in a high-energy wave zone, the efficient use of turbulent kinetic energy, and robust construction that can withstand storms. [ABSTRACT FROM AUTHOR]

Copyright of Fire Ecology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Three-dimensional (3D) computational fluid dynamics (CFD) simulations have been carried out to investigate the complex interaction of a planar shock wave (Ma = 1.22) with a cylindrical bubble. The unsteady Reynolds-averaged Navier–Stokes (URANS) approach with a level set coupled with volume of fluid (LSVOF) method has been applied in the present study. The predicted velocities of refracted wave, transmitted wave, upstream interface, downstream interface, jet, and vortex filaments are in very good agreement with the experimental data. The predicted non-dimensional bubble and vortex velocities also have great concordance with the experimental data compared with a simple model of shock-induced Rayleigh–Taylor instability (i.e., Richtmyer–Meshkov instability) and other theoretical models. The simulated changes in the bubble shape and size (length and width) against time agree very well with the experimental results. Comprehensive flow analysis has shown the shock–bubble interaction (SBI) process clearly from the onset of bubble compression up to the formation of vortex filaments, especially elucidating the mechanism on the air–jet formation and its development. It is demonstrated for the first time that turbulence is generated at the early phase of the shock cylindrical bubble interaction process, with the maximum turbulence intensity reaching about 20% around the vortex filament regions at the later phase of the interaction process. [ABSTRACT FROM AUTHOR]

Ducted rotor-only low-pressure axial fans play an integral role in automotive thermal management. The tightly packed under-hood region and down-stream heat-exchanger shape limit the fan diameter. In order to circumvent this limitation, multiple cooling fans of small diameters are tightly packaged and placed in parallel. Currently, there is limited scientific work, that study the aerodynamics of low-pressure axial fans when installed in parallel. This work aims to quantify the aerodynamic performance and the flow-field as a result of installing low-pressure axial fans in parallel through computational fluid dynamics (CFD). Publicly available experimental data from Friedrich-Alexander University is used for the validation of the numerical setup. Three-dimensional, full-annulus, unsteady Reynolds-averaged Navier-Stokes (URANS) analysis has been performed for both a single-fan and two-fans installed in parallel and their respective aerodynamic performance has been compared for the operation condition identified as the best efficiency point in experiments. Only small differences are observed in the overall aerodynamic performance of the two-fans in parallel compared to a single-fan. A circumferential nonuniformity in the form of a local high-pressure zone at the inlet of the fan is observed when the two-fans are placed in parallel. The resulting circumferential nonuniformity is quantified, both in space and time. A strong correlation is found between the pressure fields of the two-fans installed in parallel. [ABSTRACT FROM AUTHOR]

This paper presents the results of a study of self-sustained processes excited in a Helmholtz resonator after a flow over its orifice. A comparative analysis of various approaches to the numerical modeling of this problem was carried out, taking into account both the requirements for achieving the required accuracy and taking into account the resource greediness of calculations, the results of which were verified by comparison with data obtained during a special experiment. The configuration with a spherical resonator with a natural frequency of 260 Hz and an orifice diameter (about 5 cm) in an air flow with a speed of 6 to 14 m/s was considered. A comparison of the calculation results with data obtained in experiments carried out in the wind tunnel demonstrated that the accuracy of calculations of the characteristics of the self-sustained mode using the simplest URANS class model tends to the accuracy of calculations within the large eddy simulation approach formulated in the WMLES model. At the same time, when using WMLES, it is possible to better reproduce the background level of pulsations. From the point of view of resource greediness, expressed in the number of core hours spent obtaining a solution, both models of the turbulence turned out to be almost equivalent when using the same grid models. [ABSTRACT FROM AUTHOR]

In the present work, a numerical methodology is developed for affordable and accurate analysis of helicopter rotor noise in forward flight. The flow field is obtained using unsteady compressible flow analyses using commercially available computational fluid dynamics (CFD) solver FLUENT. Azimuthal variations of the flap and pitch motions of the blades are prescribed a priori as high order polynomial equations through a user-defined function. The rotor noise is predicted using Fluent Acoustic module based on the Ffowcs-Williams and Hawking's equations. Methodology is verified using HART II experimental setup and data which includes flapping, pitching, lead-lag motion and elastic torsion for each rotor blade. Individually defined blade motion is found critical for an accurate acoustic prediction based on the obtained CFD calculation. The present predictions compare very well and performs better than other previous CFD analysis both for minimum noise and baseline case at maximum sound pressure level (SPL) of HART II data, even with meshes of 7-8 million cells rather than tens of millions usually needed. [ABSTRACT FROM AUTHOR]

Wind flow on and around buildings attains more importance among architectures, builders, urban planners, structural engineers, and wind engineers. Wind tunnel experiments and wind flow assessments of full-scale buildings are expensive and complex in varied terrain conditions. Hence, wind flows are extensively assessed using Computational Fluid Dynamics (CFD). By following the turbulence parameters, CFD turbulence models create the wind tunnel and atmospheric environments. No literature has till elucidated which CFD turbulence model is more suitable for predicting the terrain wind flow on and around high-rise buildings. The efficiency of the CFD models, their performance, and their accuracy must be validated with experimental results, which is indispensable before using the turbulence model in practice. Therefore, this investigation aims to validate the Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations for a setback tall building under open terrain wind conditions enclosed within the wind tunnel dimensions. The URANS simulation is accompanied with Standard k-ε, Realizable k-ε, RNG k-ε, Standard k-ω, k-ω SST and RSM. The k -ω SST and RSM turbulence models have reproduced the wind pressure coefficients observed from the wind tunnel. However, all turbulence models failed to produce the same velocity profiles at downstream recirculation, as they vary with sampling time. The transient feature, RMS (Root Mean Square), is better reproduced by RSM and k-ω SST models, while the most unsteady features like across wind spectra and eddies were captured by Realizable, RSM and SST using iso-surface. k-ω SST and RSM models predict similar results with the experiment. Where less computational time was required for the SST, it is promising that this model provides both mean pressure and unsteady feature, encouraging more accurate simulation around the buildings. [ABSTRACT FROM AUTHOR]

