Standard Reynolds-averaged Navier-Stokes Research Articles

Stress-Equivalent Spalart–Allmaras Wall Model with Local Boundary Conditions for Reynolds-Averaged Navier–Stokes

In this paper, we present a wall model based on a modified version of the Spalart–Allmaras turbulence model. Unlike typical wall models, this method avoids the need to query the interior solution by utilizing solution information solely at the boundary, making it well-suited for unstructured grids and mesh adaptation. Furthermore, this wall-modeling method is formulated to lessen the near-wall grid resolution requirements by, below the log layer, making the eddy viscosity approach a constant, nonzero value and the velocity varying approximately linearly with distance from the wall. Using output-based mesh adaptation, this new wall-modeling method is compared to standard Reynolds-Averaged Navier-Stokes (RANS-SA). It is shown that this wall model results in accurate predictions of quantities of interest such as aerodynamic coefficients, surface pressure and temperature, skin friction, and heat transfer compared with RANS-SA, while requiring substantially less near-wall grid.

Study of nuclear reactor external vessel passive cooling using computational fluid dynamics

Nuclear energy has recently re-gained support as a reliable source of carbon-free electricity. However, the safety of nuclear plants is still questioned, particularly in the wake of the tragic events in Fukushima. In response to this, the adoption of passive safety systems, providing cooling to the plant even in the absence of any active power, has been accelerated. However, the confident assessment of their effectiveness is still constrained by the large spatial scales involved, which makes experimental validation difficult and expensive. Additionally, the many complexities intrinsic to the prediction of natural convection makes the mathematical modelling of such systems problematic. In this paper, the development of computational fluid dynamics (CFD) as a reliable method to ultimately prove, and eventually improve, the passive safety features of reactors is described. The focus of the paper is external reactor vessel cooling. In the case of a reactor core melting, the core is kept inside the reactor vessel, which is cooled from the outside by natural convection after flooding the region of the containment that houses the vessel. More specifically, a recent experiment that simulates, at a reduced scale, cooling of the horizontal calandria vessel of a pressurized heavy water reactor, submerged in a water pool, is modelled using CFD. For this situation, of major relevance is the correct prediction of boiling on the external surface of the reactor vessel, and thermal stratification in the water pool, which could prevent lower regions of the pool from performing any heat transfer duty. Results from standard Reynolds-averaged Navier-Stokes based turbulent flow simulations demonstrate that more advanced models are needed for accurate simulation, specifically in view of the excessive mixing predicted by such approaches. In contrast, the development of a more advanced tool based on large eddy simulation (LES) is started from the prediction of thermal stratification in the water pool, for which good accuracy is demonstrated through comparisons with available data. Although further improvements are needed, and discussed, it is demonstrated that LES can provide a reliable tool for the analysis and design of passive safety systems and strategies, and the assessment of passive cooling effectiveness and reactor safety. Although computational effort remains the main constraint in using LES, computation times are not prohibitive, particularly given the continual increase in available computing resources.

A Hybrid RANS-LES Computational Fluid Dynamics Simulation of an FDA Medical device benchmark

Computational fluid dynamics (CFD) is a valuable tool that complements experimental data in the development of medical devices. The reliability of CFD still presents an issue and for that reason, no standardized approaches are currently available. The United States Food and Drug Administration (FDA) has initiated the development of a program for CFD validation, and has presented an idealized nozzle benchmark model. In this study, a nozzle flow with sudden expansion has been simulated using advanced RANS-LES turbulence models. Such models partially resolve the flow and are cheaper in computer resources and time in comparison to the Large Eddy Simulation (LES). Furthermore, they are more accurate than standard Reynolds-averaged Navier-Stokes (RANS) models. A collection of hybrid turbulence models has been investigated: Detached Eddy Simulation (DES), Stress Blended Eddy Simulation (SBES), and Scale Adaptive Simulation (SAS), and compared to a standard RANS Shear Stress Transport (SST) model. Subsequently, all models were validated by experimental results already published by different research groups. Particle Image Velociometry (PIV) experiments were performed by inter-laboratory study, and the results are available online for numerical validation. The flow conditions in this study are only restricted to a turbulence flow at a Reynolds number of Re =6500. Complementing the turbulence models investigation, two advection schemes were tested: high resolution (HR) and bounded central difference scheme (BCD). Among all advanced models the SBES model with BCD scheme has the best agreement with the experimental values.

