Investigated Turbulence Models Research Articles

A Numerical and Experimental Investigation of a Confluent Jets Ventilation Supply Device in a Conference Room

In this study, confluent jets ventilation (CJV) supply devices with three different nozzle arrays (1 × 19, 2 × 19, 3 × 19) were investigated both numerically and experimentally at two different airflow and supply air temperature set-ups. The performance of the CJV supply devices was investigated concerning thermal comfort, indoor air quality (IAQ), and heat removal effectiveness in a conference room environment. A comparison between the experimental and numerical results showed that the ϑ2¯−f model had the best agreement out of the investigated turbulence models. The numerical results showed that the size of the array had a great impact both on near-field development and on the conditions in the occupied zone. A larger array with multiple rows and a lower momentum conserved the inlet temperature and the mean age of the air better than a single-row array with a higher momentum. A larger array with multiple rows had a higher IAQ and a greater heat removal effectiveness in the occupied zone because the larger array conserved the mean age of air better and the buoyancy driven flow was slightly better at removing the heat. Because of the lower inlet velocities, they also had lower velocities at ankle level, which decreased the risk of draft and thermal discomfort.

A novel strategy for comprehensive optimization of structural and operational parameters in a supersonic separator using computational fluid dynamics modeling

In this study, the effects of several structural and operational parameters affecting the separation efficiency of supersonic separators were investigated by numerical methods. Different turbulence models were used and their accuracies were evaluated. Based on the error analysis, the V2-f turbulence model was more accurate for describing the high swirling turbulent flow than other investigated turbulence models. Therefore, the V2-f turbulence model and particle tracing model were selected to optimize the structure of the convergence part, the diffuser, the drainage port, and the swirler. The cooling performance of three line-type in the convergent section were calculated. The simulation results demonstrated that the convergent section designed by the Witoszynski curve had higher cooling depth compared to the Bi-cubic and Quintic curves. Furthermore, the expansion angle of 2° resulted in the highest stability of fluid flow and therefore was selected in the design of the diffuser. The effect of incorporating the swirler and its structure on the separation performance of supersonic separator was also studied. Three different swirler types, including axial, wall-mounted, and helical, were investigated. It was observed that installing the swirler significantly improved the separation efficiency of the supersonic separator. In addition, the simulation results demonstrated that the separation efficiency was higher for the axial swirler compared to the wall-mounted and helical swirlers. Therefore, for the improved nozzle, the swirling flow was generated by the axial swirler. The optimized axial swirler was constructed from 12 arced vanes each of which had a swirl angle of 40°. For the optimized structure, the effects of operating parameters such as inlet temperature, pressure recovery ratio, density, and droplet size was also investigated. It was concluded that increasing the droplet size and density significantly improved the separation efficiency of the supersonic separator. For hydrocarbon droplets, the separation efficiency improved from 4.6 to 76.7% upon increasing the droplet size from 0.1 to 2 µm.

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.

The effects of flow separation on a lambda wing aerodynamics

Purpose In this paper, experimental and numerical results of a lambda wing have been compared. The purpose of this paper is to study the behaviour of lambda wings using a CFD tool and to consider different numerical models to obtain the most accurate results. As far as the consideration of numerical methods is concerned, the main focus is on the evaluation of computational methods for an accurate prediction of contingent leading edge vortices’ path and the flow separation occurring because of the burst of these vortices on the wing. Design/methodology/approach Experimental tests are performed in a closed-circuit wind tunnel at the Reynolds number of 6 × 105 and angles of attack (AOA) ranging from 0 to 10 degrees. Investigated turbulence models in this study are Reynolds Averaged Navior–Stokes (RANS) models in a steady state. To compare the accuracy of the turbulence models with respect to experimental results, sensitivity study of these models has been plotted in bar charts. Findings The results illustrate that the leading edge vortex on this lambda wing is unstable and disappears soon. The effect of this disappearance is obvious by an increase in local drag coefficient in the junction of inner and outer wings. Streamlines on the upper surface of the wing show that at AOA higher than 8 degrees, the absence of an intense leading edge vortex leads to a local flow separation on the outer wing and a reverse in the flow. Research limitations/implications Results obtained from the behaviour study of transition (TSS) turbulence model are more compatible with experimental findings. This model predicts the drag coefficient of the wing with the highest accuracy. Of all considered turbulence models, the Spalart model was not able to accurately predict the non-linearity of drag and pitching moment coefficients. Except for the TSS turbulence model, all other models are unable to predict the aerodynamic coefficients corresponding to AOA higher than 10 degrees. Practical implications The presented results in this paper include lift, drag and pitching moment coefficients in various AOA and also the distribution of aerodynamic coefficients along the span. Originality/value The presented results include lift, drag and pitching moment coefficients in various AOA and also aerodynamic coefficients distribution along the span.

