Reynolds-averaged Navier-Stokes Turbulence Models Research Articles

Four dimensional wave propagation simulation in liquid metal with realistic thermal fluctuation

In this paper, we performed a four-dimensional numerical simulation to calculate wave propagation in a thermal fluctuating liquid metal in order to obtain detailed knowledge on a wave propagation in coolant material of a Sodium-cooled Fast reactor (SFR). The wave and the medium are described in three spatial and one temporal dimensions. We made use of a massive data set to describe the fluctuating situation of the medium. This data set was provided by Computational Fluid Dynamics (CFD) with a Large-Eddy Simulation (LES) model, which calculated the temperature field with a higher spatial resolution than Reynolds-Averaged Navier–Stokes turbulence models (RANS). This data set was furthermore obtained from other studies on a numerical and physical experiment called PLAJEST that created mixing jets of liquid metal in order to simulate the status of running SFRs. Because of the limitation of computational resources, previous acoustic studies applied to such a medium could only use the spatial–temporally averaged fluctuating heterogeneity of a medium calculated by RANS turbulence model. This limitation may overlook wave fluctuation because of the difference of the resolution between computational fluid dynamics and acoustic wave length. Our numerical effort allowed us to study the most realistic acoustic wave propagation in liquid metal than in any former studies. A new important result was obtained in this work as we demonstrated that ultrasonic measurements could follow thermal-hydraulic fluctuations in an opaque liquid with high sensitivity. This result was obtained through the definition of descriptors to analyze medium fluctuations along the wave path. We defined a very new measurement index, called hereafter Cumulated Temperature Fluctuation Intensity (CTFI), to correlate the variations in the thermal-hydraulic conditions to the wave variations. We demonstrated a good correlation between the second derivative of this index and the second derivative of several acoustic measurements, then we discussed the easiest measurements to be used in practice in an industrial setup.

Numerical study of supercritical octane flows with multicomponent effects by pyrolysis

This study focuses on supercritical octane flow simulation with pyrolysis reactions in a heated circular pipe to determine the multicomponent effects of changes in thermophysical properties. The reaction rate of n-octane for pyrolysis was estimated by a zeroth-dimensional simulation of hydrocarbon pyrolysis. The resulting mole fractions of the decomposed components were compared with the experimental data. The mole fraction results were then used to develop three thermophysical property models for the mixture of unreacted n-octane and decomposed hydrocarbons, i.e., 1-, 4-, and 12-component models. In the supercritical octane flow simulations, the multicomponent thermophysical properties of each component model were determined using the polynomial equations in the Reference Fluid Thermodynamic and Transport Properties Database. The conventional k−ω SST model and k−ωSST+Mτ model served as the Reynolds-averaged Navier–Stokes turbulence models. In cases with a low wall temperature, the outlet temperatures in all component models were consistent with the experimental data, in which pyrolysis did not occur. In cases with a high wall temperature above 973 K, the density change and production of kinetic energy affected the turbulent thermal diffusivity in the heated pipe. In cases where the multicomponent effects of thermophysical properties and turbulence were considered, the numerical method reproduced a reasonable outlet temperature and conversion rate, whereas the experimental results were underestimated in all the other cases. These numerical results indicate that the accurate prediction of density changes and the production of turbulent kinetic energy are the key issues in reproducing thermal fluid flows of supercritical hydrocarbons with pyrolysis reactions in uniform wall temperature cases.