The presented work is dedicated to the development of the mathematical model of a closed-type wind tunnel (WT) in a two-dimensional (2D) formulation based on the computational fluid dynamics (CFD) Unsteady Reynolds Averaged Navier Strokes Equations (URANS) method. The influence of the WT walls perforation percentage on the aerodynamic characteristics of the SC1095 (SC is an abbreviation for "supercritical") airfoil in a wide range of pitch angles from 0 to 28 degrees has been studied. The obtained data are compared with the aerodynamic characteristics of the airfoil in free flow conditions. It is shown that the use of perforation makes it possible to significantly reduce the influence of the wind tunnel walls on the obtained aerodynamic characteristics of the airfoil bringing them closer to the airfoil characteristics under free flow conditions. The developed low resource-intensive 2D CFD WT model can be used to determine the optimal ratio between the walls perforation percentage, the WT test section dimensions, and the experimental model dimensions for various test modes. Support of WT experiments with the help of preliminary parametric studies based on the 2D CFD model could significantly reduce material costs and increase the accuracy of the obtained results. [ABSTRACT FROM AUTHOR]

One of the challenges of air jet impingement systems is the interaction between jets in the case of impingement with nozzle arrays. This study is focused on the wall shear stress exerted by the fluid, as it is closely related to surface heat transfer. The strengths and weaknesses of the different turbulence models for a specific case are highlighted. To examine this effect, a qualitative comparative analysis between a physical model and a computational model is designed. The computational model is evaluated with five different turbulence modeling techniques to determine which one of these predicts this effect better. The surface flow visualization technique has been used to determine the wall shear stress experimentally. The tested case is formed by a 9 nozzle square regular array at a Reynolds number of 8500 with the air outlet in minimum crossflow configuration. The traces left by the fluid and the particle density can be qualitatively related to the wall shear stress and compared with the scalar fields of the simulations. Up to four regions of interest in three nozzle positions have been studied. The results show better performance of the LES technique predicting the stagnation point location, as well as cross-fountain region shape and size. Through this work, the suitability of the techniques to the surface observations can be determined. [Display omitted] • A multiple air jet impingement system is evaluated experimentally and computationally. • The surface visualization fluid technique allows the identification of air jet impingement regions. • The detachment and reattachment lines related to the secondary vortices of the roll up zones are of major interest. • A high resolution mesh over the entire domain will be required to capture the traces observed in the jet-to-jet roll-up zone. [ABSTRACT FROM AUTHOR]

This paper combines a synthetic jet actuator (SJA) and a leeward porous coating to alter the aerodynamic forces on a cylinder in crossflow at R e = 4.2 × 10 4 . While SJAs and porous coatings are known to be effective flow control methods in isolation, their combined effect has not been studied. A 2D numerical model was created of a cylinder with a SJA at 90° and 100° leeward porous coating. The model was validated using accompanying water tunnel tests. The combined model was tested for dimensionless frequencies 0.15 < f + < 4 and compared to reference models. For f + < 1, using only the SJA increases the cylinder drag coefficient ( C d ). Combining a porous coating with the SJA in that regime lowers the C d values by 15–21%, and causes an overall reduction in C d compared to the smooth cylinder baseline case. However, using only the porous coating causes a superior 35% reduction in C d . For f + > 1, the combined SJA and porous coating configuration did not differ from the SJA only configuration, achieving the largest drag reduction of 45% at f + = 4. The flow control mechanisms of the SJA and porous coating do not combine constructively in this current setup. However, the porous coating is beneficial for f + < 1, causing an overall drag reduction even when the active SJA tends to increase drag. [ABSTRACT FROM AUTHOR]

The uniform flow over a nominally two-dimensional normal thin flat plate with blockage ratio 0.214 was numerically investigated in three dimensions by three methods: unsteady Reynolds-averaged Navier-Stokes (URANS) based on the realizable k-epsilon (RKE) turbulence model, URANS based on the k-omega shear stress transport (SST) turbulence model, and detached eddy simulation (DES). The Reynolds number based on the inlet flow velocity and the chord width of the plate was 117,000. A comprehensive comparison against earlier experimental results showed that URANS-SST method only could give a correct Strouhal number but overestimated the mean base pressure distribution and mean drag coefficient, while URANS-RKE and DES methods succeeded in giving accurate predictions of all. Moreover, by comparing the instantaneous vorticity contours and three-dimensional (3D) turbulent flow structures, it is found that DES is better suited for the present case because it can capture irregular small-scale structures and reproduce the three-dimensionality and low-frequency unsteadiness of the vortex shedding. Finally, through the volume-of-fluid (VOF)-based simulation of the free surface, it is demonstrated that the free surface has no significant effect on the mean drag coefficient and Strouhal number. [ABSTRACT FROM AUTHOR]

