One-point modelling of rapidly deformed homogeneous turbulence

1995 ◽  
Vol 451 (1941) ◽  
pp. 87-104 ◽  
Keyword(s):  
Turbulence Model ◽  
Turbulence Models ◽  
Turbulence Structure ◽  
Missing Information ◽  
Point Model ◽  
Transport Models ◽  
Special Cases ◽  
New Type ◽  
The Mean ◽  
Mean Strain

One-point turbulence models are important tools for engineering analysis. A good model should have a viscoelastic character, predicting turbulent stresses proportional to the mean strain rate for slow deformations and stresses determined by the amount of strain for rapid distortions. Our goal is to build a one-point turbulence model with this character, and this requires a one-point model for rapid distortions. Here it is shown that the turbulent stresses introduced by Osborne Reynolds do not, by themselves, provide an adequate tensorial base for one-point modelling of rapidly distorted turbulence because they do not carry critical information about the turbulence structure. The deficiency is shown to be most pronounced in flows subjected to strong mean rotation. Additional one-point tensors that do carry the missing information are introduced, and the complexities of a model that would have an adequate tensorial base are assessed. A new type of one-point structure-based turbulence model that overcomes the basic deficiency of Reynolds-stress transport models, but without the excessive complexity of multiple tensor variables, is then described. The ideas behind the rapid distortion version of this new model are presented, along with results for some special cases.

scholarly journals Assessment of CFD turbulence models for free surface flow simulation and 1-D modelling for water level calculations over a broad-crested weir floodway

Water SA ◽  
2019 ◽  
Vol 45 (3 July) ◽  
Author(s):  
Ahmed M Helmi
Keyword(s):  
Experimental Data ◽  
Free Surface ◽  
Water Level ◽  
Dimensional Analysis ◽  
Turbulence Model ◽  
Turbulence Models ◽  
Cfd Simulations ◽  
One Dimensional ◽  
The Mean ◽  
The One

Floodways, where a road embankment is permitted to be overtopped by flood water, are usually designed as broad-crested weirs. Determination of the water level above the floodway is crucial and related to road safety. Hydraulic performance of floodways can be assessed numerically using 1-D modelling or 3-D simulation using computational fluid dynamics (CFD) packages. Turbulence modelling is one of the key elements in CFD simulations. A wide variety of turbulence models are utilized in CFD packages; in order to identify the most relevant turbulence model for the case in question, 96 3-D CFD simulations were conducted using Flow-3D package, for 24 broad-crested weir configurations selected based on experimental data from a previous study. Four turbulence models (one-equation, k-ε, RNG k-ε, and k-ω) ere examined for each configuration. The volume of fluid (VOF) algorithm was adopted for free water surface determination. In addition, 24 1-D simulations using HEC-RAS-1-D were conducted for comparison with CFD results and experimental data. Validation of the simulated water free surface profiles versus the experimental measurements was carried out by the evaluation of the mean absolute error, the mean relative error percentage, and the root mean square error. It was concluded that the minimum error in simulating the full upstream to downstream free surface profile is achieved by using one-equation turbulence model with mixing length equal to 7% of the smallest domain dimension. Nevertheless, for the broad-crested weir upstream section, no significant difference in accuracy was found between all turbulence models and the one-dimensional analysis results, due to the low turbulence intensity at this part. For engineering design purposes, in which the water level is the main concern at the location of the flood way, the one-dimensional analysis has sufficient accuracy to determine the water level.

scholarly journals Prediction of flow and dispersion in cross–ventilated buildings

2019 ◽  
Vol 128 ◽  
pp. 05002
Author(s):  
Ali Cemal Benim ◽  
Michael Diederich ◽  
Ali Nahavandi
Keyword(s):  
Computational Analysis ◽  
Turbulence Models ◽  
Reynolds Stress Model ◽  
Turbulence Structure ◽  
Detached Eddy Simulation ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean ◽  
Grid Independence

The present paper presents a detailed computational analysis of flow and dispersion in a generic isolated single–zone buildings. First, a grid generation strategy is discussed, that is inspired by a previous computational analysis and a grid independence study. Different turbulence models are appliedincluding two-equation turbulence models, the differential Reynolds Stress Model, Detached Eddy Simulation and Zonal Large Eddy Simulation. The mean velocity and concentration fields are calculated and compared with the measurements. A satisfactory agreement with the experiments is not observed by any of the modelling approaches, indicating the highly demanding flow and turbulence structure of the problem.

