scholarly journals Assessment of RANS turbulence closure models for predicting airflow in neutral ABL over hilly terrain

2021 ◽  
Author(s):  
Amahjour Narjisse ◽  
Khamlichi Abdellatif
Keyword(s):  
Experimental Data ◽  
Wind Speed ◽  
Cfd Simulation ◽  
Turbulence Models ◽  
Computational Time ◽  
Recirculation Region ◽  
Leeward Side ◽  
Hilly Terrain ◽  
Steep Hill ◽  
Near Wall

AbstractImplementing wind farms in heights of a hilly terrain where wind speed is expected to be large may be viewed as a means to increase wind energy production without occupying fertile lands. Micro sitting of a wind farm in these conditions can gain dramatically from CFD simulation of fluid flow in the ABL above complex topography. However, this issue still poses tough challenges regarding the turbulence model to be used and the way to operate the near wall treatment in the presence eventually of separation. In this work, prediction capacity of RANS turbulence models was studied for a typical hill under the assumption of steady state and incompressible airflow regime in neutral ABL. Two models were analyzed by using COMSOL Multiphysics software packages. These included standard , and shear-stress transport . The most up-to-date procedures dedicated to near wall treatment were applied along with refined closer coefficients adjusted for the particular case of ABL. Considering wind tunnel test data, performance of the previous models was discussed in terms of converging mesh, computational time, reattachment point position and propensity of the model to retrieve the right level of turbulence flow in conditions of neutral stratifications. Then, a numerical simulation of the turbulent airflow over two slopes shapes of the symmetry hill by the validation of the experimental data has been then carried out. Both turbulence models agree well with air-velocity tested windward of the hills H3 and H5. Therefore, it was found that the standard model performs very well at the different positions of the low slope hill, and at the summit of a steep hill, but it over-predicts wind speed close to the wall, which requires an improvement of the near-wall treatment. However, the model in neutral case of the ABL was given consistent simulation results with experimental data for prediction of the flow separation and recirculation region at the leeward side of a steep hill, whereas standard model under the neutral condition and the model by using standard coefficients were failed to predict accurately detailed characteristics of recirculation region process.

scholarly journals Performance of Turbulence Models and Near-Wall Treatments in Discrete Jet Film Cooling Simulations

1998 ◽  
Author(s):  
Jeffrey D. Ferguson ◽  
Dibbon K. Walters ◽  
James H. Leylek
Keyword(s):  
Experimental Data ◽  
Film Cooling ◽  
Turbulence Models ◽  
Viscous Sublayer ◽  
Reynolds Stress Model ◽  
Quality Data ◽  
Wall Functions ◽  
First Time ◽  
Near Wall ◽  
Non Equilibrium

For the first time in the open literature, code validation quality data and a well-tested, highly reliable computational methodology are employed to isolate the true performance of seven turbulence treatments in discrete jet film cooling. The present research examines both computational and high quality experimental data for two length-to-diameter ratios of a row of streamwise injected, cylindrical film holes. These two cases are used to document the performance of the following turbulence treatments: 1) standard k-ε model with generalized wall functions; 2) standard k-ε model with non-equilibrium wall functions: 3) Renormalization Group k-ε (RNG) model with generalized wall functions; 4) RNG model with non-equilibrium wall functions: 51 standard k-ε model with two-layer turbulence wall treatment; 6) Reynolds Stress Model (RSM) with generalized wall functions; and 7) RSM with non-equilibrium wall functions. Overall, the standard k-ε turbulence model with the two-layer near-wall treatment, which resolves the viscous sublayer, produces results that are more consistent with experimental data.

Improved Prediction of Turbomachinery Flows Using Near-Wall Reynolds-Stress Model

2001 ◽  
Vol 124 (1) ◽  
pp. 86-99 ◽  
Author(s):  
G. A. Gerolymos ◽  
J. Neubauer ◽  
V. C. Sharma ◽  
I. Vallet
Keyword(s):  
Experimental Data ◽  
Turbulent Flows ◽  
Reynolds Stress ◽  
Turbulence Models ◽  
Reynolds Stress Model ◽  
Stress Model ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Flow Equations ◽  
Near Wall

In this paper an assessment of the improvement in the prediction of complex turbomachinery flows using a new near-wall Reynolds-stress model is attempted. The turbulence closure used is a near-wall low-turbulence-Reynolds-number Reynolds-stress model, that is independent of the distance-from-the-wall and of the normal-to-the-wall direction. The model takes into account the Coriolis redistribution effect on the Reynolds-stresses. The five mean flow equations and the seven turbulence model equations are solved using an implicit coupled OΔx3 upwind-biased solver. Results are compared with experimental data for three turbomachinery configurations: the NTUA high subsonic annular cascade, the NASA_37 rotor, and the RWTH 1 1/2 stage turbine. A detailed analysis of the flowfield is given. It is seen that the new model that takes into account the Reynolds-stress anisotropy substantially improves the agreement with experimental data, particularily for flows with large separation, while being only 30 percent more expensive than the k−ε model (thanks to an efficient implicit implementation). It is believed that further work on advanced turbulence models will substantially enhance the predictive capability of complex turbulent flows in turbomachinery.