A fully resolved (FR) NREL 5 MW turbine model is employed in two unsteady Reynoldsaveraged Navier-Stokes (URANS) simulations (one with and one without the turbine tower) of a periodic atmospheric boundary layer (ABL) to study the performance of an infinitely large wind farm. The results show that the power reduction due to the tower drag is about 5% under the assumption that the driving force of the ABL is unchanged. Two additional simulations using an actuator disc (AD) model are also conducted. The AD and FR results show nearly identical tower-induced reductions of the wind speed above the wind farm, supporting the argument that the AD model is sufficient to predict the wind farm blockage effect. We also investigate the feasibility of performing delayed-detached-eddy simulations (DDES) using the same FR turbine model and periodic domain setup. The results show complex turbulent flow characteristics within the farm, such as the interaction of large-scale hairpin-like vortices with smaller-scale blade-tip vortices. The computational cost of the DDES required for a given number of rotor revolutions is found to be similar to the corresponding URANS simulation, but the sampling period required to obtain meaningful time-averaged results seems much longer due to the existence of long-timescale fluctuations. [ABSTRACT FROM AUTHOR]

This paper is dedicated to the numerical modeling of the aerodynamic characteristics of the full-scale coaxial main rotor. The modes of forward flight in the range of values V = 30-60 m·s-1 (108-216 km·h-1) have been under consideration. The simulation has been performed with the usage of two computational fluid dynamics (CFD) approaches: the free wake model (FWM) developed by the authors and the unsteady Reynolds-averaged Navier-Stokes (URANS) equations method based on the Ansys Fluent software. The coefficients of rotor thrust and torque, vortex wake shapes and induced velocity fields have been obtained and analyzed. The FWM and the URANS modeling data match satisfactorily. The FWM demonstrates a significant advantage in computing time and resources costs with sufficient accuracy in resulting rotor's basic aerodynamic characteristics. That's why it seems appropriate to use the FWM for preliminary simulations, taking into account before using the high time and resource intensive the URANS method for comprehensive investigation of coaxial rotor aerodynamics at forward flight modes. [ABSTRACT FROM AUTHOR]

Purpose: Convective heat transfer features of a turbulent slot jet impingement are comprehensively studied using two different computational approaches, namely, URANS (unsteady Reynolds-averaged Navier–Stokes equations) and SAS (scale-adaptive simulation). Turbulent slot jet impingement heat transfer is used where a considerable heat transfer enhancement is required, and computationally, it is a quite challenging flow configuration. Design/methodology/approach: Customized OpenFOAM 4.1, an open-access computational fluid dynamics (CFD) code, is used for SAS (SST-SAS k-ω) and URANS (standard k-ε and SST k-ω) computations. A low-Re version of the standard k-ε model is used, and other models are formulated for good wall-refined calculations. Three turbulence models are formulated in OpenFOAM 4.1 with second-order accurate discretization schemes. Findings: It is observed that the profiles of the streamwise turbulence are under-predicted at all the streamwise locations by SST k-ω and SST SAS k-ω models, but follow similar trends as in the reported results. The standard k-ε model shows improvements in the predictions of the streamwise turbulence and mean streamwise velocity profiles in the zone of outer wall jet. Computed profiles of Nusselt number by SST k-ω and SST-SAS k-ω models are nearly identical and match well with the reported experimental results. However, the standard k-ε model does not provide a reasonable profile or quantification of the local Nusselt number. Originality/value: Hybrid turbulence model is suitable for efficient CFD computations for the complex flow problems. This paper deals with a detailed comparison of the SAS model with URANS and LES for the first time in the literature. A thorough assessment of the computations is performed against the results reported using experimental and large eddy simulations techniques followed by a detailed discussion on flow physics. The present results are beneficial for scientists working with hybrid turbulence models and in industries working with high-efficiency cooling/heating system computations. [ABSTRACT FROM AUTHOR]

This paper presents a comparative numerical study on liquefied natural gas (LNG) pump tower loads, while focusing on two aspects. First, are impact loads relevant for the structural design of LNG pump towers and, second, in which way does fluid-structure interaction influence these loads? Numerical simulations of the multiphase problem were conducted using viscous field methods. First, unsteady Reynolds-averaged Navier-Stokes (URANS) equations, extended by the volume of fluid (VoF) approach, were used to simulate, at model scale, the flow inside a three-dimensional LNG tank without the tower structure. Then, these results were used to validate the numerical method against model test measurements. Afterward, motion periods and amplitudes were systematically varied in the flow simulations. Flow velocities and accelerations along the positions of the main structural members of the pump tower were extracted and used as input data for load approximations based on the Morison equation. Finally, these load approximations were compared with loads determined from solving the delayed detached eddy simulation (DDES). Time histories as well as statistical measures of global loads acting on the entire and the simplified tower structure were of the same order of magnitude. However, their time evolution differed, especially at their peaks, and this was considered significant for structural design. [ABSTRACT FROM AUTHOR]

This work presents a comparative study of Unsteady Reynolds-Averaged Navier-Stokes (URANS), Detached Eddy Simulations (DES) and Delayed Detached Eddy Simulations (DDES) turbulence modeling approaches by performing numerical investigation with the ANSYS-FLUENT software package on a full-scale model of the Jetstream 31 aircraft. The lift and drag coefficients obtained from different models are compared with ight test data, wind tunnel data and theoretical estimates. The different turbulence models are also compared with each other on the basis of pressure coefficient distributions and velocity uctuations along various lines and sections of the aircraft. For the mesh and the conditions presented in this study, the DDES Spalart-Allmaras model gives the best overall results. [ABSTRACT FROM AUTHOR]

