RANS Turbulence Models Research Articles

NUMERICAL INVESTIGATION OF TURBULENT FLOW THROUGH 900 MIXING ELBOW PIPE WITH DIFFERENT REYNOLDS NUMBER

This work presents a computational investigation of turbulent flow inside a mixing elbow pipe and this study focus on the behaviour of fluid flow in a mixing elbow. Mixing elbow is a region where two types of fluid flow with different parameters and high Reynolds number is intensively mixed together and is among typical geometries exactly where velocity, as well as temperature fluctuation, happen. A CFD model of turbulent flow in the elbow pipes is implemented using the ANSYS tool. RANS turbulent models, the k-ε model are used for the simulation and the variation of axial velocity, wall shear stress, and turbulent intensity along the length of the elbow pipes are studied. The fluid used for this purpose is water. The simulations are carried out with different Reynolds numbers rangings from 2,500 to 10,000.

Assessment of different RANS turbulence models in mini-channels for the cooling of MW-class gyrotron resonators

The adoption of high-speed water flow in mini-channels is a viable option for the cooling system of the resonant cavity in gyrotrons, which are a candidate technology for the external plasma heating in nuclear fusion reactors. The evaluation of the performance of such mini-channel cooling system is a combined fluid dynamics and heat transfer phenomenon which seeks more attention to a highly accurate computational analysis. In this study, a computational-based comparative platform is proposed to evaluate the performance and fidelity of the applied turbulence models which are utilized to study the mini-channel cavity cooling systems in gyrotrons. A full-size mock-up of the gyrotron resonator equipped with mini-channels has been realized and tested in 2019 by THALES to check its total pressure drop applying a wide range of water flow rates, including that available for the gyrotron operation. In parallel, a numerical model of the mock-up has been developed using the commercial software STAR-CCM+, and simulations have been performed using different RANS turbulence closures, and namely: SST k−ω, realizable k−ε and Lag EB k−ε. The detailed comparison of the computed hydraulic characteristics (i.e., a range of pressure drop measurements at different flow rates) to the set of measured values has been addressed using a multivariate metric to assess the performance of different turbulence models in pure hydraulic simulations. This comparative platform reveals a significant clarified difference in fidelity among the RANS models. Based on the performed comparative studies against the entire set of available experimental data, the Lag EB k−ε closure provides the best performance among the other turbulence models and can be applied for the future studies of the mini-channel cavity cooling systems of the gyrotron resonators.

Evaluation of Three-Dimensional Environmental Hydraulic Modeling in Scour Hole

The main goal of this study was comparing the performance of an open-source code OpenFOAM and a commercial software Ansys Fluent in simulating the turbulent flow through a scour hole developed in a sand bed channel, which helps to give a hint in choosing the appropriate calculating tool. Both models were set with the same mesh and as similar as possible numerical settings, with RANS turbulence modeling, applying the k-ωSST model, in transient simulations. The results of flow pattern, velocity, and turbulence properties were collected and compared with laboratory experimental data. The analyzed results showed that, although both of the two models cannot perfectly reproduce the values from a laboratory experiment, they can quite well capture the flow in scour hole near the wall, with a bit higher performance coming from the OpenFOAM model application.

Computational Efficiency Assessment of Adaptive Mesh Refinement for Turbulent Jets in Crossflow

Numerical analyses of environmental discharges are commonly conducted on pre-generated numerical grids with refinements implemented in regions of interest or influence on the flow field. This approach to problem formulation relies on insights into the flow specifics so that appropriate attention is given to relevant segments of the domain. In this paper we investigated the applicability of adaptive mesh refinement (AMR) on a commonly considered environmental problem—a jet in crossflow. The assessment was made using the OpenFOAM toolbox. Several RANS turbulence models and grid generation approaches were compared in terms of accuracy to previous studies and experimental results. Main emphasis is given to the computational efficiency of the methodology with a focus on load distribution. Our findings indicate that the results are acceptable in terms of accuracy with load balancing providing significant computational savings thus enabling AMR methodology to outperform the conventional approach.

Airborne particle dispersion by high flow nasal oxygen: An experimental and CFD analysis.

