scholarly journals Mechanism and Performance Differences between the SSG/LRR-ω and SST Turbulence Models in Separated Flows

Aerospace ◽  
2021 ◽  
Vol 9 (1) ◽  
pp. 20
Author(s):  
Ruijie Bai ◽  
Jinping Li ◽  
Fanzhi Zeng ◽  
Chao Yan
Keyword(s):  
Physical Properties ◽  
Turbulence Models ◽  
Separated Flow ◽  
Viscosity Coefficient ◽  
Underlying Mechanism ◽  
Turbulent Energy ◽  
Separated Flows ◽  
Navier Stokes ◽  
Performance Differences ◽  
And Performance

Accurate predictions of flow separation are important for aerospace design, flight accident avoidance, and the development of fluid mechanics. However, the complexity of the separation process makes accurate predictions challenging for all known Reynolds-averaged Navier–Stokes (RANS) methods, and the underlying mechanism of action remains unclear. This paper analyzes the specific reasons for the defective predictions of the turbulence models applied to separated flows, explores the physical properties that impact the predictions, and investigates their specific mechanisms. Taking the Menter SST and the Speziale-Sarkar–Gatski/Launder–Reece–Rodi (SSG/LRR)-ω models as representatives, three typical separated flow cases are calculated. The performance differences between the two turbulence models applied to the different separated flow calculations are then compared. Refine the vital physical properties and analyze their calculation from the basic assumptions, modeling ideas, and construction of the turbulence models. The numerical results show that the underestimation of Reynolds stress is a significant factor in the unsatisfactory prediction of separation. In the SST model, Bradshaw’s assumption imposes the turbulent energy equilibrium condition in all regions and the eddy–viscosity coefficient is underestimated, which leads to advanced separation and lagging reattachment. In the SSG/LRR-ω model, the fidelity with which the pressure–strain term is modeled is a profound factor affecting the calculation accuracy.

scholarly journals Comparison of ω-Based Turbulence Models for Simulating Separated Flows in Transonic Compressor Cascades

1998 ◽  
Author(s):  
Tom C. Currie
Keyword(s):  
Turbulence Models ◽  
Separated Flow ◽  
Adverse Pressure Gradient ◽  
Reynolds Stress Model ◽  
Separated Flows ◽  
Navier Stokes ◽  
Test Case ◽  
Turbulent Dissipation ◽  
Transonic Compressor ◽  
Scale Variable

Separated flows in the DLR transonic compressor cascades TSG-91-8K and TSG-89-5 are simulated with a quasi-3D Navier-Stokes code using the zonal k-ω/k-ϵ “Shear Stress Transport” two-equation turbulence model of Menter and the multiscale Reynolds stress model of Wilcox. Both of these models use the specific turbulent dissipation rate ω as the length scale variable. The models are also used to simulate the low speed, separated flow, adverse pressure gradient test case of Driver. While both models predict results which are in good agreement with experiment for the latter test case, they yield relatively poor results, particularly for losses, for the cascade test cases, especially TSG-89-5 where separation occurs from both the suction and pressure surfaces. It is known from the cascade test results that the separations are laminar, so some improvement in agreement is achieved by suppressing transition to the separation points in the simulations. The poor accuracy of the models is believed to be related to severe non-equilibrium of turbulence production and dissipation predicted after the shock-induced separations.

A New PANS Model for Unsteady Separated Flow Simulations

2014 ◽  
Vol 721 ◽  
pp. 182-186 ◽  
Author(s):  
Da Hai Luo ◽  
Chao Yan ◽  
Wei Lin Zheng ◽  
Wu Yuan
Keyword(s):  
Model Performance ◽  
Separated Flow ◽  
Separated Flows ◽  
Navier Stokes ◽  
Test Case ◽  
Computational Results ◽  
Flow Simulations ◽  
Turbulent Fluctuations ◽  
Spatially Varying ◽  
Total Ratio

