Reynolds-averaged Navier-Stokes Approach Research Articles

CFD modeling of a self-oscillatory airflow regime in the test ventilated room with plane supply opening

The current contribution presents the results of numerical simulation of the airflow and heat transfer in a model room of rectangular shape ventilated with a single plane jet under the conditions of the experiment by Mataoui et al. (2001). The 2D and 3D unsteady Reynolds-averaged Navier-Stokes approaches were used. Numerical solutions were obtained with the CFD package ANSYS Fluent based on the finite volume method with the cell-centered variable arrangement. The position of the air supply nozzle corresponds to the self-oscillatory experimental conditions. The results of the computational data validation are reported. Effects of the low-frequency velocity oscillations on the heat transfer parameters are discussed.

Investigation of fine viscous flow fields in ship planar motion mechanism tests by DDES and RANS methods

The fine large separated flow is a focus in the naval architecture and ocean engineering. In the present study, the CFD solver, naoe-FOAM-SJTU, coupled with delayed detached-eddy simulation (DDES) and Reynolds Averaged Navier-Stokes (RANS) is adopted to simulate the fine viscous flow field in the ship planar motion mechanism (PMM) tests for the benchmark model Yupeng Ship. This paper compares the time histories of forces/moment acting on the hull and their oscillation frequency obtained by Fast Fourier Transform. The predicted results are in good agreement with the experimental data. By comparing the turbulent kinetic energy (TKE) and eddy viscosity coefficient calculated by both numerical methods, the results show that the TKE and eddy viscosity coefficient obtained by DDES method are much less than that achieved by RANS approach. The capacity of four vortex identification methods to capture vortex structures is analyzed in this paper, indicating that the third generation of vortex identification methods (ΩR and Liutex methods) are more suitable for analyzing the flow mechanism in the viscous large separated flow field. The axial Liutex and streamlines on the cutting planes are used to depict the flow around the hull.

Impact of natural ventilation on exposure to SARS-CoV 2 in indoor/semi-indoor terraces using CO2 concentrations as a proxy

Nowadays, it is necessary a better airborne transmission understanding of respiratory diseases in shared indoor and semi-indoor environments with natural ventilation in order to adopt effective people's health protection measures. The aim of this work is to evaluate the relative exposure to SARS-CoV 2 in a set of virtual scenarios representing enclosed and semi-enclosed terraces under different outdoor meteorological conditions. For this purpose, indoor CO2 concentration is used as a proxy for the risk assessment. Airflow and people exhaled CO2 in different scenarios are simulated through Computational Fluid Dynamics (CFD) modelling with Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach. Both spatial average concentrations and local concentrations are analyzed. In general, spatial average concentrations decrease as ventilation increases, however, depending on the people arrangement inside the terrace, spatial average concentrations and local concentrations can be very different. Therefore, for assessing the relative exposure to SARS-CoV 2 it is necessary to consider the indoor flow patterns between infectors and susceptibles. This research provides detailed information about CO2 dispersion in enclosed/semi-enclosed scenarios, which can be very useful for reducing the transmission risk through better natural ventilation designs and improving the classic risk models since it allows to check their hypotheses in real-world scenarios. Although CFD ventilation studies in indoor/semi-indoor environments have been already addressed in the literature, this research is focused on restaurant terraces, scenarios scarcely investigated. Likewise, one of the novelties of this study is to take into account the outdoor meteorological conditions to appropriately simulate natural ventilation.

