Search

Thunderstorm is a severe phenomenon which can produce downburst winds capable for causing significant damage or even the collapse of low-rise structures. Experimental tests and numerical simulations are widely used by the Wind Engineering (WE) community for gaining increased understanding of complex winds like downbursts. In this paper, the accuracy of the Unsteady Reynolds-averaged Navier-Stokes (URANS) and Scale-Adaptive Simulation (SAS) to reproduce a vertical downburst wind - previously tested in the WindEEE Dome wind-simulator facility - was investigated. CFD results were first validated with the experiments and then used to study the dynamics of the phenomenon. The most suitable CFD setting (i.e. SAS case based on the k-ω SST turbulence model) was able to reproduce the development of the primary vortex ring, shedding of secondary rings induced by Kelvin-Helmholtz (KH) instabilities and the periodic detachments of smaller secondary vortices ahead the primary vortex. The superposition of these three contributions has shown to determine the temporal variation of the characteristic nose-shaped vertical profile which was found in the experiments. The performed simulations were also compared to field measurements from the NIMROD project showing a high level of agreement.

The present research focuses on flow oscillations in a planar rectangular cavity. The compressible URANS equations in combination with transition SST k−ω model are utilized to include the turbulence effects. The simulations are carried out for low Reynolds flow with Mach number ranging from 0.2 to 0.7. Flow features are investigated and the frequency analysis is discussed. Two flow oscillation modes, namely shear-layer mode and wake mode are thoroughly identified. The flow structure and oscillation frequencies compare well with LES and DNS results presented in the literature. Furthermore, the shear mode frequency is well aligned with that predicted by Rossiter. This research is aimed to evaluate the performance of URANS as an industrially attractive tool to capture flow phenomena, previously visualized by the aforementioned sophisticated methods.

The applicability of the solution of the unsteady Reynolds-averaged Navier–Stokes equations (URANS) for the numerical simulation of the periodic quasi-switching regime of vortex generation and heat transfer in a deep conical dimple with a slope angle of 10∘ on the wall of a narrow channel is substantiated. To calculate the turbulent regime, the model of shear stress transfer by Menter 2003, modified taking into account the influence of the curvature of streamlines within the framework of the Rodi-Leshziner-Isaev approach, is used. At Reynolds number Re=104, the oscillation period of the transverse Rz and longitudinal forces Rx, as well as the total heat transfer Numm to the control section of the heated channel wall with a dimple, is set equal to 60, which corresponds to the Strouhal number St=0.0167. Computer visualization of swirling jet-vortex flows demonstrates focus-type sources on the side faces of the dimple. In the self-oscillating mode, a two-cell vortex system is formed with different intensities at the oscillation period Rz. Periodic changes in friction, Nusselt numbers and temperature are recorded in the longitudinal and transverse median sections of the dimple and reflect the oscillations of the vortex structure from left to right and from right to left. The formation of a fan jet is shown, which oscillates relative to the plane of longitudinal symmetry, causing a redistribution of power and thermal loads.

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.

The Scale-Adaptive Simulation (SAS) approach has emerged as an improved unsteady Reynolds-Averaged Navier-Stokes (URANS) formulation to bridge the gap between the less accurate commonly used URANS and the computationally expensive hybrid RANS/LES for highly separated unsteady flows, e.g. dynamic stall. However, while the SAS has been successfully used at several occasions, it has not yet been tested for the complex case of dynamic stall. Therefore, the present study analyzes the SAS predictions of dynamic stall on a vertical axis wind turbine at a chord Reynolds number of 5 × 104 and a reduced frequency of 0.125. The analysis is based on comparison of the SAS predictions of the blade aerodynamics and the turbine power performance against the corresponding URANS and hybrid RANS/LES predictions. The results show that the SAS predictions are closer to hybrid RANS/LES than URANS with respect to: (i) the instant of the bursting of the laminar separation bubble (LSB), the leading-edge suction collapse, the formation of the dynamic stall vortex (DSV) and the trailing-edge vortex (TEV) and the shedding of the TEV; (ii) the size and strength of the TEV; (iii) the DSV-TEV interaction; (iv) the drag prediction during the downstroke. On the other hand, both URANS and SAS fail to corroborate with hybrid RANS/LES with respect to: (i) the instant of the formation of the LSB and the shedding of the DSV (the stall angle); (ii) the drag jump at the stall angle; (iii) the lift values during the downstroke; and (iv) the chordwise extent of the LSB.

Unsteady RANS (URANS), hybrid LES/RANS and IDDES simulations were conducted to numerically investigate the velocity field around, and pressures distribution and forces over a square cylinder immersed in a uniform, steady oncoming flow with Reynolds number Re = 21,400. The vortex shedding responses in terms of Strouhal number, the pressure distribution, the velocity profile and the velocity fluctuations obtained by numerical simulations are compared with experimental data. Compared with 2D URANS simulation, 3D simulations using hybrid LES/RANS and IDDES models provide more accurate prediction on the responses in the wake, including mean streamwise velocity profile and rms velocity fluctuations. This also results in more accurate prediction of time-averaged surface pressure coefficient on the rear surface obtained by 3D hybrid LES/RANS and IDDES simulations than by URANS simulation. When a hybrid LES/RANS model or IDDES model is used, a more accurate prediction for either pressure coefficient or velocity profile (especially in the far wake region) is not guaranteed by increasing the mesh resolution along the spanwise direction of the square cylinder.

