Journal articles: 'URANS/SAS'

1

Yang, Xianglong, and Lei Yang. "An Elliptic Blending Turbulence Model-Based Scale-Adaptive Simulation Model Applied to Fluid Flows Separated from Curved Surfaces." Applied Sciences 12, no. 4 (February 16, 2022): 2058. http://dx.doi.org/10.3390/app12042058.

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On the basis of a previously developed elliptic blending turbulence model (SST–k–ω–φ–α model), a scale-adaptive simulation (SAS) model is developed by following Menter and Egorov’s SAS concept. An SAS source term, which is related to the ratio of the modeled turbulence scale to the von Kármán length scale, is introduced into the corresponding length-scale determining equation. The major motivation of this study is that the conventional unsteady Reynolds-averaged Navier–Stokes (URANS) models provide only large-scale unsteadiness. The introduction of the SAS term allows the proposed SAS model to dynamically adjust to resolved structures in a URANS framework because this term is sensitive to resolved fluctuations. The predictive capabilities of the proposed SAS model are demonstrated by computing the complex flow configurations in three cases with flow separation from curved surfaces, namely, three-dimensional (3D) diffuser flow, two-dimensional (2D) periodic hills flow, and 2D U-turn duct flow. For comparison, the results predicted by the SST–k–ω–φ–α model and the Menter and Egorov’s SAS model (SST–SAS) are provided. The results are also compared with the relevant experimental, direct numerical simulation, and large eddy simulation data. The results show that the SST–k–ω–φ–α model cannot capture the critical features for all three flows, and that the SST–SAS model is able to predict the results reasonably well. The proposed SAS model is capable of resolving more portions of the turbulence structures, and it yields the best results in all the cases.

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Kratzsch, Christoph, Amjad Asad, and Rüdiger Schwarze. "CFD of the MHD Mold Flow by Means of Hybrid LES/RANS Turbulence Modeling." Journal for Manufacturing Science and Production 15, no. 1 (March 31, 2015): 49–57. http://dx.doi.org/10.1515/jmsp-2014-0046.

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AbstractIn the last decades, electromagnetic braking (EMBr) systems become a powerful tool to dampen possible jet oscillations in the continuous casting mold. Further studies showed that if a EMBr is not positioned correctly, it can induce flow oscillations. Hence, the design of these braking systems can be promoted by adequate CFD simulations. In most cases, unsteady RANS simulations (URANS) are sufficient to resolve low-frequency, large-scale oscillations of these MHD flows. Alternatively, Large Eddy Simulations (LES) may also resolve important details of the turbulence. However, since they require much finer computational grids, the computational costs are much higher. A bridge between both approaches are hybrid methods like the Scale Adaptive Simulation (SAS). In this study, we compare the performance of SAS with URANS and LES. Results are validated in detail by comparison with data from a Ruler-EMBr model experiment.

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Jiménez-Varona, José, Gabriel Liaño, José L. Castillo, and Pedro L. García-Ybarra. "Roughness Effect on the Flow Past Axisymmetric Bodies at High Incidence." Aerospace 9, no. 11 (October 28, 2022): 668. http://dx.doi.org/10.3390/aerospace9110668.

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The flow at low Mach numbers and high angles of attack over axisymmetric configurations is not symmetric. The mechanism that triggers the asymmetry is a combination of a global (temporal) instability and a convective (spatial) instability. This latter instability is caused by roughness and other geometrical imperfections, which lead to roll angle dependent forces. The flow at these conditions has a complex vortex sheet structure, with two or three different flow regions. An accurate simulation by means of Computational Flow Dynamics (CFD) is thus very challenging, and many researchers have therefore employed Large Eddy Simulation (LES) codes. This study demonstrates that Unsteady Reynolds Averaged Navier-Stokes (URANS) methods are a suitable alternative, if Scale Adaptive Simulation (SAS) is used. This method is capable of capturing the main flow features, provided that fine meshes, which achieve geometrical similarity between the meshed geometry and the real object, and small-time steps are used. It is also demonstrated that, by using URANS methods in combination with SAS, strong differences in the global and local forces depending on the surface roughness of the model are obtained, a result which coincides with several wind tunnel tests.

