Performance Of Turbulence Models Research Articles

Performance of Turbulence Models on Heat Transfer and Pressure Drop with a 25° Continuous Helical Baffled Heat Exchanger

The objective of the present work is to study the influence of three turbulence models and find out the suitability of the k-ω SST turbulence model. The thermal performance of a heat exchanger with single continuous helical baffle is studied numerically and experimentally. A commercial CFD package (Ansys-Fluent),based on the finite volume method, is used to predict the performance of 25° continuous helical baffled Heat Exchanger using 3 turbulence models viz. k-ε Realizable, k-ε RNG and k-ω SST. The experiments are carried out in counter-flow mode with cold fluid (ambient water) on the shell side and hot fluid (hot water) on the tube side. In the experimentation, the variation of the shell side heat transfer coefficient and of the pressure drop with shell side mass flow rate is tested. For the validation of the numerical model, the numerical results are compared with the experimental results and with the published literature. The experimental heat transfer coefficient exhibits a deviation from the correlated values in the range of 1.5% to 5.3%, whereas pressure drop shows deviation in the range of 4.22 to 6.67%. The results indicate that all the three turbulence models are in good agreement with the experimental findings. The k-ε Realizable model is the best turbulence model for the prediction of heat transfer and k-ω SST is the best turbulence model for prediction of pressure drop. For the overall performance, k-ω SST model is suitable for the prediction of both heat transfer and pressure drop and it helps in capturing the swirling flows in helical baffle heat exchangers.

Performance of turbulence models for transonic flows in a diffuser

Eight turbulence models frequently used in aerodynamics have been employed in the detailed numerical investigations for transonic flows in the Sajben diffuser, to assess the predictive capabilities of the turbulence models for shock wave/turbulent boundary layer interactions (SWTBLI) in internal flows. The eight turbulence models include: the Spalart–Allmaras model, the standard [Formula: see text] model, the RNG [Formula: see text] model, the realizable [Formula: see text] model, the standard [Formula: see text] model, the SST [Formula: see text] model, the [Formula: see text] model and the Reynolds stress model. The performance of the different turbulence models adopted has been systematically assessed by comparing the numerical results with the available experimental data. The comparisons show that the predictive performance becomes worse as the shock wave becomes stronger. The [Formula: see text] model and the SST [Formula: see text] model perform much better than other models, and the SST [Formula: see text] model predicts a little better than the [Formula: see text] model for pressure on walls and velocity profile, whereas the [Formula: see text] model predicts a little better than the SST [Formula: see text] model for separation location, reattachment location and separation length for strong shock case.

Investigating ventilation system performance in large space building: A nozzle primary air supply with secondary airflow-relay system

In large space buildings, nozzle is usually used for ventilation and air conditioning. A secondary airflow-relay system is proposed for situations that building span is larger than the primary jet flow range. A field measurement was conducted to investigate the performance of primary airflow nozzle and secondary jet fan system; a computational fluid dynamics model was built up to research the secondary airflow velocity and relative location of primary and secondary terminal on the ventilation performance. The performance of different computational fluid dynamics turbulence models in predicting the primary jet trajectory path was evaluated by comparing predicted result with field measurements. The temperature field predicted by computational fluid dynamics method was also validated by experimentally measured vertical temperature distribution data in certain measurement points. Various indices including Air Diffusion Performance Index, percentage dissatisfied, and coefficient of variation of temperature and velocity at breathing level were used to quantitatively evaluate the ventilation performance of different configurations. It is found that re-normalization group k-ϵ turbulence model gives the best overall prediction on the primary air nozzle jet trajectory path and field temperature distribution. In a certain large space building utilizing secondary airflow system, the relative location of primary and secondary equipment and air velocity should be properly specified in order to improve the ventilation performance of a large space building. The secondary airflow-relay may potentially increase the cooling energy consumption as indicated by computational fluid dynamics result.