Numerical Investigation of Flow through a Valve during Charge Intake in a DISI -Engine Using Large Eddy Simulation

Apart from electric vehicles, most internal combustion (IC) engines are powered while burning petroleum-based fossil or alternative fuels after mixing with inducted air. Thereby the operations of mixing and combustion evolve in a turbulent flow environment created during the intake phase and then intensified by the piston motion and influenced by the shape of combustion chamber. In particular, the swirl and turbulence levels existing immediately before and during combustion affect the evolution of these processes and determine engine performance, noise and pollutant emissions. Both the turbulence characteristics and the bulk flow pattern in the cylinder are strongly affected by the inlet port and valve design. In the present paper, large eddy simulation (LES) is appraised and applied to studying the turbulent fluid flow around the intake valve of a single cylinder IC-engine as represented by the so called magnetic resonance velocimetry (MRV) flow bench configuration with a relatively large Reynolds number of 45,000. To avoid an intense mesh refinement near the wall, various subgrid scale models for LES; namely the Smagorinsky, wall adapting local eddy (WALE) model, SIGMA, and dynamic one equation models, are employed in combination with an appropriate wall function. For comparison purposes, the standard RANS (Reynolds-averaged Navier–Stokes) k- ε model is also used. In terms of a global mean error index for the velocity results obtained from all the models, at first it turns out that all the subgrid models show similar predictive capability except the Smagorinsky model, while the standard k- ε model experiences a higher normalized mean absolute error (nMAE) of velocity once compared with MRV data. Secondly, based on the cost-accuracy criteria, the WALE model is used with a fine mesh of ≈39 millions control volumes, the averaged velocity results showed excellent agreement between LES and MRV measurements, revealing the high prediction capability of the suggested LES tool for valve flows. Thirdly, the turbulent flow across the valve curtain clearly featured a back flow resulting in a high speed intake jet in the middle. Comprehensive LES data are generated to carry out statistical analysis in terms of (1) evolution of the turbulent morphology across the valve passage relying on the flow anisotropy map, (2) integral turbulent scales along the intake-charge stream, (3) turbulent flow properties such as turbulent kinetic energy, turbulent velocity and its intensity within the most critical zone in intake-port and along the port length, it further transpires that the most turbulence are generated across the valve passage and these are responsible for the in-cylinder turbulence.

A Numerical Simulation of HeartAssist5 Blood Pump Using an Advanced Turbulence Model

The need for mechanical assistance of the failing heart has increased with improvements in medicine and a rapidly aging population. In recent decades, significant progress has been made in the development and refinement of ventricular assist devices (VADs). Such devices operate in mixed laminar, transitional, and turbulent flow regime. One tool that assists in the development of VADs by facilitating understanding of the physical and mechanical properties of these flow regimes is computational fluid dynamics (CFD). In our investigation, we tested an advanced turbulence model that is a further development from standard Reynolds-averaged Navier-Stokes (RANS) models. From estimated pump flow rates (Q0) and constant rotation speed (n), pressure head (Δp) was calculated and validated with experimental data. An advanced turbulence model called scale adaptive simulation (SAS) was used in the solving of six different working cases comparing numerical SAS-SST and standard SST-kω models to experimental results.

Analysis of the Effect of the Swirl Flow Intensity on Combustion Characteristics in Liquid Fuel Powered Confined Swirling Flames

This article examines the implementation of CFD technology in the design of the industrial liquid fuel powered swirl flame burner. The coupling between the flow field and the combustion model is based on the eddy dissipation model. The choice of the LES (Large Eddy Simulation) turbulence model over standard RANS (Reynolds Averaged Navier-Stokes) offers a possibility to improve the quality of the combustion-flow field interaction. The Wall Adapting Local Eddy-Viscosity (WALE) sub-grid model was used. The reaction chemistry is a simple infinitely fast one step global irreversible reaction. The computational model was setup with the Ansys-CFX software. Through the detailed measurements of industrial size burner, it was possible to determine the natural operational state of the burner according to the type of fuel used. For the inlet conditions, axial and radial velocity components were calculated from known physical characteristics of both the fuel and air input, with the initial tangential velocity of the fuel assumed as 18% of the initial axial fuel velocity. Different swirl number (S) values were studied. Addition of a surplus (in comparison to conventional flame stabilization) of tangential air velocity component (W), the rotational component increases itself with a considerably high magnitude, contributing to the overall flame stabilization. The level of S especially influences the turbulent energy, its dissipation rate and turbulent (Reynolds) stresses. In the case of high swirl number values (S > 0,65) it is possible to divide the flow field in three principle areas: mixing area (fuel-air), where exothermal reactions are taking place, central recirculation area and outer recirculation area, which primarily contains the flow of burnt flue gases. The described model was used to determine the flow and chemical behavior, whereas the liquid atomization was accounted for by LISA (Linear Instability Sheet Atomization) model incorporating also the cavitation within injection boundary condition. The boundary conditions were determined based on the data from the experimental hot water system. Depending on system requirements, especially with continuous physical processes as well as the results of experimental measurements, the paper reports on determination of the mixing field and its intensity in the turbulent flow, the description of heat release and interaction of turbulent flow field and chemical kinetics in the case of confined swirling flames.