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Higgins, R. J., G. N. Barakos, and E. Jinks. "Estimation of three-dimensional aerodynamic damping using CFD." Aeronautical Journal 124, no. 1271 (November 12, 2019): 24–43. http://dx.doi.org/10.1017/aer.2019.135.

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AbstractAeroelastic phenomena of stall flutter are the result of the negative aerodynamic damping associated with separated flow. From this basis, an investigation has been conducted to estimate the aerodynamic damping from a time-marching aeroelastic computation. An initial investigation is conducted on the NACA 0012 aerofoil section, before transition to 3D propellers and full aeroelastic calculations. Estimates of aerodynamic damping are presented, with a comparison made between URANS and SAS. Use of a suitable turbulence closure to allow for shedding of flow structures during stall is seen as critical in predicting negative damping estimations. From this investigation, it has been found that the SAS method is able to capture this for both the aerofoil and 3D test cases.

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Decaix, Jean, Vlad Hasmatuchi, Maximilian Titzschkau, and Cécile Münch-Alligné. "CFD Investigation of a High Head Francis Turbine at Speed No-Load Using Advanced URANS Models." Applied Sciences 8, no. 12 (December 5, 2018): 2505. http://dx.doi.org/10.3390/app8122505.

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Due to the integration of new renewable energies, the electrical grid undergoes instabilities. Hydroelectric power plants are key players for grid control thanks to pumped storage power plants. However, this objective requires extending the operating range of the machines and increasing the number of start-up, stand-by, and shut-down procedures, which reduces the lifespan of the machines. CFD based on standard URANS turbulence modeling is currently able to predict accurately the performances of the hydraulic turbines for operating points close to the Best Efficiency Point (BEP). However, far from the BEP, the standard URANS approach is less efficient to capture the dynamics of 3D flows. The current study focuses on a hydraulic turbine, which has been investigated at the BEP and at the Speed-No-Load (SNL) operating conditions. Several “advanced” URANS models such as the Scale-Adaptive Simulation (SAS) SST k - ω and the BSL- EARSM have been considered and compared with the SST k - ω model. The main conclusion of this study is that, at the SNL operating condition, the prediction of the topology and the dynamics of the flow on the suction side of the runner blade channels close to the trailing edge are influenced by the turbulence model.

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Wang, Shibo, James R. Bell, David Burton, Astrid H. Herbst, John Sheridan, and Mark C. Thompson. "The performance of different turbulence models (URANS, SAS and DES) for predicting high-speed train slipstream." Journal of Wind Engineering and Industrial Aerodynamics 165 (June 2017): 46–57. http://dx.doi.org/10.1016/j.jweia.2017.03.001.

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Wang, Guangxue, Shengye Wang, Hao Li, Xiang Fu, and Wei Liu. "Comparative assessment of SAS, IDDES and hybrid filtering RANS/LES models based on second-moment closure." Advances in Mechanical Engineering 13, no. 6 (June 2021): 168781402110284. http://dx.doi.org/10.1177/16878140211028447.

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The question of which turbulence model is better for a given class of applications is always confusing for the CFD researchers and users. Comparative assessments of scale-adaptive simulation (SAS), improved delay detached-eddy simulation (IDDES) and other hybrid RANS/LES models based on eddy-viscosity models (EVMs) are thoroughly investigated. But how well they perform based on a second-moment closure needs to be answered. In this paper, a widely acclaimed Reynolds-stress model (RSM) in aeronautical engineering, SSG/LRR-[Formula: see text] model, is carried out. The relevant test cases include the NACA0012 airfoil stalled flows and turret separated flow. In order to make a more reasonable comparison, a seventh-order scheme WCNS-E8T7 is adopted to reduce the influence of the numerical dissipation and a symmetrical conservative metric method is used to ensure the robustness. By comparing with the relevant experimental data and the solutions by original SSG/LRR-[Formula: see text] model (etc. URANS), it shows that all of the three hybrid methods (SAS, IDDES and hybrid filtering methods) based on the SSG/LRR-[Formula: see text] model have a good ability to simulate unsteady turbulence. Among them, the IDDES correction has the most potential.