Computational fluid dynamics analysis of buoyancy-aided turbulent mixed convection inside a heated vertical rectangular duct

Heat transfer in buoyancy-aided turbulent mixed convection inside a vertical rectangular duct with air was investigated through steady-state computational fluid dynamics (CFD) analysis with Reynolds-averaged Navier–Stokes turbulence models. Heat transfer can occur in the passive safety system of a nuclear reactor, which uses natural circulation as an initial driving force owing to its relatively low flow rate and strong heat flux condition. One example is the reactor cavity cooling system in a very high temperature gas-cooled nuclear reactor, whose high operating temperature enables high-efficiency power generation and hydrogen production. In buoyancy-aided turbulent mixed convection, heat transfer can deteriorate under specific conditions. Therefore, heat transfer must be analyzed because its deterioration can affect the performance of the safety system, which has an influence on the safety of the nuclear reactor. In this study, the results of a CFD analysis were compared with experimental results obtained from the riser heat transfer experimental facility, which is an experimental heat transfer facility with a 4.0 vertical rectangular duct test section. The CFD analyses were performed using a realizable k–ε model and a v2–f model, and the results were compared with the experimental results. It was confirmed that the results of the v2–f model exhibited good agreement with the experimental results. Based on this fact, four additional idealized cases were calculated using the v2–f model to investigate heat transfer deterioration in the turbulent mixed-convection regime. The calculation results revealed that the gradient of the velocity profile from the wall toward the center was reduced outside the near-wall region owing to buoyancy, which led to a reduction in the shear stress, turbulence generation, and heat transfer. In addition, the velocity gradient beyond the near-wall region reached a negative value above a certain buoyancy number, and turbulence generation and heat transfer started to recover. This analysis of heat transfer deterioration can provide insight into the construction of a heat-transfer coefficient correlation for buoyancy-aided turbulent mixed convection, and it will contribute to the development of heat transfer predictions and the evaluation of safety systems in nuclear reactors.

Evaluation of wall functions for RANS turbulence models by unsteady lid-driven cavity flow

Turbulence models do not perform well near walls because the Reynolds number in this interesting area is low. The wall functions and the advantage method can be efficiently used to model fluid flow at the near wall region. The Reynolds averaged Navier-Stokes turbulence models which had the developed wall functions in Open FOAM, the open source CFD software, were evaluated by the turbulent flow. The 3-D lid-driven cavity flow at the turbulence Reynolds number of 10,000 with various spanwise aspect ratio had been referred to validate these turbulence models. The comparison shown that the realizable k-ɛ turbulence model with the standard k and ɛ wall functions of OpenFOAM was the most powerful model to represent the velocity profile of water flow in the 3-D lid-driven cavities.

Aerodynamic Response and Running Posture Analysis When the Train Passes a Crosswind Region on a Bridge

Trains running on a bridge face more significant safety risks. Based on the Unsteady Reynolds-Averaged Navier–Stokes turbulence model, a three-dimensional Computational Fluid Dynamics computational model of the train–bridge–wind barrier was proposed in this study to measure the transient aerodynamic load of the train. The transient aerodynamic load was input into the wind–train–bridge coupling dynamic system to perform dynamic analysis of running safety. Significant fluctuations in the aerodynamic coefficients were found when the train entered and exited the wind barrier due to the dramatic change in flow pattern. The maximum value of the derailment coefficient decreased with the height of wind barriers, which hardly affected the wheel load reduction rate. The 2 m high wind barrier had no evident influence on the running posture of a general high-speed train, while the 4 m high wind barrier was proven to have better protection. Over-protection was found with an even higher wind barrier.

Implied models approach for turbulence model form physics-based uncertainty quantification

A new physics-based uncertainty quantification methodology is developed for understanding model form uncertainty. This framework provides insights into the reasons and conditions under which model assumptions are wrong and is used to study model form uncertainty in two-equation Reynolds-Averaged Navier-Stokes turbulence models in wall-bounded flows. Through this analysis, the shortcomings of these models are understood with an emphasis on the role of anisotropy. Error cancellation is shown to largely benefit two-equation turbulence models in wall-bounded flows, leading to models that provide correct predictions but lack physical correctness.