A stochastic analysis of the cavitation model parameter sensitivity is carried out for internal flows relevant to injector configurations. Stochastic methodologies, namely generalized polynomial chaos and stochastic collocation, are used to obtain continuous response surfaces of the quantities of interest in the parameter space starting from a limited number of simulations. Cavitation is modeled through a transport equation for the void fraction closed by the Schnerr-Sauer relation, containing four free parameters. As for turbulence, the URANS equations are considered, together with two different closure models. The sensitivity to the cavitation model parameters is investigated, first, for a throttle geometry, for which experimental and LES data are available. First, two out of the four parameters are identified as the most important through a preliminary analysis based on 2D simulations, namely the vaporization and condensation factors. Then, the sensitivity of 3D simulation results to the previously identified most important parameters is investigated. The stochastic range of variability of the results contains the reference data. Thus, a parameter optimization is carried out in order to obtain the values giving the best agreement with the LES data. It is then shown that the cavitation parameter sensitivity is practically independent of the working fluid. Finally, it is shown that the calibrated cavitation model can be successfully applied to a different configuration, characterized by the hydraulic flip phenomenon, namely a 3-phase case in which liquid N-heptane flows from an inlet reservoir through a circular channel in an outlet reservoir where air is present.

Numerical simulations are subject to uncertainties due to the imprecise knowledge of physical properties, model parameters, as well as initial and boundary conditions. The assessment of these uncertainties is required for some applications. In the field of Computational Fluid Dynamics (CFD), the reliable prediction of hydrogen distribution and pressure build-up in nuclear reactor containment after a severe reactor accident is a representative application where the assessment of these uncertainties is of essential importance. The inital and boundary conditions that significantly influence the present buoyancy-driven flow are subject to uncertainties. Therefore, the aim is to investigate the propagation of uncertainties in input parameters to the results variables. As a basis for the examination of a representative reactor test containment, the investigations are initially carried out using the Differentially Heated Cavity (DHC) of aspect ratio 4 with Ra=2e9 as a test case from the literature. This allows for gradual method development for guidelines to quantify the uncertainty of natural convection flows in large-scale industrial applications. A dual approach is applied, in which Large Eddy Simulation (LES) is used as reference for the Unsteady Reynolds-Averaged Navier–Stokes (URANS) computations. A methodology for the uncertainty quantification in engineering applications with a preceding mesh convergence study and sensitivity analysis is presented. By taking the LES as a reference, the results indicate that URANS is able to predict the underlying mixing process at Ra=2e9 and the variability of the result variables due to parameter uncertainties.

A symmetrical NACA 0018 airfoil is often used in such applications as small-to-medium scale vertical-axis wind turbines and aerial vehicles. A review of the literature indicates a large gap in experimental studies of this airfoil at low and moderate Reynolds numbers in the previous century. This gap has limited the potential development of classical turbulence models, which in this range of Reynolds numbers predict the lift coefficients with insufficiently accurate results in comparison to contemporary experimental studies. Therefore, this paper validates the aerodynamic performance of the NACA 0018 airfoil and the characteristics of the laminar separation bubble formed on its suction side using the standard uncalibrated four-equation Transition SST turbulence model and the unsteady Reynolds-averaged Navier-Stokes (URANS) equations. A numerical study was conducted for the chord Reynolds number of 160,000, angles of attack between 0 and 11 degrees, as well as for the free-stream turbulence intensity of 0.05%. The calculated lift and drag coefficients, aerodynamic derivatives, as well as the location and length of the laminar bubble quite well agree with the results of experimental measurements taken from the literature for validation. A sensitivity study of the numerical model was performed in this paper to examine the effects of the time-step size, geometrical parameters and mesh distribution around the airfoil on the simulation results. The airfoil data sets obtained in this work using the Transition SST and the k-ω SST turbulence models were used in the improved double multiple streamtube (IDMS) to calculate aerodynamic blade loads of a vertical-axis wind turbine. The characteristics of the normal component of the aerodynamic blade load obtained by the Transition SST approach are much better suited to the experimental data compared to the k-ω SST turbulence model.

In this paper the numerical simulations of the flow around an elastically-mounted circular cylinder with one degree of freedom (1-DOF) with small damping parameter and small mass ratio are presented. The simulations are performed for subcritical Reynolds numbers ( 2 . 0 ⋅ 1 0 3 Re 1 . 3 1 0 4 ), using the URANS (Unsteady Reynolds Averaged Navier Stokes) approach in combination with the SST (Shear Stress Transport) turbulence model. The simulations are carried out on 3D as well as 2D meshes. From the overall results it seems that the bi-dimensional approach could be effectively applied in the lower branch, while major discrepancies are present in the upper branch. The detailed analysis of the wake flow characteristics shows that the three dimensionality of the flow is stronger in the transition zones between the different branches. The extensive comparison between the computational simulations and the experimental results presented in this paper could be considered the “start point” in order to select the proper grid model depending on the requested accuracy.

The evaluation of the aerodynamic admittance functions is a key step in the calculation of bridges and towers buffeting response. For streamlined bodies, such functions can be well approximated by the closed form solution developed by Sears. For bluff bodies, the evaluation of the admittance function still represents an open problem: experimental procedures require the use of grid turbulence or active wind tunnels. In the first case, a number of interrelated factors contribute to the obtained results while, in the second case, the experimental apparatus is of considerable complexity. In this paper, the use of Computational Fluid Dynamics for the evaluation of complex admittance functions is analyzed. Firstly, a discussion regarding the characterization of the flow in active wind tunnels and its reproduction in numerical simulations is presented. Such aspect has been often overlooked in previous studies, in which experimental procedures have been mimicked, so not taking full advantage of the CFD approach. Then, rectangular cylinders with aspect ratio ranging between 4 and 16.67 are studied by means of 2D URANS. It is shown that while 2D URANS usually lead to acceptable results, they can prove inaccurate in specific cases. Additional LES simulations led to a good matching of the experimental data.