Experimental and Numerical Investigation of Stationary Ribbed Ducts

2004 ◽  
Author(s):  
A. Magi ◽  
F. Montomoli ◽  
P. Adami ◽  
C. Carcasci
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Cfd Simulation ◽  
Low Cost ◽  
Turbulence Models ◽  
Data Set ◽  
Contour Maps ◽  
Accuracy Level ◽  
Coolant System ◽  
Good Agreement

Goal of this work is to define the main issues and guidelines for an accurate heat transfer CFD simulation of internal ribbed ducts. To this aim, two different ribbed ducts (AR = 1,3) have been experimentally investigated to obtain a data set useful to validate numerical analyses. Experimental HTC contour maps have been obtained using unsteady TLC technique. CFD activity deals with numerical simulation using both a commercial (Star-CD™) and an “in house” solver (HybFlow). Four different variants of the well-known two-equation turbulence models have been considered. Low cost heat transfer predictions of internal ducts are highly demanded by industry despite the uncommon complexity of modern internal coolant system. Accordingly, the main aim of the work is to provide some indications for the numerical modelling and to evaluate the accuracy level of predicted heat transfer when commercial or research packages are employed along with different grid resolution levels. Overall results are in good agreement with experimental data even if some local discrepancies are present.

Simulation of a 3D Axisymmetric Hill: Comparison of RANS and Hybrid RANS-LES Models

2016 ◽  
Author(s):  
Tausif Jamal ◽  
D. Keith Walters
Keyword(s):  
Experimental Data ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Boundary Layer Separation ◽  
Navier Stokes ◽  
Leeward Side ◽  
Mean Flow ◽  
Rans Model ◽  
Reynolds Number Flow ◽  
Low Dissipation

Computational fluid dynamics (CFD) prediction of high Reynolds number flow over a 3D axisymmetric hill presents a unique set of challenges for turbulence models. The flow on the leeward side of the hill is characterized by the presence of complex vortical structures, unsteady wakes, and regions of boundary layer separation. As a result, traditional eddy-viscosity Reynolds-averaged Navier-Stokes (RANS) models have been found to perform poorly. Recent studies have focused on the use of Large Eddy Simulation (LES) and hybrid RANS-LES (HRL) methods to improve accuracy. In this study, the capability of a dynamic hybrid RANS-LES (DHRL) model to resolve the flow over a 3D axisymmetric hill is investigated and compared to numerical results using a traditional RANS model and a conventional hybrid RANS-LES model, and to experimental data. Results show that the RANS model fails to accurately predict the mean flow features in the wake region, which is in agreement with prior studies. The conventional HRL model provides better prediction of the flow characteristics but suffers from grid sensitivity and delayed transition to LES mode. The DHRL method provides the best agreement with experimental data overall and shows least sensitivity to grid resolution. Results also highlight the importance of using a low dissipation flux formulation for flow simulations in which a portion of the turbulence spectrum is resolved, including hybrid RANS-LES.

scholarly journals Simplified Transition and Turbulence Modeling for Oscillatory Pipe Flows

Energies ◽  
2021 ◽  
Vol 14 (5) ◽  
pp. 1410
Author(s):  
Alexander Shapiro ◽  
Gershon Grossman ◽  
David Greenblatt
Keyword(s):  
Experimental Data ◽  
Pressure Gradient ◽  
Turbulence Modeling ◽  
Turbulent Viscosity ◽  
Turbulence Models ◽  
Womersley Number ◽  
Pipe Flows ◽  
Temporal Pressure ◽  
Turbulent Models ◽  
Near Wall

One-dimensional unsteady Reynolds-averaged Navier–Stokes computations were performed for oscillatory transitional and turbulent pipe flows and the results were validated against existing experimental data for a wide variety of oscillatory Reynolds and Womersley numbers. An unsteady version of the Johnson–King model was implemented with optional near-wall modification to account for temporal pressure gradient variations, and the predictions were compared with those of the Spalart–Allmaras and k–ε turbulence models. Transition and relaminarization were based on empirical Womersley number correlations and assumed to occur instantaneously: in the former case, this assumption was valid, but in the latter case, deviations between data and predictions were observed. In flows where the oscillatory Reynolds numbers are substantially higher than the commonly accepted steady critical value (~2000), fully or continuously turbulent models produced the best correspondence with experimental data. Critically and conditionally turbulent models produced slightly inferior correspondence, and no significant benefit was observed when near-wall pressure gradient effects were implemented or when common one- and two-equation turbulence models were employed. The turbulent velocity profiles were mainly unaffected by the oscillations and this was explained by noting that the turbulent viscosity is significantly higher than its laminar counterpart. Thus, a turbulent Womersley number was proposed for the analysis and categorization of oscillatory pipe flows.