Wake effects within a wind farm are identified as one of the main factors impacting wind farm power production and fatigue loading on wind turbines' structural elements. An efficient representation of wake effects is crucial for both the optimal layout design and real-time configuration of a control strategy of the wind farm to reduce the impact of wake losses. Accordingly, this study provides an overview and analysis of the recent progress in the most common existing physical and numerical modeling techniques for horizontal-axis wind turbine wakes. The techniques addressed here range from high-fidelity CFD and physical testing approaches to low-fidelity analytical models. In addition to describing the physical basis, formulations, fundamental assumptions, and limitations, this study provides a discussion of the usefulness of each modeling technique and possible further research on challenges in modeling wind turbine wakes that have not yet been adequately addressed. [Display omitted] • A wide range of methods from AWMs to CFD and physical testing are reviewed. • Physical basis, formulations, fundamental assumptions, and usefulness are discussed. • Models to capture the transient wake pattern due to dynamic disturbances are scarce. • A wider use of PIV and LiDAR is necessary for a reliable validation. • Propose what future research is required to address challenges in modeling wakes. [ABSTRACT FROM AUTHOR]

The flow in a four-stage axial compressor is investigated by numerical modeling using the Numeca FINE/Turbo software. A few approaches to modeling the rotor-stator interaction are considered: the steady statement with the use of the Mixing Plane boundary condition, the fully unsteady statement (URANS), and the non-linear harmonic (NLH) method. These approaches are compared by considering the time-averaged and instantaneous integral parameters of the compressor and the pulsations of local parameters. [ABSTRACT FROM AUTHOR]

The longitudinal motion characteristics of a slender trimaran equipped with and without a T-foil near the bow are investigated by experimental and numerical methods. Computational fluid dynamics (CFD) method is used in this study. The seakeeping characteristics such as heave, pitch and vertical acceleration in head regular waves are analyzed in various wave conditions. Numerical simulations have been validated by comparisons with experimental tests. The influence of large wave amplitudes and size of T-foil on the longitudinal motion of trimaran are analyzed. The present systematic study demonstrates that the numerical results are in a reasonable agreement with the experimental data. The research implied that the longitudinal motion response values are greatly reduced with the use of T-foil. [ABSTRACT FROM AUTHOR]

Featured Application: The present numerical investigation gives some insights regarding the challenges to compute the flow in hydraulic Francis turbines at off-design operating points. However, such simulations are useful to design hydraulic turbines that will be operated with more flexibility. Therefore, the present results can be used to determine which turbulence models are useful to compute off-design operating points, keeping in mind limitations and future improvements. Due to the integration of new renewable energies, the electrical grid undergoes instabilities. Hydroelectric power plants are key players for grid control thanks to pumped storage power plants. However, this objective requires extending the operating range of the machines and increasing the number of start-up, stand-by, and shut-down procedures, which reduces the lifespan of the machines. CFD based on standard URANS turbulence modeling is currently able to predict accurately the performances of the hydraulic turbines for operating points close to the Best Efficiency Point (BEP). However, far from the BEP, the standard URANS approach is less efficient to capture the dynamics of 3D flows. The current study focuses on a hydraulic turbine, which has been investigated at the BEP and at the Speed-No-Load (SNL) operating conditions. Several "advanced" URANS models such as the Scale-Adaptive Simulation (SAS) SST k − ω and the BSL- EARSM have been considered and compared with the SST k − ω model. The main conclusion of this study is that, at the SNL operating condition, the prediction of the topology and the dynamics of the flow on the suction side of the runner blade channels close to the trailing edge are influenced by the turbulence model. [ABSTRACT FROM AUTHOR]

A computational fluid dynamics (CFD)-based simulation using a finite volume code for a full-appendage DARPA (Defense Advanced Research Projects Agency) SUBOFF model was investigated with a sliding mesh model in a multi-zone fluid domain. Unsteady Reynolds Averaged Navier–Stokes (URANS) equations were coupled with a Menter’s shear stress transport (SST) k-ω turbulence closure based on the Boussinesq approximation. In order to simulate unsteady motions and capture unsteady interactions, the sliding mesh model was employed to simulate flows in the fluid domain that contains multiple moving zones. The pressure-based solver, semi-implicit method for the pressure linked equations-consistent (SIMPLEC) algorithm was employed for incompressible flows based on the predictor-corrector approach in a segregated manner. After the grid independence test, the numerical simulation was validated by comparison with the published experimental data and other numerical results. In this study, the capability of the CFD simulation with the sliding mesh model was well demonstrated to conduct the straight-line towing tests by analyzing hydrodynamic characteristics, viz. resistance, vorticity, frictional coefficients, and pressure coefficients.

During the optimization phase of a wave energy converter (WEC), it is essential to be able to rely on a model that is both fast and accurate. In this regard, Computational Fluid Dynamic (CFD) with Reynolds Averaged Navier–Stokes (RANS) approach is not suitable for optimization studies, given its computational cost, while methods based on potential theory are fast but not accurate enough. A good compromise can be found in boundary element methods (BEMs), based on potential theory, with the addition of non-linearities. This paper deals with the identification of viscous parameters to account for such non-linearities, based on CFD-Unsteady RANS (URANS) analysis. The work proposes two different methodologies to identify the viscous damping along the rotational degree of freedom (DOF) of pitch and roll: The first solely involves the outcomes of the CFD simulations, computing the viscous damping coefficients through the logarithmic decrement method, the second approach solves the Cummins’ equation of motion, via a Runge-Kutta scheme, selecting the damping coefficients that minimize the difference with CFD time series. The viscous damping is mostly linear for pitch and quadratic for roll, given the shape of the WEC analysed.