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Shukla, S., S. N. Singh, S. S. Sinha, and R. Vijayakumar. "Comparative assessment of URANS, SAS and DES turbulence modeling in the predictions of massively separated ship airwake characteristics." Ocean Engineering 229 (June 2021): 108954. http://dx.doi.org/10.1016/j.oceaneng.2021.108954.

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Münsterjohann, Sven, Jens Grabinger, Stefan Becker, and Manfred Kaltenbacher. "CAA of an Air-Cooling System for Electronic Devices." Advances in Acoustics and Vibration 2016 (October 20, 2016): 1–17. http://dx.doi.org/10.1155/2016/4785389.

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This paper presents the workflow and the results of fluid dynamics and aeroacoustic simulations for an air-cooling system as used in electronic devices. The setup represents a generic electronic device with several electronic assemblies with forced convection cooling by two axial fans. The aeroacoustic performance is computed using a hybrid method. In a first step, two unsteady CFD simulations using the Unsteady Reynolds-Averaged Navier-Stokes simulation with Shear Stress Transport (URANS-SST) turbulence model and the Scale Adaptive Simulation with Shear Stress Transport (SAS-SST) models were performed. Based on the unsteady flow results, the acoustic source terms were calculated using Lighthill’s acoustic analogy. Propagation of the flow-induced sound was computed using the Finite Element Method. Finally, the results of the acoustic simulation are compared with measurements and show good agreement.

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10

Maleki, Siavash, David Burton, and Mark C. Thompson. "Assessment of various turbulence models (ELES, SAS, URANS and RANS) for predicting the aerodynamics of freight train container wagons." Journal of Wind Engineering and Industrial Aerodynamics 170 (November 2017): 68–80. http://dx.doi.org/10.1016/j.jweia.2017.07.008.

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11

Hu, Ping, Tong Lin, Rui Yang, Xiaocheng Zhu, and Zhaohui Du. "Numerical investigation on flow instabilities in low-pressure steam turbine last stage under different low-load conditions." Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 235, no. 6 (February 27, 2021): 1544–62. http://dx.doi.org/10.1177/0957650921997199.

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The modern power generation system requires steam turbines operating at flexible operating points, and flow instabilities readily occur in the low-pressure (LP) last stage under low-load conditions, which may cause failure of the last stage moving blades. Some studies have shown that within this operating range, a shift of the operating point may lead to flow instabilities. Numerical simulation has gradually developed into a popular method for such researches, but it is expensive for a complex model, which has to be balanced between efficiency and accuracy. This work is divided into three parts: Firstly, one of the low-load conditions is selected to provide both URANS model and the Scale-Adaptive Simulation (SAS) model. The results of the two models are compared to evaluate specific flow phenomena; Secondly, through calculations of different low-load conditions, the flow structure and propagation characteristics of instabilities in the last stage are obtained; Finally, flow analysis is applied to explain the formation mechanism of flow instabilities in LP steam turbines. The results show that, the introduction of SAS model increases the randomness of flow over time, but does not fundamentally change the flow instabilities. Flow instabilities take different forms at different flow rate, from rotating instability to rotating stall. The formation of flow instabilities is related to the radial flow in the cascade passages.

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12

Rezaeiha, Abdolrahim, Hamid Montazeri, and Bert Blocken. "CFD analysis of dynamic stall on vertical axis wind turbines using Scale-Adaptive Simulation (SAS): Comparison against URANS and hybrid RANS/LES." Energy Conversion and Management 196 (September 2019): 1282–98. http://dx.doi.org/10.1016/j.enconman.2019.06.081.