High Flow Nasal Oxygen (HFNO) therapy offers a proven means of delivering respiratory support to critically ill patients suffering from viral illness such as COVID-19. However, the therapy has the potential to modify aerosol generation and dispersion patterns during exhalation and thereby put healthcare workers at increased risk of disease transmission. Fundamentally, a gap exists in the literature with regards to the effect of the therapy on the fluid dynamics of the exhalation jet which is essential in understanding the dispersion of aerosols and hence quantifying the disease transmission risk posed by the therapy. In this paper, a multi-faceted approach was taken to studying the aerosol-laden exhalation jet. Schlieren imaging was used to visualise the flow field for a range of expiratory activities for three healthy human volunteers receiving HFNO therapy at flow rates of 0—60 L/min. A RANS turbulence model was implemented using the CFD software OpenFOAM and used to perform a parametric study on the influence of exhalation velocity and duration on the dispersion patterns of non-evaporating droplets in a room environment. A dramatic increase in the turbulence of the exhalation jet was observed when HFNO was applied. Quantitative analysis indicated that the mean exhalation velocity was increased by 2.2—3.9 and 2.3—3 times that for unassisted breathing and coughing, respectively. A 1—2 second increase was found in the exhalation duration. The CFD model showed that small droplets (10—40 μm) were most greatly affected, where a 1 m/s increase in velocity and 1 s increase in duration caused an 80% increase in axial travel distance.

Non-zonal detached eddy simulation coupled with a steady RANS solver in the wall region

Xiao and Jenny (2012) proposed an interesting hybrid LES/RANS method in which they use two solvers and solve the RANS and LES equations in the entire computational domain. In the present work this method is simplified and used as a hybrid RANS-LES method, a wall-modeled LES. The two solvers are employed in the entire domain. Near the walls, the flow is governed by the steady RANS solver; drift terms are added to the DES equations to ensure that the time-averaged DES fields agree with the steady RANS field. Away from the walls, the flow is governed by the DES solver; in this region, the RANS field is set to the time-averaged LES field. The disadvantage of traditional DES models is that the RANS models in the near-wall region – which originally were developed and tuned for steady RANS – are used as URANS models where a large part of the turbulence is resolved. In the present method – where steady RANS is used in the near-wall region – the RANS turbulence models are used in a context for which they were developed. In standard DES methods, the near-wall accuracy can be degraded by the unsteady agitation coming from the LES region. It may in the present method be worth while to use an accurate, advanced RANS model. The EARSM model is used in the steady RANS solver. The new method is called NZ S-DES. It is found to substantially improve the predicting capability of the standard DES. A great advantage of the new model is that it is insensitive to the location of the RANS-LES interface.

Advances in complex low speed flow around a prolate spheroid

Understanding and predicting the flow around the prolate spheroid is of great engineering significance to guide the design of vehicles such as aircraft and submarines. In recent years, a lot of experimental and numerical studies have been carried out on the flow around the prolate spheroid. The qualitative description and quantitative research of flow separation around prolate spheroid at attack angle are presented, promoting the identification and topology research of three-dimensional separation. The experimental results of oil flow, smoke, dye, hydrogen bubble, and LDV are given. The flow field characteristics are analyzed, and the existing problems are pointed out. Based on the introduction of the above phenomena, the effects of separation on aerodynamic force, noise, and wake are introduced. The effects of test conditions such as transition zone, protrusion, depression, and tail support on flow are also discussed. There are obvious differences between the above steady flow and the unsteady maneuvering process. The unsteady maneuvering process can not be treated as a steady or quasi-steady problem. During the maneuvering process, the separation will be delayed obviously, and the aerodynamic force will also change obviously. The greater the angle of attack, the higher the maneuvering rate, the more noticeable this effect will be. At present, RANS turbulence model is still the primary engineering method to solve the large-scale separated flow around the prolate spheroid. However, LES, DES, and other methods have gradually been widely used. Due to the limitation of computer capability, DNS can only be used in the case of lower Reynolds number but not in high Reynolds number flow. The difference between the numerical simulation and the unsteady simulation is more significant. Finally, the research progress of prolate spheroid transition is introduced. The mechanism and identification of TS transition and cross-flow transition are more accurate. The numerical simulation results are basically consistent with the experimental results, but the understanding of reattachment transition is not clear enough, especially on the windward side. Therefore, the research of prolate spheroid transition still needs to rely on experiments.

Application of Advanced RANS Turbulence Models for the Prediction of Turbomachinery Flows

AbstractThe current trend in turbomachinery toward broader operating characteristics requires that operating points in the off-design region can be captured accordingly from the simulation models. Complex processes like separation and vortex formation/dissipation occur under these conditions. Linear two equation models are often not able to represent these effects correctly since their derivation is based on oversimplifications, such as the Boussinesq hypothesis, which makes it impossible to capture anisotropic turbulence. Advanced RANS models are usually not considered in the design process of turbomachines because (1) they are usually more delicate with regards to stability and convergence behavior and (2) require additional computational effort. To make the usage of advanced RANS models more applicable for complex turbomachinery simulations, a selected group of models were implemented into a robust framework of a pressure-based fully coupled solver. To further enhance stability, coupling terms between the turbulent transport equations were derived for several models. Anisotropic turbulence is introduced by computing an algebraic expression or by solving the transport equations for the Reynolds stress components. The evaluation of the models is performed on the RWTH Aachen “Radiver” centrifugal compressor case with vaned diffuser. For design conditions and operation points near the stability limit, all investigated turbulence models predict the compressor characteristic. Operation points in the choking region, on the other hand, are only predicted well by anisotropic models. The good results and improved convergence behavior of the advanced RANS models clearly indicates their applicability in the design process of turbomachines.