Computational fluid dynamics (CFD) simulations and stochastic validation of free-running 5415M in irregular stern-quartering sea state 7 and Froude number 0.33 are presented. Unsteady Reynolds-averaged Navier–Stokes (URANS) computations are validated against experimental fluid dynamics (EFD) tests. EFD static stability, forward speed, wave direction and spectrum were set such that roll-motion resonance were induced, causing large roll angles, significant deck-edge immersion, and static instability. Prerequisite calm-water validation covers roll decay at zero speed and self-propulsion studies for propeller RPM. Subsequent CFD stochastic validation is achieved by statistical assessment of EFD data and CFD results (expected value, standard deviation, and probability distribution) for input waves and ship response: time series are assessed by their wave energy-spectra, autocovariance (waves only), and block-bootstrap analysis (waves and motions); mean-crossing amplitudes are studied by bootstrap method. Dynamic mode decomposition is used with EFD and CFD time histories to unveil the underlying ship dynamics and the motion correlation. Overall, CFD results are in agreement with EFD. Finally, a clustering approach is used to identify wave sequences causing large roll angles. As proof of concept, the characteristics of these sequences are used for the deterministic reconstruction of severe (large roll) and rare (capsize) events via regular wave computations.

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&ndash, Stokes (URANS) equations were coupled with a Menter&rsquo, s shear stress transport (SST) k-&omega, 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.

International audience; The aim of this study is to investigate the flow pattern behaviour by using computational fluid dynamic (CFD) approach. The square profile was chosen in purpose to have a better understanding of the behaviour which is relevant to the engineering applications. Numerical simulation was performed on various turbulence models with the range of Reynolds number from 6000 to 80000 with three incidence angles of 0°, 15°, and 30°. Mesh dependency study was performed with coarse, base and fine meshes. Fine mesh and standard k–ω were chosen as the best meshing and turbulence model to perform the simulation due to the capability in terms of less absolute error on aerodynamic coefficient and clear flow visualisation capture. It was found that the average values of Strouhal number for square profile was 0.12. For this particular study, the changes of incidence angle and variation of Reynolds number gave a significant flow pattern behind a square profile. The size of the vortices became smaller and closer to the structure as the incidence angle increased. At high Reynolds number, it was also observed that the size of the vortices increased progressively. The prediction of flow pattern behind square cylinder was successfully determined by using CFD approach.

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&ndash, 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&rsquo, 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.

In near-wall turbulence modeling it is necessary to resolve a thin boundary layer containing high gradients of the solution. An accurate enough resolution of such a layer can take most of the computational time. The situation becomes even worse for unsteady problems. To avoid time-consuming computations,in the present paper a new approach is developed, which is based on a non-overlapping domain decomposition. The boundary condition of Robin type at theinterface boundary between domains is constructed by transferring the original boundary condition from the wall. For the first time, recently developed unsteady interface boundary conditions of Robin type are used for the unsteady Reynolds Averaged Navier-Stokes equations. The interface boundary conditions contain a memory term which takes into account the nonlocal effect in time to be taken into account for essential unsteady problems. In the case of stationary solutions the new approach automatically reduces to the technique earlier developed for the steady problems. The considered test cases demonstrate that the effect of the memory term can be significant for the accuracy of the near-walldomain decomposition. The criteria for importance of the memory term in the interface boundary condition are formulated and confirmed for turbulent flows.

Close-packed contra-rotating vertical-axis turbines have potential advantages in wind and hydrokinetic power generation. This paper describes the development of a numerical model of a vertical axis turbine with a torque-controlled system using an actuator line model (ALM). The developed model, coupled with the open-source OpenFOAM computational fluid dynamics (CFD) code, is used to examine the characteristics of turbulent flow behind a single two-bladed vertical-axis turbine (VAT). The flow field containing the turbine is simulated by solving the unsteady Reynolds-averaged Navier-Stokes (URANS) equations with a k - ω shear stress transport (SST) turbulence model. The numerical model is validated against experimental measurements from a two-bladed H-type wind turbine. Turbine loading is predicted, and the vorticity distribution is investigated in the vicinity of the turbine. Satisfactory overall agreement is obtained between numerical predictions and measured data on thrust coefficients. The model captures important three-dimensional flow features that contribute to wake recovery behind a vertical-axis turbine, which will be useful for future studies of close-packed rotors with a large number of blades.

Damage stability is difficult to assess due to the complex hydrodynamic phenomena regarding interactions between fluid and structures. Therefore, a detailed analysis of the flooding progression and motion responses is important for improving ship safety. In this paper, numerical simulations are performed on the damaged DTMB 5415 ship at zero speed. All calculation are carried out using CD Adapco Star CCM + software, investigating the effect of damage openings on ship hydrodynamics, including the side damage and the bottom damage. The computational domain is modelled by the overset mesh and solved using the unsteady Reynold-average Navier-Stokes (URANS) solver. An implicit solver is used to find the field of all hydrodynamics unknown quantities, in conjunction with an iterative solver to solve each time step. The Volume of Fluid (VOF) method is applied to visualize the flooding process and capture the complex hydrodynamics behaviors. The simulation results indicated that two damage locations produce the characteristic flooding processes, and the motion responses corresponding to the hydrodynamic behaviors are different. Through comparative analysis, due to the difference between the horizontal impact on the longitudinal bulkhead and the vertical impact on the bottom plate, the bottom damage scenario always has a larger heel angle than the side damage scenario in the same period. However, the pitch motions are basically consistent. Generally, the visualization of the flooding process is efficient to explain the causes of the motion responses. Also, when the damage occurs, regardless of the bottom damage or the side damage, the excessive heel angle due to asymmetric flooding is often a threat to ship survivability with respect to the pitch angle.