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13

Fossi, Alain, Alain DeChamplain, and Benjamin Akih-Kumgeh. "Unsteady RANS and scale adaptive simulations of a turbulent spray flame in a swirled-stabilized gas turbine model combustor using tabulated chemistry." International Journal of Numerical Methods for Heat & Fluid Flow 25, no. 5 (June 1, 2015): 1064–88. http://dx.doi.org/10.1108/hff-09-2014-0272.

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Purpose – The purpose of this paper is to numerically investigate the three-dimensional (3D) reacting turbulent two-phase flow field of a scaled swirl-stabilized gas turbine combustor using the commercial computational fluid dynamic (CFD) software ANSYS FLUENT. The first scope of the study aims to explicitly compare the predictive capabilities of two turbulence models namely Unsteady Reynolds Averaged Navier-Stokes and Scale Adaptive Simulation for a reasonable trade-off between accuracy of results and global computational cost when applied to simulate swirl-stabilized spray combustion. The second scope of the study is to couple chemical reactions to the turbulent flow using a realistic chemistry model and also to model the local chemical non-equilibrium(NEQ) effects caused by turbulent strain such as flame stretching. Design/methodology/approach – Standard Eulerian and Lagrangian formulations are used to describe both gaseous and liquid phases, respectively. The computing method includes a two-way coupling in which phase properties and spray source terms are interchanging between the two phases within each coupling time step. The fuel used is liquid jet-A1 which is injected in the form of a polydisperse spray and the droplet evaporation rate is calculated using the infinite conductivity model. One-component (n-decane) and two-component fuels (n-decane+toluene) are used as jet-A1 surrogates. The combustion model is based on the mean mixture fraction and its variance, and a presumed-probability density function is used to model turbulent-chemistry interactions. The instantaneous thermochemical state necessary for the chemistry tabulation is determined by using initially the equilibrium (EQ) assumption and thereafter, detailed NEQ calculations through the steady flamelets concept. The combustion chemistry of these surrogates is represented through a reduced chemical kinetic mechanism (CKM) comprising 1,045 reactions among 139 species, derived from the detailed jet-A1 surrogate model, JetSurf 2.0 using a sensitivity based method, Alternate Species Elimination. Findings – Numerical results of the gas velocity, the gas temperature and the species molar fractions are compared with their experimental counterparts obtained from a steady state flame available in the literature. It is observed that, SAS coupled to the tabulated flamelet-based chemistry, predicts reasonably the main flame trends, while URANS even provided with the same combustion model and computing resources, leads to a poor prediction of the global flame trends, emphasizing the asset of a proper resolution when simulating spray flames. Research limitations/implications – The steady flamelet model even coupled with a robust turbulence model does not reproduce accurately the trend of species with slow oxidation kinetics such as CO and H2, because of the restrictiveness of the solutions space of flamelet equations and the assumption of unity Lewis for all species. Practical implications – This work is adding a contribution for spray flame modeling and can be seen as an extension to the significant efforts for the modeling of gaseous flames using robust turbulence models coupled with the tabulated flamelet-based chemistry approach to considerably reduce computing cost. The exclusive use of a commercial CFD code widely used in the industry allows a direct application of this simulation approach to industrial configurations while keeping computing cost reasonable. Originality/value – This study is useful to engineers interested in designing combustors of gas turbines and others combustion systems fed with liquid fuels.

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Shukla, Anuj Kumar, and Anupam Dewan. "Computational analysis of convective heat transfer properties of turbulent slot jet impingement." Engineering Computations ahead-of-print, ahead-of-print (June 28, 2021). http://dx.doi.org/10.1108/ec-08-2020-0483.