Modeling of combined effects of surface roughness and blowing for Reynolds-averaged Navier–Stokes turbulence models

A new modeling strategy adapted to Reynolds-averaged Navier–Stokes turbulence models is proposed to predict combined effects of roughness and blowing boundary conditions. First, an analysis of experimental data is presented, leading to a specific description of the velocity profile in the logarithmic region of transpired turbulent boundary layers over rough walls. This analysis points out the deficiencies of existing roughness corrections to predict the effect of blowing in the presence of surface roughness. Indeed, these corrections tend to underestimate skin friction coefficients and Stanton numbers with the addition of blowing. The failure of existing models derives from an inaccurate estimation of the velocity shift of the logarithmic law given by roughness corrections. Concretely, roughness corrections underestimate the apparent velocity shift of the logarithmic law with blowing. To recover the expected law of the wall, an additional contribution on the velocity shift, characterizing blowing/roughness interactions, is integrated to standard roughness corrections. To that end, a modification of the equivalent sand grain height, adapted to k−ω based turbulence models, is proposed to take blowing effects into account. Furthermore, an extension of Aupoix's thermal correction [B. Aupoix, Int. J. Heat Fluid Flow 56, 160–171 (2015)] to blowing is presented to predict combined thermal effects of roughness and blowing. The assessment of the proposed corrections is performed using k−ω shear stress transport model on a large set of experimental data and proves the relevance of the strategy for incompressible and compressible turbulent boundary layers.

난류 모델에 따른 버터플라이 밸브 주위 유동 해석 결과 비교 연구

Nuclear Power Plant operators regularly check the performance of pumps and valves that perform safety functions through In-Service Test(IST), and monitor and evaluate the degree of vulnerability over time. Butterfly valve is one of the main types of valves for IST. Understanding of flow characteristics of butterfly valve is helpful in preventing the safety of the IST-related system from being impaired due to a transient phenomenon unintentionally caused by opening and closing of the butterfly valve. In this study, we tried to analyze the flow characteristics of butterfly valve and effect of turbulence model. Three different Reynolds averaged Navier-Stokes turbulence models available in ANSYS Fluent R19, i.e., κ-ε, κ-ε (RNG) and κ-ω (SST) model were used for the calculation and flow characteristics were analyzed by varying the disk angle from 10° to 70°.

Numerical Analysis of Turbulent Heat Transfer in the Case of Minijets Array

The presented numerical investigations show an analysis of the turbulent single-phase array of ten minijets impinging a heated surface, which lead to the intensification of heat transfer between the fluid and the surface. Attention was devoted to the comparison between phenomena occurring for the heated flat and concave surface geometry. The selection of the shapes was based on the impinging jets applications. From the numerical point of view, the focus was placed on a comparison of the Reynolds Averaged Navier–Stokes (RANS) turbulence model implementations in ANSYS Fluent software, and their impact on the modeling precision of the thermal and hydrodynamic boundary layers phenomena. The 3D numerical model was based on the continuity, momentum, and energy transport equations, together with three various RANS turbulence models: k-ω SST Kato-Launder, k-ε RNG Kato-Launder, and Intermittency Transition. The water submerged minijets, characterized by three various values of Reynolds number, were considered. Average surface Nusselt number values for all analyzed cases were compared with the experimental correlations and exhibited the same tendency but differed in detail. Numerically obtained average Nusselt number values agreed with the results of two from three correlations in the range of 10–20%. The flat surface was characterized by higher heat transfer than the concave one and an influence of the cross flow, changing the symmetrical distribution of the Nusselt number, was more visible for it. A cross flow impact was found in fuzzy hexagonal or tetragonal symmetry of this distribution. Additionally, the areas of high temperature gradient values were identified in the region of the strongest jets’ interactions, which can be important for mechanical strength analysis.