Turbulent thermal mixing of fluids at different temperature in T-junctions represents a major concern for the safety of nuclear reactors. This is due to the significant thermal fluctuations that can arise under such circumstances, consequently leading to cyclic thermal stress and thermal fatigue within the pipe wall. Computational Fluid Dynamics (CFD) can be employed in order to obtain useful insights on the characteristics of the transient behaviour of the turbulent heat transfer in T-junctions. Although LES has been found to be the most accurate approach to turbulence modelling for this type of flow, its application at high Reynolds numbers is limited by its considerable computational costs. In this respect, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach can be considered as a less computationally demanding option. In the present work, different URANS models are applied for the simulation of the thermal mixing in a T-junction at high Reynolds numbers. The numerical results are thoroughly assessed against the available experimental data. It is shown that, despite its limitations, a proper use of the URANS approach can give reasonable results for the considered flow configuration; in particular, a good prediction of the temperature fluctuations near the wall has been obtained, which is important for the evaluation of the cyclic thermal stress induced within the pipe wall. Therefore, it is concluded that URANS models can be regarded a pragmatic approach for the evaluation of temperature fluctuations in T-junction pipes. Finally, general guidelines for the application of the URANS approach for the simulation of such configurations are given. [ABSTRACT FROM AUTHOR]

We employ a three-dimensional coupled hydro-morphodynamic model, the Virtual Flow Simulator (VFS-Geophysics) in its Unsteady Reynolds Averaged Navier-Stokes mode closed with k-ω model, to simulate the turbulent flow and sediment transport in large-scale sand and gravel bed waterways under prototype and live-bed conditions. The simulation results are used to carry out systematic numerical experiments to develop design guidelines for rock vane structures. The numerical model is based on the Curvilinear Immersed Boundary approach to simulate flow and sediment transport processes in arbitrarily complex rivers with embedded rock structures. Three validation test cases are conducted to examine the capability of the model in capturing turbulent flow and sediment transport in channels with mobile-bed. Transport of sediment materials is handled using the Exner equation coupled with a transport equation for suspended load. Two representative meandering rivers, with gravel and sand beds, respectively, are selected to serve as the virtual test-bed for developing design guidelines for rock vane structures. The characteristics of these rivers are selected based on available field data. Initially guided by existing design guidelines, we consider numerous arrangements of rock vane structures computationally to identify optimal structure design and placement characteristics for a given river system. [ABSTRACT FROM AUTHOR]

High-resolution computational fluid dynamics (CFD) predictions of static and dynamic stall of a finite-span ONERA OA209 wing were validated against the wind tunnel test measurements. The freestream Mach number was 0.16 and the Reynold number was 1 million. For the dynamic stall study, a sinusoidal pitching motion was prescribed. The CFD modeling approaches employed were — Delayed Detached Eddy Simulation (DDES) modeling using the NASA OVERFLOW flow solver, Unsteady Reynolds-averaged Navier–Stokes (URANS) modeling using the ONERA elsA flow solver, and DDES modeling using the NASA FUN3D flow solver. The flow was modeled as both fully turbulent and transitional. A comparative study between predictions and the wind-tunnel test data for pre- and post-stall measurements was carried out that included wing section lift and moment, surface pressure, and velocity field at chordwise and spanwise planes. The high spatial and temporal resolutions employed resulted in good correlations with the test data, in particular with the inclusion of DDES modeling along with a turbulence transition model. The CFD modeling parameters thus establish were applied to a deep and a light stall cases, and were found to accurately capture the wing section loads, demonstrating its generalizability in capturing the stall dynamics. [ABSTRACT FROM AUTHOR]

The present paper evaluates the relative performance of scale-adaptive simulation (SAS) in modelling unsteady concentration and flow fields around a model building relative to other transient simulations such as large eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS) models. A novel application of proper orthogonal decomposition (POD) and time–frequency analysis is carried out in order to evaluate the transient behaviour and dominant structures of the flow fields predicted by SAS and LES. Results represent the outstanding performance of SAS in comparison with the URANS computation based on the SSTk–ωmodel. This better performance is related to the accurate reproduction of unsteady fluctuations around the model building by SAS. In addition, the quantitative and qualitative agreements for the shapes and magnitudes of POD modes between SAS and LES confirm the LES-like behaviour of SAS in the wake region. However, in terms of computational performance, SAS imposes an extra CPU cost as compared with LES for the same grid resolution. [ABSTRACT FROM PUBLISHER]

The accuracy of CFD simulations of vertical axis wind turbines (VAWTs) is known to be significantly associated with the computational parameters, such as azimuthal increment, domain size and number of turbine revolutions before reaching a statistically steady state condition (convergence). A detailed review of the literature, however, indicates that there is a lack of extensive parametric studies investigating the impact of the computational parameters. The current study, therefore, intends to systematically investigate the impact of these parameters, on the simulation results to guide the execution of accurate CFD simulations of VAWTs at different tip speed ratios (λ) and solidities (σ). The evaluation is based on 110 CFD simulations validated with wind-tunnel measurements for two VAWTs. Instantaneous moment coefficient, C m , and power coefficient, C P , are studied for each case using unsteady Reynolds-averaged Navier-Stokes (URANS) simulations with the 4-equation transition SST turbulence model. The results show that the azimuthal increment dθ is largely dependent on tip speed ratio. For moderate to high λ, the minimum requirement for dθ is 0.5° while this decreases to 0.1° at low to moderate λ. The need for finer time steps is associated to the flow complexities related to dynamic stall on turbine blades and blade-wake interactions at low λ. In addition, the minimum distance from the turbine center to the domain inlet and outlet is 15 and 10 times the turbine diameter, respectively. It is also shown that 20–30 turbine revolutions are required to ensure statistically converged solutions. The current findings can serve as guidelines towards accurate and reliable CFD simulations of VAWTs at different tip speed ratios and solidities. [ABSTRACT FROM AUTHOR]