A CFD Simulation of Near Wall Turbulent Flow in Concentric Annulus

2013 ◽  
Author(s):  
Aziz Rahman ◽  
Fabio Ernesto Rodriguez Corredor ◽  
Majid Bizhani ◽  
Ergun Kuru
Keyword(s):  
Experimental Data ◽  
Velocity Profile ◽  
Turbulent Flow ◽  
Water Flow ◽  
Simulation Study ◽  
Cfd Simulation ◽  
Concentric Annulus ◽  
Sst Model ◽  
Cfd Analyses ◽  
Near Wall

A CFD simulation study was conducted to analyse the near wall turbulence characteristics of water flow through concentric annulus. The continuity and momentum equations were solved by using a commercial CFD package (CFX 14) with the Shear-Stress-Transport (SST) model option. The simulation results were compared to the experimental data obtained by using high resolution Particle Image Velocimetry (PIV) analyses of water flow in a horizontal concentric annulus. A fully developed turbulent flow of water through a horizontal flow loop (ID = 9.5 cm) with concentric annular geometry (inner to outer pipe radius ratio = 0.4) was used for comparison purpose. Reynolds number ranged from 17,500 to 68,500. Annular velocity profile obtained from simulation study showed good agreement with the experimental data. Near wall velocity profile obtained from CFD simulation followed the universal wall law (u+ = y+) up to y+ = 11. CFD analyses using the SST model resulted a good number of velocity data up to y+ = 11, which is normally a very difficult task to achieve experimentally. The CFD analyses using SST model is computationally inexpensive and therefore, can be conveniently used for investigating the near wall turbulent characteristics of flow in concentric annulus.

Improved Prediction of Turbomachinery Flows Using Near-Wall Reynolds-Stress Model

2001 ◽  
Author(s):  
G. A. Gerolymos ◽  
J. Neubauer ◽  
V. C. Sharma ◽  
I. Vallet
Keyword(s):  
Experimental Data ◽  
Turbulent Flows ◽  
Reynolds Stress ◽  
Turbulence Models ◽  
Reynolds Stress Model ◽  
Stress Model ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Flow Equations ◽  
Near Wall

In this paper an assessment of the improvement in the prediction of complex turbomachinery flows using a new near-wall Reynolds-stress model is attempted. The turbulence closure used is a near-wall low-turbulence-Reynolds-number Reynolds-stress model, that is independent of the distance-from-the-wall and of the normal-to-the-wall direction. The model takes into account the Coriolis redistribution effect on the Reynolds-stresses. The 5 mean flow equations and the 7 turbulence model equations are solved using an implicit coupled O(Δx3) upwind-biased solver. Results are compared with experimental data for 3 turbomachinery configurations: the ntua high subsonic annular cascade, the nasa_37 rotor, and the rwth 1½ stage turbine. A detailed analysis of the flowfield is given. It is seen that the new model that takes into account the Reynolds-stress anisotropy substantially improves the agreement with experimental data, particularly for flows with large separation, while being only 30% more expensive than the k – ε model (thanks to an efficient implicit implementation). It is believed that further work on advanced turbulence models will substantially enhance the predictive capability of complex turbulent flows in turbomachinery.

Validation and Evaluation of Turbulence Models in Thermal Predictions of a Small Data Center Facility

2018 ◽  
Author(s):  
Beichao Hu ◽  
Long Phan ◽  
Cheng-Xian Lin
Keyword(s):  
Turbulence Model ◽  
Data Center ◽  
Data Centers ◽  
Cfd Simulation ◽  
Turbulence Models ◽  
Equation Model ◽  
Computational Time ◽  
Small Data ◽  
Resource Requirements ◽  
The Difference

Thermal management in data centers has become more and more important due to the rapid growth in power density in modern data centers. Computational fluid dynamics (CFD) is proved to be a very useful tool in data center design and analysis. However, the previous papers always utilize k-epsilon model, and has never studied on the effect of other turbulence models. This paper will demonstrate the difference between various turbulence models in terms of accuracy and computational time. The data center investigated in this paper has a floor area of 900 ft2 and comprises one rack, one CRAC unit, and several perforated tiles. This paper mainly investigates the effect of various turbulence models on CFD simulation in data center. The Turbulence model is believed to be a possible factor to improve the CFD results. The most suitable turbulence model will be identified based on a balance in both accuracy and computing resource requirements. Four turbulence models were investigated in this paper. The present investigation suggested that A&S 1-equation model yield the best accuracy and required the least computational time. Hence, 1-eqaution model should be the preferable turbulence model for CFD simulation in data center in the future.