A new Partially Averaged Navier-Stokes (PANS) model is proposed with the aim of simulating unsteady separated flows at reasonable computational expense. The unresolved-to-total ratio of kinetic energy (fk) related to PANS method is taken as a spatially varying and dynamically updating parameter in the computations. Turbulent flow past a backward-facing step is chosen as a test case in an effort to evaluate the model performance. PANS computations are compared to the experimental data and the traditional Detached Eddy Simulations (DES), showing their excellent capability of resolving turbulent fluctuations. Boundary layer shielding technique is also introduced into the PANS approach and effectively improves the computational results.

scholarly journals Numerical and Experimental Investigation of the Wake Flow Downstream of a Linear Turbine Cascade

1998 ◽  
Author(s):  
Wolfgang Sanz ◽  
Arno Gehrer ◽  
Jakob Woisetschläger ◽  
Martin Forstner ◽  
Wolfgang Artner ◽  
...  
Keyword(s):  
Turbulence Modeling ◽  
Turbulence Models ◽  
Wake Flow ◽  
Navier Stokes ◽  
Laser Doppler Velocimeter ◽  
Turbine Cascade ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
And Performance ◽  
Transonic Cascade

In turbomachinery the wake flow together with the inherent unsteadiness caused by interaction between stator and rotor has a significant impact on efficiency and performance. The prediction of the wake flow depends largely on the turbulence modeling. Therefore in this study the evolution of a viscous wake downstream of a linear turbine cascade is experimentally and computationally investigated. In a transonic cascade test stand Laser Doppler Velocimeter (LDV) measurements of velocity and turbulent kinetic energy are done in several axial planes downstream of the blade trailing edge. Two different turbulence models are then incorporated into a two-dimensional Navier-Stokes solver to calculate the turbulent wake flow and the results are compared with the experimental data to test the quality of the turbulence models. The large discrepancies between measurement and Calculation are assumed to be caused by the periodic vortex shedding from the blunt trailing edge which is not taken into account by the turbulence models. But further research is needed to resolve this issue.

Steady RANS of Flow and Heat Transfer in a Smooth and Pin-Finned U-Duct With a Trapezoidal Cross Section

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Kenny S.-Y. Hu ◽  
Xingkai Chi ◽  
Tom I.-P. Shih ◽  
Minking Chyu ◽  
Michael Crawford
Keyword(s):  
Heat Transfer ◽  
Relative Error ◽  
Turbulence Models ◽  
Separated Flow ◽  
Reynolds Stress Model ◽  
Navier Stokes ◽  
Maximum Relative Error ◽  
Flow And Heat Transfer ◽  
Pin Fins ◽  
Turn Region

Steady Reynolds-averaged Navier--Stokes (RANS) simulations were performed to examine the ability of four turbulence models—realizable k–ε (k–ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and stress-omega RSM (RSM-τω)—to predict the turbulent flow and heat transfer in a trapezoidal U-duct with and without a staggered array of pin fins. Results generated for the heat-transfer coefficient (HTC) were compared with experimental measurements. For the smooth U-duct, the maximum relative error in the averaged HTC in the up-leg is 2.5% for k–ε, SST, and RSM-τω and 9% for RSM-LPS. In the turn region, the maximum is 50% for k–ε and RSM-LPS, 14.5% for RSM-τω, and 29% for SST. In the down-leg, SST gave the best predictions and RSM-τω being a close second with maximum relative error less than 10%. The ability to predict the separated flow downstream of the turn dominated the performance of the models. For the U-duct with pin fins, SST and RSM-τω predicted the best, and k–ε predicted the least accurate HTCs. For k–ε, the maximum relative error is about 25%, whereas it is 15% for the SST and RSM-τω, and they occur in the turn. In the turn region, the staggered array of pin fins was found to behave like guide vanes in turning the flow. The pin fins also reduced the size of the separated region just after the turn.