Currently, gas turbine manufacturers frequently face the problem of strong acoustic combustion driven oscillations inside combustion chambers. These combustion instabilities can cause extensive wear and sometimes even catastrophic damages to combustion hardware. This requires prevention of combustion instabilities, which, in turn, requires reliable and fast predictive tools. This work presents a three-step method to find stability margins within which gas turbines can be operated without going into self-excited pressure oscillations. As a first step, a set of unsteady Reynolds-averaged Navier–Stokes simulations with the Flame Speed Closure (FSC) model implemented in the OpenFOAM® environment are performed to obtain the flame describing function of the combustor set-up. The standard FSC model is extended in this work to take into account the combined effect of strain and heat losses on the flame. As a second step, a linear three-time-lag-distributed model for a perfectly premixed swirl-stabilized flame is extended to the nonlinear regime. The factors causing changes in the model parameters when applying high-amplitude velocity perturbations are analysed. As a third step, time-domain simulations employing a low-order network model implemented in Simulink® are performed. In this work, the proposed method is applied to a laboratory test rig. The proposed method permits not only the unsteady frequencies of acoustic oscillations to be computed, but the amplitudes of such oscillations as well. Knowing the amplitudes of unstable pressure oscillations, it is possible to determine how these oscillations are harmful to the combustor equipment. The proposed method has a low cost because it does not require any license for computational fluid dynamics software. [ABSTRACT FROM PUBLISHER]

The seakeeping characteristics of a Small Waterplane Area Twin Hull (SWATH) vehicle equipped with fixed stabilizing fins was investigated by experimental and numerical methods The calculation methods range from viscous CFD simulation based on an unsteady RANS approach to Boundary Element Method (BEM) based on Three Dimensional Translating-pulsating Source Green Function (3DTP). Responses of ship motions in head regular waves and nonlinear effects on motion responses with increasing wave amplitude were analyzed. Numerical simulations have been validated by comparisons with experimental tests. The results indicate that the heave and pitch transfer functions depict two peaks with the increase of wave length. Comparisons amongst experimental data and different numerical calculations illustrate that the RANS method predicts ship motions with higher accuracy and allows the detection of nonlinear effects. The heave and pitch transfer functions see a downward trend with the increasing wave amplitude in the resonant zone at low speed. [ABSTRACT FROM AUTHOR]

A detailed numerical investigation of the flow behind a square cylinder at a Reynolds number of 21,400 is conducted to assess the ability of the delayed detached-eddy simulation (DDES) modeling approach to accurately predict the velocity recovery in the wake of a bluff body. Three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) and DDES simulations making use of the Spalart-Allmaras turbulence model are carried out using the open-source computational fluid dynamics (CFD) toolbox OpenFOAM-2.1.x, and are compared with available experimental velocity measurements. It is found that the DDES simulation tends to overestimate the averaged streamwise velocity component, especially in the near wake, but a better agreement with the experimental data is observed further downstream of the body. The velocity fluctuations also match reasonably well with the experimental data. Moreover, it is found that the spanwise domain length has a significant impact on the flow, especially regarding the fluctuations of the drag coefficient. Nonetheless, for both the averaged and fluctuating velocity components, the DDES approach is shown to be superior to the URANS approach. Therefore, for engineering purposes, it is found that the DDES approach is a suitable choice to simulate and characterize the velocity recovery in a wake. [ABSTRACT FROM AUTHOR]

Recent developments in Computational Fluid Dynamics (CFD) enabled common access for researchers and thus offers solutions to various complex problems in many different fields. With this motivation, in this study, two kinds of numerical methods were employed to investigate the vertical motions in variable regular waves. While the potential method is commonly known as linear "strip theory", the viscous approach is (the state of the art) named as URANS (Unsteady Reynolds Averaged Navier Stokes) solver which has a fully non-linear base. The DTMB 5415 ship model form was selected for a series of computational work. A numerical study was carried out to understand the seakeeping behaviour of the displacement hull for the stationary case Fn=0 and a high speed case Fn=0.41. The RAO (Response Amplitude Operator) graphs for the coupled heave - pitch motions and the ship's vertical accelerations were generated for five encounter frequencies. The numerical results obtained were validated with the existing experimental data and comparisons were made between the two numerical approaches with the help of RAO graphs. The obtained results showed that the limitations of the strip theory pose a handicap as the assumptions involved in the theory narrow down its application. The nonlinear viscous URANS approach tends to be a better option returning closer results to experiments in a wide Froude number range but on the other hand it does not possess the practicality of the strip theory. [ABSTRACT FROM AUTHOR]