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Purpose Convective heat transfer features of a turbulent slot jet impingement are comprehensively studied using two different computational approaches, namely, URANS (unsteady Reynolds-averaged Navier–Stokes equations) and SAS (scale-adaptive simulation). Turbulent slot jet impingement heat transfer is used where a considerable heat transfer enhancement is required, and computationally, it is a quite challenging flow configuration. Design/methodology/approach Customized OpenFOAM 4.1, an open-access computational fluid dynamics (CFD) code, is used for SAS (SST-SAS k-ω) and URANS (standard k-ε and SST k-ω) computations. A low-Re version of the standard k-ε model is used, and other models are formulated for good wall-refined calculations. Three turbulence models are formulated in OpenFOAM 4.1 with second-order accurate discretization schemes. Findings It is observed that the profiles of the streamwise turbulence are under-predicted at all the streamwise locations by SST k-ω and SST SAS k-ω models, but follow similar trends as in the reported results. The standard k-ε model shows improvements in the predictions of the streamwise turbulence and mean streamwise velocity profiles in the zone of outer wall jet. Computed profiles of Nusselt number by SST k-ω and SST-SAS k-ω models are nearly identical and match well with the reported experimental results. However, the standard k-ε model does not provide a reasonable profile or quantification of the local Nusselt number. Originality/value Hybrid turbulence model is suitable for efficient CFD computations for the complex flow problems. This paper deals with a detailed comparison of the SAS model with URANS and LES for the first time in the literature. A thorough assessment of the computations is performed against the results reported using experimental and large eddy simulations techniques followed by a detailed discussion on flow physics. The present results are beneficial for scientists working with hybrid turbulence models and in industries working with high-efficiency cooling/heating system computations.

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Ravelli, Silvia, and Giovanna Barigozzi. "Application of Unsteady Computational Fluid Dynamics Methods to Trailing Edge Cutback Film Cooling." Journal of Turbomachinery 136, no. 12 (August 26, 2014). http://dx.doi.org/10.1115/1.4028238.

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The main purpose of this numerical investigation is to overcome the limitations of the steady modeling in predicting the cooling efficiency over the cutback surface in a high pressure turbine nozzle guide vane. Since discrepancy between Reynolds-averaged Navier–Stokes (RANS) predictions and measured thermal coverage at the trailing edge was attributable to unsteadiness, Unsteady RANS (URANS) modeling was implemented to evaluate improvements in simulating the mixing between the mainstream and the coolant exiting the cutback slot. With the aim of reducing the computation effort, only a portion of the airfoil along the span was simulated at an exit Mach number of Ma2is = 0.2. Three values of the coolant-to-mainstream mass flow ratio were considered: MFR = 0.66%, 1.05%, and 1.44%. Nevertheless the inherent vortex shedding from the cutback lip was somehow captured by the URANS method, the computed mixing was not enough to reproduce the measured drop in adiabatic effectiveness η along the streamwise direction, over the cutback surface. So modeling was taken a step further by using the scale adaptive simulation (SAS) method at MFR = 1.05%. Results from the SAS approach were found to have potential to mimic the experimental measurements. Vortices shedding from the cutback lip were well predicted in shape and magnitude, but with a lower frequency, as compared to particle image velocimetry (PIV) data and flow visualizations. Moreover, the simulated reduction in film cooling effectiveness toward the trailing edge was similar to that observed experimentally.

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Da Soghe, Riccardo, Cosimo Bianchini, and Jacopo D'Errico. "Numerical Characterization of Flow and Heat Transfer in Preswirl Systems." Journal of Engineering for Gas Turbines and Power 140, no. 7 (April 20, 2018). http://dx.doi.org/10.1115/1.4038618.