Performance of turbulence and cavitation models in prediction of incipient and developed cavitation

The aim of this article is to assess the impact of turbulence and cavitation models on the prediction of diesel injector nozzle flow. Two nozzles are examined, an enlarged one, operating at incipient cavitation, and an industrial injector tip, operating at developed cavitation. The turbulence model employed includes the re-normalization group k–ε, realizable k–ε and k–ω shear stress transport Reynolds-averaged Navier–Stokes models; linear pressure–strain Reynolds stress model and the wall adapting local eddy viscosity large eddy simulation model. The results indicate that all Reynolds-averaged Navier–Stokes and the Reynolds stress turbulence models have failed to predict cavitation inception due to their limitation to resolve adequately the low pressure existing inside vortex cores, which is responsible for cavitation development in this particular flow configuration. Moreover, Reynolds-averaged Navier–Stokes models failed to predict unsteady cavitation phenomena in the industrial injector. However, the wall adapting local eddy viscosity large eddy simulation model was able to predict incipient and developed cavitation, while also capturing the shear layer instability, vortex shedding and cavitating vortex formation. Furthermore, the performance of two cavitation methodologies is discussed within the large eddy simulation framework. In particular, a barotropic model and a mixture model based on the asymptotic Rayleigh–Plesset equation of bubble dynamics have been tested. The results indicate that although the solved equations and phase change formulation are different in these models, the predicted cavitation and flow field were very similar at incipient cavitation conditions. At developed cavitation conditions, standard cavitation models may predict unrealistically high liquid tension, so modifications may be essential. It is also concluded that accurate turbulence representation is crucial for cavitation in nozzle flows.

Observational and Numerical Methods for Quantifying and Modeling of Turbulence in a Stratified Reservoir

The interplay between stratification and shear in lakes controls the vertical mixing, which is the most important mechanism affecting the transport of heat, salt, momentum and suspended and dissolved substances. This study attempts to quantify and characterize the turbulence from direct measurements conducted in a reservoir. A 3D numerical model is used to investigate the water column hydrodynamics for the duration of measurements and the performance of various turbulence models used in the CFD model are investigated via simulation of mixing in the reservoir. The drawdown curves produced by the turbulence models are formulized through linear equations. Although, use of different turbulence models do not have significant effects on the flow hydrodynamics away from the intake structure; significant effects especially on turbulence kinetic energy production are observed at the orifice. Therefore, for simulation of withdrawal flow, either use of shear stress transport (SST) k-omega models solving equations all the way to the wall or k-epsilon models with the nonequilibrium wall function is recommended to account for the changes in the pressure gradient. In this study, the methods using quantified turbulent characteristics of the flow to reformulate the Stokes’ settling velocity to be applied in turbulent flows are also investigated. An approach to predict setting velocity in turbulent flows that utilizes acoustic Doppler instruments for quantification of turbulent characteristics is presented. Modification of the Stokes’ settling velocity with the nondimensionalized turbulent kinetic energy production profiles lead better results than other turbulence characteristics (buoyancy flux and by Richardson number flux) widely used in characterizing turbulent mixing.

A comparison of the CFD simulation results in 5 × 5 sub-channels with mixing grids using different turbulence models

Abstract We evaluated the performance of various turbulence models, including eddy viscosity models and Reynolds stress models, when analyzing rod bundles in fuel assemblies using the Computational Fluid Dynamics (CFD) method. The models were assessed by calculating the pressure drop and Nusselt numbers in 5 × 5 rod bundles using the CFD software ANSYS CFX. Comparisons between the numerical and experimental results, as well as the swirl factor, cross-flow factor, and turbulence intensity utilized to evaluate the swirling and cross-flow, were used to analyze the inner relationship between the flow field and heat transfer. These comparisons allow the selection of the most appropriate turbulence model for modeling flow features and heat transfer in rod bundles.