An ability to predict the behavior of buoyant jets entering a large body of relatively stationary fluid is important in analysis of a wide variety of nuclear accidents, including for example the use of large tanks of water as heat sinks, or the release of hot gases into the secondary containment. In particular, the degree to which temperature stratification occurs is important, as it can affect markedly the effectiveness of the body of fluid as a heat sink. In this paper, we report the results of measurements on an experimental facility designed to exhibit such behavior, and the results of attempts to predict this experiment using CFD. In particular, we here investigate the effectiveness of three alternative turbulence models for this analysis; low-Re k – e , elliptic-blended k–e and Shear Stress Transport k – ω models. Both the degree of thermal stratification and the stability of the jet that were predicted differed markedly between the three models. Two of the models, the low-Re k – e and the Shear Stress Transport k – ω , tend to predict, wrongly, significant turbulent intensity in regions where fluid velocities are essentially zero. This spurious high turbulent intensity in turn causes (i) a high turbulent viscosity to be applied, wrongly stabilizing the jet, and (ii) increased turbulent diffusion of heat, causing too deep and diffuse a stratification to be predicted. [ABSTRACT FROM AUTHOR]

Bendway weirs are one of the most practical in-stream rock structures utilized to protect the outer bend of meandering streams and rivers from erosion. We present development of a simulation-based paradigm for effective design of bendway weir structures to enhance meandering stream bank stability and control lateral migration. To do so, we employ the St. Anthony Falls Laboratory Virtual StreamLab (VSL3D) code to elucidate the flow and sediment transport phenomena induced by interaction of flow, mobile bed, and in-stream structures in large rivers under prototype conditions. We carried out numerous numerical experiments to systematically simulate various arrangements of bendway weir in two river test-beds and gaining insights into the physical mechanisms via which such bendway weirs modify turbulent flow, sediment transport and scour processes. The so-gained physical insights are then taken into account to develop a set of practical physics-based design criteria for optimal placement of bendway weirs in large rivers. [ABSTRACT FROM AUTHOR]

The virtual mass and damping coefficients of an open and ducted propeller are determined using URANS computations. Time-accurate simulations are carried out for an open propeller forced to harmonic motion in two separate directions, translational x 1 - and rotational x 4 - directions. The analysis produces the diagonal coefficients of the virtual mass m 11 , m 44 together with the diagonal damping terms c 11 , c 44 . The cross-terms m 14 , m 41 and c 14 , c 41 are also evaluated. A range of advance numbers is considered in the simulations. Several excitation frequencies and amplitudes are applied in order to determine the impact of viscosity on the vibrational coefficients. Oblique flow cases are also considered and the related virtual mass and damping coefficients are analyzed. The computed cases are compared to potential flow simulations and to published semi-empirical results. The average magnitudes of the coefficients correspond quite reasonably to those computed by the CFD method. However, the viscous effects are found to have a certain impact on some coefficients. [ABSTRACT FROM AUTHOR]

International Maritime Organisation (IMO) and other bodies have been trying to set-up guidelines and regulations to reduce/limit noise levels at sea which influence marine life in particular marine mammals and certain fish types. Ships with low noise characteristics will be a must in the near future for almost all ship types. However for special ships, such as naval surface vessels, fishing vessels, submarines, etc. this has already been an issue because of their operational requirements. The propeller is one of the main sources of underwater noise generated by ships. Therefore, it is important to predict and control the underwater noise characteristics of propellers. In this respect, the main objective of this study is to calculate propeller radiated noise numerically. Propeller noise has been investigated numerically for the INSEAN E1619 submarine propeller in open water, behind a generic DARPA suboff submarine and within imposed wake cases at non-cavitating conditions due to their deeply submerged operations. Flow around the propeller is solved with a commercial CFD software using Unsteady Reynolds Averaged Navier–Stokes (URANS), while hydro-acoustic analysis is performed using a model based on Ffowcs-Williams and Hawkings equation. This paper reports the results of the study. The paper also includes the details of the bodies and discusses further improvement of the methodology applied. [ABSTRACT FROM AUTHOR]

Research on vortex-induced vibrations (VIV) mainly involves experimental science but building laboratory setups to investigate the flow are expensive and time consuming. Computational fluid dynamics (CFD) methods may offer a faster and a cheaper way to understand this phenomenon depending on the solution approach to the problem. The context of this paper is to present the author's computational approach to solve for vortex-induced vibrations which cover extensive explanations on the mathematical background, the grid structure and the turbulence models implemented. Current computational research on VIV for smooth cylinders is currently restricted to flows that have Reynolds numbers below 10,000. This paper describes the method to approach the problem with URANS and achieves to return satisfactory results for higher Reynolds numbers. The computational approach is first validated with a benchmark experimental study for rather low Reynolds number which falls into TrSL2 flow regime. Then, some numerical results up to Re = 130,000, which falls into TrSL3 flow regime, are given at the end of the paper to reveal the validity of the approach for even higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

Centrifugal compressor aeroacoustics are analyzed by means of a three-dimensional CFD model. Three operating points at nominal compressor speed are simulated ranging from best efficiency point to near-surge conditions. Experimental measurements are obtained using a steady flow rig mounted on an anechoic chamber. URANS and DES predictions of compressor global variables and pressure spectra are compared against experimental measurements. Flow-induced noise increases as the operating point moves toward surge line. Stall at the suction side of the blades exists even for high mass flow conditions, causing a high frequency boundary layer oscillation. Low momentum cells rotating at the diffuser are found at points closer to surge, causing the so-called whoosh noise . Inducer rotating stall is also present at these conditions. Point closest to surge shows a rotating tornado-type vortex at the inducer, determining a moving low pressure region that increases low frequency noise content. [ABSTRACT FROM AUTHOR]