Development of a Numerical Investigation Framework for Ground Vehicle Platooning

This paper presents a study on the flow dynamics involving vehicle interactions. In order to do so, this study first explores aerodynamic prediction capabilities of popular turbulence models used in computational fluid dynamics simulations involving tandem objects and thus, ultimately presents a framework for CFD simulations of ground vehicle platooning using a realistic vehicle model, DrivAer. Considering the availability of experimental data, the simulation methodology is first developed using a tandem arrangement of surface-mounted cubes which requires an understanding on the role of turbulence models and the impacts of the associated turbulence model closure coefficients on the prediction veracity. It was observed that the prediction accuracy of the SST k−ω turbulence model can be significantly improved through the use of a combination of modified values for the closure coefficients. Additionally, the initial validation studies reveal the inability of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach to resolve the far wake, and its frailty in simulating tandem body interactions. The Improved Delayed Detached Eddy Simulations (IDDES) approach can resolve the wakes with a reasonable accuracy. The validated simulation methodology is then applied to the fastback DrivAer model at different longitudinal spacing. The results show that, as the longitudinal spacing is reduced, the trailing car’s drag is increased while the leading car’s drag is decreased which supports prior explanations of vortex impingement as the reason for drag changes. Additionally, unlike the case of platooning involving Ahmed bodies, the trailing model drag does not return to an isolated state value at a two car-length separation. However, the impact of the resolution of the far wake of a detailed DrivAer model, and its implication on the CFD characterization of vehicle interaction aerodynamics need further investigations.

Analysis and Numerical Simulation of no Reacting Swirling Flow by Using URANS Approach

In this paper, we investigate the no-reacting swirling flow by using the numerical simulation based to the unsteady Reynolds-averaged Navier-Stokes approach. The numerical simulation was realized by using a computational fluid dynamics CFD code. The governing equations are solved by using the finite volume method with two classical models of turbulence K-epsilon and Shear Stress K-ω. The objective of this paper is therefore to evaluate the performance of the two models in predicting the recirculation zones in a swirled turbulent flow. The current models are validated by comparing the numerical results of the axial, radial and tangential velocities to the experimental data from literature.

On Determining the Critical Velocity in the Shot Sleeve of a High-Pressure Die Casting Machine Using Open Source CFD

This paper investigates the critical plunger velocity in high-pressure die casting during the slow phase of the piston motion and how it can be determined with computational fluid dynamics (CFD) in open source software. The melt-air system is modelled via an Eulerian volume-of-fluid approach, treating the air as a compressible perfect gas. The turbulence is treated via a Reynolds-averaged Navier Stokes (RANS) approach that uses the Menter SST k-ω model. Two different strategies for mesh motion are presented and compared against each other. The solver is validated via analytical models and empirical data. A method is then presented to determine the optimal velocity using a two-dimensional (2D) mesh. As a second step, it is then discussed how the results are in line with those obtained for an actual, industrially relevant, three-dimensional (3D) geometry that also includes the ingate system of the die.

CFD Evaluation of Pressure Change Along Coolant Passages in Sodium-Cooled Fast Reactor with Nek5000

To support the design efforts of advanced sodium-cooled fast reactors (SFRs), a series of computational fluid dynamics (CFD) simulations are performed to investigate the pressure change along various flow passages in the proposed SFR system. The simulations are carried out with the state-of-the-art spectral element flow solver, Nek5000. Two specific case studies are presented in this paper: the flow exiting the axial neutron reflector channels and the flow entering the fuel pin bundle. Due to the high Reynolds numbers expected, a Reynolds-averaged Navier-Stokes (RANS) approach is necessary to model the turbulence. A newly developed regularized RANS model is adopted in the related CFD calculations. The first case study explores the effect of Reynolds number on the pressure change when flow exits the reflector channels. The pressure change in this case has two major contributors: the change due to wall friction and the Bernoulli effect. It is noted that the nondimensional pressure loss follows a log-linear trend up to Re = 105, and then the trend is flattened. In the second case study, the advanced NekNek coupling capability is tested where an integral domain can be divided into multiple subdomains with coupling interfaces, which would greatly ease the meshing process of complex engineering geometries and potentially save computational resources. The preliminary results obtained so far confirm the consistency between the NekNek results and those produced by regular Nek5000 simulation. The presented work demonstrates the readiness and flexibility of the related CFD techniques, which is part of the broader effort to leverage cutting-edge CFD to inform the advanced nuclear reactor designs.