Application of LES/PDF and RANS/PDF approaches for simulation of spray combustion

This paper is addressing of a coupling Large-eddy simulation (LES) and RANS turbulence models with mixture fraction/probability density function as a combustion model. The two models have been implemented to simulate ethanol-air spray combustion. The gas phase is described with the Eulerian approach while the liquid phase is designed using a Lagrangian framework. The LES/PDF approach is obtained statistically. The sub-grid scale energy equation is used with the LES/PDF approach. The numerical results are validated with experimental data. Both LES/PDF and RANS/PDF approaches are compared with the experimental data. The LES/PDF approach shows a good agreement in predicting the average gas temperature compared with RANS/PDF approach. The LES/PDF shows a better prediction of both turbulence intensity profiles and the vortices which are generated in the turbulent flow in comparison with the RANS/PDF approach.

Numerical Study of Three-Dimensional Surface Jets Emerging from a Fishway Entrance Slot

The efficiency of a fishway is determined by the ability of immigrating fish to follow its attraction flow (i.e., its jet) to locate and enter the fishway entrance. The hydraulic characteristics of fishway entrance jets can be simplified using findings from widely investigated surface jets produced by shaped nozzles. However, the effect of the different boundary conditions of fishway entrance jets (characterized by vertical entrance slots) compared to nozzle jets must be considered. We investigate the downstream propagation of attraction jets from the vertical slot of a fishway entrance into a quiescent tailrace, considering the following boundary conditions not considered for nozzle jets: (1) slot geometry, (2) turbulence characteristics of the approach flow to the slot, and (3) presence of a lateral wall downstream of the slot. We quantify the effect of these boundary conditions using three-dimensional hydrodynamic-numeric flow simulations with DES and RANS turbulence models and a volume-of-fluid method (VoF) to simulate the free water surface. In addition, we compare jet propagation with existing analytical methods for describing jet propagations from nozzles. We show that a turbulent and inhomogeneous approach flow towards a vertical slot reduces the propagation length of the slot jet in the tailrace due to increased lateral spreading compared to that of a jet produced by a shaped nozzle. An additional lateral wall in the tailrace reduces lateral spreading and significantly increases the propagation length. For highly turbulent flows at fishway entrances, the RANS model tends to overestimate the jet propagation compared to the transient DES model.

On the Use of Computational Fluid Dynamics (CFD) Modelling to Design Improved Dry Powder Inhalers.

Computational Fluid Dynamics (CFD) simulations are performed to investigate the impact of adding a grid to a two-inlet dry powder inhaler (DPI). The purpose of the paper is to show the importance of the correct choice of closure model and modeling approach, as well as to perform validation against particle dispersion data obtained from in-vitro studies and flow velocity data obtained from particle image velocimetry (PIV) experiments. CFD simulations are performed using the Ansys Fluent 2020R1 software package. Two RANS turbulence models (realisable k - ε and k - ω SST) and the Stress Blended Eddy Simulation (SBES) models are considered. Lagrangian particle tracking for both carrier and fine particles is also performed. Excellent comparison with the PIV data is found for the SBES approach and the particle tracking data are consistent with the dispersion results, given the simplicity of the assumptions made. This work shows the importance of selecting the correct turbulence modelling approach and boundary conditions to obtain good agreement with PIV data for the flow-field exiting the device. With this validated, the model can be used with much higher confidence to explore the fluid and particle dynamics within the device.