This paper presents the results of two-dimensional numerical simulations of the flow field around a trapezoidal box-girder bridge section with later cantilevers, experiencing small-amplitude heaving or pitching harmonic oscillations. Unsteady Reynolds-averaged Navier–Stokes equations are solved in conjunction with an eddy-viscosity and an explicit algebraic Reynolds stress model. Flutter derivatives are determined and compared with wind tunnel results, showing fairly good agreement. The degree of sharpness of the deck lower edges is found to play a key role in the aeroelastic behavior of the profile. In particular, the bridge section fully behaves as a bluff body and is prone to low-reduced-wind-speed torsional galloping in the case of perfectly sharp edges. By contrast, the presence of a small radius of curvature in the section lower corners changes the nature of the instability to coupled flutter and significantly postpones the stability threshold, in line with a quasi-streamlined body behavior. Moreover, a wide sensitivity study is carried out, investigating the influence on the self-excited forces of the amplitude of oscillation, mean angle of attack and Reynolds number. In particular, the numerical simulations for the geometry with smooth lower edges highlight the regime transition occurring when the Reynolds number is varied, with significant effects on the flutter derivatives. Finally, the numerical flow visualizations provide a physical explanation of some phenomena observed in the wind tunnel experiments. [ABSTRACT FROM AUTHOR]

Maneuvering prediction tools are important resources for naval and commercial ship designers. They can enable designers to determine maneuvering characteristics of a design prior to construction or be useful in redesign if a ship is underperforming. Numerical methods allow for multiple hulls to be compared digitally, so a certain level of optimization can be achieved through evaluating trade-offs among designs before construction. In this paper, a novel maneuvering prediction method is presented that seeks to provide the accuracy available from today state-of-the-art numerical viscous-flow tools but with significantly reduced computational cost. The numerical formulation is based on the unsteady Reynolds-averaged Navier–Stokes (URANS) equations, and the unsteady ship generated waves are represented by the linear free-surface boundary conditions. The URANS equations are solved on an unstructured finite-volume discretization of the fluid domain around the hull and its appendages. The linearized free-surface approximation accounts for first-order wave effects while reducing the necessary extent of the computational domain and level of grid refinement required by fully nonlinear methods that employ an interface-capturing technique. The resulting formulation provides a marked improvement in computational efficiency with respect to nonlinear methods and can empower naval architects to obtain accurate results earlier in the design process. [ABSTRACT FROM AUTHOR]

It is analytically difficult to calculate roll damping of ships due to the effects of viscosity. Therefore, computational fluid dynamics (CFD) has become a powerful tool in predicting roll damping recently. The unsteady flow around a forced rolling hull section with bilge keels can be calculated using a commercial URANS code which includes the viscous effects. In this study, two-dimensional (2D) roll damping calculations for a S60 midsection with bilge keels including free surface effects are performed for shallow draft case. The first objective of the study is to show whether the URANS code can be used to predict roll damping coefficient correctly. The second one is to show why Ikeda's estimation method is insufficient at shallow draft case. Sinusoidal forced roll motion calculation method of roll damping moment with the help of a sliding interface and a fixed roll axis is successfully applied to predict roll damping coefficient. The calculations are carried out for different roll motion periods and amplitudes to validate the accuracy of the URANS code for different cases. Numerical results are compared with experiments, which were carried out at the towing tank facility of Osaka Prefecture University (OPU), and Ikeda's estimation method. The results show that the URANS code is capable of predicting roll damping coefficients in a good agreement with experimental results and can be used further to develop a better model for prediction of roll damping. [ABSTRACT FROM AUTHOR]

Purpose – This paper aims to simulate unsteady flows with surfaces in relative motion using a multi-block structured flow solver. Design/methodology/approach – A procedure for simulating unsteady flows with surfaces in relative motion was developed, based upon a multi-block structured U-RANS flow solver1. Meshes produced in zones of the flow field with different rotation speed are connected by sliding boundaries. The procedure developed guarantees that the flux conservation properties of the original scheme are maintained across the sliding boundaries during the rotation at every time step. Findings – The solver turns out to be very efficient, allowing computation in scalar mode with single core processors as well as in parallel. It was tested by simulating the unsteady flow on a propfan configuration with two counter-rotating rotors. The comparison of results and performances with respect to an existing commercial flow solver (unstructured) is reported. Originality/value – This paper fulfils an identified need to allow for efficient unsteady flow computations (structured solver) with different bodies in relative motion. [ABSTRACT FROM AUTHOR]

Time-periodic computational fluid dynamics simulations are widely used to investigate turbomachinery components. The triple plane pressure mode matching method developed by Ovenden and Rienstra extracts the acoustic part in such simulations. Experience shows that this method is subject to significant errors when the amplitude of pseudo-sound is high compared to sound. Pseudo-sounds are unsteady pressure fluctuations with a convective character. The presented extension to the triple plane pressure improves the splitting between acoustics and the rest of the unsteady flow field. The method is simple: (i) the acoustic eigenmodes are analytically determined for a uniform mean flow as in the original triple plane pressure mode matching method; (ii) the suggested model for convective pressure perturbations uses the convective wavenumber as axial wavenumber and the same orthogonal radial shape functions as for the acoustic modes. The reliability is demonstrated on the simulation data of a low-pressure fan. As acoustic and convective perturbations are separated, the accuracy of the results increases close to sources, allowing a reduction of the computational costs by shortening the simulation domain. The extended method is as robust as the original one-giving the same results for the acoustic modes in absence of convective perturbations. [ABSTRACT FROM AUTHOR]