Effects of Low Incoming Turbulence on the Flow around a 5 : 1 Rectangular Cylinder at Non-Null-Attack Angle

The incompressible high Reynolds number flow around the rectangular cylinder with aspect ratio 5 : 1 has been extensively studied in the recent literature and became a standard benchmark in the field of bluff bodies aerodynamics. The majority of the proposed contributions focus on the simulation of the flow when a smooth inlet condition is adopted. Nevertheless, even when nominally smooth conditions are reproduced in wind tunnel tests, a low turbulence intensity is present together with environmental disturbances and model imperfections. Additionally, many turbulence models are known to be excessively dissipative in laminar-to-turbulent transition zones, generally leading to overestimation of the reattachment length. In this paper, Large Eddy Simulations are performed on a 5 : 1 rectangular cylinder at non-null-attack angle aiming at studying the sensitivity of such flow to a low level of incoming disturbances and compare the performance of standard Smagorinsky-Lilly and Kinetic Energy Transport turbulence models.

Investigation of the Performance of Turbulence Models With Respect to High Flow Curvature in Centrifugal Compressors

The goal of this research is to evaluate the performance of three turbulence models with respect to flow with high curvature in a centrifugal compressor stage designed for an aero-engine. The effectiveness of the curvature correction terms in the two-equation turbulence models is the main focus of this study, as implemented in the curvature-corrected shear stress transport (SST-CC) model of Smirnov and Menter. The SST-CC model uses a production multiplier in the k and ω equations. SST-CC results were compared against the SST model and previous simulations by Bourgeois et al. (2011, “Assessment of Turbulence Model Predictions for an Aero-Engine Centrifugal Compressor,” ASME J. Turbomach., 133(1), pp. 1–15) using the Reynolds stress model (RSM–SSG) for stage performance characteristics, experimental velocity profiles at the impeller–diffuser interface, and velocity contours at the diffuser exit. The production multiplier was investigated in the compressor impeller. The comparisons showed that the SST-CC model better predicted the choke region in the pressure characteristic and efficiency characteristic, whereas the SST model better predicted the stall region. However, both models predicted a similar mean flow velocity field. Analysis of the production multiplier demonstrated that the term provided the expected effects near the walls of the convex and concave surfaces. However, away from the walls where turbulent production term was insignificant, the production multiplier showed abnormal predictions. The rotation effects were found to be weaker than the curvature effects near the impeller trailing edge of the current compressor.

Assessment of turbulence model performance: Severe acceleration with large integral length scales

Turbulence is substantially laminarised, when the mean flow experiences streamwise acceleration above a certain critical acceleration parameter. Recently, to essentially reveal aero engine intake acceleration scenarios, Direct Numerical Simulations (DNS) have been performed for turbulent flow through a rapidly contracting channel. On average, the streamwise acceleration parameter Ks is of the magnitude of 1×10−5. Converged statistics show that it is the streamwise acceleration that causes the first term of the production rate for u′u′¯ to be negative. This initiates the degeneration towards laminar flow and also closes the usual wall turbulence self-sustaining mechanism. Further downstream, the progressive turbulence recovery is largely streamwise dominant. Importantly, the laminarisation effects are lagging to the rate of contraction. To assess the corresponding turbulence model performance and for better modelling, for the same flow configurations, Reynolds-averaged Navier–Stokes (RANS) predictions are compared, using some available Reynolds-stress (RSM) and eddy-viscosity models. These are the second-order closure with the strain–ω formulation, the standard k–ω and the Menter’s shear-stress transport (SST) models, the standard Spalart–Allmaras (S–A) model, and that with the strain–vorticity correction. As will be shown, through the contraction, all the benchmarked models are able to predict the essential characteristics of the laminarisation; whereas, further downstream, the eddy-viscosity models tend to return the flow immediately back to the fully developed turbulence. In contrast, the RSM predicts the gradually recovery process, in spite of the lower growth rate, relative to that of the DNS. The S–A model has been modified for the lagging mechanism caused by severe acceleration. The corresponding modified predictions better match the mean flow characteristics. Moreover, all models would also benefit from sensitisation to the impact of the large integral length scales.