A 3-D numerical method is developed to investigate the spatial distribution of surface fluxes over heterogeneous surfaces in (semi-)arid regions. Quantifying the effects of changes in the momentum, thermal and moisture roughness lengths on the airflow and fluxes in the ABL is important for water resources management and local climate studies. The governing equations and turbulence models are modified to include the effects of atmospheric stability conditions on the airflow. The turbulent airflow in ABL is simulated based on the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach to understand the air flow over the non-homogeneous surfaces from dry land through the water surface and vice versa. The model can be used to study airflow in neutral and non-neutral ABL over complex and non-homogeneous surfaces. The model results were used to investigate the flow parameters and (heat) flux variations over small water surfaces considering its surrounding conditions.

© 2017 The Author(s) 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, Cm, and power coefficient, CP, 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. ispartof: Energy Conversion and Management vol:156 pages:301-316 status: published

This paper presents the results of numerical and experimental investigations on the force coefficients and Strouhal numbers of circular segments considering different corner angles or chord to sagitta ratios. The research is motivated because these geometries are becoming increasingly popular in several engineering disciplines. The so-called D-section (semi-circular cylinder with a corner angle of $ 90^{\circ } $ ) has been experimentally studied in the past, since it is a galloping prone geometry. However, there is a lack of research for cases with different corner angles, and the numerical investigations related to this topic are particularly scarce. In this work, a 2D Unsteady Reynolds Averaged Navier–Stokes approach has been adopted aiming to study the circular segments at the sub-critical regime, considering corner angles from $ 40^{\circ } $ to $ 90^{\circ } $ , and the flow parallel to the rectilinear side. These sections were found to be particularly challenging since they present massive flow separation on the rectilinear side, alongside the inherent difficulties related to modeling the flow along curved surfaces at high Reynolds numbers. The impact of introducing low-Reynolds-number and curvature corrections in the $ k-\omega $ SST turbulence model and the performance of the Transition SST model have been extensively studied.

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 x1- and rotational x4- directions. The analysis produces the diagonal coefficients of the virtual mass m11, m44 together with the diagonal damping terms c11, c44. The cross-terms m14, m41 and c14, c41 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.

In this paper, we report the results of a numerical investigation on unsteady cavitating flows around a circular leading edge (CLE) hydrofoil. The objective of this study is to properly predict the appearance of cavitation pocket, its development and its detachment causing adverse effects on industrial systems such as microscopic plastic deformations at the solid walls. For this reason it is very important to study the influence of turbulence models on simulation results. We present a closing of the hydrodynamic equation system by a transport equation of an active scalar (volume fraction of the vapor phase) with a source terms. The Computational Fluid Dynamics (CFD) code used is ANSYS CFX. Before comparing the capability of the different turbulent models to predict unsteady behavior of cavitating flow along the hydrofoil, the study of the influence of the mesh resolution was performed in cavitating condition. This investigation was performed, on CLE hydrofoil, by monitoring the influence of for progressively finer meshes on the values of the drag CD and lift CL coefficients. Moreover, a study of the influence of the normal dimensionless distance to the wall (y+) was carried out on the hydrofoil surface. For the unsteady flow, a comparison of different turbulence models with the experiment leads to study the interaction of these models with the vapor pocket (detachment and collapse of vapor pocket). Two turbulence models were tested in this study: modified k-ε model and large eddy simulation (LES). In the present work, the predictions of velocity and pressure evolutions in the vicinity of the hydrofoil are compared to experimental data.

Scale-adaptive simulation (SAS) approach is employed to investigate the complex dynamic stall phenomena occurring on a wind turbine blade. The results are com-pared with the more popular less computationally-expensive unsteady Reynolds-averaged Navier-Stokes (URANS) approach where the latter is validated using three sets of experimental data. The comparison reveals that the two approaches have similar predictions of the instant of the formation/bursting/shedding of the laminar separation bubble (LSB) and dynamic stall vortex (DSV), the size of the LSB and aerodynamic loads during the upstroke. This is while the two approach-es exhibit dissimilar predictions of the trailing-edge vortex characteristics, its in-teraction with the DSV, number of secondary vortices and aerodynamic loads during the downstroke.

A comparative analysis of seven commonly-used eddy-viscosity turbulence models for CFD simulations of VAWTs is presented. The models include one- to four-equations, namely the Spalart-Allmaras (SA), RNG k-ε, realizable k-ε, SST k-ω, SST k-ω with an additional intermittency transition model (SSTI), k-kl-ω and transition SST (TSST) k-ω models. In addition, the inviscid modeling is included in the comparison. The evaluation is based on validation with three sets of experiments for three VAWTs with different geometrical characteristics operating in a wide range of operational conditions, from dynamic stall to optimal regime and to highly-rotational flow regime. The focus is on the turbine wake, the turbine power performance, and the blade aerodynamics. High-fidelity incompressible unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are employed. The extensive analysis reveals high sensitivity of the simulation results to the turbulence model. This is especially the case for the turbine power coefficient CP. The results show that the inviscid, SA, RNG k-ε, realizable k-ε and k-kl-ω models clearly fail in reproducing the aerodynamic performance of VAWTs. Only the SST model variants (SST k-ω, SSTI and TSST) are capable of exhibiting reasonable agreement with all the experimental data sets, where the transitional SST k-ω versions (SSTI and TSST) are recommended as the models of choice especially in the transitional flow regime.

The aerodynamic performance of bridge deck girders requires a thorough assessment and optimization in the design of long-span bridges. The present paper describes a numerical investigation of the aerodynamic characteristics of a twin-box bridge girder cross section in the range of angles of attack between −10.0° and +10.2°. The simulations are performed by solving 2D unsteady Reynolds-averaged Navier−Stokes (URANS) equations together with the k−ω shear stress transport (SST) turbulence model. The investigated Reynolds number (Re) based on the free stream velocity ( U ∞ ) and the height of the deck (D) is 31,000. The predicted aerodynamic characteristics such as the mean drag, lift and moment coefficients, are generally in good agreement with the results from the wind tunnel tests. Changes of flow patterns and aerodynamic forces with different angles of attack are investigated. Flow characteristics during one vortex shedding period are highlighted. Relative contributions of each of the two bridge decks to the overall drag and lift coefficients, with respect to the angle of attack, are also discussed.