Effects of Aspect Ratio on Heat Transfer and Pressure Drop in Stationary and Rotating Two-Pass Rectangular Channels with Smooth Wall

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Applications of An Eddy-Viscosity Eliminator Based on Sigmoid Functions in Reynolds-Averaged Navier-Stokes Simulations of Sloshing Flow

Reynolds-averaged Navier-Stokes (RANS) turbulence modeling can lead to the excessive turbulence level around the interface in two-phase flow, which causes the unphysical motion of the interface in sloshing simulation. In order to avoid the unphysical motion of the interface, a novel eddy-viscosity eliminator based on sigmoid functions is designed to reduce the excessive turbulence level, and the eddy-viscosity eliminator based on polynomials is extracted from the cavitation simulations. Surface elevations by combining the eddy-viscosity eliminators and classical two-equation closure models are compared with the experiments, the ones by using the adaptive asymptotic model (AAM) and the ones by using the modified two-equation closure models. The root-mean-squared error (RMSE) is introduced to quantify the accuracies of surface elevations and the forces. The relation between the turbulence level in the transition layer and RMSEs of surface elevations is studied. Besides, the parametric analysis of the eddy-viscosity eliminators is carried out. The studies suggest that (1) the excessive turbulence level in the transition layer around the interface has a significant influence on the accuracies of surface elevations and the forces; (2) the eddy-viscosity eliminators can effectively reduce the excessive turbulence level in the transition layer to avoid the unphysical motion of the interface; (3) the k − ω SST model combined with the eddy-viscosity eliminators is appropriate for predicting surface elevations and forces in RANS simulations of sloshing flow.

Optimization of a Small Wind Turbine for a Rural Area: A Case Study of Deniliquin, New South Wales, Australia

The performance of a wind turbine is affected by wind conditions and blade shape. This study aimed to optimize the performance of a 20 kW horizontal-axis wind turbine (HAWT) under local wind conditions at Deniliquin, New South Wales, Australia. Ansys Fluent (version 18.2, Canonsburg, PA, USA) was used to investigate the aerodynamic performance of the HAWT. The effects of four Reynolds-averaged Navier–Stokes turbulence models on predicting the flows under separation condition were examined. The transition SST model had the best agreement with the NREL CER data. Then, the aerodynamic shape of the rotor was optimized to maximize the annual energy production (AEP) in the Deniliquin region. Statistical wind analysis was applied to define the Weibull function and scale parameters which were 2.096 and 5.042 m/s, respectively. The HARP_Opt (National Renewable Energy Laboratory, Golden, CO, USA) was enhanced with design variables concerning the shape of the blade, rated rotational speed, and pitch angle. The pitch angle remained at 0° while the rising wind speed improved rotor speed to 148.4482 rpm at rated speed. This optimization improved the AEP rate by 9.068% when compared to the original NREL design.

CFD Verification and Validation of Wire-Wrapped Pin Assemblies

TerraPower participated in a cooperative project among industry, a national laboratory, and a university to perform verification and validation of computational fluid dynamics (CFD) methods for predicting the flow and heat transfer within fuel assemblies with hexagonally packed wire-wrapped fuel pins. This project consisted of both experimental and numerical components and used surrogate fluids and electrically heated fuel pins to substitute for liquid metal and nuclear fuel. TerraPower performed CFD simulations of the experiments using industrial-level Reynolds-averaged Navier-Stokes (RANS) turbulence modeling. These simulations of helically wire-wrapped fuel assemblies employed meshes of bare pins without the wire-wrap geometry explicitly modeled. Instead, the effect of the wire-wrap on the flow is accounted for by introducing a momentum source (MS) into the governing fluid equations. Solution validation was conducted by benchmarking the CFD simulations to the heated bundle experiments. These simulations used the as-tested boundary and operating conditions but were conducted blind. Pressure drop measurements and local temperature measurements were compared. Axial pressure drop simulation results compared well with the experiment measurements. The vast majority of the local CFD temperatures matched thermocouple measurements within the instrument uncertainty. The good agreement between simulation and experiment supports the use of RANS-based CFD simulation methods and the specific applied MS method to model wire-wrapped fuel assemblies.