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This paper deals with a numerical study aimed at the validation of a computational procedure for the aerothermal characterization of preswirl systems employed in axial gas turbines. The numerical campaign focused on an experimental facility which models the flow field inside a direct-flow preswirl system. Steady and unsteady simulation techniques were adopted in conjunction with both a standard two-equation Reynolds-averaged Navier–Stokes (RANS)/unsteady RANS (URANS) modeling and more advanced approaches such as the scale-adaptive-simulation (SAS) principle, the stress-blended eddy simulation (SBES), and large eddy simulation (LES). Overall, the steady-state computational fluid dynamics (CFD) predictions are in reasonable good agreement with the experimental evidences even though they are not able to confidently mimic the experimental swirl and pressure behavior in some regions. Scale-resolved approaches improve the computations accuracy significantly especially in terms of static pressure distribution and heat transfer on the rotating disk. Although the use of direct turbulence modeling would in principle increase the insight in the physical phenomenon, from a design perspective, the trade-off between accuracy and computational costs is not always favorable.

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17

Abbaspour, Madjid, Navid Nemati Kourabbasloo, Pouya Mohtat, and Araz Tanha. "Numerical simulation of vortex-induced vibration of a smooth circular cylinder at the subcritical regime." Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, April 3, 2022, 147509022210884. http://dx.doi.org/10.1177/14750902221088429.

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The present paper focuses on the simulation of vortex-induced vibration (VIV) of a rigid, smooth circular cylinder with elastic supports subject to a cross-flow at the subcritical regime of Reynolds number, 30,000< Re<80,000. The circular cylinder is allowed to move in one degree-of-freedom (DOF), heave. Unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved with Menter’s k — [Formula: see text] based Shear Stress Transport based Scale-Adaptive Simulation, SAS-SST, turbulence model, and two-equation transition transport γ — θ model. The transport equations are discretized using the Finite Volume Method (FVM). The numerical amplitude and frequency ratio is compared against the experiments conducted in the Marine Renewable Energy Laboratory (MRELab) at the University of Michigan. The angle in which the computed lift leads the displacement in VIV is compared against experimental results reported by Cornell-ONR Water Channel as well. The existence of the initial, upper, and lower VIV response branches is demonstrated. Wake vortex pattern mode has been studied in the different branches of VIV. The time records of the added mass force coefficient and the vortex force coefficient are obtained. Then, the time-averaged phase angle of the vortex and added mass force coefficients are compared against the experimental results. Lastly, the time records of the phase angle in different branches of VIV are shown and analyzed.

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18

Wang, Yefang, Fan Zhang, Shouqi Yuan, Ke Chen, Xueyuan Wei, and Desmond Appiah. "Effect of URANS and Hybrid RANS-Large Eddy Simulation Turbulence Models on Unsteady Turbulent Flows Inside a Side Channel Pump." Journal of Fluids Engineering 142, no. 6 (March 5, 2020). http://dx.doi.org/10.1115/1.4045995.

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Abstract In this work, the unsteady Reynolds-averaged Navier–Stokes (URANS) and three hybrid Reynolds-averaged Navier–Stokes-large eddy simulation (RANS-LES) models are employed to resolve the vortical flows in a typical single-stage side channel pump, to evaluate the suitability of these advanced turbulence models in predicting the pump hydraulic performance and unstable swirling flows. By the comparison of the overall performance, it can be observed that the results obtained by scale-adapted simulation (SAS) are closer to test data than shear stress transport (SST), detached eddy simulation (DES) and filter-based model (FBM). Simultaneously, the distribution of axial velocity on the plane near the interface is used to describe the position and intensity of internal fluid exchange between impeller and side channel. It is obvious that the intensity of mass flow exchange is strong near the inner and outer edges. Then, the vortex core region illustrates that the vortex is easily produced near the interface due to internal fluid exchange. Finally, the evolutions of circumferential in-plane vortical structures are presented to further account for the process of fluid exchange and the main vortex flows. It reveals that the recirculation flow presents a strong instability during 6–7 blade pitches as the fluid enters into the impeller and the flow is stable in downstream 7–8 blade pitches. Besides, the flow turns to be unsteady near outlet affected by the sudden change of fluid direction. This work could provide some suggestions for the choice of appropriate turbulence model in simulating strong swirling flows.

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Journal articles on the topic 'URANS/SAS'

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