Numerical Assessment of Turbulent Flow Driving in a Two-Sided Lid-Driven Cavity with Antiparallel Wall Motion

In this paper, the case of the steady two-dimensional flow in a two-sided lid-driven square cavity is numerically investigated by the finite volume method (FVM). The flow motion is due to the top and bottom horizontal walls sliding symmetrically in the opposite direction with equal velocities, UT and UB, obtained through three respective Reynolds numbers, Re1,2=10000, 15000, and 20000. Due to the lack of availability of experimental results in this Reynolds number margin for this type of flow, the problem is first examined by considering that the flow is turbulent with the inclusion of four commonly used RANS turbulence models: Omega RSM, SST k-ω, RNG k-ε and Spalart-Allmaras (SA). Next, the regime is considered being laminar in the same range of Reynolds numbers. A systematic evaluation of the flow characteristics is performed in terms of stream-function contour, velocity profiles, and secondary vortices depth. Examination of the calculation results reveals the existence of a great similarity of the predicted flow structures between the Omega RSM model and those from the laminar flow assumption. On the other hand, the computed flow with the SST k-ω model, the RNG k-ε model, and the SA model reveals a remarkable under-prediction which appears clearly in the size and number of secondary vortices in the near-wall regions. Various benchmarking results are presented in this study.

ЧИСЛЕННОЕ ИССЛЕДОВАНИЕ ЗАКРУЧЕННОГО ТУРБУЛЕНТНОГО ТЕЧЕНИЯ В КАНАЛЕ С ВНЕЗАПНЫМ РАСШИРЕНИЕМ

A strongly swirling turbulent flow through an abrupt expansion is studied using the highly resolved DNS, LES, and SAS to shed more light on a stagnation region and spiral vortex destruction, though these methods require high computational expenses. The vortex fracture induced by a sudden expansion resembles the so-called vortex rope that occurs in hydropower draft tubes. It is known that large-scale spiral vortex structures can be captured by regular RANS turbulence models. In this paper, a numerical study of a strongly swirling flow, which abruptly expands, is carried out using the Reynolds stress models SSG / LRR-RSM and EARSM with experimental measurements implemented by Dellenback et al. (1988). Calculations are carried out using the finite volume method. The flow dynamics is studied at the Reynolds number of 3.0 × 104 at almost constant large swirl numbers of 0.6. The time-averaged velocity and pressure fields, as well as the root-mean-square values of the velocity fluctuations are recorded and studied qualitatively. The obtained results are compared with known experimental data. The aim of this work is to test the ability of the models to describe anisotropic turbulence. It is shown that the SSG / LRRRSM model is more appropriate for studying such flows.

Simulating Turbulent Air Flow Past a Hemispherical Body

ABSTRACT The flow of air past a smooth surface-mounted hemisphere is investigated numerically using six common RANS turbulence models and seeking steady flow solutions. Where possible, the turbulence models are applied using standard wall functions, resolving the viscous sublayer, and the enhanced wall treatment option in ANSYS Fluent. Results of the simulations are compared against measurements taken in a wind tunnel experiment. The comparison shows that enhanced wall treatment and resolving the boundary layer on a low Reynolds number mesh yields superior accuracy compared to standard wall functions or resolving the boundary layer on a high Reynolds number mesh, for all the turbulence models considered. The RNG k - ε model with enhanced wall treatment applied is found to yield the most accurate prediction of the static pressure distribution across the surface of the hemisphere model. Conversely, the Reynolds Stress model and the standard k - ω model are found to give the least accurate predictions, irrespective of the near-wall modelling approach applied. It is found that good agreement with the experimental data for this case offlows can be attained using each of the near-wall modelling techniques if a well-suited turbulence model is used. Keywords: hemisphere, wind tunnel, turbulence modelling, computational fluid dynamics, steady flow

LES-based Computation of Evolution of Turbulent Wakes Subjected to Adverse Pressure Gradient

Results are presented of scale-resolving computations of mean and turbulent (including the dissipation rate of turbulent kinetic energy) characteristics of the turbulent wake of a flat plate subjected to adverse pressure gradient (APG). The computations are performed in the framework of a zonal RANS-LES model for two configurations. In the first one, APG is created by a plane symmetric diffuser and in the second one by a system of two pairs of thin liner foils specially designed for this purpose. In the both cases a volumetric synthetic turbulence generator is used for creating turbulent content at the inlet of LES subdomain. High accuracy of results of the simulations is supported by their weak sensitivity to grid-refinement. Obtained detailed data on wakes’ characteristics may be used for validation and improvement of RANS turbulence models as applied to the considered class of flows.

Anisotropic RANS Turbulence Modeling for Wakes in an Active Ocean Environment

The problem of simulating wakes in a stratified oceanic environment with active background turbulence is considered. Anisotropic RANS turbulence models are tested against laboratory and eddy-resolving models of the problem. An important aspect of our work is to acknowledge that the environment is not quiescent; therefore, additional sources are included in the models to provide a non-zero background turbulence. The RANS models are found to reproduce some key features from the eddy-resolving and laboratory descriptions of the problem. Tests using the freestream sources show the intuitive result that background turbulence causes more rapid wake growth and decay.