High Resolution Overset Structured Grid RANS Simulations of Flow Past a Surface Mounted Cube Using Eddy Viscosity Closure Models

2018 ◽  
Author(s):  
Marc C. Goldbach ◽  
Mesbah Uddin
Keyword(s):  
Eddy Viscosity ◽  
Complex Structure ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Separated Flow ◽  
Industrial Applications ◽  
Separated Flows ◽  
Transitional Flow ◽  
Stream Velocity ◽  
Mean Flow

While Reynolds-averaged simulatons (RAS) have found success in the evaluation of many canonical shear flows, and moderately separated flows, their application to highly separated flows have shown notable deficiencies. This study aims to investigate these deficiencies in the eddy-viscosity formulation of four commonly used turbulence models under separated flow in an attempt to aid in the improved formulation of such models. Analyses are performed on the flow field around a wall mounted cube at a Reynolds number of 40,000 based on the cube height, h, and free stream velocity, U0. While a common occurrence in industrial applications, this type of flow constitutes a complex structure exhibiting a large separated wake region, high anisotropy, and multiple vortex structures. As well, interactions between vortices developed off of different faces of the cube significantly alter the overall flow characteristics, posing a significant challenge for the commonly used industrial turbulence models. Comparison of mean flow characteristics show remarkable agreement between experimental values and turbulence models which are capable of predicting transitional flow. Evaluation of turbulence parameters show the general underestimation of Reynolds stress for transitional models, while fully turbulent models show this value to be overestimated, resulting in completely disparate representations of mean flow structures between the two classes of models (transitional and fully turbulent).

scholarly journals Techniques for Turbulence Tripping of Boundary Layers in RANS Simulations

2021 ◽  
Author(s):  
Narges Tabatabaei ◽  
Ricardo Vinuesa ◽  
Ramis Örlü ◽  
Philipp Schlatter
Keyword(s):  
Boundary Layer ◽  
Passive Control ◽  
Target Location ◽  
Turbine Blades ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Transition Process ◽  
Navier Stokes ◽  
Test Cases ◽  
And Performance

AbstractThe exact placement of the laminar–turbulent transition has a significant effect on relevant characteristics of the boundary layer and aerodynamics, such as drag, heat transfer and flow separation on e.g. wings and turbine blades. Tripping, which fixes the transition position, has been a valuable aid to wind-tunnel testing during the past 70 years, because it makes the transition independent of the local condition of the free-stream. Tripping helps to obey flow similarity for scaled models and serves as a passive control mechanism. Fundamental fluid-mechanics studies and many engineering developments are based on tripped cases. Therefore, it is essential for computational fluid dynamics (CFD) simulations to replicate the same forced transition, in spite of the advanced improvements in transition modelling. In the last decade, both direct numerical simulation (DNS) and large-eddy simulations (LES) include tripping methods in an effort to avoid the need for modeling the complex mechanisms associated with the natural transition process, which we would like to bring over to Reynolds-averaged Navier–Stokes (RANS) turbulence models. This paper investigates the implementation and performance of such a technique in RANS and specifically in the $$k-\omega$$ k - ω SST model. This study assesses RANS tripping with three alternatives: First, a recent approach of turbulence generation, denoted as turbulence-injection method (kI), is evaluated and investigated through different test cases; second, a predefined transition point is used in a traditional transition model (denoted as IM method); and third a novel formulation combining the two previous methods is proposed, denoted $$\gamma -k$$ γ - k I. The model is compared with DNS, LES and experimental data in a variety of test cases ranging from a turbulent boundary layer on a flat plate to the three-dimensional (3D) flow over a wing section. The desired tripping is achieved at the target location and the simulation results compare very well with the reference results. With the application of the novel model, the challenging transition region can be excluded from a simulation, and consequently more reliable results can be provided.