Semi-Empirical Modeling of Turbulent Anisotropy for Airfoil Self-Noise Predictions

In the present paper an approach to derive anisotropic turbulence properties from standard Reynolds-averaged Navier-Stokes simulations is proposed. The theory includes models for anisotropic velocity spectra and scaling functions that can be used to derive other turbulence parameters required for the turbulent boundary-layer trailing-edge interaction noise prediction. This type of noise is supposed to be affected by the inhomogeneous and anisotropic flow character near the trailing edge of an airfoil or wing. For this purpose, two-point turbulent velocity correlation measurements in the near wake of two airfoils (NACA 0012 and NACA 64 3 -418) at high Reynolds numbers (Re = 1.5 x 10 6 and 2.5 x 10 6 ) were conducted in the Laminar Wind Tunnel of the University of Stuttgart. A detailed postprocessing along with assessment of the experimental data has been performed for the modeling of different isotropic and anisotropic velocity spectra and related length scales. For this modeling, an anisotropy amplitude scaling method and Kolmogorov local-isotropy hypothesis have been applied. Furthermore, two different semi-empirical functions have been correlated to the resultant anisotropy scaling factors to model the effects of turbulence anisotropy in numerically predicted Reynolds-averaged Navier-Stokes data. The proposed spectrum modeling allows an accurate evaluation of the turbulence mean dissipation rate. The new models were validated and applied to a turbulent boundary-layer trailing-edge interaction noise prediction method (Rnoise) to analyze the impact of anisotropy on the total predicted noise spectra.

Computational study of flowfield characteristics in cavities with stores

AbstractIn this paper, the results of computational studies on the unsteady flow features in three-dimensional empty cavities and cavities with a representative store are presented. Flow simulations with a turbulence model based on a hybrid method, which behaves as a standard Reynolds-averaged Navier-Stokes (RANS) model within the attached boundary layer and as a Large-Eddy Simulation LES sub-grid scale model in the rest of the flow (commonly known as Detached-Eddy Simulation (DES)) are used in this study. The time-mean flow study showed the presence of three-dimensional effects inside the cavities. The mean flowfield visualisation also clearly showed the presence of a pair of ‘tornado-like’ vortices in the upstream half of the cavity which merge to a single, large recirculation further downstream. Visualisation for the cavity-with-store case revealed that the mean flowfield was effectively divided into two halves with significant reduction of the spanwise flow across the cavity width. In the unsteady flow study, near-field acoustic spectra were computed for the empty cavity and cavity-with-store cases. Study of unsteady pressure spectra for the cavity-with-store case found the presence of many peaks and the corresponding mode frequencies were found to agree well with the Rossiter modes. The blockage effect of store and strut on the spanwise flow is thought to have reduced the interaction, and subsequent non-linear coupling, between the Rossiter modes. This may be the reason for the co-existence of multiple modes without the coupling among them.

Numerical simulation of particle-laden coaxial turbulent jets

The study of particle-laden coaxial, turbulent jets has been of interest due to its importance in several applications such as industrial burners, combustors and mixing devices. The addition of the second phase to the continuous phase jet can change the already complicated flow pattern and turbulent characteristics of the jets. Vast research efforts have been devoted to understanding such phenomena, but detailed investigation of particle-laden flows remains an active area of research. The advent of laser diagnostics has helped to quantify the myriad details of the turbulent jet flow fields in great detail. However, the diagnostic tools are very expensive to use as a research tool. As a result, computational fluid dynamics (CFD) with an acceptable level of accuracy can complement the experimental results by providing additional details that are difficult to measure. Nevertheless, even with the advancement of computational resources, modeling the turbulent characteristics remains a challenge due to its complex nature. Although recently, computational techniques have been developed to “solve” the turbulent quantities, these techniques are computationally too expensive to use in real time applications. Hence, in this work, standard Reynolds-averaged Navier-Stokes, numerical simulations are carried out to predict the flow and turbulent characteristics of coaxial jets with and without the dispersed phase. The results are compared with the experimental data measured using molecular tagging velocimetry diagnostic technique. The key objective of this work is to investigate the flow field details that are difficult, if not impossible, to measure.

Three-Dimensionality in Reynolds-Averaged Navier-Stokes Solutions Around Two-Dimensional Geometries.

The flow over two-dimensional geometries is studied via unsteady numerical simulations that are three dimensional, with periodic conditions applied along the spanwise coordinate. This framework is well accepted for direct numerical simulations (DNS), large-eddy simulations, and detached-eddy simulations (DES), but is here combined with standard Reynolds-averaged Navier-Stokes (RANS) turbulence models. This strategy, which is not new, is referred to as unsteady RANS (URANS). Limited previous evidence suggested that, in URANS, three dimensionality is suppressed by high eddy-viscosity levels. However, three dimensionality proves fairly easy to sustain with adequate initial conditions, in all three cases studied here: stalled airfoil, circular cylinder, and a rounded square, except that for one case three dimensionality failed to last from random-based initial perturbations and was sustained only when using a DES field as initial condition.

Immersed boundary technique for turbulent flow simulations

The application of the Immersed Boundary (IB) method to simulate incompressible, turbulent flows around complex configurations is illustrated; the IB is based on the use of non-body conformal grids, and the effect of the presence of a body in the flow is accounted for by modifying the governing equations. Turbulence is modeled using standard Reynolds-Averaged Navier-Stokes models or the more sophisticated Large Eddy Simulation approach. The main features of the IB technique are described with emphasis on the treatment of boundary conditions at an immersed surface. Examples of flows around a cylinder, in a wavy channel, inside a stirred tank and a piston/cylinder assembly, and around a road vehicle are presented. Comparison with experimental data shows the accuracy of the present technique. This review article cites 70 references.