Aerodynamics of the Darrieus wind turbine is an extremely complex issue requiring the use of very advanced numerical methods. Additional structural components of this device, such as, for example, a rotating shaft disturb the flow through the rotor significantly impairing its aerodynamic characteristics. The main purpose of the presented research is to validate the commonly-used unsteady Reynolds averaged Navier&ndash, Stokes (URANS) approach with the shear stress transport (SST) k-&omega, turbulence model based on the particle image velocimetry (PIV) studies of a two-bladed rotor operating at the moderate tip speed ratio of 4.5. In the present numerical studies, a two-dimensional turbine rotor with a diameter of 1 meter was considered. The following parameters were evaluated: instantaneous velocity fields, velocity profiles in the rotor shadow and aerodynamic blade loads. The obtained numerical results are comparable with the reference experimental results taken from the literature. The second purpose of this work was to examine the influence of the rotating rotor shaft/tower on the wind turbine performance. It has been proven that the cylindrical shaft reduces the power of the device by 2.5% in comparison with the non-shaft configuration.

© 2018 The Authors The current study systematically analyzes the impact of solidity (σ) and number of blades (n) on the aerodynamic performance of 2-, 3- and 4-bladed Darrieus H-type vertical axis wind turbines (VAWTs). Solidity varies within the wide range of 0.09–0.36. A large number of operational parameters, i.e., tip speed ratio (λ), Reynolds number (Re), turbulence intensity and reduced frequency (K) are investigated to provide a deeper insight into the impact of σ and n on the dynamic loads on blades, the turbine performance and the wake. High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) simulations, extensively validated with experiments, are employed. The results show that the turbine optimal tip speed ratio (λopt) is invariant to a newly-introduced parameter ‘σλ3’ regardless of the turbine geometrical and operational characteristics. In addition, a new correlation is derived to estimate λopt as a function of σ which can also be employed to predict the optimal σ for a turbine with a given λ. It is also found that: (i) for constant-speed urban VAWTs, which due to the low mean wind speed in the urban environment, frequently operate at moderate to high λ a relatively-low σ is optimal; (ii) an optimal VAWT is a moderately-high-solidity variable-speed rotor maintaining a relatively-low λ where due to the large blade chord length the resulting Re and K are favorably high; (iii) within the turbine optimal operational range, turbine power coefficient (CP) is almost independent of n. The present findings support the optimal aerodynamic design of small-to large-scale VAWTs. ispartof: ENERGY vol:165 pages:1129-1148 status: published

This study focuses on the formation and detachment of a leading edge vortex (LEV) appearing on an airfoil when its effective angle of attack is dynamically changed, inducing additional forces and moments on the airfoil. Experimental measurements of the time-resolved velocity field using Particle Image Velocimetry (PIV) are complemented by a computational study using an URANS (Unsteady Reynolds-Averaged Navier–Stokes) framework. In this framework a transition-sensitive Reynolds-stress model of turbulence, proposed by Maduta et al. (2018), which combines the near-wall Reynolds-Stress model by Jakirlic and Maduta (2015) and a phenomenological transition model governing the pre-turbulent kinetic energy by Walters and Cokljat (2008), is employed. Combined pitching and plunging kinematics of the investigated flat plate airfoil enable the effective inflow angle to be arbitrarily prescribed. A qualitative assessment of flow fields and a quantitative comparison of LEV characteristics in terms of its center position and circulation as well as an investigation of the mechanism causing the vortex to stop accumulating circulation revealed close agreement between the experimental and simulation results. Further considerations of the lift contribution from the pressure and suction side of the airfoil to the overall lift indicates that the qualitative lift evolution is reproduced even if the pressure side contribution is neglected. This reveals important characteristics of such airfoil dynamics, which can be exploited in future experimental studies, where direct aerodynamic force and moment measurements are greatly inhibited by dominating inertial forces.

Purpose– The purpose of this paper is to provide the current state of the art in the development of a computer code combining an immersed boundary method with a conjugate heat transfer (CHT) approach, including some new findings. In particular, various treatments of the fluid-solid-interface conditions are compared in order to determine the most accurate one. Most importantly, the method is capable of computing a challenging three dimensional compressible turbulent flow past an air cooled turbine vane.Design/methodology/approach– The unsteady Reynolds-averaged Navier–Stokes (URANS) equations are solved within the fluid domain, whereas the heat conduction equation is solved within the solid one, using the same spatial discretization and time-marching scheme. At the interface boundary, the temperatures and heat fluxes within the fluid and the solid are set to be equal using three different approximations.Findings– This work provides an accurate and efficient code for solving three dimensional CHT problems, such as the flow through an air cooled gas turbine cascade, using a coupled immersed boundary (IB) CHT methodology. A one-to-one comparison of three different interface-condition approximations has shown that the two multidimensional ones are slightly superior to the early treatment based on a single direction and that the one based on a least square reconstruction of the solution near the IB minimizes the oscillations caused by the Cartesian grid. This last reconstruction is then used to compute a compressible turbulent flow of industrial interest, namely, that through an air cooled gas turbine cascade. Another interesting finding is that the very promising approach based on wall functions does not combine favourably with the interface conditions for the temperature and the heat flux. Therefore, current and future work aims at developing and testing appropriate temperature wall functions, in order to further improve the accuracy – for a given grid – or the efficiency – for a given accuracy – of the proposed methodology.Originality/value– An accurate and efficient IB CHT method, using a state of the art URANS parallel solver, has been developed and tested. In particular, a detailed study has elucidated the influence of different interface treatments of the fluid-solid boundary upon the accuracy of the computations. Last but not least, the method has been applied with success to solve the well-known CHT problem of compressible turbulent flow past the C3X turbine guide vane.