A Study on Models to Simulate Boundary Layer Transition in Turbomachinery Flows

2000 ◽  
Author(s):  
M. Müller ◽  
H. E. Gallus ◽  
R. Niehuis
Keyword(s):  
Three Dimensional ◽  
Turbulence Models ◽  
Separated Flow ◽  
Boundary Layer Transition ◽  
Navier Stokes ◽  
Intermittent Flow ◽  
Flow Transition ◽  
Compressor Cascade ◽  
Layer Transition ◽  
Transition Modeling

The objective of this paper is to investigate the modeling of transition in turbomachinery flows. Under steady state conditions four modes of transition can be identified. One of them, reverse transition, can be modeled sufficiently well by turbulence models. For the other modes — natural, bypass and separated-flow transition — specific models have to be applied. These transition models are either based on stability analysis or a statistic approach that describes the production and growth of turbulent spots in the intermittent flow. This paper focuses on separated flow transition. Different models are presented and a classification is attempted. Based on these findings specific models are chosen and implemented into a Navier-Stokes solver to be studied in more detail. First an annular compressor cascade is computed two-dimensionally and the results are compared with experiments. In a second step three-dimensional computations are performed and the effect of transition modeling is analysed.

Evaluation of Turbulence Models for the Prediction of Wind Turbine Aerodynamics

2003 ◽  
Author(s):  
Sarun Benjanirat ◽  
Lakshmi N. Sankar ◽  
Guanpeng Xu
Keyword(s):  
Wind Turbine ◽  
Model Development ◽  
Turbulence Models ◽  
Separated Flow ◽  
Equation Model ◽  
Navier Stokes ◽  
Implicit Time ◽  
Horizontal Axis Wind Turbine ◽  
Attached Flow ◽  
Nrel Phase Vi

The performance of the NREL Phase VI horizontal axis wind turbine has been studied with a 3-D unsteady Navier-Stokes solver. This solver is third order accurate in space and second order accurate in time, and uses an implicit time marching scheme. Calculations were done for a range of wind conditions from 7 m/s to 25 m/s where the flow conditions ranged from attached flow to massively separated flow. A variety of turbulence models were studied: Baldwin-Lomax Model, Spalart-Allmaras one-equation model, and k-ε two equations model with and without wall corrections. It was found all the models predicted the normal forces and associated bending moments well, but most of them had difficulties in modeling the chord wise forces, power generation, and pitching moments. It was found that the k-ε model with near wall corrections did the best job of predicting most the quantities with acceptable levels of accuracy. Additional studies aimed at transition model development, and grid sensitivity studies in the tip region are deemed necessary to improve the correlation with experiments.

scholarly journals Assessment of Computational Fluid Dynamics Capabilities for the Prediction of Three-Dimensional Separated Flows: The DELFT 372 Catamaran in Static Drift Conditions

2019 ◽  
Vol 141 (9) ◽  
Author(s):  
R. Broglia ◽  
S. Zaghi ◽  
E. F. Campana ◽  
T. Dogan ◽  
H. Sadat-Hosseini ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Flow Field ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Separated Flow ◽  
Lateral Force ◽  
Separated Flows ◽  
Overlapping Grids ◽  
Wave Induced

In this paper, capabilities of state-of-the-art computational fluid dynamics (CFD) tools in the prediction of the flow-field around a multihull catamaran advancing in straight ahead motion at nonzero drift angles are investigated. CFD estimations have been provided by three research institutes by using their in-house codes: CNR-INM using Xnavis, IIHR using CFDShip-Iowa, and CNRS/ECN using ISIS. These allowed an in-depth comparison between different methodologies, such as structured overlapping grids versus unstructured grid, different turbulence models and detached eddy simulations (DES) approaches, and level-set (LS) versus volume of fluid (VoF). The activities were pursued within the NATO AVT-183 group “reliable prediction of separated flow onset and progression for air and sea vehicles,” aimed at the assessment of CFD predictions of large three-dimensional separated flows. Comparison between estimations is provided for both integral and local quantities, and for wave-induced vortices. Validation is reported by comparison against the available experimental fluid dynamics (EFD) data. Generally, all the simulations are able to capture the main features of the flow field; grid resolution effects are dominant in the onset phase of coherent structures and turbulence model affects the dynamic of the vortices. Hydrodynamic loads are in agreement between the submissions with standard deviation of about 3.5% for the resistance prediction and about 7% for lateral force and yaw moment estimation. Wave-induced vortices are correctly captured by both LS and VoF approaches, even if some differences have been highlighted, LS showing well-defined and long life vortices.

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