Improving computational fluid dynamics modeling of Direct Injection Spark Ignition cold-start

Developing a profound understanding of the combustion characteristics of the cold-start phase of a Direct Injection Spark Ignition (DISI) engine is critical to meeting increasingly stringent emissions regulations. Computational Fluid Dynamics (CFD) modeling of gasoline DISI combustion under normal operating conditions has been discussed in detail using both the detailed chemistry approach and flamelet models (e.g. the G-Equation). However, there has been little discussion regarding the capability of the existing models to capture DISI combustion under cold-start conditions. Accurate predictions of cold-start behavior involves the efficient use of multiple models - spray modeling to capture the split injection strategies, models to capture the wall-film interactions, ignition modeling to capture the effects of retarded spark timings, combustion modeling to accurately capture the flame front propagation, and turbulence modeling to capture the effects of decaying turbulent kinetic energy. The retarded spark timing helps to generate high heat flux in the exhaust for the faster catalyst light-off during cold-start. However, the adverse effect is a reduced turbulent flame speed due to decaying turbulent kinetic energy. Accordingly, developing an understanding of the turbulence-chemistry interactions is imperative for accurate modeling of combustion under cold-start conditions. In the present work, combustion characteristics during the cold-start, fast-idle phase is modeled using the G-Equation flamelet model and the RANS turbulence model. The challenges associated with capturing the turbulent-chemistry interactions are explained by tracking the flame front travel along the Borghi-Peters regime diagram. In this study, a modified version of the G-Equation combustion model for capturing cold-start flame travel is presented.

Efficient simulation of bubble dispersion and resulting interaction

In this work, an efficient model for simulating bubble dispersion and coalescence due to turbulence is developed in the Euler-Lagrange framework. The primary liquid phase is solved on the Euler grid with the RANS turbulence model. Bubble motion is computed with the force balance equations. One-way coupling between two phases is assumed and the framework is designed for the computation of disperse bubbly flows at low Eötvös number. The turbulent dispersion of the dispersed phase is reconstructed with the continuous random walk (CRW) model. Bubble-bubble collisions and coalescence are accounted for deterministically. To accelerate the time-consuming search for potential collision partners in dense bubbly flows, the sweep and prune algorithm is employed, which can be utilized in arbitrary mesh types and sizes. Validation against experiments of turbulent pipe flows demonstrates that the one-way coupled EL-CRW dispersion model can well reproduce the bubble distribution in a typical dense bubbly pipe flow. Good agreement of the bubble size distribution at the pipe outlet between the simulation and the experiment is obtained.

Impact of Underlying RANS Turbulence Models in Zonal Detached Eddy Simulation: Application to a Compressor Rotor

The present study focuses on the impact of the underlying RANS turbulence model in the Zonal Detached Eddy Simulation (ZDES) method when used for secondary flow prediction. This is carried out in light of three issues commonly investigated for hybrid RANS/LES methods (detection and protection of attached boundary layer, emergence, and growth of resolved turbulent fluctuations and accurate prediction of separation front due to progressive adverse pressure gradient). The studied configuration is the first rotor of a high pressure compressor. Three different turbulence modelings (Spalart and Allmaras model (SA), Menter model with (SST) and without (BSL) shear stress correction) are assessed as ZDES underlying turbulence model and also as turbulence model of unsteady RANS simulations. Whatever the underlying turbulence model, the ZDES behaves well with respect to the first two issues as the boundary layers appear effectively shielded and the RANS-to-LES switch is close downstream of trailing edges and separation fronts leading to a quick LES treatment of wakes and shear layers. Both tip leakage and corner flows are strongly influenced by the Navier–Stokes resolution approach (unsteady RANS vs. ZDES) but the underlying turbulence modelling (SA vs. SST vs. BSL) impacts mainly the junction flow near the hub for both approaches. ZDES underlying turbulence model choice appear essential since it leads to quite different corner flow separation topologies and so to inversion of the downstream stagnation pressure radial gradient.

Numerical Study of Turbulence Models on Boundary Layer Flow over a Flat Plate using Computational Fluid Dynamics

The work presented here has been carried out to study the effect of various RANS turbulence models and their applicability on boundary layer flow over a flat plate. The study aims to establish guidelines on proper selection of turbulence models while performing a CFD analysis. Mesh resolution and sensitivity study over y+ for various turbulence models has been presented here. Guidelines on proper selection of turbulence models with respect to y+ value ranging from viscous sub-layer to fully turbulent layer has been listed as outcome of this work. Results of various numerical analyses have also been compared with analytically derived values. Ansys Fluent 19.1 has been used as numerical tool for all CFD simulations presented here.