3D CFD Investigation of Air Flow Behind a Porous Fence Using Different Turbulence Models

2014 ◽  
Author(s):  
Yizhong Xu ◽  
Mohamad Y. Mustafa ◽  
Geanette Polanco
Keyword(s):  
Velocity Profile ◽  
Turbulence Model ◽  
Air Flow ◽  
Turbulence Models ◽  
Experimental Results ◽  
Turbulence Structure ◽  
Cfd Modelling ◽  
Lack Of Information ◽  
Vertical Velocity Profile ◽  
Flow Through

Even after many years of the application of numerical CFD techniques to flow through porous fences, still there is disagreement between researchers regarding the best turbulence model to be implemented in this field. Moreover, different sources claim to have achieved good agreement between numerical results and experimental data; however, it is not always possible to compare numerical and experimental results due to the lack of information or variations in test conditions. In this paper, five different turbulence models namely; K-ε models (standard, RNG and Realizable) and K-ω models (Standard and SST), have been applied through a 3D CFD model to investigate air flow behind a porous panel, under the same conditions (boundary conditions and numerical schemes). Results are compared with wind tunnel experiments. Comparison is based on the vertical velocity profile at a location 925 mm downstream of the fence along its center line. All models were capable of reproducing the velocity profile, however, some turbulence models over-predicted the reduction of velocity while it was under-predicted by other models, however, discrepancy between CFD modelling and experimental results was kept around 20%. Comprehensive description of the turbulence structure and the streamlines highlight the fact that the criterion for selecting the best turbulence model cannot rely only on the velocity comparison at one location, it must also include other variables.

A pdf Description of Turbulent Axisymmetric Free Jet Flow

1999 ◽  
Vol 121 (1) ◽  
pp. 73-79 ◽  
Author(s):  
Ming-Hua Chen ◽  
Zuu-Chang Hong
Keyword(s):  
Moment Method ◽  
Jet Flow ◽  
Moment Closure ◽  
Turbulence Structure ◽  
Free Jet ◽  
Closure Models ◽  
The Mean ◽  
Probability Density Function Pdf ◽  
Mean Strain ◽  
Free Jet Flow

This study presents the applications of a turbulence probability density function (pdf) equation to compute an axisymmetric turbulent free jet flow. In view of the difficulty of solving this pdf equation directly by conventional numerical methods, an approximate moment method is applied. The Calculated triple velocity correlations appearing in the second-order moments equation are calculated and compared with measured values and with those estimated by moment-closure models. The results reveal that the pdf approach gives consistency in the higher-order moments and radial budget of third moments of velocity, and that the neglect of the mean-strain production, the rapid part of the pressure correlation and the dissipation are responsible for deviations between moment-closure models and experiments. Therefore, pdf methods appear to be more suitable than conventional moment-closure models in terms of revealing turbulence structure.

scholarly journals The Development of the Mean Flow and Turbulence Structure in an Annular S-Shaped Duct

1994 ◽  
Author(s):  
K. M. Britchford ◽  
J. F. Carrotte ◽  
S. J. Stevens ◽  
J. J. McGuirk
Keyword(s):  
Reynolds Stress ◽  
Laser Doppler Anemometry ◽  
Reynolds Shear Stress ◽  
Turbulence Models ◽  
Test Facility ◽  
Turbulence Structure ◽  
Mean Flow ◽  
Mean Velocity ◽  
Curvature Effects ◽  
The Mean

This paper describes an investigation of the mean and fluctuating flow field within an annular S-shaped duct which is representative of that used to connect the compressor spools of aircraft gas turbine engines. Data was obtained from a fully annular test facility using a 3-component Laser Doppler Anemometry (LDA) system. The measurements indicate that development of the flow within the duct is complex and significantly influenced by the combined effects of streamwise pressure gradients and flow curvature. In addition CFD predictions of the flow, using both the k-ε and Reynolds stress transport equation turbulence models, are compared with the experimental data. Whereas curvature effects are not described properly by the k-ε model, such effects are captured more accurately by the Reynolds stress model leading to a better prediction of the Reynolds shear stress distribution. This, in turn, leads to a more accurate prediction of the mean velocity profiles, as reflected by the boundary layer shape parameters, particularly in the critical regions of the duct where flow separation is most likely to occur.