Investigation of pyrolysis effect on convective heat transfer characteristics of supercritical aviation kerosene

By utilizing airborne hydrocarbon fuels working as coolant, active regenerative cooling method is a significant method for maintaining the reliability of the scramjet engine components. At high temperatures, the pyrolysis of hydrocarbon fuels occurs and provides extra heat absorption capacity. Investigation on coupled heat transfer and pyrolysis of fuels is essential for the design of cooling channels. This paper aims to study the pyrolysis effects on convective heat transfer of supercritical aviation kerosene RP-3 in vertical upwards. Firstly, a simple molecular kinetic model was proposed for RP-3 pyrolysis. Then the performance of two Reynolds-averaged Navier-Stokes turbulence models with three constant turbulent Prandtl numbers in predicting the fluid-thermal behaviors of reacting flow were evaluated. At last, the mechanisms of pyrolysis effect on heat transfer in the heat transfer deterioration region and the cracked region were investigated. The results showed that the pyrolysis affected heat transfer from two aspects: providing heat absorb or release capacity through the endothermic (exothermic) pyrolysis reactions, changing the fuel properties by the variations in composition. In the heat transfer deterioration region, the effect of heat absorption dominants over the effect of additional decreased in density and specific heat capacity by pyrolysis on the heat transfer, thus a lower wall temperature and larger heat transfer coefficients were observed when considering pyrolysis. In the cracked region, under the present condition, the competitive outcome of the above two led to an improved heat transfer by pyrolysis, and the improved effect first increased and then decreased as the fuel conversion increasing. This work provides a fundamental understanding of the behaviors of hydrocarbon fuel inside the cooling channels, which is valuable for the cooling structure design.

Computational study of non-reactive swirling flow in tangentially-fired configuration gasifier

This paper presents a computational study of swirling flow in a tangentially-fired air blown entrained flow Mitsubishi Heavy Industries gasifier in a non-reactive environment. Flow fields are obtained by using the Reynolds-averaged Navier Stokes turbulence model. Shear-stress transport k-ω model is used for the modelling the effect of turbulence. Variation in length of the internal recirculation zone with the swirl number for tangentially injected flow is reported. It is observed that for the MHI gasifier the length of the internal recirculation zone in a non-reactive environment increases upon increasing the swirl number at the inlet of the gasifier.

Flow and wall shear rate analysis for a cruciform jet impacting on a plate at short distance

There are numerous turbulence models that have been developed in the past years, many of them being used in predicting flows, turbulence, mass and/or heat transfer. The particular case of an impinging jet implies all of the above. The performance of eight highly used Reynolds averaged Navier-Stokes turbulence models is examined in simulating a very sheared lobed impinging jet. The study is based on the investigation of an orthogonally lobed jet, impinging on a flat surface that flows out from a nozzle having a cruciform cross-section at a Reynolds number of 5620. Two experimental methods were implied for the comparison with numerical results. For the measurement of the wall shear rate an electrodiffusion method was employed. The velocity flow fields were measured using particle image velocimetry technique. The relative strengths and drawbacks of the SST k-ω, k-ω, TransSST, k-e realisable, RNG k-e, k-e, k-kl-ω and RSM turbulence models are compared.

Design and modelling of dust capturing system in thermally stratified flowing conditions

An improved understanding of the spreading and deposition of particles in a thermally buoyant flow environment i.e. melting hall is a key prerequisite to plan new strategies for dust capturing and reduced diffuse emissions. Accurate modeling of dust spreading and deposition requires accurate prediction of turbulence, particles dynamics, and particle-flow interactions. Widely used Reynolds-averaged Navier-Stokes (RANS) turbulence model, such as k-epsilon model, have shortcomings when modeling transient flow phenomena in thermally buoyant flow conditions. To overcome this shortcoming, a filtered VLES model (Very Large Eddy Scale model) have been applied where turbulent structures larger than a given filter size (typically grid size) is captured by the governing flow equations and smaller structures are modelled with a modified filtered k-epsilon model. This modeling approach provides more details description of the stratified flow and small turbulence structures. Both Drift-Flux Model (DFM) in a Eulerian framework and discrete phase model (DPM) in a Lagrangian framework have been applied to model the particle phase. Three coupled modelling approaches VLES-DPM, k-epsilon-DPM, and VLES-DFM are applied to understand the dust spreading and deposition. These methods are also used for designing a dust capturing system. The parameters which influence the dust capturing efficiency are also identified. The result from these three approaches are compared and the study showed that the VLES-DFM produces similar results to the VLES-DPM with lower computational time. Furthermore, the studies show that dust capturing efficiency depends on hood shape, particle size, particle density, hall-wind, and suction rate.