Assessment of turbulence model performance: Large streamline curvature and integral length scales

For the flow over curved surfaces, an extra wall-normal pressure gradient is imposed to the flow through excessive surface pressure, such that the flow turns in alignment with the surface. In turn, turbulent fluctuations are suppressed over the convex surface; whereas, they are enhanced over the concave. Recently, the direct numerical simulation (DNS) of turbulent channel flow experiencing a 60 degree circular bend shows highly complex flow phenomena. Particularly, the mean flow properties are directly related to the channel geometry; in the impulse response of the mean flow to the step change of streamline curvature, sudden changes in mean strain rate and extra rates of strain emerge. This mean flow process is prior to the response of the turbulence structures. Due to the large streamline curvature, the underlying turbulence lagging mechanism and the stress strain misalignment are difficult to model. For this, the new DNS data for the wall bounded flow with high streamline curvature and large integral length scales is used to explore RANS performance. For eddy-viscosity models, this leads to the Boussinesq approximation being questionable. Also, for a Reynolds-stress model (RSM) with closure approximations applicable to homogeneous turbulent flows that are nearly in equilibrium, the current case can result in substantial predictive error. This is because of, for example, the linear approximation for the rapid pressure–strain correlation. To help move towards better turbulence modelling, Reynolds-averaged Navier–Stokes (RANS) predictions are compared for the same flow configuration as the DNS, using some popular turbulent models. These models include the second-order closure with the stress-ω formulation, the standard k−ω and the Menter’s shear-stress transport (SST) models, the standard Spalart–Allmaras (S-A) model with and without the corresponding strain–vorticity correction. As expected, overall, the RSM provides closer predictions to the DNS data than the selected eddy-viscosity models, even though the predictive accuracy needs to be further improved. Potentially, a non-linear constitutive relation or second-order closure, incorporating a relaxation approximation for the lagging mechanism, may lead to a remedy for the current non-equilibrium flow. Moreover, all models would also benefit from sensitisation to the impact of the large integral length scales.

Evaluation of Turbulence Models Performance in Predicting Incipient Cavitation in an Enlarged Step-Nozzle

Predictive capability of RANS and LES models to calculate incipient cavitation of water in a step nozzle is assessed. The RANS models namely, Realizable k-ε, SST k-ω and Reynolds Stress Model did not predict any cavitation, due to the limitation of RANS models to predict the low pressure vortex cores. LES WALE model was able to predict the cavitation by capturing the shear layer instability and vortex shedding. The performance of a barotropic cavitation model and Rayleigh-Plesset-based cavitation models was compared using WALE model. Although the phase change formulation is different in these models, the predicted cavitation and flow field were not significantly different.

Turbulence Modeling of Flows with Extensive Crossflow Separation

The reasons for the difficulty in simulating accurately strong 3-D shock wave/turbulent boundary layer interactions (SBLIs) and high-alpha flows with classical turbulence models are investigated. These flows are characterized by the appearance of strong crossflow separation. In view of recent additional evidence, a previously published flow analysis, which attributes the poor performance of classical turbulence models to the observed laminarization of the separation domain, is reexamined. According to this analysis, the longitudinal vortices into which the separated boundary layer rolls up in this type of separated flow, transfer external inviscid air into the part of the separation adjacent to the wall, decreasing its turbulence. It is demonstrated that linear models based on the Boussinesq equation provide solutions of moderate accuracy, while non-linear ones and others that consider the particular structure of the flow are more efficient. Published and new Reynolds Averaged Navier–Stokes (RANS) simulations are reviewed, as well as results from a recent Large Eddy Simulation (LES) study, which indicate that in calculations characterized by sufficient accuracy the turbulent kinetic energy of the reverse flow inside the separation vortices is very low, i.e., the flow is almost laminar there.

Numerical validation and back-pressure effect on internal compression flows of typical supersonic inlet

AbstractA numerical study was conducted to analyse the performance of different turbulence models and different turbulence intensities and turbulence length scales specified for the boundary condition of the inflow to the internal compression flow field of a typical supersonic inlet. The effect of the back-pressure ratio on the properties of the flow field was also investigated. Computational results obtained by the commercial software FLUENT, which is used to solve the full two-dimensional Reynolds-averaged Navier-Stokes equations, were validated through both graphical and quantitative comparisons with previously published experimental data. The two-equation models that were considered in this study are the RNGk-ε, realisablek-ε, standardk-ε, and SSTk-ω turbulence models. The RNGk-ε model had the best performance among the four models and predicted good wall pressure distributions. The best agreement between the predicted results and experimental data was obtained when either the default values of the freestream turbulence intensity and length scale in the FLUENT solver were used, or the empirical formula was used to calculate the two parameters of the freestream turbulence properties. The shock wave pattern varied between the oblique mode and the fully developed normal mode with increasing back-pressure ratio, and the unstart phenomenon occurred when the back-pressure ratio was sufficiently high.