Simulations of PANDA and SPARC experiments on containment atmosphere mixing caused by vertical gas injection

Several experiments of interaction of air or steam jet with a horizontal helium rich layer in a closed vessel representing a NPP containment, which were performed in PANDA and SPARC experimental facilities, are simulated on the local instantaneous scale using the Unsteady Reynolds Averaged Navier Stokes approach. Axisymmetric two-dimensional and three-dimensional numerical models of cylindrical vessels are developed, and the effects of numerical domain reduction are studied. The mixing process is simulated as a single-phase flow with common momentum and energy equations. Consequently, an additional term accounting for enthalpy diffusion due to compositional changes is implemented into the energy equation. Included gas species mass fractions are considered as passive scalars and are calculated using the transport equation. However, the term describing the molecular diffusion cannot be neglected and had to be added to the diffusion equation implemented in the default OpenFOAM computational fluid dynamics code that was used. The k-ε turbulence model with additional turbulence generation and dissipation due to buoyancy is used for turbulence modelling. The comparison of simulation and experimental results shows that the improved physical model is able to successfully describe the phenomena in several experiments performed in different facilities with different initial and boundary conditions.

High fidelity simulations in support to assess and improve RANS for modeling turbulent heat transfer in liquid metals: The case of forced convection

This paper attempts to give a brief overview of the work conducted through some recent EU funded projects under the Euratom research for innovative nuclear systems (THINS, SESAME and MYRTE) concerning the use of high fidelity (HiFi) simulations, namely Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES), to adapt Reynolds Averaged Navier-Stokes (RANS) models for the computation of turbulent heat transfer in liquid metal flows.Here the focus is on forced convection only, that prevails in normal reactor operation, through some selected cases performed at UCLouvain, e.g. the channel flow and the impinging jet. The considered RANS approaches are those based on the simple gradient diffusion hypothesis or SGDH, and those based on the algebraic heat flux formulation (AHFM). Among the AHFM models the two possible formulations are assessed, i.e. the explicit form (k-∊-kθ-∊θ of Manservisi and Menghini (2014) still based on the eddy diffusivity concept (gradient diffusion assumption), and the implicit form of the AHFM-NRG which is essentially a recalibration of the reference model of Kenjeres et al. (2005). The Results show the overall superiority of AHFM models, although SGDH-models using the Kay correlation for the turbulent Prandtl number and the dedicated thermal wall-function developed by Duponcheel et al. (2014) provide reasonable results at a much lower effort making them interesting for industrial applications. However further research is undoubtedly required to come closer to more universal models working for a wide range of Prandtl numbers and flow conditions.

Investigations of improved cooling effectiveness for ramp film cooling with compound angle film cooling jets

PurposeThis paper aims to investigate the film cooling effectiveness (FCE) and mixing flow characteristics of the flat surface ramp model integrated with a compound angled film cooling jet.Design/methodology/approachThree-dimensional numerical simulation is performed on a flat surface ramp model with Reynolds Averaged Navier-Stokes approach using a finite volume solver. The tested model has a fixed ramp angle of 24° and a ramp width of two times the diameter of the film cooling hole. The coolant air is injected at 30° along the freestream direction. Three different film hole compound angles oriented to freestream direction at 0°, 90° and 180° were investigated for their performance on-ramp film cooling. The tested blowing ratios (BRs) are in the range of 0.9–2.0.FindingsThe film hole oriented at a compound angle of 180° has improved the area-averaged FCE on the ramp test surface by 86.74% at a mid-BR of 1.4% and 318.75% at higher BRs of 2.0. The 180° film hole compound angle has also produced higher local and spanwise averaged FCE on the ramp test surface.Originality/valueAccording to the authors’ knowledge, this study is the first of its kind to investigate the ramp film cooling with a compound angle film cooling hole. The improved ramp model with a 180° film hole compound angle can be effectively applied for the end-wall surfaces of gas turbine film cooling.