Experimental and Numerical Investigation of Turbulent High Reynolds Number Flows in a Square Duct With 90-Degree Streamwise Curvature: Part 1 — Experiment

2003 ◽  
Author(s):  
Faustin Ondore
Keyword(s):  
Turbulent Flows ◽  
Reynolds Stress ◽  
Turbulence Model ◽  
Turbulence Models ◽  
Flow Visualisation ◽  
Square Duct ◽  
Plane Flow ◽  
Convex Wall ◽  
The Mean ◽  
Stress Models

This study was aimed at obtaining a better understanding of turbulent flows in a square duct with a 90° bend, using both experimental and numerical techniques. Turbulent flows that are subjected to streamwise curvature occur in numerical engineering applications. These flows are known to experience extra rates of strain in the plane of mean shear in comparison to plane flows. Hence the gross parameters, such as the mean flow velocities, turbulence intensities and Reynolds stresses are altered dramatically from the plane flow characteristics. The flows examined are specified by the free-stream entry velocities of 12.3 m/s and 20.4 m/s measured at 1.01 duct height upstream of the bend entry plane. These velocities correspond to Reynolds numbers 3.56 × 105 and 6.43 × 105 respectively. The duct has a cross-section of 0.457 × 0.457 m 2 and the mean radius of curvature to duct height ratio is 1:21. Airflow from a wind tunnel passes through an upstream tangent of 1.31 duct height before entering the bend. The flow then exits the bend into a 7.0 duct height downstream tangent before discharging into the atmosphere. The experimental part involved hot-wire measurements. Flow visualisation was performed by smoke in a region close to the convex wall at the bend exit to confirm the numerical prediction of recirculating flow in that area. The numerical part of the investigation was based on the solution of the governing differential equations for turbulent flows in conjunction with a number of turbulence models. The discretisation of the equations was achieved using a finite-volume technique and different discretisation schemes. The main turbulence model used for the study was the Reynolds Stress Model, but the comparisons of the results were also made with those from the standard κ-ε and the RNG-κ-ε turbulence models. The boundary conditions for these simulations were obtained as part of the experimental investigations. Numerical calculations with the Reynolds stress models show a separated flow near the convex wall starting at the bend exit, which was confirmed by experiment using flow visualisation by smoke. The Reynolds stress models are observed to be superior in comparison with the standard κ-ε and the RNG-κ-ε turbulence models in terms of accuracy. Further conclusions from this work can be summarised as: 1. The proper numerical resolution of this type of flow is dependent on the turbulence model formulations as well as numerical procedures. The results highlight the limitations of the generality of turbulence models when used to model more intricate features of complex flows. 2. The need for more accurate experimental techniques in support of improvement of turbulence models is thus underlined.

Proposal of a RANS Model to Predict Both K- and D-Type Roughness

2003 ◽  
Author(s):  
Makoto Yamamoto
Keyword(s):  
Surface Roughness ◽  
Turbulence Model ◽  
Roughness Element ◽  
Turbulence Models ◽  
Mean Flow ◽  
Foreign Object Damage ◽  
Virtual Force ◽  
Physics Simulation ◽  
The Mean ◽  
Effect Of Surface

The effect of surface roughness on the aerodynamic performance of a blade and an airfoil is one of very important issues in aeronautic and mechanical engineering. Since existing turbulence models were developed for uniformly distributed roughness, the availability would be limited under a certain flow and roughness conditions. Considering a blade surface with ice accretion, foreign object damage and erosion, the roughness is not uniformly distributed, but isolated or local. Therefore, in such a multi-physics simulation, it is needed to develop a more universal turbulence model to predict the flow fields around a blade with various types of surface roughness. In the present study, employing the virtual force that represents the effect of each roughness element on the mean flow field, I develop and verify a new k–ε turbulence model that can be successfully applied to both k- and d-type roughness.