Prediction of aeroacoustic performance of counter-rotating wind turbine by changing the rotational speed of front rotor

This study aims to investigate the aerodynamic and aeroacoustic performance of a counter-rotating wind turbine (CRWT) to define the effect of rotational wind speed of the front rotor on the sound pressure level of CRWT, using CFD simulation. For this purpose, NREL Phase VI wind turbine was considered as the single rotor, and validation of aerodynamic and aeroacoustic simulation has been carried out for the solver. Then, 3D simulation has been applied on single rotor and counter-rotating wind turbines to scrutinize the acoustic level of CRWT by changing the rotational speed of the front rotor. The unsteady flow simulation was carried out with the realizable k–e Reynolds-averaged Navier–Stokes turbulence model, and the calculated flow field data were utilized using Ffowcs Williams–Hawkings model for predicting the acoustic level at receiver location, defined based on the IEC61400-11 standard. The results show that increasing the rotational wind speed of the upstream rotor leads to augmenting the noise emitted from CRWT. On the other hand, for tip speed ratios (TSR) ranging from 6 to 12, with increasing the front rotor RPM, the power coefficient of CRWT enhances, while for the TSR more than 12, an inverse behavior has been observed and power coefficient declines along with increasing front rotor’s speed of revolution.

Numerical studies on convective heat transfer of Al2O3 nanofluid in circular pipe

A numerical investigation is carried out applying a two phase approach on convective heat transfer of Al2O3 nanofluid flowing through a circular pipe. Five different Reynolds-averaged Navier-Stokes (RANS) turbulence models are employed by using the ANSYS-CFX® code. The analyses are performed for five different Reynolds number and three particle volume fractions. The results show that ω based models give very close results in themselves and that these models show better performance than e based models. SST k-ω model’s predictions are slightly better than those of other ω based models. The performance of this model increases with a decreasing particle volume fraction. It is clearly seen that a two phase approach gives satisfactory results when it is used with ω based models.

Boundary Layer Transition Models for Naval Applications: Capabilities and Limitations

We describe the implementation of several recently developed boundary layer transition models into the overset computational fluid dynamics code, REX, developed at the University of Iowa, together with an evaluation of its capabilities and limitations for naval hydrodynamics applications. Models based on correlations and on amplification factor transport were implemented in one- and two-equation Reynolds-averaged Navier-Stokes turbulence models, including modifications to operate in crossflow. Extensive validation of the transition models implemented in REX is performed for several 2- and 3-dimensional geometries of naval relevance. Standard tests with extensive available experimental data include flat plates in zero pressure gradient, an airfoil, and sickle wing. More complex test cases include the propeller, P4119, with some experimental data available, and the generic submersible, Joubert BB2, with no relevant experimental data available, to validate the transition models. Simulations for these last two cases show that extensive regions of laminar flow can be present on the bodies at laboratory scale and field scale for small vessels, and the potential effects on resistance and propulsion can be significant. 1. Introduction Progress for prediction of attached, fully turbulent flows for practical aerodynamic and hydrodynamic applications has reached a relatively mature plateau. However, according to a recent comprehensive review of pacing items (Slotnick et al. 2014), the single largest hurdle for incorporating computational fluid dynamics (CFD) into the design process in the near future is the ability to accurately predict turbulent flows with boundary layer transition and separation. Transition can impact skin friction, heat transfer, noise, propulsion efficiency, and maneuverability. This is especially true at model scale and for small craft such as unmanned or autonomous surface and underwater vehicles.