Numerical investigation of skimming flow characteristics over non-uniform gabion-stepped spillways

Stepped spillways can be defined as human-made hydraulic structures constructed to monitor and control flow release and attain high energy dissipation. Such spillways are commonly used in embankment dams; however, it is important that a sufficient chute length is provided to develop the required aerated flow; the point at which this occurs is known as the inception point. This paper focusses on the influence of the non-uniform geometry of gabion-stepped spillways (GSS) on the inception point. A numerical investigation using the Reynolds-averaged Navier Stokes (RANS) approach with the software NEWFLUME was adopted to examine the flow over the GSS. The inception point location suggested by the numerical models was compared to the location predicated in the existing formulae available in the literature. The data from the model was then used to generate two novel empirical equations. The equations were based on nonlinear multiple regression (NMR) and evolutionary polynomial regression (EPR) approaches to deliver improved results for non-uniform gabion-stepped spillways. The developed EPR correlation, respectively, scored R2 (determination coefficient) and MAE (mean absolute error) values of 0.93 and 1.66 for the training data and 0.83 and 2.7 for the testing data. In the NMR approach, a reduced R2 value of 0.91 was obtained. The outcomes of this study revealed that the numerical model proposed was able to capture the flow characteristics over the GSS accurately. Additionally, the empirical equations developed in the current investigation yielded better predictions of the inception point location compared to the existing equations. Consequently, the findings of this study can be used to improve the future design of GSS.

Full configuration drag estimation of short-to-medium range fixed-wing UAVs and its impact on initial sizing optimization

The paper presents the derivation of a new equivalent skin friction coefficient for estimating the parasitic drag of short-to-medium range fixed-wing unmanned aircraft. The new coefficient is derived from an aerodynamic analysis of ten different unmanned aircraft used for surveillance, reconnaissance, and search and rescue missions. The aircraft is simulated using a validated unsteady Reynolds-averaged Navier Stokes approach. The UAV’s parasitic drag is significantly influenced by the presence of miscellaneous components like fixed landing gears or electro-optical sensor turrets. These components are responsible for almost half of an unmanned aircraft’s total parasitic drag. The new equivalent skin friction coefficient accounts for these effects and is significantly higher compared to other aircraft categories. It is used to initially size an unmanned aircraft for a typical reconnaissance mission. The improved parasitic drag estimation yields a much heavier unmanned aircraft when compared to the sizing results using available drag data of manned aircraft.

Computational Evaluation of Shock Wave Interaction with a Cylindrical Water Column

Computational fluid dynamics was employed to predict the early stages of the aerodynamic breakup of a cylindrical water column, due to the impact of a traveling plane shock wave. The unsteady Reynolds-averaged Navier–Stokes approach was used to simulate the mean turbulent flow in a virtual shock tube device. The compressible flow governing equations were solved by means of a finite volume-based numerical method, where the volume of fluid technique was employed to track the air–water interface on the fixed numerical mesh. The present computational modeling approach for industrial gas dynamics applications was verified by making a comparison with reference experimental and numerical results for the same flow configuration. The engineering analysis of the shock–column interaction was performed in the shear-stripping regime, where an acceptably accurate prediction of the interface deformation was achieved. Both column flattening and sheet shearing at the column equator were correctly reproduced, along with the water body drift.

Oil droplets dispersion under a deep-water plunging breaker: Experimental measurements and numerical modeling

Abstract (1141370) Wave tank experiments were performed to measure the droplets size distribution under the plunging breaking wave. A deep-water plunging breaker of height 20 cm was generated using the dispersive focusing method, and a shadowgraph camera was used to take images of droplets and bubbles of different sizes. For droplets smaller than the 1000 microns, the number-based DSD matched the DS correlation (Delvigne and Sweeney 1988), which gives N(d) ~ d−2.3, but N(d) ~ d−9.7 for diameters larger than 1000 microns. A numerical method was designed to study the oil dispersion under breaking waves by coupling the computational fluid dynamic (CFD) with the Lagrangian particle tracking code (NEMO3D) and population balance model (VDROP). The wave hydrodynamics was reproduced using the Reynolds-averaged Navier Stokes approach within a commercial CFD code ANSYS Fluent. The obtained wave hydrodynamics was then used as inputs for the NEM3D code and VDROP model. The numerical results show reasonable agreement with our experimental observation. The approach adopted to produce the DSD reduces the empiricism of the DS correlation, as the approach uses oil properties and measurable wave properties. The proposed numerical method was ready to be used in other scenarios of oil spills (i.e., oil jets in deep oceans and oil dispersion in riverine systems). It could also be potentially used in large scale forecast and hindcast simulations for oil spill response and research.