Numerical Modeling of Pulsatile Turbulent Flow in Stenotic Vessels

2003 ◽  
Vol 125 (4) ◽  
pp. 445-460 ◽  
Author(s):  
Sonu S. Varghese ◽  
Steven H. Frankel
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Turbulence Model ◽  
Turbulence Models ◽  
Navier Stokes ◽  
Mean Flow ◽  
Navier Stokes Equation ◽  
Flow Through ◽  
The Mean ◽  
High Reynolds

Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-averaged Navier-Stokes equation approach. The commercially available computational fluid dynamics code (CFD), FLUENT, has been used for these studies. Two different experiments were modeled involving pulsatile flow through axisymmetric stenoses. Four different turbulence models were employed to study their influence on the results. It was found that the low Reynolds number k-ω turbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k-ε turbulence model and the standard k-ε model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow field. All models predicted a wall shear stress peak at the throat of the stenosis with minimum values observed distal to the stenosis where flow separation occurred.

Decoration technique and surface physics

1978 ◽  
Vol 36 (1) ◽  
pp. 436-437 ◽  
Author(s):  
H. Bethge
Keyword(s):  
Diffusion Path ◽  
Special Importance ◽  
Surface Physics ◽  
Step Structure ◽  
Initial Stage ◽  
Special Cases ◽  
Atomic Surface ◽  
The Mean ◽  
Bulk Structure ◽  
Decoration Technique

Besides the atomic surface structure, diverging in special cases with respect to the bulk structure, the real structure of a surface Is determined by the step structure. Using the decoration technique /1/ it is possible to image step structures having step heights down to a single lattice plane distance electron-microscopically. For a number of problems the knowledge of the monatomic step structures is important, because numerous problems of surface physics are directly connected with processes taking place at these steps, e.g. crystal growth or evaporation, sorption and nucleatlon as initial stage of overgrowth of thin films.To demonstrate the decoration technique by means of evaporation of heavy metals Fig. 1 from our former investigations shows the monatomic step structure of an evaporated NaCI crystal. of special Importance Is the detection of the movement of steps during the growth or evaporation of a crystal. From the velocity of a step fundamental quantities for the molecular processes can be determined, e.g. the mean free diffusion path of molecules.

scholarly journals Investigation of Changes in Fatigue Damage Caused by Mean Load under Block Loading Conditions

Materials ◽  
2021 ◽  
Vol 14 (11) ◽  
pp. 2738
Author(s):  
Roland Pawliczek ◽  
Tadeusz Lagoda
Keyword(s):  
Fatigue Life ◽  
Fatigue Damage ◽  
Strain Curve ◽  
Mean Value ◽  
Fatigue Properties ◽  
Stress Strain ◽  
Reference Case ◽  
Block Loading ◽  
The Mean ◽  
Mean Strain

The literature in the area of material fatigue indicates that the fatigue properties may change with the number of cycles. Researchers recommend taking this into account in fatigue life calculation algorithms. The results of simulation research presented in this paper relate to an algorithm for estimating the fatigue life of specimens subjected to block loading with a nonzero mean value. The problem of block loads using a novel calculation model is presented in this paper. The model takes into account the change in stress–strain curve parameters caused by mean strain. Simulation tests were performed for generated triangular waveforms of strains, where load blocks with changed mean strain values were applied. During the analysis, the degree of fatigue damage was compared. The results of calculations obtained for standard values of stress–strain parameters (for symmetric loads) and those determined, taking into account changes in the curve parameters, are compared and presented in this paper. It is shown that by neglecting the effect of the mean strain value on the K′ and n′ parameters and by considering only the parameters of the cyclic deformation curve for εm = 0 (symmetric loads), the ratio of the total degree of fatigue damage varies from 10% for εa = 0.2% to 3.5% for εa = 0.6%. The largest differences in the calculation for ratios of the partial degrees of fatigue damage were observed in relation to the reference case for the sequence of block n3, where εm = 0.4%. The simulation results show that higher mean strains change the properties of the material, and in such cases, it is necessary to take into account the influence of the mean value on the material response under block loads.

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