Simulation and Analysis of a Turbulent Flow Around a Three-Dimensional Obstacle

Abstract The study of flow around obstacles is devised into three different positions: above the obstacle, upstream of the obstacle, and downstream of the latter. The behaviour of the fluid downstream of the obstacle is less known, and the physical and numerical modelling is being given the existence of recirculation zones with their complex behaviour. The purpose of the work presented below is to study the influence of the inclined form of the two upper peaks of a rectangular cube. A three-dimensional study was carried out using the ANSYS CFX calculation code. Turbulence models have been used to study the flow characteristics around the inclined obstacle. The time-averaged results of contours of velocity vectors <V>, cross-stream <v> and stream wise velocity <u> and streamlines were obtained by using K-ω shear -stress transport (SST), RANG K-ε and K-ε to model the turbulence, and the governing equations were solved using the finite volume method. The turbulence model K-ω SST has presented the best prediction of the flow characteristics for the obstacle among the investigated turbulence models in this work.

Assessing Turbulence Models on the Simulation of Launch Vehicle Base Heating

Launch vehicles suffer from severe base heating during ascents. To predict launch vehicle base heat flux, the computational fluid dynamics (CFD) tools are widely used. The selection of the turbulence model determines the numerical simulation results of launch vehicle base heating, which may instruct the thermal protection design for the launch vehicle base. To assess performances, several Reynolds-averaged turbulence models have been investigated for the base heating simulation based on a four-nozzle launch vehicle model. The finite-rate chemistry model was used for afterburning. The results showed that all the turbulence models have provided nearly identical mean flow properties at the nozzle exit. Menter’s baseline (BSL) and shear stress transport (SST) models have estimated the highest collision pressure and have best predicted base heat flux compared to the experiment. The Spalart-Allmaras (SA) model and the renormalization group (RNG) model have performed best in temperature estimation, respectively, in around r/rb=0~0.2 and r/rb=0.6~1. The realizable k‐ε (RKE) model has underestimated the reverse flow and failed to correctly reflect the recirculation in the base region, thus poorly predicted base heating. Among all the investigated turbulence models, the BSL and SST models are more suitable for launch vehicle base heating simulation.

Numerical Modeling of Stall and Poststall Events of a Single Pitching Blade of a Cycloidal Rotor

In the present work, a numerical study is carried out to compare the performance of seven turbulence models on a single pitching blade of cycloidal rotor operating in deep dynamic stall regime at moderate Reynolds number. The investigated turbulence models were: (i) kω-shear stress transport (SST), (ii) kω-SST with γ, (iii) transition SST (γ–Reθ), (iv) scale adaptive simulation (SAS), (v) SAS coupled with transition SST, (vi) SAS with γ, and (vii) detached eddy simulation (DES) coupled with transition kω-SST. The wake vortices evolution and shedding analysis are also carried out for the pitching blade. The performance of the investigated turbulence models is evaluated at various critical points on the hysterias loop of lift and drag coefficients. The predictions of the investigated turbulence models are in good agreement at lower angle of attack, i.e., αu ≤ 20 deg. The detailed quantitative analysis at critical points showed that the predictions of SAS and transition SST-SAS turbulence models are in better agreement with the experimental results as compared to the other investigated models. The wake vortices analysis and fast Fourier transport analysis showed that the wake vortex characteristics of a pitching blade are significantly different than those for the low amplitude oscillating blade at the higher reduced frequency.

On characteristics of a non-reacting and a reacting turbulent flow over a backward facing step (BFS)

The turbulent reacting flow over a backward facing step shares some essential characteristics of premixed combustion occurring in a typical gas turbine combustor, while it is a simpler configuration to observe and model. For this reason and to explore the characteristics of the turbulent flow, in this study the combustion and flow dynamics in a backward facing step as a most elementary part of a combustor is studied numerically in atmospheric conditions. Two different configurations representing two laboratory devices are considered. As a first necessary step, the accuracy of predicted results is validated through the detailed comparison of numerical predictions and experimental measurements for a non-reacting flow. First, based on these non-reacting calculations, the turbulent model is selected and then the reacting simulations are done using a standard combustion model (available in CFX). Calculations are well supported with experimental data available from literature. Among the investigated turbulence models (k−ω, SST and SAS–SST), SAS–SST model showed the best agreement with the experimental data. The chosen turbulence model was used for the calculation of well documented case of turbulent flow over a backward facing step with the heated wall, showing satisfactory results compared to experimental data. For modeling of the reacting flow, the BVM combustion model was used. The predicted results using this model showed accurate results with an error about 2% on prediction of reattachment length.