Data assimilation for turbulent mean flow and scalar fields with anisotropic formulation

This work presents an anisotropic formulation for data assimilation (DA) of turbulent flows using the continuous adjoint system. This DA scheme serves as a tool to complement flow measurement data (the observations), which are usually limited in spatial range and measurable quantities, with the help of a predictive model in the framework of the Reynolds-averaged Navier–Stokes approach (the estimator). Herein, the measured data profiles at several locations are used as observations. In the estimator, a forcing term $${\varvec{F}}$$ is added to the momentum equation to compensate for the contribution of the anisotropic eddy viscosity, whilst the isotropic part of the eddy viscosity is determined by conventional turbulence models. $${\varvec{F}}$$ is optimised using the continuous adjoint equations, driving the predicted flow quantities towards the observations. Similar treatments are conducted for the scalar prediction. Three test cases are used for assessment and validation of the present DA scheme. The results of a circular jet demonstrate that the mean flow and scalar fields can be perfectly reproduced from the observations due to the $${\varvec{F}}$$ optimisation. The flow over a blunt plate demonstrates the ability of the present DA scheme to reconstruct global flow fields from several measured velocity profiles. The flow and heat transfer on a ribbed wall reveal that once the global flow field is well recovered, the wall heat transfer characteristics can be correctly determined with a limited number of observations. The present anisotropic DA approach is quite practical for complex flow applications, serving as a complement to our previous isotropic adjoint-based data assimilation for further DA work.

Effect of in-service burnout effect on the transonic leakage flows over cavity tip model

Increasing the gas temperature at the inlet to the high pressure turbine of gas turbine engines is known as a proven method to increase the efficiency of these engines. However, this will expose the blades’ surface to very high heat load and thermal damages. In the case of the un-shrouded turbine blades, the blade tip will be exposed to a significant thermal load due to the developed leakage flows in the tip gap, this leads to in-service burnout which degrades the blade tip and shortens its operational life. This paper studies the in-service burnout effect of the transonic tip flows over a cavity tip which is a configuration commonly used to reduce the tip leakage flows. This investigation is carried out experimentally within a transonic wind tunnel and computationally using steady and unsteady Reynolds Averaged Navier Stokes approaches. Various flow measurements are established and different flow behaviour including separation bubbles, shockwave development and distinct flow interactions are captured and discussed. It is found that when the tip is exposed to the in-service burnout, leakage flow behaves in a significantly different way. In addition, the effective tip gap becomes much larger and allows higher leakage mass flow rate in comparison to the sharp-edge tip (i.e. a tip at the beginning of its operational life). The tip leakage losses are found much higher for the round-edge cavity tip (i.e. a tip exposed to burn-out effect). Experimental and computational flow visualisations, surface pressure measurements and discharge coefficient variation are given and analysed for several pressure ratios across the tip gap.

Validation of a three-phase Eulerian CFD model to account for cavitation and spray atomization phenomena

Cavitation phase change phenomenon appears in many engineering applications, often eroding and damaging surfaces, so deteriorating the performance of devices. Therefore, it is a phenomenon of great interest for the research and industry communities. In this work, three different cavitation models, the Homogeneous Relaxation Model (HRM), the Schnerr and Sauer, and the Kunz, are implemented in a Eulerian multiphase homogeneous flow Computational Fluid Dynamics (CFD) solver previously developed for simulating fully atomized sprays. The improved solver can be used then to study not only cases with cavitation, such a hydrofoil, but also situations where cavitation occurs together with liquid atomization, such as high pressure injection systems. Validation of this solver is carried out for three different cases under diverse operating conditions: a two-dimmensional throttle, a hydrofoil and a single-hole fuel injector. The Reynolds-Averaged Navier-Stokes (RANS) approach is employed for taking into account the turbulence effects. Simulation results are compared to experimental data available in the literature. Among the tested cavitation models, the HRM is the one that provides the best accuracy in the three validation cases. Nevertheless, the onset of cavitation and the area occupied by the vapor cavities are always underpredicted, by all cavitation models in all validation cases. This can be associated to the unsteady and turbulent nature of the cavitation phenomenon. Even so, the computational prediction of several parameters, such as mass flow rate through the nozzles or spray spreading angle, has an error below 5-10%, which proves the capabilities of the solver.