Predicting the cavitation impact loads on a propeller surface using numerical tools is becoming essential, as the demand for more efficient designs, stretched to the limit, is increasing. One of the possible design limits is governed by cavitation erosion. The accuracy of estimating such loads, using a URANS approach, has been investigated. We follow the energy balance approach by (Schenke and van Terwisga, 2019), (Schenke et al., 2019), where we take account of the focusing of the potential energy into the collapse center before it is radiated as shock wave energy in the domain. In complex flows, satisfying the total energy balance, when reconstructing the radiated energy, has always been an issue in the past. Therefore, in this study, we investigate different considerations for the vapor reduction rate, in order to minimize the numerical errors, when estimating the local surface impact power. We show that when the vapor volume reduction rate is estimated using the mass transfer source term, then all the energy is conserved and the total energy balance is satisfied. The model is verified on a single cavitating bubble collapse, and it is further validated on a model propeller test case. The obtained surface impact distribution agrees well with the experimental paint test results, illustrating the potential for practical use of our fully conservative method to predict cavitation implosion loads on propeller blades.

The work focuses on the calibration of cavitation model parameters for the numerical simulation of three-phase injector flows. Cavitation is modeled through a transport equation for the void fraction closed by the Schnerr-Sauer relation. The vaporization and condensation factors contained in this model are considered for calibration against experimental data available for a test-case characterized by fuel injection in a reservoir filled of air through an axisymmetric channel. In spite of the simplified geometry, this flow configuration is representative of a real injector and contains most of the complex physical phenomena which may be encountered in injector flows, as turbulence, cavitation and hydraulic flip, i.e. a back flow of air from outside the injector along the whole length of the channel replacing the cavitating regions. Since a direct calibration would imply huge computational costs, not affordable in practical applications, response surfaces of the quantities of interest are built through generalized Polynomial Chaos. These response surfaces, which can be obtained starting from only a few deterministic simulations, have been then used to carry out the parameter calibration. The quantities of interest taken into consideration are the critical cavitation point (CCP), i.e. the value of the outlet pressure at which the flow inside the injector can be considered choked (for a fixed inlet pressure), and the mass-flow-rate (MFR) at the CCP. To further reduce the computational costs, calibration is carried out by using axisymmetric simulations. It has been then checked that the obtained cavitation model gives accurate results also in three-dimensional simulations of the actual geometry. Moreover, this set-up has been applied to two different complex one-hole injector geometries, i.e. a sector of real injector geometries, and the results have been compared against available experimental data. The calibrated cavitation model set-up appears to be robust, giving good predictions also in conditions significantly different from those in which it has been obtained.

In the current work a detailed investigation and a performance assessment of two eddy viscosity and two Reynolds stress turbulence models for modelling the transitional flow on a double circular arc (DCA) compressor blade is presented. The investigation is focused on the comparison of the obtained computational results with available experimental data for a specific DCA compressor blade cascade which can be found in the European Research Community on Flow, Turbulence and Combustion (ERCOFTAC) experimental database. The examined flow field is very challenging for the performance assessment of the turbulence models. The blade inlet angle departs +5&deg, from the compressor blade design conditions resulting in a complex flow field having large regions of boundary layer transition both on the suction and pressure sides of the blade with the presence of an unsteady wake. The presented results include velocity and turbulence intensity distributions along the pressure, the suction sides, and the wake region of the blade. From the comparison with the available experimental data, it is evident that in order to accurately compute such complex velocity and turbulence fields that are met in aero engine components (compressors and turbines), it is obligatory to use more advanced turbulence models with the Unsteady Reynolds Averaged Navier Stokes Equations (URANS) adoption, or other simulation and hybrid methodologies which require unsteady calculations.

International audience; Highlights• A numerical analysis on a C-class-like geometry is performed in unsteady wind conditions at different wind frequencies.• LES and URANS models agree in predicting the global aerodynamic coefficients as well as the flow structures of the wingsail.•Stall cells appear on the wingsail in case of unsteady wind conditions and their features depend on wind frequency.•The wind frequency, affecting the stall cells, leads to a modification of the aerodynamic coefficients of the wingsail.

[Abstract:] Flexible structures, such as cable-supported bridges, are prone to suffer from vortex-induced vibrations (VIV) under wind flow, as their span lengths are steadily growing in the last decades. VIV is a phenomenon that takes place at reduced wind speeds. The movements of the structure at VIV are self-limited and their frequency corresponds with the natural frequency of the structure (lock in). Therefore, VIV affects the structure’s serviceability and can cause fatigue related damage. Hence, the need for identifying and avoiding this phenomenon at the early design stages is a key issue in long-span bridges design. In the present study a rectangular cylinder of width to depth ratio 4:1, which is a common simplification of a bridge deck cross section, is analysed for the static case as well as undergoing free vibration in the vertical direction under wind flow. These analyses have been carried out by 2D URANS CFD simulations, adopting two different turbulence models: the k − ω SST, which is based upon the Boussinesq eddy-viscosity approximation, and the Reynolds Stress Model, which directly calculates the components of the specific Reynolds stresses. For the static case the force coefficients, Strouhal number and the pressure coefficient distributions were calculated and compared with the available experimental data. In the case of the free-to-oscillate 4:1 rectangular cylinder, the oscillation amplitudes are compared with wind tunnel data reported in the literature. In addition, the frequencies and phase-lags between the time-dependent lift coefficient and the vertical oscillations are studied. Ministerio de Economía y Competitividad; BIA2016-76656-R Ministerio de Economía y Competitividad; BES-2014-068418 Ministerio de Economía y Competitividad; BIA2013-41965-P Xunta de Galicia; ED431C 2017/72