Analysis of RCCS Heat Exchange for Benchmarking of CFD Codes

2010 ◽  
Author(s):  
Angelo Frisani ◽  
Victor M. Ugaz ◽  
Yassin A. Hassan
Keyword(s):  
Experimental Data ◽  
Heat Exchange ◽  
Reynolds Stress ◽  
Turbulence Models ◽  
Experimental Results ◽  
Scaling Analysis ◽  
Experimental Facility ◽  
Cfd Model ◽  
Forced Circulation ◽  
Near Wall

One of the main concerns for modular Very High Temperature Gas-Cooled Reactors (VHTR) is the design of passive heat removal systems from the reactor vessel cavity. The Reactor Cavity Cooling System (RCCS) is an important heat removal system during normal and up-normal conditions. The design and validation of the RCCS is necessary to demonstrate that HTGRs can survive the postulated accidents. Here we investigate this using the Computational Fluid Dynamics (CFD) STAR-CCM+ V3.06.006 code to simulate the Pressurized Conduction Cooling (PCC) and Depressurized Conduction Cooling (DCC) accident scenarios. Heat is transported by radiation and free convection from the Reactor Pressure Vessel surface to the cooling panels or standpipes. The standpipes are cooled by natural circulation of air or forced circulation of water flowing through the pipes. A representative VHTR RCCS configuration was considered, represented experimentally by a 180° scaled model facility that was used to measure temperature and velocity distributions inside the cavity. The CFD model constructed incorporated the features of the experimental facility. Using the vessel temperature profile obtained from the experimental facility as boundary conditions in the CFD simulations, different tests were performed increasing the vessel average wall temperature progressively. Grid independence was achieved and different turbulence models and near-wall treatments were tested. For the standpipes, simulations with both natural circulation of air and forced circulation of water were performed. A reasonable agreement between the experimental results and the CFD simulations was achieved for the temperature distributions in the RCCS cavity. Also the standpipes external wall temperature was close to the experimental data. The fraction of heat exchange due to radiation determined by STAR-CCM+ code was in reasonable agreement with the experimental results. The k-ε turbulence models results were compared against the other turbulence models (i.e., the k-ω, Reynolds Stress Transport, and Spalart-Allmaras). Some differences were found between the turbulence models used. The k-ε turbulence models showed in general better performance than the k-ω and Spalart-Allmaras models if compared with the Reynolds Stress Transport (RST) results and experimental data. Among the k-ε turbulence models, the Realizable k-ε turbulence models with two-layer all-y+ near wall treatment performed better than the standard and the Abe-Kondoh-Nagano (AKN) k-ε models with Low-Reynolds Number low-y+ and all-y+ near wall treatments, if compared to both the RST and experimental results. The RST model was expected to perform better than the other models considering the strong anisotropy of the Reynolds stress tensor close to the vessel wall. The discrepancy between the experimental data with the RST model predictions may be due to the need for finer computational mesh model. A scaling analysis was developed to address the distortion introduced by the experimental facility and CFD model in simulating the physics inside the RCCS system with respect to the real plant configuration. The scaling analysis demonstrated that both the experimental facility and CFD model give a satisfactory reproduction of the main flow characteristics inside the RCCS cavity region, with convection and radiation heat exchange phenomena being properly scaled from the real plant to the model analyzed.

CFD Simulation of Round Impinging Jet and Comparison With Experimental Data

2016 ◽  
Author(s):  
H. Arabnejad ◽  
A. Mansouri ◽  
S. A. Shirazi ◽  
B. S. McLaury
Keyword(s):  
Fluid Dynamics ◽  
Experimental Data ◽  
Cfd Simulation ◽  
Turbulence Models ◽  
Impinging Jet ◽  
Reynolds Stress Model ◽  
Ansys Fluent ◽  
Target Wall ◽  
Computational Fluid Dynamics Cfd ◽  
Better Than

In this work, fluid dynamics of a turbulent round impinging jet has been studied using Computational Fluid Dynamics (CFD) and the results have been compared with experimental data from the literature. The fluid was water with density of 1000 kg/m3 and the average velocity of the submerged jet was kept constant at 10.7 m/s while the liquid viscosity varied from 1 cP to 100 cP. Different turbulence models including k-ε, k-ω and Reynolds Stress Model (RSM) have been employed in ANSYS FLUENT and the predicted axial and radial velocity profiles at various distances from the wall are compared with LDV data. It was observed that at locations away from the target wall, predicted velocities are comparable to the measured velocities for all the viscosities. However, near the wall, the deviation between the CFD predictions and experimental measurements become noticeable. The performance of k-ω model and RSM are found to be better than the k-ε model especially for the highest viscous fluid, but no model was found to be superior for all conditions and at all locations.

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