Aerodynamic and aeroacoustic performance investigations on modified H-rotor Darrieus wind turbine

The present paper investigates the aerodynamic and aeroacoustic characteristics of the H-rotor Darrieus vertical axis wind turbine (VAWT) combined with very promising energy conversion and steering technology; a fixed guide-vanes. The main scope of the current work is to enhance the aerodynamic performance and assess the noise production accomplished with such enhancement. The studies are carried out in two phases; the first phase is a parametric 2D CFD simulation employing the unsteady Reynolds-averaged Navier-Stokes (URANS) approach to optimize the design parameters of the guide-vanes. The second phase is a 3D CFD simulation of the full turbine using a higher-order numerical scheme and a hybrid RANS/LES (DDES) method. The guide-vanes show a superior power augmentation, about 42% increase in the power coefficient at λ = 2.75, with a slightly noisy operation and completely change the signal directivity. A remarkable difference in power coefficient is observed between 2D and 3D models at the high-speed ratios stems from the 3D effect. As a result, a 3D simulation of the capped Darrieus turbine is carried out, and then a noise assessment of such configuration is assessed. The results show a 20% increase in power coefficient by using the cap, without significant change in the noise signal.

Analysis and optimization of the curved trapezoidal winglet geometry in a high-efficiency compact heat exchanger

The present work is undertaken to optimize the geometry of the curved trapezoidal winglet mounted over each cooling tube to enhance the overall performance of the heat exchanger. Before conducting the optimization study, the most critical geometric entities have been identified, and this has helped in significantly reducing the computational overheads. It has been found that the three most critical design parameters are the arc radius (R), the angle subtended (θ) and the winglet's leading-edge height (h1), and the same are chosen for optimization. To plan the experiment, we make use of the Latin hypercube sampling (LHS) scheme. The system responses viz. the Colburn factor (j) and the friction factor (f) ‒ for different combinations of the independent variables ‒ are computed using the numerical simulations together with the conjugate heat transfer approach. To get the best outcome from computations, both fluid and solid domains have been discretized using hexahedra. We make use of the Reynolds-averaged Navier-Stokes (RANS) approach ‒ with SST k–ω (a 2-equation) turbulence model as a closure to RANS ‒ for evaluating the dependent variables. The artificial neural network (ANN) is trained using the data from the DoE table to build the metamodel necessary for conducting the multiobjective optimization. The genetic algorithm (GA) is employed to generate the optimized data sets. The study reveals that the optimized HE variants outperform the baseline model not only at the on-design condition but also at the off-design conditions. The resulting Pareto front points reveal very interesting results, and these data would facilitate the designers to make choices over a wide range of the VG geometry of the heat exchanger.

Turbulent flows over real heterogeneous urban surfaces: Wind tunnel experiments and Reynolds-averaged Navier-Stokes simulations

Wind tunnel experiment and steady-state Reynolds-averaged Navier-Stokes (RANS) approaches are used to examine the urban boundary layer (UBL) development above Kowloon Peninsula, Hong Kong Special Administrative Region (HKSAR). The detailed urban morphology is resolved by computational fluid dynamics (CFD) and is fabricated by 3D-printing (reduced scale) for wind tunnel experiments. Different from the majority existing results based on idealized, homogeneous urban geometries, it was found that the wind and turbulence in the UBL over downtown Kowloon are characterized by the wake behind several high-rise buildings. In particular, local maxima of turbulence kinetic energy (TKE) and shear stress are found at the roof level of those high-rise buildings. In the downstream region where the flows are already adjusted to the urban surfaces, the urban roughness sublayer (URSL) can be further divided into two layers based on the structures of the mixing length lm, effective drag Dx and dispersive stress. In the lower URSL (z ≤ 100 m), lm is rather uniform, and the Reynolds stress and dispersive stress are comparable. In the upper URSL (100 m ≤ z ≤ 300 m), on the contrary, lm is peaked at the mid-height and the magnitude of dispersive stress is smaller than that of the Reynolds stress (< 30%). The effective drag Dx is negligible in the upper URSL.