Purpose The development of reusable space launchers requires a comprehensive knowledge of transonic flow effects on the launcher structure, such as buffet. Indeed, the mechanical integrity of the launcher can be compromised by shock wave/boundary layer interactions, that induce lateral forces responsible for plunging and pitching moments. Design/methodology/approach This paper aims to report numerical and experimental investigations on the aerodynamic and aeroelastic behavior of a diamond airfoil, designed for microsatellite-dedicated launchers, with a particular interest for the fluid/structure interaction during buffeting. Experimental investigations based on Schlieren visualizations are conducted in a transonic wind tunnel and are then compared with numerical predictions based on unsteady Reynolds averaged Navier–Stokes and large eddy simulation (LES) approaches. The effect of buffeting on the structure is finally studied by solving the equation of the dynamics. Findings Buffeting is both experimentally and numerically revealed. Experiments highlight 3D oscillations of the shock wave in the manner of a wind-flapping flag. LES computations identify a lambda-shaped shock wave foot width oscillations, which noticeably impact aerodynamic loads. At last, the experiments highlight the chaotic behavior of the shock wave as it shifts from an oscillatory periodic to an erratic 3D flapping state. Fluid structure computations show that the aerodynamic response of the airfoil tends to damp the structural vibrations and to mitigate the effect of buffeting. Originality/value While buffeting has been extensively studied for classical supercritical profiles, this study focuses on diamond airfoils. Moreover, a fluid structure computation has been conducted to point out the effect of buffeting.

International audience; 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.

CFD has become an essential tool for researchers to analyze centrifugal compressors. Tip leakage flow is usually considered one of the main mechanisms that dictate compressor flow field and stability. However, it is a common practice to rely on CAD tip clearance, even though the gap between blades and shroud changes when compressor is running. In this paper, sensitivity of centrifugal compressor flow field and noise prediction to tip clearance ratio is investigated. 3D CFD simulations are performed with three different tip clearance ratios in accordance to expected operating values, extracted from shaft motion measurements and FEM predictions of temperature and rotational deformation. Near-surge operating conditions are simulated with URANS and DES. DES shows superior performance for acoustic predictions. Cases with reduced tip clearance present higher pressure ratio and isentropic efficiency, but no significant changes in compressor acoustic signature are found when varying clearance. In this working point, tip clearance is immersed in a region of strongly swirling backflow. Therefore, tip leakage cannot establish any coherent noise source mechanism., The equipment used in this work has been partially supported by FEDER project funds "Dotacion de infraestructuras cientifico tecnicas para el Centro Integral de Mejora Energetica y Medioambiental de Sistemas de Transporte (CiMeT), (FEDER-ICTS-2012-06)", framed in the operational program of unique scientific and technical infrastructure of the Ministry of Science and Innovation of Spain. The authors wish to thank Mr. Pau Raga for his worthy assistance during the meshing process.

© 2017 The Authors The central shaft is an inseparable part of a vertical axis wind turbine (VAWT). For small turbines such as those typically used in urban environments, the shaft could operate in the subcritical regime, resulting in large drag and considerable aerodynamic power loss. The current study aims to (i) quantify the turbine power loss due to the presence of the shaft for different shaft-to-turbine diameter ratios δ from 0 to 16%, (ii) investigate the impact of different operational and geometrical parameters on the quantified power loss and (iii) evaluate the impact of the addition of surface roughness on turbine performance improvement. Unsteady Reynolds-averaged Navier-Stokes (URANS) calculations are performed on a high-resolution computational grid. The evaluation is based on validation with wind-tunnel measurements. The results show that the power loss increases asymptotically with increasing δ due to the higher width and length of the shaft wake as the blades pass through a larger region with lower velocity in the downwind area. A maximum power loss of 5.5% compared to the hypothetical case without shaft is observed for δ = 16%. The addition of surface roughness is shown to be an effective approach to shift the flow over the shaft into the critical regime, reducing the shaft drag and wake width as a result of a delay in separation. For an optimal dimensionless equivalent sand-grain roughness height of 0.08, the turbine power coefficient at δ = 4% improves by 1.7%, which is equivalent to a 69% recovery of the corresponding turbine power loss. The results are found to be virtually independent of the shaft-to-turbine rotational speed ratio. ispartof: Energy Conversion and Management vol:149 pages:616-630 status: published

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.

International audience; This paper investigates flows around a free surface piercing cylinder with Froude number F

Active flow control over the rudder shoulder of a vertical tail plane is applied to increase its side force coefficient. The rudder of the infinite span model is deflected. This leads without actuation to separated flow over the rudder. Several investigation are conducted. First a full-span constant blowing slot is used. With sufficient high jet velocity the flow on the rudder can be attached. By using in a next step several discrete slots, the required mass flow rate can be reduced for a similar increase in the side force coefficient. Using then pulsed blowing from the discrete slots is advantageous since the mass flow rate can be reduced. However, this is only true for quite small momentum coefficients. For cases with high side force coefficients requirements only the constant blowing approach is capable to achieve them, albeit at the correspondingly large mass flow rates and momentum coefficients.