Investigation on the relative performance of various low-Reynolds number turbulence models for buoyancy-driven flow in a tall cavity

The present study deals with the numerical investigation of turbulent buoyancy driven flow in a differentially heated rectangular cavity with adiabatic horizontal walls. The aspect ratio of cavity is 5 and the Rayleigh number based on the cavity height is 4.56 × 1010. The computations have been carried out using the finite volume method on a staggered grid and SIMPLEC algorithm for pressure–velocity coupling. The low-Reynolds number \(k-\epsilon\) model proposed by Yang and Shih (YS), low-Reynolds number \(k-\omega\) model proposed by Wilcox, and \(k-\omega\) shear stress transport (SST) model of Menter have been applied for turbulence closure. The performance comparison of different models have been carried out using the experimental, LES and various RANS results available in the literature. The computation of turbulent natural convection flow is numerically challenging due to complex flow involving laminar, transition and turbulent regions, coupling of velocity with the energy equation, and some other problems reported in literature e.g. grid dependency of solution, numerical stability problem, etc. The flux Richardson number is calculated to get an estimate of relative importance of buoyancy and shear force in different regions of flow. The shearing and swirling zones have been identified in the entire flow domain using the \(\lambda _{2}\) criterion. Based on the comparison of mean flow, heat transfer and turbulence characteristics with the available results, it has been found that YS model performs better. The better performance obtained from YS model may be due to peculiarity of model that takes into account the Kolmogorov time scale near the wall and the conventional time scale \((k/\epsilon )\) away from the wall.

Numerical investigation on heat transfer of supercritical CO2 in heated helically coiled tubes

Numerical simulation on heat transfer of supercritical CO2 in heated helically coiled tubes is performed to evaluate the performance of turbulence models in predicting heat transfer of supercritical CO2 in the helically coiled tube, and to help better understanding the heat transfer mechanism. All turbulence models yield similar tendencies in heat transfer coefficient in the helically coiled tube. The SST (shear-stress transport) model gives the best prediction to the experimental data due to accurate predictions of flow separation under adverse pressure gradients. The parameter of Gr/Re2.7 is incapable of predicting the buoyancy effect onset of supercritical CO2 in the helically coiled tube. The turbulent Prandtl number has little influence on the calculated heat transfer coefficient.

Modelling turbulent vertical mixing sensitivity using a 1-D version of NEMO

Abstract. Through two numerical experiments, a 1-D vertical model called NEMO1D was used to investigate physical and numerical turbulent-mixing behaviour. The results show that all the turbulent closures tested (k+l from Blanke and Delecluse, 1993, and two equation models: generic length scale closures from Umlauf and Burchard, 2003) are able to correctly reproduce the classical test of Kato and Phillips (1969) under favourable numerical conditions while some solutions may diverge depending on the degradation of the spatial and time discretization. The performances of turbulence models were then compared with data measured over a 1-year period (mid-2010 to mid-2011) at the PAPA station, located in the North Pacific Ocean. The modelled temperature and salinity were in good agreement with the observations, with a maximum temperature error between −2 and 2 °C during the stratified period (June to October). However, the results also depend on the numerical conditions. The vertical RMSE varied, for different turbulent closures, from 0.1 to 0.3 °C during the stratified period and from 0.03 to 0.15 °C during the homogeneous period. This 1-D configuration at the PAPA station (called PAPA1D) is now available in NEMO as a reference configuration including the input files and atmospheric forcing set described in this paper. Thus, all the results described can be recovered by downloading and launching PAPA1D. The configuration is described on the NEMO site (http://www.nemo-ocean.eu/Using-NEMO/Configurations/C1D_PAPA). This package is a good starting point for further investigation of vertical processes.