Optimization of a Lobed Perforated Panel Diffuser - A Numerical Study of Orifice Arrangement

Heating Ventilating and Air Conditioning (HVAC) systems are primarily designed for ensuring good indoor air quality and thermal comfort. However, building energy requirements tend to put demand on reducing air change rates. Passive control of jet flows in order to enhance mixing and entrainment may be a solution to this problem. Our purpose is to develop new air diffusers, in order to ameliorate the users’ thermal comfort and air quality. When the diffuser is a lobed perforated panel (Meslem et al, 2010), the optimization of jet entrainment consists of optimizing the spacing between neighboring orifices and their relative arrangement on the panel. The objective is to conceive a perforated panel diffuser with high entrainment and high perforation rate. In a recent study (Meslem et al, 2011) the flow field of a turbulent twin cross-shaped jet was investigated experimentally and numerically using different turbulence models. In comparison to Particle Image Velocimetry (PIV) measurements, it was shown that among the investigated turbulence models, the SST k-ω model is capable of reproducing reasonably well jet interaction, global expansion and ambient air entrainment when the flow is numerically resolved through the lobed diffuser. Based on the previous experimental validation of the SST k-ω turbulence model, in this work, numerical simulations of parallel cross-shaped jets in different flow configurations are analysed using this model. This study deals with the effect of orifice arrangement and spacing on jet entrainment. The parametric study put into evidence an optimal configuration which will be transferred to the scale of a real air diffuser.

CFD analysis of the HYPER spallation target

KAERI (Korea Atomic Energy Research Institute) is developing an accelerator driven system (ADS) named HYPER (HYbrid Power Extraction Reactor) for a transmutation of long-lived nuclear wastes. One of the challenging tasks for the HYPER system is to design a large spallation target with a beam power of 15–25 MW. The paper focuses on a thermal–hydraulic analysis of the active part of the HYPER target. Computational fluid dynamics (CFD) analysis was performed by using a commercial code CFX 5.7.1. Several advanced turbulence models with different grid structures were applied. The CFX results reveal a significant impact of the turbulence model on the window temperature. Particularly, the k– ε model predicts the lowest window temperature among the five investigated turbulence models.

Numerical and Experimental Analysis of the Intake Flow in a High Performance Four-Stroke Motorcycle Engine: Influence of the Two-Equation Turbulence Models

An experimental and numerical analysis of the intake system of a production high performance four-stroke motorcycle engine was carried out. The aim of the work was to characterize the fluid dynamic behavior of the engine during the intake phase and to evaluate the capability of the most commonly used two-equation turbulence models to reproduce the in-cylinder flow field for a very complex engine head. Pressure and mass flow rates were measured on a steady-flow rig. Furthermore, velocity measurements were obtained within the combustion chamber using laser Doppler anemometry (LDA). The experimental data were compared to the numerical results using four two-equation turbulence models (standard k-ε, realizable k-ε, Wilcox k-ω, and SST k-ω models). All the investigated turbulence models well predicted the global performances of the intake system and the mean flow structure inside the cylinder. Some differences between measurements and computations were found close to the cylinder head while an improving agreement was evident moving away from the engine head. Furthermore, the Wilcox k-ω model permitted the flow field inside the combustion chamber of the engine to be reproduced and the overall angular momentum of the flux with respect to the cylinder axis to be quantified more properly.

A comparative study of two-equation turbulence models for simulating microscale wind flow

Numerical modelling of turbulence plays a crucial role in providing accurate microscale wind fields which are necessary to predict transport and dispersion of pollutants in the vicinity of buildings reliably. Although more elaborate turbulence models have been developed, two-equation turbulence models are widely applied in atmospheric flow and dispersion models. The large CPU time required for direct numerical simulation (DNS) and large eddy simulation (LES) still prohibits their application to microscale wind flow simulations of practical interest. In this study, the results obtained by the microscale model MIMO for the three-dimensional flow around a surface mounted cubical obstacle, using different two-equation turbulence models, are discussed. The accuracy of the investigated turbulence models is assessed by comparing the numerical results with wind tunnel measurements.