Due to growing interest in wind energy harvesting offshore as well as in the urban environment, vertical axis wind turbines (VAWTs) have recently received renewed interest. Their omni-directional capability makes them a very interesting option for use with the frequently varying wind directions typically encountered in the built environment while their scalability and low installation costs make them highly suitable for offshore wind farms. However, they require further performance optimization to become competitive with horizontal axis wind turbines (HAWTs) as they currently have a lower power coefficient (CP). This can be attributed both to the complexity of the flow around VAWTs and the significantly smaller amount of research they have received. The pitch angle is a potential parameter to enhance the performance of VAWTs. The current study investigates the variations in loads and moments on the turbine as well as the experienced angle of attack, shed vorticity and boundary layer events (leading edge and trailing edge separation, laminar-to-turbulent transition) as a function of pitch angle using Computational Fluid Dynamics (CFD) calculations. Pitch angles of −7° to +3° are investigated using Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations while turbulence is modeled with the 4-equation transition SST model. The results show that a 6.6% increase in CP can be achieved using a pitch angle of −2° at a tip speed ratio of 4. Additionally, it is found that a change in pitch angle shifts instantaneous loads and moments between upwind and downwind halves of the turbine. The shift in instantaneous moment during the revolution for various pitch angles suggests that dynamic pitching might be a very promising approach for further performance optimization. ispartof: Applied Energy vol:197 pages:132-150 status: published

In the present work, an in-house UnSteady Double Wake Model (USDWM) is developed for simulating general flow problems behind bodies. The model is presented and used to simulate flows past a circular cylinder at subcritical, supercritical, and transcritical flows. The flow model is a two-dimensional panel method which uses the unsteady double wake technique to model flow separation and its dynamics. In the present work the separation location is obtained from experimental data and fixed in time. The highly unsteady flow field behind the cylinder is analyzed in detail. The results are compared with experiments and Unsteady Reynolds-Averaged Navier Stokes (URANS) simulations and show good agreement in terms of the vortex shedding characteristics, drag, and pressure coefficients for the different flow regimes.

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., The equipment used in this work has been partially supported by the Spanish Ministerio de Economia y Competitividad through grant no. TRA2012-36954 and by FEDER project funds "Dotacion de infraestructuras cientifico tecnicas para el Centro Integral de Mejora Energetica y Medioambiental de Sistemas de Transporte (CiMeT), (FEDER-ICTS-2012-06)" framed in the operational program of unique scientific and technical infrastructure of the Spanish Ministerio de Economia y Competitividad. Part of the computational resources used in this work have been provided by Super computing Center of Universitat Politecnica de Valencia and are thus gratefully acknowledged.

This paper presents numerical study of an oval-sail, a bluff-body equipped with a grid all along the span. Suction based flow control is applied to this body that is developed for wind assisted ship propulsion. First, a choice of numerical turbulence model is discussed through results of an oval-sail without suction. Three turbulence models are applied: the Ri j SSG, the Ri j EBRSM and the v2 f model. Then, computations are performed for the oval-sail fitted with suction grid. These last simu- lations are carried out with the low-Reynolds-number Ri j EBRSM turbulence model. The influence of the grid geometry on the oval-sail aerodynamic performances is highlighted. All simulations are carried out for the sail set at zero incidence. The Reynolds number based on the free stream velocity and the profile chord is Re = 5105. Results are compared to available experimental data.

[Abstract:] In the wind engineering field, the so-called D-section (semi-circular cylinder) has attracted some attention from researchers, since it is a galloping prone geometry. In fact, a modest number of references where this type of cross-section has been studied in the wind tunnel can be found, such as Novak and Tanaka (1974) or Weaver and Veljkovic (2005). However, to the authors’ knowledge, references in the literature concerning the CFD-based simulation of the aerodynamic response of circular segments are particularly scarce. In this work a 2D Unsteady Reynolds-Averaged Navier–Stokes (URANS) approach has been adopted with the purpose of computing force coefficients and Strouhal numbers of static circular segments at the subcritical regime considering corner angles of 90, 80, 70, 60, 50 and 40. Since the motivation of this work is to study circular segments as a simplified bridge deck geometry, the reference flow direction is parallel to the rectilinear side. It has been found that this kind of cross-section is particularly challenging since it presents massive flow separation on the rectilinear side, as well as the inherent difficulties in modeling the aerodynamic response on curved surfaces at high Reynolds numbers. For certain geometrical configurations low-Reynolds-number and curvature corrections in the k − ω Shear-Stress Transport (SST) turbulence model had to be introduced, as well as considering transition from laminar to turbulent flow, in order to obtain results similar to the experimental data. Ministerio de Economía y Competitividad; BIA2013-41965-P

International audience; Nowadays, unsteady RANS methods are the only techniques usable in the industry to design a control device based on a periodic excitation of the flow. This paper aims at evaluating the potential of such methods by comparing their performances with respect to LES computations taken as a reference. The chosen test case is a rounded backward facing step proposed by Dandois et al. [1]. The frequency response of this flow to the periodic excitation of a synthetic jet is computed both with LES and with four different URANS models. It is demonstrated that the optimal frequency identified by best URANS models is close to the one of LES. Nevertheless, the amplitude of separated bubble surface reduction is underestimated by URANS models.

HyperVapotrons are two-phase water-cooled heat exchangers able to receive high heat fluxes (HHF) by employing a cyclic phenomenon called the “Vapotron Effect”. HyperVapotrons have been used routinely in HHF nuclear fusion applications. A detailed experimental investigation on the effect giving rise to the ability to sustain steady state heat fluxes in excess of 10 MW/m2 has not yet been possible and hence the phenomenon is not yet well understood. The coolant flow structures that promote the effect have been a major point of interest, and many investigations based on computational fluid dynamic (CFD) simulations have been performed in the past. The understanding of the physics of the coolant flow inside the device may hold the key to further optimisation of engineering designs. However, past computational investigations have not been experimentally evaluated. Isothermal flow velocity distribution measurements of the fluid flow in HyperVapotron optical models with high spatial resolution are performed in this paper. The same measurements are subsequently calculated via commercial CFD software. The isothermal CFD calculation is compared to the experimental velocity measurements to deduce the accuracy of the CFD investigations carried out. This unique comparison between computational and experimental results in HyperVapotrons will direct future efforts in analysing similar devices.