Aerodynamic Investigation of RAE2822 Airfoil in Ground Effect

The aerodynamic characteristics of RAE2822 in ground effect were investigated with finite volume method (FVM) based on the averaged Navier-Stokes equations. The performance of several eddy-viscosity turbulence models were assessed by comparing with the available experimental data of NACA4412 in ground effect. Realizable k-epsilon model demonstrates with good capability of predicting the flow characteristics. In study of the ground effect on supercritical airfoil RAE2822, high lift and ratio of lift and drag only could be achieved in medium angle of attack (AOA). The ground effect would induce the upfront separation of boundary layer on the airfoil. In study of the ground boundary condition and compressibility effect, these factors all show significant impact on the aerodynamic characteristics of RAE2822 in ground effect, which should be considered in numerical or experimental studies.

Appropriate CFD Turbulence Model for Improving Indoor Air Quality of Ventilated Spaces

Abstract Accurate assessment of air-flow in ventilated spaces is of major importance for achieving healthy and comfortable indoor environment conditions. The CFD (Computational Fluid Dynamics) technique is nowadays one of the most used approaches in order to improve the indoor air quality in ventilated environments. Nevertheless, CFD has still two main challenges: turbulence modeling and experimental validation. As a result, the objective of this study is to evaluate the performance of different turbulence models potentially appropriate for the prediction of indoor airflow. Accordingly, results obtained with 6 turbulence models (standard k-ε model, RNG k-ε model, realizable k-ε model, LRN SST k-ω model, transition SST k-ω model and low Reynolds Stress-ω model) are thoroughly validated based on detailed experimental data. The configuration taken into account in this work corresponds to isothermal and anisothermal airflows produced by mixing ventilation systems in small enclosures at low room air changes per hour. In general, the transition SST k-ω model shows the better overall behavior in comparison with measurement values. Consequently, the application of this turbulence model is appropriate for air flows in ventilated spaces, being an interesting option to more sophisticated LES (Large Eddy Simulation) models as it requires less computational resources.

Performance of turbulence models for dense gas release in computational fluid dynamics

The prediction of temporal and spatial concentration profiles of gas cloud is of great importance in safety issues. Existing models for predicting dense gases release are used for flat plates, and they cannot usually involve the complex environments, hence in this article, computational fluid dynamics have been used for prediction of dense gases behavior. The selection of turbulence model shows its significance in the results of computational fluid dynamics. In order to select the best turbulence model, the models of k–ɛ and k–ω have been studied. Experimental data of the test no. 26 of Thorney Island Series data have been extracted. The results show that k–ɛ realizable model is closest to the experimental data. This model has the closest and most appropriate prediction of the spatial and temporal profile. The model is also able to predict the phenomenon of gravity slumping associated with dense gas dispersion.

Wind flow simulations on idealized and real complex terrain using various turbulence models

The effect of topographic features on wind speed and wake turbulence is evaluated by conducting Computational Fluid Dynamics (CFD) simulations using an in-house CFD program that features various turbulence models. The simulation results are assessed by computing Fractional Speed Up Ratio (FSUR) along longitudinal lines at different elevations. Such information is useful for evaluating wind loads on long span structures and micro-siting of wind turbines on complex terrain. Simulations are conducted on both idealized and real topographic features in both 2D and 3D domain. The turbulence structure behind hills is examined using several turbulence models such as the mixing-length, standard k–∊, RNG k–∊, realizable k–∊ and Smagorinsky LES models. All turbulence models predicted FSUR values on upstream side of hills adequately; however, the performance of simple turbulence models, such as mixing length, is found to be insufficient for characterizing wakes behind hills. RANS turbulence models gave results close to one another; however, those models that incorporate modifications to account for adverse pressure gradient conditions performed better at wakes behind hills. LES conducted at full scale dimensions, and using wall functions, failed to give results that are comparable to the other turbulence models. Re-conducting the simulations at model scale dimensions, hence at relatively small Reynolds number, and without using wall functions gave results that are comparable to those found in the literature. Therefore, use of wall functions can degrade quality of results in LES of high Reynolds number flows of practical interest.